Investigation on Design and Device Modeling of High Performance CH₃NH₃PbI_3-xCl_x Perovskite Solar Cells

Naman Shukla^*, Dharam Lal, Sanjay Tiwari

School of Studies in Electronics and Photonics, Pt. Ravishankar Shukla University, Raipur-492010, C.G., India

*Corresponding author: naman.shukla43@gmail.com

[Received: 27 April 2021; Accepted: 31 May 2021]

Abstract: Perovskite solar cell fabricated with inexpensive & simple technology exhibits high efficiency has witnessed worldwide boom in research. The optimization of solar cell can be done through modeling and simulation. The optical and electrical modelings are the ways to optimize different parameter such as thickness, defect density, doping density and material selection for fabricating stable and highly efficient perovskite solar cells. In this research work, electrical modeling of solar cell is done through Solar Cell Capacitance Simulator (SCAPS-1D). The architecture of the solar cell is n-i-p device structure. CH₃NH₃PbI_3-xCl_x acts as light absorber active layer, TiO₂ as electron transport layer and Spiro-OMeTAD as hole transport layer with device structure FTO/ TiO₂/ CH₃NH₃PbI_3-xCl_x/ Spiro-OMeTAD /Au. The open circuit voltage V_oc, short circuit current density Isc, fill factor and power conversion efficiency are 1.28 V, 21.63 mA/cm², 0.78 and 21.53% respectively. The result showed that the optimize parameter can be applied for fabrication of the solar cell experimentally. Various metal contact materials of the anode are also studied and analyzed.

Keywords: Perovskite solar cell, modeling, simulation, SCAPS-1D, fabrication.

Introduction

Organic Inorgnic lead halide Perovskites are promising solar materials with better light absorption coefficient (Green et al., 2014)and tunable band gap of 1.5 to 2.0 eV (Eperon et al., 2014; Tan et al., 2014). Among the various perovskite materials, organic inorganic halide perovskite has the common molecular formula ABX₃, where A is an organic cation such as CH₃NH₃⁺, B is a metal cation (e.g. Sn²⁺ or Pb²⁺ etc.) and X is a halogen anion (e.g. F^-, Cl^-, Br^- or I^- etc.) (Giorgi et al., 2013). Methyl ammonium lead iodide (CH₃NH₃PbI₃) is the most commonly used perovskite material, which is synthesized by organic compounds i.e. methylammonium iodide (MAI) and the lead salt (PbI₂ or PbCl₂). An efficiency of around 3.8% was reported by Miyasaka and his colleagues in 2009 (Kojima et al., 2009). This was the first time when organometal halide perovskite (CH₃NH₃PbI₃) used as solar material. It was used as a dye in dye-sensitized solar cell (DSSC) architecture. Within three years, Mike Lee et al. achieved 9.7% PCE by replacing the DSSC liquid electrolyte from a solid transport layer (Lee et al., 2012). In this device, mp-TiO2 and spiro-MeOTAD were used as electron transport layer and hole transport layer respectively. Burshka and his colleagues (Burschka et al., 2013)presented a technique through which the cell efficiency increased by   more than 6% (η_{cell ~}15%). In this technique, two-step layer processing is employed in sensitized architectures. In 2019, Zhou et al. reported 19.3% efficiency by controlling the carrier mobility of the charge transport layer in a planar heterojunction perovskite solar cell. Yang et al. achieved an efficiency of more than about 20% based on formamidinium lead iodide (FAPbI3) by controlling intramolecular exchange and error. It was used on Planner Thin Film Architecture (Yang et al., 2015; Yang et al., 2017).Approximate 25% power conversion efficiency has been reported recently using organic-inorganic halide Perovskite material (National Renewable Energy Laboratory). Multifold increase in work efficiency during short span of research period attracts the interest of researcher across the globe. The solar cell manufactured using simplest technological process is low cost and proposed as the most suitable candidate for solar cell materials. Properties like transparent layer enables Perovskite to be used with silicon solar cell in tandem structure (Filipic et al., 2015; Shi et al., 2015). Besides having numerous favorable properties the Perovskite has possesses limitation for working in humid condition which adversely affect the efficiency and abruptly reduces the stability. Exposure to UV and temperate zone degrades the perovskite material which ultimately reduces the generation. This gives rise to research for scientist and researcher working on Perovskite worldwide. Generally, Methyl ammonium (MA+) and formamidinum (FA+) are used as organic cation and lead is used as inorganic cation (Pb⁺⁺). We used various halide (Cl^-, I⁻, Br⁻ etc.) for band gap modulation which assist in energy harvesting. Thin transparent film of ZnO and TiO₂ is used widely as electron transport layer and PEDOT:PSS, Spiro-OMeTAD, CNT is used as hole transport layer. The stable and efficient perovskite solar cells can be developed through optimization of the solar cell device through experiments supported by data which is obtained by optical and electrical simulations. In this research work, the numerical simulation of the CH₃NH₃PbI_3-xCl_x based perovskite solar cell was done by using Solar Cell Capacitance Simulator (SCAPS-1D). It is a one dimensional solar cell simulator based on the drift diffusion physical model(Burgelman et al., 2000). The model adopted is built on transfer matrix method (TMM) (Tiwari et al., 2018) for the simulation of PSC. This software tool has executed the semiconductor equation, the continuity equation of carrier, poisson equation, carrier transport equations etc..

