LAUSR

We study the thermal stability in air of the mixed cation organic-inorganic lead halide perovskites Cs0.17FA0.83Pb(I0.83Br0.17)3 and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)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 (T < 150 °C). For the second stage, we find that both decomposition rate and the activation energy have similar values for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 and Cs0.17FA0.83Pb(I0.83Br0.17)3, which suggests that the first stage mainly involves reaction of MA+ and the second stage mainly FA+.