Solvent-Assisted Crystallization of an α-Fe 2 O 3 Electron Transport Layer for Efficient and Stable Perovskite Solar Cells Featuring Negligible Hysteresis.

Inorganic-organic metal halide perovskite solar cells (PSCs) show power conversion efficiency values approaching those of state-of-the-art silicon solar cells. In a quest to find suitable charge transport materials in PSCs, hematite (α-Fe 2 O 3 ) has emerged as a potential electron transport layer (ETL) in n-i-p planar PSCs due to its low cost, UV light stability, and nontoxicity. Yet, the performance of α-Fe 2 O 3 -based PSCs is far lower than that of state-of-the-art PSCs owing to the poor quality of the α-Fe 2 O 3 ETL. In this work, solvent-assisted crystallization of α-Fe 2 O 3 ETLs was carried out to examine the impact of solvents on the optoelectronic properties of α-Fe 2 O 3 thin films. Among the various solvents used in this study (deionized water, ethanol, iso-propanol, and iso-butanol), optimized ethanol-based α-Fe 2 O 3 ETLs lead to champion device performance with a power conversion efficiency of 13% with a reduced hysteresis index of 0.04 in an n-i-p-configured PSC. The PSC also exhibited superior long-term inert and ambient stabilities compared to a reference device made using a SnO 2 ETL. Through a series of experiments spanning structural, morphological, and optoelectronic properties of the various α-Fe 2 O 3 thin films and their devices, we provide insights into the reasons for the improved photovoltaic performance. It is noted that the formation of a pinhole-free compact morphology of ETLs facilitates crack-free surface coverage of the perovskite film atop an α-Fe 2 O 3 ETL, reduces interfacial recombination, and enhances charge transfer efficiency. This work opens up the route toward novel ETLs for the development of efficient and photo-stable PSCs.