Organic Host Material HOMO Alignment for Light-Emitting Element Efficiency
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Solution Overview
Problem
Current light-emitting elements face challenges in achieving high emission efficiency due to inadequate carrier injection properties, particularly when the difference in highest occupied molecular orbital (HOMO) levels between host and guest materials is significant, leading to trapped holes at the anode interface and reduced distribution within the light-emitting layer.
Innovation Solution
A light-emitting element structure is developed using a host material with a 5-membered ring-based compound, which increases electron density and maintains a high triplet excited energy level, ensuring a small difference in HOMO levels between the host and guest materials, thereby enhancing carrier balance and emission efficiency. The host material, represented by the general formula (G1), is used in conjunction with a phosphorescent guest material to optimize the light-emitting layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the HOMO level of the host material is significantly lower than the HOMO level of the guest material, then the light-emitting element can be fabricated with conventional materials, but holes are selectively trapped at the anode interface and carrier balance deteriorates
Solution Approach 1:
The patent applies parameter changes by adjusting the HOMO level of the host material to be within 0.5 eV of the guest material's HOMO level. This specific parameter range prevents hole trapping at the anode interface while maintaining ease of manufacture with available phosphorescent compounds.
Solution Approach 2:
The patent applies local quality by optimizing the host material's HOMO level specifically at the anode interface region where hole trapping occurs. This localized parameter optimization ensures proper carrier distribution without affecting other regions of the light-emitting layer.
2Illumination intensity
If a phosphorescent guest material with short wavelength emission is used, then the light-emitting element can achieve blue light emission, but the T1 level becomes high when HOMO level is high, reducing emission efficiency
Solution Approach 1:
The patent applies parameter changes by simultaneously optimizing both the HOMO level and T1 level of the host material. The HOMO level is set within 0.5 eV of the guest material to ensure proper carrier injection, while the T1 level is maintained above the guest material's T1 level to prevent energy loss and maintain high emission efficiency for blue light emission.
3Reliability
If the HOMO level of the host material is increased to match high HOMO level guest materials, then carrier injection property improves, but the T1 level also increases, potentially reducing emission efficiency
Solution Approach 1:
The patent applies parameter changes by precisely controlling both HOMO and T1 levels of the host material within specific ranges. The HOMO level is optimized to be within 0.5 eV of the guest material for improved carrier injection, while the T1 level is simultaneously maintained above the guest material's T1 level to ensure high emission efficiency is preserved.
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 results in a light-emitting element with improved emission efficiency, characterized by lower driving voltage and higher external energy efficiency, while maintaining consistent chromaticity and carrier balance, thus enhancing the overall performance of the light-emitting device.
Implementation Method 1
it is very important to increase a property of injecting carriers to a light-emitting layer
Implementation Method 2
a magnitude relation of the highest occupied molecular orbital level (hereinafter referred to as a HOMO level) of the host material and the HOMO level of the guest material is considered to affect a property of injecting carriers
Implementation Method 3
an organic compound capable of providing light emission by application of an electric field is provided between a pair of electrodes
Implementation Method 4
Luminescence from the singlet excited state (S1) is referred to as fluorescence
Implementation Method 5
Luminescence from the triplet excited state (T1) is referred to as phosphorescence
Implementation Method 6
electrons injected from the cathode and holes injected from the anode are excited in a light emission center of the EL layer, and energy is released and light is emitted when the excited state returns to a ground state
Data Source
AI summary
A light-emitting element emitting phosphorescence and having high emission efficiency, in which a property of injecting holes to a light-emitting layer is increased, is provided. The light-emitting layer of the light-emitting element includes a first organic compound represented by the following general formula (G1) and a second organic compound which is a phosphorescent compound. The difference between the HOMO level of the first organic compound and the HOMO level of the second organic compound is lower than or equal to 0.3 eV.