DEVELOPMENT OF NON-TOXIC AND LATTICE MATCHED BUFFER LAYER FOR CZTS SOLAR CELLS

Presentation by: Faiazul Haque (P67226)

Supervisor: Prof. Dr. Nowshad AminCo-Supervisor: Prof. Dato’ Dr Kamaruzzaman

Sopian

M.Sc VIVA Presentation

Solar Energy Research InstituteThe National University of Malaysia

(@Universiti Kebangsaan Malaysia, UKM)

Outline Introduction

- Energy, Environment & Solar Cells

- Classification of solar cells

- CZTS Schematic & Buffer Layer Literature Review Problem Statement Objectives Methodology

- Numerical Analysis by AMPS-1D

- Practical Fabrication Results and Discussions

- Part 1: Numerical simulation by AMPS 1D

- Part 2: ZnS Thin Films Characterization

- Part 3: n-ZnS/p-CZTS Full Device Results Conclusion2

3

• Carbon dioxide emission• Global warming

• Energy demand is ever increasing• Main energy production is by fossil fuels

Energy versus Environment

Renewable Energy SourcesSolar

A solar cell is a semiconductor device designed to convert sunlight into electricity. The conversion of light into electricity in a solar cell is called the photovoltaic (PV) effect. Photovoltaic stands for photo, meaning “light”, and voltaic, meaning “electricity”.

INTRODUCTION: ENERGY, ENVIRONMENT & SOLAR CELLS

Wind

Geothermal

Biomass

Wave

Among all the renewable energy resources, solar energy is considered the most consistent and abundant renewable energy source.

PV System → Solar Cell

Solar CellsSolar Cells

First Generation:(Mainly Si, 200 - 600

µm thick)

First Generation:(Mainly Si, 200 - 600

µm thick)

Third Generation:Organic / Hybrid Solar Cells (Dye Sensitized Solar Cell, Quantum

dot, Tandem, Perovskite etc.)

Third Generation:Organic / Hybrid Solar Cells (Dye Sensitized Solar Cell, Quantum

dot, Tandem, Perovskite etc.)

Classification of Solar Cells

Second Generation:Thin Film Solar Cells: 1.5 - 5 µm thick (a-Si,

CdTe, CIS, CIGS, CZTS etc)

Second Generation:Thin Film Solar Cells: 1.5 - 5 µm thick (a-Si,

CdTe, CIS, CIGS, CZTS etc)

CZTS → Cu2ZnSnS4

Group I2-II-IV-VI4 p-type quarternarycompound semiconductor. Optimal direct bandgap energy (1.4 ev - 1.5 eV) High absorption coefficient. (α > 1 x 104 cm-1) Has similar material properties like CIGS ( CuInxGa1-xS(Se)2 ) Abundant in nature (Cu=50 ppm, Zn=75 ppm, Sn=2.2 ppm, S=260 ppm compared to In=0.049 ppm and Se=0.05 ppm) Non toxic and Inexpensive

CZTS Schematic & Buffer Layer

Substrate Configuration

Attributes of a good buffer layer

1. Good lattice matching property with absorber layer – enables the formation of interface with low defect density.

2. High transparency and wide bandgap material: to allow photons to pass through to the absorber layer.

3. Optimum thickness (around 50 nm) to avoid short circuit effects.

4. High conductivity to minimize carrier loss.

5. Free from toxic elements.

Roles of the buffer layer

• Serves as a heterojunction partner for p-CZTS layer. Serves as a protection layer during TCO sputtering. Insulates pin holes to avoid shunting.

