Role of Water in Suppressing Recombination Pathways in CH₃NH₃PbI₃ Perovskite Solar Cells

Moisture degradation of halide perovskites is the Achilles heel of perovskite solar cells. A surprising revelation in 2014 about the beneficial effects of controlled humidity in enhancing device efficiencies overthrew established paradigms on perovskite solar cell fabrication. Despite the extensive studies on water additives in perovskite solar cell processing that followed, detailed understanding of the role of water from the photophysical perspective remains lacking; specifically, the interplay between the induced morphological effects and the intrinsic recombination pathways. Through ultrafast optical spectroscopy, we show that both the monomolecular and bimolecular recombination rate constants decrease by approximately 1 order with the addition of an optimal 1% H 2 O by volume in CH 3 NH 3 PbI 3 as compared to the reference (without the H 2 O additive). Correspondingly, the trap density reduces from 4.8 × 10 17 cm –3 (reference) to 3.2 × 10 17 cm –3 with 1% H 2 O. We obtained an efficiency of 12.3% for the champion inverted CH 3 NH 3 PbI 3 perovskite solar cell (1% H 2 O additive) as compared to the 10% efficiency for the reference cell. Increasing the H 2 O content further is deleterious for the device. Trace amounts of H 2 O afford the benefits of surface trap passivation and suppression of trap-mediated recombination, whereas higher concentrations result in a preferential dissolution of methylammonium iodide during fabrication that increases the trap density (MA vacancies). Importantly, our study reveals the effects of trace H 2 O additives on the photophysical properties of CH 3 NH 3 PbI 3 films.