High-Index Electroluminescent Device Light Extraction
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Solution Overview
Problem
Conventional organic light-emitting devices (OLEDs) face significant challenges in achieving high external quantum efficiencies due to total internal reflection caused by refractive-index mismatches, resulting in only 20-25% out-coupling efficiency of internal radiation, which is inadequate for power-efficient applications.
Innovation Solution
The introduction of a high-index layer with a refractive index greater than 1.8, such as TiO2, combined with a low-refractive-index electrode and optimized layer thicknesses, along with a substrate or superstrate treated for improved light extraction, enhances optical coupling efficiencies by reshaping the radiation pattern and inducing a microcavity effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional ITO electrode with high refractive index is used, then device structure is simple, but light out-coupling efficiency is low (20-25%) due to total internal reflection
Solution Approach 1:
The device is segmented into multiple functional layers with different refractive indices. The ETL is divided into sub-layers (ETL1, ETL2, ETL3) with progressively increasing refractive indices (1.7-1.8, 1.8-1.9, 1.9-2.0), creating a gradient structure that facilitates gradual light out-coupling and reduces total internal reflection at interfaces
Solution Approach 2:
Different regions of the device are assigned different refractive index properties. The ETL sub-layers have locally optimized refractive indices that increase from bottom to top, matching the gradient from the high-index ITO electrode (n=1.8-2.1) to the lower-index organic layers and air, thereby optimizing light extraction at each interface
2Reliability
If high refractive index materials (ITO n=1.8-2.1, organic layers n=1.7-1.8) are used, then internal quantum efficiency reaches nearly 100%, but external quantum efficiency is limited by refractive-index mismatch at interfaces
Solution Approach 1:
The ETL sub-layers act as intermediary layers between the high-index ITO electrode and the lower-index organic emitting layers. These intermediate layers with graded refractive indices (1.7-2.0) serve as optical mediators that facilitate the transition of light from the high-index ITO to the lower-index organic layers, reducing reflection losses at each interface
Solution Approach 2:
The refractive index parameter is changed progressively across the ETL sub-layers. By varying the refractive index from 1.7-1.8 in ETL1 to 1.8-1.9 in ETL2 to 1.9-2.0 in ETL3, the device optimizes optical coupling at each interface while maintaining electrical functionality, thereby improving external quantum efficiency
3Device complexity
If conventional OLED structure with ITO electrode is used, then manufacturing process is simple, but most internal radiation is trapped and waveguided inside the device
Solution Approach 1:
The ETL is segmented into three sub-layers with different refractive indices and thicknesses optimized for light extraction. This segmentation creates multiple interfaces with graded refractive indices that reduce total internal reflection and enable more radiation to escape the device rather than being trapped and waveguided
Solution Approach 2:
The device employs a composite structure combining ITO electrode, multiple ETL sub-layers with different refractive indices, and organic emitting layers. This composite material approach creates a refractive index gradient that facilitates light out-coupling while maintaining the electrical and optical functionality of each component
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly increases external quantum efficiency to nearly 40% without external out-coupling techniques, and up to 57% with additional surface treatments, surpassing conventional ITO and PEDOT devices.
Implementation Method 1
the high-index layer with a refractive index greater than 1.8, such as TiO2, combined with a low-refractive-index electrode and optimized layer thicknesses, along with a substrate or superstrate treated for improved light extraction, enhances optical coupling efficiencies by reshaping the radiation pattern and inducing a microcavity effect
Implementation Method 2
due to the significant refractive-index mismatches at air/substrate and substrate/ITO interfaces in typical OLEDs, OLED internal emission usually suffers total internal reflection and hence most of internal radiation is trapped and waveguided inside the device
Data Source
AI summary
An electroluminescent (EL) device is disclosed, comprising a high-index layer, having a first refractive index more than 1.8; a first electrode, which is transparent and disposed adjacent to the high-index layer; one or more functional layers, disposed adjacent to the first electrode and opposite to the high-index layer, including a light emitting layer; and, a second electrode, disposed adjacent to the one or more functional layers and opposite to the first electrode; wherein the first electrode has a second refractive index less than 1.7.