Binary cations minimize energy loss in the wide-band-gap perovskite toward efficient all-perovskite tandem solar cells

Enhancing the performance of perovskite solar cells relies crucially on the surface post-treatment of the perovskite film using large spacer cations. These cations play multifunctional roles, encompassing bulk/surface defect passivation, interfacial energy-level alignment, and the creation of protective low-dimensional phases. Particularly in perovskite-based tandem solar cells that extend beyond the detailed balance (DB) limit, wide-band-gap perovskite front cells encounter significant open-circuit voltage (VOC) and fill factor (FF) losses, constraining overall device performance.
In this study, we present a novel approach involving a mixed spacer cation system (i.e., GABr and F-PEAI) applied to the perovskite surface. This innovative treatment leads to a substantial increase in both VOC and FF. A comprehensive experimental-theoretical synergy elucidates that the primary mechanisms behind the enhanced performance are surface defect passivation and interfacial energetic alignment induced by the mixed cations, with a noteworthy exclusion of contributions from low-dimensional phases. These findings deepen our comprehension of the surface passivation mechanism employing large spacer cations on the perovskite surface, offering a pioneering and dependable strategy to mitigate energy losses, thereby advancing the path toward the commercialization of perovskite photovoltaic technologies.