Tönu Pullerits

With organic solar cells surpassing 20% efficiency, the focus is shifting toward understanding the more complex mechanisms in ternary blends. This work investigates the distinct working principles of ternary organic solar cells based on a polymer donor (D) and a nonfullerene acceptor (NFA), with a fullerene acceptor (FA) as the third component. A systematic comparison between ternary D:NFA:FA systems, with different components and compositions, and D:NFA:NFA systems was conducted. In all systems, the open-circuit voltage increased with a higher fullerene ratio, correlating with the fullerene's LUMO position, indicating its involvement in charge transfer (CT) state formation. Various analytical methods and simulations reveal that the investigated D:NFA:FA systems follow an alloy model, where the CT state is delocalized over both acceptors, even in systems with strong surface energy differences between the acceptors. Notably, recombination behavior is largely unaffected by the nature of the third component and is primarily linked to the CT state energy. Based on the internal quantum efficiency characteristics, we propose that the positive effect of fullerenes as third components arises not from reduced nonradiative recombination as often suggested but from more efficient exciton splitting. (Figure presented.).