Structured ZnO Growth Using Electrochemical Deposition for Light Management in Solar Cells

8/2/2019 Structured ZnO Growth Using Electrochemical Deposition for Light Management in Solar Cells

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Structured ZnO growth usingelectrochemical deposition for lightmanagement in Solar cells

Graduate Seminar

Nanomolecular Science Program

Jacobs University Bremen, Germany

Uddin Md. Jalal

March 02, 2012


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Outline

Basic science of Solar Energy conversion

Thin-films in Solar Cell

Overview of conventional light trappingtechnology and their limitations

Proposed research project


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What is Photovoltaics (PV)?

Photovoltaics (PV) is a technology to generate electrical energy

in the form of current and voltage from semiconductor when

they are illuminated by photons.

Why Photovoltaics?

Direct conversion of sunlight to electricity is possible usingSolar cells

Environment-friendly renewable energy source

Guaranteed energy availability


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Basic Process of Photovoltaics

Energy Generation

The basic processes behind the PV

effect are:

Generation of the charge carriersdue to the absorption of photons

in the materials that form a junction

Subsequent separation of the

photo-generated charge carriers

in the junction

Collection of the photo-generated

charge carriers at the terminals of

the junction

Fig.1 Illuminated semiconductor illustratingincoming, reflected and absorbed light in thesemiconductor and light passing through thesemiconductor.


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Fig.3 Boosting electron at higher energyFig.2 Electron-hole pair generation


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Fig.4 Transfer electron to contact terminals Fig.5 Transfer of energy to the external circuit


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Conversion of Solar Energy

Fig.6 Solar spectrum at different airmasses

The solar spectrum shows that forAM1.0, a maximum intensity is at

the wavelength l = 0.5 mm and itfalls to half at l = 1 mm.

AM = 1/cos, is the angle of theposition of the sun with a vertical

line.

Stephen J. Fonash (2010) Solar Cell Devices Physics, Elsevier, USA, 2nd Ed., 1-7


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Fig.7 Cross-section of a typical Solar Cell

The total input power PIN :

The output power POUT :

POUT = JV

Fig.8 J-V Characteristic of Solar Cell underillumination

The best efficiency of the PV energy

conversion process for the cell of Fig. 4:

The fill factor:

cs AAWhen

in

mm

P

VJ

cs

c

s

AAWhenA

A

in

mm

P

VJ

ocsc

mpmp

VI

VIFF

Stephen J. Fonash (2010) Solar Cell Devices Physics, Elsevier, USA, 2nd Ed., 1-7


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Motivation with Thin-film Silicon

Material

Silicon is most commonly used material for photovoltaic device.Because-

Silicon is abundant and easy to obtain from earth crust

Well characterized and use of Si material in thin-film leads to the costminimization

The thickness varies for a few mm to 10 mm with diffusion lengthof 10-20 nm

Best efficiency of single Si is 24.7% with maxm theoretical value of 30%

K. L. Chopra et al. (2004) Prog. Photovolt: Res. Appl. 12, 69


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Thin film Si forms active layer of the thickness of a few mm

Most of the photons dont contribute to the electron-hole pair generation

Requires additional light trapping techniques

Major Challenge with Si Solar cell


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Overview of Light Trapping Technology

Light trapping mechanism is required

To increase the optical path length inside the active layer To improve absorption in the active layer

To prevent light that otherwise would be lost


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Methods to trap light are-

Anti reflection coating Use of back reflector

Surface texturing

Methods to Trap Light Inside Solar Cell


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Materials that has lower refractive index (nARC)

Materials with refractive index of 1.87

Materials thickness of d1= lo/4n1 Prominent ARC materials are ZnO, Si3N4

Fig.9 Simplified schematic of thin film Si solarcell showing anti-reflecting coating

Anti Reflection Coating (ARC):

Albert Polman,Light management in thin- film solar cells, Center for Nanophotonics

FOM-Institute AMOLF, Amsterdam, The Netherlands


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Fig.10 (a) Thin film Si solar cell showing the effect of thicknessof anti-reflecting coating on (b) Comparison of surface reflectionwith and without antireflection coating

(a) (b)

http://pveducation.org/pvcdrom/design/anti-reflection-coatings


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Back Reflector & Surface Texturing:

Fig.11 Cross-section of microcrystalline thin film Si solar device showing (a) Flat backreflector (b) Textured back reflector

Kenji Yamamoto (2003)JSAP International, 7, 12-19


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Fig.12 Efficiency of microcrystalline thin film Si solar device having (a) Flat backreflector (b) Textured back reflector


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Limits of the Conventional Light Trapping

The maximum possible enhancement has an upper limit which is given

by the absorption coefficient of 4n2/ Sin2,

where, n = the refractive index of the active material

= the angle of emission cone in the medium

surrounding the solar cell

But this limit is no applicable in nanophotonic regime using nanostructures


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Different Approaches to Enhance Light

Trapping to Avoid Conventional Limits

Different approaches:

Metal nanostructures

Photonic crystals

Optical interference in multilayer stacks


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Metal Nanostructures to Enhance

Light Trapping

Fig.13 Light trapping by (a) scattering from metallic or dielectric nanostructures at theSurface of the solar cell, (b) scattering of light from a corrugated metal back surfacecouple to surface plasmon polariton

H.A. Atwater and A. Polman (2010),Nature Materials 9, 205


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Proposed ModelStructured ZnO growth using electrochemical

deposition for light management in Solar cell

Why ZnO?

Its a wide band gap (3.3 eV) material with semi conducting properties

The optical and chemical properties as well as the structures of ZnO

rely on its preparation method

Solution chemistry can be suitably used for ZnO growth with low

temperature of less than 100C.


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Methodologyfor Fabrication of Solar Cell

Step 1:Collection of Indium Tin Oxide (ITO) on glass from the commercial source

Step 2:Growth ZnO nanorods on top of ITO using electrochemical deposition

Fig.14 Schematic setup for the ZnO growth experiment


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Fig.15 Proposed design of nanorod-based hybrid solar cell

Proposed Structure


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Working mechanism of Proposed Model

Fig.16 Explanation of working of Proposed model with charge separating diagram


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Working Mechanism of Proposed Model

Top metal electrode should be transparent

Light comes through the top transparent electrode

and falls on the P3HT semiconductor

Electrons from HOMO in P3HT get excited to LUMO

level and then they go to the LUMO level of ZnO

and get collected by the transparent top electrode

Absence of electrons in the HOMO level of P3HT

creates holes, and they are collected at the ITO

Thus charge carriers generate current


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Conclusion

Use of indium-tin oxide (ITO) as the cathode

It has good conducting properties

Use of poly-3-hexylthiophene (P3HT) as active layer

It possesses high charge-carrier mobility

It has good optical absorption properties

Use of P3HT in organic solar cell as donar material provides the

efficiency of about 5%


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Further Research to Continue

To replace highly cost contact materials with lower cost materials such ascopper or carbon-based materials (nanotubes)

Low-cost (i.e. abundant) materials must be incorporated into reliable, highperformance PV modules and systems


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Thanks for your kind attention!!!

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Structured ZnO Growth Using Electrochemical Deposition for Light Management in Solar Cells