Passivating defects via 4-cyanobenzenaminium iodide enables 22.44% efficiency perovskite solar cells

Defects can easily form at the surfaces and grain boundaries of solution-processed organic-inorganic halide perovskites films, leading to undesired non-radiative recombination and ion migration in the device. Consequently, this limits the performance of perovskite solar cells (PSCs). Herein, a new organic ammonium halide salt with cyano (-CN) functional group, 4-cyanobenzenaminium iodide (CNBAI), was developed to passivate the defects in the bulk of perovskite films. It was found that the CNBAI can improve the film morphology of the perovskite layer and passivate the various types of defects in the perovskite. On the one hand, the -CN group could bind with the under-coordinated Pb2+ and form intermolecular halogen bonds with the iodide atom in perovskite, as illustrated by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy measurements. On the other hand, CNBAI could fill the iodide vacancy with its own iodide ions. For the first time, we demonstrate experimentally that the -CN group can simultaneously interact with Pb2+ and form CN…I halogen bond in the perovskite. In addition, the introduced CNBAI improved energy alignment between the perovskite and adjacent charge transport layers. Finally, the PSCs fabricated with CNBAI-based perovskite films achieved the best PCE of 22.4% with negligible hysteresis and displayed good long-term stability.