*Corresponding author: naman.shukla43@gmail.com

Recently, organic-inorganic perovskite-based solar cells have become a revolution in photovoltaic field due to their unique properties.  Several studies were focused on perovskite solar cells based on Pb perovskite layer as lead provides strong absorption of photons and have high efficiency. However, the factor of toxicity, stability and ecological challenges of these devices is the main challenge to the progress in commercial production. In this, study and numerical modeling of perovskite solar cells using an alternative candidate which is tin as a perovskite material has been carried out. This later is investigated in order to overcome the toxicity, stability and ecological challenges effects on perovskite solar cells, as they exhibit similar photovoltaic performances as Pb-perovskite solar cells. Therefore, the effect of single and double absorbent i.e. CH3NH3SnI3 and CH3NH3SnBr3 and no Hole Transport Layer is studied and investigated to enhance the conversion efficiency of perovskite devices. The obtained simulation results illustrate that perovskite solar cells based on no HTL and double absorbent layer exhibit 21.3% of power conversion efficiency compared to that with other HTL materials. Thus, adding double absorbent layer in perovskite solar cell design possibly will be considered as novel designing for future Sn-perovskite solar cells. The numerical simulation was performed using 1DSolar Cell Capacitance Simulator (1D- SCAPS).

Keywords:

Perovskite solar cells, power conversion efficiency, double absorbent layer, 1D- SCAPS.  

Figure:1. Comparison between lead based and tin based perovskite

Figure:2. Structure of solar cell                                   

As technology is advancing, the need of renewable sources of energy resources is increasing at international level, especially with concerns about the depletion of fossil resources globally. Researchers are trying to expand their area of research to take advantage of these natural resource as Renewable energies allow the delivery of permanent resources, and. Solar energy is One of the most promising renewable energies present (Kabir et al., 2018; Dickinson et al., 2017; gong et al., 1864). So, significant effort is required to develop a new solar cell technology with increased power conversion efficiency (PCE) and reduced processing costs, less harmful to the environment and ecology. Recently, organic/inorganic halide perovskite solar cells have given promising results by providing essential advantages like as high absorption coefficient, long carrier-diffusion length, high carrier mobility, easy fabrication, and in various areas as well as satisfying above proprieties (hima et al.,2019). Which make them a very attractive topic for future solar cell technologies. However, the first perovskite solar cell was developed by Kojima et al. From the Tokyo-based group of Tsutomu Mi yasaka with a PCE of 2.2% in 2006 (kemerchou et al.,2019) and in few years they enhance it to 3.8% (kojima et al.,2009). Hence, in less than one decade, PCE of perovskite-based solar cell jumped from 2.2% 25.7% in 2021 . The rapid progress of Power Conversion Efficiency (PCE) from unstable 3.8% to stable 22.7% within a short span of time, due to the incorporation of hybrid halide perovskite make them promising candidate for commercialization(feng et al.,2018;Il seok et al.,2018). Enhancement in the device structure and perovskite material is the key solution for improvement in the performance of perovskite-based solar cell. Several studies are focused on use of perovskite material methyl ammonium lead triiodide (MAPbI3) perovskite based solar cells. Perovskite is based on ABX3 structure, where A represents methyl ammonium (MA, CH3NH3), B is the lead (Pb) and X represents a halide material anion, iodide (I). in spite the high performance provided by lead halide perovskite material, the factor of instability and toxicity and other ecological factor may hamper its commercial production (oiong et al.,2019; rui et al.,2019). The better way to improve these factors is by introducing an alternative candidate which is ecological and have better performance. The Sn -perovskite material may potentially provide analogous photovoltaic performances similar to Pb-perovskite devices. In this context, our work presents numerical simulations of lead free methylammonium tin tri-iodide (MASnI3)-based solar cell using 1D-Solar Cell Capacitance Simulator (1D-SCAPS) developed at the Department of Electronics and Information Systems (ELIS), University of Gent, Belgium which is a one dimensional solar cell simulator based on the drift diffusion physical model (burgelman et al.,2016). The photovoltaic performances of Sn-perovskite solar cell are compared to that of Pb-perovskite solar cell and the obtained results are validated by experimental ones taken from literature showing a good agreement between them. Therefore, the work is extended to study and investigate the HTL material effect on Sn-perovskite solar cell design.

