Surface-Activated Corrosion in Tin-Lead Halide Perovskite Solar Cells

Mixed tin-lead halide perovskite solar cells have promising power conversion efficiencies, but long-term stability is still a challenge. Herein we examine the stability of a 60:40 tin-lead perovskite to better understand diminished device performance upon thermal treatment, both in ambient and inert atmosphere. Operando X-ray diffraction shows a stable bulk structure of the perovskite absorber, leading to the hypothesis that surface chemistry dominates the degradation mechanism. X-ray photoelectron spectroscopy reveals two new observations post-thermal annealing that accompany previously reported Sn4+ evolution: (i) the formation of I-3(-) intermediates preceding I, loss at the surface and (ii) evidence of under-coordinated tin and lead surface sites (Sn delta<2+ and Pb delta<2+, respectively) in inert and ambient conditions. These two species indicate an activated corrosion (i.e., both oxidation and reduction) process at the surface as a possible chemical pathway for degradation, which is expected to be accelerated under operando voltage and light biases.