Organic semiconductor bulk heterojunctions for solar-to-chemical conversion: recent advances and challenges

Solar fuel production involving the conversion of solar energy directly into chemical fuels such as hydrogen and valuable chemicals using photoelectrochemical (PEC) cells and photocatalysts (PCs) offers a promising avenue for sustainable energy while reducing carbon emissions. However, existing PEC cells and PCs fall short of economic viability due to their low solar-to-chemical (STC) conversion efficiency associated with the employed semiconductors, highlighting the clear need for identifying ideal semiconductor materials. Organic semiconductors (OSs), π-conjugated carbon-based materials, have emerged as promising candidates for enhancing STC conversion efficiency due to their remarkable optoelectrical properties, which can be readily adjustable through molecular engineering. In particular, the use of OS bulk heterojunctions (BHJs) consisting of intermixed electron-donating and electron-accepting OSs facilitates efficient charge generation under illumination, thereby contributing to enhanced STC conversion efficiency. This review explores the recent advancements in the rational design of OS materials and approaches aimed at enhancing the performance of BHJ-based PEC cells and PCs for solar-driven production of hydrogen and valuable chemicals. The discussion also introduces new perspectives to address the remaining challenges in this field.