Nonfullerene Bulk Heterojunction‐Based Photocathodes for Highly Efficient Solar Hydrogen Production in Acidic and Neutral Solutions

Highly efficient photocathode with onset potential beyond 0.6 V is very desirable in order to construct photoelectrochemical (PEC) device for stand‐alone overall water splitting. In this work, it is reported a nonfullerene PBDB‐T:ITIC bulk heterojunction (BHJ) based photocathode delivering unprecedented positive onset potential of 0.8 V and optimal photocurrent density of −11.7 mA cm −2 at 0 V in pH 1 solution. The yielded high ideal ratiometric power‐saved efficiency (Φ saved,ideal ) of 4.1% is a record among the previous organic photocathodes. When the electrolyte pH moves to neutral, the Φ saved,ideal is still to be 3.2%, yet being twice more than the currently reported highest value. Pairing with TiO x , the hole transport layer of CuO x with more efficient charge carrier transportation and hole extraction ability excels MoO 3 and NiO x , contributing to the highly efficient solar hydrogen production in both acidic and neutral solutions. Premixing the BHJ blend precursor with an anti‐oxidative agent is proved to be a beneficial strategy to improve the PEC stability by suppressing the photo‐oxidation degradation of organic polymer materials. Subsequently a conceptual PEC cell comprising of tandem CuO x /BHJ/TiO x /Pt photocathode and BiVO 4 /NiFeO x photoanode with a high solar‐to‐hydrogen efficiency up to 1.1% is fabricated in neutral solution without assistance of extra bias.