Exciplex-Mediated Energy Transfer in Stacked OLED Layers
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Solution Overview
Problem
Current light-emitting elements have limited external quantum efficiency and lifetime due to inefficient energy transfer and carrier management, particularly in the use of phosphorescent compounds where the fluorescent and phosphorescent spectra do not overlap effectively, leading to reduced emission efficiency and increased carrier density.
Innovation Solution
A light-emitting element with a stacked-layer structure incorporating a first and second light-emitting layer, where the first layer contains a phosphorescent compound and an electron-transport organic compound, and the second layer contains another phosphorescent compound and an electron-transport organic compound, forming an exciplex that enhances energy transfer efficiency by aligning the emission spectrum with the absorption spectrum of the phosphorescent compound, thereby increasing external quantum efficiency and reducing carrier-related degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If phosphorescent compounds are used in light-emitting elements, then light emission can be obtained through triplet excited states, but external quantum efficiency is limited to approximately 25% at most due to light absorption by electrodes and inefficient energy transfer
Solution Approach 1:
The patent introduces an exciplex-forming compound as an intermediary between the electron-transport compound and phosphorescent compound. This intermediary forms an exciplex that emits light with a spectrum overlapping the absorption spectrum of the phosphorescent compound, enabling efficient energy transfer. The exciplex acts as a mediator that bridges the energy gap and spectral mismatch between the electron-transport material and phosphorescent emitter, thereby improving external quantum efficiency beyond the conventional 25% limit
Solution Approach 2:
The patent modifies the energy transfer mechanism by changing the spectral parameters. Specifically, it selects compounds where the exciplex emission spectrum overlaps with the phosphorescent compound absorption spectrum, creating an efficient energy transfer pathway. This parameter optimization—matching emission and absorption spectra—enables near-unity energy transfer efficiency and dramatically improves external quantum efficiency
2Duration of action of stationary object
If conventional light-emitting layer structures are used, then device simplicity is maintained, but carrier density increases leading to reduced lifetime and degradation
Solution Approach 1:
The exciplex-forming compound serves as a mediator that facilitates balanced carrier recombination. By forming exciplexes at the interface between electron-transport and hole-transport materials, it creates dedicated recombination zones that efficiently convert carrier density into light emission. This prevents carrier accumulation and degradation while extending device lifetime through controlled energy transfer to phosphorescent emitters
Solution Approach 2:
The patent replaces direct carrier recombination in conventional layers with exciplex-mediated energy transfer. Instead of relying on direct electron-hole recombination in the phosphorescent layer, the system uses exciplex formation and subsequent energy transfer as an alternative mechanism. This substitution enables more efficient carrier management, reduces degradation pathways, and extends operational lifetime
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 high external quantum efficiency and extended lifetime by optimizing energy transfer and carrier management through the formation of an exciplex, which aligns the emission spectrum with the absorption spectrum of the phosphorescent compound, improving light emission efficiency and preventing degradation from increased carrier density.
Implementation Method 1
A combination of the first organic compound and the second organic compound forms an exciplex. The emission wavelength of the exciplex formed by the first organic compound (host material) and the second organic compound (assist material) is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the first and second organic compounds (host and assist materials). Therefore, by formation of the exciplex, the fluorescent spectrum of the first organic compound (host material) and the fluorescent spectrum of the second organic compound (assist material) can be converted into an emission spectrum which is located on the longer wavelength side.
Implementation Method 2
The light-emitting layer has a stacked-layer structure including a first light-emitting layer, which contains at least a first phosphorescent compound (guest material)... and a second light-emitting layer, which contains at least a second phosphorescent compound (guest material)
Implementation Method 3
when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, electrons injected from the cathode and holes injected from the anode are recombined in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state
Data Source
AI summary
A light-emitting element having high external quantum efficiency is provided. A light-emitting element having a long lifetime is provided. A light-emitting layer is provided between a pair of electrodes. The light-emitting layer is a stack of a first light-emitting layer, which contains at least a first phosphorescent compound, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property and is provided on the anode side, and a second light-emitting layer, which contains at least a second phosphorescent compound and the first organic compound having an electron-transport property. A combination of the first organic compound and the second organic compound forms an exciplex.