Numerical analysis of graded band gap CZT(S,Se) solar cells using AMPS-1D

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1 Numerical analysis of graded band gap CZT(S,Se) solar cells using AMPS-1D Abdulkader Jaleel Muhammad 1, Ali Ismail Salih 2, Ginan Fadel 3 1 Assistant Professor/University of Kirkuk 2 Assistant Professor /University of Kirkuk 3 B. Sc. / Dept. of Physics, College of Science, University of Kirkuk, Kirkuk, Iraq. ABSTRACT The one-dimensional simulation program called Analysis of Microelectronic and Photonic Structures (AMPS-1D), was used to simulate the performance of some novel solar cell structures. The cells based on Copper-zinc-tin-sulphide (selenide), Cu 2ZnSn(S,Se) 4 as absorber layer, cadmium sulfide (CdS) as a buffer layer, un-doped ZnO (i-zno) and F-doped SnO 2 (FTO) as a window layer. The effect of grading the band gap through changing the value of x in the composition CZTS X, Se 1-X is investigated. Improved performance, in terms of efficiency was observed. An efficiency of about 30% for a graded-band cell is reported relative to non-graded cells where the efficiencies are around 20%. Keywords: Solar cell, CZT(S,Se) thin film, graded band gap, AMPS-1D. 1. INTRODUCTION Copper-zinc-tin-sulphide (selenide), Cu 2 ZnSn(S,Se) 4, composition is usually obtained by replacing the indium (In) in chalcopyrite CuInS2 with zinc (Zn) and tin (Sn) [1]. It is one of the most promising materials for the newer generation of solar cells, because it depends on materials which are abundant and inexpensive (compared with CIGS and CdTe). CZTS(Se) is a quaternary semiconductor which can form crystal structures that are classified as kesterite type and stannite type. The difference between these two types is in the order of cation layers. In kesterite type the cation layers of CuSn, CuZn, CuSn, and CuZn alternating at z=0, 1/4, 1/2, and 3/4, planes respectively (z=fractional coordinate along the long c-axis of the structure), while in the stannite type the ZnSn layers alternate with Cu 2 layers. Chen et al. [2] using first-principle calculations showed that the kesterite structure has a lower energy than the stannite structure for CZTS(Se), so that most CZTS(Se) crystals crystallize in kesterite structure. CZTS(Se) is a semiconductor that have optimal direct band gap in the range ev for CZTS (0.9eV -1eV for CZTSe), and absorption coefficient larger than 10 4 cm -1 [3,4], so that it can be highly promising absorber layer in solar cell applications. CZTS(Se) is self-doping through the formation of intrinsic defects that include the vacancies (V cu, V zn, V sn, V s ), interstitials (Cu i, Zn i, Sn i, S i ), and the antisites (Cu Zn, Zn Cu, Zn Sn, Sn Zn, Cu Sn, etc.) [5]. Existence of point defects lead to the self-doping and intrinsic p-type conductivity of CZTS(Se), Chen and co-workers have thoroughly studied the defect characteristics of CZTS(Se) [6] and calculated the defect formation energies of various isolated point defects (vacancies, interstitials and antisites) as a function of Fermi energy, All investigations converge to the general agreement that the acceptor defects such as Cu Zn or Cu Sn antisites and the Cu vacancy (V Cu ) have lower energy of formation with the lowest formation energy for Cu Zn antisite defect then donor defects, Such low formation energies of the acceptor defect levels, particularly Cu Zn antisite, make p-type conductivity is more possible of Cu2ZnSnS4 [5,7,8] and Cu2ZnSnSe4 [9]. CZT(S,Se) thin film based solar cells has achieved great interest during the past period [10]. The performance of the solar cell can improve by incorporating graded band gap which helps to absorb a larger amount of radiation of different wavelengths. In this paper we study the improvement which might result from using graded band gap by changing x in the composition; CZT(S X, Se 1-X ) ; where x represents the ratio {[S]/( [S] + [Se])}. Through changing x from 0 to 1, the band gap of CZTSSe is tuned from 0.9eV for Cu 2 ZnSnSe 4 (CZTSe) to 1.5eV for Cu 2 ZnSnS 4 (CZTS). We will use AMPS-1D (Analysis of Microelectronic and Photonic Structures) [11] to perform the numerical simulation. Figure 1 shows the schematic of the geometrical structure of the solar cell design studied in this work. The aim of this study is to improve upon the performance of CZT(S,Se) thin film solar cells. Volume 4, Issue 10, October 2016 Page 28

