An Organic Semiconductor Photoelectrochemical Tandem Cell for Solar Water Splitting

Abstract Photoelectrochemical cells employing organic semiconductors (OS) are promising for solar‐to‐fuel conversion via water splitting. However, despite encouraging advances with the half reactions, complete overall water splitting remains a challenge. Herein, a robust organic photocathode operating in near‐neutral pH electrolyte by careful selections of a semiconducting polymer bulk heterojunction (BHJ) blend and organic charge‐selective layer is realized. The optimized photocathode produces a photocurrent density of >4 mA cm −2 at 0 V vs the reversible hydrogen electrode ( V RHE ) for solar water reduction with noticeable operational stability (retaining ≈90% of the initial performance over 6 h) at pH 9. Combining the optimized BHJ photocathode with a benchmark BHJ photoanode leads to the demonstration of a large‐area (2.4 cm 2 ) organic photoelectrochemical tandem cell for complete solar water splitting, with a predicted solar‐to‐hydrogen (STH) conversion efficiency of 0.8%. Under unassisted two‐electrode operation (1 Sun illumination) a stabilized photocurrent of 0.6 mA and an STH of 0.3% are observed together with near unity Faradaic efficiency of H 2 and O 2 production.