Melamine holding PbI2 with three “arms”: an effective chelation strategy to control the lead iodide to perovskite conversion for inverted perovskite solar cells

Abstract

It is now well-known that moderate amounts of lead iodide (PbI₂) in organic–inorganic hybrid perovskite films are capable of passivating defects and stabilizing the material. However, contrarily, excessive PbI₂ instead leads to rapid degradation and thus destabilizes the perovskite solar cells (PSCs). To address this challenge, we propose to use melamine (MEA) additive to control the concentration of PbI₂ in perovskite films fabricated with sequential deposition method. As demonstrated by our calculations and NMR measurement results, MEA has both donor and acceptor regions which combine well with the PbI₂'s surface topology: the triazine core units are capable of binding to uncoordinated lead while the amino groups of MEA are capable of coordinating with the iodide anions and effectively “trichelate” PbI₂, thus passivating the defects and promoting carrier separation. Furthermore, the simultaneous introduction of MEA and cesium iodide regulated the crystallization of perovskite films, improved the degree of (111) crystal orientation, and enabled the formation of high-quality perovskite films without pinholes. As such, based on the synergistic effect of MEA and cesium iodide, we prepared inverted PSCs by sequential deposition method with a PCE of 25.66% (certified at 25.06%) and high V_OC approaching 1.2 V with a steady state PCE of 25.19%. The optimized device can maintain more than 90% of the initial efficiency at the maximum power point for 1000 h. In addition, through this strategy, we also prepared a flexible device with an efficiency of up to 24.03%, which can maintain more than 90% of the initial performance after 5000 bending cycles, thus demonstrating an excellent mechanical stability.