Degradation Kinetics of Inverted Perovskite Solar Cells

We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA)_0.83Cs_0.17Pb(I_0.8Br_0.2)₃, CH₃NH₃PbI₃, and CH₃NH₃PbI_3-xCl_x perovskite solar cells. We continuously test the devices in-situ and in-operando with current-voltage sweeps, transient photocurrent, and transient photovoltage measurements, and find that degradation in the CH₃NH₃PbI_3-xCl_x solar cells due to oxygen exposure occurs over shorter timescales than FA_0.83Cs_0.17Pb(I_0.8Br_0.2)₃ mixed-cation devices. We attribute these oxygen-induced losses in the power conversion efficiencies to the formation of electron traps within the perovskite photoactive layer. Our results highlight that the formamidinium-caesium mixed-cation perovskites are much less sensitive to oxygen-induced degradation than the methylammonium-based perovskite cells, and that further improvements in perovskite solar cell stability should focus on the mitigation of trap generation during ageing.