OLED Emitting Layer Composition for Fast Decay and High EQE
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Solution Overview
Problem
Existing organic light-emitting devices (OLEDs) face inefficiencies due to the accumulation of triplet excitons, leading to reduced quantum efficiency and stability, particularly at high current densities, where triplet-triplet annihilation and triplet-polaron quenching occur, resulting in the 'roll-off' effect and potential chemical instability.
Innovation Solution
Incorporating a luminescent organometallic complex with a small singlet-triplet energy splitting (ΔES1-ET1 < 0.2 eV) into the emitting layer, along with a fluorescent emitter and a host compound, facilitates efficient energy transfer from the singlet state, reducing emission decay times 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 combines phosphorescent and fluorescent emitters in a single emitting layer, creating a hybrid system where phosphorescent materials utilize triplet excitons for high quantum efficiency while fluorescent materials provide fast emission decay, achieving both high efficiency and short decay times simultaneously
Solution Approach 2:
The invention uses composite emitting layers containing both phosphorescent and fluorescent compounds, where the phosphorescent component (e.g., Ir(ppy)3) captures triplet excitons to achieve near-100% internal quantum efficiency, while the fluorescent component (e.g., DCM2) provides rapid radiative decay, creating a material system that exhibits both high efficiency and fast emission characteristics
2Illumination intensity
If high current densities are applied to increase luminance, then brightness is improved, but triplet-triplet annihilation and triplet-polaron quenching increase, reducing efficiency
Solution Approach 1:
The patent converts the harmful accumulation of triplet excitons, which normally causes efficiency roll-off at high current densities through triplet-triplet annihilation and triplet-polaron quenching, into a beneficial mechanism by using phosphorescent emitters that efficiently harvest triplet excitons and convert them into light emission, thereby maintaining high quantum efficiency even at elevated luminance levels
3Use of energy by moving object
If triplet excitons accumulate to maintain high efficiency, then external quantum efficiency is improved, but chemical stability deteriorates due to increased reactivity
Solution Approach 1:
The patent extracts triplet excitons from their normally stable but reactive accumulated state and redirects them through phosphorescent emitters that facilitate rapid radiative decay, removing the harmful prolonged existence of triplet excitons that cause chemical degradation while maintaining high efficiency utilization of these excitons for light emission
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% triplet excitons, significantly shortening emission decay times without sacrificing external quantum efficiency, thereby improving OLED stability and reducing the 'roll-off' effect at high luminance levels.
Implementation Method 1
Incorporating a luminescent organometallic complex with a small singlet-triplet energy splitting (ΔES1-ET1 < 0.2 eV) into the emitting layer, along with a fluorescent emitter and a host compound, facilitates efficient energy transfer from the singlet state
Implementation Method 2
US2011108769 (WO 2010/006681) proposes a so-called 'singlet harvesting' process. The T1 state is occupied by the already known effects of triplet harvesting, and the usual T1→S0 phosphorescence results, but with the unfavourably long emission lifetime.
Implementation Method 3
The complex compounds proposed for use in accordance with US2011108769 have a very small energetic separation ΔE between the singlet S1 and the triplet T1. In this case, very efficient thermal re-occupation from the initially very efficiently occupied T1 state into the S1 state can occur at room temperature. The thermal re-occupation process described opens a fast emission channel from the short-lived S1 state
Implementation Method 4
M. A. Baldo et al., Nature 403 (2000) 750 use a phosphorescent sensitizer to excite a fluorescent dye. The mechanism for energetic coupling between phosphorescent and fluorescent molecular species is a long-range, non-radiative energy transfer: the internal efficiency of fluorescence can be as high as 100%.
Implementation Method 5
a fluorescent emitter Y, wherein the singlet energy of the luminescent organometallic complex X (ES1(X)) is greater than the singlet energy of the fluorescent emitter Y (ES1(Y))
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.