Oxidative CVD of side-chain-free benzo[1,2-b:3,4-b′:5,6-b″]trithiophene polymer for photoelectrochemical energy conversion

Conjugated polymers are attractive photoelectrode materials because their π-conjugated structures, light absorption, and redox properties can be tuned by molecular design. However, many promising conjugated building blocks are poorly soluble and difficult to process into uniform conjugated polymer thin films, often requiring solubilizing side chains that can compromise packing and charge transport. Here we report the direct fabrication of side-chain-free poly(benzo[1,2-b:3,4-b′:5,6-b″]trithiophene) (pBTT) photocathodes by oxidative chemical vapor deposition (oCVD), a solvent-free route that converts unsubstituted BTT into adherent conjugated polymer coatings. HRMS, XPS, UV–vis–NIR spectroscopy, SEM, and conductivity measurements confirm the conversion of BTT into a conjugated polymer thin film, evidenced by oxidative coupling, visible-light absorption, fibrous film growth, and a ca. two-order-of-magnitude increase in electrical conductivity. Among the oCVD conditions examined, pBTT(12) provides the best structure–property balance, whereas excess oxidant loading introduces Fe/Cl-rich residues that suppress photocurrent. FeCl 3 -derived reference electrodes showed substantial dark cathodic current but negligible photocurrent, whereas EDTA treatment enhanced the pBTT photoresponse without major morphological disruption. Together with O₂- and pH-dependent controls, these results assign the light-driven ORR response primarily to the pBTT photocathode. Without sacrificial electron donors, pBTT(12)-EDTA gave the strongest response, reaching 69 μA cm⁻² at 0.581 V vs RHE and producing 2.2 μmol H₂O₂ with an apparent Faradaic efficiency over 90% during 2 h of illumination. Upon reuse, it retained comparable performance, producing 2.1 μmol H₂O₂ with ca. 89% efficiency. Post-operation imaging and SEM revealed pH-dependent film stability, the microfiber morphology remained largely intact after 6 h of chronoamperometry under acidic conditions, whereas greater degradation occurred at higher pH. This operational stability likely contributed to the preserved H₂O₂ production upon reuse, indicating that film integrity, alongside the intrinsic properties of the polymer, is an important factor governing sustained photoelectrocatalytic activity. These results establish oCVD-grown pBTT as a stable side-chain-free organic photocathode for selective photo-assisted H₂O₂ generation.