Numerical Simulation of CIGS Thin Film Solar Cells ... - IEEE Xplore

2013 IEEE Conference on Sustainable Utilization and Development in Engineering and Technology

Numerical Simulation of CIGS Thin Film Solar Cells Using SCAPS-1D Nima Khoshsirat and Nurul Amziah Md Yunus Micro and Nano Electronic Systems Unit, Department of Electrical and Electronic Engineering Faculty of Engineering University Putra Malaysia Serdang, 43400, Selangor, Malaysia Email: [email protected] and [email protected]

of CIGS band gap to have high conversion efficiency is from 1.12eV to 1.26eV [4]. Recently a successful effort has been done to improve the energy conversion efficiency of CIGS solar cells with band gaps up to 1.45eV [5]. The other important layer in a heterojunction structure is the buffer layer. The role of a buffer layer is to form a junction with the absorber layer while leading maximum amount of incoming light to the absorber layer. Accordingly, the buffer layer should have minimal absorption loss, low surface recombination and electrical resistance in driving out the generated carriers. To satisfy such desired features, the buffer layer should be as thin as possible and should have wider band gap in comparison with the CIGS absorber layer. A number of materials have been tested for the buffer layer since the beginning of chalcopyrite-based thin film solar cells. So far the CIGS solar cell found with the highest efficiency was fabricated using CdS buffer layer. However, since the Cadmium (Cd) is classified as toxic material, the development of Cdfree buffer layers became an interesting research area in the field of CIGS solar cell [6]. Some Nontoxic and wide-band gap materials were proposed and tested as alternative buffer layers in CIGS solar cells such as Zn(O,S), Zn1−x Mgx O, In2 S3 , etc. [7]– [9]. Among these materials, the Indium sulfide has promising features for a desired buffer layer because of its stability; higher band gap and lower absorption edge in comparison with CdS [10]. In this paper the cell structure n-ZnO/i-ZnO/In2 S3 /CIGS is numerically simulated and the simulation performed is aimed to observe the effect of buffer and absorber layer thickness on the cell performance.

Abstract—The performance of copper indium gallium diselenide (CIGS) thin film solar cell has been numerically simulated with different buffer and absorber layers thickness. The cell structure based on CIGS compound semiconductor as the absorber layer, indium sulfide as a buffer layer, un-doped (i) and n-doped zinc oxide as a window layer has been simulated using the simulation program called SCAPS-1D. This study aimed to find the optimum thickness of buffer and absorber layer for a CIGS thin film solar cells with indium sulfide buffer layer. It is found that the optimum thickness of the buffer layer is from 40nm to 50nm and for the absorber layer is in the range of 2000nm to 3000nm. Index Terms—CIGS, Inx Sy , SCAPS-1D, Thin film solar cells.

I. I NTRODUCTION The interest in CIGS thin-film solar cells has increased significantly due to its promising characteristics for high performance and low cost. The highest reported efficiency for a laboratory scale CIGS solar cell is 20.3% which was gained by P.Jackson et al. [1] . Lately, E.Wallin et al. reported a new certified word record of 17.4% for the CIGS thin-lm sub-module [2]. The structure of CIGS cells is heterojuction which is formed of different semiconductor materials. A typical CIGS thin film solar cell structure consists of a p-type wide-band gap absorber layer (CIGS) which is deposited on the Molybdenum (Mo) coated back glass substrate and an n-type buffer/window layer. The band gap of copperindiumgalliumdiselenide absorber can be varied over the range of 1eV to 1.7eV depending on the ratio of indium and gallium in the layer composition [3]. Undoubtedly the variation of absorbers band gap does affect the cell performance. Experimental studies show that the optimum range

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II. N UMERICAL S IMULATION

1 shows the material properties which are used for each layer. The transmission and reflection of the back and front contacts should also be set before running the simulation. One of the SCAPS beneficial capabilities is its batch option which is used for exploring the influence of one or few parameters variation over a certain range. The absorber and buffer layers thickness range were set in batch-set up window as they are shown in Table 1.

A. SCAPS-1D Numerical Simulation Program This numerical analysis used SCAPS (a Solar Cell Capacitance Simulator) software (version 3.2.00), a numerical simulation tool written and introduced at the University of Gent [11]. It is generally developed for polycrystalline thin-film devices and especially used for CdTe and CIGS solar cells. In comparison with other simulation software, SCAPS has the largest number of AC and DC electrical measurements which can be simulated including open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF%), quantum efficiency (QE%), capacitance voltage spectroscopy C(V), capacitance frequency spectroscopy C(f), efficiency percentage, generation and recombination profiles, spectral response, heterojunction energy band structure, distribution of electric field, carrier current densities, etc. All these measurements can be calculated and obtained in dark and light condition and also at different temperatures and illuminations. Up to seven layers can be added to the cell structure in SCAPSs problem setting window. The physical and electronic properties of these layers and contacts should be imported into their own specific sections.

