Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

The fill factor of polymer bulk heterojunction solar cells (PSCs), which is mainly governed by the processes of charge carrier generation, recombination, transport and extraction, and the competition between them in the device, is one of the most important parameters that determine the power conversion efficiency of the device. We show that the fill factor of PSCs based on thieno[3,4-b]-thiophene/ben zodithiophene (PTB7):[6,6]-phenyl C-71-butyric acid methylester (PC71BM) blend that only have moderate carrier mobilities for hole and electron transport, can be enhanced to 76% by reducing the thickness of the photoactive layer. A drift-diffusion simulation study showed that reduced charge recombination loss is mainly responsible for the improvement of FF, as a result of manipulating spatial distribution of charge carrier in the photoactive layer. Furthermore, the reduction of the active layer thickness also leads to enhanced built-in electric field across the active layer, therefore can facilitate efficient charge carrier transport and extraction. Finally, the dependence of FF on charge carrier mobility and transport balance is also investigated theoretically, revealing that an ultrahigh FF of 80-82% is feasible if the charge mobility is high enough (similar to 10(-3)-10(-1) cm(2)/V s). (C) 2015 Elsevier B.V. All rights reserved.