Quasi‐2D Bilayer Surface Passivation for High Efficiency Narrow Bandgap Perovskite Solar Cells

The combination of comprehensive surface passivation and effective interface carriers transfer plays a critical role in high‐performance perovskite solar cells. A 2D structure is an important approach for surface passivation of perovskite film, however, its large band gap could compromise carrier transfer. Herein, we synthesize a new molecule 2‐thiopheneethylamine thiocyanate (TEASCN) for the construction of bilayer quasi‐2D structure precisely on a tin‐lead mixed perovskite surface. This bilayer structure can passivate the perovskite surface and ensure effective carriers transfer simultaneously. As a result, the open‐circuit voltage (Voc) of the device is increased without sacrificing short‐circuit current density (Jsc), giving rise to a high certified efficiency from a credible third‐party certification of narrow band gap perovskite solar cells. Furthermore, theoretical simulation indicates that the inclusion of TEASCN makes the bilayer structure thermodynamically more stable, which provides a strategy to tailor the number of layers of quasi‐2D perovskite structures. TEASCN (2‐thiopheneethylamine thiocyanate) was synthesized to construct a bilayer structure on a Sn‐Pb perovskite surface, which can passivate perovskite and ensure effective carrier transfer, enabling the device to reach a certified efficiency of 21.1 %. The mechanism for the growth of the uniform bilayer structure is revealed by simulation based on density functional theory.