Simulation of CZTS thin film solar cell for different

South Asian Journal of Engineering and Technology Vol.2, No.52 (2016) 1–10

Simulation of CZTS thin film solar cell for different buffer layers for high efficiency performance Rafee Mahbub, Md. Saidul Islam, Farhana Anwar, Sakin Sarwar Satter, Saeed Mahmud Ullah* Department of Electrical and Electronic Engineering, University of Dhaka, Dhaka-1000, Bangladesh

E-mail: [email protected] Received: 13/12/2016, Revised: 02/01/2017 and Accepted: 08/01/2017

Abstract: Cu2ZnSnS4 (CZTS) absorber layer research shows extensive influential factors to replace expensive Copper Indium Gallium Selenide (CIGS) absorber layer due to its high efficiency, low-cost, non-radioactive and environmental friendly behavior. Potential buffer layers for CZTS solar cells like ZnO, ZnS, In 2S3 and ZnSe along with conventional CdS buffer layer are numerically analyzed. Among these structures, ZnS/CZTS structure shows an optimum efficiency of 26.82% (with Voc = 0.724 V, Jsc = 53.312 mA/cm2 and fill factor = 69.44 %).This paper explicitly reveals the most favorable CZTS layer thickness around 2.5 μm, whereas buffer layer thickness lies just below 50 nm. Absorber carrier density has its effect on Voc and Jsc and so on efficiency. With increasing carrier density Jsc decreases while Voc increases. An optimum density of 5×1017 cm-3 to 1×1018 cm-3 shows a great result. The achieved results can lead to the development of higher efficiency CZTS thin film solar cells. (Avoid the words in abstract Ex. Recently in this work research) Keywords: CZTS; CIGS; Carrier density; Defect density; Thin film solar cell; SCAPS-1D Introduction: Thin film solar cells have attracted a great deal of attention for a couple of decades especially the second generation solar cells such as Cu(In,Ga)Se2 (CIGS) and Cadmium Teluride (CdTe). A very good amount of study has made these solar cells commercially available for recent years.1 But these cells are made of rare & toxic elements such as Cadmium and selenium which are toxic at even tiny doses, while tellurium and indium are extremely rare. A reliable alternative material such as Cu 2ZnSnS4 (CZTS) is being extensively studied as a possible replacement absorber material for thin-film solar cells.2 CZTS has none of the toxicity problems of its two thin-film rivals, CdTe (cadmium-telluride) and CIGS (copper-indium-gallium-selenide). CZTS is a quaternary semiconductor and have Kesterite type structure. Its excellent material properties such as direct and tunable band gap energy in the range of 1.4-1.6 eV and large absorption coefficient over 104 cm-1 have made it a very favorable and promising material.3 Several physical and chemical deposition methods have been investigated for the fabrication of CZTS thin film absorber layer including thermal evaporation,4 electron beam evaporation,5 RF/DC magnetron sputtering,6 hybrid sputtering,7 pulsed laser deposition (PLD),8 electrochemical deposition,9 solution-processed,10 nanoparticle-based synthesis,11 spray pyrolysis,12 successive ion layer adsorption and reaction (SILAR). 13 The main driving force for exploring different deposition methods is to develop a suitable absorber preparation technique to realize commercially viable low-cost and high-efficiency CZTS (Se) thin film solar cells.14

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In 2012, Todorov et. al have developed CZTS based solar cell capable of converting 11.1% of solar energy into electricity.15 Wei Wang et.al demonstrated a record cell efficiency of around 12.6% via a hydrazine pure-solution process for Cu2ZnSnS4 based solar cells.16 In order to improve the performance of a photovoltaic solar cell, it is necessary to optimize the absorber/buffer interface involving hetero-junction structure. As heterostructure like CZTS is complex in nature, numerical simulation is an efficient way to predict the effect of changes of different electrical & optical properties on the output performance. A device model for CZTS solar cell is presented and a numerical simulation based on SCAPS-1D is performed to investigate the effect of CZTS layer thickness, carrier density, defect density and thickness of different buffer layers on the cell performance. Numerical observations (Efficiency, Jsc, FF and Voc) for different solar cells have been obtained and among them ZnS as a buffer layer has shown the best performance.

