Solution-processed asymmetry conjugated reticular oligomers for large-area panel photoelectrodes in photoelectrochemical devices

Covalent organic frameworks (COFs) show potential in photoelectrochemical (PEC) water reduction. Herein, we synthesize solution-processed benzoxazole-based conjugated reticular oligomers (BBO-CROs) in micelle nanoreactors, acting as an “electronic paint” for fabricating consecutive panel-type photoelectrodes for PEC water reduction. The small-molecule BBO-CROPDA, BBO-CRODHTA, and BBO-CROBPY exhibit a narrower energy bandgap and prolong the fluorescence lifetimes of photoinduced charge carriers compared to bulk BBO-COFs. Colloidal COFs in PEC water reduction are illustrated through the creation of flexible full devices via spinning coating. The optimized photoelectrode CuI/BBO-CROBPY+P27/SnO2/Pt result in ΔJ of 86.5 μA cm−2 at 0.4 V vs. RHE, which is 86.5 times higher than that of bulk BBO-COFBPY. The CuI/BBO-CROBPY+P27/SnO2/Pt photoelectrode with an area of 100 cm² displays an impressive ΔJ of 82.0 μA cm−2 with good durability. A flat photoelectrode surface and the generation of photogenerated charge carriers under light are observed using scanning electrochemical microscopy (SECM) as an in-situ technique. Density Functional Theory (DFT) calculation indicates that the asymmetry of the BBO unit prompts electrons to preferentially move in a defined direction upon light excitation in the small-molecular framework, thereby suppressing electron backflow. This work overcomes COF processability limitations in PEC water reduction and paves the way for using flexible devices. [Display omitted] •Sub-5 nm CROs with excellent solution processability were synthesized, overcoming the poor processability of COFs.•We integrated an asymmetrical building block incorporating a benzoxazole unit into CROs to prevent exciton recombination.•The high solubility of CROs allows direct film production via a coating, ideal for large-scale industrial manufacturing.•The CuI/BBO-CROBPY+P27/SnO2/Pt photoelectrode with an area of 100 cm2 displays a ∆J of 82.0 μA cm‒2 with good durability.