Relationship between Energetic Disorder and Reduced Recombination of Free Carriers in Organic Solar Cells

Reducing non-radiative recombination is key to achieve high fill factors (FFs) in organic solar cells. While it is generally accepted that recombination proceeds via charge transfer (CT) states at the donor:acceptor interface, the underlying principles that dictate the decay kinetics of these CT states are not yet well understood. Here, a study on the effect of energetic disorder is presented. Based on a data set of 10 representative donor:acceptor blends, clear correlations between disorder, the recombination coefficient of free charge carriers, and the non-radiative voltage loss are found. It is suggested that a narrower distribution of CT energies leads to a longer CT decay time and thus reduces non-radiative losses. This leads to a simultaneous improvement of the FF and open circuit voltage and highlights the importance of having materials with low energetic disorder on the way to the commercialisation of organic photovoltaics.