Investigation the effect of π bridge and side chain on photovoltaic properties of benzodithiophene and quinoxaline based conjugated polymers

A series of donor-acceptor type alternative polymers containing 2D-BDT and 1D-BDT as the donors and quinoxaline as the acceptor were successfully obtained by Stille coupling reactions. Synthesized polymers containing selenophene and thiophene as pi-bridges in their backbones and 2-ethylhexylselenophene and 2-ethylhexylalkoxy side groups on the BDT structure were named as P1, P2, P3 and P4, respectively. E-onset(ox) and E-onset(red) were investigated for P1, P2, P3, and P4, respectively. Onset oxidation potentials and onset reduction potentials for P2 and P4 containing selenophene pi-bridge in their structure were measured at lower levels than P1 and P3 containing thiophene pi-bridge. Maximum absorptions for P1 and P2 containing 2D-benzodithiophene were observed at 595 nm and 610 nm, while maximum absorption peaks were observed at 575 nm and 610 nm for P3 and P4 polymers. It was observed that the absorption onset of P2 and P4 polymers containing selenophene pi bridges were relatively red-shifted to 735 nm and 732 nm, respectively. According to the kinetic studies, the maximum optical contrast was observed in the bipolaron region for all polymers and was measured as 57% for P1 and P2, 52% for P3, and 55% for P4, respectively. Theoretical calculations were executed on model tetramer structures to shed light on how changing the pi-bridge and substituent on benzodithiophene along the building block influenced the electronic, structural and optical properties of the designed polymers. All polymers were used as electron rich donor materials for bulk heterojunction organic solar cells with ITO/PEDOT:PSS/Polymer:PC71BM/LiF/Al device architecture. P3-based solar cell device resulted the highest power conversion efficiency as 4.47% with V-OC, J(SC) and FF values of 0.80 V, 10.08 mA/cm(2) and 55.5%, respectively.