Dispersive Charge Transfer State Electroluminescence in Organic Solar Cells

Abstract

The notion of quasi-equilibrium is central to most solar cells; however, it has been questioned in organic photovoltaics (OPVs) owing to strong energetic disorder that frustrates efficient relaxation of electrons and holes within their respective density of states (DOS). Here, modulation electroluminescence (EL) spectroscopy is applied to OPVs and it is found that the frequency response of charge transfer (CT) state EL on the high energy side of the spectrum differs from that of the low energy side. This observation confirms that static disorder contributes substantially to the linewidth of the steady-state EL spectrum and is unambiguous proof that the distribution of CT states formed by electrical injection in the dark is not in quasi-equilibrium. These results emphasize the need for caution when analyzing OPV cells on the basis of reciprocity models that assume quasi-equilibrium holds, and highlight a new method to study this unusual aspect of OPV operation.Disorder in organic solar cells leads to an energetic distribution of charge transfer (CT) states. Modulation electroluminescence spectroscopy shows that higher energy CT states in this distribution respond to a voltage perturbation faster than lower energy CT states, confirming that they do not exist in quasi-equilibrium, even for devices operated in the dark.