The open-circuit voltage ͑V oc ͒ of polymer:fullerene bulk heterojunction solar cells is determined by the interfacial charge-transfer ͑CT͒ states between polymer and fullerene. Fourier-transform photocurrent spectroscopy and electroluminescence spectra of several polymer:fullerene blends are used to extract the relevant interfacial molecular parameters. An analytical expression linking these properties to V oc is deduced and shown to be valid for photovoltaic devices comprising three commonly

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... sed conjugated polymers blended with the fullerene derivative ͓6,6͔-phenyl-C61-butyric acid methyl ester ͑PCBM͒. V oc is proportional to the energy of the CT states E CT . The energetic loss q⌬V between E CT and qV oc vanishes when approaching 0 K. It depends linearly on T and logarithmically on illumination intensity. Furthermore q⌬V can be reduced by decreasing the electronic coupling between polymer and fullerene or by reducing the nonradiative recombination rate. For the investigated devices we find a loss q⌬V of ϳ0.6 eV at room temperature and under solar illumination conditions, of which ϳ0.25 eV is due to radiative recombination via the CT state and ϳ0.35 eV is due to nonradiative recombination.