Optical signatures of the interplay between intermolecular and intramolecular coupling in plastic semiconductors

Polymeric semiconductors such has regioregular poly(3-hexylthiophene) have electronic proprieties that can be tuned by proper control of the solid-state microstructure. We process thin films of P3HT of different molecular weight ranging from 2 kg/mol to 341 kg/mol. The polymer undergo a transition from a paraffinic, non-entangled microstructure to a two-phase microstructure defined by entangled chains embedded in amorphous regions at around 50 kg/mol. We observe an abrupt decrease in the intermolecular coupling from an average of similar to 20 meV for molecular weight below 50 kg/mol to similar to 5 meV above 50 kg/mol. We assign this decrease in the interchain coupling and associated free-exciton bandwidth at higher molecular weight to a transition from a one-phase morphology to a two-phase morphology defined above. In steady-state photoluminescence, we associate the lower Huang-Rhys factors at higher molecular weight to more planar backbone.