Methylammonium lead halide perovskites are attracting intense interest as promising
materials for next-generation solar cells, but serious issues related to long-term stability need
to be addressed. Specifically, perovskite films based on
CH
3
NH
3
PbI
3
undergo fast oxygen-
and light-induced degradation. However, the mechanism of such degradation and its relation
to particle size and oxygen transport are poorly understood. Here, we report new mechanistic
insights through the combined use of isothermal gravimetric analysis, photoluminescence,
secondary ion mass spectrometry and ab initio simulation techniques. We find fast oxygen
diffusion into CH
3
NH
3
PbI
3
films is accompanied by superoxide formation, which are critical
to degradation by oxygen in the atmosphere.
Perovskite films composed of small crystallites
show high yields of photo-induced superoxide species and low stability. Ab initio simulations
indicate that iodide vacancies are the preferred sites in mediating the photo-induced
formation of superoxide species from oxygen. We also show that thin-film passivation with
iodide salts can lead to enhanced film stability and thus device stability. The understanding of degradation phenomena gained from this study is important for the future design and
optimisation of perovskite solar cells with greater stability.