2 Figure 1 CZT(S x, Se 1-x ) solar cell structures used in the simulation, a single absorber-layer (non-graded band) on the left and a 3-absorber-layer (graded band) on the right. 2. DESCRIPTION OF AMPS-1D AMPS is a software that has been developed by Stephen Fonash [12] and his collaborators at the Pennsylvania state university to solve the system of highly complex and nonlinear equations that characterize a photovoltaic device under different operation conditions (thermodynamic equilibrium, voltage bias, etc.). The AMPS-1D program can operate in two different modes: the density of state (DOS) mode or the lifetime mode, the details can be found in the AMPS manual [11]. In this study we used the DOS mode. To model the charge transport processes in the solar cell device shown in the figure 1 the three nonlinear equations (the Poisson's equation, continuity equation for free electrons, and continuity equation for free holes) must be solved, the Poisson's equation [13] is: Where ε is dielectric permittivity of semiconductor, Ψ is electrostatic potential, q is electron charge, n, p are the concentrations of free electron and free hole respectively, ND +, NA are the concentrations of ionized donors and acceptors, pt. nt are the concentrations of trapped holes and electrons. While the continuity equations in steady state conditions are: Where J n, J p are electron and hole current density, R n, R P are electrons and holes recombination velocities of direct bandto-band and indirect transitions, G is the optical generation rate which can be expressed by: Volume 4, Issue 10, October 2016 Page 29

3 where, FOR i. REV i are, respectively, the photon flux of the incident light and the light reflected from the back surface at wavelength, λ of i at some point x, depending on the light absorption coefficient, and the light reflectance in the forward and reverse direction. The three governing equations (1), (2), and (3) must hold at all point in the solar cell, and the solution to these equations includes determining the state variables Ψ(x) and the electron quasi-fermi level E Fn, and the hole quasi-fermi level E Fp or, equivalently, Ψ(x), n(x), and p(x), which are known as a function of depth. There must be boundary conditions imposed on the set of equations. The solution to equations (1), (2), and (3) must satisfy the following boundary conditions. Where,, are effective interface recombination speeds for holes and electrons at x=0 and x=l. Newton-Raphson technique is used in AMPS-1D. Solving these equations we can get the carrier concentrations, electric fields and currents, and device parameters like the short circuit current (Jsc), the open circuit voltage (Voc), the fill factor (FF) and efficiency which defines the performance of the solar cell. 3. MODELING AND SIMULATION To simulate the solar cell model that seen in the figure 1 by AMPS-1D and obtain the results, we must enter the parameters for each layer in the structure. Figure 2 displays the user interface of AMPS-1D. In this study we investigated four different cases (solar cell structures). The first case consists of 4-layers plus the contacts; that is: TCO/i-ZnO/n-CdS/pCZTS with no selenium (x=1 in the general formula CZTS X Se 1-X ). The second case consists of 5layers plus the contacts; that is: TCO/i-ZnO/n-CdS/p-CZTSe/p-CZTS. In this structure the ptype material is made of two layers (x=0) and (x=1). This introduces a gradient in the energy gap from (0.9 ev) to (1.5 ev). In the third case we add a layer with x=0.5 between the x=0 and the x=1 layers so that the cell consists from 6-layers plus the contacts; that is: TCO/iZnO/n-CdS/p-CZTSe/p-CZTS 0.5 Se 0.5 /p-czts. The final case consists of a 4-layer cell plus the contacts, that is: TCO/i-ZnO/n-CdS/p-CZTSe with no Sulphur (x=0 in the general formula CZTS X Se 1-X ). TCO layer is transparent conductive oxide in this study we take it F-doped SnO 2 (FTO). the thickness of the p-layer (the absorber layer) are kept constant even when using gradient structures. The parameters and the band gap for CZTS X Se 1-X were calculated using the known parameters of (CZTS), (CZTSe) plus the following equations: Eg(x) =x Eg(CZTS) + (1-x) Eg(CZTSe) b x (1-x) (6) K(x) = x K(CZTS) + (1-x) K(CZTSe) (7) Where Eg(CZTS) is the band gap energy for CZTS, Eg(CZTSe) is the band gap energy for CZTSe, and b is a band bowing parameter and it is equal to 0.07 for CZTSSe [14], K in eq.7 represents any parameter we want to calculate, e.g. absorption coefficient. Table 1 show the description of the parameters required in the simulation together with the values which are used in this study. While table 2 shows the thickness of the layers. The total absorber layer thickness is kept 3000nm for all the cases. Table 3 is the parameters needed for the front and back contacts in the AMPS-1D program, the contacts are ohmic contacts. Volume 4, Issue 10, October 2016 Page 30