Literature Review:Timeline of CZTS Development

6

1967

1967: CZTS single crystal was synthesized and

analyzed for the first time

1988 1996 1997 1999 2006 2008 2010 2013

1988: K. Ito and T. Nakazawa confirmed suitability of CZTS by Atomic Beam Sputtering

technique

1996: Katagiri et al. first time fabricated CZTS on SLG by E-B evaporation technique followed

by sulfurization η = 0.66% [SLG/Mo/CZTS/CdS/AZO]1997: Friedlmeier et al.

achieved η = 2.3% by using the same structure & technique of

Katagiri’s group

1999: Katagiri et al. used Mo coated SLG &

broke the previous record with η = 2.63%

2006: co-sputtering technique was introduced

by K. Jimbo et al and achieved η= 5.74%

2008: Katagiri et al. reported η= 6.77%

efficiency by RF co-sputtering followed by

sulfurization

2010: IBM reported η=9.6% with CZTSSe by using the wet process/non vacuum

process

2013: Todorov et al. reported η=11.1% with

CZTSSe using spin coating technique.

So far a conversion efficiency of 12.6% has

been achieved using a CZTSSe absorber

layer by hydrazine chemical process.In December 2014, CZTS

solar cell efficiency reached

8.8% by Toyota Centre R&D

using co-sputtering

technique

Still there are challenges!!!

Literature Review

CdS: Conventional buffer layer.

Efficiency achieved: more than 12.6% (on CZTS absorber)

But has the following disadvantages: Presence of toxic element

Cadmium. Bandgap ~ 2.45 eV. Limits the blue response of the

photovoltaic cells. CBD (Chemical Bath Deposition)

is not desired in commercialization stage, as it requires tedious and costly waste disposal plan and treatment.

ZnS: Prospective candidate

as an alternative buffer layer.

Efficiency achieved: 18.6% (on CIGS absorber)

Advantages: Non toxic layer. Bandgap ~ 3.1 eV – 3.7 eV

(Higher than CdS). Enhances the blue response of

the photovoltaic cells. Relatively cheaper and

abundant. Deposition by sputtering is

suitable for in-line vacuum large scale commercialized production.

Possible compound semiconductors as buffer layer are: CdS, ZnS, ZnSe, In2S3, CdSe, ZnxCd1-xS etc.

7

Problem Statement:

The presence of toxic Cadmium (Cd) in the buffer layer will raise questions regarding the environmental risk.

This technology will face a marketing problem due to legal regulations of Cd usage in electrical and electronic devices in different countries.

Due to the relatively smaller bandgap of CdS, there is considerable amount of photon loss.

CdS is commonly deposited by Chemical Bath Deposition (CBD) technique, which is tedious and not desired for commercialization stage.

If the CdS can be replaced by another material having higher bandgap, the absorption loss can also be minimized and hence can contribute to an improved cell efficiency.8

Objectives

9

SLG Substrate

Metal Back Contact (Mo)

p type absorber layer(CZTS)

n type buffer layer(CdS)

TCO Window Layer(ZnO:Al)

Grid

n type buffer layer(ZnS)

Conventional Buffer

Alternative Buffer

9

Main Objective:To assess the

performance of ZnS as a non-

toxic and lattice matched buffer layer for CZTS

solar cells

Objective 1:

To analyze the performance of n-ZnS and p-CZTS

thin film solar cells by AMPS-1D, as a feasibility study or

theoretical validation.

Objective 2:

To fabricate, characterize and optimize ZnS thin films deposited by RF magnetron sputtering

technique and study the effects of different

annealing temperatures.Objective 3:

To fabricate complete basic n-ZnS and p-CZTS

solar cell and analyze the

performance.

Methodology:Numerical Analysis by AMPS-1D

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1. AMPS-1D is an abbreviation for “Analysis of Microelectronic and Photonic Structures,”.

2. In general AMPS 1D is a free software and widely used for solar cell and detector design, material parameter sensitivity studies, and parameter extraction.

3. The program has been successfully used by 70 groups to study detector and solar cells.

4. It has several features which make it especially attractive: It can analyze the transport in a variety of crystalline, polycrystalline,

or amorphous solar-cell materials, and device structures including homojunction, heterojunction, or multijunction solar cells and detectors.

