Organic EL Emitter Layer Using TADF Host for High Efficiency
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Solution Overview
Problem
Current organic electroluminescence devices have limitations in achieving high luminous efficiency, particularly in utilizing triplet excitons, which are not fully harnessed in fluorescent devices, leading to suboptimal performance.
Innovation Solution
Incorporating a specific combination of materials in the emitting layer, including a fluorescent material, a thermally activated delayed fluorescent material, and a third material with a higher singlet energy, to enhance the utilization of both singlet and triplet excitons through the TADF mechanism, thereby improving luminous efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a fluorescent organic EL device uses only singlet excitons for emission, then the device structure is simple, but the internal quantum efficiency is limited to 25% at maximum
Solution Approach 1:
The emitting layer uses a composite material system consisting of a TADF host material (e.g., mCP or TCTA) and a fluorescent dopant material. This composite structure enables the host to generate triplet excitons that can undergo inverse intersystem crossing to singlet excitons, which then transfer energy to the dopant for fluorescent emission, thereby utilizing both singlet and triplet excitons to achieve internal quantum efficiency exceeding 25% while maintaining a relatively simple device structure
Solution Approach 2:
The invention changes the energy level parameters of the host material by selecting TADF materials with specific singlet-triplet energy gaps (ΔEST). By controlling this energy parameter to be small enough to allow thermal activation but large enough to maintain material stability, the device can efficiently convert triplet excitons to singlet excitons and achieve high internal quantum efficiency without significantly complicating the device structure
2Use of energy by moving object
If a compound with a small energy difference (ΔST) between singlet and triplet energy levels is used to generate TADF mechanism, then the internal quantum efficiency is improved, but the material selection becomes more restrictive
Solution Approach 1:
The TADF host material acts as an intermediary between the injected carriers and the fluorescent dopant. It first generates excitons (both singlet and triplet) from carrier recombination, then facilitates inverse intersystem crossing of triplet excitons to singlet excitons, and finally transfers the singlet exciton energy to the dopant for light emission. This intermediary role allows the system to achieve high efficiency while using materials with appropriate but not extremely restrictive energy level matching
Solution Approach 2:
The invention specifies parameter ranges for the TADF host material (singlet energy level ES of 2.5-3.5 eV and small ΔEST) that balance the competing requirements of efficient TADF mechanism and material versatility. These parameter ranges are wide enough to accommodate multiple material choices while being specific enough to ensure proper TADF functionality and energy transfer to the dopant
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 proposed configuration significantly enhances the internal quantum efficiency of the organic electroluminescence device by effectively utilizing both singlet and triplet excitons, potentially reaching theoretical maximum efficiency of 100% through delayed fluorescence.
Implementation Method 1
a thermally activated delayed fluorescent material, the third material having a singlet energy larger than a singlet energy of the second material
Implementation Method 2
The TADF mechanism uses such a phenomenon that inverse intersystem crossing from triplet excitons to singlet excitons thermally occurs when a material having a small energy difference (ΔST) between singlet energy level and triplet energy level is used
Implementation Method 3
When a voltage is applied to an organic electroluminescence device (hereinafter, occasionally referred to as 'organic EL device'), holes and electrons are injected into an emitting layer respectively from an anode and a cathode. The injected holes and electrons are recombined to generate excitons in the emitting layer
Data Source
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AI summary
An organic electroluminescence device includes: an anode; an emitting layer; and a cathode, the emitting layer containing a first material, a second material and a third material, the first material being a fluorescent material, the second material being a delayed fluorescent material, the third material having a singlet energy larger than a singlet energy of the second material.