Precise control of PbI₂ excess into grain boundary for efficacious charge extraction in off-stoichiometric perovskite solar cells

Excess PbI₂ has been widely introduced in halide perovskite precursor solution to passivate the grain boundary defects in high efficient perovskite solar cells (PSCs). However, the passivation effect is highly sensitive to the PbI₂ content and overmuch PbI₂ affects the crystalline growth and charge transport. We report here a precious control of excess PbI₂ into perovskite grain boundaries by methylamine treatment for efficacious defects passivation and charge extraction. The crystallization of the deposited perovskite films was carefully evaluated and the charge recombination dynamics were systematically analyzed through time-resolved photoluminescence spectroscopy, space charge limited current, electrochemical impedance spectroscopy and Mott-Schottky analysis. The precise PbI₂ introduction to the grain boundaries results in a significantly lower defect state density, efficient charge extraction and prolonged carrier lifetime, causing a notable increase of short-circuit current density and negligible hysteresis in the corresponding device. Our work demonstrates a reliable method to alleviate the rigorous tolerance of PbI₂ excess in high efficient PSCs.