Low band gap copolymers were prepared by incorporating thiophene modified benzo[1,2-b:4,5-b’]dithiophene and alkyloxy or fluorine modified benzothiadiazole as donor and accepter units. These copolymers showed desired deep HOMO and LUMO levels allowing the high Voc and Jsc. PCEs of over 6% are demonstrated in solar cell studies indicating their promising photovoltaic application as donor materials.