Enhancing outcoupling efficiency of indium-tin-oxide-free organic light-emitting diodes via nanostructured high index layers

Abstract

We fabricated organic light-emitting diodes with one-dimensional Bragg gratings as light extraction elements for substrate and waveguide modes. A Ta2O5 layer was introduced to obtain a high refractive index contrast to the subsequent anode layer. As anode we employed a highly conductive polymer. Laser interference lithography and physical plasma etching were used to pattern gratings into the Ta2O5 layer with a lattice constant of 370 nm and various grating depths. Mainly attributed to the outcoupling of the substrate modes, the structured devices exhibit a luminous flux which is up to four times higher compared to the unstructured reference devices.

White organic light emitting diodes with enhanced internal and external outcoupling for ultra-efficient light extraction and Lambertian emission

“…Figure 2(a) shows the I-V characteristics of the devices. The structured devices all show a slight shift towards higher current densities, which can be explained by the field enhancement resulting from the corrugated layers [19,23]. Even though the devices were solution processed, this did not completely smoothen the corrugation resulting from the grating structure (see Fig.…”

“…In this report we have used a high index material to optimize the extraction. However, Bragg-gratings lead to a high angular dependence of the emission spectrum [18][19][20]. This is a serious drawback for both, illumination and display applications.…”

“…The observed enhancement by a factor of about 1.4 however, is expected to be surpassed by the use of internal diffractive nanostructure which leads to a stronger outcoupling. Periodic nanostructures have been integrated in monochrome devices [18][19][20][21][22] to enhance the outcoupling. The enhancement factors achieved however did not reach the optimum as a high contrast in the refractive indices between the grating and the surrounding is needed for high extraction efficiencies.…”

Enhanced light outcoupling of polymer light-emitting diodes with a solution-processed, -flattening photonic-crystal underlayer

“…Nanoscale PC structures were engraved in quartz substrates using helium-cadmium (He-Cd) laser-based holographic lithography, which has been widely used in combination with a reactive-ion-etching (RIE) process to obtain a one-or two-dimensional reciprocal nanostructure [3,10,29,30]. Quartz substrates were carefully cleaned with acetone and isopropyl alcohol.…”

“…The TIR at the anode/organiclayer, transparent substrate/anode and transparent substrate/air interfaces has been pointed out to be the main origin of the limited outcoupling efficiencies, and it is known that only 20% of the light generated from the EML can be extracted to the outside if no light extraction structures are utilized [1,2]. To relieve the indexmismatching issue, index-matching approaches such as the use of a high-refractive index coating material and the making of a low index grid in organic layers have been employed [3][4][5]. In addition to these index-matching approaches, photonic structures like the microlens array have been introduced to the backside of the substrate [6][7][8].…”