Organic Electroluminescent Device with Dual Emissive Layers
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Solution Overview
Problem
Conventional organic electroluminescent devices with phosphorescent host-dopant systems suffer from reduced luminance efficiency due to triplet-triplet annihilation caused by long exciton lifetimes of triplet excitons, which declines rapidly with increasing electric current.
Innovation Solution
An organic electroluminescent device is designed with both fluorescent and phosphorescent emissive layers, where the fluorescent excitons dilute the concentration of triplet excitons, reducing their collision probability and incorporating an exciton blocking layer to prevent triplet-triplet annihilation, thereby maintaining high luminance efficiency at high operating brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a phosphorescent host-dopant system is used to improve luminance efficiency, then the internal quantum efficiency increases due to triplet exciton utilization, but the exciton lifetime becomes too long causing triplet-triplet annihilation and rapid efficiency decline at high current
Solution Approach 1:
The emissive layer is divided into multiple sub-layers with different phosphorescent dopants having different triplet energy levels. This segmentation creates energy barriers that prevent triplet-triplet annihilation while maintaining high internal quantum efficiency through sequential energy transfer from higher to lower energy dopants.
Solution Approach 2:
The patent changes the energy level parameters of the phosphorescent dopants by selecting materials with different triplet energy levels (Et values). This parameter variation creates an energy cascade structure where excitons transfer from high-Et to low-Et dopants, extending effective exciton lifetime without causing annihilation.
2Illumination intensity
If the concentration of triplet excitons is increased to improve phosphorescence emission, then the phosphorescence intensity increases, but the collision probability between triplet excitons increases leading to triplet-triplet annihilation
Solution Approach 1:
The phosphorescent dopants are segmented into multiple species with different triplet energy levels distributed across the emissive layer. This spatial and energetic segmentation allows high overall phosphorescence intensity while maintaining low local triplet exciton concentrations in each sub-layer, preventing annihilation.
Solution Approach 2:
Phosphorescent dopants with intermediate triplet energy levels act as mediators, accepting excitons from high-Et dopants and transferring to low-Et dopants. This intermediary structure provides an energy transfer pathway that reduces direct triplet-triplet collisions while maintaining high phosphorescence output.
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 device achieves improved brightness and luminance efficiency by emitting both visible fluorescence and phosphorescence simultaneously, outperforming conventional devices with either fluorescent or phosphorescent systems, as the phosphorescent emissive layer's higher efficiency is combined with the stabilizing effect of fluorescent excitons.
Implementation Method 1
the phosphorescent dopants can transfer the energy from the singlet excitons of the host material to the triplet excitons of the dopants
Implementation Method 2
when returning to the ground state, the triplet excitons emit visible phosphorescence
Implementation Method 3
the singlet excitons emit light which can be transferred to phosphorescence through internal system crossing (ISC)
Data Source
AI summary
An organic electroluminescent device (OELD) is provided. The OELD includes a substrate, a first electrode, a second electrode, a hole transport layer, an electron transport layer and two emissive layers. The first electrode and the second electrode are disposed over the substrate. The hole transport layer is disposed between the first electrode and the second electrode. The electron transport layer is disposed between the second electrode and the hole transport layer. The emissive layers are disposed between the hole transport layer and the electron transport layer. One of the emissive layers is a fluorescent emissive layer and another one of the emissive layers is a phosphorescent emissive layer. The visible light of the fluorescent emissive layer and the phosphorescent emissive layer are not absorbed by each other and the visible light spectrums of the fluorescent emissive layer and the phosphorescent emissive layer are not affected by each other.