Correlation of structure and photovoltaic performance of benzo[1,2-b:4,5-b′]dithiophene copolymers alternating with different acceptors†
Four π-conjugated benzo[1,2-b:4,5-b′]dithiophene (BDT) based polymers were synthesized for application in polymer solar cells. These polymers possessed desirable HOMO/LUMO levels for polymer photovoltaic applications. PBDTT–TTz and PBDTT–DTBT displayed strong absorption in the range of 300–650 nm, while PBDTT–DPP and PBDTT–TTDPP showed a further 100 nm extended absorption band. The lowest unoccupied molecular orbital energy levels of polymers were tuned effectively from −3.34 eV to −3.81 eV by fusing with different accepting units. A maximum power conversion efficiency of 2.60% was obtained from photovoltaic cells with a PBDTT–TTz : PC61BM (1 : 2, w/w) blend film as the active layer, with a short circuit current density of 8.37 mA cm−2, an open circuit voltage of 0.70 V, and a fill factor of 44.3%.