Determination of the singlet exciton fraction in polymer based organic light emitting diodes

This report discusses a method to determine the exciton singlet fraction for organic semiconductors in single-layer polymer based OLEDs. The approach is an extension of a method originally developed by Segal et al. in 2003, and it is based on the experimental measurement of the electroluminescence and photoluminescence quantum efficiency of the device, carried out in the same experimental set-up. The singlet fraction determination method is applied to two material classes. In first place, to a blue-emitting polyuorene (PF-0.5mol%TAA) based OLED; secondly, to the orange-red emitting polymer NRS-PPV. Other emitting polymers have also been studied, namely a different polyuorene copolymer, OC1C10-PPV , and MEH-PPV, but the corresponding devices showed stability problems, therefore it hasn't been possible to determine the singlet exciton fraction. The determined value for the singlet fraction is in the range 8% - 24% for the PF- 0.5mol%TAA-based copolymer, and 8% - 14% for the NRS-PPV, in all cases independent of the experimental conditions but slightly emitting layer-thickness dependent. The emitting layer thickness dependence of the singlet fraction is under investigation, and we noticed that a recent theoretical study has shown that a singlet fraction significantly lower than the standard statistical 25% can be expected under specific conditions. The determined singlet fraction values have been successfully exploited to predict the external quantum efficiency of the analyzed devices, substantially in agreement with the measured values. In future, it would be of interest to apply this method to fluorescent small-molecule organic semiconductors, which are of interest for high efficiency OLED applications, as well as to those polymers for which large deviations from the standard statistical behavior are claimed in the literature.