The basic perovskite solar cell p-i-n structure is presented in Figure. 1. It consists of three different layers, a perovskite absorbing layer (PAL) which is sandwiched between an electron transporting layer (ETL) and a hole transporting layer (HTL). The PAL represents one of both perovskite materials, MAPbI3 or MASnI3. Whereas, the CH3NH3SnBr3 is used as second absorbent layer and TiO2 is inserted as ETL. In perovskite devices, ETL is connected at metal back contact (Ag) and HTL at the transparent conducting indium tin oxide (ITO). Note that the structure is illuminated under AM1.5G solar spectrum with 100 mW/cm2 incident power density. In order to carry out our simulations, different parameter materials related to each layer and collected from recent experimental and simulation studies (zhao et al,.2017;umari et al,.2014; chen et al,.2015; zuo et al,.2015; leguy et al,.2015; zhu et al.,2016) are summarized in Table 1. The pre-factor values,

Aα, of both ETL and HTL is set to 105 to achieve the desired curve of absorption coefficient (α) which calculated using the following expression: 

Besides, optical models used for different PAL materials are taken from experimental results found in literatures. To investigate and study the Sn -perovskite solar cells, numerical simulations are performed to various p-i-n perovskite structures in order to show the influence of each perovskite material on perovskite solar cell performances. Using the methylammonium lead iodide (MAPbI3) as perovskite material in p-i-n perovskite design, simulation results is compared with experimental ones found in literature. Then, the lead-perovskite layer is changed into tin perovskite material at the same p-i-n perovskite design showing the effect of Sn-perovskite material on electrical performance of perovskite solar cell. In addition, the HTL material effect on Sn-perovskite solar cell performance is studied and investigated in order to ameliorate the power conversion of solar cell. Moreover, the appropriate ETL material-based perovskite structure may be considered as novel designing for future Sn-perovskite solar cells.

Table: 1 Material properties relating different layers of perovskite solar cell

Numerical modeling was carried out using 1D-SCAPS software and including various material proprieties given in Table-1. Firstly, the p-i-n solar cell structure, ITO/CuI /MAPbI3/TiO2/Ag, based on Pb-perovskite material ITO/CuI/MAPbBr3/TiO2/Ag, and corresponds to layer thicknesses of 30 nm, 400 nm and 30 nm for HTL, PAL and ETL, and double PAL, ETL respectively is studied and analyzed. Figure. 2 plots the J-V characteristics of both simulated and experimental Pb-based perovskite solar cell structure measured under reverse voltage. As can be seen from the Figureure, the obtained simulated results are very close to experimental ones taken from literature. Therefore, the extracted photovoltaic parameters of both simulated and experimental results are presented in Table 2. These results indicate that no big difference between both simulated and experimental values which validate our model and the parameters used in the simulation. In second time, using p-i-n solar cell structure, ITO/MASnI3/MASnBr3/PCBM/Ag, based on Sn-perovskite material and corresponds to the layer thicknesses of 50 nm, 400 nm and 50 nm for PAL and ETL, respectively. Figure. 3 displays the variation of J-V characteristic of the Sn-perovskite solar cell. The photovoltaic parameters Jsc, Voc, FF and PCE resultant were 31.26 mA/cm2, 0.88 V, 73.89% and 20.38%, respectively. It is necessary to carry out an optimization process of the perovskite device. This is can be down by varying the diverse layer thicknesses, absorber, HTL and ETL layers. In this context, the optimization process consists to fix two-layer thicknesses and vary the remaining one. Then, in each case we select the layer thickness that gives the optimal PCE value. Figure. 4 shows the optimized J-V characteristics of the Sn-based perovskite solar cell. It is evident that the optimized perovskite device shows a noticeable enhancement in PCE from 20.38% to 21.93%. Sn-based perovskite device shows analogous photovoltaic performance comparable to Pb-based perovskite device. Using the same optimization method mentioned above, our simulation is performed on the power conversion efficiency as function of absorber layer thickness, temperature, bandgap, electron affinity of absorbent layer in order to find the optimum values. The optimized value of absorber layer thickness, temperature, bandgap, electron affinity gives higher PCE. Evidently, the PCE increases with increasing of absorber thickness and then decreases after reaching a specified value as shown in Figure. 4. This increasing is due to more photons absorbed by carrier concentration in this layer creating more electron hole pairs and led to high short-circuit current density in the device. In addition, PCE is measured for different temperature and it is noted that with increase in temperature efficiency is decreases as shown in Figure. 5. Lastly, measurement of PCE for different bandgap is carried out and it is noted that with increase in bandgap efficiency is decreases and it works well between 1.5-2.2 eV as shown in Figure. 6.

  Table: 3 Electrical parameters of CH3NH3SnI3 perovskite solar cell with different HTMs

In this paper, no HTL and double absorbent layer of Tin-based perovskite solar cell is studied and investigated using 1D-Solar Cell Capacitance Simulator (1D-SCAPS). The obtained simulation results of Sn-based perovskite solar cell demonstrate similar photovoltaic performance compared to Pb-based perovskite solar cell and there is no big difference between both structures. In addition, double absorbent layer is used and HTL layer is removed and its different effects with change of temperature, bandgap, thickness are studied and discussed. The performance of Sn-based device was effectively improved by inserting extra absorbent layer. Therefore, the perovskite device with CH3NH3SnI3 as second absorbent and no HTL exhibits PCE of 21.93% compared to that with other HTL materials. The obtained results designate that double absorbent layer can be used for improvement of perovskite devices in near future for enhanced efficiency and low costing keeping in mind.