Attainment of Stable (FTO/ZnO/CdS/CH3NH3SnI3/GaAs/Au) Perovskite Solar Cell With Above 23% Photovoltaic Performance Using Solar Capacitance Simulator

Solar energy is found to be low cost and abundant of all available energy resources and needs exploration of highly efficient devices for global energy requirements. We have investigated methyl ammonium tin halide (CH 3 NH 3 SnI 3 )-based perovskite solar cells (PSCs) for optimized device performance using solar capacitance simulator SCAPS-1D software. This study is a step forward towards availability of stable and non-toxic solar cells. We explored all necessary parameters such as metal work functions, thickness of absorber and buffer layers, charge carrier’s mobility and defect density for improved device performance. Calculations revealed that for the best efficiency of device the maximum thickness of the perovskite absorber layer must be 4.2 μm. Furthermore, o ptimized thickness values of (ZnO =0.01 μm) as electron transport layer (ETL), GaAs as hole transport layer (HTL=3.0 2 μm ) and (CdS=10 nm) and buffer layer have provided power conversion efficiency (PCE) of 23.53%. have also been reported in this study as they can play a crucial for the device performance. Insertion of ZnO layer and CdS buffer layers have shown improved device performance and PCE. Current investigations may prove to be useful for designing and fabrication of climate friendly, non-toxic and highly efficient solar cells.


Introduction
Nowadays energy consumption has been increased steadily with civilization development, and in order to keep up with the pace of the development of modern society in the near future, the energy or power consumption will be further increased which might result a crisis situation. In recent years, Inorganic and organo-halides solar cells with perovskite absorber materials have drawn tremendous considerations in the photovoltaic community because of their outstanding photoelectric performance, high electrical parameters such as current density and quantum efficiency and low manufacturing cost [1][2][3]. The physical, mechanical and optoelectronic properties of perovskite materials are recommended for the PV application. Researchers have analyzed some of these properties by using first-principle method with density functional theory (DFT) [4,5]. A typical perovskite solar cell employed organic-inorganic halide material as active material [6,7]. Although, these types of materials exhibit efficiency of more than 20% but, there are two main concerns associated with it. First, these kinds of materials consist of organic cation which create instability and thereby suppressing the life time of the perovskite material and second is the toxicity of lead (Pb) which is highly hazardous to our environment [8]. On the other hand, the energy band gap of inorganic-halide perovskite (Sn, Ag, Sb, Bi, Cu) based solar cells is more than 2eV which is less suitable for the PV application [9,10]. Furthermore, some other challenges are also linked with such perovskite materials. For example, the open-circuit voltage of Sn 2+ cation and Sb is quite low, oxidation of Ge 2+ cation makes it unstable, Bi has poor charge transport ability; Cu has inferior PV properties etc. To date, the maximum PCE of perovskite based solar cell is recorded 25.2% which is quite greater than the efficiency recorded 3.8% in 2009 [11,12]. This tremendous achievement has brought perovskite solar cells in photovoltaic market as compare to Si-based solar cells [13]. However, some hole-transporting material materials (HTM) like Spiro-OMETAD has restricted the perovskite absorber materials based solar cells to market because of their heavy manufacturing cost and suppression of long time stability [14,15]. Scientists in the field of perovskite solar cells have obtained better advancement of high mobility, absorption co-efficient and tunable band gap of the absorber layer but still some main problems like stability degradation, toxic nature of lead, hysteresis, high cost and high-power conversion efficiency are challenged. So, there is inexorable need to optimize the parameters of perovskite solar cells for better performance at low manufacturing cost. An active way for achieving the required results is to optimized performance of Perovskite solar cells, typically Organo-Halide Perovskite absorber materials e.g., CH3NH3XY3 (X=Pb, Sn, Ge & Y= Cl, Br, I) in which the major focus is to reduce toxicity and enhance stability. Several compositional and structural derivatives of perovskite family including layered Ruddlesden-Popper perovskites and double perovskites are needed to be explored on the basis of best available computational and experimental resources. Hui-Jing Du et.al, studied the electrical parameters and device structure stability of CH3NH3SnI3 perovskite absorber layer by simulations. The electrical parameters such as Voc, Jsc, FF and PCE was found to be 0.92V, 31.59 mA/cm 2 , 79.99% and 23.36% respectively. Furthermore, CH3NH3SnI3 perovskite absorber material was found more efficient than CH3NH3PbI3 due to non-toxicity and Sn +2 stability of Sn element in CH3NH3SnI3 structure [16]. Nacereddine Lakhdar.et [22]. In lead free MAGeI3 based perovskite solar cell showed better combination of device parameters with 18.03% efficiency in ITO/ZnO/MAGeI3/Spiro-OmeTAD/Au configuration [23]. CH3NH3SnI3 is a lead-free inorganic perovskite material. This material is suitable for light absorption layer due to its low energy gap of 1.35 eV, high absorption coefficient and high hole mobility of 10 4 cm −1 and 585 cm 2 / V −1 s −1 respectively at room temperature. Therefore, Sn 2+ is highly recommended in halide perovskite solar cells because of their excellent photoelectric performance and also Sn 2+ is a non-toxic cation as compared to Pb 2+ ions [24][25][26].
In our current research work, CH3NH3SnI3 based PSCs have been studied and the influences of temperature, thickness of absorbing different layers i.e., absorber, HTL, ETL, defect density and interface defects, carrier recombination and energy band gaps have been reported for optimum device performance. The fundamental objective of our current research work is to optimize all possible characteristic parameters of PSCs precisely in such a way that we can get a high PCE of the device and reduce the fabrication cost.

