OLED Light-Emitting Layer Using Exciplex Energy Transfer
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Solution Overview
Problem
Organic electroluminescence (EL) elements face limitations in external quantum efficiency and lifetime due to inefficient energy transfer from host to guest materials, particularly when using phosphorescent compounds, where the singlet and triplet excitation energies differ significantly, leading to reduced emission efficiency.
Innovation Solution
Incorporating a combination of first and second organic compounds that form an exciplex, which has a small energy difference between singlet and triplet excited states, allowing for efficient energy transfer to a phosphorescent compound, thereby enhancing external quantum efficiency and extending the element's lifetime.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a phosphorescent compound is used as the light-emitting material, then the internal quantum efficiency can theoretically reach 100%, but the external quantum efficiency is limited to approximately 25% due to light extraction efficiency of 20-30%
Solution Approach 1:
The patent introduces a fluorescent compound as an intermediary material between the phosphorescent compound and the host material. This fluorescent compound acts as a mediator that receives energy from the host material and transfers it to the phosphorescent compound, enabling more efficient energy transfer pathways and improving overall light extraction efficiency while maintaining high internal quantum efficiency.
2Reliability
If the phosphorescent compound is dispersed in a host material matrix to suppress concentration quenching, then the emission efficiency is improved, but the energy transfer from host to guest becomes inefficient due to significant energy difference between singlet and triplet excited states
Solution Approach 1:
The fluorescent compound serves as an energy transfer intermediary that bridges the host material and phosphorescent compound. The host material transfers energy to the fluorescent compound's singlet excited state, which then transfers to the phosphorescent compound's triplet excited state. This two-step energy transfer process overcomes the inefficiency caused by direct host-to-phosphorescent compound energy transfer.
Solution Approach 2:
The patent changes the energy level parameters of the light-emitting layer by introducing a fluorescent compound with appropriate singlet excited state energy levels. This parameter adjustment creates an efficient energy transfer pathway where the fluorescent compound's singlet state energy matches well with the host material's triplet state energy, enabling effective energy transfer to the phosphorescent compound.
3Illumination intensity
If conventional phosphorescent compounds are used, then high phosphorescence quantum yield is achieved, but the lifetime of the organic EL element is reduced due to inefficient energy transfer and deactivation of excitation energy
Solution Approach 1:
The fluorescent compound acts as a protective intermediary that facilitates efficient energy transfer from the host material to the phosphorescent compound. This reduces the residence time of excitation energy in the host material, minimizing deactivation pathways and extending the operational lifetime of the element while maintaining high phosphorescence quantum yield.
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 use of an exciplex in the light-emitting layer enables high external quantum efficiency and prolonged lifetime by ensuring effective energy transfer and minimizing deactivation of excitation energy, surpassing the theoretical limits of conventional phosphorescent-based organic EL elements.
Implementation Method 1
a combination of the first organic compound and the second organic compound forms an exciplex, which has a small energy difference between singlet and triplet excited states, allowing for efficient energy transfer to a phosphorescent compound
Implementation Method 2
light emission from the triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different spin multiplicities
Implementation Method 3
a combination of the first organic compound and the second organic compound forms an exciplex, which has a small energy difference between singlet and triplet excited states
Data Source
AI summary
A light-emitting element having high external quantum efficiency is provided. A light-emitting element having a long lifetime is provided. A light-emitting element is provided which includes a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, in which a combination of the first organic compound and the second organic compound forms an exciplex (excited complex). The light-emitting element transfers energy by utilizing an overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound and thus has high energy transfer efficiency. Therefore, a light-emitting element having high external quantum efficiency can be obtained.