Multifunctional Guanidine\nIonic Liquid with Lactate\nAnion-Assisted Crystallization and Defect Passivation for High-Efficient\nand Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) are receiving tremendous\nattention\namong other photovoltaic devices for their high power conversion efficiency\n(PCE), facile fabrication technique, eco-friendliness, and low cost.\nUnfortunately, the intrinsic defects and the quality of the perovskite\nfilms arising from the halide ion migration and undercoordinated Pb<sup>2+</sup> are still considered to be the bottleneck for long-term\noperational stability. Herein, the introduction of task-specific ionic\nliquid (IL) tetramethylguanidine lactate (GuLAC) demonstrates excellent\ndefect passivation effects and crystal growth. More specifically,\nthe formation of hydrogen bonds between –NH<sub>2</sub> in\nGA<sup>+</sup> and I<sup>–</sup> passivates cation defects,\nwhile strong chemical interaction of lactate anion passivates the\nundercoordinated Pb<sup>2+</sup>. Both experimental observations and\ntheoretical simulation confirm the strong interaction of GuLAC with\nthe perovskite, which is responsible for restricting ion migration,\nimproving grains’ size, and elongating the carrier lifetime.\nAs a result, the IL-modified device exhibits improved PCE and superior\nlong-term stability compared to the control device. The incorporation\nof IL additives proves to be a viable approach for achieving both\nhigh PCE and stable PSC devices.