Ultrafast Exciton Dissociation in Block Copolymer toward Efficient Single Material Organic Solar Cells

The study reports for the first time on the ultrafast dynamics of charge transfer (CT) and exciton dissociation in block copolymer PBDB-T-b-PTY6-based state-of-the-art single-material organic solar cells (SMOSCs). From the transient absorption spectroscopy of the dilute PBDB-T-b-PTY6 in the insulating polystyrene, exciton dissociation and hole transfer (HT) processes at the intramolecular interface of covalent linkage between the donor and acceptor segment are achieved. In comparison to the charge generation in blend PBDB-T:PTY6 films, it is found that the HT rate in the isolated block copolymer chain via the intramolecular channel is approximately an order of magnitude higher than that via the intermolecular channel. Much faster exciton dissociation in the dilute PBDB-T-b-PTY6 film than in the blend film from electro-absorption and polaron-absorption signals is also verified. The intrachain chemical interface in the block copolymer is thus more conducive to the HT path than the traditional interface in the bulk heterojunction. Moreover, though the PBDB-T-b-PTY6 film has weak molecular ordering, its overall CT efficiency is comparable to that of the PBDB-T:PTY6 film. These findings portend that further molecular design with optimized ordering toward fast intramolecular exciton dissociation may contribute to SMOSCs with higher power conversion efficiency. The existence of hole transfer channels at the chemical interface in dilute PBDB-T-b-PTY6 block copolymer films is rigorously demonstrated. It is found that the intramolecular hole transfer rate is approximately an order of magnitude higher than the intermolecular one in the traditional blend films. The intramolecular channel contributes significantly to the hole transfer efficiency (30.5%), comparable to the intermolecular channel in PBDB-T-b-PTY6 films.image