The Photocurrent in Organic Solar Cells does not Vanish at Open-Circuit

Organic solar cells (OSCs) had been around for several decades while scoring considerably worse efficiencies than their inorganic counterparts. Recently discovered materials have closed the gap considerably and optimized OSCs can now reach close to 100% conversion yield between incident photons to extracted charge carriers. Still, they suffer from losses to the open-circuit voltage and fill factor, and conventional equilibrium charge carrier dynamics fail to exactly reproduce their current-voltage-characteristic. Instead, non-thermalized charge carrier distributions—due to incomplete relaxation in a disordered density of states—provide better agreement, yet with at times uncommon ramifications: here, we will show that a non-neglectable photogenerated current under open-circuit conditions is a general feature of OSCs.<br/><br/>In more detail, we have investigated the internal quantum efficiency of photogenerated charges in non-fullerene acceptor OSCs in the steady state. Therefore, we have first measured their external quantum efficiency spectrum under different experimental conditions, including a constant background illumination. Utilizing a small signal light source, modulated at low frequencies, and varying the working conditions from reverse (collection) to forward (injection regime) fields, allowed us to determine how efficient photogenerated charges can be extracted at any point on the current-voltage-curve. We find that while the yield decreases towards open-circuit, it does not vanish completely and some 10-20% of the short circuit yield remains; therefore, the resulting photocurrent has to be compensated by charge injection in order to reach zero net current. In a subsequent step, we were able to reproduce these experimental findings with kinetic Monte-Carlo device simulations involving non-equilibrium dynamics with decoupled photo- and injection currents.