Oxidant-controlled band structure and charge transport in benzothiadiazole-based conjugated polymers prepared by oxidative chemical vapor deposition

Oxidative chemical vapor deposition enables the solvent-free synthesis of conjugated polymers with tuneable optoelectronic properties. Here, within a material design perspective, the use of two different solid oxidants, i.e. iron(III) chloride (FeCl3) and iron(III) trifluoromethanesulfonate (Fe(OTf)3) is investigated to elucidate their effect on the optoelectronic properties and structural arrangement of the 4,7-Di(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)benzo[1,2,5]thiadiazole oCVD-polymerized film, namely pDEDOT-BT. Both the polymerization yield and the insertion of either chlorine or fluorinated substituents, are dictated by the oxidant choice, which strongly influences the frontier orbital energies. The structure–property relationships have been thoroughly investigated using a wide range of techniques, revealing that oxidant-dependent reaction kinetics also modulate film morphology, with a significant impact on charge-transport properties. FeCl3–derived pDEDOT-BT exhibited uniform film-formation, enhanced structural organization, and charge carrier transport, which resulted in a superior mobility of 0.3 cm2 V−1 s−1 and conductivity of ≈ 1 S cm−1. Conversely, the Fe(OTf)3-derived pDEDOT-BT showed a heterogenous structure where large, isolated domains hinder macroscopic charge transport, resulting in very low conductivities (' 10-3 S cm−1) and negligible hole mobility. Overall, oxidant selection in oCVD offers a simple handle to tune band structure, morphology, and transport in benzothiadiazole-based conjugated polymers.