Host-Guest OLED Emission Layer for Efficient Energy Transfer
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Solution Overview
Problem
Organic electroluminescence (EL) elements face limitations in external quantum efficiency and lifetime due to light absorption by electrodes and concentration quenching in phosphorescent compounds, with existing solutions struggling to achieve high efficiency and long lifespan.
Innovation Solution
A light-emitting element design featuring a light-emitting layer with a guest material and a host material, where the emission spectrum of the host overlaps with the absorption spectrum of the guest, particularly an organometallic complex like iridium, to enhance energy transfer efficiency and reduce deactivation, thereby increasing external quantum efficiency and extending the element's lifetime.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If phosphorescent compounds are used to achieve high internal quantum efficiency, then light emission efficiency is improved, but concentration quenching occurs reducing external quantum efficiency
Solution Approach 1:
The light-emitting layer is segmented into multiple layers with different functions: a first light-emitting layer containing phosphorescent compound for high internal quantum efficiency, and a second light-emitting layer without phosphorescent compound to prevent concentration quenching and improve light extraction. This segmentation allows each layer to optimize its function independently.
Solution Approach 2:
A microlens array is introduced as an intermediary component between the light-emitting layers and the external environment. The microlenses focus and extract light from the light-emitting layers, significantly improving external quantum efficiency by directing light that would otherwise be trapped or absorbed.
2Illumination intensity
If phosphorescent compound concentration is increased to improve emission intensity, then light output is improved, but concentration quenching increases reducing efficiency
Solution Approach 1:
The patent divides the light-emitting structure into multiple layers with different phosphorescent compound concentrations. The first light-emitting layer has higher concentration for strong emission, while the second layer has lower or zero concentration to eliminate quenching effects, allowing the system to achieve high light output without excessive quenching losses.
Solution Approach 2:
Different regions of the light-emitting structure have different local compositions: the first light-emitting layer is optimized for high emission intensity with higher phosphorescent compound concentration, while the second light-emitting layer is optimized for light extraction with lower concentration, creating local quality variations that resolve the contradiction between intensity and quenching.
3Illumination intensity
If light-emitting layer thickness is increased to improve light output, then emission intensity is improved, but light absorption by electrodes increases reducing external efficiency
Solution Approach 1:
The light-emitting layer is divided into multiple thinner layers (first and second light-emitting layers) rather than using a single thick layer. This segmentation reduces the total absorption path length for light, allowing more light to escape before being absorbed by electrodes, while still achieving high emission intensity through the combined output of multiple layers.
Solution Approach 2:
The microlens array acts as an intermediary that extracts light from the light-emitting layers before it can be absorbed by the electrodes. The microlenses focus and direct light outward, significantly reducing the impact of electrode absorption and improving external quantum efficiency.
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 high external quantum efficiency and a long lifetime for the light-emitting element by optimizing energy transfer between the host and guest materials, surpassing the theoretical limits of previous technologies.
Implementation Method 1
an emission spectrum of the host material overlaps with an absorption spectrum of the guest material, and phosphorescence is emitted by conversion of an excitation energy of the host material into an excitation energy of the guest material
Implementation Method 2
light emission from the triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different spin multiplicities
Implementation Method 3
by application of voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make a light-emitting substance excited
Data Source
AI summary
Provided is a light-emitting element with high external quantum efficiency, or a light-emitting element with a long lifetime. The light-emitting element includes, between a pair of electrodes, a light-emitting layer including a guest material and a host material, in which an emission spectrum of the host material overlaps with an absorption spectrum of the guest material, and phosphorescence is emitted by conversion of an excitation energy of the host material into an excitation energy of the guest material. By using the overlap between the emission spectrum of the host material and the absorption spectrum of the guest material, the energy smoothly transfers from the host material to the guest material, so that the energy transfer efficiency of the light-emitting element is high. Accordingly, a light-emitting element with high external quantum efficiency can be achieved.