Interface Energy‐Level Management toward Efficient Tin Perovskite Solar Cells with Hole‐Transport‐Layer‐Free Structure

Abstract Lead‐free tin perovskite solar cells (PSCs) have emerged as a promising candidate toward high‐performance and eco‐friendly photovoltaic technology with great potential for future application. However, tin PSCs with over 10% efficiency usually feature an organic hole transport layer (HTL) at the illumination side that may induce device degradation during long‐term operation. Removing the unstable organic HTL is an important way to solve these stability issues, but the efficiency of HTL‐free tin PSCs is still much lower than that of the completed cells. Herein, it is demonstrated that formamidinium tin iodide doped with heterogeneous ammonium salts can form an upward band‐bending structure to selectively extract the hole in the HTL‐free devices. By using this band‐bending structure, a promising efficiency of over 10% is first achieved for the lead‐free PSCs with a HTL‐free structure. More importantly, the optimized cell is highly stable, keeping 95% and 90% of the initial efficiency after continuous light soaking for 40 days and 80 °C annealing for 300 h, respectively. This work paves a route toward the development of efficient, eco‐friendly, and highly stable perovskite photovoltaics.