Interfacial Engineering of Perovskite Solar Cells with Evaporated PbI₂ Ultrathin Layers

Perovskite solar cells are one of the most promising thin-film photovoltaic techniques, which have an unprecedented progress in the last decade. It is well-recognized in the perovskite community that nonradiative recombination losses and the open-circuit voltage deficit are the dominant limiting factors to further improve the device efficiency. Recently, multiple groups have reported that lead iodide can effectively passivate both perovskite grain boundaries and the interfaces between perovskite and charge transport layers. However, most of the excess PbI₂ was processed with solution methods and formed PbI₂ grains, which cannot cover perovskite layers completely. It is also very challenging to spin-coat PbI₂ layers directly on perovskites, which requires orthogonal solvents. In this work, we deposit additional PbI₂ thin layers directly on perovskite thin films via thermal evaporation. The impact of PbI₂ layers on the perovskite thin films and devices is systematically investigated. It was found that the evaporated PbI₂ thin films can effectively reduce the nonradiative recombination and enhance the device performance. The optimized thickness of the PbI₂ layer was determined to be around 10 nm, which results in a relatively high V_oc of 1.18 V and power conversion efficiency of 21.52%.