Carrier losses in non-geminate charge-transferred states of nonfullerene acceptor-based organic solar cells

To understand the current limitations of nonfullerene-based organic solar cells (OSCs), the early-time dynamics of the carrier generation in the high performance bulk heterojunction (BHJ) blend of a semiconducting polymer, PBDB-T, and the low bandgap nonfullerene acceptor, ITIC-m, are investigated. After photoexcitation, photo-induced excitons are separated through the ultrafast (~200 fs) electron transfer process from PBDB-T to ITIC-m and through the fast (3-6 ps) hole transfer process from ITIC-m to PBDB-T. However, a part of the separated charges recombines in the non-geminate (long-range) charge-transferred (CT) states. The yield of mobile carriers is correspondingly decreased by recombination in the CT states. In our measurements, the carrier recombination loss in the CT state is decreased by optimizing the BHJ morphology, especially for showing better electron mobility using a processing additive (1,8-diiodooctane) during the fabrication of the composite film, as evidenced by the decreased CT band intensity at ~30 ps and the increased polaron band intensity, which eventually improve power conversion efficiencies (PCEs).