The effect of morphology on the temperature-dependent photoluminescence quantum efficiency of the conjugated polymer poly(9, 9-dioctylfluorene)

We have measured the temperature-dependent photoluminescence quantum yields (PLQYs) of poly(9, 9-dioctylfluorene) (PFO) films with four morphologies, namely as-spin-coated (SC) glass, quenched nematic glass, crystalline, and vapour-treated SC glass containing a fraction of 21 helix conformation (β-phase) chains. We find that the room temperature PLQYs of the as-SC, crystalline, and quenched films all increase as the temperature is reduced. However, the PLQY of the film containing β-phase chains decreases at temperatures below 150 K. Via temperature-dependent photoinduced absorption measurements, we show that the polaron population in films containing β-phase PFO chains grows as the temperature is reduced, and is significantly larger than in films with any of the other morphologies. Because of the smaller HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) energy gap of the β-phase chains compared to chains in the surrounding glassy PFO matrix, they act as recombination sites for excitons, and as traps for polarons. Hence at low temperatures, the polarons become strongly localized on these chains, where they quench the singlet excitons and reduce the PLQY.