Relating the open-circuit voltage to interface molecular properties of donor:acceptor bulk heterojunction solar cells

The open-circuit voltage $({V}_{oc})$ of polymer:fullerene bulk heterojunction solar cells is determined by the interfacial charge-transfer (CT) 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 used conjugated polymers blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). ${V}_{oc}$ is proportional to the energy of the CT states ${E}_{CT}$. The energetic loss $q\ensuremath{\Delta}V$ between ${E}_{CT}$ and $q{V}_{oc}$ vanishes when approaching 0 K. It depends linearly on $T$ and logarithmically on illumination intensity. Furthermore $q\ensuremath{\Delta}V$ 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 $q\ensuremath{\Delta}V$ of $\ensuremath{\sim}0.6\text{ }\text{eV}$ at room temperature and under solar illumination conditions, of which $\ensuremath{\sim}0.25\text{ }\text{eV}$ is due to radiative recombination via the CT state and $\ensuremath{\sim}0.35\text{ }\text{eV}$ is due to nonradiative recombination.