III. R ESULTS AND D ISCUSSION A. Impacts of Buffer Layer Thickness Variation

The simulation started by choosing the thickness of the absorber layer set to 2µm and the thickness of In2 S3 buffer layer was changed from 20nm to 90nm and the variation of the cell performance was reviewed. As it can be seen from the Fig.1, although the increase of buffer layer thickness causes reduction in Voc, Jsc and the fill factor (FF%), it is not significant for Voc and fill factor. These two parameters can be considered almost constant for all amounts of buffer layer thickness. The small changes and reductions of fill factor with increase of buffer thickness can arise from the valance band discontinuities at the interfaces that B. Cell Structure and Materials Properties appear as spikes. Unlike the changes in Voc and The cell structure that is simulated in this study FF%, the Jsc reduction caused by the increase of is an n-ZnO/i-ZnO/In2 S3 /CIGS. For each layer the In2 S3 buffer layer is significant. The reason for this material properties should be given to software dependency of Jsc to the buffer layer thickness is as inputs. Furthermore, the test conditions includ- noticeably fewer photons can reach absorber layer ing the temperature, illumination, bias voltage, etc. in a cell with thicker buffer layer. More photons are should be set before starting the simulation. Table being absorbed within buffer layer especially those with the wave length around the In2 S3 absorption edge, thus fewer photons can contribute to quantum TABLE I efficiency. Fig. 2 shows the changes in the quantum L AYER ’ S M ATERIAL P ROPERTIES efficiency of the cell due to increase of In2 S3 buffer CIGS In2 S3 i-ZnO n-ZnO Properties layer thickness. Therefore, it is obvious that the 1.5-4.5 0.02-0.09 0.08 0.2 Thickness (µm) resultant cell efficiency has a downward trend while 1.1 2.8 3.3 3.3 Bandgap (eV) the buffer layer thickness is increasing. Although Electron affinity (eV) 4.5 4.7 4.6 4.6 Permittivity 13.6 13.5 9.0 9.0 the cell with thinner buffer layer shows higher CB (1/cm3 ) 2.2E+18 1.8E+19 2.2E+18 2.2E+18 performance, thicknesses less than 40nm currently VB (1/cm3 ) 1.8E+19 4.0E+13 1.8E+19 1.8E+19 is not reachable because of fabrication techniques 2 Electron mobility (cm /Vs) 1.0E+2 4.0E+2 1. 0E+2 1. 0E+2 2 and instruments limitation. Thus, the range of 40nm Hole mobility (cm /Vs) 2.5E+1 2.1E+2 2.5E+1 2.5E+1 3 ND (1/cm ) 0 1.00E+18 1.0E+16 1.0E+18 to 50nm is the preferred and the optimized thickness NA (1/cm3 ) 2.0E+16 1.00E+1 0 0 of the buffer layer in CIGS thin film solar cell.

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Fig. 1. Cell’s performance degradation induced by increase of In2 S3 buffer layer thickness: (a)Open- circuit voltage (Voc), (b) Short circuit current density (Jsc), (c) Fill Factor, (d) Efficiency.

Fig. 3. Cell’s performance enhancement induced by increase of CIGS absorber layer thickness: (a) Open- circuit voltage (Voc), (b)Short circuit current density (Jsc), (c) Fill Factor, (d) Efficiency.

the cell performance was investigated. The simulation results show that the general performance of the cell increases while the thickness of absorber layer is increased. The entire measured parameters including Voc, Jsc and the fill factor (FF%) almost follow a same pattern. Fig. 3 shows the variation of the cell performance due to the absorber thickness changes. To have better understanding from the dependency of cell performance on the absorber layer thickness, it is needed to look at the band Fig. 2. Quantum efficiency of the cells with different buffer layer diagram of the studied CIGS thin film solar cell. thickness. The QE% is degraded by increase of buffer layer thickness. The Fig. 4 shows the cells band diagram. There are four recombination regions in the band diagram which are shown in Fig.4. The region 1 represents B. Impacts of Absorber Layer Thickness Variation the recombination at back contact and region 2 In this stage, the thickness of In2 S3 buffer layer shows the Quasi-Neutral recombination (bulk rewas set to 50nm, the thickness of absorber layer was combination) in the absorber layer. The back contact varied from 500nm to 4500nm, and the variation of is located near the depletion region in a cell with

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Fig. 4. The cells band diagram: (1) Back contact recombination, (2) Quasi-Neutral recombination, (3) space charge region recombination, (4) Buffer/ Absorber interface recombination

Fig. 6. Quantum efficiency enhancment at long wavelength due to the increase of absorber layer thickness