Device Simulation
The device modeling and optimization of PSC can be done by using SCAPS-1D software version (3.3.08) [27]. It was developed by university of Gent, Belgium in which the algorithm is based on three different coupled partial differential-equations (PDEs), namely, Poisson's equation, and Up (n, P) are the recombination rates of electrons and holes respectively. Furthermore, carrier current density may also be obtained from; Here q is the charge, are carrier mobilities, and , are the diffusion coefficients.
We can find different parameters of a solar cell through simulations such as current density (Jsc),

Simulation methodology
The architecture model and device structure used for simulated solar cell And the thickness of each layer was optimized for maximum output.
The interfacial defect layers CdS/CH3NH3SnI3-Peroskite and CH3NH3SnI3-Peroskite/GaAs were also taken for simulations purpose with parameters shown in  The carrier mobility of the absorber layer has a remarkable effect on the device performance.  Fig.6 shows the effect of interface defect density versus PCE curves for optimized perovskite solar cell all interface layers. From the graphs, it can be deduced that increasing interface defect densities, the recombination rate also improves which in turn reduces the PCE.
The higher defect densities of the interfaces bring more traps and recombination centers, and deteriorate the performance of cells. So, it can be realized from the simulated results that interface defect density of 1 × 10 10 cm −3 is optimum for device simulation. Fig.7 shows the relationship of carrier's recombination (cm -3 .s) and diffusion length (µm) of the optimized solar cell. Geh of 1.346×10 22 cm -3 .s is found to be maximum at 7.22 µm. Also, it is clear that with the enhancement of diffusion length, carrier's recombination also increases and thus will degrade the performance of solar cell [31]. Furthermore SHR recombination and total recombination are approximately equal and are found to be maximum at 9.

4.2Effects of ZnO Electron Transport Layer Thickness
The effect of thickness of the electron transport layer (ZnO) on device performance has been studied using SCAPS-1D. It has been observed that as the thickness of ZnO Layer increases from 0.010 μm to 2.000 μm in seven steps, the open circuit voltage (Voc), short-circuit current density (Jsc), Fill Factor (FF) and quantum efficiency (eta%) is slightly reduced and then become constant from 2.00 μm to 4.00 μm. The variation of electrical parameters (Voc, Jsc, FF% and PCE%) versus thickness for ZnO layer correspond to layer thickness is illustrated in Fig. 9. The maximum values of electrical parameters such as efficiency, short-circuit current density, open circuit voltage and Fill Factor were recorded 15.65%, 29.758527 mA/cm 2 , 0.7572 V and 69.44% respectively at 0.010 μm. It means that the electrical parameters of the organic-inorganic perovskite-based solar cell are not too much affected by electron transport layer (ETL) [32]. This is explained on the basis of the fact that perovskite compound itself could help the generation of charge carriers by photon excitation and ETL layer is just a charge transport layer.

Effects of CdS Buffer Layer Thickness
The variation of thickness of the CdS (buffer layer) corresponds to different electrical parameters were checked and presented in Fig. 10.  because the enhanced efficiency is due to the increase in the current density as the light-absorber layer thickness is increased. The increase in absorber layer thickness helps in enhancing the carrier generation due to more exposure of the absorber material to light [33][34][35]. More electronhole pairs are generated and thereby electron mobility is increased as shown by the increased Jsc.

Effects of CH3NH3SnI3 Absorber Layer Thickness
The only electrical parameter Fill Factor (FF%) initially increases up to 80.26% at 3.40 μm but then gradually decreases as the layer thickness is increased up to 5.0 μm. This can be due to the recombination of the charge carriers and reduction in lifetime of the charge carrier in the CH3NH3SnI3-perovskite layer [36]. The recorded electrical parameters for various layer thicknesses were rechecked and confirmed.

4.5Effects of GaAs Hole Transport Layer Thickness
Properties like high efficiency, flexibility and light weight, resistance to UV radiation and moisture and a low temperature coefficient make gallium arsenide (GaAs) more favorable than the ubiquitously-used silicon for solar cells. In this study, GaAs is acting as a hole transport layer in device structure as shown in Fig. 10. The variation of thickness of GaAs layer from 0.050 μm to 6.00 μm has been studied on perovskite solar cell. The maximum power conversion efficiency FF% and PCE% can be seen in Fig. 12. All the electrical parameters such as Jsc, PCE, Voc and FF% correspond to layer thickness of GaAs layer were rechecked and confirmed.

Effects of Temperature on Optimized CH3NH3SnI3-based Solar Cell
Solar cells are greatly affected by temperature T(K) when exposed to sunlight. It is important to investigate the performance of optimized solar cells as a function of temperature T(K). The temperature dependence electrical parameters of optimized perovskite solar cells have been shown in Fig.13. There is initially increasing trend in the FF% and after 300K there is continuous decrease in its value. Practically, the rise in temperature give rise to carrier creation but, at the

Conclusion
In