Organic Electroluminescence Device Host Material Energy Gap Optimization
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Solution Overview
Problem
Phosphorescent organic electroluminescence devices face limitations in luminous efficiency due to the use of materials with large energy gaps, which increase drive voltage and reduce lifetime, and require specific material selection and device design different from fluorescent devices to control exciton recombination effectively.
Innovation Solution
Selecting a host material for the emitting layer that satisfies specific conditions such as a small energy gap difference between singlet and triplet energies, a large refractive index difference, and a broad photoluminescence spectrum, along with a phosphorescent dopant material, to enhance exciton trapping and reduce energy transfer, while using an electron transporting layer with high mobility to improve luminous efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a phosphorescent organic EL device uses materials with large energy gaps to achieve efficient triplet exciton trapping, then luminous efficiency is improved, but drive voltage increases and device lifetime decreases
Solution Approach 1:
The patent applies parameter changes by carefully selecting host and dopant materials with specific energy gap relationships. The host material is chosen to have a triplet energy slightly larger than the dopant's triplet energy, while the singlet energy gap is optimized to be not excessively large. This parameter optimization allows efficient triplet exciton trapping without requiring excessively large overall energy gaps, thereby maintaining lower drive voltages while achieving high luminous efficiency.
Solution Approach 2:
The patent employs composite materials by combining specific host materials (such as Alq3 or BCP) with phosphorescent dopant materials (such as Ir(ppy)3 or PtOEP) in controlled ratios. This composite approach allows the host material to provide the necessary energy gap for triplet exciton confinement while the dopant material provides efficient phosphorescent emission, achieving a balance between luminous efficiency and drive voltage requirements.
2Productivity
If a phosphorescent organic EL device uses materials with large energy gaps to achieve efficient triplet exciton trapping, then luminous efficiency is improved, but device lifetime decreases
Solution Approach 1:
The patent optimizes the energy gap parameters of the host and dopant materials to achieve a balance between luminous efficiency and device lifetime. By selecting materials where the triplet energy difference is optimized rather than maximized, the device achieves efficient exciton trapping without excessive energy gaps that would increase drive voltage and accelerate material degradation, thereby extending device lifetime while maintaining high luminous efficiency.
3Device complexity
If a fluorescent device technique is simply used for designing the phosphorescent organic EL device, then device design is simplified, but highly efficient phosphorescent emission cannot be obtained
Solution Approach 1:
The patent applies local quality by implementing specific design features in the phosphorescent emitting layer that differ from conventional fluorescent device designs. This includes using host materials with appropriately optimized triplet energies, controlling dopant concentration ratios, and selecting materials with specific energy level alignments. These localized design modifications in the emitting layer enable efficient phosphorescent emission while keeping other device structures relatively simple.
4Productivity
If a compound with large energy gap is used in phosphorescent organic EL device, then triplet exciton trapping is improved, but recombination region control becomes more difficult
Solution Approach 1:
The patent optimizes the energy gap parameters of the host and dopant materials to achieve a balance between triplet exciton trapping efficiency and recombination region control. By selecting materials where the triplet energy difference is optimized rather than maximized, the device achieves efficient exciton trapping without excessive energy gaps that would broaden the recombination region and make control more difficult.
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 approach results in a phosphorescent organic electroluminescence device with improved luminous efficiency and reduced drive voltage, achieving efficient exciton trapping and emission by optimizing the host and dopant material properties and electron mobility.
Implementation Method 1
in a phosphorescent EL device which uses emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons
Implementation Method 2
it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons
Implementation Method 3
electrons are injected from a cathode into the emitting layer. The injected holes and electrons are recombined in the emitting layer to form excitons
Implementation Method 4
When a voltage is applied to an organic electroluminescence device (hereinafter referred to as an organic EL device), holes are injected from an anode into an emitting layer and electrons are injected from a cathode into the emitting layer
Data Source
AI summary
An organic electroluminescence device includes a pair of electrodes and an organic compound layer between the pair of electrodes. The organic compound layer includes an emitting layer including a host material and a phosphorescent dopant material. The host material is selected from a compound satisfying the following formula (1) with respect to a difference ΔST between singlet energy EgS and an energy gap Eg77K at 77K and satisfying the following formula (2) with respect to the singlet energy EgSΔST=EgS−Eg77K<0.4 (eV) (1)EgS≧2.90 (eV) (2)