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   Numerical Analysis of Stacked Layered CZTS and CZTSe-Based Thin Film Solar Cell Mohammad Wahidur Rahman 1,2* , Saad Mohammad Abdullah 1 , Mohammed Akhyar Bakth 1 ,   Quazi Nafees Ul Islam 1  and Md. Ashraful Hoque 1 1 Department of Electrical and Electronic Engineering Islamic University of Technology Board Bazar, Gazipur-1704, Bangladesh 2 Department of Electrical and Computer Engineering The Ohio State University Columbus, OH 43210, USA *  Abstract   — In this paper, single CZTSe layer and stacked layer of CZTS and CZTSe have been optimized as absorber layers in thin film solar cell and the variation of cell performance was observed. CZTSe layer was optimized by varying thickness, carrier density and defect density and obtained J SC  of 27.87 mAcm -2 , Voc of 0.702V, fill factor of 65.41% and efficiency of 12.79%. By varying the thickness and carrier density of both CZTS and CZTSe together, solar cell was optimized with stacked layer of CZTS and CZTSe. Solar cell with the stacked absorber layer was found to have better cell performance among different CZTS-based solar cells with J SC  of 29.23 mAcm -2 , V OC  of 1.1188 V, FF of 71.70% and efficiency of 23.45%.  Keywords  —    thin film; solar cell; CZTS; CZTSe; SCAPS;   I.   I  NTRODUCTION  Thin Film Solar cells, which are commercially available like a-Si are associated with problems such as low efficiency. Due to shortage of Te and toxicity of Cd, CdTe based solar cells are also lagging behind moreover shortage and high cost of In for the case of CIGS technology is its main drawback. Whereas, quaternary semiconductors like Cu 2 ZnSnS 4  (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) can be a potential alternative to the available technology with the advantages such as non-toxicity, earth abundance and low cost of raw materials and potential for  providing high-efficiency. Moreover, other properties such as high absorption co-efficient of over 10 4  cm -1  and a direct band gap value about 1.4-1.5 eV for CZTS and  1.0 eV for CZTSe make these solar cells as attractive candidates [1]. There are various techniques like chemical bath deposition (CBD), nanocrystal (NCs)-based fabrication method, thermal evaporation, DC/RF sputtering, electrochemical deposition,  pulsed laser deposition, co-evaporation, spray pyrolysis, and sol–gel formation for deposition of CZTS, CZTSe thin film solar cells. By sputtering method, efficiency of 6.77% and by evaporation method 6.81% was also achieved for CZTS thin film solar cell [2, 3]. Shin et al. prepared CZTS (Al:ZnO/i-ZnO/CdS/CZTS) structure with an efficiency of 8.4% using vacuum process [4]. Mono-grain Layer (MGL) technique and sol-gel sulfurization process were used to prepare CZTSe solar cell with efficiencies of 2.16% and 2.76% respectively, which are very low [5, 6]. Highest 9.15% efficiency for CZTSe solar cell was obtained using evaporation method by Repins et al. [7]. A single junction solar cell with a band gap of 1.34 eV has maximum Shockley–Queisser limit (SQL) of 33.7% whereas solar cells with CZTSe may have SQL limit of at least 30.9% (with a band gap of  1.0 eV) [8]. So enough scope is available for improving the efficiency of different CZTS-based thin film solar cells. Efficiency of CZTS-based solar cells can be improved by adding high Se content due to which more facile  p-type and n-type doping will be possible. Due to the quaternary properties of CZTS or CZTSe semiconductors; the structural, crystallographic, and electrical  properties can be influenced by stoichiometric composition, which will result in forming native defects [9]. As a result of these defects and due to the small grain size of CZTS or CZTSe, short diffusion length of carriers are resulted, which result in low efficiency [2]. The higher series resistance and lower shunt resistance of the cell also cause the cell efficiency to reduce. In order to improve the efficiency of CZTS-based thin solar cells and fully understand their performance, it is necessary to systematically explore the influence of the basic factors in the performance of the cells.  