NANOSTRUCTURED CARBON NANOTUBE SCHOTTKY JUNCTION SOLARCELLS

By

MAUREEN K. PETTERSON

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOLOF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2013

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c© 2013 Maureen K. Petterson

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To my family

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ACKNOWLEDGMENTS

I would like to thank my adviser, Dr. Andrew Rinzler, for his unwavering support

and guidance throughout the past few years. His knowledge of both experimental and

theoretical aspects of physics research is comprehensive and inexhaustible; he was always

present to answer theoretical questions or help design and build experimental apparatus.

His ability to place contemporary research in historical perspective dissuades a myopic

view of graduate research, instead fostering an appreciation for the wide applicability of

experimental results. Dr. Rinzler gave me freedom to spend days, weeks, and months

tediously troubleshooting experiments and his encouragement and availability expedited

the successful results, while his patience and commiseration alleviated frustration over the

failures.

I’d like to thank my committee members for their ongoing support. Dr. Stanton and

Dr. Tanner were among the first professors I met within the physics department, and

I’m glad to have had their counsel over the past several years. Collaborations with Dr.

Hebard and his group have been enlightening and fruitful, culminating in some excellent

published work. Even before he was on my committee, Dr. Biswas lent a lively atmosphere

to the department without undermining the ethos of graduate research and discussions

with him regarding professional pursuits have been beneficial. I’m thankful to Dr. So for

his excellent objective input; his own research gave him particularly good insight into my

research projects.

I also owe a lot of gratitude to my labmates: Dr. Mitchell McCarthy, Dr. Bo Liu,

Dr. Rajib Das, Dr. Svetlana Vasilyeva, Dr. Max Lemaitre, Dr. Pooja Wadhwa, Dr. Evan

Donoghue, Dr. Po-Hsiang Wang, Yu Shen, Xiao Chen, Nan Zhao, Jie Hou, Matt Gilbert,

and Kyle Dorsey. Information gleaned from discussions with them greatly facilitated my

understanding of physics and chemistry. They were eager to help with any problems I

encountered and offered their expertise and guidance on many aspects of my experiments.

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Not relegated to just professional cohorts, they also livened up the atmosphere and made

coming into lab an enjoyable experience.

I’d like to thank Darlene Latimer and Pam Marlin for wading through the academic

bureaucracy on my behalf and always ensuring I was on track to graduate. Darlene’s

genuine concern for all of the graduate students is heartwarming and her contributions to

the physics department are beyond measure. I’d like to thank Jay Horton, Tim Noland,

and the machine shop for providing their technical expertise to our lab and for fabricating

and fixing most (if not all) of our experimental apparatus. I’d also like to thank Pete

Axson and the rest of the electronics shop for keeping our solar simulator in working

order.

I’d like to thank my friends for offering unlimited and unconditional support,

encouragement, and advice; without whom I never would have appreciated the camaraderie

induced by sporting events or tolerated summers in Florida. I’d especially like to thank

Evan Donoghue for bestowing upon me the lessons he learned in his time during graduate

school and for helping me find the perfect balance between hard work and personal

development. Physicists are not generally known for their social aptitude or enthusiastic

inclusion of newcomers, but the friends I have gained through the department defy the

stereotypes and have made my graduate school years ones of both academic and personal

growth.

Finally, I’d like to thank my family. My parents, John Petterson and Loretta Kelley,

for supporting me emotionally, intellectually, and financially for the past three decades.

Both having experienced the trials and tribulations of earning an advanced degree, their

empathy and understanding was greatly appreciated. I owe much to my siblings, Alyssa

and Carey Petterson, for shaping my personality and ultimately putting me on the path to

being a successful physicist.

I’d like to acknowledge the National Science Foundation for funding support under

award ECCS 1232018.

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TABLE OF CONTENTS

page

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

LIST OF ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

CHAPTER

1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 INTRODUCTION TO CARBON NANOTUBES . . . . . . . . . . . . . . . . . 21

2.1 History and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.2 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3 INTRODUCTION TO SOLAR CELLS . . . . . . . . . . . . . . . . . . . . . . . 30

3.1 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.1.1 Generation and Solar Spectrum . . . . . . . . . . . . . . . . . . . . 303.1.2 Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.1.2.1 Radiative . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.1.2.2 Auger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.1.2.3 Shockley Reed Hall . . . . . . . . . . . . . . . . . . . . . . 34

3.1.3 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.1.4 Series and Shunt Resistance . . . . . . . . . . . . . . . . . . . . . . 37

3.2 Theoretical Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.3 Types of Solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.3.1 P-N Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.3.2 Organic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.3.3 Photoelectrochemical Devices . . . . . . . . . . . . . . . . . . . . . 453.3.4 Multi-junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.3.5 Schottky junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.3.6 Inversion Layer Cells . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4 INTRODUCTION TO SCHOTTKY BARRIERS . . . . . . . . . . . . . . . . . 50

4.1 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504.1.1 Basic Schottky Model . . . . . . . . . . . . . . . . . . . . . . . . . . 504.1.2 Current Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.1.2.1 Thermionic Emission . . . . . . . . . . . . . . . . . . . . . 524.1.2.2 Thermionic Field Emission and Field Emission . . . . . . . 524.1.2.3 Minority Carrier Injection . . . . . . . . . . . . . . . . . . 53

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4.1.3 Beyond Schottky-Mott . . . . . . . . . . . . . . . . . . . . . . . . . 534.1.3.1 Fermi Level Pinning: Bardeen Model and Metal Induced

Gap States . . . . . . . . . . . . . . . . . . . . . . . . . . 554.1.3.2 Bond Polarization . . . . . . . . . . . . . . . . . . . . . . 56

4.2 Schottky Junction Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . 574.2.1 Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . 574.2.2 CNT on Silicon Schottky Junction Solar Cells . . . . . . . . . . . . 58

4.2.2.1 Experimental Details and Equipment . . . . . . . . . . . . 584.2.2.2 Electronic Gating . . . . . . . . . . . . . . . . . . . . . . . 604.2.2.3 Inversion Layer Modeling . . . . . . . . . . . . . . . . . . 60

5 NANOSTRUCTURING FOR ENHANCED LIGHT ABSORPTION . . . . . . . 66

5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665.2 Potassium Hydroxide Etching . . . . . . . . . . . . . . . . . . . . . . . . . 665.3 Silicon Nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

5.3.1 Procedure and Characterization . . . . . . . . . . . . . . . . . . . . 695.3.2 Integration in solar cells and initial performance . . . . . . . . . . . 71

5.3.2.1 Remote Gating . . . . . . . . . . . . . . . . . . . . . . . . 735.3.2.2 Passivation of Nanowire Sidewalls . . . . . . . . . . . . . . 745.3.2.3 SWNT film transfer on SiNW . . . . . . . . . . . . . . . . 76

5.3.3 Discussion of inversion layer in SiNWs . . . . . . . . . . . . . . . . . 785.4 Effect of Oxygen and Water on Device Performance . . . . . . . . . . . . . 79

5.4.1 Effect of ambient oxidation . . . . . . . . . . . . . . . . . . . . . . . 795.4.2 Reversible doping in ambient environment . . . . . . . . . . . . . . 815.4.3 Water vapor and oxygen contamination . . . . . . . . . . . . . . . . 82

5.4.3.1 CV measurements showing IL contamination . . . . . . . . 835.4.3.2 Exclusion on planar device . . . . . . . . . . . . . . . . . . 84

5.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

6 PASSIVATION OF SILICON . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

6.1 Atomic Layer Deposition of Al2O3 and HfO . . . . . . . . . . . . . . . . . 906.1.1 Al2O3 and HfO results . . . . . . . . . . . . . . . . . . . . . . . . . 93

6.2 Hydroquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966.3 Sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 986.4 Discussion and Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

7 ADDITIONAL PROJECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

7.1 TFSA Doping of Graphene-Si and Carbon Nanotube-Si Devices . . . . . . 1047.1.1 Graphene-Si Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . 1047.1.2 TFSA with carbon nanotubes . . . . . . . . . . . . . . . . . . . . . 107

7.2 Backside Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1087.3 Concluding Remarks and Path Forward . . . . . . . . . . . . . . . . . . . . 110

APPENDIX

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A FULL SIMULATIONS FOR THE INVERSION LAYER CELL . . . . . . . . . 113

B SOLAR CELL PARAMETERS WITH INCREASING OXIDATION TIME . . . 114

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

BIOGRAPHICAL SKETCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

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LIST OF TABLES

Table page

3-1 Current maximum efficiencies for various photovoltaic devices [1] . . . . . . . . . 49

4-1 Theoretical vs Experimental Schottky Barrier Heights: Barrier heights measuredat 300K, theoretical values determined from Schottky-Mott relation . . . . . . . 65

5-1 Performance for various film deposition techniques and thicknesses . . . . . . . . 88

6-1 Performance for ALD devices for VG = -1.0 V . . . . . . . . . . . . . . . . . . . 103

7-1 Performance summary for TFSA doped graphene and SWNT solar cells . . . . . 112

7-2 Performance for backside doped substrates . . . . . . . . . . . . . . . . . . . . . 112

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LIST OF FIGURES

Figure page

2-1 Graphene lattice and geometric classifications for carbon nanotubes . . . . . . . 22

2-2 Band structure for carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . 23

2-3 Density of states for semiconducting and metallic carbon nanotubes . . . . . . . 24

2-4 Electronic gating of SWNT film . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2-5 Dedoping of SWNT film during high temperature bake . . . . . . . . . . . . . . 29

3-1 The solar spectrum received both outside the Earth’s atmosphere and at thesurface of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3-2 Three recombination routes within semiconductors. . . . . . . . . . . . . . . . . 34

3-3 Maximum generated power density (blue box), defined by P = VMJM . . . . . . 36

3-4 Circuit equivalent showing series and shunt resistance. . . . . . . . . . . . . . . 38

3-5 The effects of series and shunt resistance on the J-V curve . . . . . . . . . . . . 39

3-6 The Shockley-Queisser limit showing the maximum theoretical efficiency as afunction of band gap for a single p-n junction solar cell. . . . . . . . . . . . . . 41

3-7 Schematic for a p-n junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3-8 Band diagram of P-N junction with no bias, reverse bias, and forward bias . . . 43

3-9 Schematic of the bilayer and bulk heterojunction solar cells. . . . . . . . . . . . 45

3-10 Schematic of simple semiconductor/liquid junction solar cell showing redox reactionsoccurring both at the semiconductor surface and at a metal counterelectrode. . . 46

4-1 Schottky barrier band diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4-2 Interface states shown in the realistic model of a Schottky junction. . . . . . . . 55

4-3 Carbon nanotube-silicon Schottky junction cell . . . . . . . . . . . . . . . . . . 59

4-4 Schematic and results for electronically gated SWNT-Si cell . . . . . . . . . . . 61

4-5 Schematic and performance for grid cell . . . . . . . . . . . . . . . . . . . . . . 62

4-6 Simulations showing inversion layer in silicon extending across entire surface inbetween carbon nanotube strips . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5-1 KOH schematic and performance. . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5-2 The mechanism for silicon nanowire growth . . . . . . . . . . . . . . . . . . . . 69

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5-3 Silicon nanowires grown in an HF/AgNO3 solution . . . . . . . . . . . . . . . . 70

5-4 Orientation of silicon nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

5-5 Reflectance of the silicon nanowire substrates as compared to untextured silicon. 72

5-6 Initial performance of the SWNT-SiNW device . . . . . . . . . . . . . . . . . . 73

5-7 Schematic for remote gating and SEM of SWNT-SiNW active area . . . . . . . 74

5-8 J-V of a SWNT-SiNW device showing the effect of sidewall passivation via oxidationon the performance of the device. . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5-9 J-V curves for VG = -1.0 V, 0 V, +1.0 V on the SiNW device. . . . . . . . . . . 77

5-10 Evolution of J-V curve with oxidation in ambient atmosphere. . . . . . . . . . . 80

5-11 Reversibility of the J-V curve upon alternating exposure to argon and ambientatmospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5-12 Cyclic voltammograms of the glassy carbon electrode in EMI-BTI ionic liquidat 50 mV

s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5-13 Stability of planar device with oxygen and water excluded by gating in inertatmosphere with VG=-1.0 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

5-14 Degradation of the planar SWNT-SiNW device upon exposure to atmospherewith VG=-1.0 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

6-1 ALD growth process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

6-2 SEM image of ALD deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6-3 J-V curves for the ALD Al2O3 coated SWNT/SiNW cell . . . . . . . . . . . . . 93

6-4 J-V curves for ALD SWNT-SiNW device vs device without ALD . . . . . . . . 94

6-5 J-V for the ALD HfO device showing a lowering JSC due to the high reflectanceof the device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

6-6 Silicon substrate and hydroquinone molecule . . . . . . . . . . . . . . . . . . . . 97

6-7 J-V curve of the HQ treated planar cell before, during, and after electronic gatingwith EMI-BTI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

6-8 J-V curves for sulfur and hydroquinone passivated devices . . . . . . . . . . . . 101

7-1 JV curve for the monolayer graphene device. . . . . . . . . . . . . . . . . . . . . 106

7-2 Schematic and performance for graphene PV cell. . . . . . . . . . . . . . . . . . 107

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7-3 J-V curves showing effect of TFSA doping and subsequent gating on SWNT-Sidevice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

7-4 Blistering on the surface of the silicon following a high temperature bake to dopethe backside. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

A-1 Modeling of the inversion layer at the silicon surface in the carbon nanotubegrid solar cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

B-1 FF, JSC , VOC , and PCE for a SWNT-SiNW device for various oxidation timesin the lab atmosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

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LIST OF ABBREVIATIONS

A Richardson Constant

ALD Atomic Layer Deposition

AM1.5G AirMass 1.5 Global

CNT Carbon Nanotube

CVD Chemical Vapor Deposition

Dn/p Electron or hole diffusion constant

DOS Density of States

EF Fermi energy

EMI −BTI 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

FF Fill Factor

JM Current density at maximum power point

Jphoto Photocurrent density

JO Saturation current density/dark current

JSC Short circuit current density

MIGS Metal induced gap states

MIS − IL Metal-insulator-semiconductor inversion layer cell

PM Maximum power density

PCE Power conversion efficiency

φm Work function of metal

φBn0 Schottky barrier height to n-type semiconductor

φ0 Neutral level (above EV ) of interface states

∆ Potential across interfacial layer

χ Electron affinity of semiconductor

ψbi Built-in potential

δ Thickness of interfacial layer

q Electron charge

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Qsc Space-charge density in semiconductor

Qss Interface-trap charge

QM Surface-charge density on metal

Dit Interface-trap density

ǫi Permittivity of interfacial layer (vacuum)

ǫs Permittivity of semiconductor

PLV Pulsed Laser Vaporization

RS Series Resistance

RSH Shunt Resistance

SBH Schottky Barrier Height

SRH Shockley-Reed Hall Recombination

SiNW Silicon Nanowire

SWNT Single-Wall Nanotube

µe/p Electron or hole mobility

VM Voltage at maximum power point

VOC Open Circuit Voltage

VG Gate Voltage

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Abstract of Dissertation Presented to the Graduate Schoolof the University of Florida in Partial Fulfillment of theRequirements for the Degree of Doctor of Philosophy

NANOSTRUCTURED CARBON NANOTUBE SCHOTTKY JUNCTION SOLARCELLS

By

Maureen K. Petterson

August 2013

Chair: Andrew G. RinzlerMajor: Physics

This dissertation explores and exploits the physical processes uncovered during

experiments aimed at improving solar cell efficiency in a novel electronically gated solar

cell through surface texturing. Besides the increased device efficiency, the findings shed

light on the previous limitations in similar devices and may have implications for other

semiconductor based devices.

Silicon nanowires have long been known for their excellent antireflection properties,

but have suffered substantially from recombination at the surface. Here, we deposit a

disperse carbon nanotube network on the tips of a forest of vertical silicon nanowires and

exploit electronic gating in a novel Schottky junction solar cell. Previous experiments on

carbon nanotube- silicon solar cells made use of an ionic liquid to modulate the nanotube

Fermi level via electronic gating. This modulation changed the Schottky barrier height of

the device and decreased the carbon nanotube film resistance, leading to power conversion

efficiencies of up to 12% for a gate voltage of -0.75V. Further experiments uncovered

an additional mechanism in which the ionic liquid induced an inversion layer within the

silicon, greatly facilitating hole extraction by repelling electrons from the silicon surface

(and consequently reducing recombination). We exploit this induced inversion layer within

our silicon nanowire solar cells and show a greatly increased power conversion efficiency

exceeding 15%, the highest reported efficiency for silicon nanowire based devices to date.

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We also investigate the physical and chemical processes responsible for degradation

in these devices. We show that contamination of the ionic liquid with oxygen or

water leads to redox reactions for gate voltages previously thought to be well within

the electrochemical window. We subsequently demonstrate that by excluding these

contaminants, stable performance of the electronically gated nanotube/silicon solar cell

is possible. Advanced passivation techniques are used to alleviate such degradation.

Specifically, deposition of aluminum oxide via atomic layer deposition was used to create

a high quality, conformal, dielectric layer that inhibits electrochemical reactions between

the ionic liquid and the silicon, leading to minimal reduction in performance as the gate

voltage is applied.

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CHAPTER 1INTRODUCTION

Photovoltaics have been an extremely active area of research since the early 1970s, yet

only a few device structures have passed the test of affordability, longevity, and efficiency.

Volatile markets, decreasing resources, and a penchant for innovation fuel the continued

research into different novel architectures utilizing a variety of organic and inorganic

materials. Concomitant with the increase in solar cell efficiency is a deeper understanding

of the underlying physical processes present in such devices, something which also has

more general scientific value. The high efficiency of the solar cells presented in this thesis

was achieved by efforts to understand the underlying physics of the devices and using that

knowledge to improve light absorption while minimizing losses and degradation. The high

efficiencies were realized through multiple methods, as discussed in detail in Chapters 4-7.

A brief summary of the dissertation is presented below.

Chapter 2 starts off by discussing the theoretical background of single wall carbon

nanotubes (SWNT), with particular emphasis on the ability to modulate the Fermi level

of the SWNTs due to their low density of states. This modulation can be experimentally

verified by observing the change in transmittance of the film during electronic gating, as

demonstrated in the work of Dr. Zhihong Chen and Dr. Zhuangchun Wu, who led that

effort in the Rinzler group.[8]

Chapter 3 gives a brief introduction to solar cells. Testing, characterization, and

different types of solar cells are presented, along with some of the challenges confronting

researchers in their pursuit to develop high efficiency devices. Historical information on

solar cells is mentioned with the purpose of showing how work described later in this

thesis can address and solve problems encountered in the photovoltaic devices developed in

the 1970s.

