Surface termination of perovskite with 1-naphthylmethylammonium tetrafluoroborate for efficient perovskite solar cells

Constructing low-dimensional/three-dimensional (LD/3D) perovskite heterostructures through post-treatment of 3D perovskites is an effective strategy for passivating surface defects in perovskite solar cells. Organic ammonium halide salts are among the most widely used materials for converting the surface layer of 3D perovskite to LD perovskite via cation exchange. However, the formed LD perovskites typically adopt an in-plane orientation and exhibit high exciton binding energy, which can adversely impact interfacial charge transport and reduce the effectiveness of molecular defect passivation. Herein, we design and synthesize a non-halide ammonium salt of 1-naphthylmethylammonium tetrafluoroborate (NMABF₄) to inhibit the transformation reaction, enabling surface termination of perovskite films with functional molecules. The use of a relatively large cation enhances the barrier of cation exchange between ammonium salt and 3D perovskite. Moreover, the non-halide anion with a strong interaction with Pb²⁺ is introduced to prevent the formation of Pb-I octahedra, thereby preventing the formation of 2D perovskite. Therefore, NMABF₄ effectively terminates the perovskite surface. This approach not only simultaneously passivates defects with opposing charges on the perovskite surface but also prevents potential drawbacks associated with heterostructure-induced intercalation. As a result, a champion device efficiency of 25.38% can be achieved using NMABF₄-terminated perovskite, compared to 23.52% for the control device. Moreover, the unencapsulated device demonstrates excellent operational stability, retaining 80% of its initial efficiency after 2001 h of aging at the maximum power point under continuous one-sun illumination.