Numerical modelling of CZTS by SCAPS-1D
June- October, 2018
Kinjal K. Joshi
Dr. Prashant Ghediya
Department of Physics
In this work, I have simulated Cu2ZnSnS4 solar cell with the help of solar cell capacitance simulation in one dimension (SCAPS-1D). In SCAPS-1D software almost all parameters can be graded, recombination mechanics like Auger, SRH type etc, shows tunnelling effect, generation effect, illumination effect, batch calculation possible, a built-in curve fitting facility As there are so may simulation software like PC-1, AMPS-1D, AFORS-HET, SCAPS-1D, among them SCAPS-1D software is more operative then other software so I used SCAPS-1D. Thin film solar cell based on CZTS absorber layer were simulated with CdS as buffer layer and ZnO as window layer using SCAPS-1D software to study the influence of series resistance, band to band recombination, defects and interfaces, thickness of CZTS absorber layer, CdS buffer layer ad transparent conductive oxide layer ZnO on the photovoltaic cell parameters.
1. Introduction……………………………………………………………………… 04
2. Literature review………………………………………………………………… 05
3. Methodology……………………………………………………………………. 06
4. Result and discussion…………………………………………………………… 09
5. Conclusion……………………………………………………………………… 10
6. Acknowledgement……………………………………………………………… 10
Over recent years, a good efforts have been made to study an earth-abundant, cost effective, non-toxic Copper Zinc Tin Sulphide (Cu2ZnSnS4) thin film solar cells (TFSC) 1-3. Cu2ZnSnS4 is p-type material and have 12.6% stagnate efficiency 1. The SQ limit also known as Schockley-Quisser limit refers to the maximum efficiency of a solar cell using a single p-n junction to collect power from the cell. The limit is that the maximum solar conversion efficiency is around 33.7% for a single p-n junction photovoltaic (PV) cell. Modern commercial mono crystalline solar cell produce about 24% conversion efficiency, the losses due largely to practical concerns like reflection of the front of the cell surface from the thin wires on the cell surface 4-6.
To understand the performance of cell and to improve efficiency of solar cell, systematically investigation of basic factors in the performance of solar cell is necessary 7. The process of solving physical problems by appropriate simplification of reality is called numerical modelling. Now a days it is required to operate PV array at its highest energy conservation output by continuously utilizing the maximum available solar power of the array. The electrical system PV modules are powered by solar arrays requires special design considerations due to varying nature of the solar power generated resulting from unpredictable and sudden changes in weather conditions which change the solar irradiation level as well as the cell operating temperature 8. The need for numerical modelling is relevant as the absorber/buffer interface involving heterojunctions is more complex in nature, for thin film polycrystalline solar cells 9.
For the numerical modelling purpose of thin film solar cell there are several software like PC-1D, AMPS-1D, AFORS-HET, SCAPS-1D 10-20. One of them is SCAPS-1D (Solar cell capacitance simulator), which is one dimensional solar cell simulation programme developed at the solar cell simulation programme Department of Electronics and Information Systems (ELIS) of the University of Gent, Belgium by Marc Burgelman et al. 17. The original programme was developed for cell structures of CuInSe2 ad CdTe families. Recent developments make the programme now also applicable to crystalline solar cells ad amorphous cells 21.
AMPS-1D is able to handle ay defects, doping energy gap, special distribution, Boltzmann and Fermi Dirac statistics, having material properties. While SCAPS-1D give data analysis for I-V, C-V, C-f characteristics of solar cell. It have well developed user interface , convenient scripting facilities and the package evolved over the years to include additional mechanics eg., Auger recombination, tunnelling, multiple enhancement to user interface.
SCAPS-1D software is developed to measure diverse properties of different materials like CIGS, CdTe, CuInSe2, Cu(InGa)Se2, CZTS, CZTSSe, etc by numerical simulation14-20,22,23. Among all the new emerging materials CZTS is p-type semiconductor having direct ad gap of about 1.5 eV ad absorption coefficient of above 104 cm-1 in the solar spectrum. Tale 1.1 shows the comparison between dissimilar materials 24-28.