Chapter 4 presets a more thorough description of Schottky junction solar cells,

with emphasis on the physics of Schottky barriers, including how surface preparation

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affects device performance. The carbon nanotube-silicon Schottky junction solar cell

is introduced, along with the work done by Dr. Pooja Wadhwa in which such a device

was electronically gated. Modulating the gate voltage between the active area film and

a gate film modulates the SWNT Fermi level, changing the Schottky built-in potential

and modulating device performance. Lastly, the inversion layer grid cell is described. This

device, in which I made my first contributions to this class of devices, demonstrated a

new mechanism by which the ionic liquid used during electronic gating of the SWNT film

simultaneously forms an inversion layer within the silicon.[10] This allowed for efficient

collection of photogenerated carriers far from the SWNT gridlines, boosting the efficiency

of the device from 10.9% to 12%.

Chapter 5 details my work on nanostructuring the silicon surface to improve

light absorption. This work was motivated by the results of the inversion layer device

discussed in Chapter 4. The ability to induce an inversion layer within the silicon no

longer constrains us to have the carbon nanotube film touching the entire silicon surface,

allowing exploration of alternative architectures. Silicon nanowires (SiNW), known for

their excellent anti-reflection properties, were integrated into a SWNT-SiNW device and

took full advantage of the ionic liquid-induced inversion layer along the nanowire sidewalls.

The greatly increased surface area of the nanowires required modifications to the solar

cells, specifically an increased surface area gate film to compensate for the ions needed

to induce the inversion layer, and a SWNT spray deposited film to improve connection

between the silicon nanowires and the carbon nanotubes. Integration of these two led to a

greatly improved power conversion efficiency of over 15%, the highest PCE for any silicon

nanowire device to date.

The latter part of Chapter 5 addresses the stability of these devices during electronic

gating. Experiments on the carbon nanotube-silicon Schottky junctions solar cells

showed a reduction of the PCE as the device was electronically gated. Characteristics

of degradation suggested that redox reactions were facilitating oxidation of the silicon

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surface, forming a barrier to carrier extraction and decreasing performance. The greatly

increased surface area of the silicon nanowires, versus the planar devices, exhibited a

greater degradation (and hence more redox reactions), as evidenced by an increased

parasitic gate current. Experiments to test the electrochemical window of our ionic liquid

showed a substantial reduction of the window due to contamination of water and oxygen.

Aware that testing in ambient atmosphere would lead to immediate contamination,

I tested a planar device in a glovebox using dried ionic liquid and observed stable

performance over numerous hours. A dramatic reduction in gate currents indicating

negligible redox reactions conclusively demonstrated that the degradation was due to

contamination of water and oxygen at the silicon surface.

Chapter 6 describes my work to reduce degradation of the SWNT-SiNW devices

during electronic gating. Though simple encapsulation can eliminate degradation, I

explored both atomic layer deposition and chemical passivation as a means to elicit stable

performance. Hydroquinone and sulfur passivation led to improved device performance

prior to electronic gating, but ultimately proved to be incompatible with the ionic

liquid. Atomic layer deposition of aluminum oxide on the fully fabricated device limited

contact between the ionic liquid and the silicon surface during electronic gating without

sacrificing the inversion layer. This reduced contact limited redox reactions and lowered

the gate current by a factor of 60. Though the device was tested in the atmosphere with

”contaminated” ionic liquid, the ALD layer improved the stability of the devices and still

produced a high power conversion efficiency of 14.8%.

Finally, Chapter 7 discusses two side projects: graphene-silicon Schottky junction

solar cells and backside doping of the silicon substrates. The former demonstrated a

greatly enhanced power conversion efficiency upon introduction of the organic dopant,

bis(trifluoromethylsulfonyl)amide (TFSA). The improvement in efficiency is attributed to

an increase in the Schottky barrier height, decrease in series resistance, and the ability of

the TFSA to act as an anti-reflection layer. Lastly, backside doping was found to improve

19

the power conversion efficiency by limiting recombination at the back contact. A spin

on dopant deposited onto the backside of the silicon substrates produced a think, highly

doped region on the backside of the silicon. The power conversion efficiency of the planar

devices was improved from 9.7% to 13.4%.

20

CHAPTER 2INTRODUCTION TO CARBON NANOTUBES

2.1 History and Structure

First structurally interpreted in 1991 by Sumio Iijima at the Nippon Electric

Company (NEC), carbon nanotubes have been the focus of intense research and

development for the past two decades. Fundamental properties and novel applications

have been explored in the physical sciences since their discovery, while within the medical

and biological sciences much work has been done to incorporate carbon nanotubes into

various devices ranging from prosthetics to molecular transporters.[2–5] The wide range of

potential applications is derived from the unique electrical, physical, and optical properties

possessed by these fullerenes. A quasi-one dimensional structure, carbon nanotubes can be

thought of as a sheet of graphene rolled into a seamless tube with diameters approximately

1-10 nm and aspect ratios up to 105.

The electronic and optical properties of carbon nanotubes can be calculated from the

band structure of graphene due to the local structural similarity of the two. Graphene is a

simple two dimensional hexagonal lattice composed of sp2 bonded carbon atoms. A long,

narrow rectangular strip cut from this lattice and rolled up along the narrow dimension

(with bonds reformed across the seam) generates the structure of a single wall nanotube

(SWNT). Depending upon the orientation of the strip direction relative to the graphene

lattice SWNTs of three distinct structural classifications can form: armchair, zigzag, or

chiral. Aside from their geometric classifications, SWNTs can be subdivided into either

metallic or semiconducting types based on the nanotube n,m index (defined below).[6, 7]

Figure 2-1A shows the graphene lattice and corresponding unit cell with primitive

vectors, −→a1 and −→a2 along with the chiral vector−→C . Defined as

−→C = n−→a1 +m−→a2 for integer n, m, (m < n), (2–1)

21

A Graphene lattice B Carbon nanotube geometries

Figure 2-1. Graphene lattice and geometric classifications for carbon nanotubes. Figure A:Graphene lattice showing primitive vectors −→a1 and −→a2 that comprise the unitcell. Also shown is the chiral vector that determines nanotube type. Figure B:The three geometric types of carbon nanotubes. Reprinted with permissionfrom R. Saito, G. Dresselhaus, and M. S. Dresselhaus, Physical properties ofcarbon nanotubes (Imperial College Press, 1998)

the chiral vector is a linear combination of primitive vectors governing the directions in

which a graphene sheet can be rolled up, subsequently determining the specific properties

of the nanotube; n=m results in an armchair nanotube, m=0 corresponds to a zigzag

nanotube, and all other combinations of n,m result in a chiral nanotube. The conditions

for a metallic nanotube require that (n − m) = 3j, where j is an integer. Consequently,

all armchair nanotubes are metallic while zigzag and chiral nanotubes can be either type;

these selection rules lead to an overall 2:1 ratio of semiconducting to metallic nanotubes

across all allowed n, m.

The energy dispersion relations and density of states (DOS) for nanotubes are derived

by placing the appropriate circumferential boundary conditions on the energy dispersion

relation for graphene and solving for the allowed k values and associated energy states.

For a single sheet of graphene,

22

Eg2D(kx, ky) = ±t{

1 + 4 cos

(√3kxa

2

)

cos

(kya

2

)

+ 4 cos2(ky

a

)}1

2

, (2–2)

where t = -3.033eV and kx and ky correspond to the x and y axes in reciprocal space.[7]

The boundary conditions require ka to be multiples of π, with the exact dispersion

relation for a given nanotube depending on the chiral vector. Figure 2-2 shows the band

structure for both a metallic and semiconducting nanotube. Shown in Figure 2-3 is the

density of states for both a semiconducting and metallic nanotube, with the graphene

DOS overlayed as a dotted line.

Figure 2-2. Band structure for carbon nanotubes. Shown is a metallic a) (5,5) and b) (9,0)nanotube and c) a semiconducting (10,0) nanotube. Reprinted with permissionfrom Saito, et al. Physical properties of carbon nanotubes (Imperial CollegePress, 1998)

The DOS of graphene goes to exactly zero at the six Dirac points in the Brillioun

zone, making graphene a zero gap semiconductor. The cyclical symmetry of carbon

nanotubes restricts the allowed wave vectors along the circumference. Divergences of the

density of states, called Van Hove singularities, are due to the one dimensionality of the

carbon nanotubes and are the visible spikes in Figure 2-3.

23

Figure 2-3. The density of states for a semiconducting nanotube (left), and a metallicnanotube (right). The dotted line overlayed in both images is the density ofstates for graphene. Note the low DOS for the metallic nanotubes near theFermi energy. γ0 is 3.13 eV and corresponds to the C-C tight binding overlapenergy. Reprinted with permission from Saito, et al. Physical properties ofcarbon nanotubes (Imperial College Press, 1998)

Carbon nanotubes are notable for their relatively low density of states and easily

manipulated Fermi level. Both electronic gating and chemical charge transfer doping can

be utilized to shift the carrier concentration of the nanotubes, allowing integration into

devices that demand control over conductance.[8–10] This change in electronic populations

concomitant with a Fermi level modulation can be demonstrated by a change in the

film transmittance. Figure 2-4B shows the experimental set up for demonstrating such

modulation via electronic gating. Two carbon nanotube films are transferred to a quartz

substrate and then baked at 600C to dedope the films, after which the entire substrate

is submerged in a liquid electrolyte and a gate voltage is applied between the two films,

driving the ions in the electrolyte to either film in a response to Coluombic forces.

Inclusion of ionic liquid boosts the capacitance of the nanotube film, akin to inserting a

dielectric between a parallel plate capacitor. Figure 2-4A shows the transmittance data

taken with a UV-Vis spectrophotometer as a gate voltage between the test film and gate

film was held at incremental voltages from -1.8V to +1.8V. The S1, S2, and M1 peaks

corresponding to electronic transitions between Van Hove singularities. As a negative gate

24

voltage is applied, the test film becomes more p-doped as the Fermi level is pushed farther

from vacuum, resulting in an increased transmittance as electrons are depleted from the

Van Hove singularities that comprise the ground state for the corresponding transition.

As a positive gate voltage is applied, the trend reverses and the transmittance decreases.

This contradicts the intuitive idea that any applied gate voltage (positive or negative) will

result in an increased transmittance due to a mid-gap Fermi level at zero applied voltage.

In the idealized case, any applied gate voltage will decrease the number of available states

for electronic transitions, lowering the transmittance and leading to ambipolar behavior.

The experimental results seem to belie the theory. Though initially attributed to possible

contamination effectively p-doping the carbon nanotubes, we now believe this discrepancy

is attributed to electrons from the carbon nanotubes being donated to an oxygen/water

redox couple, as was studied in depth by Martel in 2009.[11] The hydrophilic quartz

substrate and water contaminated ionic liquid provide ample amounts of water and oxygen

to facilitate electrochemical reactions, leading to p-type behavior

A Electronic Gating B Experimental Setup

Figure 2-4. Electronic gating of SWNT film. Figure A: Change in transmittance of a 45nmthick SWNT film with various applied gate voltage. Figure B: Experimentalset-up for transmittance measurements.

25

2.2 Synthesis

Carbon nanotubes can be synthesized through four different methods: arc discharge,

pulsed laser vaporization (PLV), plasma torch, and chemical vapor deposition (CVD).

The initial procedure for carbon nanotube fabrication, arc discharge growth is achieved by

applying a potential between two graphite/carbon electrodes with the resultant discharge

heating the carbon targets, catalyzing the formation of carbon nanotubes and other

fullerenes.[12] PLV also uses a high energy beam to form carbon nanotubes, but employs a

green and infrared laser to ablate a carbon target.[13, 14] Environmental conditions within

the carbonaceous vapor affect the specific properties of the carbon nanotubes; temperature

relates to the diameter while the presence of metal catalysts particles facilitate formation

of single wall nanotubes instead of multi-walled nanotubes.[15–17] During the growth,

several individual carbon nanotubes adhere together via Van der Walls forces to form

nanotube bundles tens of nanometers in diameter. Plasma torch growth is another

permutation of growth via thermal decomposition of catalyst precursors in a carbonaceous

gas. In this method, a carbon source and a metal catalyst source are simultaneously fed

through a plasma torch, producing carbon nanotubes in the heated vapor.[18]

CVD growth comprises a few different methods: conventional CVD, plasma enhanced

CVD (PECVD), HiPCo (high-pressure conversion of carbon monoxide), Ferrocene

injection, and floating catalyst method, to name a few. The first generation of CVD

growth developed by Nikolaev, et al. used flowing carbon monoxide in conjunction with a

metal catalyst containing gas in a continuous flow reactor. Growth of carbon nanotubes

occurred though thermal decomposition of the metal catalyst within the heated carbon

monoxide flow, after which they would adhere to the sides of the quartz tube. [19].

Alternatively, metal catalyst particles can be pre-deposited onto a carrier substrate and

then placed into the growth chamber whereupon the carbon containing reactant gas would

be fed. Decomposition in the high temperature growth chamber would be followed by

carbon nanotube growth facilitated by the metal particles on the carrier substrate.[20–22]

26

The carbon nanotubes used for the experiments described in the following chapters

were synthesized by PLV and subsequently underwent an extensive purification process

to eliminate residual amorphous carbon, catalyst particles, and other non-nanotube

contaminants. A 2.4M nitric acid reflux removed amorphous carbon (and other forms

of carbon exhibiting relatively weak sp3 bonding) and metal impurities in addition to

p-doping the carbon nanotubes. Multiple centrifugations at 6000 RPM followed by

decantation of the acidic supernatant neutralized the carbon nanotube/acid solution and

allowed their dispersion in a 1% Triton-X surfactant solution. Next, crossflow filtration

eliminated reaction products and fine particulates by repeatedly passing the solution

through long, hollow fibers with small pores along the sidewalls. These small pores allow

the passage of small particulates while preventing the permeation of carbon nanotubes.

This filtration continued until the permeate is clear, indicating a majority of small

particles have been removed from the solution. An additional centrifugation (6000-10000

RPM), this time retaining the supernatant, separated out contaminants with a density

greater than that of the surfactant buoyed carbon nanotubes.. Lastly, a filtration though

a 650 µm membrane removed large particulates not broken down by the nitric acid reflux

or eliminated by previous steps, ultimately producing a purified, surfactant based carbon

nanotube suspension.[8, 23–25]

In the studies discussed in this work the SWNTs were typically used in the form

of thin transparent films. Such film formation proceeded as follows. Starting with the

purified solution, the carbon nanotubes are vacuum filtered onto a mixed cellulose

(MCE) membrane and copiously rinsed with deionized water to remove residual Triton-X

surfactant. After drying under an incandescent lamp, the carbon nanotube film is ready

to be transferred to a substrate or stored in an inert atmosphere until needed. To transfer

to a substrate (such as glass, ITO, PET, silicon, etc), the nanotube film is first placed

against the substrate (MCE side up), on top of which is placed a porous Teflon membrane

followed by a hydrated sheet of porous plastic. This assembly is sandwiched between

27

two aluminum plates that are clamped together to supply pressure between the carbon

nanotube film and the substrate. Upon being placed in a 100C oven, the water in the

porous plastic evaporates and wets the nanotube film. As the steam slowly diffuses out

of the assembly with continued heating, the nanotubes are brought into intimate contact

with the substrate and are thereby retained via Van der Walls forces. After several hours,

the assembly is removed and the substrate/film is placed in an acetone vapor bath to

dissolve the MCE membrane, leaving behind the carbon nanotube film. Subsequent liquid

acetone baths ensure complete removal of the cellulose, after which the substrate is placed

in an isopropol bath (a generally cleaner solvent than acetone, in which the latter is

miscible), removed, and thoroughly dried in a nitrogen stream.

As mentioned above, the carbon nanotubes are charge transfer p-doped during the

nitric acid reflux. Nitrogen based cations, NOx, intercalate the nanotube bundles and

sequester electrons, effectively shifting the Fermi level towards the valence band edge

and hole doping the nanotubes.[26] This results in a change in transmittance in both the

optical and IR regimes that can be observed with a UV/VIS/NIR spectrophotometer.

Baking at 600C provides enough thermal energy to de-adsorb the dopant species, resulting

in a lower conductivity and reduced transmittance at wavelengths corresponding to Van

Hove singularities, as shown in Figure 2-5.

Controlled doping combined with the ability to modulate the Fermi level via

electronic gating make it possible to tailor carbon nanotube’s electrical and optical

properties on an as-needed basis. The photovoltaic devices presented in this dissertation

take full advantage of this malleability and exhibit superior performance compared to

similar photovoltaic devices. Additionally, information regarding interactions between

the carbon nanotubes and other materials (crystalline semiconductors and electrolytes)

has been gleaned through experiments aimed at improving PV efficiency, elucidating

fundamental physical interactions within the device.

28

Figure 2-5. Dedoping of SWNT film during high temperature bake. The shift intransmittance for a 45 nm single walled nanotube film baked at 600C,indicating dedoping. The inset shows the density of states of a semiconductingand metallic nanotube, with the shaded regions indicating electronicpopulations (in this case, p-type doping).

29

CHAPTER 3INTRODUCTION TO SOLAR CELLS

3.1 Fundamentals

A rise in gas prices coupled with heightened environmental awareness has prompted

increasing amounts of research into alternative energy sources. Though much research

has been put into the advancement of various types of solar cells, silicon based devices

continue to dominate the market. Silicon is readily available, well understood, and a staple

of the electronics manufacturing industry. Combined with nuclear, wind, and hydro, solar

power promises to become one of the front contenders in the energy market. It’s modest

and localized installation requirements make it attractive for individuals who desire

more control over their energy use or for urban locations with minute amounts of spare

real estate. Other forms of alternative energy require vast tracts of land or are highly

geographic dependent, while solar power can be efficiently harvested most anywhere south

of the mid-latitudes. A single type of solar cell will not be sufficient to meet the needs of

every application, and as such continuing research in all types of solar cells is vital to the

future of alternative energy.

Unlike a battery, which supplies a constant voltage, a solar cell is more akin to a

current generator that is limited by the photon flux incident on the active area of the

device. Excluding those devices exhibiting multiple exciton generation, every photon

with energy above the bandgap that is absorbed within the active material generates an

electron-hole pair. The primary aims of R & D in this area are to both absorb all photons

hitting the solar cell and to collect all the photogenerated carriers. Table 3-1 at the end of

the chapter shows the current progress of various types of photovoltaic devices.

