Two-Layer Organic EL Light Emitting Structure for Driving Voltage Reduction
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Solution Overview
Problem
Organic electroluminescence (EL) elements emitting blue light face low light emitting efficiency due to the need for high driving voltage and large energy gaps between molecular orbitals, which hinders efficient hole and electron transport and recombination.
Innovation Solution
The organic EL element is designed with a two-layer light emitting structure where the first light-emitting layer has a host material with a shallower highest occupied molecular orbital (HOMO) and a deeper lowest unoccupied molecular orbital (LUMO) than the organic light emitting material, and the second light-emitting layer has a host material with a deeper HOMO and shallower LUMO, optimizing the energy gap to enhance recombination probability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a phosphorescent light emitting material with maximum internal quantum yield of 100% is used in blue light emitting organic EL elements, then light emitting efficiency should be improved, but the energy gap between HOMO and LUMO must be extremely increased, making it difficult to use materials with high carrier mobility and conjugated molecules, resulting in low light emitting efficiency
Solution Approach 1:
The patent divides the light emitting layer into two distinct layers: a first light emitting layer containing a phosphorescent light emitting material and a second light emitting layer containing a fluorescent light emitting material. This segmentation allows each layer to be optimized independently - the first layer can use materials with appropriate HOMO/LUMO levels for high internal quantum yield while the second layer facilitates efficient carrier transport, thereby resolving the contradiction between achieving 100% internal quantum yield and maintaining low driving voltage.
2Reliability
If the energy gap between HOMO and LUMO is increased to confine excited triplet (T1) within the phosphorescent light emitting material, then light emitting efficiency should improve, but it becomes difficult to use materials with high carrier mobility and conjugated interactions
Solution Approach 1:
The patent applies local quality by assigning different functional characteristics to different layers. The first light emitting layer is designed with specific HOMO/LUMO energy levels to confine excited triplet states and achieve high internal quantum yield, while the second light emitting layer is designed with materials that have high carrier mobility and appropriate energy levels to facilitate efficient charge transport. This localized optimization of material properties in different regions resolves the contradiction between achieving high internal quantum yield and maintaining material selection flexibility.
3Device complexity
If a single light emitting layer with phosphorescent material is used, then device structure is simplified, but achieving both high internal quantum yield and low driving voltage simultaneously is difficult
Solution Approach 1:
The patent segments the light emitting layer into two functional layers to simultaneously achieve high internal quantum yield and low driving voltage. The first layer with phosphorescent material and optimized HOMO/LUMO levels achieves high internal quantum yield, while the second layer with fluorescent material provides efficient carrier transport pathways. This segmentation resolves the contradiction between device simplicity and high efficiency by distributing different functions across multiple layers rather than attempting to achieve all functions in a single layer.
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 reduces the driving voltage and increases the internal quantum yield, improving light emitting efficiency by preventing unnecessary movement of holes and electrons between layers and confining excited energy within the phosphorescent light emitting material.
Implementation Method 1
The organic EL element emits light by use of light generated when the exciton deactivates
Implementation Method 2
The light emitting layer is made from an organic light emitting material such as a phosphorescent light emitting material or a fluorescent light emitting material
Data Source
AI summary
An organic EL element (11) includes a light emitting layer (15) which has a two-layer structure, and a first light-emitting layer (15a) is made from a host material which includes LUMO shallower than (i) LUMO (19) of a phosphorescent light emitting material and (ii) LUMO of a host material, from which a second light-emitting layer (15b) is made. Further, the second light-emitting layer (15b) is made from a host material which includes HOMO deeper than (i) HOMO (18) of the phosphorescent light emitting material and (ii) HOMO of the host material, from which the first light-emitting layer (15a) is made. This makes it possible (i) to block holes from moving to the second light-emitting layer (15b) and (ii) to block electrons from moving to the first light-emitting layer (15a). As a result, a probability that the holes and the respective electrons recombine with each other is increased. It is therefore possible to reduce a driving voltage of an organic EL element (11). This improves light emitting efficiency.