Defect Passivation of Organic-Inorganic Hybrid Perovskites by Diammonium Iodide toward High-Performance Photovoltaic Devices

The polycrystalline feature of solution-processed perovskite film and its ionic nature inevitably incur substantial crystallographic defects, especially at the film surface and the grain boundaries (GBs). Here, a simple defect passivation method was exploited by post-treating CH₃NH₃PbI₃ (MAPbI₃) film with a rationally selected diammonium iodide. The molecular structure of the used diammonium iodide was discovered to play a critical role in affecting the phase purity of treated MAPbI₃. Both NH₃I(CH₂)₄NH₃I and NH₃I(CH₂)₂O(CH₂)₂NH₃I (EDBE) induce three-dimensional (3D) to two-dimensional (2D) perovskite phase transformation during the treatment while only NH₃I(CH₂)₈NH₃I (C8) successfully passivates perovskite surface and GBs without forming 2D perovskite because of the elevated activation energy arising from its unique anti-gauche isomerization. Defect passivation of MAPbI₃ was clearly confirmed by scanning Kelvin probe microscopy (SKPM) and time-resolved photoluminescence (TRPL) studies, which results in the reduced recombination loss in derived devices. Consequently, the perovskite solar cell with C8 passivation showed a much improved power conversion efficiency (PCE) of 17.60% compared to the control device PCE of 14.64%.