  The wide band gap methylammonium lead bromide perovskite is promising for
applications in tandem solar cells and light-emitting diodes. Despite its
utility, there is only a limited understanding of its reproducibility and
stability. Herein, the dependence of the properties, performance, and shelf
storage of thin films and devices on minute changes to the precursor solution
stoichiometry is examined in detail. Although photovoltaic cells based on these
solution changes exhibit similar initial performance, the shelf-storage depends
strongly on the precursor solution stoichiometry. While all devices exhibit
some degree of healing, the bromide-deficient films show a remarkable
improvement, more than doubling in their photoconversion efficiency.
Photoluminescence spectroscopy experiments performed under different
atmospheres suggest that this increase is due in part to a trap healing
mechanism that occurs upon exposure to the environment. Our results highlight
the importance of understanding and manipulating defects in lead halide
perovskites to produce long-lasting, stable devices.
