  We explore the degradation behaviour under continuous illumination and direct
oxygen exposure of inverted unencapsulated
formamidinium(FA)0.83Cs0.17Pb(I0.8Br0.2)3, CH3NH3PbI3, and CH3NH3PbI3-xClx
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 CH3NH3PbI3-xClx
solar cells due to oxygen exposure occurs over shorter timescales than
FA0.83Cs0.17Pb(I0.8Br0.2)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.
