Light extraction from OLEDs with (high) index matched glass substrates

In a conventional bottom emitting organic light emitting diode only about half of the generated photons are emitted into the glass substrate (out of which 25% are extracted into air), the other half being wave-guided and dissipated in the OLED stack. This is due to the refractive index mismatch between the organic layers (n=1.7-1.9) and the glass substrate (n=1.5). By matching the refractive index of the substrate (n=1.8) and organic layers and augmenting the distance of the emission zone to the cathode to suppress plasmonic losses light extraction into the substrate can be increased to 80-90%. This is shown by simulation and experiment. Furthermore the effect of pyramidal structures on the light extraction from the substrate into air is studied by experiment and simulation. Ultimately it is limited by the reflectance of the OLED stack. The experimental results for monochromatic light are well corroborated by simulations. The main conclusion is that most photons can be out-coupled from the organic stack into an index matched substrate. The OLED light extraction problem is thus reduced to an effective extraction from the substrate into air.