A hybrid bulk-heterojunction photoanode for direct solar-to-chemical conversion

Abstract

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 H₂ and I₃⁻ from simulated sunlight without applied bias.