Triplet exciton losses in polymer: fullerene-free acceptor blends

In the past, most organic solar cells (OSCs) employed fullerenes as electron acceptors. Recently, the synthesis of novel non-fullerene acceptors (NFAs) resulted in high power conversion efficiencies (PCE) of 18.2%. The remarkably high VOC in NFA-based OSCs has been achieved by increasing charge transfer energy (CT) via closer matching of acceptor (A) and donor (D) energy levels. Moreover, an increase in CT energy can lead to a new loss pathway resulting from charge recombination to the now energetically favorable D and A triplet exciton (TE) states. Formation of TEs is not only causing a new loss pathway and reduced short-circuit current but can also lead to enhanced degradation of the active layer. We studied the polymer donor PBDB-T and its fluorinated version PM6, and the thiophene-based acceptor molecule ITIC and a novel benzothiadiazole-based acceptor molecule Y6. With low temperature spin-sensitive photoluminescence measurements formation of TEs is studied.