Fig. 5. Back contact recombination current density decrease due to the increase of absorber layer thickness. Fig. 7. Quasi-Neutral recombinaton current density increase due to the increase of absorber layer thickness

thin absorber layer. This leads to significant increasing of back contact recombination. Thus, a large number of photo-generated carriers recombine in the back contact and less photo-generated electrons can contribute to the quantum efficiency. Therefore, with increasing of the absorber thickness the back contact recombination current decreases and the performance of the cell increases consequently. Fig.5 shows the back contact recombination current density versus the voltage for cells with different thickness of CIGS absorber layer. As mentioned above, an increase of the absorber layer thickness can enhance the cell performance but this needs an optimization. However, the effect of absorber layer thickness on the cell characteristics cannot be increased excessively by looking at Fig.3. This justify that for thicknesses more than 3000nm the Voc, Jsc, FF% and the efficiency will only increase slightly and it can be considered almost persistent or constant. This is due to the fact that the photons at higher wavelength (>750nm) were absorbed deep down in the absorber layer, far from the depletion region. Fig.6 shows the increase of the

photons depth absorption at wavelengths more than 750nm. The resultant carriers recombine in absorber bulk before reaching to the depletion region. It means that increasing absorber layer thickness can raise the possibility of quasi-neutral recombination (region 2 in Fig.4). Undeniably, an increase of absorber layer thickness causes photon absorption especially at high wavelength but the resultant carriers cannot be used to improve the cell performance and recombine before reaching to the depletion region. Fig.7 shows the quasi-neutral recombination current density of the CIGS cells with different absorber layer thickness. Hence the results show that the optimum range for the absorber layer thickness is between 2000nm to 3000nm and cell fabrication with the absorber thicknesses more than this range is not reasonable because of more material consumption without any significant effect on the cell characteristics and efficiency.

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IV. C ONCLUSION

[9] N. Naghavi, E. Chassaing, M. Bouttemy, G. Rocha, G. Renou, E. Leite, A. Etcheberry, and D. Lincot, “Electrodeposition of In2S3 buffer layer for Cu(In,Ga)Se2 solar cells,” Energy Procedia, vol. 10, no. 0, pp. 155–160, 2011. [10] K. Ernits, D. Br´emaud, S. Buecheler, C. Hibberd, M. Kaelin, G. Khrypunov, U. M¨uller, E. Mellikov, and A. Tiwari, “Characterisation of ultrasonically sprayed InxSy buffer layers for Cu(In,Ga)Se2 solar cells,” Thin Solid Films, vol. 515, no. 15, pp. 6051–6054, May 2007. [11] M. Burgelman, P. Nollet, and S. Degrave, “Modelling polycrystalline semiconductor solar cells,” Thin Solid Films, vol. 361362, no. 0, pp. 527–532, 2000.

The cell performance is analyzed and simulated by the function of buffer layer and absorber layer thickness. The optimum thickness of absorber layer and buffer of a CIGS thin film solar cell with In2 S3 buffer layer are found in the range of 2000nm to 3000nm and between 40nm and 50nm respectively. There is a compromise between having a thin and a minimum recombination current density in these ranges of layer thickness. Although these results can help us to fabricate a desired CIGS thin film solar cell, there are some other effective parameters, which can affect the cell performance and need to be investigated in further studies. ACKNOWLEDGMENT The authors gratefully acknowledge the University of Gent, Belgium for providing the opportunity to use SCAPS-1D simulation software. R EFERENCES [1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and M. Powalla, “New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%,” Progress in Photovoltaics: Research and Applications, vol. 19, no. 7, pp. 894–897, 2011. [2] E. Wallin, U. Malm, T. Jarmar, O. L. M. Edoff, and L. Stolt, “World-record Cu(In,Ga)Se2-based thin-film sub-module with 17.4% efficiency,” Progress in Photovoltaics: Research and Applications, vol. 20, no. 7, pp. 851–854, 2012. [3] V. S. Saji, S.-M. Lee, and C.-W. Lee, “CIGS Thin Film Solar Cells by Electrodeposition,” Journal of the Korean Electrochemical Society, vol. 14, no. 2, pp. 61–70, 2011. [4] M. Gloeckler and J. R. Sites, “Band-gap grading in Cu(In,Ga)Se2 solar cells,” Journal of Physics and Chemistry of Solids, vol. 66, no. 11, pp. 1891–1894, 2005. [5] M. A. Contreras, L. M. Mansfield, B. Egaas, J. Li, M. Romero, R. Noufi, E. Rudiger-voigt, and W. Mannstadt, “Wide bandgap Cu(In,Ga)Se2 solar cells with improved energy conversion efficiency,” Progress in Photovoltaics: Research and Applications, vol. 20, no. 7, pp. 843–850, 2012. [6] D. Hariskos, S. Spiering, and M. Powalla, “Buffer layers in Cu(In,Ga)Se2 solar cells and modules,” Thin Solid Films, vol. 480481, no. 0, pp. 99–109, 2005. [7] C. Platzer-Bjorkman, J. Kessler, and L. Stolt, “Atomic layer deposition of Zn(O,S) buffer layers for high efficiency Cu(In,Ga)Se/sub 2/ solar cells,” in Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on, vol. 1, 2003, pp. 461–464 Vol.1. [8] C. Yoshiyuki, F. Y. Meng, Y. Akira, and K. Makoto, “Study on Phase Transition of Zn1-XMgXO Thin Films Grown by MOCVD Process,” in Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on, vol. 1, 2006, pp. 567–570.

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