OLED Charge Transport Layer Structuring for Light Outcoupling
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Solution Overview
Problem
Existing organic light-emitting devices (OLEDs) suffer from significant light trapping due to total internal reflection at layer interfaces, limiting their external quantum efficiency.
Innovation Solution
Incorporating a patterned or surface-structured charge transport layer and an alignment layer between the charge transport and emissive layers to enhance light outcoupling efficiency by aligning optical dipoles and modifying refractive properties, thereby increasing the critical angle of total internal reflection and reducing light trapping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional planar OLED structure is used, then device simplicity is maintained, but light outcoupling efficiency is limited due to total internal reflection
Solution Approach 1:
The charge transport layer is provided with a periodic surface structure featuring curved undulations rather than a flat planar surface. This curvature modifies the interface geometry to reduce total internal reflection and enhance light extraction efficiency while maintaining structural integrity and manufacturability.
Solution Approach 2:
The periodic surface structure creates local variations in the charge transport layer surface, where different regions have different orientations and curvatures. These local geometric variations are strategically designed to manipulate light propagation paths and improve outcoupling at specific locations without compromising the overall device structure.
2Loss of energy
If alignment layer is added between charge transport layer and emissive layer, then light outcoupling is improved through dipole alignment, but device complexity increases
Solution Approach 1:
An alignment layer is introduced as an intermediary component between the charge transport layer and the emissive layer. This intermediate layer serves the dual function of facilitating charge transport while simultaneously aligning the optical dipoles of emitter molecules to enhance light outcoupling efficiency.
Solution Approach 2:
The alignment layer performs multiple functions: it acts as a charge transport interface, provides surface for molecular alignment, and serves as an optical element for dipole orientation. By combining multiple functions in a single layer, the overall device complexity is minimized while achieving the desired light outcoupling improvement.
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
This approach significantly improves light outcoupling efficiency by directing emitted light away from trapped paths, resulting in enhanced external quantum efficiency and reduced light loss within the device.
Implementation Method 1
adapting the refractive properties of the emissive layer/charge transport layer interface, e.g. to increase the critical angle of total internal reflection for light incident on this interface from the emissive layer
Implementation Method 2
adapting the refractive properties of the emissive layer/charge transport layer interface
Implementation Method 3
utilizing photonic crystal characteristics of the charge transport layer, said characteristics including suppressed optical density of states in the in plane direction of the charge transport layer and enhanced optical density of states in directions corresponding to incidence angles
Implementation Method 4
alignment of the optical dipoles of the emitter molecules in a common preferred direction thereby directing emission of light in directions having incidence angles which does not result in substantial or total reflection at the charge transport layer interface
Data Source
Figure 1~4
AI summary
The invention relates to an organic light-emitting device (OLED) comprising at least: a first electrode (102); a second electrode (105); an organic light emissive layer (104) arranged between said first electrode and said second electrode; and an organic charge transport layer (103) arranged between said first electrode and said emissive layer, wherein i) the charge transport layer is patterned or provided with a periodic surface structure on a surface of the charge transport layer facing the emissive layer, and/or ii) an alignment layer which allows for charge transport to the emissive layer is provided between said charge transport layer and said emissive layer, which alignment layer promotes alignment of the optical dipoles of molecules of said light emissive layer towards a common preferred direction of the molecular axes. The use of the patterned or structured charge transport layer and/or the alignment layer provides improved light out coupling from the OLED layer stack, i.e. increased external quantum efficiency.