In situ growth of a 2D/3D mixed perovskite interface layer by seed-mediated and solvent-assisted Ostwald ripening for stable and efficient photovoltaics

2D/3D mixed perovskites have been recognized as very promising materials for simultaneously achieving high efficiency and stability in solar cells. In this work, we developed a seed-mediated and solvent-assisted Ostwald ripening method to form 2D PEA2PbI4 with a gradient distribution in the underlying 3D perovskite layer by a two-step deposition process with DMF as an additive. Using this method, the reaction of PEAI with MAPbI3 seed grains is prone to produce pure PEA2PbI4 which gives a graduated cladding on the 3D perovskite grains rather than a traditional 2D/3D planar heterostructured perovskite. We demonstrate that this type of perovskite simultaneously delivers a better optical performance and a higher stability than traditional perovskites. The devices made by our method exhibit an average enhancement of 15.5% in power conversion efficiency (PCE) compared with the perovskite solar cells (PSCs) made with 2D/3D planar heterojunction perovskites. Besides, the PCE of the proposed cell without encapsulation remains at above 88% of its original value after 15 days of storage in an air environment at 35–55% humidity, which is higher than the 83% delivered by the 2D/3D planar heterojunction PSCs. Moreover, this kind of cell shows superior operational stability at maximum power under AM 1.5G illumination at 100 mW cm−2 over 1 h relative to the regular 3D PSCs. Our results demonstrate an important strategy for the preparation of highly efficient and stable PSCs for future photovoltaic applications.