Organic Electroluminescence Element Ionization Potential Optimization
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Solution Overview
Problem
Organic electroluminescence devices face challenges in prolonging lifetime, improving luminous efficiency, and inhibiting roll-off when driven at high current densities, particularly due to the instability of delayed fluorescent compounds and the competition between emission processes in emitting layers.
Innovation Solution
An organic electroluminescence device is designed with an emitting layer containing a thermally activated delayed fluorescent compound and a fluorescent compound, where the ionization potential difference between the two compounds is optimized to inhibit trap emission and promote energy transfer, thereby enhancing luminous efficiency and reducing roll-off.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a thermally activated delayed fluorescence (TADF) mechanism is used to improve luminous efficiency, then triplet excitons can be converted to singlet excitons, but the delayed fluorescent compound exhibits instability and trap emission occurs
Solution Approach 1:
The patent introduces a fluorescent compound as an intermediary material in the emitting layer. This fluorescent compound acts as a mediator that receives energy from the delayed fluorescent compound through energy transfer, converting the unstable delayed fluorescence emission into stable fluorescent emission. The fluorescent compound thus serves as a bridge that transforms the harmful instability of TADF compounds into useful stable light emission, resolving the contradiction between improving luminous efficiency and maintaining reliability.
Solution Approach 2:
The patent converts the harmful trap emission and instability of delayed fluorescent compounds into beneficial effects. By utilizing the energy transfer from the delayed fluorescent compound to the fluorescent compound, the patent transforms the unstable TADF emission (which causes reliability issues) into stable fluorescent emission. The previously harmful trap emission is converted into a mechanism that can still contribute to overall emission while the stability issue is resolved through the fluorescent compound's stable emission characteristics.
2Productivity
If high current density is applied to improve productivity, then light output increases, but roll-off occurs and lifetime decreases
Solution Approach 1:
The patent changes the energy level parameters of the emitting layer materials to resolve the roll-off issue. By carefully selecting materials with appropriate energy levels (where the fluorescent compound's energy level is lower than the delayed fluorescent compound's energy level), the patent enables efficient energy transfer while maintaining stable emission characteristics at high current densities. This parameter optimization allows the device to operate at high productivity without the severe roll-off that would otherwise occur, thereby extending device lifetime.
3Loss of energy
If delayed fluorescent compound is used to utilize triplet excitons, then luminous efficiency improves, but competition between emission processes reduces overall performance
Solution Approach 1:
The patent segments the emission function into two distinct components: the delayed fluorescent compound is responsible for harvesting triplet excitons and transferring energy, while the fluorescent compound is responsible for the actual light emission. This segmentation allows each material to perform its specialized function optimally - the delayed fluorescent compound efficiently converts triplet excitons without suffering from stability issues, while the fluorescent compound provides stable, controllable emission. This division of labor resolves the competition between emission processes by making them sequential rather than competing.
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 combination of delayed fluorescent and fluorescent compounds in the emitting layer prolongs the device's lifetime, improves luminous efficiency, and inhibits roll-off at high current densities by optimizing energy transfer and carrier balance.
Implementation Method 1
a thermally activated delayed fluorescent compound... inverse intersystem crossing from triplet excitons to singlet excitons thermally occurs
Implementation Method 2
energy transfer from a delayed fluorescent compound to a fluorescent compound... ionization potential difference between the two compounds is optimized to inhibit trap emission and promote energy transfer
Implementation Method 3
When a voltage is applied to an organic electroluminescence 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, in which the emitting layer includes a first compound and a second compound, the first compound is a delayed fluorescent compound, the second compound is a fluorescent compound, an emission quantum efficiency of the first compound is 70% or less, and an ionization potential Ip1 of the first compound and an ionization potential Ip2 of the second compound satisfy a relationship of 0 ≤ Ip2 - Ip1 ≤ 0.8 eV.