2D/3D heterojunction engineering for hole transport layer‐free carbon‐based perovskite solar cells

Hole transport layer (HTL)-free carbon-based perovskite solar cells (C-PSCs) own outstanding potential for commercial applications due to their attractive advantages of low cost and superior stability. However, the abundant defects and mismatched energy levels at the interface of the perovskite/carbon electrode severely limit the device efficiency and stability. Constructing a 2D layer on the surface of 3D perovskite films to form 2D/3D heterojunctions has been demonstrated to be an effective method of passivating surface defects and optimizing the energy level alignment in almost all kinds of PSCs. Due to the unique structure of HTL-free C-PSCs, 2D/3D heterojunctions play especially important roles. This review article summarizes the reports of 2D/3D perovskite heterojunctions in HTL-free C-PSCs. It describes the contributions of 2D/3D heterojunctions in terms of their roles in defect passivation, energy level optimization, and stability improvement. Finally, challenges and prospects of 2D/3D heterojunction for further development of HTL-free C-PSCs are highlighted.