3.1.1 Generation and Solar Spectrum

Not all semiconducting materials are suitable for broadband absorption of the solar

spectrum; a large bandgap is ideal for collecting high energy photons, while a smaller

bandgap wastes much of the energy of higher energy photons as the photogenerated

30

electrons decay to the bottom of the conduction band by generating heat within the cell.

As such, small bandgap materials are generally relegated to an IR absorption layer within

tandem cells. The sun’s spectrum is that of a blackbody with an average temperature of

5800K, however the radiation received at the top of the Earth’s atmosphere differs from

that in space due to enhanced absorption and scattering from gas and water vapor and the

oblique angle at which the radiation hits the terrestrial surface. Shown in Figure 3-1 are

the relevant solar spectra at the top and bottom of earth’s atmosphere compared to a 5250

C black body spectrum..[27]

Figure 3-1. The solar spectrum received both outside the Earth’s atmosphere and at thesurface of the Earth. Image created by Robert A. Rohde for Global WarmingArt.

Comparisons of solar cell efficiency are only valid if all the devices have been tested

under the same conditions. In order to create consistency among reported performance,

the community has agreed upon a solar spectrum corresponding to the average radiation

received at the mid-latitudes, designated Air Mass 1.5 Global (AM1.5G). Equivalent to

roughly 100 mWcm2 , this spectrum approximates the solar irradiance at the Earth’s surface

when the sun is 48.2◦ off zenith and accounts for scattering by the atmosphere. AM0

31

is the solar irradiance at the top of the atmosphere (roughly equivalent to a blackbody

spectrum at 5800K), a higher fluence that overestimates photon flux for solar installations

in the United States.

A photon incident on a solar cell that is absorbed within the semiconducting

material will excite an electron to the conduction band, leaving behind a hole in the

valence band. Extraction of this electron only occurs if there is a driving force moving

the electron or hole towards the electrodes. Extraction of the charges is predicated on

transport of the charge to the electrodes, necessitating an internal mechanism to move the

photogenerated carriers. While both drift (due to an internal electric field) and diffusion

(due to concentration gradient) can contribute to the current, the former is more efficient

at quickly separating electron hole pairs and ferrying them to their irrespective electrodes.

This driving force is found within diodes, and the total current in a solar cell can be

represented by the Shockley diode equation with an additional term corresponding to the

photocurrent.[27]

Jtotal = Jphoto + Jdiode (3–1)

Jdiode = Jo

[

exp

(qV

kT

)

− 1

]

,

where Jo is referred to as the dark current and yields information regarding recombination

at the junction (a more detailed analysis of the Shockley equation is given in Chapter 3).

The binding energy of the electron hole pair varies depending on the absorber

material: in silicon it is only 14.7 meV, low enough for the pair to be dissociated at room

temperature. In contrast, the binding energy in organic materials is on the order of 0.5 -

1 eV, necessitating a mechanism for dissociation in order to separate and then extract the

photogenerated carriers.[28, 29]

3.1.2 Recombination

Recombination is the combining of a photogenerated electron and hole. It remains the

primary loss mechanism within solar cells; carriers that recombine cannot be extracted as

32

usable electrical energy and overall cell efficiency decreases. There are three recombination

types pertinent to solar cells: Radiative, Auger, and Shockley-Reed-Hall (SRH). The

dominant source of recombination is dependent on the type of material used: electron

hole pairs in direct band gap semiconductors are predisposed to combine radiatively,

while carriers in indirect band gap semiconductors are more likely to under go SRH

recombination. Auger recombination is possible in both types of semiconductors, though

typically negligible at most doping densities.[30] The type of band gap isn’t the only

contributing factor to recombination rates; crystallinity plays a large role as well.

Amorphous silicon, which can possess hugely varying densities of defects and dangling

bonds, has carrier lifetimes on the order of 10−9 s, far shorter than the value for it’s

crystalline counterpart, which has a minority carrier lifetime of 2.5 x 10−3 s.[30, 31]

3.1.2.1 Radiative

Radiative recombination is a two body interaction in which an electron combines

with a hole and produces a photon. Conservation of crystal momentum dictates that

this process only occur in a direct band gap semiconductor where a direct band to

band transition may occur without an additional interaction. Hence, direct band gap

materials are prone to having high levels of recombination in the bulk, necessitating

thinner semiconductor layers in order to extract photogenerated carriers before they

recombine. This results in a relatively short diffusion length and smaller carrier lifetime.

One of the most popular direct bandgap semiconductors, GaAs, has a minority carrier

lifetime of approximately 10−8 s, several orders of magnitude shorter than that of silicon.

Consequently, direct band gap materials are often relegated to multi-junction solar cells.

Shown in Figure 3-2 are the schematics for the three types of recombination.

3.1.2.2 Auger

Unlike radiative recombination, Auger recombination is a three carrier interaction

that can occur in both direct and indirect band gap semiconductors. An electron and

hole recombine with a kinetic energy transfer to a third carrier which is subsequently

33

Figure 3-2. Three recombination routes within semiconductors. Adapted from Principlesof Semiconductor Physics, Van Zeghbroeck, B. unpublished.

excited to a higher energy level. The third carrier then relaxes down via phonon emission.

Although this process is permitted in all semiconductors, it requires high carrier densities

to contribute significantly to recombination, especially in direct band gap semiconductors.

Though the majority of recombination in indirect band gap semiconductors is due to

defects (mostly at the surface), a majority of recombination in the bulk can be attributed

to Auger if the defect density is low.

3.1.2.3 Shockley Reed Hall

Shockley Reed Hall (SRH) recombination is recombination of an electron and hole

that is catalyzed by a defect or trap state. This remains the dominant recombination

mechanism in indirect band gap semiconductors, as purely radiative recombination is

impossible and Auger is only likely with high carrier densities. Defects in the crystal

create new energy states, allowing an electron and hole to recombine radiatively or

non-radiatively while still conserving crystal momentum. Trap states can ”trap” carriers

for a finite amount of time, during which they can recombine with another carrier or

be thermally excited out of the trap. This recombination mechanism explains the long

diffusion length in indirect band gap monocrystalline semiconductors, as photogenerated

34

carriers are lost to recombination only at the surface. This allows relatively thick solar

cells modules capable of absorbing lower energy photons. Comparatively, amorphous and

polycrystalline silicon possess a high density of grain boundaries that act as defects and

facilitate recombination, limiting the optimal absorber thickness.

The recombination rate at the surface can be quantified by the recombination velocity

parameter, S:

Sn = vthσsNn (3–2)

where vth is the thermal velocity (typically ∼107 cms), σs is recombination cross section

(typically ∼10−15 cm2), and Nn is the density of trap states at the semiconductor surface.

A higher surface recombination velocity indicates increased carrier recombination.[30]

Highly quality passivated silicon can achieve surface recombination velocities as low as

0.25 cm2

vsfor undoped silicon.[32], as compared to 2590 cm2

vsfor polycrystalline silicon.[33]

3.1.3 Characterization

In lieu of attaching a variable resistive load to the solar cell to calculate efficiency, the

device is connected to a power source and the voltage is ramped from negative to positive

voltages. The current at each voltage is recorded and plotted as a J-V curve. Each point

in the fourth quadrant represents a particular load resistance and an identical J-V curve

can be generated in that quadrant by connecting the solar cell to a variable resistor and

recording the current and voltage at each resistance value. Though the device generates

power only in the fourth quadrant, biasing the device over a full range of voltages yields

important information about the solar cell in regards to both photogeneration and diode

behavior.

The four parameters used to evaluate a solar cell’s performance are power conversion

efficiency (PCE), short circuit current (JSC), open circuit voltage (VOC), and fill factor

(FF). The bias voltage where the photocurrent is equal and opposite to the diode current

(ie, Jtotal = 0) is called the open circuit voltage, VOC. The output current when Vbias= 0 V

is defined as the short circuit current density, JSC . An overall measure for the efficiency of

35

Figure 3-3. Maximum generated power density (blue box), defined by P = VMJM

a cell, PCE is defined as a ratio of the maximum power generated by the cell to the power

of the incident radiation on the active area of the device.

The short circuit current density is taken directly from the J-V curve and is

approximately equal to the photocurrent (the dark diode current is generally orders of

magnitude smaller). The VOC is defined as the voltage at which the photocurrent is equal

and opposite to the diode current, Jphoto = Jdiode:

Jphoto = JO

[

exp

(qVOC

kT

)

− 1

]

(3–3)

Solving for VOC yields:

VOC =kT

qln

(Jphoto

JO+ 1

)

(3–4)

The VOC is extracted directly from the J-V and can be used to calculate other

parameters, such as the dark current (and subsequently Schottky barrier height).This

equation elucidates the factors contributing to a high or low VOC , namely and increase in

36

short circuit current and/or a decrease in dark current. Increasing VOC by increasing the

light intensity is a well known phenomena exploited through solar concentrators, though

the semiconducting material must have high enough mobilities to avoid carrier saturation

(and inversion). The inverse logarithmic dependence on dark current points to a reduction

in junction recombination as another way to increase the VOC .

The fill factor is a measure of how much the solar cell functions as an ideal diode,

with a FF of 1 corresponding to a completely square shaped J-V curve in the fourth

quadrant. Table 3-1 in the beginning of the chapter shows high performing solar cells

possessing a FF of approximately 0.8-0.9.

FF =VMJM

VOCJSC(3–5)

VM and JM are the voltage and current density corresponding to the maximum power

point: the point on the J-V curve where the maximum power is generated by the solar

cell. This is found by graphing the power density vs voltage and solving for dPdV

= 0.

The power generated at the maximum power point is represented by the blue square in

Figure 3-3.

With those parameters defined, we can now solve for the PCE:

η =JMVM

Pincid

=JSCVOCFF

Pincid

(3–6)

Note that this is the efficiency at the maximum power point; the efficiency will be lower

at other equivalent loads in the fourth quadrant. Consequently, maximizing the power

extracted from a solar cell involves matching the load to the resistance at the maximum

power point.

3.1.4 Series and Shunt Resistance

Both series and shunt resistance have a detrimental effect on solar cell performance,

with the best performance extracted by minimizing the former and maximizing the latter.

37

Figure 3-4. Circuit equivalent showing series and shunt resistance.

The expression for the total current can be rewritten to include both series and shunt

resistance:

Jtotal = Jphoto − JOexp

[q(V + IRS)

ηkT

]

− V + IRS

RSH

(3–7)

Series resistance is the resistance encountered by carriers as they are extracted from

the device. Series resistance should ideally be as low as possible; high series resistances

lead to a lower FF and ultimately lower PCE. High series resistances can be due to

employing poorly conductive contacts or having insufficient electrical contact such that

the photogenerated carriers cannot be efficiently extracted. Shunt resistance is ideally as

high as possible, as it represents all current paths that carry the charges through a circuit

in parallel with the load, i.e., the photogenerated carriers do no useful work and do not

contribute to the overall efficiency. One possible contributor to low shunt resistance in

both p-n junctions and Schottky junctions is losing photogenerated carriers out the edge of

a device. A circuit schematic depicting series and shunt resistances is shown in Figure 3-4,

while Figure 3-5 shows the degradation of the J-V curve with large series and small shunt

resistances.

38

Figure 3-5. The effects of series and shunt resistance on the J-V curve. Optimal values areRseries = 0 ohm cm2 and Rshunt = ∞ ohm cm2.

3.2 Theoretical Limitations

No physical, biological, or chemical process is 100% efficient due to thermodynamic

limitations, and solar cells are no exception. In 1961 Shockley and Queisser published

their exhaustive calculation on the maximum theoretical efficiency of single p-n junction

solar cells, often called the detailed balance limit. This maximum efficiency is attributed

to three primary mechanisms: emission of blackbody radiation, spectrum losses, and

recombination.[34, 35] All objects emit blackbody radiation as a function of their

temperature. Solar cells operating at room temperature emit radiation corresponding

to 300K, and this emission accounts for loses of 7%.

Spectral losses are the losses of photon energy exceeding the band gap of the absorber

material. Creation of a single electron-hole pair only requires energy equal to the band

gap of the semiconductor, anything in excess is carried away by the charges as kinetic

energy, subsequently lost to phonons (heat) as the electrons relax to the bottom of the

conduction band and holes to the top of the valence band. Heating of the device can be

especially degrading to performance as the dark current is exponentially dependent on

temperature, leading to a lowered VOC and lowered PCE with an increase in temperature.

Mitigation of spectral losses is achieved by introducing multi-junction cells composed of

individual layers, each specifically tailored to collection photons of a particular frequency.

39

Lastly, up or down converters, luminescent layers that absorb high(low) frequencies and

re-emit low(high) frequency photons, can be utilized to better match the photon frequency

to the band gap and minimize energy loss and avoid heating. Up converters, typically

placed at the backside of the cell, are theorized to increase the maximum theoretical

efficiency to 47.6% for non-concentrated solar light, a noticeable improvement from the

Shockley-Quiesser limit. Down converters, which must be placed on the top of the cell,

reap benefit due to avoidance of thermalization of the photogenerated carriers, leading

to less heating of the device. Theoretical efficiency for these reach 38.6%, only a modest

improvement from the Shockley-Quiesser limit. [36]

Recombination is inevitable in solar cells and much effort is aimed at minimizing

losses. Photogenerated electrons and holes must travel to the electrodes to be extracted as

usable electrical energy, and poor construction of PV cells or using incompatible materials

lead to high rates of recombination. Even with careful control over surface properties

and use of compatible materials, recombination will still occur due to inherent properties

of solar cells. Differing effective masses of electrons and holes lead to different diffusion

lengths. A fast moving electron can collide with a slow moving hole leftover from a

previous photon absorption, recombining through one of the mechanisms discussed above.

An increase in photon flux correlates with an increase in the density of photogenerated

carriers, increasing the probability of recombination. Materials which possess a higher

minority carrier diffusion length are less likely to suffer from increased recombination at

higher photon fluxes.

Combining these three loss mechanisms together leads to the maximum theoretical

efficiency for a single p-n junction solar cell as a function of bandgap (Figure 3-6). The

maximum possible efficiency is 33.7% for a band gap of 1.34 eV. With a band gap of 1.12

eV, silicon can attain a peak efficiency of only 29%. Utilizing more complex architectures

such as tandem solar cells with concentrated light, increases the theoretical efficiency

40

Figure 3-6. The Shockley-Queisser limit showing the maximum theoretical efficiency as afunction of band gap for a single p-n junction solar cell.

efficiency dramatically; an infinitely layered solar cell under concentrated light can

theoretically reach 86%.[37]

3.3 Types of Solar cells

3.3.1 P-N Junction

The most prevalent solar cells are composed of a p-type and n-type semiconductor

brought into contact to form a p-n junction. P-N junctions can be both heterojunctions or

homojunctions, though in the case of the former, care much be taken to match the lattice

constants to minimize strain and defects at the junction. When a p-type semiconductor

is brought into contact with an n-type semiconductor, mobile charges rearrange in order

to establish equilibrium of Fermi levels. The offset in work function between the two

materials drives electrons in the n-type semiconductor towards the p-type semiconductor,

where they combine with the holes and leave the region with a net negative charge.

The n-side is left with a net positive charge. This continues until the resulting built-in

potential, Vbi, prevents any further migration of charge due to Coloumbic repulsion. The

built in potential and altered carrier density near the junction are manifested as a bending

of the semiconductor conduction and valence bands, with the region over which the band

bending occurs called the depletion region (Figure 3-7).[30, 38]

41

Figure 3-7. Schematic for a p-n junction. Shown is the space charge (depletion) region inboth p and n side. Electron transfer driven by Fermi level offset establishes abuilt in potential (yellow arrow) opposing further transfer of charge.

Solving Poisson’s equation yields the width of this region in both the p and n-type

semiconductor;

wp =1

Na

√√√√

2ǫsVbi

q(

1

Na+ 1

Nd

) , (3–8)

wn =1

Nd

√√√√

2ǫsVbi

q(

1

Na+ 1

Nd

) , (3–9)

where the total width is the sum of the two:

wtotal =

√

2ǫsq

(1

Na+

1

Na

)

Vbi (3–10)

where Vbi is the built in potential, and Nd and Na are the donor and acceptor densities,

respectively. Note that as the carrier density increases, the depletion width decreases; in

the case where one side is heavily doped, the depletion layer exists almost entirely in the

other side. This built in potential acts as a barrier to majority carriers while facilitating

transport of minority carriers, making solar cell efficiency of p-n based devices more

sensitive to minority carrier diffusion lengths.

42

Figure 3-8. Band diagram of P-N junction with no bias, reverse bias, and forward bias. Vbiis the built in potential and Vbias is the bias voltage. In reverse bias, thebarrier increases and the depletion layer expands. Under forward bias, thebarrier decreases and the depletion layer shrinks, leading to a higher current.

Under illumination, electron hole pairs are created within the bulk; UV photons tend

to be absorbed near the surface while IR photons are absorbed deeper in the substrate.

Minority carriers generated close to the depletion layer are carried by the built in potential

across the junction, where they become majority carriers and must diffuse to an electrode

to be extracted. Carriers generated farther from the depletion layer either diffuse to the

junction and are swept by the built in potential to the electrodes, or they recombine.

In p-n solar cells, the doped semiconductor material does not possess a high enough

conductivity to function as an electrode, necessitating metal grid lines evenly spaced

across the top of the device. These gridlines prevent incoming photons from reaching the

bulk, lowing the short circuit current and PCE commensurate with the percentage of

43

surface area covered by the metal. Increases in carrier lifetime can mitigate these losses by

allowing gridlines to be spaced further apart.

3.3.2 Organic

Organic based photovoltaics have become increasingly investigated due to low

temperature processing and the potential lower cost of materials, an easy way to reduce

the cost of manufacturing that would allow solar energy to be more competitive on the

open market. Organic molecules are flexible and can be tailored to possess the desired

electronic characteristics (e.g. tailored band gaps), two attributes distinctly lacking in

silicon. However, excitons in organic materials generally have binding energies on the

order of 0.5 - 1 eV1 .[39] Separation cannot occur within the bulk, but must occur at the

junction of the donor and acceptor materials.The strongly bound electron hole pairs have

a high probability of recombining, leading to diffusion lengths of tens of nanometers within

most organic photovoltaic materials, leading to recombination losses. A balance must be

struck between having sufficient absorber thickness to capture the majority of the light

incident on the device and minimizing the path to the junction.[29, 40, 41]

Two main architectures for organic solar cells are thin bilayer devices and bulk

heterojunction devices. The former relies on using thin layers of high conductivity p and

n-type conjugated polymers (mobilities comparable to amorphous silicon) to form a p-n

junction, while the latter uses a single solution mixture of the same polymers to minimize

the distance between the bulk and the junction.

