Charge carrier mobility of the organic photovoltaic materials PTB7 and PC₇₁BM and its influence on device performance

The mobility is an important parameter for organic solar cell materials as it influences the charge extraction and recombination dynamics. In this study, the time of flight technique is used to investigate the charge mobility of the important organic photovoltaic materials PC71BM, PTB7 and their blend. The electron mobility of PC71BM is in the region of 1 × 10−3 cm2/Vs for the neat fullerene film, and has a positive electric field dependence. The hole mobility of PTB7 is 1 × 10−3 cm2/Vs for the neat film and 2 × 10−4 cm2/Vs for their blend. The hole mobility of the blend reduces by a factor of a thousand when the sample is cooled from room temperature to 77 K. This finding is compared with the device performance of efficient PTB7:PC71BM solar cells for varying temperature. At 77 K the solar cell efficiency halved, due to losses in fill factor and short circuit current. Bimolecular and trap-assisted recombination increase at low mobility (low temperature) conditions, whereas at high mobility conditions the open circuit voltage reduces. The power conversion efficiency as a function of temperature has a maximum between 260 K and 295 K, revealing an optimized mobility at room temperature.