Stable Perovskite Solar Cells Enabled by Simultaneous Surface and Bulk Defects Passivation

It is challenging to passivate defects that are buried in the depth of perovskite films; most of the reported passivation methods cannot reach the deep defects. Herein, methanol is adopted as a dual-functional reagent to not only act as a solvent but also help the dissolved ions penetrate the depth of perovskite films. By treating the as-prepared perovskite films with CsBr/methanol solution, Br(-)ions can react with the undercoordinated Pb2+, and Cs(+)ions can fill in the cation vacancies. This strategy enables surface and bulk defects passivation to be achieved simultaneously. The nonradiative recombination of the double-passivated devices is significantly suppressed and the migration of charged defects is remarkably hindered. As a result, an improved power conversion efficiency of 19.5% and an open-circuit voltage of 1.183 V is achieved. Moreover, the passivated device can retain approximate to 80% of the initial efficiency after working for 500 h at maximum power point under 1-sun illumination, whereas the pristine device reaches 80% of the initial efficiency after only 90 h. This work demonstrates that surface and bulk defects passivation is critical to improve the efficiency and long-term operational stability of the perovskite solar cells.