It also has a very flexible plotting program, so, user can generate output plots such as:

1. J-V curves, 2. Spectral response, 3. Band diagrams, 4. Carrier concentrations and 5. Currents, and recombination profiles under various

bias conditions.

Parameters n-ZnO:Al

n-ZnS p-CZTS

Thickness, W (μm) 0.2 0.03-0.08

1-4

Dielectric ratio, ε/εo 9.0 10 10Electron mobility, μn (cm2/Vs)

100 100 100

Hole mobility, μp (cm2/Vs) 25 25 25

Carrier concentration, n/p (cm-3)

1018 9.0×1017 2×1015

Bandgap, Eg (eV) 3.30 3.10-3.40

1.40

CB Density of states, NC (cm-3)

2.2×1018 1.5×1018 2.2×1018

VB Density of states, NV (cm-3)

1.8×1019 1.8×1018 1.8×1019

Electron affinity, χ (eV) 4.60 4.50 4.10

The main aim of numerical simulation of solar cells is to validate its performance with respect to

various input parameters and to observe the trend

Methodology:Numerical Analysis by AMPS-1D

Numerical Simulation by AMPS 1D

Variation of CZTS absorber thickness

Variation of ZnS buffer thickness

Variation of ZnS buffer bandgap

Variation of operating temp.

Analysis & Optimization of the simulated parameters

To accomplish objective 1

Methodology: Practical Fabrication

13

Sputter ZnS on SLG substrates

Variation of growth temp. (RT – 400˚C)

Anneal each films at (300˚C, 400˚C & 500˚C)

Anneal each films at (300˚C, 400˚C & 500˚C)

Characterization of the as grown and annealed samples

Optimization of the deposition and annealing conditions

Full device fabrication with optimized buffer layer parameters

Variation of RF power (40W – 80W)

To accomplish objective 3

To accomplish objective 2

Methodology: Practical Fabrication

Results and Discussion:

14

Part 1• Numerical Simulation to Validate ZnS as

Alternative Buffer

Part 2

• ZnS Thin Films Deposition & Characterization• Effect of Growth Temp.• Effect of RF Power

Part 3• n-ZnS/p-CZTS Complete Solar Cell

Fabrication & Characterization

Results & Discussion:Numerical Simulation

15

Ultra thin absorber layer is

sufficient to absorb the solar

spectrum.

Absorber layer thickness variation

Variation of operating temp.

Additional thermal energy

gained by the electrons

causes to recombine before

reaching the depletion region.

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Results & Discussion:Numerical Simulation

Buffer layer thickness variation

Thinner buffer layer allows

the majority of the photons

to be transmitted towards

the absorber layer.

Buffer layer bandgap variation

Bandgap engineering can be

employed to tune the

bandgap according to the

subsequent TCO layer.

Optimized bandgap:

3.10 – 3.25 eV

Obtained final J-V curve considering the optimized parameters

17

Part 1• Numerical Simulation to Validate ZnS as

Alternative Buffer

Part 2

• ZnS Thin Films Deposition & Characterization• Effect of Growth Temp.• Effect of RF Power

Part 3• n-ZnS/p-CZTS Complete Solar Cell

Fabrication & Characterization

Objective 1:Accomplished

Results and Discussion:

As grown (RT – 400˚C)

Results & Discussion: XRD

Grown at RT + AnnealedGrown at 100˚C + AnnealedGrown at 200˚C + AnnealedGrown at 300˚C + AnnealedGrown at 400˚C + Annealed

Growth temperature plays a significant

role in the crystallinity of the ZnS films

@ various growth temp. + subsequently annealed

19

Results & Discussion: XRD

As Grown (40 W – 80 W)Grown at 60 W + AnnealedGrown at 70 W + AnnealedGrown at 80 W + Annealed

Highest RF power produced

better crystalline films.

