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 ₆₀BM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) blends and blends of low band gap polymers with PC₇₀BM. 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.