The Importance of Microstructure in Determining Polaron Generation Yield in Poly(9,9-dioctylfluorene)

Understanding the structure-property relationships that govern exciton dissociation into polarons in conjugated polymers is key in developing materials for optoelectronic applications such as light-emitting diodes and solar cells. Here, the polymer poly(9,9-dioctylfluorene) (PFO), which can form a minority population of chain segments in a distinct, lower-energy "β-phase" conformation, is studied to examine the influence of conformation and microstructure on polaron generation in neat thin films. Through use of ultrafast transient absorption spectroscopy to probe PFO thin films with glassy-phase and β-phase microstructures and selectively exciting each phase independently, the dynamics of exciton dissociation are resolved. Ultrafast polaron generation is consistently found to be significantly higher and long-lived in thin films containing β-phase chain segments, with an average polaron yield that increases by over a factor of three to 4.9% vs 1.4% in glassy-phase films. The higher polaron yield, attributed to an increased exciton dissociation yield at the interface between conformational phases, is most likely due to a combination of the significant energetic differences between glassy-phase and β-phase segments and disparities in electronic delocalization and charge carrier mobilities between phases.