The routes and kinetics of the degradation of thin films of methylammonium (MA)/formamidinium (FA) lead iodide perovskites (MA 1Àx FA x PbI 3 , 0 # x # 1) under dry atmospheric conditions have been investigated. MA-rich phases decompose to the precursor iodide salts and PbI 2 , while FA-rich phases convert mainly to the yellow hexagonal phase. The reactivity is strongly inhibited for mixed cation phases of MA 1Àx FA x PbI 3 , for x ¼ 0.4 to 0.6, where the decomposition routes available to end member phases become less favourable. It is shown that for pristine films with x ¼ 0.6, PbI 2 formation can be completely suppressed for up to 10 days. Kinetic analysis reveals that the rate of PbI 2 formation decays exponentially with increasing FA content until x ¼ 0.7, beyond which the FA containing perovskite transforms rapidly to the hexagonal phase. Ab initio simulations of the decomposition reaction energies fully support the increased kinetic stability found experimentally for the mixed A-cation perovskites.Hybrid lead halide perovskites have recently risen to prominence as highly versatile materials for optoelectronic technologies, particularly high efficiency, solution processed solar cells.1-4 They display near optimal band gap for solar light absorption, high absorption coefficients, steep absorption onsets and signicant charge carrier lifetimes (and thus diffusion lengths).2,5 These properties, combined with the processability of these materials, 6,7 have resulted in solar cell device efficiencies rising rapidly to over 22%.8 However, the commercial applicability of these materials is hampered by their relative lack of stability compared to established inorganic and organic semiconductors.9-11 By chemical site-substitution at any of the A, B or X sites of the ABX 3 perovskite structure, it has been found that it is possible to tune the properties and, most pertinently, the stability of the resultant material.
12-14The archetypal hybrid perovskite methylammonium (MA) lead iodide (CH 3 NH 3 PbI 3 ), MAPI, crystallises in the tetragonal space group I4/mcm at room temperature.15 Formamidinium (FA) lead iodide (CH(NH 2 ) 2 PbI 3 ), FAPI, by contrast preferentially crystallises in the yellow, 2H hexagonal perovskite type polymorph at room temperature, which can be driven to transform to the black cubic perovskite polymorph (required for photovoltaic applications) by heating above room temperature.
16-19Such varying phase behaviour can be accounted for by the different sizes and hydrogen bonding mechanisms of these dissimilar organic cations, MA and FA.Both MA and FA cations display directional hydrogen bonding, a factor of increasing importance in controlling phase behaviour as the system temperature is reduced and also for dening the relative stability of the two polymorphs of FAPI.
20Stabilisation of mixed A site compounds relative to the end members of the series can be rationalised using geometrical arguments in which the perovskite tolerance factor is tuned using differently sized cations, as well as an entrop...
