Femtosecond spectroscopic study of photoinduced charge separation and recombination in the donor-acceptor co-oligomers for solar cells

Femtosecond transient absorption and time-resolved fluorescence techniques are employed to study the formation and recombination processes of the charge transfer (CT) state in a series of liquid-crystalline conjugated donor acceptor (D-A) co-oligomers F3T4-hP, F4T6-hP, and F5T8-hP. It is revealed that the CT state of the D-A co-oligomer film forms faster but recombines slower than that of its solution, which is beneficial for the charges to be extracted in the D-A co-oligomer-based photovoltaic cells. It is also found that all the films made from different D-A co-oligomers, different processing conditions, and the blend of the donor anD-Acceptor moieties show exactly the same CT dynamics, even though the films show very different film morphologies and photovoltaic performances, which indicate that the CT state property is only decided by the intrinsic molecular nature but not the film morphologies. The film morphology only affects the charge transport process but does not affect the CT state formation and recombination processes.