All polymer bulk heterojunction solar cells with high fill factors based on blends of poly3hexylthiophene: poly(perylene diimidealtterthiophene)

The photovoltaic characteristics of all polymer bulk heterojunction solar cells made of P3HT and a perylene diimide-based copolymer (PEK3) have been studied. Thermal annealing is needed to improve the performances. Annealing optimization induces an enhancement of the power conversion efficiency from 0.06 to 1%, Jsc from 0.24 to 2.9 mA/cm2 and FF from 0.32 to 0.59. The origin of such improvements has been investigated by studying the P3HT:PEK3 blend morphology, by means of absorption and emission spectroscopy and charge transport, from single carrier measurements on P3HT:PEK3 diodes. Upon annealing we have observed an increase in phase segregation and a 100-fold enhancement of the hole and electron mobilities, that favor the dissociation of bound electron-hole pairs and their transport to the electrodes. This explains the high FF of the annealed P3HT:PEK3 solar cell.