4 Figure 2 AMPS-1D simulation front panel contains the device and general layer parameters. Table 1: Material parameters needed for the layers that used in this study [15,16] Layers FTO i-zno CdS CZTS CZTSe CZTS 0.5 Se 0.5 parameters Dielectric constant, Conductivity type N N N P P P Electron mobility, µn (cm 2 /V s) Hole mobility, µp (cm 2 /V s) Free Carrier concentration, N A or N D (cm -3 ) (D) 1e+20 (D) 1e+18 (D) 1.1e+18 (A) 4e+16 (A) 5e+16 (A) 4.5e+16 Band gap, Eg (ev) density of state, Nc (cm -3 ) 1.2e e e e e e+18 density of state, Nv (cm -3 ) 7e e e e e e+19 Electron affinity, א (ev) Table 2: Thickness of each layer in the cases used in this study. The thickness of FTO, i-zno and CdS layers are kept constant at 130, 80, and 60 nm respectively. Layer thickness Case 1 Case 2 Case 3 Case 4 CZTS 3000 (nm) 2950(nm) 1950(nm) 0 CZTS0.5 Se (nm) 0 CZTSe 0 50 (nm) 50(nm) 3000 (nm) Volume 4, Issue 10, October 2016 Page 31

5 Table 3: Parameters used for the front and back contacts (note: the back contact work function in the case4 equal to 0.7) parameters Front contact (O) Work function, PHIBO, or PHIBL (ev) 0 1 Back contact (L) Surface recombination speed for electrons, SNO, or SNL (cm/sec) 1e+7 1e+7 Surface recombination speed for holes, SPO, or SPL (cm/sec) 1e+7 1e+7 Reflection coefficient, RF, or RB RESULTS AND DISCUSSION 4.1 Effect of graded band gap Table 4 summarizes the results of calculating the important parameters of the solar cell that is Voc, Jsc, FF and the efficiency. Figure 3 shows the J-V characteristics and Figure 4 displays the energy band diagrams in the graded band gap in case 3. The cases are as follows: case 1 is TCO/i-ZnO/n-CdS/p-CZTS, case2 is TCO/i-ZnO/n-CdS/p-CZTSe/p- CZTS, case3 is TCO/i-ZnO/n-CdS/p-CZTSe/p-CZTS 0.5 Se 0.5 /p-czts, and case 4 is TCO/i-ZnO/n-CdS/pCZTSe. By taking different cases for CZTS X, S 1-X thin film solar cell with graded absorber layer (cases 2 and 3) we can observe from the results of the numerical simulation a notable increase in the efficiency of solar cell of structure of case 3. The efficiency of case 3 is about 30% meanwhile that of the cases 1 and 2 is about 20% and 22% and that of case 4 is about 17.2%. The increase of the efficiency of case 3 can be attributed to a noticeable increase in Jsc. The increase in Jsc observed in case 3 which is a graded band gap structure; (Eg varies from 0.9 to 1.2 to 1.56 ev in three consecutive layers), may be due to an effective electric field introduced due to the variation in Eg. This field will cause the drift of electrons which are minority carrier in a p-type material toward the n-type material and increase the collection, which result in an increase in the observed current density. Table 4: Performances parameters of different solar cell cases, the shaded area represents graded-band cells. cases No. of absorber layers Voc Jsc (ma/cm^2) FF Eff % Case Case Case Case Figure 3 JV- curve for graded band gap solar cells (case-3) obtained using AMPS-1D. Volume 4, Issue 10, October 2016 Page 32

