Nonfullerene Acceptor-Iridium Coordination Polymer as a Triplet Sensitizer for Efficient Organic Photovoltaics

Utilizing triplet excitons has been recognized as a promising method to boost performances of organic solar cells (OSCs). Although long lifetime and diffusion length of triplet excitons have been observed in some systems, efficiencies of triplet-sensitized OSCs are still moderate because of the lack of an efficient way to generate and transfer triplet excitons. In this study, we report a nonfullerene acceptor-iridium coordination polymer (BTP-N-Ir) as a triplet sensitizer in ternary OSCs for utilizing triplet excitons to improve photocurrents. The triplet polymer sensitizer shows a small singlet-triplet energy gap, facilitating the generation of long-lived triplet excitons, which transfer to an intra-moiety excited (i-EX) state of the acceptor in ternary films, dissociating into free charges to contribute photocurrents. Such a new triplet-exciton transfer pathway with the i-EX as an intermediate state is more effective than those in previous systems, leading to the highest efficiency of 20.37% for OSCs with a triplet sensitizer.