Small molecule donors T0–T2 with thiazolothiazole as acceptor unit, triphenylamine as donor unit and thiophene with different number as bridge are synthesized. With increasing thiophene number and conjugation length, these molecules exhibit red-shifted absorption (300–600 nm), slightly up-shifted HOMO levels (−5.31 to −5.28 eV), slightly down-shifted LUMO levels (−2.64 to −2.75 eV), and reduced optical band gaps (2.55–2.11 eV). Solution processed organic solar cells based on T0–T2/PC71BM (1:4, w/w) after thermal annealing exhibit power conversion efficiency up to 2.19%, 3.73% and 4.05% under AM 1.5G illumination (100 mW/cm2), respectively. Effects of thermal annealing on morphology, charge transport and photovoltaic property of blend films are investigated.