Numerical analysis is an effective way to predict the effect of changes in material properties, measure the potential merits of cell structures and then optimize the structure of cells. So,  by numerical analysis based on SCAPS (Solar Cell Capacitance Simulator), in our previous study, a solar cell was optimized with CZTS as absorber layer. The structure used for simulation was n-Al:ZnO/i-ZnO/n-CdS/p-CZTS (Fig. 1). After optimization of CZTS absorber layer and CdS buffer layer, a conversion efficiency of 21.79% and fill factor of 68.79% (with V OC  of 1.0017 V and J SC  of 31.624 mAcm -2 ) has  been achieved at 300 K for CZTS thin film solar cell [10]. The optimized values of different layer parameters of CZTS and CdS are shown in Table I. In extension to that work, in this study, the performance dependency of solar cells with only CZTSe as absorber layer and then using a stacked absorber layer containing both CZTS and CZTSe on a set of geometric and physical parameters such as absorber layer’s thickness, carrier density, and defect density has been observed using SCAPS-1D. II.   M ETHODOLOGY SCAPS is an one dimensional solar cell simulation  program developed at the department of Electronics and Information Systems (ELIS) of the University of Gent, Belgium [11]. SCAPS can simulate up to 7 semiconductor layers and can calculate energy bands, concentrations and currents at a given working point, J-V characteristics, AC characteristics (C and G as function of V and/or f), and 978-1-5090-5627-9/17/$31.00 ©2017 115 International Conference on Electrical, Computer and Communication Engineering (ECCE), February 16-18, 2017, Cox’s Bazar, Bangladesh  spectral response both in light and dark condition which is  based on the hole and electron continuity equations together with Poisson equation. SCAPS was used for our simulation as its simulation results have good agreement with experimental results [12]. The structure used for solar cell with CZTSe absorber layer was n-Al:ZnO/i-ZnO/n-CdS/p-CZTSe (Fig. 1). While simulating the solar cell with combination of CZTS and CZTSe as absorber layer, the structure used was n-Al:ZnO/i-ZnO/n-CdS/p-CZTS/p-CZTSe (Fig. 1). The baseline values of the physical parameters used in this study for different material layers were all cited from experimental study, reasonable estimates in some cases [3, 10, 13, 14], or literatures, which are summarized in Table I. By incorporating these material parameters into SCAPS for all of the analysis aspects, changes in the values for efficiency, open circuit voltage, short circuit current and fill factor were observed. In each material layer only Gaussian type of single level defects was introduced in order to make the simulation model as simple as possible. These defects were all compensating defects that positioned at the intrinsic level which is close to the mid-gap. Furthermore, series resistance, (R  S ) = 4.25   and shunt resistance, (R  Sh ) = 350   were used in the simulation to measure the device performances. These values were chosen with reference to different experimental works as shown in Table II. Here operating temperature was set to 300 K, illumination condition was set to global AM 1.5 standard. III.   R  ESULTS A  ND D ISCUSSIONS  A.    Effect of CZTSe (absorber layer) thickness, carrier density and defect density At the beginning of our study, the thickness of the CZTSe layer was varied from 1000 nm (1 μ m) to 5000 nm (5 μ m); while for other material parameters we used the optimized values obtained in our previous study, for the solar cell with conversion efficiency of 21.79% using   CZTS as absorber layer [10] as shown in Table I. Initially, the carrier density of CZTSe was considered as 10 15 cm -3  and that of defect density as 10 16 cm -3 . The cell performance with varied CZTSe layer thickness is shown in Fig. 2. It was found that short circuit current density (J SC ), open circuit voltage   (V