Heterojunction Based Hybrid Si Nanowire Solar Cell Assets/Pages/20th...Obstacles on the Road Obstacles for nanowire growth and surface modifications: SiO2 Native oxides Gold diffusion

Muhammad Y. BashoutiAlexandre Yersin Department of Solar Energy and Environmental Physics

Sde Boqer Campus, BGU

Material Growth Surface Engineering Devices prototype

M. Y. Bashouti , BGU, 26.09.2016

Characterization

Heterojunction Based Hybrid Si Nanowire Solar Cell

Table of Contents

IntroductionImportant Features of NanowiresObstaclesMolecules

First Chapter: Growth of Silicon NanowireTop Down

Second Chapter: Surface Treatment & CharacterizationTermination with Molecules: Grignard ReactionSurface Transfer Doping

Third Chapter: DevicesSolar Cell


Introduction

Introduction

Important features of NanowiresObstaclesMolecules


Integration & Devices

Important features of Nanowires

They represent the smallest efficient transport

nm

µm

A Nanowire

Orthogonal Junction

Increased surface scattering

Increased surface area

Reflection ~<10%

λ<1000nm

na

no

wir

e


Obstacles on the Road

Obstacles for nanowire growth and surface modifications:

Native oxides

Gold can behave as an catalyst. However:(i) has a high diffusion coefficient 10-6 cm-2sec (10-9-10-14) at 1100°C (ii) Induces deep states inside the band gap

Gold diffusion

SiO2

Lowest surface densities ever reported with thermal oxides! ~1010

cm-2 (Si surface density ~1014 cm-2). However:

Amorphous oxides: (i) Dangling bonds

(ii) Induces trap states at the SiO2/Si interface

(iii) Limits the effect of gate voltage

(iv) No direct chemistry with Si

100

100

100

100

N. Lewis at. el. JACS, (2006): 128, 8990-8991.

H. Haick, at. el. ACS Appl. Mater. Interf. (2012): 4, 4251.


Obstacles on the Road

Obstacles for nanowire growth and surface modifications:

SiO2

Native oxides

Gold diffusion Ec

Ev0.33ev

0.54ev

Au-

Au+

Eg

Lowest surface densities ever reported with thermal oxides! ~1010

cm-2 (Si surface density ~1014 cm-2). However:

Amorphous oxides: (i) Dangling bonds

(ii) Induces trap states at the SiO2/Si interface

(iii) Limits the effect of gate voltage

(iv) No direct chemistry with Si

100

100

100

100

N. Lewis at. el. JACS, (2006): 128, 8990-8991


Advantages of using Molecules

A promising approach to overcome the aforementionedobstacles is to use molecules. Main advantages are asfollows:

http://www.dailygalaxy.com/

Positive dipoles

+

- +

-

Si

Negative dipoles

Stabilizing the surface (superior oxidation resistance)Molecular surface transfer doping: Controlling the doping level and type (p or n) through organic molecules.

Controlling the molecular density

Controlling cross-linking

Terminating dangling bonds low surface state densitySystematic dipoles (work function design, surface dipoles)


Chapter I

Realizing Silicon Nanowires

Chapter 1

Realizing Si Nanowires (Si NWs)

Top Down (Wet and dry etching)


Realizing Si Nanowire: Top down

Top down

BottomupChemical Vapor Growth

Self Assembly

Etching (dry or wet)

Main Advantages wet chemistryNo need for special equipment's Fabricating Si NWs in a few minutesLarge areas (4 inch)High aspect ratioDiameter of sub-micro

Bashouti, Y. M.; at. el. (2013): Progress in Surface

Science, 88, 39-60. (Invited Review)

Top down approach: Wet etching


Bottom Up: CVD process

Realizing Si Nanowire

Top down

BottomupChemical Vapor Growth

Self Assembly

Etching (dry or wet)Top down approach: Wet etching(two step process)

“Si NW”

Si Wafer(100)

Si Wafer(100)

Solution 1 (masking)AgNO3/HF

Step 1(5-60)s

Solution 2 (etching)

H2O2/HFStep 2

(0.5-60)min

Si Wafer (100)

HNO3

Ag layer

Si Wafer (100)

Main AdvantagesNo need for special equipment's Fabricating Si NWs in a few minutesLarge areas (4 inch)High aspect ratio


Top down approach(wet etching) Cont.

Top down approach: Wet etching(two step process: step II – Etching time)

0 20 40 60 80 100 120

0

10

20

30

40

R6

00

nm

(%

)Etching Time (min)

Lowest R<2%

0 20 40 60 80 100 120

0

5

10

15

20

25

30

35

Si N

W a

rray

s le

ngt

h (

m)

Time (min)

Length Vs Time

0 10 20 30 40 50

30

40

50

60

70

Po

rosit

y (

a.u

.)Time (min)

Porosity Vs Time

4 inch Si wafer

300 400 500 600 700 800 900 1000 11000

10

20

30

40

50

60

45 min15 min5 min

SiNW for 2 min

Polished

R

nm)



Top down approach: Wet etching(the mechanism)

(i) 15sec

(ii) 30sec

4Ag+ 4Ag(s) + 4h+

Cathode Reaction:

