Remarkable attention has been committed to the recently discovered cost effective and solution processable lead-free organic-inorganic halide perovskite solar cells. Recent studies have reported that, within five years, the reported efficiency has reached 9.0%, which makes them an extremely promising and fast developing candidate to compete with conventional lead-based perovskite solar cells. The major challenge associated with the conventional perovskite solar cells is the toxic nature of lead (Pb) used in the active layer of perovskite material. If lead continues to be used in fabricating solar cells, negative health impacts will result in the environment due to the toxicity of lead. Alternatively, lead free perovskite solar cells could give a safe way by substituting low-cost, abundant and non toxic material. This review focuses on formability of lead-free organic-inorganic halide perovskite, alternative metal cations candidates to replace lead (Pb), and possible substitutions of organic cations, as well as halide anions in the lead-free organic-inorganic halide perovskite architecture. Furthermore, the review gives highlights on the impact of organic cations, metal cations and inorganic anions on stability and the overall performance of lead free perovskite solar cells.

Perovskite solar cells have attracted tremendous attention owing to its rapid increase in power conversion efficiency. This work designed and simulated lead-based perovskite solar cells in planar structure; TCO/ TiO2/ CH3NH3PbI3/Spiro-OMeTAD/Au. To study the effect of various transparent conductive oxides (TCOs) on power conversion efficiency of the devices, Solar Capacitance (SCAP) simulating software was used. To achieve an optimum efficiency, the influence of thickness and band-gap energy of the absorber layer were varied and investigated. The optimized power conversion efficiency (PCE) is achieved using MoO3/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au architecture with PCE of 22.44 % and Voc, Jsc, and FF of 1.0842 V, 25.57 mA/cm2 and 80.94 % respectively. The numerical simulation shows the potential of substituting the conventional FTO and ITO used in perovskite solar cells with MoO3 as a promising transparent conductive oxide layer.

Substrates used in perovskite solar cells as front contact are usually transparent conductive oxide (TCO) to allow light to pass through the device. The dominating TCO employed in perovskite solar cells are indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO). However, it is imperative to investigate alternative TCOs due to the scarcity of indium metal, relatively low electrical conductivity and high leakage current in ITO and FTO. In this study, simulation has been carried out using Solar Capacitance Simulator (SCAPS) to investigate the efficiency of methyl-ammonium tin iodide (CH₃NH₃SnI₃) based solar cells including various TCOs such as boron-doped zinc oxide (BZO), molybdenum trioxide (MoO₃) and zinc oxide (ZnO). TCO parameters such as thickness, donor concentration and operating temperature were varied to study their influence on device performance. The best device performance was achieved using MoO₃ with power conversion efficiency of 25.83 % and J_sc, V_oc and FF of 32.44 mA/cm², 0.979 V and 81.38 % respectively. The work shows the potential of fabricating an improved CH₃NH₃SnI₃ perovskite solar cell with MoO₃ as front contact.

Perovskites solar cells (PSC) constitutes of different thin layers deposited on top of a substrate. Hole Transporting Layer (HTL) plays a significant role on the efficiency of the device owing to its numerous advantages such as good conductivity, solution processable, and suitable energy level that matches with perovskite absorber layer. However, Spiro-OMeTAD is extremely expensive and suffers from low hole mobility. However, it is imperative to search for an alternative to the expensive Spiro-OMeTAD commonly used as HTL in PSC. In this work, the influence of various hole transporting layers (such as CuI, CuSCN, Cu₂O, and PEDOT:PSS) on the electrical performance of methyl-ammonium tin-iodide based perovskite solar cells was numerically examined using Solar Capacitance (SCAPS) simulator. To optimize the device performance, the HTL and absorber layer thickness were varied and investigated. The simulation results show that the device performance strongly depends on the absorber layer thickness. Among the four selected HTLs, the best performance was achieved using Cu₂O with V_oc, J_sc, FF, and PCE of 0.9676 V, 32.8344 mA/cm², 80.97 %, and 25.72 % respectively. The results obtained from this numerical investigation show the potential of fabricating efficient, inexpensive and stable tin-based perovskite solar cells with inorganic HTL.

In organometallic halide perovskites solar cells, electron transporting layer has significant impact on the optical, structural and optical properties of these promising solar cells. In this work, influence of electron transporting layer on the electrical performance of tin-based perovskite solar cells were numerically investigated using Solar Capacitance (SCAPS) simulator. In order to achieve an optimal efficiency, the effect of ETL thickness and doping concentration were varied and investigated. The results show that PCE is significantly dropped with increased ETL thickness using C60 and CdS from 50 nm to 300 nm. Conversely, a steady PCE as the thickness of Cd_0.5Zn_0.5S and ZnO increased were observed. Furthermore, varied ETL donor concentration significantly influenced the performance of the simulated devices. Generally, moderate doping concentration is required as heavy concentration leads to increased recombination rate and subsequently reduced the PCE of the devices. Cd_0.5Zn_0.5S and ZnO exhibited best PCE at doping concentration value of 10¹⁹ cm^-3 and 10²⁰ cm^-3 respectively. It can be concluded that Cd_0.5Zn_0.5S and ZnO are potential alternative ETL in perovskite solar cells.