Green OLED Delayed Fluorescence Microcavity Voltage Reduction
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Solution Overview
Problem
Organic electroluminescent (EL) devices using phosphorescent light emitting materials face challenges in maintaining low driving voltage and high color purity due to the expansion of the host material's band gap, requiring redesign of layer constitution and selection of optimal materials for adjacent layers.
Innovation Solution
Incorporating a thermal excitation-type delayed fluorescent material in the emission layer of green organic EL devices with a microcavity and common thickness for hole transport layers across green and blue devices, which narrows the emission spectral bandwidth and reduces the driving voltage, while allowing for increased emission layer thickness without significant voltage increase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If phosphorescent light emitting material is used in the emission layer, then internal quantum efficiency can reach 100%, but the host material's band gap expands causing high driving voltage
Solution Approach 1:
The patent changes the emission mechanism from phosphorescence to delayed fluorescence, altering the physical parameters of the emission layer. This parameter change allows the system to achieve high internal quantum efficiency without the band gap expansion problem that occurs with phosphorescent materials, thereby maintaining low driving voltage.
Solution Approach 2:
The patent uses a fluorescent material with a short-lived excited state that undergoes thermal excitation to produce delayed fluorescence. This approach replaces the need for expensive rare-earth phosphorescent materials and their associated complex host material requirements, achieving similar efficiency benefits without the voltage penalty.
2Loss of energy
If phosphorescent light emitting material is used in the emission layer, then internal quantum efficiency can reach 100%, but color purity deteriorates due to expanded band gap
Solution Approach 1:
By changing from phosphorescent to delayed fluorescent emission mechanism, the patent alters the energy level structure and emission characteristics. This parameter change enables maintenance of narrow emission bandwidth (high color purity) while achieving high internal quantum efficiency through the delayed fluorescence process.
3Loss of energy
If emission layer thickness is increased to improve light extraction, then light extraction efficiency improves, but driving voltage increases significantly
Solution Approach 1:
The patent employs delayed fluorescent materials with short-lived excited states that enable efficient carrier recombination and light emission. This allows the emission layer to be made thicker for improved light extraction without the proportional increase in driving voltage that would normally occur, because the delayed fluorescence mechanism maintains efficient recombination over the thicker distance.
4Reliability
If different hole transport layers are used for green and blue devices to optimize performance, then device performance improves, but manufacturing complexity increases
Solution Approach 1:
The patent uses the same hole transport layer material for both green and blue organic EL devices. This universal approach simplifies the manufacturing process and reduces device complexity while maintaining optimized performance for both device types, demonstrating that a single material can serve multiple functions across different device configurations.
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 solution enables low-power operation with high color purity and improved light extraction efficiency, simplifying the production process by allowing common hole transport layers for blue and green devices, and reducing the driving voltage even with thicker emission layers.
Implementation Method 1
the emission layer of each of the green organic electroluminescent devices has a thermal excitation-type delayed fluorescent material
Implementation Method 2
a delayed fluorescent material is used in an organic EL device
Implementation Method 3
each of the green organic electroluminescent devices has a microcavity between the pair of electrodes
Implementation Method 4
the emission spectral bandwidth of light emitted from the thermal excitation-type delayed fluorescent material is narrowed
Data Source
Figure 1~2
Figure 3
AI summary
An organic electroluminescent display apparatus having organic electroluminescent devices each of which is excellent in color reproducibility and has high emission efficiency in which green organic electroluminescent devices each have a delayed fluorescent material and a microcavity, and the hole transport layer of each of the devices has the same thickness as that of the hole transport layer of each of blue organic electroluminescent devices.