Phase-Dependent Photocurrent Generation in Polymer/Fullerene Bulk Heterojunction Solar Cells

Voltage-dependent white-light bias external quantum efficiency (EQE) measurements are employed to compare the relative voltage-dependence of photocurrent generation of polymer and fullerene phases in polymer bulk heterojunction solar cells. By measuring EQE spectra as a function of applied bias, voltage dependent changes in the shape of EQE spectra can be detected and compared with the absorption profiles of polymer and fullerene. Several high-efficiency systems are studied including poly(3-hexylthiophene):PC 60 BM ([6,6]-phenyl-C 61 -butyric acid methyl ester) blends and blends of low band gap polymers with PC 70 BM. For a number of systems (but not all), voltage-dependent but light-intensity-independent changes in the shape of the EQE spectrum are observed that originate from differences in the voltage dependence of photocurrent for polymer and fullerene absorption. For one system, a stronger voltage-dependence of photocurrent generation following fullerene absorption is found, whereas for another system a stronger voltage-dependence of photocurrent generation following polymer absorption is observed. We attribute this effect to differences in the character of interfacial electron–hole pairs formed immediately after charge transfer, sensitive to differences in the nature of the initial excitonic state and the conformation nature of the interface.