Graphene-Based Hole-Selective Layers for High-Efficiency, Solution Processed, Large-Area, Flexible, Hydrogen-Evolving Organic Photocathodes

Regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the workhorse of organic photovoltaics, has been recently exploited in bulk heterojunction (BHJ) configuration with phenyl-C61-butyric acid methyl ester (PCBM) for solution-processed hydrogen-evolving photocathodes, reaching cathodic photocurrents at 0 V vs RHE (J(0) (Vvs) (RHE)) of up to 8 mA cm(-2). The photoelectrochemical performance of these photocathodes strongly depends on the presence of the electron- (ESL) and hole- (HSL) selective layers. While TiO2 and its substoichiometric phases are consolidated ESL materials, the currently used HSLs (e.g., MoO3, CuI, PEDOTT:PSS, WO3) suffer electrochemical degradation under hydrogen evolution reaction (HER) working conditions. In this work, we use solution-processed graphene derivatives as HSL to boost the efficiency and durability of rr-P3HT:PCBM-based photocathodes, demonstrating record-high performance. In fact, our devices show cathodic J(0) (Vvs) (RHE) of 6.01 mA cm(-2), onset potential (V-o) of 0.6 V vs RHE, ratiometric power-saved efficiency (phi(saved)) of 1.11%, and operational activity of 20 h in 0.5 M H2SO4 solution. Moreover, the designed photocathodes are effectively working in different pH environments ranging from acidic to basic. This is pivotal for their exploitation in tandem configurations, where photoanodes operate only in restricted electrochemical conditions. Furthermore, we demonstrate the scalability of our all solution-processed approach by fabricating a large-area (similar to 9 cm(-2)) photocathode on flexible substrate, achieving a remarkable cathodic J(0) (Vvs) (RHE) of 2.8 mA cm(-2), V. of 0.45 V vs RHE, and phi(saved) of 0.31%. This is the first demonstration of highly efficient rr-P3HT:PCBM flexible photocathodes based on low-cost and solution-processed manufacturing techniques.