Regulate excess PbI₂ distribution on perovskite film via amphiphilic surfactant for efficient and stable device

Lead iodide (PbI₂) as a defect passivation in perovskite thin films can reduce halide vacancies and improve the performance of perovskite solar cells. However, excess PbI₂ can hurt the device's performance. In the case of perovskite films prepared by two-step sequential deposition, excess PbI₂ is distributed on the surface of perovskite film. The photo-decomposition of PbI₂ and the formation of surface energy barriers can hinder charge transfer in the device, leading to reduced device performance and accelerated device degradation. Inspired by the use of surfactants to control the morphology of perovskite nanocrystals, a dual-functional surfactant (oleamide) was introduced to control the distribution of excess PbI₂ on perovskite films. By adding oleamide, the aggregation distribution of PbI₂ was successfully achieved at the perovskite grain boundaries (GBs), forming a Type-I band alignment and reducing the trap density, thereby prolonging the carrier lifetime. The results show that the power conversion efficiency (PCE) of the device increased from 21.23% to 23.12% after the introduction of oleamide. Compared with the control and oleamide-modified devices, the deep-level defects were reduced by about five orders of magnitude due to the Lewis-base nature of amide groups in oleamide. In addition, due to the aggregation distribution of excess PbI₂ at GBs and the hydrophobicity of oleamide, the modified device showed better stability, maintaining 83% of its initial performance after 1000 h in air.