Engineering multi-phase metal halide perovskites thin films for stable and efficient solar cells

The intrinsic instability of lead halide perovskite materials in ambient atmosphere is one of the most critical issues that impede perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers emerged as promising candidates for stable photovoltaics. The resulting perovskite thin films present complex morphologies, difficult to predict, which will directly affect the device efficiency. Here, by combining in-depth morphological and spectroscopic characterization, we show that both the ionic size and relative concentration of the organic cation drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final morphology. Based on such studies we propose a fine-engineered multi-phases perovskite by employing two different large cations, namely ethyl ammonium and butyl ammonium. The first one takes part to the 3D perovskite phase formation, the second one works as surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. Our findings propose a general approach to achieve reliability of perovskite-based optoelectronic devices.