Organic EL Layer Stack With Segmented Hole Transport
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Solution Overview
Problem
Existing light-emitting elements face challenges with hole injection difficulties due to mismatched energy levels between organic acceptors and hole-transport layers, leading to reduced efficiency, lifetime, and increased power consumption.
Innovation Solution
A novel light-emitting element structure with multiple hole-transport layers and electron-transport layers, where the HOMO levels are carefully aligned to facilitate smooth carrier injection, using specific materials with heteroaromatic ring skeletons and organic acceptors to enhance hole injection and reduce barriers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a substance with a shallow HOMO level is used as the organic compound in the hole-transport layer to reduce the difference between HOMO levels, then hole injection from the hole-transport layer into the host material becomes difficult
Solution Approach 1:
The hole-transport function is divided into multiple layers with different HOMO levels. The first hole-transport layer has a HOMO level closer to the organic acceptor for effective hole injection, while the second hole-transport layer has a HOMO level closer to the host material for efficient hole transport into the light-emitting layer. This segmentation resolves the contradiction by assigning different energy level characteristics to different functional zones.
Solution Approach 2:
Different regions of the hole-transport structure are given different energy level properties. The first hole-transport layer is designed with specific HOMO level characteristics optimized for interface with the organic acceptor, while the second hole-transport layer is designed with HOMO level characteristics optimized for interface with the host material. This local differentiation of energy levels allows each layer to perform its specific function optimally.
2Reliability
If the LUMO level of the organic acceptor is distanced from the HOMO level of the organic compound in the hole-transport layer, then hole injection into the EL layer is difficult
Solution Approach 1:
The first hole-transport layer acts as an intermediary between the organic acceptor and the host material. Its HOMO level is positioned to bridge the energy gap between the organic acceptor's LUMO level and the host material's HOMO level, facilitating smooth hole transfer. This intermediary layer with appropriately positioned energy levels enables efficient hole injection while maintaining low power consumption.
3Reliability
If the organic acceptor has low hole-injection capability, then lifetime decreases or roll-off occurs at high luminance
Solution Approach 1:
The hole-transport function is segmented into two layers with distinct material properties. The first hole-transport layer uses materials optimized for hole injection from the organic acceptor, while the second hole-transport layer uses materials optimized for hole transport to the host material. This segmentation allows selection of materials that maximize lifetime and prevent roll-off without overly constraining manufacturing options.
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 results in a light-emitting element with improved efficiency, longer lifetime, and lower power consumption, achieving better emission properties and reliability.
Implementation Method 1
The organic acceptor can be easily deposited by evaporation and thus is suitable for mass production and has become widely used. However, the injection of holes into an EL layer is difficult when the LUMO level of the organic acceptor is distanced from the HOMO level of an organic compound included in a hole-transport layer.
Implementation Method 2
Light-emitting elements (organic EL elements) including organic compounds and utilizing electroluminescence (EL) have been put to more practical use. Carriers are injected by application of voltage to the element, and light emission can be obtained from the light-emitting material by using the recombination energy of the carriers.
Data Source
AI summary
A novel light-emitting element is provided. A light-emitting element with a long lifetime is provided. A light-emitting element with high emission efficiency is provided. In the light-emitting element, an EL layer includes a hole-injection layer, a first hole-transport layer, a second hole-transport layer, a third hole-transport layer, a light-emitting layer, a first electron-transport layer, and a second electron-transport layer in this order; the hole-injection layer includes an organic acceptor; the LUMO level of the host material is higher than that of the first electron-transport layer; the LUMO level of the second electron-transport layer is higher than that of the first electron-transport layer; the host material is a substance including a condensed aromatic ring skeleton; and the first and second electron-transport layers each include a substance having a heteroaromatic ring skeleton.