OLED Bilayer Buffer Layer Exciton Diffusion Control
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Solution Overview
Problem
The lifetime of organic light-emitting diodes (OLEDs) comprising blue emitters is limited due to electrical stress, degradation, and exciton diffusion issues when paired with narrower bandgap emitters, leading to unsatisfactory Color Rendering Index (CRI) and luminance degradation, particularly in white OLEDs.
Innovation Solution
Incorporating a conductive buffer layer composed of a bilayer of electron and hole transport layers between the electroluminescent layers, allowing for efficient carrier transport and reducing exciton diffusion, thereby enhancing the diode's conductivity and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If blue emitters with wide bandgap are used to achieve satisfactory CRI, then Color Rendering Index is improved, but lifespan is reduced due to shorter lifetime of blue organic emitters
Solution Approach 1:
The patent introduces a buffer layer as an intermediary component between the blue emitter and other electroluminescent layers. This buffer layer mediates the interaction by providing a transition zone that reduces electrical stress on the blue emitter while maintaining color rendering performance, thus extending the overall device lifespan without sacrificing CRI
Solution Approach 2:
The patent modifies the structural parameters of the OLED by introducing a buffer layer with specific thickness (5-20 nm) and material composition. This parameter change alters the electrical field distribution and carrier transport characteristics, reducing the stress on blue emitters and thereby extending their operational lifetime while preserving the desired color output
2Use of energy by moving object
If exciton diffusion is allowed to occur between electroluminescent layers, then energy transfer may occur, but luminance degradation and color point instability increase
Solution Approach 1:
The patent segments the electroluminescent structure by inserting a buffer layer between different emitter layers. This segmentation physically separates the electroluminescent zones, preventing unwanted exciton diffusion between layers while maintaining controlled energy transfer within each layer, thus stabilizing the color point and reducing luminance degradation
Solution Approach 2:
The buffer layer serves as an intermediary zone that controls exciton behavior. It allows controlled energy transfer when needed while primarily functioning to block exciton diffusion between adjacent electroluminescent layers, thereby preventing color instability and luminance degradation
3Illumination intensity
If blue emitters are subjected to higher electrical stress to maintain luminance, then luminance level is maintained, but lifespan is further reduced
Solution Approach 1:
The buffer layer acts as a mediator that redistributes electrical stress away from the blue emitter. By providing an additional layer for carrier transport and recombination, it reduces the electrical stress concentration on the blue emitter, allowing it to maintain luminance output without being subjected to excessive stress that would accelerate degradation
Solution Approach 2:
The introduction of the buffer layer changes the electrical parameters of the device, specifically the voltage distribution and current density profile. This parameter change allows the blue emitter to operate at lower stress conditions while maintaining the required luminance level, thereby extending its operational lifetime
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 the lifetime of OLEDs by limiting exciton diffusion and ion formation, maintaining luminance and CRI, with a notable improvement in diode longevity, typically extending the 50% luminance reduction time by a factor of 5 to 10.
Implementation Method 1
an OLED consists of a stack of layers of organic materials, among which there is at least one electroluminescent, fluorescent, or phosphorescent layer
Implementation Method 2
said buffer layer is a bilayer comprising an electron transport layer and a hole transport layer
Implementation Method 3
the excitons generated in the large bandgap emitter ELB tend to diffuse towards the smaller bandgap emitter ELR
Data Source
Figure 1~2
Figure 3
AI summary
The invention relates to an organic light-emitting diode which includes at least two electroluminescent layers (ELR, ELB), both of which are fluorescent or phosphorescent and emit at different wavelengths, as well as a hole- and electron-conducting buffer layer (T) arranged between said electroluminescent layers, characterised in that said buffer layer is a bi-layer comprising an electron-transport layer (T2) and a hole-transport layer (T1), each one of said hole- and electron-transport layers being made of one or more materials in which the HOMO level(s) are comprised between or equal to the HOMO levels of said electroluminescent layers, and in which the LUMO levels are comprised between or equal to the LUMO levels of said electroluminescent layers, with a tolerance of 0.3 eV.