Exploration of charge-transfer exciton formation in organic semiconductors through transient photoconductivity measurements

This project investigated transient photoconductivity in organic donor-acceptor (D-A) systems, where a flourinated anthradithiophene, ADT-TES-F, acts as the donor. Various acceptor molecules were used, with different sidegroups, and different HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy level offsets relative to the acceptor. Thin film composites of donor and acceptor materials were excited with sub-nanosecond 355 nm laser pulses, and the photoconductivity of the sample was measured up to several microseconds after excitation. Comparing the response of acceptor molecules with different packing properties and energy level offsets allowed insight into the generation and dynamics of charge carriers in the sample, including the formation of CT (charge-transfer) excitons. In pristine films and in samples with a small donor-acceptor spatial separation, the primary contribution to the peak photocurrent was simply the dissociation of a precursor state to the donor exciton. In systems where the spatial separation was greater, CT exciton formation dominated charge carrier generation at low E-fields. There was up to a factor of two increase in the efficiency of charge carrier generation in these samples due to CT exciton dissociation. When higher electric fields were applied, fast charge carrier generation dominated and the presence of the acceptor did not lead to an improvement in charge carrier generation efficiency.\ud \ud These results allow us to infer the following picture of charge carrier dynamics within the sample. When the excitation pulse hits the sample, a precursor state is formed, which in pristine samples, gives rise to a donor exciton and fast generation of charge carriers. In D-A systems, however, there is a competition process between the formation of donor excitons and charge transfer (CT) exciton states. CT exciton formation is followed by a relatively slow process of charge carrier generation via exciton dissociation. In systems with a large D-A separation, this CT state is more likely to make an important contribution to the photoconductivity, because it dissociates more easily