OLED Emitting Layer Doping for Fast Decay and High EQE
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Solution Overview
Problem
Existing organic light-emitting devices (OLEDs) face inefficiencies due to the limited utilization of triplet excitons, leading to reduced efficiency and stability, particularly at high luminance levels, as a result of long decay times and triplet-triplet annihilation or triplet-polaron interactions.
Innovation Solution
Incorporating a luminescent organometallic complex with a small singlet-triplet energy difference (Δ(ES1(X))−(ET1(X))<0.2 eV) and a fluorescent emitter Y in the emitting layer, where ES1(X) > ES1(Y), to facilitate efficient energy transfer from the singlet state, thereby shortening the emission decay time below 100 ns and enhancing stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If phosphorescent emitters are used to utilize triplet excitons, then external quantum efficiency is improved, but emission decay time increases significantly
Solution Approach 1:
The patent introduces a fluorescent emitter as an intermediary species that accepts energy from triplet excitons generated by the electroluminescent complex. The fluorescent emitter has a shorter natural decay time than phosphorescent emitters, thus achieving fast decay while still utilizing triplet excitons through energy transfer. This mediator approach resolves the contradiction between efficiency and decay time by decoupling the triplet exciton utilization mechanism from the emission process itself.
Solution Approach 2:
The patent changes the key parameter of the emitting species from phosphorescent (long-lived) to fluorescent (short-lived), while maintaining triplet exciton utilization through energy transfer. By selecting a fluorescent emitter with appropriate energy levels and optimizing the concentration ratio between the electroluminescent complex and fluorescent emitter, the system achieves both high external quantum efficiency and short decay times below 100 ns.
2Use of energy by moving object
If triplet excitons are utilized efficiently, then external quantum efficiency is improved, but triplet-triplet annihilation and triplet-polaron interactions increase at high luminance
Solution Approach 1:
The fluorescent emitter acts as a mediator that rapidly converts triplet excitons into singlet excitons with short lifetimes. This prevents the accumulation of long-lived triplet excitons that would otherwise undergo triplet-triplet annihilation or interact with polarons at high current densities. The intermediary fluorescent emitter thus protects the system from these harmful interactions while maintaining high efficiency.
Solution Approach 2:
The system rushes through the triplet exciton utilization process by using fluorescent emitters with decay times below 100 ns. This rapid conversion of triplet excitons to photons prevents the build-up of triplet population that leads to annihilation and degradation processes, enabling stable operation at high luminance levels.
3Duration of action of moving object
If fluorescent emitters are used alone, then emission decay time is shortened, but triplet exciton utilization is limited to 25%
Solution Approach 1:
The patent merges the advantages of both phosphorescent and fluorescent emitters by combining them in a single emitting layer. The electroluminescent complex generates triplet excitons (utilizing 100% of injected carriers), which are then transferred to the fluorescent emitter for rapid emission. This combination achieves both short decay times and high external quantum efficiency by integrating the triplet generation capability with the fast emission characteristic.
Solution Approach 2:
The emitting layer is designed as a composite system containing both the electroluminescent complex (for triplet exciton generation) and the fluorescent emitter (for fast emission). This composite material approach allows the system to exploit the complementary properties of both emitter types, achieving superior performance that neither component could achieve alone.
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 enables the utilization of 100% of triplet excitons, resulting in increased stability and reduced roll-off at high luminance, with decay times below 100 ns without sacrificing external quantum efficiency.
Implementation Method 1
facilitate efficient energy transfer from the singlet state, thereby shortening the emission decay time below 100 ns
Implementation Method 2
very efficient thermal re-occupation from the initially very efficiently occupied T1 state into the S1 state can occur at room temperature
Data Source
AI summary
The present invention relates to organic light-emitting devices comprising (a) an anode, (i) a cathode, and (e) an emitting layer between the anode and cathode, comprising 2 to 40% by weight of a triplet emitter X having a difference of the singlet energy (ES1(X)) and the triplet energy (ET1(X)) of less than or equal to 0.4 eV [Δ(ES1(X))−(ET1(X))≤0.4 eV], 0.05 to 5.0% by weight of a fluorescent emitter Y and 55 to 97.95% by weight of a host compound(s), wherein the amount of the triplet emitter X, the fluorescent emitter Y and the host compound(s) adds up to a total of 100% by weight and the singlet energy of the triplet emitter X (ES1(X)) is greater than the singlet energy of the fluorescent emitter Y (ES1(Y)) [(ES1(X))>ES1(Y)]. By doping, for example, an emitting layer containing a luminescent organometallic complex having a small S1-T1 splitting, with a fluorescent emitter the emission decay time can significantly be shortened without sacrificing external quantum efficiency (EQE) because of very efficient energy transfer.