OC ) and efficiency increases with increase in thickness, but fill factor decreases. As thickness is increased more electron-hole  pairs are generated because thicker layer can absorb more  photons which results in higher efficiency. Now, if thickness is increased too much, then photons will be absorbed deeper into the absorber layer. As a result generated electron-hole  pairs recombine before they can reach the depletion region. That is why after a certain thickness there is no significant improvement in cell performance. Moreover, higher thickness is not cost effective also. Therefore, from Fig. 2, thickness of 3000 nm was chosen as the optimum value for CZTSe. TABLE   II.   E XPERIMENTAL V ALUES OF S ERIES AND S HUNT R  ESISTANCE   Efficiency (%) R  s (  cm 2 )   R  sh  (  cm 2 )   Reference 5.74 6.41 424 [15] 6.77 4.25 370 [2] 8.4 4.5 - [4] For optimized thickness of CZTSe the results found are J SC  of 29.77 mAcm -2 , V OC  of 0.483V, FF of 51.01% and efficiency of 7.34%. As the efficiency obtained by optimizing the thickness is very low,   using this optimized thickness next the carrier density of CZTSe was varied from 10 12 cm -3  to 10 19 cm -3 . The cell performance is shown in Fig. 3.   It is observed that if carrier density is increased, J SC  increases for a while then decreases as for higher carrier density there is high number of carriers, so the chances for recombination of carriers increase. As a result, J SC  decreases at higher carrier density. But for increasing carrier density reverse saturation current (I 0 ) decreases, so V OC  increases according to (1). Now due to variation of J SC  and V OC  values, there were changes in FF and efficiency curve. So, optimum carrier density was chosen as 10 18 cm -3  because beyond this value the cell performance degrades due to carrier scattering. And carrier density below the optimized value gives lower performance as seen from Fig. 3.                                    The results obtained by optimizing carrier density at 10 18 cm -3  are J SC  of 23.86 mAcm -2 , V OC  of 0.664V, FF of 63.17% and efficiency of 10.01%.  Next optimization of the defect density of CZTSe was done using the optimized thickness and carrier density and keeping parameter of other materials unchanged. Defect density was varied from 10 12 cm -3  to 10 18 cm -3 . The cell  performance is shown in Fig. 4, where it has been found that as defect density increases, J SC , V OC , FF and efficiency decreases because increase in defect density means addition of recombination carrier in absorber layer. TABLE   I. P ERFORMANCE P ARAMETER U SED IN  N UMERICAL A  NALYSIS   Parameters p-CZTSe p-CZTS n-CdS i-ZnO n-ZnO Thickness,W (nm) Relative permittivity Electron affinity (eV) Eg (eV)  Nc (cm -3 )  Nv (cm -3 ) μ n  (cm 2 V -1 s -1 ) μ  p  (cm 2 V -1 s -1 ) Donor density (cm -3 ) Acceptor density(cm -3 ) Defect density (cm -3 ) 3000 8.6 4.6 1 7.9×10 17 4.5×10 18 40 10 0 10 18  10 15  3000 10 4.5 1.5 2.2×10 18  1.8×10 19  100 25 0 10 18  10 15  60 10 4.2 2.4 2.2×10 18  1.8×10 19  100 25 10 18 0 10 16  50 9 4.6 3.3 2.2×10 18  1.8×10 19  100 25 10 5  0 10 16  200 9 4.6 3.3 2.2×10 18  1.8×10 19  100 25 10 18  0 10 16   Fig. 1. Solar Cell Structures used in simulation with (a) CZTS, (b) CZTSe and (c) Stacked layer of CZTS and CZTSe 116    From Fig. 4 it can be observed that defect density of less than or equal 10 15 cm -3 will provide good cell performance. So, 10 15 cm -3 can be considered as the optimized defect density. For optimized thickness, carrier density and defect density of CZTSe, the results obtained are J SC  of 27.87 mAcm -2 , V OC  of 0.702 V, FF of 65.41% and efficiency of 12.79%. The efficiency obtained by using CZTSe as absorber layer is less than that of CZTS. This is because the bandgap of CZTSe is smaller than bandgap of CZTS. In fact the band gap of CZTSe is less than the theoretical optimal bandgap of 1.4 eV. In an attempt to improve the cell performance combined absorber layer (containing both CZTS and CZTSe) was used which is discussed in the next section.  