Device structure & simulation: In this work, we have used SCAPS (solar cell capacitance software) as our simulation software. SCAPS is a one dimensional solar cell simulation program developed with Lab Windows/CVI of National Instruments at University of Gent, Belgium. Marc Burgelman, Alex Niemegeers, Koen Decock, Johan Verschraegen, Stefaan Degrave have contributed to the development of this software.18 This software provides a panel to input different physical parameters for individual layers in order to analyze the overall cell performance. In our study we have considered the structures:[CZTS/CdS/iZnO/ZnO:Al],[CZTS/ZnO/iZnO/ZnO:Al],[CZTS/ZnS/iZnO/ZnO:Al],[CZTS/In2S3/i-ZnO/ZnO:Al], [CZTS/ZnSe/i-ZnO/ZnO:Al] for a comparison study of Kesterite based thin film photovoltaic devices(TFPV). Figure 1 shows the structure of the solar cell where CZTS is the p-type absorber layer, CdS, ZnO, ZnS, ZnSe, In 2S3 are the different n-type buffer layers for different cells. On top of that a highly conductive n-type Al-doped ZnO (ZnO:Al) and an intrinsic ZnO (i-ZnO) have been considered as the window layers. A thin layer of MoS2 has also been considered for the simulation between the Mo and absorber layer.

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Figure 1: Device structure of CZTS solar cell

Here, an illumination of 1000 W/m2, temperature of 250C and an ‗air mass of 1.5, global‘ spectrum has been considered for simulation. The device and material parameters used in the simulation are all cited from experimental study, literature values or in some cases reasonable estimation and they are listed in Table 1 and Table 2. 17, 19, 20, 21 Table 1: Material parameters for different layers of solar cell structure Parameter

MoS2

CZTS

CdS

ZnO

ZnS

ZnSe

In2S3

i- ZnO

Zno:Al

Thickness (nm)

100

2500

80

80

80

80

80

100

200

Band gap(eV)

1.7

1.45

2.4

3.3

3.5

2.9

2.8

3.3

3.3

Electron affinity(eV)

4.2

4.5

4.2

4.6

4.5

4.1

4.7

4.4

4.6

Dielectric permittivity

13.6

10

9

9

10

10

13.50

9

9

CB effective density of states (cm-3)

2.2E+18

2.2E+18

2.2E+19

2.2E+19

1.8E+18

1.8E+18

2.2E+17

2.2E+18

2.2E+18

VB effective density of states (cm-3)

1.8E+19

1.8E+19

1.8E+18

1.8E+19

1.8E+19

1.8E+19

1.8E+19

1.8E+19

1.8E+19

Electron thermal velocity (cms-1)

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

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Hole thermal velocity (cms-1)

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

1.0E+7

Electron mobility (cm2/Vs)

100

100

100

100

100

100

100

100

100

Hole mobility(cm2/Vs)

25

25

25

25

25

25

25

25

25

Shallow uniform donor density , ND (cm-3)

0

0

3.0E+16

1.0E+14

5.0E+15

5.0E+18

1.0E+14

1.0E+18

1.0E+18

Shallow uniform acceptor density , NA (cm-3)

1.0E+16

1.0E+17

1.0E+1

1.0E+1

1.0E+1

1.0E+1

1.0E+1

1.0E+18

0

Donor

Acceptor

Acceptor

Acceptor

Acceptor

Acceptor

Acceptor

Acceptor

Defect type

_

Table 2: Device parameters used for the simulation Cell properties

Value

Cell temperature

300K

Series resistance

1.50 Ω cm2

Shunt resistance

6.00x102 Ω cm2

Back metal contact properties Electron work function of Mo

5.0 eV

Surface recombination velocity of electron

1.00x107 cm/s

Surface recombination velocity of hole

1.00x107 cm/s

Front metal contact properties Electron work function

Flat band

Surface recombination velocity of electron

1.00x107 cm/s

Surface recombination velocity of hole

1.00x107 cm/s

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Effect of absorber (CZTS) thickness vs. efficiency: CZTS has a high absorption coefficient (>104 cm-1) and it has been found that it shows better absorption in the blue region than other absorber materials.22 CZTS thickness from 2000 nm to 3000 nm has been considered in this simulation and the result is illustrated in Figure 2. It has been found through the simulation that the thickness of 3000 nm would be sufficient enough for almost complete absorption of AM 1.5G radiation, due to high absorption coefficient and direct bandgap of the material. 19 Both the Jsc and Voc of the solar cell have also increased with the increasing thickness of CZTS layer, thus the efficiency has also increased. This is due to the fact that the thicker absorber layer will absorb more photons with longer wavelength, which will in turn make a contribution to the generation of electron-hole pairs. But the cell efficiency has a much slower increasing rate when the layer thickness is over 2500 nm because that enhances the recombination rate as well. However, through the simulation process it has been found that when the CZTS layer thickness is less than 1000 nm, a linear decrease of Jsc can be found. This happens due to the incomplete absorption of the incident photons. So, this work suggests that the thickness of 2500 nm would be enough to absorb most of the incident photons and also for better efficiency.

