Organic EL Display Electrode Layering for Uniform Light Emission
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Solution Overview
Problem
Organic electroluminescence (EL) display devices experience local brightness unevenness due to current concentration at the end portions of the lower electrode, leading to reduced luminous efficiency and color purity, despite previous attempts to mitigate this issue through electrode shaping and processing.
Innovation Solution
A display device design featuring a lower electrode with a first layer for light reflection and a second layer with enhanced charge injection properties, where the second layer is strategically placed to avoid end portions, ensuring uniform current distribution and emission across the pixel.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the lower electrode is formed with a thickness of 30 nm or more to obtain high reflectance, then the light extraction efficiency is improved, but the organic compound layer becomes thin at end portions of the lower electrode causing current concentration
Solution Approach 1:
The lower electrode is segmented into multiple layers: a first lower electrode layer (30 nm or more thickness) for high reflectance and light extraction efficiency, and a second lower electrode layer (5-20 nm thickness) with good charge injection properties. This segmentation allows each layer to fulfill its specific function without compromising the other, resolving the contradiction between achieving high reflectance and maintaining uniform organic compound layer thickness.
Solution Approach 2:
Different regions of the lower electrode structure are assigned different properties: the first layer provides high reflectance for light extraction, while the second layer provides good charge injection. This local differentiation of functional properties allows the system to simultaneously achieve high light extraction efficiency and uniform current distribution without requiring the entire electrode structure to compromise either property.
2Shape
If the upper end portion of the lower electrode is processed (rounded or tapered) to prevent electric field concentration, then the electrode shape is improved, but the organic compound layer thickness remains smaller at end portions causing current concentration
Solution Approach 1:
The lower electrode is divided into a first layer and a second layer, where the second layer is selectively positioned to cover end portions. This segmentation allows the electrode shape to be processed (rounded or tapered) for electric field distribution while the second layer compensates for the reduced organic compound layer thickness at end portions, preventing current concentration.
Solution Approach 2:
The second lower electrode layer is deposited in advance on the first lower electrode layer, specifically covering the end portions before the organic compound layer is formed. This preliminary action ensures that even if the organic compound layer becomes thin at end portions due to electrode shape processing, the second layer provides sufficient charge injection properties to prevent current concentration.
3Ease of manufacture
If the organic compound layer thickness varies due to lower electrode state, then the device structure is formed, but the current efficiency and chromaticity of the organic EL element deteriorate
Solution Approach 1:
The lower electrode is segmented into a first layer for light extraction and a second layer for charge injection. This segmentation allows the device structure to be formed with varying organic compound layer thickness while the second layer compensates for thickness variations, maintaining reliable current efficiency and chromaticity.
Solution Approach 2:
The invention changes the parameters of the lower electrode by introducing a second layer with specific thickness (5-20 nm) and material properties (good charge injection). This parameter change compensates for variations in organic compound layer thickness, ensuring stable current efficiency and chromaticity regardless of manufacturing variations.
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 improves luminous efficiency and color purity by reducing current flow at end portions and facilitating uniform light emission, regardless of electrode thickness or shape, thereby addressing the issue of local brightness unevenness.
Implementation Method 1
a first lower electrode layer provided on the substrate and having a small charge injection property into the organic compound layer
Implementation Method 2
a second lower electrode layer provided in a region on the first lower electrode layer, the region avoiding an end portion, and having a large charge injection property into the organic compound layer
Implementation Method 3
an organic EL element obtained by laminating a lower electrode provided on the substrate, an organic compound layer, and an upper electrode in the stated order
Data Source
AI summary
Provided is a display device including: a substrate; and multiple pixels provided on the substrate, the pixels each having an organic EL element obtained by laminating a lower electrode provided on the substrate, an organic compound layer, and an upper electrode in the stated order, and the lower electrode including an electrode independently placed for each of the pixels, in which: the lower electrode is formed of a first lower electrode layer provided on the substrate and a second lower electrode layer provided on the first lower electrode layer; the organic compound layer and the upper electrode cover the first lower electrode layer and the second lower electrode layer; and charge injection property from the second lower electrode layer into the organic compound layer is larger than charge injection property from an end portion of the first lower electrode layer into the organic compound layer.