Molecular Orientation of Polymer Acceptor Dominates Open-Circuit Voltage Losses in All-Polymer Solar Cells

Low open-circuit voltage (Voc) induced by energy loss in organic solar cells is considered to be one of the most influencing factors limiting device performance, in which morphology of the active layer plays a crucial role in determining energy loss. By employing a bilayer structure of the P3HT:N2200 all-polymer system, we have identified the isolated impact of a molecular packing structure on device Voc with analysis of energy loss processes. Thermal annealing and various solvents were used to control molecular orientation in P3HT:N2200 bilayer devices, in which different Voc spanning from 0.45 to 0.54 V could be obtained. It was found that energy of charge-transfer state (Ect) differed in these bilayer devices. Besides, increased charge recombination could be observed in bilayer devices when N2200 layers exhibited face-on orientation, which caused an additional energy loss and decreased Voc. Our results suggest that rational control of polymer molecular orientation is essential to reduce the energy loss and ultimately achieve high Voc in all-polymer solar cells.