@ different RF Powers + subsequently annealed

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Results & Discussion: UV-Vis

Average transmittance of over 80%

confirming excellent photovoltaic

properties for buffer layer.

Overall bandgap increased from

3.1 eV to 3.8 eV with the increase

of substrate temp.

TransmittanceBandgap

@ various growth temp.

After annealing, very small change was observed in the bandgap.

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Results & Discussion: UV-Vis

Average transmittance of over 80%

recorded.The bandgap remained in the

range of 3.20 eV to 3.45 eV.

TransmittanceBandgap

@ different RF Powers

After annealing, very negligible amount of change was observed in the bandgap.

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T=100 T=200 T=300

T=400

T=RT

Fig: Roughness trend w.r.t. substrate temperature.

Results & Discussion: AFM

Roughness values remained in the range of

4 nm to 14 nm which is acceptable for buffer

layer applications.

@ various growth temp.

23Fig: Roughness trend w.r.t. RF Power.

RF40 RF50 RF60 RF70

RF80

Roughness values remained in the range of

4 nm to 25 nm which is acceptable for buffer

layer applications.

Results & Discussion: AFM@ different RF Powers

24

10

0˚C

30

0˚C

40

0˚C

Subsequently annealedat 500˚C

Subsequently annealedat 500˚C

Subsequently annealedat 500˚C

The grain size of the annealed films are

found to be bigger along with a compact &

fine grained morphology.

Results & Discussion: SEM@ various growth temp. + subsequently annealed

25

60

W7

0 W

80

W

Subsequently annealedat 500˚C

Subsequently annealedat 500˚C

Subsequently annealedat 500˚C

Results & Discussion: SEM

With the increasing RF power, the grains

became more closely packed and possessed

spherical shapes compared to the films

grown at lower RF power.

@ different RF Powers + subsequently annealed

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1. Conductivity Type: n type

2. Bulk Carrier Density: 1012 cm-3

3. Surface Carrier Density: 107 cm-2

4. Highest Carrier Concentration: 400˚C

5. Lowest Resistivity: 300˚C

Fig. Carrier Concentration and Resistivity w.r.t substrate temp.

Results & Discussion: HALL Measurements@ various growth temp.

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1. Conductivity Type: n type

2. Bulk Carrier Density: 1012 cm-3

3. Surface Carrier Density: 107 cm-2

Fig. Carrier Concentration and Resistivity w.r.t RF Power.

Results & Discussion: HALL Measurements@ different RF Powers

Results and Discussion:

28

Part 1• Numerical Simulation to validate ZnS as

alternative buffer

Part 2

• ZnS Thin Films Deposition & Characterization• Effect of Growth Temp.• Effect of RF Power

Part 3• n-ZnS/p-CZTS Complete Solar Cell

Fabrication & Characterization

Objective 1:Accomplished

Objective 2:Accomplished

Results and Discussion: Full Working Device Deposition

SLG Substrate

Metal Back Contact (Mo)

p type absorber layer(CZTS)

TCO Window Layer(ZnO:Al)

FC

n type buffer layer(CdS)

Cell Voc (Volts)

Jsc (mA/c

m2)

FF η (%)

Cell 1 0.075 22.092 0.278 0.46 %

Cell 2 0.4 5.875 0.341 0.71 %

CZTS/CdS

Cell ZnS deposition parameters

Annealing parameters

Growth Temp.

(˚C)

RF (W)

Temp. (˚C)

Time (min)

Cell 1 100 80 N.A. N.A.Cell 2 300 80 N.A. N.A.Cell 3 RT 80 300 20

SLG Substrate

Metal Back Contact (Mo)

p type absorber layer(CZTS)

n type buffer layer(ZnS)

TCO Window Layer(ZnO:Al)

FC

CZTS/ZnSCell Voc

(Volts)Jsc

(mA/cm2)

FF η (%)

Cell 1 0.51 7.44 0.43 1.63 %

Cell 2 0.57 6.69 0.45 1.72 %

Cell 3 0.4 5.88 0.27 0.64 %

The J-V curves clearly show the formation of a p-n junction any solar cell should have and it is only a matter of optimization of many layers or deposition parameters to improve the curve as well as the efficiency.

