Organic EL Cathode Spatial Frequency Design
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Solution Overview
Problem
Organic electroluminescent (EL) devices suffer from low light extraction efficiency due to surface plasmon absorption and total reflection at the cathode, with existing solutions like two-dimensional lattice and circular/annular structures limiting light extraction directions and failing to match with external films for effective light extraction.
Innovation Solution
An organic EL device with a cathode having a surface featuring recesses or protrusions forming a Fourier transform image with specific spatial frequencies for surface plasmon absorption suppression and light scattering areas, optimized using Equation (I) to enhance light extraction efficiency across all directions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a two-dimensional lattice structure is provided at the cathode to convert surface plasmons to light, then light extraction efficiency is improved in specific directions, but light extraction is limited in other directions where the lattice interval is not optimum
Solution Approach 1:
The invention transitions from a symmetric two-dimensional lattice structure to an asymmetric circular or annular Fourier transform image structure. This asymmetric design allows the cathode surface to have varying spatial frequency characteristics in different directions, enabling optimal surface plasmon to light conversion across all extraction directions rather than being limited to specific orientations.
Solution Approach 2:
The invention moves from considering only the spatial arrangement (2D lattice) to incorporating the Fourier transform domain (frequency dimension). By designing the cathode surface based on its Fourier transform image being circular or annular, the patent adds a frequency domain dimension to the design, enabling comprehensive control over light extraction in all directions simultaneously.
2Adaptability or versatility
If an uneven structure with circular or annular Fourier transform image is provided on the cathode for all-direction light extraction, then surface plasmon conversion is optimized in every direction, but matching with external film is insufficient due to total reflection at the external film-air interface
Solution Approach 1:
The invention merges two previously separate functions into a unified cathode surface structure: (1) surface plasmon to light conversion optimized in all directions, and (2) light extraction enhancement through constructive interference. By designing the cathode's Fourier transform image to be circular or annular with specific spatial frequency characteristics, both functions are achieved simultaneously, eliminating the need for separate optimization of external film matching.
Solution Approach 2:
The invention changes the key design parameter from the physical geometry of the cathode surface to the spatial frequency distribution in its Fourier transform image. By specifying that the Fourier transform image should be circular or annular with particular spatial frequency characteristics, the patent enables optimal light extraction across all directions while maintaining compatibility with external films, as the spatial frequency parameters can be tuned to match the external film's optical properties.
3Loss of energy
If external film is attached to improve light extraction from transparent substrate, then light extraction efficiency increases to about 30%, but total reflection loss remains significant
Solution Approach 1:
The invention performs preliminary action by optimizing the cathode surface structure before light reaches the external film interface. By designing the cathode's Fourier transform image to be circular or annular, the patent pre-conditions the light to have optimal spatial frequency characteristics for extraction, which then works synergistically with the external film to achieve high extraction efficiency without requiring complex multi-layer external film structures.
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 significantly improves light extraction efficiency by effectively suppressing surface plasmon absorption and total reflection, allowing for higher light extraction ratios, even with external films, by using spatial frequencies tailored to the emission spectrum of the luminescent layer.
Implementation Method 1
the surface of the cathode part has a two-dimensional lattice structure in which a plurality of recesses are arranged at a certain interval in two dimensions. This two-dimensional lattice structure functions as a diffraction grating and converts the surface plasmons generated by light at the cathode into light again
Implementation Method 2
This two-dimensional lattice structure functions as a diffraction grating and converts the surface plasmons generated by light at the cathode into light again
Implementation Method 3
providing a light scattering structure is being considered in order to alleviate total reflection due to the difference in refractive index between the transparent substrate and the transparent electrode
Implementation Method 4
The light scattering structure alleviates the total reflection of light and allows more light to be extracted
Implementation Method 5
the cathode has the recesses or protrusions formed such that the surface has a Fourier transform image which includes one or more surface plasmon absorption suppression areas having a spatial frequency v
Implementation Method 6
an organic EL (electroluminescent) device
Data Source
AI summary
An organic EL (electroluminescent) device includes a translucent substrate, a transparent electrode, a luminescent layer, and a cathode placed over one surface of the translucent substrate, and a light extraction film having unevenness placed on the other surface. The surface of the cathode facing the luminescent layer has a plurality of recesses or protrusions. The Fourier transform image of the surface of the cathode facing the luminescent layer has a surface plasmon absorption suppression area including a spatial frequency v obtained from Eq. (I) and a light scattering area not including spatial frequencies equal to or greater than the spatial frequency v.