The bilayer device, shown in Figure 3-9A, has the two layered donor and acceptor

materials sandwiched between electrodes. In most organic devices, the anode is constructed

of indium tin oxide (ITO) and the cathode of thin aluminum. Poly(ethylene-dioxythiophene)

(PEDOT:PSS) is often deposited on top of the ITO to minimize band offset. Due to the

1 Measurements of some organics, MEH-PPV and PPV have yielded binding energies from zero to ∼1eV. Discrepancies are possibly attributed to inaccurate treatment of electron-phonon coupling or otherelectronic effects.

44

A Bilayer Organic Solar Cell B Bulk Heterojunction Solar Cell

Figure 3-9. Schematic of the bilayer and bulk heterojunction solar cells.

short diffusion length of excitons, only those generated within 20nm of the junction

will be dissociated and collected, leading to relatively poor efficiencies. The bulk

heterojunction cell, shown in Figure 3-9B address this issue by having the acceptor

and donor material mixed together prior to deposition, improving efficiencies by combing a

thicker absorber layer with a donor-acceptor junction distributed throughout the bulk.

More recently there has been interest in organic-inorganic hybrid devices. In addition

to low temperature fabrication, these take advantage of the long diffusion lengths within

silicon and the excellent conductivity and antireflection properties of the polymer to

produce a solar cell with a PCE of 13%.[42]

The major barrier to large scale implementation of organic solar cells is their lack of

long term stability. Inorganic devices continue to work decades after installation, whereas

organic polymers suffer from both light induced degradation and oxygen/water vapor

degradation. Simple encapsulation solves the latter problem, but avoiding the degradation

associated with light involves using filters to block out selective wavelengths, which also

lowers the fraction of the solar spectrum being absorbed and turned into available energy.

[29]

3.3.3 Photoelectrochemical Devices

In the electrolyte of a photoelectrochemical cell, the electrochemical (Nernst)

potential of the incorporated redox couple sets the equilibrium distribution of the couple

45

Figure 3-10. Schematic of simple semiconductor/liquid junction solar cell showing redoxreactions occurring both at the semiconductor surface and at a metalcounterelectrode.

between its reduced and oxidized states. When the electrolyte comes into contact with

the semiconductor, the two exchange charge, simultaneously shifting the electrochemical

potential of the redox couple and the Fermi level of the semiconductor until they are

in equilibrium (thus establishing the depletion layer in the semiconductor). Under

illumination photocarriers are created in the semiconductor and charge of one sign is

repelled from the junction while charge of the other sign is driven to the junction where it

reacts with one member of the redox couple. That charge is then delivered by ion diffusion

to a counter electrode that comprises the second terminal of the cell. Though initially

promising, silicon based liquid junction (so called) solar cells suffer from electrochemical

reactions at the silicon surface, creating a high surface defect density in addition to

creating new species which would contaminate the electrolyte.

Another popular type of photoelectrochemical cell is the dye sensitized solar cell,

or Gratzel cell. These use as the semiconductor titanium dioxide which has too large a

bandgap to itself be useful as an absorber but this is ”sensitized” by the incorporation

of smaller bandgap dye molecules that are deposited onto the surface of the TiO2.

46

Otherwise they operate as discussed above with a redox couple ferrying charge to a

counterelectrode.[43] Similar to organic cells, Gratzel cells depend in high surface areas for

charge separation and must employ a bulk heterojunction structure to achieve appreciable

power conversion efficiencies.

3.3.4 Multi-junction

Multi-junction cells address the need to capture the incident radiation without

wasting above bandgap energy to non-radiative relaxation by having multiple layers.Tandem

devices are two, three, or more distinct layers, with the top layers absorbing shorter

wavelengths and the bottom cells absorbing at longer wavelengths. Due to their multiple

layers, these devices are not held to the Shockley-Quiesser limit imposed on single junction

Schottky and p-n junction solar cells, already achieving over 40% efficiency. The biggest

drawback is cost due to the complicated, multi-step processing and use of expensive

semiconducting materials, relegating these devices to specialized high value applications

(satellites and military).

3.3.5 Schottky junction

Schottky junction solar cells are similar to p-n junction solar cells with one of

the semiconductors replaced with a metal. The equilibration of the metal with the

semiconductor creates the depletion layer within the latter must generally be made so

thick that it is opaque. To get light to the semiconductor the metal is patterned as a

grid of narrow lines allowing the light to get into the semiconductor between the opaque

lines. Schottky junctions also benefit from being majority carrier devices fabricated

with low temperature processes, but up until the mid-1970s suffered from a lower open

circuit voltage than p-n junction solar cells.[44] This low VOC is partly due to the higher

dark current inherent in devices that rely on thermionic emission of majority carriers

for transport across the junctions (as in Schottky junctions). Additionally, surface

states that pin the Fermi level are also responsible for increased recombination and a

corresponding increase in dark current. A VOC and corresponding low PCE kept Schottky

47

junction devices from competing with p-n solar cells until Godfrey and Green developed a

17.6% metal-insulator-semiconductor (MIS) PV cell with an open circuit voltage of over

0.65V.[45, 46] This excellent performance was realized though a thin (<2nm) insulating

layer between the metal and semiconducting layer, passivating the silicon surface and

reducing recombination while negligibly affecting current transport. The carbon nanotube

- silicon solar cells presented in this dissertation are Schottky junction devices, and a much

more complete description of the underlying physics is presented in the following chapter.

3.3.6 Inversion Layer Cells

Inversion layer cells induce an inversion layer within the device to enhance performance.

The first of these devices was developed by RL Call in the early 1970s and was composed

of a grid based Schottky junction cell possessing larger spacing between the metal grid

lines and a thick insulating layer sandwiched between a transparent electrode and the

semiconductor. A voltage between the transparent electrode and the semiconductor

formed an inversion layer at the surface. This so called induced junction cell benefited

from efficient charge collection due to the surface depletion layer, which repelled the

majority carriers while attracting the minority carriers avoiding their recombination as

the minority carriers diffused to the widely spaced electrodes.[47] Call abandoned such

electronically induced junctions because of the challenges of getting pinhole free insulators

over the large areas needed but discovered that the deposition of certain insulators

simultaneously trapped charge of a sign that created the inversion layer without needing

to do so actively. Such cells were further refined by Godfrey and Green in the late ’70s.

These devices suffered from impermanence of the trapped charge and over time the

inversion layer would disappear.[44] The devices presented in this dissertation constitute in

some sense a rediscovery of the phenomena exploited by Call, Godfrey and Green.

48

Table 3-1. Current maximum efficiencies for various photovoltaic devices [1]

Type Voc (V) Jsc(mAcm2

)FF (%) Efficiency (%)

Monocrystalline Silicon (PERL) 0.706 42.7 82.8 25.0Polycrystalline Silicon 0.664 38.0 80.9 20.4Commercial Silicon - - - 13Monocrystalline GaAs 1.107 28.3 86.7 28.3Triple Junction GaInP/GaInAs/Ge 2.691 14.7 86.0 34.1Dye Sensitised 0.714 21.93 70.3 11.0Organic Tandem 0.899 16.75 66.1 12.1

49

CHAPTER 4INTRODUCTION TO SCHOTTKY BARRIERS

4.1 Fundamentals

Schottky junction solar cells are composed of a metal in contact with a semiconductor

and display rectifying behavior. Analogous to a p-n junction in which one side is

degenerately doped, the depletion layer exists solely on one side of the junction.

Semiconductor doping, Schottky barrier height, and interface dynamics affect the

transport of carriers within the device, affecting the functionality and PCE. The

highly conductive metal contact also functions as the electrode, eliminating the need

for additional gridlines to extract photogenerated carriers.

4.1.1 Basic Schottky Model

To first order, the Schottky barrier height formed at the interface between a metal

and semiconductor can be approximated by the Schottky-Mott model. Assuming a metal

of work function φm and a semiconductor of work function φn and electron affinity χ, upon

placing the two into electrical contact the energy difference between the work function

of the metal and the Fermi level of the semiconductor drives electrons to move from one

material to the other. Charge rearrangement continues until electrostatic equilibrium

is established, forming a potential gradient within the semiconducting material that

opposes any further transfer of electrons. This induces band bending of the conduction

and valence bands in the semiconductor, with the depletion layer width determined by

the spatial region over which the band bending occurs. A higher doping density yields

steeper band bending and a shorter depletion width. Shown in Figure 4-1 below are the

band profiles for an n-type semiconductor and a metal for which φm > χ, and for a p-type

semiconductor and metal for which φm < χ. For an n-type semiconductor, electrons

approaching the metal-semiconductor junction from the semiconductor side see a potential

of qVbi. Electrons approaching the junction from the metal see a potential barrier equal to

the Schottky barrier height. [30, 38, 48]

50

A N-Type Semiconductor

B P-Type Semiconductor

Figure 4-1. Schottky barrier band diagrams. Adapted from Ayalew, T, ”SiCSemiconductor Devices Technology, Modeling, and Simulation”, 2004

To first order, the barrier height is given by the Schottky-Mott relation:

qφSBH = q(φm − χ) for n-type semiconductor (4–1)

qφSBH = Eg − q(φm − χ) for p-type semiconductor (4–2)

4.1.2 Current Transport

Schottky barriers function as diodes, allowing negligible current under reverse bias

and exhibiting an exponential current with forward bias. Under reverse bias, the built

in potential increases and very little charge flows until breakdown. Under forward bias,

the band bending decreases as the metal is raised to a higher potential relative to the

semiconductor, resulting in an exponential increase of current, principally as carriers spill

over the barrier. In both cases the idealized Schottky barrier remains unchanged. Carrier

51

transport in Schottky diodes occurs through several mechanisms: thermionic emission,

thermionic field emission, field emission (tunneling), and minority carrier injection.[49]

4.1.2.1 Thermionic Emission

Thermionic emission, the primary transport mechanism for moderately doped

semiconductors and the one most relevant to the work presented in the following chapters,

is the transport of energetic carriers over the potential barrier at the junction. Thoroughly

investigated by Hans Bethe in the 1940s, the total current is a simple sum of the current

flowing from the semiconductor into the metal plus the current flowing in the opposite

direction.[50] The former is given by

Js→m = A∗T 2exp

(

−qφSBH

kT

)

exp

(qV

kT

)

(4–3)

A∗ =4πqm∗k2

h3

where V is the bias voltage and A is the Richardson constant which is dependent on the

effective mass (m∗)of the carriers. A = 120 Acm2K2 and

(A∗

A

)

n−Si= 2.1 for n-type silicon.

Current flowing from the metal into the semiconductor is independent of the bias voltage

and is given by

Jm→s = −A∗T 2exp

(

−qφSBH

kT

)

(4–4)

Summing the two equations yields

Jtotal = Jo

[

exp

(qV

kT

)

− 1

]

(4–5)

with Jo = A∗T 2exp(− qφSBH

kT

)as the saturation current density (often called dark current).

4.1.2.2 Thermionic Field Emission and Field Emission

For heavily doped (> 1017) semiconductors at low temperature, tunneling begins to

contribute significantly to carrier transport. Field emission is the tunneling of carriers

through the potential barrier at the semiconductor/metal junction. These carriers

52

generally possess little kinetic energy and lie near the Fermi level. Thermionic field

emission considers the tunneling of moderately thermally excited carriers across the

junction. Carries that already possess some thermal energy (though less than that

required to surmount the barrier) ”see” a smaller potential width than carriers that are

not thermally excited. A parameter for evaluating the transport regime is given as:

E00 ≡q~

2

√

N

m∗ǫs(4–6)

With kT ≫ E00, thermal emission is the dominant transport mechanism. If kT ≪ E00,

field emission (tunneling) is the primary mode of transport across the barrier. Finally, if

kT ≈ E00, then thermal field emission dominates. [49]

4.1.2.3 Minority Carrier Injection

Schottky junction diodes are primarily thought of as majority carrier devices due

to the extremely small contribution from minority carrier diffusion. At large forward

bias, however, drift of minority carriers becomes comparable to the thermal emission of

majority carriers over the barrier. The current contribution due to minority carriers is the

sum of the drift and diffusion processes:

J = qµnE︸ ︷︷ ︸

Drift

− qDdn

dx︸ ︷︷ ︸

Diffusion

(4–7)

Where µ is the mobility, n is the number concentration, D is the diffusion coefficient, and

dndx

is concentration gradient, and E is the electric field. The total current is limited by

minority carrier recombination within the depletion layer.

4.1.3 Beyond Schottky-Mott

Decades of experimental results have shown that the Schottky-Mott theory is not

an accurate measure of Schottky barrier heights for most metal-semiconductor interfaces,

motivating alternate theories to explain the physics and chemistry of these junctions.

53

Shown in Table 4-1 at the end of the chapter are the theoretical and measured Schottky

barrier heights for various semiconductor-metal interfaces.[51, 52]

Clearly the experimental results deviate from the theoretical values predicted by the

Schottky-Mott theory. For simplicity, an equation to quantitatively predict barrier heights

on n-type silicon has been adopted based on experimental results: qφSBH = 0.27qφm−0.52.

A more complete picture of the ”real” junction is shown in Figure 4-2. Included

are interface states occurring in the bandgap of silicon, denoted Qss, the origin of

which is explored in more detail below. It’s clear from this graphic that the underlying

physics of metal-semiconductor junctions is far more complicated than captured by the

Schottky-Mott model.

The graphic represents the junction without any external applied bias. Upon

introduction of an electric field, the combination of the applied field and the induced

image charges result in a small Schottky barrier lowering. Consequently, real junction are

somewhat bias dependent, with the amount the barrier is lowered given by

δφ =

√qǫm

4πǫs(4–8)

ǫm =

√

2qN |ψs|ǫs

and ψs is the surface potential. In addition to Schottky barrier lowering, devices can

suffer from leakage of carriers out the sides of the active area. This edge leakage often

occurs at the sharp corners of metal electrodes where highly concentrated electric field

lines facilitate tunneling. Mitigation of this effect is achieved in both p-n junctions and

Schottky junctions by using guard rings.

54

Figure 4-2. Interface states shown in the realistic model of a Schottky junction.

4.1.3.1 Fermi Level Pinning: Bardeen Model and Metal Induced Gap States

In 1947 John Bardeen published his paper on interface states at the semiconductor-metal

junction and their role in creating a barrier independent of the Schottky-Mott model. A

property of the semiconductor surface, interface states can affect the Schottky barrier

height dramatically, producing a barrier height completely independent of the metal work

function. The semiconductor surface can possess a very high density of surface states

- far higher than in the bulk. This leads to Fermi level pinning, a process in which the

55

extremely high density of surface states prevent the semiconductor Fermi level from

shifting in response to the metal contact, even after reaching equilibrium. Not only do

these surface states pin the Fermi level and create a Schottky barrier independent of

φm, but they also act as trap sites, facilitating recombination of electrons and holes.[53]

Nearly 30 years after Bardeen published his work on Fermi level pinning at semiconductor

surfaces, Tersoff published his theory exploring alternate sources of Fermi level pinning

in order to explain the discrepancy in the Bardeen model for predicting the SBH within

ionic semiconductors. At a free semiconductor surface (not an MS interface), the Fermi

level can be pinned by a relatively small amount of surface defects. The screening

length of these defect states within the semiconductor is relatively large, leading to a

correspondingly large surface dipole and shift in EF . At an MS interface, the charges in

the metal screen the surface defects, leading to a smaller local dipole and smaller shift in

EF . His theory, based on metal induced gap states (MIGS), posits that the continuum of

states existing in the metal at the MS interface ”leak” into the semiconductor, leading to

gap states that decay into the bulk and consequently pin the Fermi level. The barrier

height is the sum of both the dipole due to metallic screening of the MIGS and a

surface dipole, either of which may dominate depending on the bulk properties of the

semiconductor.[53]

4.1.3.2 Bond Polarization

A more contemporary theory pioneered by Raymond Tung looks to chemical bonds

formed across the metal-semiconductor junction as the source of interface states that cause

the barrier height to deviate from the Schottky-Mott approximation. All properties of

the Schottky junction, including barrier height and interface dipole, are a consequence of

the chemical bonds formed between the semiconductor and metal. The interface dipole

is simply due to the polarization of these bonds. This model assumes an extremely small

interface region and predicts the barrier height using the following equation:

56

φSBH,n = γB (φm − χs) + (1− γB)Eg

2(4–9)

γB = 1− e2NBdMS

ǫit|β|and β is the bond integral, NB is the density of chemical bonds, dMS is the distance

between the atoms at the metal surface from the atoms at the semiconductor surface, ǫit is

the permittivity of the interface, and κ accounts for charges ”hopping” from one atom to

another (metal-metal, metal-semiconductor, semiconductor-semiconductor).[54–56]

Though bond polarization is an excellent model for chemically active metals, carbon

nanotubes are not chemically reactive. Due to this lack of chemical reactivity of the

carbon nanotubes and the reluctance to form chemical bonds with substrates, the Bardeen

model provides more insight into the behaviors relevant to our solar cells.

4.2 Schottky Junction Solar Cells

4.2.1 Historical Background

Built by Charles Fritts in 1894, the first solar cell was a Schottky junction device

constructed from selenium sandwiched between gold and another metal. Solar cell research

and development activity remained low until the 1950s, when high quality silicon became

available, spurring advancements in homojunction structures. Initially engineered for

space applications, it wasn’t until the energy crisis in the 1970s that solar cell development

began to focus on production for commercial use. Both p-n junctions and Schottky

junction solar cells were extensively researched, with metal-insulator-semiconductor

inversion layer (MIS-IL) devices achieving 17.6%.[45, 46] In the decades since Godfrey

and Green’s high performing MIS structure, solar cell designs have expanded dramatically

by exploiting advances in polymer science for organic devices, using new technologies to

fabricate high quality thin film structures, developing novel light trapping techniques, and

creating hybrid structures exploiting the strengths of varied materials. Concomitant with

those developments was a greater understanding of the physical and chemical processes

57

pertinent to photovoltaics. A discussion of all PV types is outside the scope of this thesis,

but much of the work pioneered in the previous decades greatly influenced the design of

the devices presented below.

