Formation of charge-transfer complexes significantly improves the performance of polymer solar cells based on PBDTTT-C-T: PC71 BM

By adding appropriate proportions of nitrobenzene (C6H5NO2) to the blended solution of poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2′-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]}:[6,6]-phenyl-C71-butyric acid methyl ester (PBDTTT-C-T: PC71BM), we substantially improved the power conversion efficiency from the best reported value of 7.48–8.88%. Experiments and simulations show that nitrobenzene and PBDTTT-C-T form stable coplanar charge-transfer complexes through hydrogen bonds. Formation of the PBDTTT-C-T-C6H5NO2 complex simultaneously increases the external quantum efficiency. The underlying mechanisms of increased external quantum efficiency are attributed to the following: (i) higher lowest unoccupied molecular orbital (LUMO) of PBDTTT-C-T-C6H5NO2 for more efficient photoinduced electron transfer to the LUMO of PC71BM and (ii) efficient quenching of fluorescence in the active layers due to formation of the PBDTTT-C-T-C6H5NO2 complex. This discovery clearly illustrates the potential of hydrogen-bonded complexes as a new route for efficient polymer-based photovoltaic devices. Copyright © 2014 John Wiley & Sons, Ltd.