OLED Emissive Layer with High Stokes Shift Phosphors
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Solution Overview
Problem
Organic light emitting devices (OLEDs) face efficiency roll-off at high brightness due to triplet-triplet annihilation (TTA), primarily caused by triplet diffusion, which is not effectively addressed by existing technologies.
Innovation Solution
Incorporating a phosphorescent compound with a Stokes Shift overlap greater than 0.3 eV and a triplet-triplet annihilation rate constant of less than 1×10−12 cm3s−1 into the emissive layer of OLEDs, along with a hole transport layer and an electron transport layer, to minimize TTA and maintain high efficiency at high current densities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If conventional phosphorescent compounds with long lifetimes are used in OLEDs, then device stability is improved, but triplet-triplet annihilation increases causing efficiency roll-off at high brightness
Solution Approach 1:
The patent changes the phosphorescent lifetime parameter from conventional long lifetimes (μs range) to ultra-short lifetimes (sub-100 ns range). This parameter change fundamentally alters the device behavior: the ultra-short lifetime prevents triplet accumulation that leads to TTA, while the large Stokes shift (greater than 0.3 eV) ensures efficient energy transfer from host to guest. This resolves the contradiction by decoupling the traditional relationship between lifetime and TTA susceptibility.
Solution Approach 2:
The patent introduces local quality differentiation through the choice of phosphorescent compounds with specific properties (ultra-short lifetime and large Stokes shift) in the emissive layer. The host-guest system is designed with specific energy level alignments where the host has higher triplet energy than the guest, creating a localized energy transfer pathway that prevents triplet diffusion and annihilation while maintaining stable operation.
2Loss of energy
If phosphorescent compounds with ultra-short lifetimes are used, then triplet-triplet annihilation is minimized, but device efficiency may be compromised
Solution Approach 1:
The patent ensures continuous efficient energy transfer from the organic host to the phosphorescent guest through optimized host-guest energy level alignment. The large Stokes shift (greater than 0.3 eV) creates a continuous, efficient energy transfer pathway that compensates for the ultra-short lifetime, maintaining high external quantum efficiency (greater than 10%) while preventing triplet accumulation and TTA.
Solution Approach 2:
The patent employs a composite host-guest system where the organic host material and phosphorescent guest compound are carefully selected and combined. The host provides the energy transfer pathway and structural matrix, while the guest provides the ultra-short lifetime phosphorescent emission. This composite approach synergistically combines the advantages of both components to achieve high efficiency and minimal TTA simultaneously.
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 solution achieves a peak external quantum efficiency of greater than 10% with minimal roll-off at current densities above 100 mA/cm2, outperforming devices with comparative phosphorescent compounds having lower Stokes Shift overlap and longer phosphorescent lifetimes.
Implementation Method 1
an emissive layer disposed between the anode and the cathode, the emissive layer comprising an organic host compound and a phosphorescent compound exhibiting a Stokes Shift overlap greater than 0.3 eV
Data Source
AI summary
An organic light emitting device including a) an anode; b) a cathode; and c) an emissive layer disposed between the anode and the cathode, the emissive layer comprising an organic host compound and a phosphorescent compound exhibiting a Stokes Shift overlap greater than 0.3 eV. The organic light emitting device may further include a hole transport layer disposed between the emissive layer and the anode; and an electron transport layer disposed between the emissive layer and the cathode. In some embodiments, the phosphorescent compound exhibits a phosphorescent lifetime of less than 10 μs. In some embodiments, the concentration of the phosphorescent compound ranges from 0.5 wt. % to 10 wt. %.