Degradation Kinetics of Inverted Perovskite Solar Cells

We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA) 0.83 Cs 0.17 Pb(I 0.8 Br 0.2 ) 3 , CH 3 NH 3 PbI 3 , and CH 3 NH 3 PbI 3−x Cl 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 CH 3 NH 3 PbI 3−x Cl x solar cells due to oxygen exposure occurs over shorter timescales than FA 0.83 Cs 0.17 Pb(I 0.8 Br 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.