4.2.2 CNT on Silicon Schottky Junction Solar Cells

Carbon nanotubes were first incorporated into devices as conductive electrodes for

either supercapacitors or organic photovoltaics.[57, 58] In 2002, a carbon nanotube/polymer

solar cell with an efficiency of 0.04% was developed, catalyzing the use of carbon

nanotubes in photovoltaics.[59] Low performance in CNT/organic PV cells encouraged

development using inorganic materials, and in 2007 a double walled carbon nanotube -

silicon device with a PCE of 1.38% was published.[60]

SWNT-Si solar cells have achieved high efficiencies in recent years, due in part to

the malleability of the electronic and optical properties of carbon nanotubes. In 2010, it

was demonstrated that an ionic liquid could be used to electronically modulate the Fermi

level of the carbon nanotube film, changing the built in potential, junction dynamics, and

PCE.[9] The devices presented below have taken the original solar cells, shown below in

Figure 4-3 and further optimized the device performance by exploiting the effect of the

ionic liquid.

4.2.2.1 Experimental Details and Equipment

Substrate were diced from a 500 µm thick, <100>/<111>, n-Type silicon wafer

possessing either a 200nm or 1000 nm thick thermal oxide. The <100> wafers had a

resistivity of 0.5-0.7 Ohm-cm, while the <111> wafers were slightly less doped with a

resistivity of 4-10 Ohm-cm. Onto the surface of the oxide was defined a square 12x12 mm2

Au/Cr (60/10 nm) pad, possessing a 2x4 mm2 rectangular window at its center. This

Au/Cr pad served several functions: it provided an etch mask for a BOE etch of the oxide

in the window and served as the electrical contact to the nanotubes that were draped as

a thin film from the Au/Cr layer down across the silicon; and finally, it provided a literal

shadow mask, limiting the collimated, simulated solar radiation to the window area in

58

Figure 4-3. Carbon nanotube-silicon Schottky junction cell. Included are both the activearea film and the gate film

which the active area was defined. After a BOE etch removed the thermal oxide within

the gold framed window, a carbon nanotube film was transferred following the procedure

in Wu, et al..[8] Another film transferred to a secondary gold electrode functioned as the

gate film and was insulated from the silicon by the thermal oxide. Ohmic contact between

the Si wafer backside and a stainless steel sheet was made by a gallium-indium (Ga/In)

eutectic spread between the two.

Illumination was provided by a 150W xenon lamp (Oriel 6255) in an Oriel 6136

housing powered by a model 8500 power supply. An Oriel 81094 AM1.5G filter approximated

the solar spectral distribution. Light from the inhomogeneous source was focused into the

aperture of a 150 mm long, fused silica Homogenizing Rod (Edmund Optics P65-837) by

a 50 mm diameter fused silica lens with a 65 mm focal length. The output face of the

Homogenizing Rod was imaged in the horizontal focal plane of the sample by a 50 mm

diameter, 100 mm focal length fused silica lens after rotation by 90 degrees with a broad

band mirror (Newport 66225). The intensity at the sample plane was adjusted to 100 mWcm2

by translation of the 65 mm FL lens, cutting down on the fraction of the light entering the

59

Homogenizing Rod. Homogeneity of the light intensity over the 1 cm2 central region of

the homogenized beam at the sample plane was measured to be within 5%.

4.2.2.2 Electronic Gating

These devices differed from conventional Schottky junction solar cells by inclusion of

an ionic liquid 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-BTI)

to alter the electronic properties of the nanotube film, which in turn modifies the

characteristics of the Schottky barrier and built in potential. The ionic liquid saturates

both the active area SWNT film and a secondary SWNT film. A gate voltage is applied

between the two films, polarizing the ions in the ionic liquid and increasing or decreasing

the carrier density of the SWNT film in the active window. This capacitive doping alters

the Fermi level of the film and subsequently changes the built in potential at the SWNT-Si

junction. Unlike photoelectrochemical cells where the ionic liquid actually transfers

charge between electrodes, there is no net current flow through the ionic liquid. EMI-BTI

has a large electrochemical window spanning nearly 5 V, allowing the application of

appreciable voltages between the two SWNT films without electrochemistry occurring at

the electrodes. This process, akin to charging an electrolytic capacitor, draws no current

once equilibrium is established at each gate voltage. To characterize the cell, a fixed gate

voltage is applied to the SWNT gate film and the voltage across the electrodes is swept

from -1V to 1V and the resulting J-V curve is analyzed to extract device performance.

Then another gate voltage is applied and a new J-V curve is generated. Previous results

demonstrated an optimized performance at a gate voltage of -0.75V; positive gate voltages

result in a smaller PCE, fill factor, and VOC . [9] The resulting J-V curves for gate voltages

between +/-0.75 V are shown in Figure 4-4B. The native device PCE without gating was

8.5%. Gating modulated the PCE between 3.6 and 10.9

4.2.2.3 Inversion Layer Modeling

The previous work by Dr. Wadhwa elucidated the mechanism by which the ionic

liquid improved the efficiency of the solar cells. The intended function of the electrolyte

60

A Schematic of the during electronic gating.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-40

-30

-20

-10

0

10

20

30

40

50

60

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

-0.75V -0.6V -0.45V -0.3V -0.15V 0V 0.15V 0.3V 0.45V 0.6V 0.75V

B Resultant J-V curves at various gate voltages.

Figure 4-4. Schematic and results for electronically gated SWNT-Si cell. Gate voltage isthe potential difference between the active area film and the gate film

was to change the Fermi level of the CNT film, increasing or decreasing the charge

transferred between the silicon and CNT in order to establish equilibrium of Fermi

levels in addition to increasing the conductivity of the film. However, an experimental

architecture in which the carbon nanotube film was pattered into thin strips via

photolithography demonstrated another mechanism at play. In this device, the carbon

nanotube strips measured 100 µm across and were evenly spaced every 300 µm, as shown

in Figure 4-5A below. The active area of the device has less than 30% film coverage.

As expected, the photocurrent prior to addition of ionic liquid was reduced due to the

decreased film coverage. Though reduced by a factor of 2, the photocurrent did not

decrease directly in proportion to the reduced SWNT film coverage; the long diffusion

lengths in single crystal silicon allows collection of photocarriers generated outside the

junction. Upon addition of ionic liquid the photocurrent increased dramatically, indicating

an alteration of the silicon surface in a way to facilitate charge collection. Applying a gate

voltage further improved device performance, consistent with previous planar devices. [10]

61

A The active area of the film com-prising of 2 x 4 mm silicon windowwith patterned carbon nanotube film.

B Figure B: J-V curve for the same device. Note reduced pho-tocurrent prior to addition of EMI-BTI.

Figure 4-5. Schematic and performance for grid cell.

We inferred, and simulations by J. Guo and J. Seol in the Electrical and Computer

Engineering department at the University of Florida confirmed, that the ionic liquid

spontaneously forms a layer of charge at the silicon surface where no nanotubes are

present, inducing an inversion layer in the silicon outside the SWNT-silicon junction. The

positive carriers that diffused into the depletion region traveled along the surface of the

silicon until they encountered a nanotube and were extracted. This is manifested as an

increased photocurrent and corresponding increase in efficiency. This behavior can be

qualitatively understood as follows. When the nanotubes and the n-Si are first placed

in intimate contact, the free energy of electrons in the n-Si (work function:φSi = 4.3 eV)

is reduced by their transfer to the carbon nanotubes (work function: φSWNT = 4.9 eV).

Such transfer stops when Coulombic restoring forces due to the charge imbalance raise

the local potential (the built in potential) to prevent further charge exchange, establishing

equilibrium. In the presence of electrolyte ions, the ions migrate to compensate the

transferred charge and thus permit the exchange of substantially more charge before

62

the equilibrium is reached. Additional electrons are transferred to the nanotubes from

the Si regions between the nanotube grid lines compensated by positive electrolyte ions

surrounding the nanotubes, while the positive charge left behind in the n-Si inversion layer

is compensated by negative electrolyte ions accumulated at the Si surface. The electrolyte

here serves much as it does in an electrolytic capacitor to raise the capacitance of the

system with a self-potential provided internally by the original Fermi level offset between

the nanotubes and the n-Si, or externally by the gate field. Shown in Figure 4-6 below

are the simulated results at a bias voltage of 0V for VG = -0.75V and VG = +0.75V. Full

simulation results are included in Appendix A. [61]

Exploitation of this discovery opened the doors for alternative architectures designed

to optimize device performance. Guo and Seol showed that the ionic-induced extension

of the depletion region occurred over hundreds of microns, allowing a reduction in

the fraction of nanotube film covering the silicon active area without sacrificing hole

extraction. The positive carriers that diffused into the depletion region traveled along the

surface of the silicon until they encountered a nanotube and were collected. The reduction

in the nanotube film area enhanced the number of photons being absorbed in the silicon

surface to increase both Jsc and the PCE.

63

A Model Parameters

B Simulations at VG = -0.75V and VG = +0.75V

Figure 4-6. Simulations showing inversion layer in silicon extending across entire surface inbetween carbon nanotube strips. Note the reduction in the inversion layer at+0.75V; at more positive bias voltages, this results in a reduced collection ofcarriers, as shown in previous J-V curves for gated devices (Figure 4-4B)

64

Table 4-1. Theoretical vs Experimental Schottky Barrier Heights: Barrierheights measured at 300K, theoretical values determined fromSchottky-Mott relation

Semiconductor Metal Theoretical SBH (eV)1 Experimental SBH (eV)2

n-Si Al 0.01-0.2 0.81n-Si Au 1.05-1.4 0.83n-Si Pt 1.1-1.9 0.9n-GaAs Al 0.03-0.2 0.93n-GaAs Au 1-1.38 1.05n-GaAs Pt 1-1.8 0.981 The theoretical value is given as a range as the work function of metals differs

depending on crystallographic orientation.2 Highest measured values.[62, 63]

65

CHAPTER 5NANOSTRUCTURING FOR ENHANCED LIGHT ABSORPTION

5.1 Overview

Texturing of photovoltaics to improve photon absorption has been aggressively

investigated for several decades as a means to improve solar cell efficiency.[28, 64–72] In

2000, Li et al. demonstrated the feasibility of using a simple metal-assisted chemical etch

to create porous silicon, opening the doors to facile production of optically absorbent

substrates with a high surface area.[73] While fabrication of porous structures using

chemical etching is a simple and low cost process, there is little control over the size and

uniformity of the resulting features (though spatial periodicity can be detrimental due to

diffraction losses). CVD growth is often utilized for applications where complete control

over diameter and depth is needed. In 2005, Peng et al. demonstrated a 9.31% efficient

silicon nanowire, p-n homojunction solar cell.[74] Continued innovation in nanostructured

devices has culminated in an 18.2% efficient porous silicon p-n homojunction cell.[75]

Considering the reflectivity of the devices presented in the previous chapter (silicon

+ SWNT + ionic liquid) is roughly 20% over wavelengths greater than the band gap of

silicon, optimizing light absorption can result in significant increases of the PCE. Methods

for texturing the silicon to enhance absorption are presented in this chapter, along with

complications inherent to alteration of the semiconductor substrate.

5.2 Potassium Hydroxide Etching

An aqueous solution of potassium hydroxide (KOH) is a basic solution frequently

used to etch pyramidal structures in silicon. At room temperature, the ratio of etch

rates between <100> and <111> planes is roughly 100:1 for a 30% KOH solution. This

anisotropy leads to regular pyramidal structures with the exposed faces corresponding

to the <111> plane.[76] Though the pyramids are excellent at reducing reflection from

the silicon surface, the pointed tops would provide little surface for contact to the SWNT

film and prompted fabrication of pyramidal grooves instead. Fabrication consisted of first

66

defining the areas to be etched through photolithography on a <100> substrate with

a 200 nm thermal oxide. SiO2 is largely unaffected by KOH, permitting its use as an

etch mask. A BOE etch removes the thermal oxide in the regions not protected by the

photoresist, after which the photoresist is removed and the substrate is then placed in the

KOH for the desired length of time. Following a DI rinse and N2 dry, the remaining oxide

is stripped with BOE and a SWNT film is transferred to the silicon following standard

procedures and subsequently tested. The resulting grooves were 10 µm wide, 14 µm deep,

and extended across the entire active area.

A Resultant grooves from KOHetch

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-40

-30

-20

-10

0

10

20

30

40

50

60

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

Trenches No IL Trenches -0.75V No Trenches No IL No Trenches -0.75V

B J-V curves for KOH device

Figure 5-1. KOH schematic and performance.

Similar to the grid film, there is a reduced photocurrent without the ionic liquid and

an increased photocurrent with addition of the EMI-BTI, as shown in Figure 5-1. As

with planar devices, a marked improvement in efficiency occurs for negative gate voltages.

The maximum PCE obtained was 11.4%, an improvement of 12% over a planar, full film

<100> device (10.6% PCE). The fill factor remained roughly the same: 0.77 for the KOH

etched device versus 0.76 for the full film planar device. The increased performance can

67

therefore be attributed to the increased JSC resulting from the enhanced absorption of the

grooved structure. Note, however, that the overall PCE is still lower than that achieved

by the grid film. The dimensions of the grooves are far larger than the wavelength of

visible light, leading to a relatively poor increase in absorption compared to other textured

devices. Additionally, the periodic spacing of the grooves leads to diffraction at oblique

angles, making these structures inferior for light collection. Nonetheless, this result

reinforces the improved performance attained with a reduction in SWNT-Si contact and

encouraged continued experiments in surface texturing.

5.3 Silicon Nanowires

Due to their excellent absorption and easily tailored geometries, silicon nanowires

arrayed in vertical forests on a substrate have been extensively studied for integration

into photovoltaic devices. As in commercially available solar cells, p-n homojunctions

of various designs are found within silicon nanowire based devices. Fabrication involves

taking a doped semiconducting nanowire forest supported on a bulk substrate and using

a high temperature diffusion process to dope the surface of the nanowires. This doping

process can be optimized to dope just the nanowire, leaving the supporting substrate

with the original doping type, or the doping can be done coaxially, creating a radial

p-n junction.[72, 77] The first generation p-n nanowire solar cells suffered from high

series resistance, reduced light absorption due to metal finger electrodes, and diffraction

at oblique angles due to the regular spacing of nanowires.[72, 78, 79] Aside from p-n

homojunctions, silicon nanowires have been integrated into photoelectrochemical cells

and even devices utilizing a carbon nanotube top layer.[80, 81] The latter, however, were

poorly conceived and did not likely operate as photoelectrochemical cells as claimed,

but rather likely along the lines of the mechanisms discussed here with a PCE of only

1.3%. Despite impressive progress in the efficiency of silicon nanowire cells in general, the

maximum efficiency for any silicon nanowire based devices has not yet passed 10%.[78, 82,

83] The general thought in the community for the reason behind this limit is the challenge

68

of controlling the trap states (and resulting recombination) at the large surface area

created in the nanowire surfaces. Given the grid SWNT film results we reasoned that we

could avoid recombination via the electrolyte inversion layer and thus resolved to pursue

such Si nanowire devices.

5.3.1 Procedure and Characterization

Fabrication of the silicon nanowires was achieved via a simple chemical etch adapted

from KQ Peng, et al..[80, 84] Substrates were prepared by evaporating a gold electrode

frame surrounding an open window to become the active cell area, and then painting

photoresist everywhere save for the active area window. The photoresist protects the

substrate from the aggressive etch while the gold electrode also functions as an etch mask

to keep the nanowire ”growth” contained within the active area. The substrates were

placed in 6:1 BOE to remove the thermal oxide layer, rinsed with DI water, dried in an N2

stream, and submerged into a 4.6M/0.02M HF/AgNO3 solution for a varying amount of

time. At room temperature, a 4 minute yielded 1 µm long nanowires.

Figure 5-2. The mechanism for silicon nanowire growth. A: Silver ions adsorb ontosurface. B: Oxidation of silicon. C: Etching of SiO2 and sinking of silverparticle. Adapted from KQ Peng et al.

The etching solution produces silicon nanowires via a three step process, as shown in

Figure 5-2 silver ions (dissociated from the AgNO3) adsorb onto the silicon surface and

oxidize the silicon directly underneath the silver particle. This locally oxidized region of

silicon is etched away by the HF, creating a spatial vacancy that the silver particle sinks

into. The reduced silver undergoes a redox reaction and returns to its oxidized state,

upon which the process repeats until the reaction is quenched. The resulting structure is

69

Figure 5-3. Silicon nanowires grown in an HF/AgNO3 solution

dependent on both the molar concentration of the two active chemicals (HF and AgNO3)

and the length of etch, with the concentrations listed above producing the high aspect

ratio nanowires. Alternative wet chemical etches also explored involve electroless metal

deposition or sputtering/evaporation of molecular silver/gold, respectively, onto the

substrate and using an oxidant solution (typically H22O2 and HF) to catalyze redox

reactions at the metal conglomerates. The hole produced by the oxidation of the metal is

injected into the valence band of the silicon, oxidizing the region directly below the metal.

This alternative has the advantage of controlling metal deposition using photolithography

to control the etching geometry. Although these structures were initially considered

for integration into our SiNW devices, SEM images showed less uniform growth of

nanowires with this method. Additionally, a comprehensive review of silicon nanowires

in photovoltaic cells concluded that nanowire forests fabricated through in HF/AgNO3

solution had a higher VOC than those fabricated through other wet chemical etches.[80].

These two factors led us to use silicon nanowires fabricated in a HF/AgNO3 solution.

Immediately upon removal from the etchant solution, residual silver residing at

the bottom of the nanowires or extending across the nanowire surface are removed in

an 8M nitric acid bath, followed by two DI baths and a gentle N2 dry. The nanowires

70

A Silicon Nanowires on <100> crystallographicorientation.

B Silicon Nanowires on <111> crystallographicorientation.

Figure 5-4. Orientation of silicon nanowires.

agglomerate upon removal from the last DI bath, as shown in Figure 5-3. While growth

was initially performed on both <100> and <111> silicon substrates, SEM images showed

a distinct difference between the wires formed on different crystallographic orientation

substrates; <100> gave an isotropic distribution of vertical nanowires, while <111>

formed well defined regions of nanowires at other angles corresponding to the easily

etched <100> and <110> crystallographic planes, as shown in Figure 5-4. It has been

demonstrated that the selective etch orientation is related to oxidant concentration, with

low concentrations leading to etching along <100> planes and high concentrations leading

to vertical etching independent of crystalline orientation.[85]

Reflectance from the nanowires near normal incidence to the substrate on both

<100> and <111> were measured using a UV/VIS/NIR spectrophotometer from

400-1200nm. Though it was far lower than that for untextured silicon, the <111> still

exhibited higher reflectance than <100>, most likely due to the angled nanowires.