6 Figure 4 The energy band diagram for graded band gap solar sell (case 3; 3-absorber-layers). 4.2Effect of CZTS 0.5 Se 0.5 layer thickness As it was shown in the previous paragraphs, the cell structure called case-3, that is the 6layer plus the contacts structure: TCO/i-ZnO/n-CdS/p-CZTSe/p-CZTS 0.5 Se 0.5 /p-czts, exhibit the highest efficiency of about 30%, therefore it was desirable to investigate the effect of the thickness of the intervening layer (CZTS 0.5 Se 0.5 ) on the efficiency and on the overall performance of the cell. For this purpose, we simulated the characteristics of cells with various (CZTS 0.5 Se 0.5 )- thickness, ranging from 50 nm to 2000 nm. Figure 5 displays the efficiency-versus- thickness characteristic of these cells. We can distinguish two regions of CZTS 0.5 Se 0.5 thickness which influence the electrical parameters (especially Jsc & Eff). The first region where the thickness is > 1000 nm; in this region the short circuit current density Jsc and the efficiency Eff increase with thickness, while the second region where the thickness is < 1000 nm, we observe that Jsc and Eff are almost constant. This tendency towards a saturation value may be explained on the basis that as the thickness increases, more and more incident photons are absorbed, hence Jsc and Eff increase till where almost all the incident photons are absorbed, then any further increase in the thickness is an economic loss because it does not improve the efficiency. The optimum thickness is something about 1000 nm. For the sake of comparison, we investigated the effect of thickness of (CZTS 0.5 Se 0.5 ) absorber layer on the performance of cells consisted from 4-layers plus the contacts, e.g.: TCO/i-ZnO/n-CdS/p-CZTS 0.5 Se 0.5, the absorber layer thickness is varied from 50 to 4000 nm, while keeping everything else constant. Figure 6 shows the result of the simulation, only curves of efficiency are shown due to space limitations. Voc increases from about 0.38 V to about 0.79 V, Jsc increases from about 17.6 to about 32.9 ma/cm 2, while the efficiency increases from about 5% to about 22%. Note Eff for this structure at a thickness of 1000 nm is about 19%, while its value for the same thickness for case-3 cell (TCO/i-ZnO/n- CdS/pCZTSe/p-CZTS 0.5 Se 0.5 /p-czts) is about 30%. This result indicates the advantage of the gradedband structure over the single-layer structure. The increase in Voc in the single-layer structure with the increase of thickness may be due to increase of the depletion region thickness available for charge redistribution. In comparison Voc in the gradedband cell was almost constant at about V. Volume 4, Issue 10, October 2016 Page 33

7 Figure 5 Effect of the thickness of CZTS 0.5 Se 0.5 layer on the graded-band cell efficiency Eff (%) CONCLUSIONS thickness(nm) Figure 6 Effect of CZTS 0.5 Se 0.5 absorber thickness on the efficiency of a single-layer cell. We have used AMPS-1D to study the performance of CZT (S X, Se 1-X ) thin film solar cells. It was observed that the efficiency will increase by using graded band gap. The band gap can be changed through the change of the composition parameter x {x= [S]/([S]+[Se])}. A triple ptype absorber layer of 1950nm thick CZTS (Eg=1.56), 1000nm thick CZT (S 0.5, Se 0.5 ) (Eg=1.2 ev) and a 50nm thick CZTSe (Eg=0.96 ev) layers exhibit an efficiency of about 30% relative to non-graded structures where the efficiencies are around 20%. This simulation shows that grading the energy gap could have a remarkable impact on the increase in the photogenerated current and reduce the recombination in the back contact resulting in a significantly improved efficiency. Volume 4, Issue 10, October 2016 Page 34

