Abstract
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. Perovskite films based on CH3NH3PbI3 undergo rapid degradation when exposed to oxygen and light. Here, we report mechanistic insights into this oxygen-induced photodegradation from a range of experimental and computational techniques. We find fast oxygen diffusion into CH3NH3PbI3 films is accompanied by photo-induced formation of highly reactive superoxide species. Perovskite films composed of small crystallites show higher yields of superoxide and lower stability. Ab initio simulations indicate that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen. Thin-film passivation with iodide salts is shown to enhance film and device stability. The understanding of degradation phenomena gained from this study is important for the future design and optimization of stable perovskite solar cells.
Highlights
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
CH3NH3PbI3 and CH3NH3PbI3(Cl) were chosen for a comparative study, since both have been widely tested in photovoltaic devices. (Note that the formula CH3NH3PbI3(Cl) refers to a perovskite material fabricated from a combination of iodide and chloride precursors.) Of the two systems, CH3NH3PbI3(Cl) films are reported to be more stable[59,60] but the origin(s) of this superior stability remains unclear
The results demonstrate that iodide salt treatment can be employed to reduce the number of problematic iodide vacancies, thereby hindering the electron transfer reaction that generates superoxide species
Summary
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. We recently demonstrated[38,39] that exposure of CH3NH3PbI3 photoactive layers to light and oxygen leads to the formation of superoxide (O2À ) species This oxygen-induced degradation pathway has been shown to affect the stability of both CH3NH3PbI3 photoactive layers and solar cell devices[40]. It is reasonable to suppose that the particle size and defect chemistry of the films may influence oxygen diffusion into the perovskite layer and its susceptibility to oxidative reactions
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