Determination of the exciton singlet-to-triplet ratio in single-layer organic light-emitting diodes

The efficiency of fluorescent organic light-emitting diodes (OLEDs) is strongly affected by the fraction of singlet excitons formed. While the standard statistical value of the singlet-to-triplet ratio is 1: 3, significant deviations have been reported for several materials, in particular for polymers. We developed a method to determine the singlet fraction with high accuracy for organic semiconductors in single-layer OLEDs by extending a method introduced by Segal et al. [Phys. Rev. B 68, 075211 (2003).] within which the analysis is based on a combination of electroluminescence (EL) and reverse bias photoluminescence (PL) measurements. We carefully determine from a combined experimental and modeling approach the PL and EL emission profiles and light outcoupling efficiencies, which are generally quite different for single-layer devices. The approach is demonstrated for the case of OLEDs based on a blue-emitting polyfluorene-based copolymer, for which the singlet fraction is found to be in the range 10%-25%, increasing with increasing emitting layer thickness but independent of the applied voltage.