Recombination kinetics in wide gap electroluminescent conjugated polymers with on-chain emissive defects

We analyze the experimental dependence of temporally and spectrally resolved electroluminescence (EL) from polyfluorene-based light-emitting diodes on electric field and temperature. The blue band in the EL spectrum is caused by emission from the polymer backbone, while the low-energy green emission results from on-chain keto defects, which act as traps for electrons. Although the time and temperature dependencies of the blue and green emission could be explained by the kinetics of redistribution of trapped holes and electrons in energy and space, the increase of the blue-to-green delayed emission ratio with increasing field strength appears to result from a difference in the nature of recombination. We propose that the formation of blue-emitting singlet excitons from close electron-hole pairs after turn-off is impeded by an energy barrier, in contrast to the formation of green-emitting excitons.