Insights towards optimizing electron injection in BHJ photoanodes
© 2022 EPFL
The use of a bulk heterojunction (BHJ) of organic semiconductors to drive solar-to-fuel energy con version via 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. Now in a new paper, Dr. Arvindh Sekar of the LIMNO lab at EPFL has revealed important factors that determine efficient electron injection.
Dr. Sekar (who recently defended his PhD thesis), together with colleges in the LIMNO lab prepared 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 up to 4.1 mA cm–2 was observed at 1.23 V vs reversible hydrogen electrode in optimized photoanodes. Moreover, this work established that an energetically lower-lying donor polymer leads to improved performance due to both improved exciton separation and better charge collection. Similarly, lower-lying acceptors also gave 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. Thus, future work on BHJ-based photoanodes should reasonably focus on developing materials with low-lying HOMO and LUMO values and the stable attachment of oxygen evolution reaction (OER) co-catalyst overlayers that have low overpotential for O2 evolution at acidic-to-neutral pH, because obtaining high photocurrent under alkaline pH while also efficiently injecting into an oxide ETL is a significant challenge for OSC-based photoanodes for water splitting.