Bilayers, Gradients and Funnels: Harvesting Excess Energy to Surpass The Shockley-Queisser Limit

Compared to inorganic solar cells, the energy losses in organic photovoltaics (OPV) are still (too) large. Fortunately, a significant fraction of these losses results from a relatively slow relaxation of photogenerated charges in the disorder-broadened density of states. In previous work, we have already shown that the slowness of the mentioned thermalization leads to a 0.1-0.2 V higher open circuit voltage (Voc) in OPV devices than would be expected for instantaneous thermalization. Here, we will discuss strategies to harvest even larger parts of this excess energy, even beating the Shockley-Queisser limit for the constituent material.<br/>Specifically, we experimentally and numerically show how inhomogeneous distributions of electron donating and accepting phases can be used to entropically drive positive and negative charge carriers towards the anode and cathode, respectively. First, we analyze simple step-like bilayer devices without any built-in voltage (Vbi) to single out the contribution of said mechanism to Voc. Next, we discuss how more suitably chosen gradient profiles can be used to improve actual devices that do have a finite Vbi. Combining these concepts, we demonstrate that in optimized, funnel-shaped morphologies the enhanced, non-equilibrium diffusivity of photogenerated charges can be rectified, allowing to surpass the Shockley-Queisser limit for the same material in absence of gradients and under near-equilibrium conditions. Finally, we use a cellular automaton model to show a realistic pathway to generate the proposed funnel-like morphologies in actual donor-acceptor blends.