Poly(1,4‐Diethynylbenzene) Gradient Homojunction with Enhanced Charge Carrier Separation for Photoelectrochemical Water Reduction

: As appealing photoelectrode materials for photoeletrochemical hydrogen evolution reaction (PEC HER), conjugated polymers still show poor PEC HER performance as a result of their serious recombination of photogenerated electrons and holes. Herein, a novel design of gradient homojunction is demonstrated by controlled copolymerization of 1,4-diethynylbenzene (DEB) and 1,3,5-triethynylbenzene (TEB). The as-built gradient distribution of TEB monomer in poly(1,4-diethynylbenzene) (pDEB) leads to continuous band bending engineering, which constitutes a gradient homojunction. Under AM 1.5G irradiation and in 0.1 m Na2 SO4 aqueous solution, the as-fabricated pDEB gradient homojunction exhibits a charge separation efficiency of 0.27% at 0.3 V versus reversible hydrogen electrode (RHE), which is 3.4 and 1.7 times higher than those for pure pDEB and the traditionally designed pDEB homojunction. As a result, the photocurrent of the pDEB gradient homojunction unprecedentedly reaches 55 µA cm-2 at 0.3 V versus RHE, which is much higher than 19 µA cm-2 for pure pDEB, 32 µA cm-2 for the pDEB homojunction, and state-of-the-art organic photocathodes, e.g., g-C3 N4 (≈1-32 µA cm-2 ). This work opens up a new window for the design of gradient homojunctions and will advance the exploration of high-performance organic photoelectrodes.