OLED Emitting Layer for Fast Organometallic–Fluorescent Energy Transfer
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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 quantum efficiency and stability issues, particularly at high luminance, as well as long decay times that can lead to triplet-triplet annihilation and chemical reactions.
Innovation Solution
Incorporating a luminescent organometallic complex with a small singlet-triplet energy splitting (Δ(E S1 (X) - E T1 (X)) of less than 0.2 eV into the emitting layer, along with a fluorescent emitter, enables efficient energy transfer from the singlet state, significantly shortening the emission decay time to below 100 ns and improving stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a phosphorescent emitter is used to utilize triplet excitons, then external quantum efficiency is improved, but emission decay time becomes excessively long (microsecond range)
Solution Approach 1:
The patent changes the energy parameter ΔE between singlet and triplet states to be less than 0.2 eV, enabling efficient thermal re-occupation from T1 to S1 states. This parameter modification allows the system to achieve both high quantum efficiency (utilizing triplet excitons) and short decay times (below 100 ns) by creating a fast emission channel from the short-lived S1 state.
Solution Approach 2:
The patent introduces a luminescent organometallic complex as an intermediary species with specific energy level characteristics. This complex acts as a mediator that facilitates rapid energy transfer from triplet to singlet states through thermal re-occupation, thereby enabling both efficient triplet exciton utilization and short emission lifetimes.
2Illumination intensity
If triplet excitons are accumulated at high current densities to maintain luminance, then brightness is improved, but triplet-triplet annihilation and chemical reactions increase, reducing stability
Solution Approach 1:
The patent implements a rapid decay mechanism that allows the system to quickly pass through the triplet state population before triplet-triplet annihilation can occur. By enabling fast thermal re-occupation from T1 to S1 states and creating a short emission lifetime (below 100 ns), the system rushes through the exciton population cycle, preventing accumulation and associated harmful reactions even at high current densities.
3Speed
If a fluorescent emitter with high singlet energy is used, then emission speed is improved, but triplet excitons cannot be effectively utilized, limiting quantum efficiency to 25%
Solution Approach 1:
The patent merges the advantages of fluorescent emitters (fast emission speed) and phosphorescent emitters (triplet exciton utilization) by using a luminescent organometallic complex with small singlet-triplet energy splitting. This combination enables both rapid emission (maintaining fluorescent characteristics) and efficient triplet exciton harvesting (achieving near-100% quantum efficiency).
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 allows for the utilization of 100% of triplet excitons, enhancing external quantum efficiency and reducing roll-off at high luminance, while improving the stability and longevity of the OLEDs.
Implementation Method 1
enables efficient energy transfer from the singlet state, significantly shortening the emission decay time to below 100 ns
Implementation Method 2
very efficient thermal re-occupation from the initially very efficiently occupied T 1 state into the S 1 state can occur at room temperature. The thermal re-occupation process described opens a fast emission channel from the short-lived S 1 state
Data Source
Figure 1a~1b
Figure 2
Figure 3
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 luminescent organometallic complex X having a difference of the singlet energy (ES1(X)) and the triplet energy (ET1(X)) of smaller than 0.2 eV [Δ (ES1(X)) - (ET1(X)) < 0.2 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 organometallic complex 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 luminescent organometallic complex 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.