Abstract
Formic acid (FA) was used as a novel additive in bulk heterojunction (BHJ) solar cells, which contains blends of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The effect of FA on the performance of PTB7:PC71BM based BHJ solar cells is investigated. By the incorporation of FA, the device with the ratio of 6 vol % shows the best power conversion efficiency (PCE) of 9.04%, along with the short-circuit current density (J(sc)), open-circuit voltage (V-oc), and fill factor (FF) being 24.11 mA/cm(2), 0.72 V, and 52.11%, respectively. Experimental results suggest that FA has a strong influence on charge carrier dynamics with a significant increase in J(sc) by similar to 65% and the dramatically enhanced PCE is mainly due to the increase of absorption and exciton generation of the active layers and the improved charge-carrier mobility of the devices. (C) 2015 Elsevier B.V. All rights reserved.