5.3.2 Integration in solar cells and initial performance

The silicon nanowires exhibit a very low reflectance over photon energies greater

than the band gap of silicon, but single-wall carbon nanotube on silicon nanowire

(SWNT-SiNW) devices suffer from a reduced contact between the silicon and SWNT

71

400 500 600 700 800 900 1000 11000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Reflectivity

Wavelength (nm)

Bare silicon w\IL 20nm SWNT on Si w/IL 20nm SWNT on <100> SiNW w/IL 20nm SWNT on <111> SiNW w/IL

Figure 5-5. Reflectance of the silicon nanowire substrates as compared to untexturedsilicon.

film. Initial devices performed relatively poor. In fact, there is practically no photocurrent

in the absence of the ionic liquid and the PCE is only 0.02%. Addition of the ionic liquid

yields a maximum gated PCE of 4.6%, relatively low compared to other devices, but

the increase emphasized how enhanced absorption coupled with the inversion layer were

able to partially compensate for the minimal SWNT-SiNW junctions at the tips of the

nanowires.

Though this efficiency is poor in comparison to the planar gated devices, it was good

enough to encourage further investigation and optimization. An increase in the gate

electrode film area commensurate with the increase in silicon surface area was imperative

for inducing the inversion layer in the nanowire sidewalls. Employing a gate film of a size

used in the planar devices evidently limited the capacitance and partially explained the

poor performance. Additionally, employing better transfer methods to enhance contact

between the carbon nanotube film and the silicon nanowires would further improve carrier

collection. Finally, the increased surface area provided more recombination centers,

72

Figure 5-6. Initial performance of the SWNT-SiNW device. Note that the blue curve isthe illuminated J-V prior to addition of ionic liquid. The inset shows minutepower generation prior to the addition of the ionic liquid.

making the performance highly sensitive to the surface properties of the silicon and

encouraged experiments aimed at passivating the silicon surface.

5.3.2.1 Remote Gating

The need for a larger capacitance gate electrode and desire to avoid taking up

(potentially useful) area on the face of the solar cell necessitated the development of

a remote gate electrode that still retained a high surface area while occupying a small

volume. This gate electrode consisted of a coiled Pt wire onto which a thick layer of

SWNTs had been deposited, placed within a 2 mm inner diameter polyethylene tube. The

small tube was filled with the viscous EMI-BTI electrolyte, rendered immobile though

capillary forces. The SWNTs on the Pt wire amounted to a 1cm x 1cm, 1 µm thick

film, over 2 orders of magnitude larger than the previous gate films. The end of this

gate electrode was touched to the EMI-BTI electrolyte drop (over the Au pad to avoid

shadowing light from the active area) connecting the electrolyte reservoirs. Note that this

remote gate electrode improves on the previous design where the gate electrode occupied

front surface real estate of the Si (thus, in principle, precluding that areas availability for

73

light capture). A schematic cross-section of the device architecture (not to scale) and the

wiring diagram for testing is shown in Figure 5-7A.

A Experimental set-up for electronic gating withthe remote gate

B SEM image of SWNT-SiNW activearea, showing the nanotube filmlaying across the SiNW.

Figure 5-7. Schematic for remote gating and SEM of SWNT-SiNW active area

5.3.2.2 Passivation of Nanowire Sidewalls

The benefits of a thin native oxide passivation layer in planar SWNT-Si solar cells

was noted in the supporting information of Wadhwa et al. and studied in some detail for

double walled carbon nanotube/planar-Si cells by Jia et al.[9, 86] Such passivation is also

critical for the silicon nanowire devices. If the oxide layer becomes too thick, however,

it presents a tunneling barrier that degrades the cell performance. An initially poor

performance of SWNT-SiNW cells tested immediately after deposition of the nanotube

layer suggested that the native oxide had grown too thick during the device fabrication

steps. Accordingly, a brief BOE etch of the SiNWs through the porous SWNT network

was implemented to strip away the oxide, followed by an oxide regrowth in the ambient

air. Figure 5-8 shows J-V curves for a SWNT-SiNW cell (no EMI-BTI) as a function of

time in ambient air following the BOE etch. The initial measurements exhibited very poor

74

performance; however, the short circuit current density, JSC , open circuit voltage, VOC,

and fill factor, FF, were all seen to improve with time up to 96 hr, after which the trends

reversed. The series resistance, RS, obtained from the slope at the highest forward bias,

was found to grow monotonically while still being low enough at 96 hr that the native

device performance was maximized at that time. Plots of the solar cell parameters with

increasing oxidation time are shown in Appendix B. Electrolyte gating was also found to

be optimized following such 96 hr oxidation. It has been shown that water plays an active

role in silicon oxidation, so it was reasoned that its exclusion by the hydrophobic ionic

liquid used during gating would avoid further silicon oxide formation once the electrolyte

was added.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-40

-30

-20

-10

0

10

20

30

40

50

60

Cur

rent

Den

sity

(mA/

cm2 )

Voltage (V)

post BOE etch 5 hrs 24 hrs 30 hrs

Figure 5-8. J-V of a SWNT-SiNW device showing the effect of sidewall passivation viaoxidation on the performance of the device.

The effect of gating with the EMI-BTI electrolyte at gate voltages of +1.0, 0, and

-1.0 V under 100 mWcm2 , AM1.5G illumination are shown in Figure 5-9. The gate voltage

75

induced modulation of the SWNT Fermi level relative to that of the n-Si, modulating

the built-in potential (Vbi) at the junction is indicated by the dramatic shift of the open

circuit voltage (VOC) from 0.15 V (at VG = +1.0 V) to 0.58 V (at VG = -1.0 V). At the

gate voltage of VG = +1.0 V, positive ionic charge in the electrolyte at the silicon surface

attracts majority carriers (electrons, in the n-Si) to the surface and into the nanowires.

Screened by these excess majority carriers from the positive ionic charge, photogenerated

holes can also approach the silicon surface, resulting in an enhanced surface recombination.

Combined with the simultaneous decrease in the built-in potential in the SiNWs at

their junctions with the nanotubes, the recombination losses lead to a fill factor that is

essentially zero. At the gate voltage of VG = -1.0 V, negative ions in the electrolyte at

the silicon surface repels the majority carriers, creating an inversion layer at the surface

which limits surface recombination in a major fraction of the SiNWs. Combined with the

enhanced Vbi in the SiNW at the SiNW/SWNT junctions, the fill factor increases to 0.76

and maximizes the cell performance. The 35 mAcm2 short circuit current density here is much

greater than that in the planar, gated SWNT/Si cells (mAcm2 ), consistent with the additional

light absorption due to the vertical SiNW array.

5.3.2.3 SWNT film transfer on SiNW

Two distinct methods for depositing the SWNT layer were explored: ultrasonic

spraying from an ethanol suspension and transfer of a pre-formed SWNT film made

by the filtration route.[8] Purely sprayed SWNT layers had to be made substantially

thicker than what is seen in Figure 5-7B to attain low resistance continuity to the Au/Cr

electrode. In our experience, however, photons absorbed in the nanotubes contribute

little, if at all, to the power generation, so that thicker nanotube layers degraded cell

performance.[10] A good compromise was to spray a thin layer of nanotubes followed by

the transfer of a 10 nm thick filtration fabricated film. The roughly optimized quantity

of nanotubes deposited by the combined method had a surface nanotube concentration of

1.3 µgcm2 , approximately equivalent to that in a 20 nm thick, entirely filtration formed and

76

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

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-10

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10

20

30

40

50

60

Cur

rent

Den

sity

(mA/

cm2 )

Voltage (V)

VG = -1V

VG = 0V

VG = +1V

Figure 5-9. J-V curves for VG = -1.0 V, 0 V, +1.0 V on the SiNW device.

transferred film; however, the transfer of a 20 nm thick film without the sprayed layer did

not yield devices that performed as well as the partly sprayed, partly transferred layers.

Table 5-1 at the end of the chapter compares the performance of several SWNT/SiNW

cells at VG = -1.0 V for which the principle differences were the deposition method and

thickness of the SWNT layer. Device D (J-V curve shown in Figure 5-9) was the best, for

which the power conversion efficiency was 15.1%. Devices with a thicker (and hence more

light absorbing) net SWNT layer, (with correspondingly reduced light transmission into

the Si) exhibited poorer performance.

To explain the differences between the wholly transferred versus the partially

spray deposited films we note that thicker filtration fabricated SWNT films possesses

a greater mechanical stiffness. When such a film is transferred across vertical nanowires

with varying heights, the stiffness limits the films ability to conform over short length

scales, preventing contact to the shorter nanowires. This motivated the use of mixed

77

sprayed/transferred films and is consistent with the data in Table 5-1. Indeed it is this

ability of the nanotubes to touch and extract photocurrent from virtually every nanowire

tip, while providing a direct (non-tortuous), low impedance pathway to the gold electrode

(along with the gate induced inversion that avoids surface recombination) that explains

the dramatically improved performance in these cells over other silicon nanowire based

cells reported to date.

5.3.3 Discussion of inversion layer in SiNWs

The improvement garnered from the inversion layer in planar cells is impressive,

though predictable; the long diffusion length in single crystal silicon ensures adequate

collection of carriers with an inversion layer stretching hundreds of microns long. The

passivating native oxide layer coupled with the inversion layer prevent recombination at

the surface. The silicon nanowires, however, are known to have extremely high rates of

recombination. Indeed, this has historically been one of the limiting factors in efficiency

as the high surface area and long path to the electrode result in significant loss of carriers.

While oxidizing the substrate for numerous hours partially passivates the nanowire

sidewalls, performance is still extremely poor due to recombination. The induction of ionic

liquid immediately improves performance and allows carriers to then diffuse thought the

single crystal silicon nanowires until they reach a carbon nanotube. The geometries of the

silicon nanowires are such that it’s possible the entire nanowire is inverted. Simulations

presented in the previous chapter showed the inversion layer reaching a depth of over

1 µm, while the silicon nanowires are only tens of nanometers wide. Were the entire

nanowire to be inverted and there existed no potential gradient, there is no driving force

to repel the electron from the nanowire, eventually leading to recombination. It is thus

possible that further improvements could be obtained with thicker nanowires. Efforts to

experimentally test this are presently part of a collaboration with the CNMS at Oak Ridge

National Labs.

78

5.4 Effect of Oxygen and Water on Device Performance

Encapsulation of photovoltaics is universal; oxygen, water, and other atmospheric

contaminants shorten the lifetime of such devices. Especially vulnerable are organic

solar cells, which also suffer from UV light induced degradation.[29] Though silicon solar

cells typically function for decades, deleterious interactions with the environment require

encapsulation to ensure stability. The devices presented above showed some degradation

over time during continued testing with the lab atmosphere. Experiments to pinpoint the

source of degradation were carried out.

5.4.1 Effect of ambient oxidation

Our initial foray into passivation started with ambient grown oxide. The passivating

effect of silicon oxide has been utilized on the first SWNT-Si devices fabricated by Dr.

Wadhwa and were explored more in depth due to the results in the preceding chapter.[9,

10] Investigations into the kink that is seen in both SWNT-Si cells and MIS solar cells

showed a complex connection between cell performance and oxide thickness at the

SWNT-Si junction. Immediately upon addition of the ionic liquid, a hump shows up

in the J-V curve for all solar cells tested. This was initially attributed to a modulation

of the interface dipole between the carbon nanotube film and the silicon, but further

experiments with the inversion layer cell indicated another possible mechanism: ions

from the ionic liquid situated at the silicon surface within the interstitial regions between

carbon nanotube bundles create a field that either enhances or counteracts the built in

potential. As the depletion layer decreases with an increase in bias voltage, the effect

of these ions increases. Furthermore, the gate voltage affects the relative populations of

cations versus anions within these interstitial regions. Though the kink can be caused by

the mechanisms explained above, this same feature also arises from a sub-optimal oxide

thickness at the silicon-oxide junction.[10, 87] Planar, full film solar cells were constructed

using both <100> and <111> silicon and tested continuously for up to 49 hours to test

the degradation of the cell over time. No gating was performed; only the atmosphere was

79

exposed to the active area. As shown in the plot below, a kink is formed between 7 and 22

hours.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-30

-20

-10

0

10

20

30

40

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage

5 min 15 min 30 min 1 hr 2 hr 7hr 22 hr 49 hr

Figure 5-10. Evolution of J-V curve with oxidation in ambient atmosphere.

The kink feature occurs for both little to no oxidation, and for too much oxidation.

We hypothesize that when the silicon surface is freshly etched, it possess shallow trap

states that inhibit carrier extraction and lead to a saturation in current density when the

bias voltage is close to the built in potential. As the bias voltage is increased, the carriers

have enough energy to be excited out of the traps and the J-V curve exhibits exponential

behavior following the Shockley diode equation. Conversely, too much oxide results in

a barrier. At room temperature, the oxidation rate slows dramatically in an ambient

environment after the first few nanometers as subsequent oxidation requires the oxygen

to diffusion through the already formed silicon oxide to reach the silicon as stated in the

Deal-Grove model. This leads to the formation of a kink that progressively becomes more

severe as the oxide layer grows.

80

5.4.2 Reversible doping in ambient environment

In 2009, Martel et al. explored the inability to n-dope carbon nanotube-silicon

devices. It was demonstrated that reduction-oxidation reactions were occurring between

the carbon nanotubes and water vapor present on the substrate surface. Though the

substrate (silicon oxide insulating layer on top of silicon) itself was not participating in the

redox couple, its hydrophilic surface facilitated such reactions through adsorption of water.

These redox reactions transferred electrons from the carbon nanotubes into H2O/O2

redox couple, effectively p-doping the carbon nanotubes.[11] Efforts to n-dope the carbon

nanotubes resulted in immediate transfer of electrons to the redox couple. Replacing the

silicon oxide with a hydrophobic dielectric, such as parylene, diminished this effect and

allowed electron conductance within the carbon nanotubes. We believe this same effect is

occurring in our solar cells during testing in ambient conditions. A reversible reduction in

VOC has been observed during characterization in an inert (argon) atmosphere relative to

testing in the ambient lab environment.

After a BOE etch to remove native oxide, the devices were placed in a small vacuum

chamber that was evacuated and subsequently filled with argon gas. J-V curves were

taken while the substrate sat in this environment for 2 hours under illumination by a high

intensity fiber optic lamp. The resulting J-V curve is characteristic of a device without

residual water vapor in the active area and without a passivating oxide layer, i.e. there is

still a kink due to surface trap states. The initial VOC is lower than what is normally seen

in the SWNT-Si planar solar cells, consistent with both Martel’s conclusions and also a

lack of a passivating layer. The chamber door was opened and the substrate exposed to

ambient atmosphere for numerous hours. The VOC steadily increased, albeit at a slower

rate than usual due to the initial exclusion of water vapor (thus slowing the rate of oxide

growth) and saturated at 16 hours. The chamber was then closed, pumped out, and

backfilled with argon gas. A notable decrease in the VOC resulted, indicating a shift in

81

nanotube Fermi level. When the chamber was opened back to ambient atmosphere, the

VOC increased to its previous value.

-0.2 0.0 0.2 0.4 0.6-5

-4

-3

-2

-1

0

1

2

3

4

5

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

post BOE Argon 2 hrs Atmosphere 16 hrs Argon 3 hrs Atmosphere 2 hrs Argon overnight

Figure 5-11. Reversibility of the J-V curve upon alternating exposure to argon andambient atmospheres. The legend is ordered vertically based on the order ofmeasurements.

This reversible modulation implicates chemical reactions at the silicon/nanotube

surface in the presence of water or oxygen. Shown in Figure 5-11 are the J-V curves

for the device in both argon and ambient atmospheres. Concurrent with the change in

VOC was a development of a kink when water and oxygen were excluded. This can be

explained as the water having a moderate electrolytic affect on the device similar (but

more subtle) than what is seen with the EMI-BTI. Though this redox initially seems to be

beneficial to device performance by increasing the open circuit voltage, it became apparent

in subsequent experiments that the moisture adsorbed onto the silicon surface oxidizes the

silicon during electronic gating with the EMI-BTI, causing a degradation in performance.

5.4.3 Water vapor and oxygen contamination

Electrolyte gated cells do suffer a serious problem analogous to one that plagued

initially very promising liquid junction solar cells: chemical reactions at the silicon surface

82

degrade cell performance.[88] In the gated cells such degradation was accelerated by the

applied gate voltage so that when held, even for a few minutes at VG= 1.0 V, the J-V

curves began to exhibit an increasing series resistance and decreasing fill factor. Such

characteristics for the degradation suggested a continued growth of the oxide layer between

the SWNTs and the silicon, implying that water/oxygen had access to the junction despite

the hydrophobicity of the electrolyte. This deleterious effect is even more pronounced in

the SiNW devices, presumably attributable to the high surface area available for such

chemical reactions.

5.4.3.1 CV measurements showing IL contamination

To fully exclude water from the devices during electronic gating and ensure that

minimal electrochemical reactions were occurring at the silicon surface, measurements

confirming the wide electrochemical window of the EMI-BTI were carried out. Although

the as-received, EMI-BTI electrolyte was always stored and sampled from an inert

atmosphere glove box (argon, H2O, O2 each < 0.1 ppm), cyclic voltammetry measurements

on the electrolyte performed within the glove box (glassy carbon working-electrode, Ag

wire pseudo reference, Pt counter-electrode) revealed an electrochemical window of only

2.7 V, greatly reduced from its literature reported window of 4.4 V, but consistent with

being contaminated with water.

”Drying” the ionic liquid involved using activated molecular sieves (1:1 mixture of

3A and 4A) to trap water molecules. The sieves were submerged in the ionic liquid for 4

hours, after which cyclic voltammograms were carried out to check the electrochemical

window. As expected, the CV measurements showed a marked improvement after drying,

with the electrochemical window increasing by roughly 600 mV. Removing the dried

EMI-BTI from the glove box and repeating the CV measurement in ambient atmosphere

showed a gradual shrinking of the electrochemical window over the course of a few hours

as the ionic liquid became re-contaminated with water.

83

Figure 5-12. Cyclic voltammograms of the glassy carbon electrode in EMI-BTI ionic liquidat 50 mV

s: black line before treatment; red line after drying over the

molecular sieves. Black and red dotted lines specify the electrochemicalwindows of the EMI-BTI before and after drying, respectively.

The peaks in the CV measurement indicate reduction and oxidation reactions. It’s

unclear exactly what species are being created in the ionic liquid, and applying varying

potentials outside the electrochemical window produce different compounds with their

own associated redox potentials, leading to chemical reactions inside of the EMI-BTI

electrochemical window and degrading device performance. This is apparent in the black

curve in Figure 5-12. The spike at -0.25V is an oxidation reaction corresponding to the

reduction that occurred as the voltage was swept beyond -1.25 V.