Final Optimized Parameters:

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Deposition Parameters ConditionTarget ZnS (99.99% pure)Substrate TemperatureRF powerBase PressureWorking Pressure

300°C80 Watt

6.2 × 10-5 Torr4.0 × 10-2 Torr

Sputtering GasAnnealing TimeAnnealing Temperature

Pure argon (5 SCCM)20 minutes

300°C

Results and Discussion:

31

Part 1• Numerical Simulation to validate ZnS as

alternative buffer

Part 2

• ZnS Thin Films Deposition & Characterization• Effect of Growth Temp.• Effect of RF Power

Part 3• n-ZnS/p-CZTS Complete Solar Cell

Fabrication & Characterization

Objective 1:Accomplished

Objective 2:Accomplished

Objective 3:Accomplished

CONCLUSION

Objective 1 • η = 14.49% (Voc=0.81 V, Jsc=28.85 mA/cm2, FF=0.67) is achievable with ZnS/CZTS solar cell

Objective 2

• Higher growth temperature yielded better quality films• Optimized growth temperature: 300 °C• Optimized RF Power: 80 Watt• Post deposition annealing temperature has significant effect

on the structural, optical and electrical properties • Optimized annealing temperature: 300 °C.

For Al/ZnO:Al/ZnS/CZTS/Mo solar cell• The highest conversion efficiency =1.72 % • Voc = 0.57 V• Jsc = 6.69 mA/cm2

• Fill factor = 0.45

Objective 3

This study has investigated the potentials of ZnS as an alternative non-toxic and lattice matched buffer layer that opened up the usage

for CZTS solar cells as a result of theoretical and practical investigation, which can play a crucial role in the commercialization

process.

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PUBLICATIONSJournals:

[1] F. Haque, K.S. Rahman, M.A. Islam, M.J. Rashid, M. Akhtaruzzaman, M.M. Alam, Z.A. Alothman, K. Sopian & N. Amin. 2014. Growth Optimization of ZnS Thin Films by RF Magnetron Sputtering as Prospective buffer Layer in Thin Film Solar Cells. Chalcogenide Letters 11(4): 189-197.[ISI: Q3: IF=1.184]

[2] P. Chelvanathan, Y. Yusoff, F. Haque, M. Akhtaruzzaman, M.M. Alam, M.J. Rashid, K. Sopian & N. Amin. 2014. Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application. Applied Surface Science. In Press, Corrected Proof. (Available online: 11 Oct 2014). doi:10.1016/j.apsusc.2014.08.155 [ISI: Q1: IF=2.538]

[3] M.A. Islam, K.S. Rahman, F. Haque, M. Akhtaruzzaman, M.M. Alam, Z.A. Alothman, K. Sopian & N. Amin. 2014. Properties of Low Temperature Vacuum Annealed CZTS Thin Films Deposited on Polymer Substrate. Chalcogenide Letters 11(5): 233-239. [ISI: Q3: IF=1.184]

[4] F. Haque, K.S. Rahman, N.A. Khan, M.A. Islam & N. Amin. 2014. Analysis of the Structural and Optical Properties of Thermally Evaporated Zinc Sulphide (ZnS) Thin Films for Photovoltaic Application. Australian Journal of Basic and Applied Sciences 8(19): 264-267. [SCOPUS]

International Conferences:

[1] F. Haque, N.A. Khan, K.S. Rahman, M.A. Islam, M.M. Alam, K. Sopian & N. Amin. 2014. Prospects of Zinc Sulphide as an Alternative Buffer Layer for CZTS Solar Cells from Numerical Analysis. 8th International Conference on Electrical and Computer Engineering (ICECE 2014) Dhaka, Bangladesh. In IEEE Xplore Digital Library. Page(s): 504-507. DOI: 10.1109/ICECE.2014.7026855