8 Acknowledgements We would like to acknowledge the use of AMPS-1D program that was developed by Dr. Fonash's group at Pennsylvania State University (PSU). We also would like to express our thanks to University of Kirkuk, and one of us (G. F.) would like to thank university of Kirkuk, college of Science for the study grant. References [1] Nowshad Amin, Mohammad Istiaque Hossain, Puvaneswaran Chelvanathan, A.S.M. Mukter Uzzaman, Kamaruzzaman Sopian, " Prospects of Cu2ZnSnS4 (CZTS) Solar Cells from Numerical Analysis ", 6th International Conference on Electrical and Computer Engineering ICECE, pp , [2] Shiyou Chen, X. G. Gong, Aron Walsh, and Su-Huai Wei, " Crystal and electronic band structure of Cu2ZnSnX4 X=S and Se photovoltaic absorbers: First-principles insights", APPLIED PHYSICS LETTERS, Vol. 94, No , [3] Kentaro ITO, Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells, John Wiley & Sons, Ltd, Nagano, Japan, [4] Jonathan J. Scragg, Copper Zinc Tin Sulfide Thin Films for Photovoltaics, Springer, Ph.D. theses, University of Bath, UK, [5] Shiyou Chen, Ji-Hui Yang, X. G. Gong, Aron Walsh, Su-Huai Wei," Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4", PHYSICAL REVIEW B, Vol. 81, No , [6] Shiyou Chen, Aron Walsh, Xin-Gao Gong, and Su-Huai Wei," Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth-Abundant Solar Cell Absorbers", Adv. Mater. Vol. 25, pp , [7] Leitao, J.P., Santos, N.M., Fernandes, P.A., Salome, P.M.P., da Cunha, A.F., Gonzalez, J.C,. Ribeiro, G.M., Matinaga, F.M.," Photoluminescence and electrical study of fluctuating potentials in Cu2ZnSnS4-based thin films", PHYSICAL REVIEW B 84, No , [8] Katagiri, H., Ishigaki, N., Ishida, T., Saito, K.," Characterization of Cu2ZnSnS4 thin films prepared by vapor phase sulfurization", Jpn. J. Appl. Phys. Vol.40, pp , [9] Wibowo, R.A., Kim, W.S., Lee, E.S., Munir, B., Kim, K.H. "Single step preparation of quaternary Cu2ZnSnSe4 thin films by RF magnetron sputtering from binary chalcogenide targets", J. Phys. Chem. Solids, Vol. 68, pp , 2007 [10] Xiangbo Song, Xu Ji, Ming Li,Weidong Lin, Xi Luo, and Hua Zhang " A Review on Development Prospect of CZTS Based Thin Film Solar Cells ", International Journal of Photoenergy, No , [11] S.J. Fonash, " A manual for One-Dimensional Device Simulation Program for the Analysis of Microelectronic and Photonic Structures (AMPS-1D)", (The Center for Nanotechnology Education and Utilization, The Pennsylvania State University, University Park, PA 16802). [12] Stephen J. Fonash, Solar Cell Device Physics", Second Edition, Elsevier Inc [13] S.M. Sze and Kwok K. Ng, Physics of semiconductor devices, Third Edition, John Wiley & Sons, Inc., [14] Jianjun Li, HongxiaWang, MiaoLuo, JiangTang, ChengChen, WeiLiu, FangfangLiu, YunSun, JunboHan, YiZhang," 10%Efficiency Cu2ZnSn(S,Se)4 thin film solar cells fabricated by magnetron sputtering with enlarged depletion region width ", Solar Energy Materials & Solar Cells, No.149,pp , [15] Ramprasad Chandrasekharan," NUMERICAL MODELING OF TIN-BASED ABSORBER DEVICES FOR COST- EFFECTIVE SOLAR PHOTOVOLTAICS ", John and Willie Leone Family Department of Energy and Mineral Engineering, Ph.D. theses, Pennsylvania State University, [16] O.K. Simya, A. Mahaboobbatcha, K. Balachander, " A comparative study on the performance of Kesterite based thin film solar cells using SCAPS simulation program", Superlattices and Microstructures, Volume 4, Issue 10, October 2016 Page 35

9 AUTHOR Abdulkader Jaleel Muhammad was born in Kirkuk, Iraq, in He received the B.S. and M.S. degrees in physics from University of Baghdad, Iraq, in 1971, and 1978, respectively. He obtained the Ph.D. degree in Electronic Materials and Devices, from Department of Electrical & Electronic Engineering, University of Leeds, UK, in He worked as a staff member in University of Salahaddin-Hawler in Arbil, Kurdistan Region-Iraq. From he worked as a senior scientist in a Scientific Research Center in Baghdad, Iraq. From he was an Assistant professor in Department of Electrical and Electronic Engineering, University of Garyounis, Benghazi, Libya. From he held the post of Head of the Department of physics, College of science, University of Kirkuk, Kirkuk, Iraq. His current interest includes electronic device physics especially MOSFETs, electrical and optical properties of electronic materials, high temperature superconductivity, solar cells, photovoltaics, and power electronics. He recently retired from University of Kirkuk. Ali Ismail Salih has been awarded the degree of B.Sc. in Applied Physics from University of Technology in academic year ( ). M. Sc. degree in Applied physics (Material science) from University of Technology in 1998 Title of Thesis: decolourization of Iraqi glass. Ph. D. degree in Applied physics (Material techniques) from University of Technology in 2008 Title of Thesis: Preparation of metal matrix composites- MMCs by different techniques and comparison between them). He has been a Member of Iraqi physics and Mathematics Society in Between years ( ) worked as anengineering inspector in Iraqi North oil company (NOC). In years ( ) worked as Assistant lecturer at Kirkuk University-College of Science-Physics Department. In (2012) as a Lecturer at Kirkuk University-College of Science-Physics Department. From as Assistant professor till this date.he has published more than ten papers research in Applied physics. Ginan Fadel Muhammad Ali has been awarded the degree of B.Sc. in physics from University of Kirkuk in academic year ( ), She prepares her MSc. thesis in physics in college of science university of Kirkuk Volume 4, Issue 10, October 2016 Page 36