The attraction of organic -inorganic metal halide perovskites stem from their low cost processability and desirable optoelectronic properties. Band gap tunability and inherent low band gaps of Pb-Sn based hybrid halide perovskites make them ideal candidates for use in all-perovskite tandem solar cells which plays a key role in developing photovoltaic technologies that can achieve high efficiencies at very low fabrication costs. However, at present their progression into commercial markets have been limited due to the lack of stability of Pb-Sn mixed perovskites because of Sn oxidation.

This thesis focuses on the optimization of a triple cation Pb-Sn perovskite with composition Cs_0.05(FA_0.83MA_0.17)_0.95Pb_0.5Sn_0.5I₃ from careful selection of the precursor sourcing materials to optimization of the fabrication process to achieve perovskite solar cells (PSCs) with high photovoltaic performances . Firstly, the use of metallic tin powder to minimize the oxidation of Sn²⁺ ions in the perovskite precursor is investigated and it is found that a redox reaction between the tin powder and oxidized Sn⁴⁺ is induced leading to reduction of Sn⁴⁺ ions back to their original Sn²⁺ oxidation state in the precursor solution. Next, the additives used in the triple cation Pb-Sn perovskite are studied and their performances are compared to identify the optimum compositions. Furthermore, the use of ethylenediamine (EDA) as a passivating layer to prevent oxidation of Sn²⁺ ions showed improvement in the obtained J-V curves of the solar cells fabricated as well as the champion power conversion efficiency achieved to increase to 13.5% from 12.0%. Then, this work focuses on the optimization of the perovskite architecture by considering the selection of charge transport layers in order to eliminate the voltage losses that occur at interlayers leading to poor device performances. Replacement of the hole transport layer from poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) lead to the fabrication of a narrow bandgap Pb-Sn PSC with an inverted planar architecture and having the configuration glass/ITO/PTAA/PFN-P1/perovskite/EDA/PC₍₆₀₎BM/C₆₀/BCP/Ag. This device showed impressive improvements to the open circuit voltage by increasing from 0.66 V to 0.83 V and achieved a champion power conversion efficiency of 19.4% along with improved reproducibility of results.