Figure 2: Absorber thickness vs. efficiency

Figure 3: Buffer thickness vs. efficiency

Effect of buffer layer thickness vs. efficiency: The role of the buffer layer is very important for the creation of the electric field resulting from the junction in the space charge zone. In this simulation the thickness of buffer layer has been changed from 30 nm to 100 nm and simulation result is illustrated in Figure 3. It can be seen that increasing the buffer layer thickness decreases efficiency. The buffer layer absorbs light in the UV region of the spectrum. If the thickness of buffer layer is increased, a large number of photons are absorbed into the n-type buffer layer before arriving in the p-type absorber layer, which causes the current to be reduced and thus the efficiency. Another factor that causes the reduction of efficiency is that as the buffer layer thickness is increased, the recombination rate gets higher due to the low diffusion length of the minority carriers of the buffer layer. However, for very thin buffer layer (< 10 nm) a reduction of Voc, Jsc and Efficiency is found. Too thin buffer layer may result into leakage current and too thick one could lead to low carrier separation rate. So, the thickness of buffer layer can be considered close to 50nm to get good uniformity.

Effect of absorber carrier density: Even though for the changes of absorber and buffer layer thickness, the Jsc and Voc changes as the same manner as the efficiency, that does not happen for carrier density of absorber layer. For increasing absorber doping, Jsc decreases whereas Voc increases.

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Higher absorber carrier density enhances the recombination process which consequently reduces the chances of collection of photo generated carriers. As a result the value of the Jsc gets low. In this simulation we have considered acceptor density from 1x1015cm-3 to 1x1018 cm-3 and our simulation results match up with these phenomenons and are illustrated in Figure 4. It shows the overall efficiency increases with absorber carrier density but the rate slows down for higher value of density. In CZTS films, the ratio of Cu/Zn determines the carrier density. As Cu- rich Zn-poor films have extremely high value of carrier density, it is desirable to use Zn-rich and Cu-poor films.

Figure 4: (a) Absorber carrier density vs. Jsc (b) Absorber carrier density vs. Voc (c)Absorber carrier density vs. efficiency Effect of absorber defect density: Defect states introduce additional carrier recombination center in the solar cell. Higher defect density enhances recombination process of photo generated carriers which leads to reduction in efficiency. Gaussian defect in the absorber layer has been considered and the defect density value ranges from 1x1010 cm-3 to 1x1017 cm-3 in this simulation. Through the simulation it can be found that defect density up to 1x1012 cm-3 affects the cell performance very little. But above that range the cell performance

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decreases dramatically with increasing defect density. This is mainly because defect increases reverse saturation current density and reduction of Voc and thus the efficiency.

J-V characteristics: Finally, based on the best possible optimization, the J-V characteristics for different solar cells have been simulated and Figure 5 reveals it.

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Figure 5: J-V curves for different buffer layers (a) CdS, (b) ZnO, (c) ZnS, (d) In2S3 & (e) ZnSe

Along with the curves the best output parameters has been obtained and are given below. Table 3: Output parameters for different cells Thin film solar cell

Efficiency (%)

Fill (%)

CZTS/CdS/i-ZnO/Zno:Al

22.63

CZTS/ZnO/i-ZnO/ZnO:Al

Factor

Jsc(mA/cm)

Voc(V)

55.83

51.925

0.780

26.13

67.26

53.597

0.725

CZTS/ZnS/ i-ZnO/ZnO:Al

26.82

69.44

53.312

0.724

CZTS/In2S3/i-ZnO/ZnO:Al

25.47

65.06

54.059

0.724

CZTS/ZnSe/i-ZnO/ZnO:Al

15.90

48.00

48.441

0.684

In order to obtain these results the optimal values considered in this paper are, absorber layer thickness of 2500 nm, buffer layer thickness of 50 nm and absorber carrier density of 5×1017cm3. Simulating all the parameters it has been found that the solar cell with ZnS buffer layer has the highest efficiency (efficiency= 26.82%, FF= 69.44%, Jsc= 53.312 mA/cm2 and Voc= 0.724V) among all other considered buffer layers. Conclusion: The numerical simulation has been done by varying absorber thickness, carries density, defect density and buffer thickness. In order to obtain an improved performance, the optimal values have been considered for J-V characteristics. The solar cell with ZnS as a buffer layer has shown the best performance (efficiency- 26.82%, FF- 69.44%, Jsc - 53.312 mA/cm2 and Voc - 0.724V). The above optimization can lead to develop higher efficiency CZTS thin film solar cells.

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Acknowledgement: The authors acknowledge Dr. Marc Burgelman at the University of Gent. We would like to thank him for providing SCAPS.

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