Molecular and Heterojunction Device Engineering of Solution‐Processed Conjugated Reticular Oligomers: Enhanced Photoelectrochemical Hydrogen Evolution through High‐Effective Exciton Separation

Abstract

Covalent organic frameworks (COFs) face limited processability challenges as photoelectrodes in photoelectrochemical water reduction. Herein, sub‐10 nm benzothiazole‐based colloidal conjugated reticular oligomers (CROs) are synthesized using an aqueous nanoreactor approach, and the end‐capping molecular strategy to engineer electron‐deficient units onto the periphery of a CRO nanocrystalline lattices (named CROs‐Cg). This results in stable and processable “electronic inks” for flexible photoelectrodes. CRO‐BtzTp‐Cg and CRO‐TtzTp‐Cg expand the absorption spectrum into the infrared region and improve fluorescence lifetimes. Heterojunction device engineering is used to develop interlayer heterojunction and bulk heterojunction (BHJ) photoelectrodes with a hole transport layer, electron transport layer, and the main active layers, using a CROs/CROs‐Cg or one‐dimensional (1D) electron‐donating polymer HP18 mixed solution via spinning coating. The ITO/CuI/CRO‐TtzTp‐Cg‐HP18/SnO2/Pt photoelectrode shows a photocurrent of 94.9 µA cm‒2 at 0.4 V versus reversible hydrogen electrode (RHE), which is 47.5 times higher than that of ITO/Bulk‐TtzTp. Density functional theory calculations show reduced energy barriers for generating adsorbed H* intermediates and increased electron affinity in CROs‐Cg. Mott‐Schottky and charge density difference analyses indicate enhanced charge carrier densities and accelerated charge transfer kinetics in BHJ devices. This study lays the groundwork for large‐scale production of COF nanomembranes and heterojunction structures, offering the potential for cost‐effective, printable energy systems.