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

Abstract

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.

Extraction of surface plasmons in organic light-emitting diodes via high-index coupling

“…Furthermore, the used index matching fluid was strongly absorbing in the short wavelength range so that the substrate light has a brownish appearance. Nevertheless, this is a clear demonstration that the concept of high-index coupling is not only applicable to extract WGMs in bottom-emitting OLEDs, as was demonstrated before by different authors [8][9][10], but moreover to get access to all trapped modes -including the surface plasmons -in a top-emitting structure.…”

“…The use of glass substrates with higher refractive index than the organic layers (including ITO) is possible [7][8][9][10], so that a large fraction of the light from WGMs goes into the HI glass substrate, from where it can be extracted more easily. However, due to momentum mismatch (see below for details) the energy contained in SPPs traveling at the metal-organic interface of the top electrode is not accessible in this way.…”

Rising advancements in the application of PEDOT:PSS as a prosperous transparent and flexible electrode material for solution-processed organic electronics

“…Furthermore, the refractive index of ITO electrodes is higher than that of any other layer of an OLED, including the glass substrate [33]. It is around 1.8-2.2, depending on the ITO fabrication and treatment methods [37,78,82,88], while the refractive indices of the organic layers and the glass substrates of OLEDs are usually around 1.7-1.9 and 1.5, respectively [82,89]. This refractive index mismatch between ITO and the overlying organic layers as well as the underlying glass substrate leads to the total internal reflection (TIR) of light.…”

Realization of high efficacy white organic light emitting diode by controlling internal light absorption and angular distribution

“…For example, micro‐sized texture, nano‐sized gratings, and scattering layers have been fabricated on the surface of the substrate or interface of substrate/transparent electrode . On the other hand, index‐matching micro‐patterned texture or hemisphere macro‐extractor with specifically high refractive index (n ~ 1.8) substrate achieved excellent LEE (over 40 %) because of the improved transmission from thin layers to the substrate . Besides, LEE enhancement effect with controlling orientation of dipole is actively discussed.…”