Maria S. Kotova: Triplet Exciton Loss Pathways in Non-Fullerine Organic Solar Cells?

In the past, most organic solar cells (OSCs) employed fullerenes as electron acceptors. Recently, the synthesis of novel non-fullerene acceptors (NFA) resulted in higher power conversion efficiencies (PCE) of over 13%. It has been shown, that in fullerene-based OSCs the formation of triplet excitons (TE) opens up an additional recombination pathway and may cause enhanced degradation. In this work, the formation of TEs in NFA-based OSCs is investigated. We studied optical, electrical and spin properties of the polymer donor PBDB-T and the NFA acceptor ITIC in pristine films, in blend films and OSC devices based on the blend. OSCs show a PCE of 9.8 % and external quantum yield of up to 73 %. Low temperature spin-sensitive photoluminescence measurements reveal the formation of highly localized TEs in pristine films of donor (D) and acceptor (A), as well as in the blend films. This can occur either via intersystem crossing (ISC) or electron back transfer (EBT). ISC is likely for low mobility singlet excitons that cannot reach D-A interfaces and dissociate into charge transfer (CT) states. The EBT forms TE from the CT states. However, no TE signal was observed by electrically detected magnetic resonance measurements in OSCs under working conditions, thus no significant ISC or EBT occurs and the TE formation loss channel is not very efficient. The lack of TEs matches well with the high PCE of NFA-based OSCs and their stability.