Pb/Sn-Based Perovskite Heterojunction for Hole Transport Layer-Free Carbon-Based Perovskite Solar Cells

The construction of heterojunctions has been proven to be effective in optimizing the interface energy structure and passivating defect states in perovskite solar cells (PSCs). Tin (Sn)-based perovskites have been demonstrated to have good hole extraction capability and are promising candidates to form heterojunctions with Pb-based perovskites to facilitate the charge transfer. Here, we propose a strategy for the in situ construction of Pb/Sn-based perovskite heterojunctions by a solution method. First, Pb-based perovskite films with an FAI-rich surface are prepared. Subsequently, a thin layer of FA2SnI6 is in situ formed by SnI4 post-treatment, resulting in the construction of a Pb/Sn-based perovskite heterojunction. The formation of the heterojunction reduces the defect states and also yields an optimized interfacial energy level alignment, which facilitates the carrier separation process and reduces the nonradiative recombination loss. Finally, an efficiency of 19.89% is achieved for the planar carbon-based perovskite solar cells without a hole transport layer.