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 (V-oc), 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 half of the 3-hexylthiophene units (3HT) being replaced by 3HOT, and the trend is directly reflected in the V-oc measured in polymer:PC61BM solar cells. High absorption coefficient and semicrystallinity are retained for all of the copolymers. The semi-random P3HTT-HOT-DPP copolymers with low percentage of 3HOT (up to 15%) show J(sc) of above 10 mA/cm(2), which is comparable to the parent P3HTT-DPP. Importantly, this study demonstrates that significant changes in polymer electronic properties can be induced with only small percentage of comonomers in random and semi-random conjugated polymers.