Intact 2D/3D halide junction perovskite solar cells via solid-phase in-plane growth

The solution process has been employed to obtain Ruddlesden–Popper two-dimensional/three-dimensional (2D/3D) halide perovskite bilayers in perovskite solar cells for improving the efficiency and chemical stability; however, the solution process has limitations in achieving thermal stability and designing a proper local electric field for efficient carrier collection due to the formation of a metastable quasi-2D perovskite. Here we grow a stable and highly crystalline 2D (C4H9NH3)2PbI4 film on top of a 3D film using a solvent-free solid-phase in-plane growth, which could result in an intact 2D/3D heterojunction. An enhanced built-in potential is achieved at the 2D/3D heterojunction with a thick 2D film, resulting in high photovoltage in the device. The intact 2D/3D heterojunction endow the devices with an open-circuit voltage of 1.185 V and a certified steady-state efficiency of 24.35%. The encapsulated device retained 94% of its initial efficiency after 1,056 h under the damp heat test (85 °C/85% relative humidity) and 98% after 1,620 h under full-sun illumination.