5.4.3.2 Exclusion on planar device

Knowing that water contamination of the ionic liquid could lead to electrochemical

reactions inside the theoretical electrochemical window, we proceeded to fully gate a

device in a dry, inert atmosphere. After transferring a SWNT film, a final BOE etch was

84

followed by optimal oxidation in ambient atmosphere. To terminate further oxidation

and to evaporate residual surface water, the device was placed into the argon glove box

where it was stored for 4 days. Concurrently, a sample of the electrolyte was thoroughly

dried in an activated molecular sieve. At the end of this time the active cell area was

saturated with the thoroughly dried electrolyte and J-V measurements were periodically

recorded under illumination, in the glove box, with the gate voltage initially maintained

at a constant VG = -0.75 V. No degradation in any of the J-V characteristics was observed

even after 5 hours at this gate voltage. The gate voltage was subsequently raised to VG

= -1.0 V for an additional 5 hours with still no degradation observed. The gate voltage

was turned off and the device left in the glove box overnight. The next day a gate voltage

of -1.0 V was again applied, with no change in the J-V curve, as shown in Figure 5-13.

The device was subsequently moved into the laboratory ambient atmosphere and retested.

Degradation became noticeable within one hour of exposure to the ambient atmosphere

(at VG = 1.0 V), becoming progressively worse with further exposure, as shown in

Figure 5-14 These experiments strongly implicate water as the source of the degradation

in ambient atmosphere and indicate that by avoiding it such rapid degradation can be

overcome in the gated cells.

Interestingly, the VOC of the planar device was higher outside of the glove box, both

before and after exposure to the inert atmosphere. This reversible behavior is thought to

be caused by oxygen and water redox reactions with the carbon nanotube/silicon junction,

as discussed in Section 5.4.2. Exposure to the atmosphere, even after ionic liquid is

applied, results in an increase in VOC, indicating contamination of water and/or oxygen at

the junction. This theory is corroborated by the degradation in the J-V curve consistent

with water/oxygen contamination.

85

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-1V 1hr 2hr 3hr 4hr 5hr next day

Figure 5-13. Stability of planar device with oxygen and water excluded by gating in inertatmosphere with VG=-1.0 V.

5.5 Concluding Remarks

While metal assisted chemical etching of nanowires is a simple, low cost procedure,

the resultant nanowires are rather fragile. Their small diameter not only makes them

susceptible to breakage or agglomeration, but possibly allows complete inversion under

electronic gating. Current collaborations with Oak Ridge National Laboratory aim to

produce silicon nanowire devices with controlled pitches and diameters optimized for

optical absorption. This controlled growth should produce robust, uniform nanowires that

resist agglomeration. The ability to randomly space the nanowires inhibits diffraction and

further increases the optical absorption and uniform heights might eliminate the need for a

sprayed SWNT film.

Producing a record high 15.1% silicon nanowire device is a major advance over

the previous 10%, but only practical if the performance is stable. The propensity for

recombination at the nanowire sidewalls inhibit carrier collection while the high surface

area and severe topology exacerbate any chemical reactions with the ionic liquid. Locally

86

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Post glovebox 1hr 3hrs 5hrs Next day

Figure 5-14. Degradation of the planar SWNT-SiNW device upon exposure to atmospherewith VG=-1.0 V.

strong electric fields can catalyze local redox reactions, producing varied chemicals

deleterious to the device. Proof of stability with water exclusion was a critical first step in

proving the viability of these devices, but improved passivation is also imperative to avoid

long term degradation.

87

Table 5-1. Performance for various film deposition techniques and thicknesses

Device VOC (V) JSC(mAcm2

)FF Efficiency (%) Notes

A 0.58 32.5 0.74 13.9 20 nm transferredB 0.58 32.0 0.73 13.5 5 nm sprayed/25 nm transferredC 0.58 32.5 0.71 13.2 45 nm transferredD 0.58 35.0 0.76 15.1 10 nm sprayed/10 nm transferred

88

CHAPTER 6PASSIVATION OF SILICON

The need for excellent passivation of semiconductors has been extensively researched

in regards to both MIS and P-N junction devices. Inherent to p-n homojunctions is

a near perfect boundary when the junction is formed by high temperature diffusion

on a crystalline substrate. Heterojunctions and Schottky junctions suffer from abrupt

boundaries that facilitate recombination through defects and mid gap trap states. As

such, much effort has been put into various chemical and mechanical methods aimed at

creating defect free surfaces and junctions.[32, 89–91] Passivating silicon with an insulating

layer, such as silicon nitride, is a facile method to reduce recombination, but often requires

thicknesses inhibiting charge transport across the junction.[92] The challenge with the

silicon nanowire devices is to passivate the tips in such a way that holes can still be

transferred to the carbon nanotube film, while still providing robust passivation along

the nanowire sidewalls to limit recombination. Limitations on thickness constrain the

passivation layer between the SWNTs and SiNW to be less than a couple nanometers,

while the passivation of the sidewalls must not interfere with the ionic liquid induced

inversion layer.

Our experiments have shown increased degradation within the nanowire devices

as compared to the planar devices upon electronic gating. The planar devices, after

being held at a gate voltage of -0.75V for 20 minutes, showed a small decrease in power

efficiency: 10.12% to 10.06%, a 0.6% decrease. The nanowire based device degraded from

15.1% to 14.4% during the same period at a gate voltage of -1.0 V, an overall decrease

of 4.6%. The increase in surface area simultaneously creates more sites for recombination

of photogenerated carriers and adverse electrochemical reactions between the silicon and

ionic liquid. Excluding water from the junction mitigates the latter, but providing a high

quality passivation layer could help alleviate both problems. Presented below are several

methods of passivation achieved through chemical treatments and atomic layer deposition.

89

Figure 6-1. ALD growth process. 1. Hydroxide terminated substrate is exposed toreactant Trimethylaluminum, which chemiadsorbs onto the substrate,producing methane as a byproduct. 2. System is purged, removing unreactedprecursors and chemical byproducts. 3. Oxidant (water vapor) is fed into thesystem, where it reacts with the methyl ligand, forming OH groups. 4. Systemis again purged of excess species, and the process repeats.

6.1 Atomic Layer Deposition of Al2O3 and HfO

One means to avoid ambient water adsorption is to encapsulate the cells in an inert

atmosphere as must presently be done for other water/oxygen sensitive systems (e.g.

organic solar cells). Alternatively, a thin dielectric barrier layer coating the nanotubes

and SiNWs at the junctions may be sufficient to prevent oxidation due to the water

entrained in the electrolyte. In an attempt to create such a barrier we turned to

atomic layer deposition (ALD) of Al2O3. ALD deposited aluminum oxide has received

increasing interest as a silicon surface passivation layer since the availability of commercial

ALD systems.[93] The layer by layer deposition of vapor phase reactants (sequentially

trimethylaluminum and water) implies a conformal coating even through the pre-deposited

nanotube layer.

ALD is a self limiting process that deposits a monolayer by monolayer conformal

coating of by alternating reactant gasses. In between each reactant gas, the chamber is

purged to vacate any reactants not chemisorbed onto the substrate. Both thermal and

90

plasma/ozone ALD form conformal layers, though the use of unstable and reactive species

in the latter often create small defects in the coating and is only utilized for substrates

that are sensitive to high temperatures.[94] Exposure to ozone for even a couple minutes

causes extreme sidewall damage to the nanotubes, resulting in a substantial loss of

conductivity. Consequently, efforts were made to avoid plasma while still using a relatively

low deposition temperatures. The acid purified nanotubes we use are p-doped by it which

increases the built-in potential against n-type silicon so low temperatures were preferred to

prevent nanotube dedoping.

Figure 6-1 shows the four main phases of ALD Aluminum Oxide growth. Once loaded

into the ALD system and contained within an inert environment, the first reactant gas,

trimethylaluminum (TMA), is fed into the chamber. Reactions with the (OH)− on the

substrate result in chemiadsorbtion of methly terminated aluminum (bonded to oxygen),

creating methane gas as a byproduct. A purge removes any unreacted TMA along

with the methane. Water vapor is then fed into the chamber where it reacts with the

methyl terminated aluminum, releasing methane and resulting in a hydroxide terminated

aluminum. Another purge removes the excess water and methane, leaving the substrate

in a similar chemical state as it was in the first step. The process then repeats, each time

growing a single layer of aluminum oxide.

Prior to growth of the Al2O3 the SWNT/SiNW device underwent a final BOE

etch, followed by oxidation in ambient for the time that optimized device performance

(96 hr). Al2O3 was grown for 110 reactant cycles at a substrate temperature of 80

C.[95] Ellipsometry performed on such a film deposited on a flat silicon chip under these

conditions gave a film thickness of 8.8 nm (coverage in the first several cycles is incomplete

so the initial growth does not give complete layers). Figure 6-2 shows an SEM image of

the Al2O3 coated device. Bright spots in the image are enhanced secondary emission from

where the SiNW tips underlie the dielectric coated nanotubes.

91

Figure 6-2. SEM image of ALD deposition. ALD Al2O3 on SWNT-SiNW device. Brightspots are enhanced secondary emission from the tips of the underlying SiNWtips.

Measurement of parasitic gate currents can quantify the reactions occurring at

the Si surface, some portion of which should correspond to deleterious redox reactions

(other electrolyte or impurity reactions that don’t degrade the SWNT/Si interface can

also occur). For SWNT-SiNW devices without the ALD dielectric coating, the steady

state gate current at VG = -1.0 V in the non-dried electrolyte was typically 2.7 µA. For

the dielectric coated device this was reduced by a factor of 60 to 45 nA. Unfortunately,

this was still a factor of 110 greater that observed for the device measured in the glove

box using the dried electrolyte for which the gate current was 0.4 nA, and while the

rate of degradation of the coated device was greatly reduced over the uncoated device,

degradation was evident over the course of several hours (measured in the ambient

lab atmosphere in the non-dried EMI-BTI electrolyte). This implies that the ALD

layer remains permeable to water at the thickness used. A thicker layer may prevent

this, although a hydrophobic coating (e.g. Parylene) may be preferred to the naturally

hydrophilic oxide in such an application.

92

6.1.1 Al2O3 and HfO results

Figure 6-3 shows the J-V curves for the device before and after electrolyte addition

at gate voltages for the latter of zero and -1.0 V. At VG = -1.0 V the open circuit voltage,

short circuit current density and fill factor were, VOC = 0.62 V, JSC = 33.4 mAcm2 , FF =

0.73 resulting in a PCE of 14.8%. The slightly lower JSC and PCE over the uncoated

device is likely due to an increased light scattering due to the Al2O3.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-40

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Figure 6-3. J-V curves for the ALD Al2O3 coated SWNT/SiNW cell. Without electrolyte(red) and with the electrolyte at the indicated gate voltages (black, blue). Theenhanced photocurrent on addition of the electrolyte is attributed to refractiveindex matching reducing the scattering.

The ALD results demonstrated good passivation of the SiNWs. The quantitative

reduction in the gate current indicating reduced redox occurring at the silicon/nanotube

junction is corroborated by the decreased degradation of the J-V curve, shown in

Figure 6-4. The decrease in performance for a non-ALD SiNW device is shown for

comparison, albeit the measurements are taken for different times (48 hours vs 72 hours),

93

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No ALD -0.75V initial No ALD -0.75V 72hrs No ALD -0.75V 72hrs hold for 20min ALD -0.75V initial ALD -0.75V 48hrs ALD -0.75V 48hrs hold for 60min

Jsc

(mA

/cm

^2)

Voltage (V)

Figure 6-4. J-V curves for ALD SWNT-SiNW device vs device without ALD. Included isthe degradation occurring both with applied gate voltage and time spent inambient without the gate voltage applied. The ”No ALD -0.75V 72 hours” and”ALD -0.75V 48 hours” graphs indicate the device was sitting in ambientwithout the gate voltage on (but ionic liquid still in place) for the specifiedamount of time.

so an exact comparison is not provided. provided. The Al22O33 coating clearly helps but

it is clearly also not sufficient to overcome the electrochemical degradation. While it is

possible that more water had diffused into the device at 72 hours, and hence could exhibit

a higher degradation due to enhanced redox, we do not think that is the primary factor for

the increase in degradation relative to the ALD device.

Noting that Hafnium Oxide (HfO) possesses a dielectric constant twice that of Al2O3,

we also deposited a 25 nm thick AD HfO layer onto SWNT-SiNW substrates (already

oxidized for 96 hours in the lab atmosphere). It was reasoned that the thicker layer

would further reduce electrolyte access to the silicon surface while still providing the

same capacitance as the Al2O3. Device stability, specifically gate currents, were markedly

94

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25 nm HfO ALD 8.8 nm Al2O3

Figure 6-5. J-V for the ALD HfO device showing a lowering JSC due to the highreflectance of the device.

smaller than for the 8.8 nm Al2O3. The gate current at VG = -1.0 V was 13 nA, as

opposed to the 60 nA for the thinner aluminum oxide ALD layer, indicating that a thicker

ALD inhibits redox reactions by limiting contact between the ionic liquid and the silicon

surface. The thicker ALD layer wasn’t without drawbacks; JSC was only 30 mAcm2 due to the

higher scattering induced by the thicker HfO layer, which had an associated haze visible to

the naked eye. Reduced degradation in the J-V curve of the HfO device was quantifiable

- After 30 minutes of continuously held gate voltage at -1V, the reduction in PCE was

measured to be 0.98%, compared to a degradation of 2.1% for the 8.8 nm Al2O3, which

was held at a lower gate voltage of -0.75V.1

1 Some of the measurements made on different devices were not identical, leading to discrepancies whentrying to compare degradation. Care was taken to try and compare results under similar gating condi-tions. Early devices, unfortunately, were not as extensively tested as later devices, resulting in less data forcomparison.

95

To assess the ability of the ALD layer to protect the silicon from redox reactions

with the ionic liquid and to fully evaluate the effect of the deposited dielectric, we also

deposited Al2O3 onto a planar SWNT-Si device. As for the previous devices, the solar

cell was allowed to oxidize in the ambient atmosphere for 2 hours prior to ALD growth,

which was found to be the optimum time for planar devices. All solar cell parameters

improved in the gated ALD device as compared to non-ALD devices. Most notable is the

increase in JSC both with and without ionic liquid, indicating the ALD layer acts as an

index matching layer that reduces reflection. With a refractive index of 1.55[96], the Al2O3

is well below the refractive index of silicon, n= 3.96, and similar to refractive index of the

ionic liquid, n = 1.42.2 Data are shown in Table 6.4 at the end of the chapter.

The ALD devices clearly exhibit superior performance relative to the non ALD

devices, though there is still some degradation. Current efforts are aimed at testing the

ALD devices in the glove box with the dried EMI-BTI. Complete exclusion of water

in conjunction with high quality passivation should afford excellent means to avoid all

sources of degradation. Despite the promising results of incorporating ALD into the

device, we also explored other methods of producing the same stability. Photovoltaics

must be stable in addition to inexpensive in order to compete on the open market,

motivating a search to find a facile, scale-able, and inexpensive means to protect the

modules from degradation.

6.2 Hydroquinone

Hydroquinone, an organic compound commonly used as a bleaching agent and

photographic developer, has recently been shown to produce ideal Schottky junctions

when used to treat to a silicon substrate.[97] Molecules attached to the dangling silicon

bonds create a surface dipole, changing the effective electron affinity of the semiconductor.

Through use of different additives, the length of the dipole can be altered, leading to a

2 Taken from TCI Chemicals website: http://www.tcichemicals.com/eshop/en/us/commodity/E0599/

96

A Silicon surface showing danging bonds afterremoval of oxide layer.

B Hydroquinone moleculewith attached methoxy (CH3)group, forming a surface dipoleand simultaneously passivatingthe surface.

Figure 6-6. Silicon substrate and hydroquinone molecule. Reprinted with permission fromR. Har-Lavan, et al., AIP Advances 2, 012164 (2012).

tunable effective electron affinity and subsequent change in Schottky barrier height. In the

paper by Har-Lavan, et al., it was shown that methanol yields that largest negative dipole,

yielding an effective electron affinity of 3.05 eV. This created a barrier height of nearly 1

eV, remarkable in that it is comparable to the band gap of silicon. Due to the dipole, the

silicon at the junction became strongly inverted, leading to carrier transport dominated

by minority carrier generation and recombination. Most important for our purposes, the

surface of the silicon was passivated, with stability shown over several days (as opposed to

hydrogen termination which is stable up to a few hours in ambient).

The preparation was modified slightly from the procedure done by Har-Lavan, et al.

A SWNT-Si device was prepared following the standard procedure, finishing with a final

BOE etch to remove any native oxide growth and leaving the silicon hydrogen terminated

at the surface. The substrate was placed in a 0.01M Hydroquinone in methanol solution

for 3 hours in a dark, ambient environment. The original paper called for sonication

in ethyl acetate, but the violence of such a process would have damaged the carbon

nanotube film. Treating the substrate prior to transferring the carbon nanotube film

produced devices with a higher series resistance and poor fill factor, indicating adverse

reactions between the hydroquinone and solvents used in the transfer process. As such,

97

HQ treatments were done after the film had been transferred, after which the substrate

was dipped into boiling dichloromethane for 30 seconds to remove any excess molecules

not firmly adsorbed onto the surface. This was followed by drying in an N2 stream and

construction of the backside contact.

Shown in Figure 6-7 are the J-V curves for the hydroquinone device. Initial

performance is comparable to that achieved without HQ treatment, with a VOC of

0.56V, FF =0.74 , JSC = 23.6 mAcm2 , and PCE of 9.73%. Addition of EMI-BTI to the active

area results in immediate degradation, with severe, irreversible degradation after electronic

gating, indicating that HQ passivation is not compatible with inclusion of ionic liquid.

Gated results for VG = -0.75V are VOC of 0.52V, FF =0.21 , JSC = 8.6 mAcm2 , and PCE

of 0.93%. Though the previous experiments on water and oxygen exclusion indicate that

the problem could be attributed to additional chemical interactions between the ionic

liquid and water or the hydroquinone and water, ultimately forming several compounds

deleterious to device stability. Stability measurements of the hydroquinone passivated

devices without electronic gating are presently under investigation.

