Stacked-Layer Light-Emitting Element Exciplex Energy Transfer
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Solution Overview
Problem
Current light-emitting elements have limited external quantum efficiency and lifetime due to inefficient energy transfer from host to guest materials, primarily because their fluorescent and phosphorescent spectra do not overlap effectively, leading to reduced emission efficiency and shorter device lifespan.
Innovation Solution
A light-emitting element with a stacked-layer structure containing a phosphorescent compound and organic compounds that form an exciplex, where the emission wavelength of the exciplex overlaps with the absorption spectrum of the phosphorescent compound, enhancing energy transfer efficiency and balancing carrier distribution to prevent exciton density increases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a phosphorescent compound is used as the luminous body in a light-emitting element, then light emission can be obtained through triplet excited state, but the 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 changes the energy transfer mechanism by utilizing exciplex formation between host and assist materials. The exciplex emission spectrum is designed to overlap with the phosphorescent compound's absorption spectrum, creating an efficient energy transfer pathway that overcomes the conventional 25% efficiency limit. This parameter change in the energy transfer mechanism enables external quantum efficiency exceeding 25%.
Solution Approach 2:
The patent introduces an 'assist material' as an intermediary component that forms an exciplex with the host material. This exciplex acts as a mediator to transfer energy to the phosphorescent compound (guest material). The exciplex emission spectrum is positioned to overlap with the guest material's absorption spectrum, creating an efficient energy transfer bridge that improves overall energy transfer efficiency and external quantum efficiency.
2Reliability
If conventional host and guest materials are used without spectral overlap, then the device structure remains simple, but energy transfer is inefficient leading to reduced emission efficiency and shorter device lifespan
Solution Approach 1:
The patent optimizes the energy level parameters by selecting host and assist materials whose exciplex emission spectrum overlaps with the phosphorescent compound's absorption spectrum. This parameter optimization ensures efficient energy transfer from the exciplex to the phosphorescent compound, improving both emission efficiency and device lifespan by reducing energy loss and exciton accumulation.
Solution Approach 2:
The exciplex formed between host and assist materials serves as an intermediary energy transfer medium. This intermediary enables efficient energy transfer to the phosphorescent compound while preventing direct harmful interactions between electrons/holes and the phosphorescent material, thereby extending device lifespan and improving emission efficiency simultaneously.
3Duration of action of stationary object
If the light-emitting layer uses a single-layer structure, then the manufacturing process is simpler, but carrier distribution is unbalanced leading to exciton density increases and reduced lifetime
Solution Approach 1:
The patent divides the light-emitting layer into multiple sub-layers with different compositions. The first light-emitting layer contains a higher proportion of the assist material, while the second light-emitting layer contains a lower proportion. This segmentation creates a gradient structure that optimizes carrier distribution and exciton management, extending element lifetime without excessive complexity.
Solution Approach 2:
The patent applies local quality by creating regions with different material compositions within the light-emitting layer. The first light-emitting layer (with higher assist material content) and second light-emitting layer (with lower assist material content) have different local properties that optimize carrier injection and recombination in different regions, preventing exciton density accumulation and extending device 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 balance, leading to improved light emission efficiency and device reliability.
Implementation Method 1
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... 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
In the light-emitting layer (the first light-emitting layer and the second light-emitting layer), a combination of the first organic compound and the second organic compound forms an exciplex
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. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states
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 element includes a light-emitting layer between a pair of electrodes. The light-emitting layer contains at least a phosphorescent compound, a first organic compound (host material) having an electron-transport property, and a second organic compound (assist material) having a hole-transport property. The light-emitting layer has a stacked-layer structure including a first light-emitting layer and a second light-emitting layer, and the first light-emitting layer contains a higher proportion of the second organic compound than the second light-emitting layer. In the light-emitting layer (the first light-emitting layer and the second light-emitting layer), a combination of the first organic compound and the second organic compound forms an exciplex.