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Metal halide perovskites of the general formula ABX3—where A is a monovalent cation such as caesium, methylammonium or formamidinium; B is divalent lead, tin or germanium; and X is a halide anion—have shown great potential as light harvesters for thin-film photovoltaics1,2,3,4,5. Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI3) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells6,7,8,9, and maximizing the performance of this material in such devices is of vital importance for the perovskite research community. Here we introduce an anion engineering concept that uses the pseudo-halide anion formate (HCOO−) to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the perovskite films and to augment the crystallinity of the films. The resulting solar cell devices attain a power conversion efficiency of 25.6 per cent (certified 25.2 per cent), have long-term operational stability (450 hours) and show intense electroluminescence with external quantum efficiencies of more than 10 per cent. Our findings provide a direct route to eliminate the most abundant and deleterious lattice defects present in metal halide perovskites, providing a facile access to solution-processable films with improved optoelectronic performance.

Organic semiconductors (OSs) are emerging candidates as light-harvesting materials in photoelectrochemical (PEC) cells for direct solar-to-chemical conversion. Despite significant recent progress with OS-based photocathodes, the development of efficient and stable OS-based photoanodes remains a challenge. Here, we demonstrate the concept of an in situ formed covalent polymer network (CPN) in a hybrid CPN:SnO2 bulk-heterojunction (BHJ) to increase the photocurrent density (J(ph)) and stability of OS-based photoanodes for PEC splitting of hydroiodic acid (HI). Our results indicate that the CPN:SnO2 BHJ overcomes the limited exciton diffusion length in OSs and provides a J(ph) improvement of more than three orders of magnitude compared to equivalent bilayer heterojunctions. Furthermore, insight into the operation of the hybrid BHJ in direct contact with aqueous electrolyte is gained with electrochemical impedance spectroscopy and PEC measurements under varying pH. With 1 M HI (pH 0) as the electrolyte, an optimized CPN:SnO2 photoanode without catalyst or protection layer delivers a J(ph) of 3.3 mA cm(-2) at the thermodynamic potential of iodide oxidation (+0.54 V vs. the normal hydrogen electrode) and a continuous operation for 27 h (J(ph) loss of 12%), representing a new benchmark for OS photoanodes for solar-to-chemical conversion. Complete HI splitting is further demonstrated in an all-OS photocathode/photoanode PEC cell to produce H-2 and I-3(-) from simulated sunlight without applied bias.