The Origin of the High Voltage in DPM12/P3HT Solar Cells

Organic solar cells made using a blend of DPM12 and P3HT are studied. The results show that higher V-oc can be obtained when using DPM12 in comparison to the usual mono-substituted PCBM electron acceptor. Moreover, better device performances are also registered when the cells are irradiated with sun-simulated light of 10-50 mW cm(-2) intensity. Electrochemical and time-resolved spectroscopic measurements are compared for both devices and a 100-mV shift in the density of states (DOS) is observed for DPM12/P3HT devices with respect to PCBM/P3HT solar cells and slow polaron-recombination dynamics are found for the DPM12/P3HT devices. These observations can be directly correlated with the observed increase in V-oc, which is in contrast with previous results that correlated the higher V-oc with different ideality factors obtained using dark-diode measurements. The origin for the shift in the DOS can be correlated to the crystallinity of the blend that is influenced by the properties of the included fullerene.