Influence of the acceptor on electrical perfomance and charge carrier transport in bulk heterojunction solar cells with HXS-1

Enhancing the open-circuit voltage (VOC) is one way to increase the efficiency of organic solar cells. In cells with the polymer poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1) this can be achieved by replacing the acceptor [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) with indene-C60-bis-adduct (ICBA). The lowest unoccupied molecular orbital (LUMO) of ICBA is located at a higher energy, which leads to an increase of VOC from 0.86 to 1.05 V. However, the short-circuit current density (JSC) and fill factor (FF) are significantly lower in HXS-1:ICBA cells when compared with HXS-1:PC71BM cells, and thus the overall efficiency drops from almost 5% to 2.5%. Despite the smaller LUMO-LUMO offset between HXS-1 and ICBA, strong photoluminescence quenching as well as transient absorption studies indicate efficient and fast exciton dissociation in cells with either fullerene. The slope of the current-voltage characteristics of HXS-1:ICBA cells at short-circuit conditions and the lower dark current in forward direction suggest poor charge carrier transport. These findings were reproduced by a reduction of the electron mobility in electrical simulations. Furthermore, results from Suns-VOC measurements reveal a dramatically increased transport resistance in cells with ICBA when compared with devices using PC71BM. The observed effects could at least be partially due to a finer morphology in the HXS-1:ICBA layer, which is supported by AFM images.