Hybrid lead-halide perovskites have risen to become promising materials for use in thin-film photovoltaic (PV) devices, with efficiencies in excess of 22% achieved after just a few years research1. However, further development of these devices has been hampered by the lack of stability of the materials, especially when compared with those of incumbent technologies. It has recently been found that chemical site-substitution at one or more of the A, B or X sites in the perovskite’s ABX3 structure can be used to tune both the physical properties and the stability of the materials2,3. Results reported here focus on the degradation routes and kinetics of thin films of methlyammonium (MA) / formamidinium (FA) lead iodide perovskites from the solid solution MA1−xFAxPbI3, 0 ≤ x ≤ 1 in low humidity conditions.4 The degradation route favoured for MA-rich phases is decomposition to PbI2 and precursor organic cation salts, whereas FA-rich phases quickly transform to the non-perovskite hexagonal δ-phase. However, kinetic analysis of PbI2 formation, measured, using powder X-ray diffraction, shows an exponential decay in the rate of formation up to x = 0.6, for which negligible amounts of PbI2 was produced even after 10 days exposure to the atmosphere. The observed increase in stability was supported by ab initio simulations of the decomposition reaction energies. This combined experimental – computational study provides a greater understanding of favoured degradation routes in hybrid lead-halide perovskites, leading to likely improvements in the long-term stability of perovskites solar cells.
1 M. A. Green et al., Prog. Photovolt. Res. Appl. 2017, 25, 3-13
2 M. T. Klug et al., Energy Environ. Sci. 2017, 10, 236-246
3 C. C. Stoumpos et al., Inorg. Chem. 2013, 52, 9019-9038
4 B. Charles, J. Dillon, O.J. Weber, M. S. Islam and M.T. Weller J. Mat. Chem. A, 2017, DOI: 10.1039/C7TA08617B