Thermal Stability of Mixed Cation Metal Halide Perovskites in Air

We study the thermal stability in air of the mixed cation organic–inorganic lead halide perovskites Cs_0.17FA_0.83Pb­(I_0.83Br_0.17)₃ and Cs_0.05(MA_0.17FA_0.83)_0.95Pb­(I_0.83Br_0.17)₃. For the latter compound, containing both MA⁺ and FA⁺ ions, thermal decomposition of the perovskite phase was observed to occur in two stages. The first stage of decomposition occurs at a faster rate compared to the second stage and is only observed at relatively low temperatures (<i>T</i> < 150 °C). For the second stage, we find that both decomposition rate and the activation energy have similar values for Cs_0.05(MA_0.17FA_0.83)_0.95Pb­(I_0.83Br_0.17)₃ and Cs_0.17FA_0.83Pb­(I_0.83Br_0.17)₃, which suggests that the first stage mainly involves reaction of MA⁺ and the second stage mainly FA⁺

Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells

A semiconductor that can be processed on a large scale with a bandgap around 1.8 eV could enable the manufacture of highly efficient low cost double-junction solar cells on crystalline Si. Solution-processable organic–inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells, and though it is possible to tune the bandgap of (CH₃NH₃)­Pb­(Br_<i>x</i>I_1–<i>x</i>)₃ between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photoinduced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, it is possible to synthesize a mixed halide absorber material with improved optical and thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV

Chloride in Lead Chloride-Derived Organo-Metal Halides for Perovskite-Absorber Solar Cells

Organo-metal halide perovskites are an intriguing class of materials that have recently been explored for their potential in solar energy conversion. Within a very short period of intensive research, highly efficient solar cell devices have been demonstrated. One of the heavily debated questions in this new field of research concerns the role of chlorine in solution-processed samples utilizing lead chloride and 3 equiv of methylammonium iodide to prepare the perovskite samples. We utilized a combination of X-ray photoelectron spectroscopy, X-ray fluorescence, and X-ray diffraction to probe the amount of chlorine in samples before and during annealing. As-deposited samples, before annealing, consist of a crystalline precursor phase containing excess methylammonium and halide. We used in situ techniques to study the crystallization of MAPbI₃ from this crystalline precursor phase. Excess methylammonium and chloride evaporate during annealing, forming highly crystalline MAPbI₃. However, even after prolonged annealing times, chlorine can be detected in the films in X-ray fluorescence measurements