Table 1.1 Comparison of dissimilar materials
Properties of absorber CZTS CIGS CdTe
Band gap 1.4-1.5 eV 1.48 eV 1.5 eV
Absorption coefficient >104cm-1105cm-1>103cm-1Conduction type p-type p-type p-type
Mobility High Low High
Diffusion length of minority carriers – 10?m1.75?mAvailability Earth abundant Earth abundant Earth abundant
Toxicity Non toxic Non toxic Non toxic
2. Literature review
In this report I reviewed that by changing different parameters of the material of thin film solar cell or by changing properties of material the efficiency of solar cell is affected. The influence of series resistance, band to band recombination, defects and interfaces, thickness of CZTS/CZTSe/CZTSSe absorber layer, CdS buffer layer and transparent conductive oxide layer ZnO on the photovoltaic cell parameters were studied in detail and the efficiency is improved as 12.03%, 13.16% and 15.77% respectively by changing the back contact metal work function was studied O.K. Simya et al. 14. P. Lin et al. studied that y increasing the temperature, the optimal photovoltaic property has been achieved with an efficiency of 19.28% 15. M. A. Olopade et al. investigate that by changing buffer layer of CdS with ZnS or ZnSe, the efficiency they get is nearer to the efficiency of CdS based solar cell, which is 6.52% and 6.76% respectively 17. M. Patel et al. descried that by optimizing the layer thickness, BMWF ad acceptor concertation performance of the CZTS is improved and efficiency is increased up to 13.41% determined by one dimension simulation of solar cell structure by incorporating radiative and Auger recombination in the CZTS layer 19. N. Amin et al. studied that by increasing the thickness of absorber layer ad buffer layer, the efficiency is increased up to 7.9% 20. The above information is given in tabularised form in table 2.1.
Tale 2.1 Summary of CZTS solar cells simulated using SCAPS-1D
Author Solar cell material Properties Efficiency
O.K. Simya et al.
(2015) 14 CZTS, CZTSe, CZTSSe Comparative study of efficiency of different material 12.03%, 13.16%, 15.77%
P. Lin et al.
(2014) 15 CZTS Study the impact
of buffer layer thickness and other parameters on the cell performance 19.87%
M.A. Olopade et al.
(2012) 17 CZTS Study the change in efficiency by changing buffer layer 6.77%
M. Patel et al.
(2012) 19 CZTS To study enhancement of output performance of cell 13.41%
N. Amin et al.
(2010) 20 CZTS To study the Vision of CZTS solar cell 7.55%
In SCAPS-1D, excluding front and back contact we can input up to seven layers. A solar cell structure of sodalime glass (SLG) | Molybdenum | CZTS | CdS | ZnO:Al | Flat contact was implemented in this study I the SCAPS 3.3.07 environment. Illumination of light is from the front contact with air mass 1.5 global spectrum with a light power of sun (1000 W/m2) 14, 29. Under illumination excess, free carriers are generated ad a Fermi level split into quasi Fermi level, as this is hetero-junction based thin films device 19. This separation between quasi-Fermi level is responsible for the open circuit voltage in the device. The transmission ad reflection of the back and front contact were set before starting the simulation 29.
Fig 3.1 Representation of SCAPS-1D software
For every layer, the material properties were given as an input to SCAPS software. The contact properties were considered for each design. The metal work function of the contacts varies accordingly, depending upon the metal used. In this simulation, for the front contact, flat ad were chose, for the back contact, flat ad were utilised for simulation.
Fig 3.2 Schematic representation of TFPV device
For the flat ad calculations, the layer adjacent to the contact is considered, depending upon whether it is n-type or p-type or intrinsic type layers, and SCAPS automatically calculates the metal work function 29. Furthermore, the simulation, as descried in Table 3.1.