[2] F. Haque, K.S. Rahman, M.A. Islam, P. Chelvanathan, T.H. Chowdhury, M.M. Alam & N. Amin. 2013. A Comparative Study of ZnS Thin Films Grown by Thermal Evaporation and Sputtering. IEEE Student Conference on Research and Development (SCOReD 2013), Putrajaya, Malaysia. In IEEE Xplore Digital Library. Page(s): 260-264. DOI: 10.1109/SCOReD.2013.7002584 

PUBLICATIONS

[3] F. Haque, K.S. Rahman, N.A. Khan, M.A. Islam, M. J. Rashid, M. Salim, M. Akhtaruzzaman, M.M. Alam & N. Amin. 2014. A Comprehensive Study on Undoped and In-doped ZnS Thin Films Prepared by Co-sputtering Technique. Poster Presentation in International Conference on Electronic Materials and Nanotechnology for Green Environment (ENGE 2014), Jeju, Korea.

[4] K.S. Rahman, F. Haque, N.A. Khan, M.A. Islam, M.M. Alam, Z.A. ALOthman, K. Sopian & N. Amin. 2014. Influence of Thermal Annealing on CdTe Thin Film Deposited by Thermal Evaporation Technique. 3rd International Conference on the Developments in Renewable Energy Technology (ICDRET'14), Dhaka, Bangladesh. In IEEE Xplore Digital Library. Page(s): 1-4. DOI: 10.1109/ICDRET.2014.6861722

[5] K.S. Rahman, F. Haque, M.A. Islam, M.M. Alam, Z.A. Alothman & N. Amin. 2013. Effect of Growth Techniques on the Properties of CdTe Thin Films for Photovoltaic Application. IEEE Student Conference on Research and Development (SCOReD 2013), Putrajaya, Malaysia. In IEEE Xplore Digital Library. Page(s): 265-268. DOI: 10.1109/SCOReD.2013.7002585

[6] K.S. Rahman, F. Haque, N.A. Khan, M.A. Islam, M.M. Alam, M. Akhtaruzzaman, K. Sopian & N. Amin. 2014. Properties of Cu Incorporated CdTe Thin Films for Photovoltaic Application. Poster Presentation in International Conference on Electronic Materials and Nanotechnology for Green Environment (ENGE 2014), Jeju, Korea.

[7] N.A. Khan, K.S. Rahman, F. Haque, N. Dhar, M.A. Islam, M. Akhtaruzzaman, K. Sopian & N. Amin. 2014. Design Optimization of CdTe Thin Film Solar Cells from Numerical Analysis. 8th International Conference on Electrical and Computer Engineering (ICECE 2014) Dhaka, Bangladesh. In IEEE Xplore Digital Library. Page(s): 508-511. DOI: 10.1109/ICECE.2014.7026862 

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REFERENCES1. Siebentritt, S., 2004. Alternative bu ers for chalcopyrite solar cells. ff Sol. Energy 77: 767.

2. Fella, C.M., Stuckelberger, J.A., Uhl, A.R., Romanyuk, Y.E. and Tiwari, A.N. 2013. Influence of annealing conditions on non-vacuum deposited Cu2ZnSn(S,Se)4 absorber layers for thin film solar cells. Poster Presented at MRS Spring Meeting, San Francisco.

3. Wadia, C., Alivisatos, A.P. and Kammen, D.M. 2009. Materials availability expands the opportunity for large-scale photovoltaics deployment. Environ. Sci. Technol. 43 (6): 2072-2077.

