Traps and Transport Resistance—The Next Frontier for Stable State-of-the-Art Non-Fullerene Acceptor Solar Cells

Stability is one of the most important challenges facing organic solar cells (OSC) on their path to commercialization. In this work, we use a combination of experiments and simulation to investigate the mechanisms leading to degradation of devices based on the state-of-the-art PM6:Y6 material system. We find that material properties and interface energetics remain remarkably stable upon aging by thermal stress in the dark. However, the devices suffer from trap state formation in the bulk. The increased charge carrier trapping causes larger non-radiative recombination losses and thus reduces the open-circuit voltage, as well as the charge carrier mobility. Trap formation in the photoactive layer increases large initial transport resistance even further. We conclude that despite the mostly stable material properties, aging-induced trap states with their impact on non-radiative recombination and transport limitations remain the key issue to be tackled in order to improve state-of-the-art OSC stability. For organic devices to make the leap from the lab to large scale production the community must be able to better understand and control the link between chemical structure/morphological/stability and the density/distribution of transport trap states – this should be the next frontier.