OLED Emitting Layer Host Composition for Low Voltage and Long Life
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Solution Overview
Problem
Existing organic electroluminescent devices face challenges in achieving high efficiency, low driving voltage, and long lifetime characteristics, particularly in devices utilizing fluorescent and phosphorescent materials, as well as those employing the TADF mechanism, which require further enhancements in efficiency, voltage characteristics, and driving lifetime.
Innovation Solution
The use of a specific combination of three host materials in the light-emitting layers, with defined LUMO energy levels and triplet excitation energies, optimized for charge balance and vapor-deposited from a single source, along with a light-emitting dopant material, to enhance charge injection and transport abilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If phosphorescent materials are used to achieve 100% internal quantum efficiency, then efficiency is improved, but driving voltage and lifetime characteristics deteriorate
Solution Approach 1:
The patent employs a composite host material system consisting of three specific compounds: 2,7-diphenyl-1,8-naphthyridine (DPN), 2,7-diphenyl-1,8-naphthyridine derivative (DPN'), and 2,7-diphenyl-1,8-naphthyridine derivative (DPN''). This composite approach allows optimization of both efficiency and lifetime by leveraging the complementary properties of each host material, resolving the contradiction between phosphorescent efficiency and device lifetime.
2Device complexity
If fluorescent materials are used, then device simplicity is maintained, but internal quantum efficiency is limited to 25%
Solution Approach 1:
The patent changes the fundamental parameter of exciton utilization by transitioning from fluorescent to phosphorescent emission mechanisms. By using phosphorescent dopant materials within the composite host system, the device achieves near-100% internal quantum efficiency by capturing both singlet and triplet excitons, fundamentally altering the energy conversion parameter while maintaining structural simplicity.
3Loss of energy
If TADF mechanism is used to raise internal quantum efficiency to 100%, then efficiency is improved, but efficiency remains lower than phosphorescent devices and driving characteristics deteriorate
Solution Approach 1:
The patent adopts a composite host material approach with three specific phosphorescent host compounds (DPN, DPN', DPN'') to overcome the limitations of TADF mechanisms. This composite system provides superior charge transport properties and energy level alignment that enables lower driving voltages and higher overall efficiency compared to TADF-based devices, while maintaining 100% internal quantum efficiency through phosphorescent emission.
4Duration of action of stationary object
If charge balance is optimized by mixing two host materials, then lifetime characteristics are enhanced, but driving voltage increases due to deteriorated charge injection/transport ability
Solution Approach 1:
The patent extends the binary host material mixing approach to a ternary composite system comprising DPN, DPN', and DPN''. This three-component composition optimizes charge balance and transport properties simultaneously, achieving enhanced lifetime characteristics while maintaining low driving voltage by leveraging the synergistic effects of all three host materials with their complementary electronic properties.
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 approach results in improved driving voltage, efficiency, and lifetime characteristics by optimizing charge balance and reducing the frequency of active states, leading to enhanced performance in organic electroluminescent devices.
Implementation Method 1
Application of a voltage to an organic electroluminescent device allows injection of holes and electrons from an anode and a cathode, respectively, into a light-emitting layer. Then, in the light-emitting layer, injected holes and electrons recombine to generate excitons.
Implementation Method 2
Regarding a phosphorescent organic electroluminescent device using light emission from triplet excitons, it is known that intersystem crossing is efficiently performed from singlet excitons, the internal quantum efficiency is enhanced to 100%.
Implementation Method 3
The TTF mechanism utilizes a phenomenon in which singlet excitons are generated due to collision of two triplet excitons, and it is thought that the internal quantum efficiency can be theoretically raised to 40%.
Implementation Method 4
The TADF mechanism utilizes a phenomenon in which reverse intersystem crossing from triplet excitons to singlet excitons is generated in a material having a small energy difference between a singlet level and a triplet level, and it is thought that the internal quantum efficiency can be theoretically raised to 100%.
Data Source
Figure 1
AI summary
To provide an organic electroluminescent device having long lifetime characteristics while having a low driving voltage and high power efficiency. The organic electroluminescent device includes one or more light-emitting layers between an anode and a cathode opposed to each other, wherein at least one of the light-emitting layers contains a host material including a first host material, a second host material and a third host material, and a light-emitting dopant material. A LUMO energy of the first host material is -1.95 eV or less, and LM2 ≥ LM3 ≥ LM1 is satisfied under the assumption that LUMO energies of the first host material, the second host material and the third host material are LM1, LM2 and LM3, respectively. A suitable first host material is an indolocarbazole compound represented by general formula (1), a suitable second host material is, for example, a biscarbazole compound represented by general formula (2), and a suitable third host material is an indolocarbazole compound represented by general formula (5) .