|
As one of the major candidates for future lighting technologies, organic light-emitting diodes (OLEDs) have reached a mature status fulfilling industrial performance requirements for display applications [1]. Their overall efficiency is currently mainly limited by the large amount of initial power dissipated into optical loss modes due to the high refractive index of the organic layers. A key method of reducing the trapping of light inside the OLED is to orient the transition dipole moments (TDM) of the light-emitting molecules parallel to the substrate interface.
The most established experimental technique of determining the so-called anisotropy coefficient as a measure of the average TDM orientation of the molecules are angular resolved photoluminescence measurements [2]. Although this method has been applied by several groups with varying experimental realizations, a quantitative discussion of the effect of specific setup configurations is - to the best of our knowledge - missing so far. For instance, the accurate positioning and size of the optical excitation spot is one of the crucial requirements of this measurement. Also, the distance between the rotation center and the detector is important.
In this work, we present a ray optics model which accounts for such macroscopic effects. By solving the developed equations numerically, we show that already a small displacement of the excitation spot leads to remarkable changes in the measured emission spectra. For specific setup configurations the accuracy of experimental data fits can be drastically improved using the mentioned corrections.
Since some of the requirements for almost ideal experimental conditions are hard to realize, for instance due to varying substrate thicknesses, this numerical model might be a step towards better comparability of determined anisotropy coefficients. Additionally, it enables for the first time to quantify measurement deviations caused by non-idealities of the setup and with that to optimize them in a controlled fashion.
[1] T. Tsujimura, OLED display fundamentals and applications, John Wiley and Sons (2017)
[2] T. D. Schmidt, T. Lampe, D. Sylvinson M. R., P. I. Djurovich, M. E. Thompson and W. Brütting, Phys. Rev. Applied 8, 037001 (2017)
|
