Morphology control of nanofibril donor-acceptor heterojunction to achieve high photoconductivity: exploration of new molecular design rule.

Donor-acceptor nanofibril composites have been fabricated, and the dependence of their photocurrent response on the structure and morphology of the donor part has been systematically investigated. The nanofibril composites were composed of template nanofibers, assembled from an electron acceptor molecule, perylene tetracarboxylic diimide (PTCDI), onto which (through drop-casting) various electron donor molecules (D1-D4) were coated. The donor molecules have the same π-conjugated core, but different side groups. Due to the different side groups, the four donor molecules showed distinctly different propensity for intermolecular aggregation, with D1-D3 forming segregated phases, while D4 prefers homogeneous molecular distribution within the film. It was found that the nanofibril composites with D4 exhibit the highest photocurrent, whereas those with aggregation-prone D1-D3 exhibited much lower photocurrent under the same illumination condition. Solvent annealing is found to further enhance the aggregation of D1-D3 but facilitate more uniform molecular distribution of D4 molecules. As a result, the photocurrent response of PTCDI fibers coated with D1-D3 decreased after vapor annealing, whereas those coated with D4 further increased. The detrimental effect of the aggregation of donor molecules on the PTCDI fiber is likely due to the enhanced local electrical field built up by the high charge density around the aggregate-nanofiber interface, which hinders the charge separation of the photogenerated electron-hole pair. The results reported in this study give further insight into the molecular structural effect on photoconductivity of hybrid materials, particularly those based on donor-acceptor composites or interfaces, and provide new molecular design rules and material processing guidelines to achieve high photoconductivity.