Perovskite processing for photovoltaics: a spectro-thermal evaluation

Abstract

Thermal analysis (TGA and DSC), coupled with evolved gas FTIR spectroscopy, has been used to study the changes occurring during, and differences between materials after, the annealing step of mixed-halide methylammonium lead halide perovskites. This is important because, to date, the material is the most efficient light harvester in highly efficient, 3^rd generation perovskite photovoltaic devices, and processing plays a significant role in device performance. TGA-FTIR data show only solvent evolution during the annealing step, whilst post-annealing analysis shows that the resulting material still contains a significant amount of residual solvent; however, efficient DMF removal was possible using a silica gel desiccant for a period of 3 days. The data also show that methylammonium halide decomposition does not occur until temperatures well above those used for perovskite processing, suggesting that this is not a significant issue for device manufacture. The absence of a well-defined, reversible tetragonal – cubic phase change around 55 °C in the DSC data of the annealed material, and the presence of HCl in evolved gas analysed following thermal decomposition, demonstrates that CH₃NH₃I_3−xCl_x does retain some Cl after annealing and does not simply form stoichiometric CH₃NH₃PbI₃ as has been suggested by some workers.