Significant influence of doping effect on photovoltaic performance of efficient fullerene-free polymer solar cells

The modification mechanism of the water/alcohol cathode interlayer is one of the most complicated problems in the field of organic photovoltaics, which has not been clearly elucidated yet; this greatly restricts the further enhancement of the PCE for polymer solar cells. Herein, we clarified the different effects of PFN and its derivatives, namely, poly[(9,9-bis(3'-(N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) in modifying fullerene-free PSCs. It is found for the first time that doping on IT-4F by the amino group of PFN leads to the unfavorable charge accumulation, and hence, forms a dense layer of electronegative molecule due to the poor electron transport capacity of the non-fullerene acceptor IT-4F. The electronegative molecular layer can block the electron transfer from the active layer to the interlayer and cause serious charge recombination at the active layer/cathode interface. This mechanism could be verified by the ESR measurement and electron-only devices. By replacing PFN with PFN-Br, the excessive doping effect between the cathode interlayer and IT-4F is eliminated, by which the charge transport and collection can be greatly improved. As a result, a high PCE of 13.5% was achieved in the fullerene-free PSCs. (C) 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.