The SCAPS software gives us the results by solving the basic semiconductor equations, Poisson equations, the continuity equations for electrons and holes and other carrier transport constraints (Burgelman et al., 2013). Organometal halide perovskite is used as active layer in this simulation. Device simulations have been performed according to the parameters given in the literature summarized in Table 1 (Lin et al., 2014; Hima et al., 2019; Calado et al., 2016; Isoe et al., 2020).The n-i-p device structure is FTO/ TiO₂/ CH₃NH₃PbI_3-xCl_x/ Spiro-OMeTAD /Au. For optimized design of the solar cell for above structure, short circuit current density J_sc, open circuit voltage V_oc, fill factor FF and efficiency are 21.63 mA/cm², 1.28 V, 0.78 and 21.53% respectively. 

Table 1. Parameter of different layers used for simulation

 Result and discussion

The structure consists of a light absorbing Perovskite layer, n-type material as electron transporting layer (ETL) and p-type material as hole transporting layer (HTL). The light absorbing layer was sandwiched between ETL and HTL. ETL is connected at metal back contact (Au) and HTL at the transparent conducting FTO. The whole simulation of the PSC was done under Standard Testing conditions (AM1.5G, solar irradiance of 1000 W/m² and 25°C). The different parameters for the materials used in the solar cell for simulation are summarized in Table 1. The architecture of FTO/ TiO₂/ CH₃NH₃PbI_3-xCl_x/ Spiro-OMeTAD /Au perovskite solar cell is n-i-p type and shown in figure 1. The layer thicknesses of ETL, light absorbing material and HTL were taken as 40 nm, 350 nm and 200 nm respectively. Energy band diagram and quantum efficiency was plotted as shown in figure 3 and figure 4 respectively. Figure 3 shows the energy band diagram for the fabricated solar cell obtained from the simulation software. It is observed that the most of radiation can absorb by the lower band gap. This is cause of more electrons and holes are generated in the cell.  This will lead to significant improvement in current generated as well as device parameter (fill factor and efficiency). Quantum efficiency is an important parameter for the characterization of solar cell.  In Fig. 4, It showed that the good percentage of photons that are being converted to electrons in the device. A photon energy spectrum of the cell is shown in the figure 5. It establishes the relationship between photon energy and wavelength for the solar cell. It clearly shows that solar cell device has capable of capturing a large amount of photon energy. We see (in figure 6 Inset), when light travels through the transparent electrode is absorbed by an active layer of perovskite generating electron hole pair. These charges (electron and hole) dissociate due to low binding energy. Electron and hole charge reaches the connected electrode via the electron and hole charge transport layer, generating electric current. The simulated values of short circuit current density J_sc, open circuit voltage V_oc, fill factor FF and efficiency are 21.63 mA/cm², 1.28 V, 0.78 and 21.53% respectively. These computed results of simulation are verified by the experimental results of the structure. From the J-V computation, a higher efficiency of 21.53% was attained which is an improvement from the reported efficiency (Zhou et al., 2014).

 The role of active materials with different band gap is observed to see the change in device characteristics (FF, efficiency). Efficiency and fill factor varies with energy band gap of active material as shown in figure 7 and 8 respectively. As clearly shown in figure 7, PCE decreases with increasing the energy band gap of the active layer material. Although fill factor increases with increase in energy band gap and maximum value at 1.66 eV as shown in figure 8. The Role of different electrode materials (anode) is shown in figures 9 and 10. Our objective was to study and analysis the behavior of various metal contact materials of the anode(Guo et al., 2019). Therefore, Ag, Cu, Au and Pt materials are used in the simulation. The work function of anode Ag, Cu, Au and Pt are 4.74 eV, 5.0 eV, 5.1 eV and 5.7 eV respectively(Ming et al., 2017). Figure 9 represents the effect of anode material on J-V characteristics of perovskite solar devices.  PCE and fill factor decreases with decreasing the work function of the anode as shown in the Figure 10. Low work function of the contact materials formed a schottky junction in between HTL and anode interface.  Therefore, the ohmic contact of platinum (high work function material) is best among all other anode material.

Conclusion

Perovskite solar cell with the structure glass/FTO/TiO₂/CH₃NH₃PbI_3-xCl_x/Spiro-OMeTAD/Au was successively simulated using SCAPS-1D software. Optimization of several PV parameter like thickness, active layer’s energy band gap (tunability), work function of anode material were performed to investigate their influence on the device performance. The simulation showed that the variation of thickness of the electron transport layer TiO₂ showed no significant change in the performance of the device. The simulation results further revealed thatthe layer thicknesses of ETL, light absorbing perovskite CH₃NH₃PbI_3-xCl_x and HTL were taken such as 40 nm, 350 nm and 200 nm respectively, gave a better cell performanceas compared to report in the literature. The simulation results (efficiency) of a cell fabricated using an active material (of band gap approximately 1.66 eV) and anode (of work function more than 5.2 eV) are 20.2% and 23.7% respectively.  The reported CH₃NH₃PbI_3-xCl_x based PSC provide a viable path to realizing low cost and high performance PSC. The optimized parameters can be applied experimentally to construct a solar device.

Acknowledgement

I am thankful for research facilities provided by Photonics Research Laboratory, School of Studies in Electronics and Photonics, Pt. Ravishankar Shukla University, Raipur, Chhattisgarh, India. I would like to acknowledge Dr. Marc Burgelman and the researcher team associated with Department of Electronics and Information Systems, University of Gent, Belgium for providing the simulation software SCAPS-1D.

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