It is known that photogenerated charge carriers in organic solar cells can be extracted before they are fully thermalised. The reason is that the relaxation to the quasi-equilibrium energy is carried out by hopping in a disorder-broadened density of states, which is a slow process. Until now, however, it was not clear whether this effect has a – possibly positive – impact on the overall device performance of the solar cell.

In a recent preprint, we show that the open-circuit voltage (Voc), and thus also the efficiency, benefits from the non-equilibrium effects. For this purpose, we have carried out kinetic Monte Carlo simulations, which take the slow relaxation fully into account, and compared them with two different organic donor-acceptor blend systems (TQ1:PC71BM and PM6:Y6). After experimental calibration of the model, it can excellently describe Voc as well as its dependence on layer thickness and temperature. This is not the case when using a drift-diffusion model, which explicitly assumes that charge carriers to be in quasi-equilibrium.

The kinetic Monte Carlo (KMC) model describes the experimental current-voltage curve, while the quasi-equilibrium drift-diffusion (DD) model does not.

Our work gives a clear indication that organic solar cells are an example of ‘hot carrier’ solar cells. This opens up new perspectives for device optimisation and how the usual layer thickness limitations can be overcome.

T. Upreti, S. Wilken, H. Zhang, M. Kemerink, Hot carriers boost open-circuit voltage of organic solar cells, arXiv:2105.11305 []