Influence of energetic disorder on electroluminescence emission in polymer:fullerene solar cells

Electroluminescence (EL) spectroscopy and imaging can be useful techniques to analyze various loss mechanisms in solar cells, but the interpretation of the results is not trivial in solar cells made from disordered materials such as organic semiconductors. In this case the interpretation of EL measurements may be affected by the presence of a tail of localized states. Here, we study several polymer: fullerene systems and show that, despite the presence of tail states, the shape of the EL spectrum is insensitive to the applied voltage. This indicates that the emission originates mainly from mobile charges in higher lying states recombining at the polymer: fullerene interface and that most charges in deeper tail states do not contribute to the EL spectrum. The consequence of our finding is that simple models of EL emission in ideal semiconductors can be applied to polymer: fullerene solar cells and can therefore be used to evaluate the potential of different material systems in terms of recombination losses and to study resistive losses using luminescence imaging.