4. Haque, F., Rahman, K.S., Islam, M.A., Rashid, M.J., Akhtaruzzaman, M., Alam, M.M., Alothman, Z.A., Sopian, K. and Amin, N. 2014. Growth Optimization of ZnS thin films by RF magnetron sputtering as prospective buffer layer in thin film solar cells. Chalcogenide Letters 11: 189-197.

5. Katagiri, H., Saitoh, K., Washio, T., Shinohara, H., Kurumadani, T. and Miyajima, S. 2001. Development of thin film solar cell based on Cu2ZnSnS4 thin films. Solar Energy Materials Solar Cells. 65: 141-148.

6. Katagiri, H., Jimbo, K., Yamada, S., Kamimura, T., Maw, W.S., Fukano, T., Ito, T. and Motohiro, T. 2008. Enhanced Conversion Efficiencies of Cu2ZnSnS4-Based Thin Film Solar Cells by Using Preferential Etching Technique. Appl. Phys. Express. 1 (4).

7. Jimbo, K., Kimura, R., Kamimura, T., Yamada, S., Maw, W.S., Araki, H., Oishi, K. and Katagiri, H. 2007. Cu2ZnSnS4-type thin film solar cells using abundant materials. Thin Solid Films. 515: 5997-5999.

8. Ito, K. and Nakazawa, T. 1988. Electrical and optical properties of stannite-type quaternary semiconductor thin films. Jpn. J. Appl. Phys. 27: 2094-2097.

9. Martin Green. 1982. Solar cells operating principles, technology, and system applications. New Jersey: Prentice-Hall.

10. Shahi, A., Chelvanathan, P., Hossain, M.I., Zaman, M., Sopian, K. and Amin, N. 2012. Performance Analysis of Cu2ZnSnS4 (CZTS) Solar Cell By Solar Cell Capacitance Simulator (SCAPS). Proceedings of The 22nd International Photovoltaic Science and Engineering Conference.

11. Sze, S.M. 2002. Semiconductor devices, physics and technology. 2nd edition. New Jersey: John Wiley & Sons, Inc.

12. Hwang, D.H., Ahn, J.H., Hui, N., Hui, K.S. and Son, Y.G. 2012. Structural and optical properties of ZnS thin films deposited by RF magnetron sputtering. Nanoscale Res Lett. 7: 26.

13. Intechopen URL http://www.intechopen.com/books/solar-cells-research-and-application-perspectives/cu2znsns4-thin-film-solar-cells-present-status-and-future prospects

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Thank You….

37

FUTURE WORK

1. Effect of Operating Pressure, Ar flow rate, etc can be varied to see their effects on the films.

2. Metal doping effects on the properties of ZnS thin films can be studied.

3. Prospects of Hydrogenated and Oxygenated ZnS (O,H) can be studied.

4. Variation on the duration of annealing can be explored.

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AMPS-1D To evaluate device performance with respect to various

input parameters. All the numerical programs used for modeling the thin film

solar cells work by solving the two basic semiconductor equations :

(i) Poisson’s equation for the distributions of (a) electric field (φ) inside the device.

(ii) the equation of continuity for conservation of (b) electrons and (c) holes current.

The physics of device transport can be achieved by solving these three governing equations (indicated by a, b, & c) along with the appropriate boundary conditions.

39

DRAWBACKS OF AMPS-1D AMPS-1D is a free software. It is not perfect as it only uses few basic

equations which doesn’t allow to manipulate many phenomena. Different type of defects cannon be included in the simulation, such

as anti-site, vacancy, mismatch, interface etc. It results in only ideal cases under standard condition most of the

times. May provide unrealistic results. During practical fabrication process, loss mechanism or generation-

recombination happen which is not possible to include in AMPS-1D. It has discreet grid points instead of being continuous. The objective of performing the simulation is to acquire knowledge on the

trend of the output parameters with respect to other parameters of the solar cells. The values of output parameters derived from the simulators are not necessary to believe strictly but the trend is much more important.

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BASIC EQUATIONS