Low Cost Corrugated Substrates for High Efficiency OLEDs (Final Report)
- North Carolina State Univ., Raleigh, NC (United States)
The external quantum efficiency (EQE) of a planar OLED is limited to 25%. Around 75% of the light is trapped in the substrate, waveguided and SPP modes. In this project, we demonstrated that nano-structures can significantly improve the EQE of OLEDs. The objective is to improve the EQE to 70% using corrugated substrate, low index layer incorporation and micro lens arrays. We conducted optical study of the operating devices using ARES measurement, and used the result to guide the design of corrugated substrates as well as the OLED stack for better performance. During the first year of the program, we have demonstrated over 60% EQE in a corrugated green OLED with half-ball lens for light extraction. By comparing, the optical characteristics of OLEDs fabricated on single and poly crystal corrugated structures, we confirm that the enhancement using corrugated substrates comes from waveguided and SPP modes extraction. To push the device EQE to 70%, further optimization of the corrugated substrate is needed. The extraction of waveguided and SPP modes are influenced by the corrugation pitch, depth and profile. Therefore, we developed a series of processes to precisely control the corrugation morphology. Through comparison of different corrugated substrates, we observed that while an abrupt surface profile results in stronger mode extraction, it also introduces strong leakage current; while a smooth profile are less efficient in mode extraction, it preserves the quality of an OLED. Based on these observations, we conclude that a single crystal hole corrugation results in optimum EQE % and low leakage current when planarized with PEDOT. To improve the extraction of waveguided mode, we also introduce a low index contrast layer (ICL) to increase the index contrast between corrugated ITO and glass interface. We optimized our recipe for the index contrast layer processing and reduced its refractive index from 1.50 to 1.38. As a result, the optimized device shows 47.6% EQE, which is 1.64 times higher than the reference planar device. By extracting the substrate mode with a hemispherical lens, we obtained an EQE of 71.6%, which is a 2.46 times enhancement. The cross-section SEM image reveals the high EQE is due to the formation of an air gap at the interface of the ICL, resulting in an increase of the index contrast to Δn ~ 1. We use angle-resolved electroluminescence spectroscopy (ARES) to demonstrate that the ICL improves the diffraction efficiency at the ITO/glass interface, extracts TM, TE waveguide and SPP mode more efficiently, and leads to the observed high efficiency. We also address the directional emission issue with the single crystalline microlens arrays (MLAs) by fabricating polycrystalline MLAs. The process uses colloidal assembly method to create a mono-layer of silica spheres, then uses PDMS stamps to replicate the pattern for multi-use imprinting. The MLAs are close-packed in short range but have random orientations in the long range. This avoids stronger extraction along the optical axes of single crystal MLAs and prevents the directional emission issue. We also demonstrate that the color shift induced by the corrugation can be minimized using MLAs. In this scenario, the MLAs work as diffuser to randomize the directional emission from extracted waveguide and SPP modes.
- Research Organization:
- North Carolina State University, Raleigh, NC (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- DOE Contract Number:
- EE0007624
- OSTI ID:
- 1514255
- Report Number(s):
- DE-EE0007624; 9195137060
- Country of Publication:
- United States
- Language:
- English
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