Fine Tuning of Polymer Properties by Incorporating Strongly Electron-Donating 3-Hexyloxythiophene Units into Random and Semi-random Copolymers

Two series of copolymers containing varying percentages of strongly electron-donating 3-hexyloxythiophene units (3HOT), namely the random poly(3-hexylthiophene-co-3-hexyloxythiophene)s (P3HT-co-3HOTs) and the semi-random poly(3-hexylthiophene–thiophene–3-hexyloxythiophene–diketopyrrolopyrrole)s (P3HTT-HOT-DPPs), were synthesized via Stille polymerization. The influence of 3HOT content on UV–vis absorption, HOMO energy levels, polymer crystallinity, and polymer:PC61BM solar cell performance, especially the open-circuit voltage (Voc), was investigated. Importantly, introduction of the strong donor 3HOT results in regularly decreased band gaps and broadened absorption compared to the corresponding parent polymers, regioregular poly(3-hexylthiophene) (P3HT) and semi-random poly(3-hexylthiophene–thiophene–diketopyrrolopyrrole) (P3HTT-DPP). The HOMO energies of both random P3HT-co-3HOT and semi-random P3HTT-HOT-DPP copolymers increase significantly with 3HOT incorporation, from −5.2 eV to around −4.95 eV with h...