Low-Refractive-Index Aromatic Capping Layers for OLED Light Extraction
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Solution Overview
Problem
Existing organic electroluminescent (EL) devices face challenges in controlling light propagation due to the complexity of forming low refractive index layers through co-evaporation of organic semiconductor materials and additives, which complicates additive control and deposition conditions, thereby limiting light extraction efficiency.
Innovation Solution
The use of an aromatic compound with specific structural features, such as phenylene groups with amide or ester structures, allows for the formation of a low refractive index layer that, when stacked with a high refractive index layer, enhances light extraction efficiency in organic EL devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a low refractive index layer is formed by co-evaporation of organic semiconductor material and additive, then light extraction efficiency is improved, but process complexity and control difficulty increase
Solution Approach 1:
The invention extracts the refractive index adjustment function from the co-evaporation process by using a separate low refractive index material layer. This layer is deposited independently after the organic semiconductor layer, allowing refractive index control without complicating the co-evaporation process. The low refractive index material is specifically selected to have a refractive index of 1.70 or less, which is lower than conventional organic semiconductor materials, thereby improving light extraction efficiency while maintaining process simplicity.
Solution Approach 2:
The invention segments the device structure into distinct functional layers: an organic semiconductor layer and a separate low refractive index material layer. This segmentation allows each layer to be optimized independently - the organic semiconductor layer for charge transport and the low refractive index material layer for light extraction. The segmentation eliminates the need for complex co-evaporation processes while achieving both electronic and optical functionality.
2Illumination intensity
If a low refractive index layer is formed by co-evaporation with additive, then light propagation control is achieved, but deposition condition control becomes complicated
Solution Approach 1:
The invention extracts the light propagation control function into a separate low refractive index material layer that is deposited independently. This eliminates the need to control additive concentrations and co-evaporation parameters, simplifying deposition conditions. The low refractive index material layer is formed by simple vacuum deposition or spin coating, which are easier to control than co-evaporation processes.
3Device complexity
If conventional organic semiconductor materials are used, then device structure is simple, but refractive index is too high for effective light extraction
Solution Approach 1:
The invention uses a composite structure consisting of an organic semiconductor layer and a low refractive index material layer. The organic semiconductor material maintains its electronic functionality while the low refractive index material layer provides optical optimization. This composite approach allows the device to benefit from both the simplicity of conventional organic semiconductors and the light extraction enhancement of low refractive index materials.
Solution Approach 2:
The invention applies local quality optimization by placing a low refractive index material specifically at the light extraction interface (the outer surface of the device). This localized application of low refractive index material targets the specific region where light extraction occurs, without requiring changes to the bulk organic semiconductor material properties. This approach maintains overall device simplicity while improving light extraction efficiency at the critical interface.
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
The aromatic compound achieves improved light emission efficiency in organic EL devices by optimizing the refractive index properties, leading to enhanced light extraction and utilization.
Implementation Method 1
effective light propagation control is achieved by utilizing the light interference effect of the stacked film
Implementation Method 2
effective light propagation control is achieved by utilizing the light interference effect of the stacked film
Data Source
Figure 1
AI summary
The aromatic compound represented by the following general formula (a) or (b) has a low refractive index, and therefore, by stacking a low refractive index layer containing the aromatic compound and a high refractive index layer to use the stack as a capping layer, an organic EL device with improved light extraction efficiency can be realized. A1 and A2 each represent an alkanediyl, cycloalkanediyl or fluorenediyl group, L1 to L4 each represent a single bond, -O-, -NH-, or an alkanediyl group, Cy1 to Cy4 each represent an alkyl group, a cycloalkyl group, or a monovalent aromatic hydrocarbon group, and X1 to X4 each represent -O- or -NH-.