A facile strategy to enhance the fill factor of ternary blend solar cells by increasing charge carrier mobility

Two conjugated polymers based on benzo[1,2-b: 4,5-b'] dithiophene (BDT) with triethylene glycol (TEG) and ethylhexyl side chains (abbreviated as PBDTT-TEG and PBDTT-EH, respectively) were designed and synthesized. A polymer field-effect transistor of PBDTT-TEG exhibited a charge carrier mobility nearly one order of magnitude higher than that of PBDTT-EH. Measurements on polymer solar cells (PSCs) based on polymer:[6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) binary blends showed the PBDTT-TEG-based devices had higher short-circuit current density (J(sc)) than PBDTT-EH. The effect of the incorporation of PBDTT-TEG into PBDTT-EH on the photovoltaic properties has been investigated by blending them together with different ratios. The optimization of the devices showed an obvious increase in fill factor (FF) values and an improvement in efficiency compared with their binary ones, which is ascribed to a higher charge mobility and higher crystallinity of PBDTT-TEG.