Table 3.1 Device parameters used in the simulation
Cell properties Cell temperature 300K
Series resistance Rs 4.25 ? cm2
Shunt resistance Rsh 370 ? cm2
Contacts Back metal contact properties Front metal contact properties
Metal work function Flat band Mo-5 eV Flat band SRV of electros 107 cm/s 105 cm/s 105 cm/s SRV of hole 105 cm/s 105 cm/s 105 cm/s The individual materials parameters have to be inputted for all the associated values, which includes ad gap (Eg), electro affinity (?), dielectric permittivity (?), conduction band density of states (Nc), valence band density of states (Nv), electro thermal velocity (Vthn), hole thermal velocity (Vthp), electron mobility (?n), hole mobility (?p), door density (NA), acceptor density (ND) as descried in Table 3.2.
Table 3.2 Material parameters used in this simulation
Parameters p-CZTS n-CdS i-ZnO ZnO:Al
Thickness w (nm) 2500 5000 50 200
Relative permittivity 10 10 9 9
Electron affinity 4.5 4.2 4.4 4.6
Eg(eV) 1.5 2.4 3.3 3.3
Nc(cm-3)2.2×10182.2×10182.2×10182.2×1018Nv(cm-3)1.8×10191.8×10191.8×10191.8×1019Electron mobility(cm2V-1S-1)100 100 100 100
Hole mobility(cm2V-1S-1)25 25 25 25
Donor concertation(cm-3)0 0 1×1051×1018Acceptor concertation(cm-3)1×10171×10170 0
The most common defects in semiconductors are either door or acceptor defects, whereas neutral defect is also included to specify electro and hole life-time 29. There exist an interface between different layers, absorber/buffer and buffer/window interface which were also considered in this simulation for the present study. The device ad material parameters used in this simulation were selected based on literature values, theory, or in some cases leading to reasonable estimation.
4. Result and discussion
In this simulation, the effect of absorber doping on solar cell performance is investigated. As the absorber doping increases while Voc increases, the value of Jsc decreases. The influence of carrier densities (NA) in the CZTS absorbing layer on the J-V characteristics of the cells is illustrated in Figure 4.1.
Fig 4.1 J-V of cells with different carrier densities in the CZTS absorber 15
When the absorber carrier densities are increased, the saturation current I0 will be reduced, and the resulting in the increase of Voc. However, the short-circuit current will decrease with increasing of carrier densities. The reason behind this is the enhancement of recombination process and reduced probability of photo-generated electron by the higher carrier densities. The resistivity and carrier densities of CZTS films dependent on the ratio Cu/ (Zn+Sn). Higher efficiency cells can be obtained using Zn-rich, Cu-poor CZTS absorber layers because Cu-rich ad Zn-poor films have low resistivity and high carrier densities, which are not suitable for fabricating solar cells 15.
ZnS and ZnSe could be alterative buffers to the CdS in the production of CZTS solar cells. This is due to their efficiencies of 6.52% and 6.76% respectively which is closer to the established experimental efficiency. The value of Voc, Jsc and FF of ZnS is nearer to the value to CdS while ZnSe have quite different values comparatively ZnS 17.
Fig 4.2 Light characteristic curve J-V graph of simulation with ZnS buffers 17
In this report, an effort was done to learn the new simulation software SCAPS-1D for the modelling of chalcogenide/chalcopyrite solar cells. SCAPS provide in depth physics of CdTe/CIGS/CZTS solar cells. Within a current semester, I have tried to understand the mechanisms of SCAPS. This will help fabrication of solar cells using experimental and theoretical data produced at the end of the final semester minor research project. This report facilitates up-coming graduates who are interested in simulation and modelling of solar cells.
I am grateful to Prof. Burgelman, University of Gent, Belgium for providing SCAPS-1D software.
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