Bi-molecular kinetic competition for surface passivation in high-performance perovskite solar cells

Surface post-treatment using larger organic spacer cations is widely recognized as one of the most effective approaches to suppress the defects and reconstruct microstructures in perovskite films. However, larger organic spacer cations always cause structural transformation to an uncontrollable low-dimensional phase at the perovskite surface, significantly restricting charge transport across interfaces within perovskite solar cells (PSCs). In this study, we introduce a bi-molecular competitive adsorption strategy using phenylmethylammonium iodide (PMAI) and octylammonium iodide (OAI) as co-modifiers from molecular dynamics perspectives. It is revealed that OA+ preferentially adsorbs on the surface of perovskite films owing to its much greater molecular polarity and steric hindrance effects, thereby inhibiting PMA+-induced surface layer transformation into a low-dimensional structural phase. This strategy therefore prevents restriction of charge separation and extraction at the perovskite upper interface, resulting in a remarkable PCE of up to 25.23% (certified: 25.0%) for the final device. Such a bi-molecular kinetic competition strategy is also universal for flexible PSC devices that have achieved a superior PCE of 23.52%. After 1000 hours of maximum power point tracking (MPPT) under continuous illumination, unencapsulated cells functionalized with bi-molecular ligands maintained 88% of their initial performance, thus demonstrating excellent operational stability under the improved efficacy of competitive-style dual ligand modulation. Our work points to an avenue for harnessing the important role of kinetic competition between surface ligands in further advancing the performance and applicability of PSCs. A bi-molecular kinetic competitive adsorption strategy was proposed to circumvent the dimensional structural transformation which has been widely observed on the film surface passivated by conventional methods.