2D-3D mixed tin halide perovskites are outstanding candidate materials for lead-free perovskite solar cells (PSCs) due to their improved stability and decreased trap density in comparison with their pure 3D counterparts. However, the mixture of multiple phases may lead to poor charge transfer across the films and limit the device efficiency. Here, a stacked quasi-2D (down)-3D (top) double-layered structure in perovskite films prepared via vacuum treatment is demonstrated, which can result in a planar bilayer heterojunction. In addition, it is found that the introduction of guanidinium thiocyanate (GuaSCN) additive can improve the crystallinity and carrier mobility in the 2D perovskite layer and passivate defects in the whole film, leading to a long carrier lifetime (>140 ns) in photoluminescence measurements. As a result, the PSCs show a high open circuit voltage (V OC ) up to 1.01 V with a voltage loss of only 0.39 V, which represents the record values ever reported for tin-based PSCs. The champion device exhibits a power conversion efficiency (PCE) of 13.79% with decent stability, retaining 90% of the initial PCE for 1200 h storage in N 2 -filled glovebox.

Metal halide perovskite solar cells (PSCs) have become one of the most promising next‐generation photovoltaic technologies due to their low‐cost fabrication, solution processability, and superior optoelectronic properties. Although state‐of‐art PSCs demonstrate a power conversion efficiency record comparable to that of silicon solar cells, there are still many challenges toward commercialization. PSCs are devices based on various semiconductor heterojunctions that all play important roles in device performance. The device operation relies on a combination of multiple heterojunctions to offer a delicate control of photocarrier generation, separation, and transport to respective electrodes. Hence, advanced heterojunction design in PSCs is crucial for the further improvement of device performance. Notably, the conversion efficiency records for PSCs are mainly ascribed to optimized heterojunction engineering. Considering the significance of this topic, a comprehensive review of the recently developed heterojunction designs is presented. Following a brief introduction to PSC architectures, operation, and fundamental heterojunction design theories, the recent progress on perovskite/electron transport layer, perovskite/hole transport layer, and perovskite/perovskite heterojunction engineering is elaborated. Finally, conclusions and perspectives on this research field are addressed.