Multifunctional ionic liquid boosts the efficiency of hole transport layer‐free carbon‐based perovskite solar cells beyond 20%

Abstract The planar triple‐layer hole transport layer (HTL)‐free carbon‐based perovskite solar cells have the advantages of low cost and high stability, but their low efficiency hinders the commercialization process. Here, a dual regulation strategy for bulk defects and interface defects has been developed. After selecting dimethylamine (DMA) as the fixed cation and selecting the anion site (I − , Br − , Cl − , HCOO − ), an ionic liquid additive, DMAFo, was synthesized, achieving multiple functions such as energy level regulation, removal of residual PbI 2 at the buried interface, retarding crystallization through the intermediate phase DMAPbI (3−x) HCOO x , filling halide vacancy defects, and releasing residual stress. This effectively reduces energy loss during carrier transport and obtains higher‐quality perovskite films. Under the synergistic effect of DMA + and HCOO − , both bulk defects and interface defects in the perovskite film were simultaneously addressed. The device prepared using DMAFo as an additive achieved the best device efficiency of 20.77%, and after continuous maximum power point tracking for 1200 h, the device efficiency remained almost unchanged, demonstrating excellent operational stability.