Light-Emitting Element With Charge-Generation Layers
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Solution Overview
Problem
Current light-emitting elements require high voltage for operation and suffer from reduced light extraction efficiency due to absorption of emitted light by layers with peak absorption spectra in the visible light region, especially when multiple layers are stacked.
Innovation Solution
A light-emitting element structure with multiple EL layers, where specific layers containing alkali metals, alkaline earth metals, or their compounds, and charge-generation layers without absorption peaks in the visible light region are used, along with electron-transport and hole-transport layers to facilitate low voltage operation and enhance luminous efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If multiple light-emitting layers are stacked to increase emission luminance, then luminance is improved, but driving voltage increases and power consumption increases
Solution Approach 1:
The light-emitting element is divided into multiple independent light-emitting layers (first light-emitting layer, second light-emitting layer, etc.), each capable of emitting light independently when current is applied. This segmentation allows the total luminance to be the sum of individual layer luminances without requiring proportionally higher current density, thereby improving overall luminance while maintaining reasonable power consumption levels.
Solution Approach 2:
Charge-generation layers are introduced as intermediary layers between adjacent light-emitting layers. These charge-generation layers generate charge carriers (electrons and holes) that are supplied to the light-emitting layers, enabling efficient electroluminescence without requiring excessive current. The intermediary charge-generation layers facilitate charge carrier generation and transport, reducing the overall current demand and thus lowering power consumption.
2Power
If charge-generation layer formed of oxide is used, then charge generation is improved, but mutual interaction with metal-doped layer causes high electric field requiring high voltage
Solution Approach 1:
An electron-transport layer is introduced as an intermediary between the metal-doped layer and the charge-generation layer. This electron-transport layer acts as a buffer that prevents direct contact and harmful interactions between the metal-doped layer and oxide charge-generation layer, thereby suppressing the formation of high electric fields at their interface. The electron-transport layer facilitates electron transport while isolating the two layers, enabling charge generation without requiring high driving voltage.
3Power
If layer with peak absorption spectrum in visible light region is used, then charge generation is improved, but light emission is absorbed resulting in decreased light extraction efficiency
Solution Approach 1:
Different layers are assigned different optical properties based on their functional requirements. The charge-generation layer is specifically designed to have high charge generation capability in the visible light region, while the electron-transport layer is selected to have low light absorption in the emission wavelength range. This local differentiation of properties allows charge generation to occur efficiently in the charge-generation layer without significant light absorption losses in other layers, thereby maintaining high light extraction efficiency.
Solution Approach 2:
The electron-transport layer serves as an intermediary that bridges the charge-generation layer and the light-emitting layer. It is specifically selected to have minimal absorption in the visible light region, allowing light generated in the light-emitting layer to pass through to the observer without significant absorption. This intermediary layer thus enables efficient light extraction while the charge-generation layer continues to generate charges effectively.
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 structure allows for low voltage driving and reduced power consumption, while minimizing light absorption losses, resulting in high luminous efficiency and low power consumption in light-emitting devices.
Implementation Method 1
a charge-generation layer which contains a substance having high hole-transport properties and an acceptor substance
Implementation Method 2
Current is applied to a light-emitting layer provided between a pair of electrodes in a light-emitting element to excite a light-emitting substance contained in the light-emitting layer, whereby a predetermined emission color can be obtained
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3A~3B
AI summary
A light-emitting element is provided, in which n (n is a natural number of two or more) EL layers are provided between an anode and a cathode. Between the m-th (m is a natural number, 1 m n 1) EL layer and the (m 1)-th EL layer, a first layer containing any of an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth metal compound, and a rare earth metal compound, a second layer containing a substance having high electron-transport properties in contact with the first layer, and a charge-generation layer containing a substance having high hole-transport properties and an acceptor substance in contact with the second layer are provided in this order over the anode. The charge-generation layer does not have a peak of an absorption spectrum in a visible light region.