Bulk Heterojunction Organic Semiconductor Photoanodes: Tuning Energy Levels to Optimize Electron Injection

The use of a bulk heterojunction of organic semiconductors to drive photoelectrochemical water splitting is an emerging trend; however, the optimum energy levels of the donor and acceptor have not been established for photoanode operation with respect to electrolyte pH. Herein, we prepare a set of donor polymers and non-fullerene acceptors with varying energy levels to probe the effect of photogenerated electron injection into a SnO₂-based substrate under sacrificial photo-oxidation conditions. Photocurrent density (for sacrificial oxidation) up to 4.1 mA cm^-2 was observed at 1.23 V vs reversible hydrogen electrode in optimized photoanodes. Moreover, we establish that a lower-lying donor polymer leads to improved performance due to both improved exciton separation and better charge collection. Similarly, lower-lying acceptors also give photoanodes with higher photocurrent density but with a later photocurrent onset potential and a narrower range of pH for good operation due to the Nernstian behavior of the SnO₂, which leads to a smaller driving force for electron injection at high pH.