Effect of side chains on phenanthrene based D-A type copolymers for polymer solar cells

A series of low band gap conjugated copolymers containing 9,10-modified phenanthrene and diketo-pyrrolopyrrole (DPP) units were synthesized as electron donor materials for bulk heterojunction organic solar cells. These donor-acceptor type PDPP copolymers have varying solubilizing groups on their identical conjugated backbones. The optical bandgap of PDPP copolymers is about 1.6 eV which corresponds to the long wavelength region of the solar spectrum. Through the incorporation of phenanthrene units into the conjugated backbone instead of commonly used thiophene derivatives, a higher opencircuit voltage of about 0.8 V could be achieved, as a result of their deeper HOMO level. Of all the devices, the P4: PC61BM BHJ system showed the best performance with a V-oc of 0.79 V, a J(sc) of 5.97 mA cm(-2), a fill factor of 0.62 and a power conversion efficiency of 2.73% due to superior nanoscale phase separation between the electron donor and electron acceptor materials than in the other polymers arising from short-branched solubilizing groups on the phenanthrene side of its conjugated backbone. (C) 2017 Published by Elsevier B. V.