(Invited) Organic Photoelectrochemical Cells for Overall Solar Water Splitting

Solar-driven water splitting to produce hydrogen and oxygen offers a promising avenue for reducing reliance on fossil fuels as hydrogen can be converted into electrical energy using a fuel cell or transformed into useful chemical feedstocks. Despite its potential, the search for cost-effective light harvesting semiconductors suitable for industrial-scale deployment remains a challenge. Organic photoelectrochemical cells (OPECs) utilizing organic semiconductors (OSs) coupled with co-catalysts have recently attracted great attention as alternative photoelectrodes for solar water splitting, considering the unique features of OSs such as precisely tunable optoelectrical properties and solution-processability at low temperatures. However, the conversion efficiency and stability of OPECs (both photocathode and photoanode) have remained particularly poor. Herein, I present high-performance and robust organic photoelectrochemical cells by employing a bulk heterojunction (BHJ) blend of semiconducting polymers as a photoactive layer. The in-depth study using sacrificial agents for photoreduction and photooxidation unveils critical parameters that significantly affect the performance and operational stability of OPECs: (i) rational selection of semiconducting polymer donor and acceptor to generate free charges efficiently and ensure chemical stability upon illumination, (ii) large surface roughness of interlayers to improve interfacial adhesion, and (iii) mitigation of charge accumulation at the interfaces. By leveraging these findings, the optimized polymer BHJ photocathode and photoanode show outstanding performance and robustness compared to previous OPECs, demonstrating a new benchmark of OPECs in solar water splitting. Consequently, this advancement, combined with the simplicity of the polymer blending process, establishes the use of polymer BHJs as a promising route for efficient and scalable solar-driven water splitting technology.