Organic EL Device Host-Dopant Energy Alignment for Roll-Off
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Solution Overview
Problem
Organic electroluminescence devices using the TADF mechanism exhibit reduced luminous efficiency at high current densities, necessitating an improvement in light emission efficiency in practical usage ranges.
Innovation Solution
An organic electroluminescence device is designed with a specific configuration of electrodes and an organic compound layer, including a first host material with a small energy difference between singlet and triplet states, a second host material with a fused aromatic structure, and a fluorescent dopant material, optimizing energy transfer and intersystem crossing to enhance efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If TADF mechanism is used to improve internal quantum efficiency, then internal quantum efficiency can be raised up to 100%, but luminous efficiency decreases in high current density range due to roll-off phenomenon
Solution Approach 1:
The patent changes the energy level parameters of the host and dopant materials. Specifically, it sets the singlet energy of the dopant (EgS(D)) to be lower than that of the host (EgS(H)), and controls the triplet energy difference (ΔST) to be within 0.1-0.5 eV. This parameter optimization enables efficient reverse intersystem crossing while maintaining high luminous efficiency even at high current densities, resolving the roll-off problem.
Solution Approach 2:
The patent uses a composite material system consisting of a host material and a dopant material with specifically designed energy level structures. The host material has high triplet energy and the dopant material has appropriate singlet and triplet energy levels to enable TADF mechanism. This composite approach allows simultaneous achievement of high internal quantum efficiency and maintained luminous efficiency at practical current densities.
2Loss of energy
If delayed fluorescence mechanism is used to increase internal quantum efficiency beyond 25%, then internal quantum efficiency improves, but the device still cannot achieve phosphorescent-level efficiency
Solution Approach 1:
The patent optimizes the energy level parameters by setting EgS(D) < EgS(H) and controlling ΔST within 0.1-0.5 eV. This creates optimal conditions for reverse intersystem crossing from triplet to singlet states, enabling the TADF mechanism to achieve near-100% internal quantum efficiency while maintaining stable efficiency characteristics that match phosphorescent device performance.
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 luminous efficiency and internal quantum efficiency, exceeding theoretical limits, particularly in high current density ranges, by leveraging the TADF mechanism and efficient energy transfer within the emitting layer.
Implementation Method 1
The TADF mechanism utilizes a phenomenon in which reverse intersystem crossing from triplet excitons to singlet excitons is generated by using a material having a small energy difference (ΔST) between the singlet level and the triplet level. Reverse intersystem crossing from the triplet level to the singlet level is caused by thermal energy.
Implementation Method 2
singlet energy EgS(H1) of the first host material, singlet energy EgS(H2) of the second host material, and singlet energy EgS(D) of the dopant material satisfy a relationship of numerical formulae (1) and (2) below
Data Source
AI summary
An organic EL device includes a pair of electrodes and an organic compound layer between pair of electrodes. The organic compound layer includes an emitting layer including a first material, a second material and a third material, in which singlet energy EgS(H) of the first material, singlet energy EgS(H2) of the second material, and singlet energy EgS(D) of the third material satisfy a specific relationship.