6.3 Sulfur

Sulfur has been used to successfully passivate GaAs <100> and InP substrates and in

2007 it was shown that sulfur provides an excellent passivation for <100> silicon, resulting

in near ideal Schottky barriers with both low and high work function metals.[98] Though

the adherence of a foreign molecule to dangling silicon bonds forms an interface dipole,

experiments show the effect of this interface dipole on the SBH to be negligible relative

to the reduced Fermi level pinning achieved through a reduction of surface states. With

an electronegativity of 2.58, sulfur attracts electrons when covalently bonded to silicon

(electronegativity of 1.9),[99] resulting in a surface dipole in opposition to the built in

potential in the silicon at the SWNT-Si junction. Nonetheless, the passivation afforded by

this chemical treatment reduces the surface recombination velocity and subsequently the

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pre EMI-BTI 0V -0.75V 0V post gating

Figure 6-7. J-V curve of the HQ treated planar cell before, during, and after electronicgating with EMI-BTI.

dark current, negating any decrease in the Vbi and leaving VOC largely unaffected, shown

in Figure 6-8A.

Early experiments with sulfur passivation following the procedure in Ali et al.

resulted in poor performance due to low FF and high RS. In light of research conducted

by Aibin, et al., we concluded that the time spent in the sulfur solution allowed growth

of large crystallites, thereby contaminating the silicon surface with large particulates and

impeding charge collection. Shortening the time in solution by a factor of 4 produced a

uniform monolayer of sulfur, passivating the surface without creating a physical barrier to

hole extraction.[100]

Similarly to the HQ experiments, sulfur passivation was done both prior to and after

SWNT film transfer, with optimal results for treatment after film transfer. After the

prepared devices underwent a final BOE etch to remove native oxide, they were placed in

99

a 0.33M/2.4M NH4)2S/NH4OH solution at 60C for 5 minutes. This was followed by two

DI baths of 5 minutes each, and an N2 dry. J-V curves were taken immediately after the

back contact had been assembled. Initial results showed excellent performance relative

to unpassivated devices. Addition of EMI-BTI and subsequent gating indicated negative

interactions of the sulfur with the ionic liquid. Initial gating curves were excellent, but

continued application of the gate voltage resulted in irreversible degradation, as shown in

Figure 6-8A.

A likely culprit in the poor performance is the presence of water, which was shown in

Chapter 4 to degrade device performance. Further experiments performed in a glove box

using dried ionic liquid will be used to check if this is correct.

6.4 Discussion and Summary

Given that encapsulation or a better optimized passivation layer should prevent the

electrochemical degradation, we consider other aspects of our present devices that could

limit their performance (suggesting means to increase their PCEs beyond the present

15%). One limitation concerns excess recombination at the back contact. It has long been

known that a back surface field induced by appropriate doping of the Si at the device back

contact can reduce recombination there, with corresponding improvements in the device

performance.[101] Another factor also limiting the PCE in our present design is their

geometry. In the present construction the Si wafer thickness (550 µm) is large relative

to the active area width (2 mm) meaning that photocarriers created near the edge of

the active window region have an appreciable cross-section for escape out the sides of

that region, thus contributing to the losses. Capturing those carriers could significantly

boost the device PCE. Quite recently PCEs comparable to what we demonstrate here

were obtained from planar, chemically charge transfer doped nanotube/Si solar cells

exploiting a TiO2 antireflection coating.[102] The broad-band reflectance due to that

coating was not as low as our vertical NW arrays , exhibiting a minimum of 5% at 600

nm, but rising smoothly on either side of the minimum to over 10% at 500 nm and 800

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A Performance of the sulfur passivated device (post SWNT film transfer)

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HQ pre EMI-BTI HQ at -0.75V S pre EMI-BTI S at -0.75V

B Hydroquinone versus Sulfur passivation upon electronic gating.

Figure 6-8. J-V curves for sulfur and hydroquinone passivated devices.

101

nm, respectively and to over 20% at the extremes of the relevant solar spectrum (400-1100

nm). The comparable performance, despite our reduced reflectance, suggests that their

devices exhibited lower losses which could be due to their more optimized geometry.

They used thinner (400 µm) wafers and larger active cell area (15 mm2 vs. our 8 mm2)

consistent with a reduced carrier leakage out the sides of their active region. Combing the

results presented here with future strategies for further improvement bode well for rapid

advancement of nanotube-silicon devices since their initial development in 2007.[60]

102

Table 6-1. Performance for ALD devices for VG = -1.0 V

Device ALD type and thickness Voc (V) Jsc(mAcm2

)FF Efficiency (%)

SiNW 8.8 nm Al2O3 0.62 33.2 0.73 14.84SiNW 25 nm HfO 0.58 29.7 0.72 9.74Planar 8.8 nm Al2O3 0.6 28 0.8 13.45

103

CHAPTER 7ADDITIONAL PROJECTS

7.1 TFSA Doping of Graphene-Si and Carbon Nanotube-Si Devices

Graphene has long been used as an electrode for organic photovoltaics, [103, 104]

but only in the past couple years has it been demonstrated in Schottky junction solar

cells.[105, 106] The first generation of devices achieved a PCE of approximately 1.7%,

as the as prepared graphene introduced high levels of resistance into the device which

reduced both the FF and JSC while increase RS. Nonetheless, we decided to integrate

graphene into our architecture in an attempt to mitigate the deleterious effects of excessive

oxide growth at the junction and to limit redox reactions between the silicon and ionic

liquid. With it’s hexagonally close packed carbon atoms and relatively low permeability

of water vapor and oxygen, the graphene lattice is suitable for hindering silicon oxidation

at the surface of the active area. This should stabilize the series resistance and hinder the

formation of the kink that accompanies an oxide induced barrier. Unless highly doped, a

single monolayer of graphene is too resistive act as an electrode, resulting in the low PCE

reported above. Additionally, a graphene-CNT hybrid device could offer the best of both

worlds: retention of the optimal oxide thickness and a low series resistance. Additionally,

graphene would be able to screen the silicon surface from the ions in ionic liquid, avoiding

redox reactions and leading to more stable performance with electronic gating.

7.1.1 Graphene-Si Solar Cells

The fabrication process for the graphene-Si device was modified slightly due to

the impermeability of the graphene; oxygen and water vapor cannot easily penetrate

the lattice to readily oxidize the silicon surface. The SWNT devices underwent a final

BOE etch to remove native oxide, followed by oxidation in ambient atmosphere for 2

hours until the JV curve exhibited stable, maximum performance. This process is not

practical for graphene devices, as the BOE cannot permeate the graphene to etch the

underlying oxide (though a tear in the graphene could allow etching of the underlying

104

oxide, reaction products could not diffuse out). Our modified fabrication process involved

BOE etching the gold framed active area window and exposing the substrate to the

ambient atmosphere for 2 hours to facilitate oxide growth before transfer of the graphene.

A drop of isopropanol (IPA) was placed on the window and a PMMA supported graphene

sheet, synthesized and provided by Dr. Max Lemaitre and Dr. Art Hebard, was placed

on top of the IPA (graphene side against the silicon). Pressure to adhere the graphene

to the silicon was supplied by aluminum blocks. After 4 hours at room temperature, the

IPA had evaporated and the graphene was held tightly to the substrate via Van der Walls

forces. An acetone vapor bath followed by several acetone soaks removed the PMMA

membrane. Fabrication of the back contact and characterization proceeded following

standard procedures.

As expected, the device exhibited a higher series resistance relative to the carbon

nanotube devices. Though graphene has high mobility its carrier density is low resulting

in a resistivity that makes for a high series resistance. Prior to addition of the EMI-BTI,

PCE was 1.9%. Addition of EMI-BTI and subsequent gating at -0.75V increased the

efficiency to 4.6%, an overall increase of over 140%. While this huge improvement

indicates facile manipulation of graphene’s electronic properties, the performance was

several factors below that of the carbon nanotube-silicon devices. We hypothesized

that a hybrid-graphene-SWNT device would exhibit better performance though a

confluence stability from the graphene layer and improved conductivity from the carbon

nanotube layer. Unfortunately the results for the hybrid device were worse than for the

graphene-only device, presumably due to silicon surface states formed during the excessive

amount of processing involved in transferring graphene and SWNTs. The JV curves are

shown in Figure 7-1 and cell characteristics shown in line one of Table 7-1.

In collaboration with Dr. Xiaochang Miao and Dr. Seffattin Tongay, a graphene-Si

solar cell was fabricated and then doped with Trifluoromethanesulfonyl-amide (TFSA,

(CF3SO2)2NH). Extensive research on dopants for graphene has been done the past several

105

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-40

-30

-20

-10

0

10

20

30

40

50

60

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

pre EMI-BTI VG = 0 V VG = -0.75V

Figure 7-1. JV curve for the monolayer graphene device.

years, with TFSA (alternatively, TFSI) proving to be highly p-doping and stable due

to its hydrophobic nature.[107] Graphene devices were fabricated following the method

outlined above, with PCEs of 1.5-3%. A 20 mM solution of TFSA in nitromethane was

spun coated onto the graphene at 1000-1500 RPM for 1 minute, after which the devices

were characterized. Immediate improvement occurred for all solar cell parameters, as

summarized in Table 7-1 at the end of the chapter and shown in Figure 7-2B. The PCE

jumped to 8.6% due to a dramatic increase in JSC , VOC , and FF. The strong charge

transfer doping achieved with the TFSA proved to be fairly stable for the graphene device,

resulting in negligible efficiency decreases over several days.[106, 108]

The TFSA doping increased the Schottky barrier height by approximately 0.11eV,

from 0.79eV to 0.89eV, resulting in an increase of the built in potential and a reduction

of dark current due to reduced recombination at the junction. Additionally, the external

quantum efficiency increased by roughly 30% over visible wavelengths, indicating a more

106

A Architecture of the graphene cell.Identical to the SWNT-Si planar deviceswithout the on chip gate film.

B J-V curves showing effect of TFSA doping

Figure 7-2. Schematic and performance for graphene PV cell.

efficient collection of carriers most likely attributed to the increased built in potential. The

thin layer of TFSA potentially acts as an antireflection layer, as darkening of the active

area window post TFSA treatment was noticeable.

7.1.2 TFSA with carbon nanotubes

Given the excellent results with the TFSA doped graphene device, we set out to

incorporate the same treatment for the carbon nanotube films. Though our as-purified

SWNTs are already p-doped, the TFSA provides a higher level of doping as evidenced

by the increased VOC and smaller RS. Following the standard procedure for fabricating

SWNT-Si devices, the device was oxidized in the ambient atmosphere for 2 hours until

the JV curve revealed stable performance, after which the TFSA was spun coated onto

the device. Immediate testing revealed enhanced performance relative to the undoped

and ungated performance. Unlike the graphene device, the TFSA doping of the SWNTs

appeared to be transitory and a slow degradation in all parameters was noticed over

time. While we do not know the exact cause of the discrepancy between the stability

of the graphene devices vs. the carbon nanotube devices, we believe it is attributed to

the reduced, but still finite, access of atmospheric water to the line contact between the

107

nanotubes and the silicon, permitting oxidation there. The TFSA dopant was shown to

be highly incompatible with the ionic liquid (as expected due to their chemical reactivity),

and as such this method of doping would only be viable for ungated encapsulated devices.

Shown in Figure 7-3 are the JV curves for the TFSA doped carbon nanotube device.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-40

-30

-20

-10

0

10

20

30

40

50

60

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

Post BOE etch and oxidation Post TFSA TFSA at 50 min EMI-BTI VG = -0.75V

Figure 7-3. J-V curves showing effect of TFSA doping and subsequent gating on SWNT-Sidevice.

7.2 Backside Doping

Several experiments were performed attempting backside doping of the silicon

substrates to create a back surface field there. This strongly doped region has been used

extensively in p-n junction solar cells to reduce recombination.[109] An n+ - n junction

exhibits similar properties of a p-n junction: in both cases carriers diffuse from one region

to another until electrostatic equilibrium is reached. The resulting electric field serves

to sweep carriers of opposite charges in opposite directions. When employed on the

backside of photovoltaic cells, this secondary built in potential serves to reduce carrier

108

recombination at the back contact. The doping procedure is fairly straightforward: a

phosphorus spin on dopant (Filmtronix P509) is spun coated at 2500 RPM onto the

backside of the silicon substrate and baked at 1150 C in ambient atmosphere for 2

hours (ramping at 1 degree C per minute). The high temperature allows the dopant to

diffuse into the silicon, creating a shallow, gradated region of n+ doping. Preliminary

testing yielded a sheet resistance of 1.5-3 ohm/square on the backside of the wafers

(versus 20-60 ohm/square for the undoped wafers) and a modest increase in PCE was

obtained. The wafers used were also thinned to 250 µm before doping the backside, a

modification that should minimize recombination by reducing the cross-section for carrier

leakage out the sides of the active area. Additionally, the diffusion length of electrons in

lightly doped n-type silicon is on the order of hundreds of microns (depending on silicon

grade, growth parameters, and doping density) so reducing the path of travel should

result in an increased photocurrent. This improvement in comes at a price: slightly less

photoconversion from photons near the band gap of silicon. The absorption length of

silicon for light with energy 1.1eV is approximately 6.6 mm, with higher energy photons

having a shorter absorption length.[110] However, the irradiance of infrared photons in the

solar spectrum is relatively low, so the net change in photons absorbed within the silicon is

only marginally affected.

The higher VOC is attributed to a reduction in recombination, which subsequently

lowers the dark current and increases the open circuit voltage. Additionally, less

recombination of electron hole pairs generated near the backside increased JSC, as shown

in Table 7-2 at the end of the chapter. Though this method was highly successful at

increasing the PCE, the process was only viable on silicon with a thick (>1 µm) oxide

layer. The high temperature bake formed blisters on the surface of the substrate, creating

shorts within the device, shown in Figure 7-4. Altering the temperature and ramp rate

were unsuccessful at preventing the defects. Future projects should look to alternative

methods of implanting a highly doped backside region to reliably boost performance.

109

Figure 7-4. Blistering on the surface of the silicon following a high temperature bake todope the backside.

7.3 Concluding Remarks and Path Forward

The improvements discussed in the previous chapters indicate the potential for

further improvement of the SWNT-SiNW devices by increasing both efficiency and

longevity. The ALD proved to be an excellent barrier between the silicon and the ionic

liquid without sacrificing the inversion layer needed to reduce recombination. Though

degradation did eventually occur in the ALD devices, parasitic gate currents indicate that

the electrochemical reactions were greatly reduced compared to the non-ALD SiNW based

devices. Additionally, it was demonstrated that water vapor and oxygen contamination at

the surface of the device were responsible for the decrease in performance during electronic

gating, necessitating testing within an inert atmosphere to elicit stable performance.

Lastly, a highly doped region at the back contact was effective at reducing recombination

and boosting PCE. Incorporating these three mechanisms into photovoltaic designs

promises to deliver devices that achieve a PCE above 15%, but also stable.

Future projects are currently underway, including a collaboration with Oak Ridge

National Laboratory to develop more ordered silicon nanopillar devices. These new

architectures will be more robust than the nanowire. The ability to selectively control

diameter will allow us to explore the charge collection efficiency when the entire nanopillar

is either completely or partially inverted. Though beyond the scope of this thesis, the

110

ability of the carbon nanotube film to conform to the substrate surface would make it an

idea candidate for forming Schottky junctions with amorphous silicon. A short diffusion

length due to multiple grain boundaries necessitate excellent connection between the

electrode and the silicon. The SWNT film within our Schottky junction devices also

acts as a transparent electrode, making it an ideal candidate for a-Si based devices. The

flexibility of the carbon nanotube film compliments the malleability of amorphous silicon,

raising the possibility of a flexible, thin film solar cell.

The vast majority of experimental breakthroughs realized at the academic level

never reach the consumer market. As daunting and discouraging as it may seem to the

budding scientist, no experiment is completely disconnected from either contemporary

innovations or the research conducted in the past. The primary goal of science is to

increase the knowledge of how the world works, disseminate that knowledge to those who

are interested, and ultimately use that knowledge to advance society. The advances made

during these experiments in photovoltaic cells will hopefully benefit other scientists and

contribute to technological innovation.

111

Table 7-1. Performance summary for TFSA doped graphene and SWNT solar cells

Device Voc (V) Jsc(mAcm2

)FF Efficiency (%) Notes

Graphene 0.42 14.2 0.32 1.9Graphene w/TFSA 0.54 25.3 0.63 8.6Graphene w/ EMI-BTI 0.52 26.84 0.34 4.6 VG = -0.75VSWNT 0.51 24.0 0.72 8.97SWNT w/TFSA 0.54 26.0 0.75 10.43SWNT w/TFSA and EMI-BTI 0.45 25.65 0.21 2.45 VG = -0.75V

Table 7-2. Performance for backside doped substrates

Device Voc (V) Jsc(mAcm2

)FF Efficiency (%) Notes

<111> 500 µm 0.54 25 0.74 9.98 VG = -0.75V<111> 250 µm 0.52 26.7 0.65 9.74 VG = -0.75V<111> 250 µm doped 0.58 29.6 0.79 13.42 VG = -0.75V

112

APPENDIX AFULL SIMULATIONS FOR THE INVERSION LAYER CELL

Figure A-1. Modeling of the inversion layer at the silicon surface in the carbon nanotubegrid solar cell

113

APPENDIX BSOLAR CELL PARAMETERS WITH INCREASING OXIDATION TIME

Figure B-1. FF, JSC , VOC , and PCE for a SWNT-SiNW device for various oxidation timesin the lab atmosphere.

114

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BIOGRAPHICAL SKETCH

Maureen Petterson was born in San Francisco, California in 1983. The youngest of

three children, she found support and encouragement to explore the universe through

her parents, who both believed in inquiry based learning and always forced her to

think instead of giving easy answers. She received a Bachelors of Science in Physics

(Astrophysics) from the University of California, Santa Cruz in 2006. Though her degree

focused on Astrophysics, a senior year thesis in Medical Physics working under Dr.

Hartmut Sadrozinski led to an interest in more hands on research. After graduation

she worked at the Santa Cruz Institute for Particle Physics for two years, an experience

that was invaluable for how it shaped her into an analytic and patient researcher and

ultimately cemented her decision to pursue an advanced degree in physics. After sending

in her graduate school applications, she left the United States and spent the next 6

months backpacking around Southeast Asia, the Middle East, and Eastern Europe with

Griffiths’ Quantum Mechanics in tow. She enrolled at the University of Florida in the

fall of 2008. Active within the physics community, she was the Graduate Student Council

representative for the physics department, served on the physics Graduate Student

Advisory Committee, and participated in numerous outreach events. In her second year

she started working with Dr. Andrew Rinzler on carbon nanotube based photovoltaics, a

field in which she could combine both gratifying research and exciting physical principles

with a desire to impact and contribute to society in a meaningful and positive way.

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