Woojin Lee School of Energy and Chemical Engineering

Abstract

Organic semiconductors are considered promising light absorbers for photoelectrochemical (PEC) hydrogen production, due to their strong optical absorption, tunable energy levels, and compatibility with low-temperature solution processing. However, the development of organic-based photocathodes is significantly hindered by their inherent instability in aqueous environments, where electrolyte permeation and interfacial degradation accelerate the deterioration of the bulk heterojunction (BHJ) structure. To overcome these intrinsic stability issues, it is necessary to develop protection strategies that can form uniform, pinhole-free barrier layers while maintaining efficient charge transport. In this study, an integrated stabilization is implemented, combining highly colloidally stable ZnO nanoparticle interlayers with conformal atomic layer deposition (ALD) metal oxide coatings, to realize durable and efficient organic photocathodes. The ligand-engineered ZnO nanoparticles demonstrate superior stability in polar solvents, which enhances the formation of smooth and adherent interlayers, thereby substantially improving the adhesion between the organic active layer and the ALD overlayer. This enhanced interfacial compatibility permits the deposition of compact, defect-free ALD films, which is critical for long-term PEC operation, without compromising efficient electron extraction. The robust interface architecture enabled the systematic evaluation of three representative classes of light absorbers. Fullerene-based photocathodes demonstrated a photovoltage of 1.2 V and retained operational stability for up to 12 hours (T50) when combined with ALD TiO2 and RuO2 surface catalysts. Non-fullerene acceptor (NFA) systems incorporating Ru-based interlayers achieved a photocurrent density of 14.9 mA cm-2 and demonstrated enhanced durability. Perovskite/organic tandem photocathodes with a K2CO3-modified 2PACz-K interface yielded a photocurrent density of 7.1 mA cm-2 and a photovoltage of 2.16 V, constituting the first reported case of a transparent, metal-free tandem PEC photocathode. Overall, these results provide a generalizable and scalable strategy for stabilizing organic-based photocathodes via ZnO interfacial engineering in conjunction with ALD protection layers. Moreover, the successful implementation of transparent tandem architecture highlights the significant potential of organic and hybrid photocathodes for bias-free water splitting in future integrated systems.