Anode Reaction:

Si(s) + 2H2O + 4h+ SiO2 + 4H+

H2SiF6 + 2H2O

6HF

Total Reaction

Si(s) + 4Ag+ + 6HF 4Ag(s) + H2SiF6 + 4H+

Nano local electrochemical reaction



Top down approach: Wet etching(the mechanism)

(i)

(ii)

(iii)

4Ag+ 4Ag(s) + 4h+

Cathode Reaction:

Anode Reaction:

Si(s) + 2H2O + 4h+ SiO2 + 4H+

H2SiF6 + 2H2O

6HF

Total Reaction

Si(s) + 4Ag+ + 6HF 4Ag(s) + H2SiF6 + 4H+

Nano local electrochemical reaction


Surface Treatment & Characterization

Chapter 2

Surface Treatment & Characterization


Surface Modification

Nan

op

arti

cles

Direct Laser Writing

Molecules

Phase Transfer(From Cubic to Wurtzite)

Molecules on Surfaces(Grignard Reaction, Electrografting)

Phase Transfer

Bashouti, Y. M. at el. (2015): Lab-on-Chip, 15, 1742-1747. DOI: 10.1039/c4lc01376j

Bashouti, Y. M. at el. (2016): Sci. Rep. Accepted

Si NW


Molecules on Surfaces: Grignard Reaction

Molecules on Surfaces: Grignard Reaction

1) Characterization? 2) Replacing oxide shell?3) Si-C bonds?4) Maximum Coverage?5) Stability? 6) Oxidation Mechanism

Two step chlorination/alkylation process

Etching : Removes the native oxide layer (makes H-Si)

Chlorination: Replace the H by Cl

Alkylation: Terminate the Si-Cl to Si-R

Two Steps Reaction

M. Y. Bashouti, at. el. (2008): JPCC, 112, 19168-19172.

M. Y. Bashouti, at. el. (2009): JPCC, 113, 14823-14828.


Alkylation

Si2p & C1s of Terminated Si NW

5 n m2 0 n m 2 0 n m

5 n m

[112]

SiO2

SiO2

Bicrystalline N

SiO2

M. Y. Bashouti, at el. (2012): Phys. Chem.

Chem. Phys., 14, 11877-11881.

Before Alkylation Si2p

No oxides

108 104 100 96

No

rma

lize

d C

ou

nts

(a

.u.)

Binding Energy (eV)M. Y. Bashouti , BGU, 26.09.2016

Si2p & C1s of Terminated Si NW


290 288 286 284 282 280

No

rma

lize

d C

ou

nts

(a

.u.)

Binding Energy (eV)

Alkylation

H-C-HH

Si NW

C

Si

C1s

C-S

i

C-C

or

C-H

C-O

Impact on Density of State (DOS)

In Suite Measurements

cs = Φ-Eg+ (EF-EV)

d=cs - cB

Band diagram of CH3-SiNW, H-SiNW and SiO2-SiNW surfaces. All the numbers are in eV unit.

We followed the DOS by using the following:

1. Emission of Si2p2. Photo-electron Yield (PYS)3. Work-Function

M. Y. Bashouti at. .el. (2014): Prog. Photovolt: Res. Appl. 2, 1050-1061.

Si2p EF-EVBM F

SiO2 99.77 0.83 4.32

H 99.70 0.98 4.26

CH3 99.55 1.05 4.22


d=100nm

hλ=(2-6eV)

Si NW

C H

H

H

Si

e-Si N

W

λ =10nm

Evac

Conduction band

Valance band

EV

EC

e- e- e-

h+h+

h+

Photoemission of Hybrid Si NW

goc= occupied density of statesEVL = Vacuum Levelħw = optical excitation


Third Chapter

Chapter 3: Devices

Solar Cells


Publications:1. M. Bashouti, et al. (2009): Small, 88:39-60. 5:2761-2769

2. M. Bashouti, et al. (2015): J. Phys. Chem. Lett., 6:3988–3993.

3. M. Bashouti, et al. (2015): ACS Appl. Mater. Interfaces, 7: 21657−21661.4. M. Bashouti, et al. (2016): Scientific Reports. (Accepted).5. M. Bashouti, et al. (2016): J. Mat. Chem. (Under revision).

devices: FET Surface modifications

Devices: FETs, Solar Cells, Diodes &

Transparent electrodes

Projects: Brief Summary


Hybrid Solar Cells: PEDOT:PSS/ Si NW

320nm

M. Bashouti, et al. (2014): Prog. Photovolt: Res. Appl. 2, 1050.

M. Bashouti, et al. (2014): JPCC, 117, 9049−9055.











M. Bashouti, et al. (2015): ACS Appl. Mater. Interfaces, 7: 21657−21661.

M. Bashouti, et al. (2016): Langmuir, (Under revision).

Transparent electrodes


Summary

10µm

Wet etching Termination


Thank you for your attention!


Documents

Heterojunction Based Hybrid Si Nanowire Solar Cell Assets/Pages/20th...Obstacles on the Road Obstacles for nanowire growth and surface modifications: SiO2 Native oxides Gold diffusion