B.    Effect of combining CZTS and CZTSe as a stacked absorber layer Since low efficiency was obtained by using CZTSe as absorber layer, so stacked layer of CZTS and CZTSe was used as absorber layer and observed the effect of variation of thickness and carrier density of absorber layer on cell  performance. The overall cell performance for different thickness of CZTS and CZTSe has been shown in Table III. Here, thickness of both CZTS and CZTSe layer was varied. At first, thickness of CZTS was kept smaller than CZTSe thickness. As thickness of CZTS was made to increase and simultaneously CZTSe thickness was made to decrease, it was observed that there is increase in efficiency and fill factor. But it is difficult to practically fabricate thinner absorber layer. Therefore, thickness of 1000nm was chosen for CZTSe layer and kept increasing thickness of CZTS layer. But keeping in mind that thicker layer increases the fabrication cost, thickness of 3000nm was selected for CZTS, as for higher thickness the efficiency is not increasing significantly. For the optimized values of thickness, the J-V characteristics of the cell are shown in Fig. 5. The obtained results are J SC  of 30.92 mAcm -2 , V OC  of 0.9913 V, FF of 68.56% and efficiency of 21.02%.  Next for these values of optimized thickness, variation of carrier density for both CZTS and CZTSe was made and observed their combined effect on cell performance as shown in Table IV. First, the carrier density of CZTSe was kept at 10 18  cm -3  which was optimized in previous section and varied the carrier density of CZTS from 10 16  cm -3  to 10 19  cm -3 . The efficiency and fill factor increases up to 10 18  cm -3 and then decreases for 10 19  cm -3 carrier density of CZTS. So, 10 18  cm -3  was chosen   as the optimum value of CZTS carrier density.  Now keeping the carrier density of CZTS at 10 18  cm -3  and changing the carrier density of CZTSe, no significant variations were found in efficiency and fill factor. Therefore, 10 18  cm -3  was selected as the optimized carrier density for  both CZTS and CZTSe. (a) (b) Fig. 2. Effect of variation of CZTSe absorber layer thickness on (a) Jsc, Voc; (b)Fill factor, Efficiency   Fig. 4. Effect of variation of CZTSe defect density on (a) Jsc, Voc; (b)Fill factor, Efficiency   Fig. 3. Effect of variation of CZTSe carrier density on (a) Jsc, Voc; (b)Fill factor, Efficiency 117  After optimizing both thickness and carrier density, the obtained J-V characteristics of the cell is shown in Fig. 5. The obtained results are J SC  of 29.23 mAcm -2 , V OC  of 1.1188 V, FF of 71.70% and efficiency of 23.45%. Bandgap and wavelength are inversely related (2). As CZTSe has lower bandgap than CZTS it can absorb photons of higher wavelength. As CZTSe was used at the bottom of the structure, it could absorb those photons of higher wavelength, which CZTS could not absorb. As a result higher efficiency was obtained.           IV.   C ONCLUSIONS  In this paper, we carried out numerical simulation to observe the cell performance by varying thickness, carrier density and defect density of CZTSe as absorber layer and then by varying thickness and carrier density of stacked absorber layer composed of both CZTS and CZTSe. Firstly we detected the optimum thickness of 3000 nm, carrier density of 10 18 cm -3  and defect density of 10 15 cm -3  for CZTSe keeping other layer parameters unchanged where the obtained fill factor was 65.41% and efficiency was 12.79%. Then for the stacked layer, we observed the optimum thickness of 1000 nm for CZTSe and 3000 nm for CZTS, carrier density of 10 18 cm -3  for both layers by keeping other layer parameters unchanged. Finally we achieved fill factor of 71.70% and efficiency of 23.45% which is better than other CZTS-based thin film solar cells. R  EFERENCES   [1] Nowshad Amin, Mohammad Istiaque Hossain, Puvaneswaran Chelvanathan, A.S.M. Mukter Uzzaman, and K. Sopian, Prospects of Cu 2 ZnSnS 4  (CZTS) Solar Cells from Numerical Analysis, in 6th International