Ultralong-range exciton transport in submillimeter-scale spherulite film of π-conjugated polymers.

Abstract

Long-range exciton transport in organic semiconductors is essential for the performance of optoelectronic devices. However, solution-processed π-conjugated polymers films typically exhibit short exciton diffusion lengths (<20 nm) due to local imperfections or variations in interchain packing. Here, large-area submillimeter-scale spherulites are achieved by treating the spin-coated polydiarylfluorene film under solvent vapor annealing. The exciton diffusion length, visualized using transient photoluminescence microscopy, is determined to be an average of 186 nm, with a corresponding diffusion coefficient of 0.14 cm²s^-1. Notably, the maximum value of exciton diffusion lengths and diffusion coefficient can reach up to approximately 396 nm and 0.63 cm²s^-1, respectively. Well-ordered hierarchical structure with an outstanding chain alignment in spherulite provides a uniform excitonic energy landscape, enabling ultralong exciton diffusion. The reduced defect density in the spherulite film may result in shallower trap states, facilitating exciton diffusion and radiative recombination. Polymer light-emitting diodes based on submillimeter-scale spherulite films exhibit deep-blue electroluminescence with high brightness (4897 cd m^-2) at low current density and good color purity. These findings demonstrate that the long-range ordered spherulite structure can significantly enhance the excitons transport and improve the overall optoelectronic property.