Synthesis of new 2,6-bis(6-fluoro-2-hexyl-2H-benzotriazol-4-yl)-4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene based D-A conjugated terpolymers for photovoltaic application

Abstract

Two D1-A-D2-A conjugated terpolymers with same A (fluorinated benzotriazole, fBTz), D2 (dithienosilole, DTS) and different D1 (bis(undecyl)benzo[2,1-b:3,4-b']dithiophene for P1 and didodecyloxy-benzo[1,2-b:4,5-b']dithiophene for P2) were synthesized and their optical and electrochemical properties were investigated. Compared to P1, the P2 with didodecyloxy side chains in benzodithiophene showed strong interchain π-π interaction, more redshifted absorption spectrum and higher molar extinction coefficient. The X-ray diffraction results indicated their excellent crystallinity and molecular stacking features, especially P2 containing didodecyloxy side chains in BDT. These two terpolymers were used as donor along with PC71BM for the fabrication of bulk heterojunction polymer solar cells. After the optimization of active layers, i.e. weight ratios of terpolymer to PC71BM and concentration of solvent additive (DIO) in the host solvent, the resultant polymer solar cells showed overall power conversion efficiency of 6.95% (Jsc = 12.23 mA/cm2, Voc = 0.93 V and FF = 0.61) and 8.14% (Jsc = 14.08 mA/cm2, Voc = 0.85 V and FF = 0.68) for P1 and P2 based bulk heterojunction active layers, respectively. The higher power conversion efficiency of P2 based polymer solar cell than P1 based counterpart, may be attributed to the more exciton dissociation, better nanoscale morphology and higher hole mobility in P2:PC71BM active layer than P1:PC71BM.