Conference on Electrical and Computer Engineering ICECE 2010, Dhaka, Bangladesh, 2010. [2] Hironori Katagiri, Kazuo Jimbo, Satoru Yamada, Tsuyoshi Kamimura, Win Shwe Maw, Tatsuo Fukano  , et al. , Enhanced Conversion Efficiencies of Cu 2 ZnSnS 4 -Based Thin Film Solar Cells by Using Preferential Etching Technique, Applied Physics Express  , vol. 1, pp. 041201-041202, 2008. [3] K. Wang, O. Gunawan, T. Todorov, B. Shin, S. J. Chey, N. A. Bojarczuk   , et al. , Thermally evaporated Cu 2 ZnSnS 4  solar cells, Applied Physics Letters  , vol. 97, p. 143508, 2010. [4] Byungha Shin, Oki Gunawan, Yu Zhu, Nestor A. Bojarczuk, S. Jay Chey, and S. Guha, Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu 2 ZnSnS 4  absorber., Progress in Photovoltaics: Research & Applications  , vol. 21, pp. 72-76, 2011. [5] E. Mellikov, D. Meissner, T. Varema, M. Altosaar, M. Kauk, O. Volobujeva  , et al. , Monograin materials for solar cells, Solar Energy Materials & Solar Cells  , vol. 93, pp. 65–68, 2009. [6] Gabriele M. Ilari, Carolin M. Fella, Carmen Ziegler, Alexander R. Uhl, Yaroslav E. Romanyuk, and A. N.Tiwari, Cu 2 ZnSnSe 4  solar cell absorbers spin-coated from amine-containing ether solutions, Solar Energy Materials & Solar Cells  , vol. 104, pp. 125–130, 2012. [7] I.L. Repins, J.V. Li, A.Kanevce, C.L. Perkins, K.X. Steirer, J. Pankow  , et al. , Effects of deposition termination on Cu 2 ZnSnSe 4  device characteristics, Thin Solid Films  , vol. 582, pp. 184-187, 2015. [8] Tang Jiao Huang, Xuesong Yin, Guojun Qi, and H. Gong, CZTS- based materials and interfaces and their effects on the performance of thin film solar cells, Physica Status Solidi RRL  ,  pp. 1–28, 2014. [9] Kunihiko Tanaka, Masatoshi Oonuki, Noriko Moritake, and H. Uchiki, Cu 2 ZnSnS 4  thin film solar cells prepared by non-vacuum processing, Solar Energy Materials and Solar Cells  , vol. 93, pp. 583–587, 2009. [10] Mohammad Wahidur Rahman, Quazi Nafees Ul Islam, Saad Mohammad Abdullah, Mohammed Akhyar Bakth, and M. A. Hoque, Numerical Optimization of Absorber and Buffer Layers of CZTS Thin Film Solar Cells, in International Conference on Advances in Electrical, Electronic and Systems Engineering (ICAEESE 2016), Malaysia, 2016. [11] Simulation programme SCAPS-1D for thin film solar cells developed at ELIS, University of Gent. Available: [12] D. K. Marlein J, Burgelman M, Analysis of electrical properties of CIGSSe and Cd-free buffer CIGSSe solar cells, Thin Solid Films  , vol. 517, pp. 2353-2356, 2009. [13] D. Cozza, C.M. Ruiz, D. Duché, M. Neuschitzer, E.Saucedo, J.J. Simon  , et al. , 1D and 2D numerical simulations of Cu 2 ZnSnSe 4  solar cells. [14] G. Altamura, Development of CZTSSe thin films based solar cells, PhD, Material chemistry, Universite Joseph-Fourier-Grenoble, HAL, 2014. [15] Kazuo Jimbo, Ryoichi Kimura, Tsuyoshi Kamimura, Satoru Yamada, Win Shwe Maw, Hideaki Araki  , et al. , Cu 2 ZnSnS 4 -type thin film solar cells using abundant materials, Thin Solid Films  , vol. 515, pp. 5997– 5999, 2007. (a) (b) Fig. 5. J-V characteristics of solar cell with stacked absorber layer (containing  both CZTS and CZTSe) after optimizing (a) Thickness; (b) Carrier Density   TABLE   IV P ERFORMANCE ANALYSIS BY VARYING ABSORBER LAYER CARRIER DENSITY   CZTS carrier density (cm -3 ) CZTSe carrier density (cm -3 ) J SC (mAcm -2 )   V OC  (V) FF (%) Efficiency (%) 10 16  10 18 30.92 0.9913 68.56 21.02 10 17  10 18 29.73 1.0577 70.60 22.21 10 18  10 18 29.23 1.1188 71.70 23.45 10 19 10 18 23.96 1.1758 25.96 7.31 10 18 10 17 29.23 1.1187 71.70 23.44 10 18 10 19 29.23 1.1189 71.71 23.45 TABLE   III P ERFORMANCE ANALYSIS BY VARYING ABSORBER LAYER THICKNESS   CZTS thickness (nm) CZTSe thickness (nm) J SC (mAcm -2 ) V OC  (V) FF (%) Efficiency (%) 1000 2000 24.05 0.9506 66.34 15.17 1500 1500 26.69 0.9675 67.61 17.46 2000 1000 28.49 0.9778 68.14 18.99 3000 1000 30.92 0.9913 68.56 21.02 4000 1000 32.53 0.9987 68.64 22.30 5000 1000 33.68 1.0043 68.66 23.22 6000 1000 34.52 1.0085 68.55 23.87 118
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