Improvement of the photovoltaic properties for P3HT:PCBM system annealed at the temperature in liquid crystalline regions induced by liquid crystal molecules

Abstract

The poly(3-hexylthiophene):[6, 6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic photovoltaics (BHJ OPVs) showed a maximum value of 3.5 % by incorporating of 6 wt% nematic liquid crystals (NLCs) of 4-octyloxy-4′-cyanobiphenyls (8OCB) after annealing at the temperature of 70 °C in liquid crystalline transitions. The 8OCB was immiscible with P3HT, and the melting temperature of P3HT was not influenced at 8OCB doping amount below 10 wt%. Based on the UV–vis analysis, the conjugation length and chain order of P3HT should be the highest at 8OCB content of 6 wt% after annealing at 70 °C. The reflection peak of P3HT shifted to lower positions as the annealing temperature increasing from 70 to 130 °C, indicating that PCBM and 8OCB molecules could be incorporated into the lamellar spacing of P3HT crystallites at high annealing temperatures. The improvement of the solar cell device after annealing at 70 °C is probably due to the optimized phase separation morphology and relatively higher hole mobility in P3HT:PCBM:8OCB system. The 8OCB molecules in the liquid crystalline state are supposed to be a good template for the crystallization of P3HT, as well as the phase separation between P3HT and PCBM.