White OLED Optical Path Control for Viewing Angle Color Shift
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Solution Overview
Problem
Conventional white organic light emitting devices with tandem structures experience greater brightness reduction in fluorescent light emitting layers compared to phosphorescent layers as the viewing angle increases, leading to warmer white color coordinates and noticeable color shifts.
Innovation Solution
A white organic light emitting device is designed with specific optical path conditions and layer thicknesses between the cathode and anode, including a first stack emitting blue light and a second stack with a phosphorescent dopant emitting longer wavelengths, to maintain consistent brightness variations and prevent color shifts by optimizing the refractive indices and thicknesses of the layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a tandem white organic light emitting device is implemented using known light emitting materials, then the device structure is established with fluorescent and phosphorescent light emitting stacks, but the brightness reduction rate of the fluorescent light emitting layer becomes greater than that of the phosphorescent light emitting layer as viewing angle increases, causing color shift
Solution Approach 1:
The patent changes the optical path length parameter by adjusting the thickness of the optical compensation layer and the positions of light emitting layers. This parameter modification compensates for the differential brightness reduction between fluorescent and phosphorescent stacks at oblique viewing angles, thereby maintaining color stability without altering the fundamental tandem device structure
Solution Approach 2:
The patent introduces an optical compensation layer as an intermediary element between the substrate and the light emitting stacks. This intermediary layer serves to equalize the optical paths for different emission types, mediating the color shift problem by balancing the brightness reduction rates of fluorescent and phosphorescent emissions across varying viewing angles
2Use of energy by moving object
If the optical path length is increased to improve light extraction efficiency, then more light can be extracted from the device, but the color shift due to viewing angle changes becomes more pronounced
Solution Approach 1:
The patent optimizes the thickness parameter of the optical compensation layer and the vertical positions of light emitting layers to achieve a balanced optical path length. This optimized parameter configuration simultaneously improves light extraction efficiency while preventing excessive color shift at oblique viewing angles by equalizing the optical paths for different emission wavelengths
3Stability of the object's composition
If different thicknesses of organic material layers are used to optimize optical paths, then color stability can be improved, but the device structure becomes more complex with precise thickness control requirements
Solution Approach 1:
The patent identifies specific numerical ranges for layer thicknesses (optical compensation layer: 50-200 nm, light emitting layers: 20-100 nm from electrode interfaces) that achieve color stability. By providing concrete parameter specifications, the patent simplifies the manufacturing process while maintaining color consistency, reducing the complexity of thickness control through standardized dimensional guidelines
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 ensures improved color stability and efficiency by maintaining similar brightness reductions for both blue fluorescent and phosphorescent emissions across varying viewing angles, preventing color shifts and enhancing the overall brightness and quantum efficiency of the white light emitted.
Implementation Method 1
a second stack disposed between the charge generation layer and the second electrode and including a second light emitting layer that includes at least one host doped with a phosphorescent dopant emitting light with a longer wavelength than blue light
Implementation Method 2
a structure including the first electrode and the layers disposed between the first electrode and the second electrode satisfies an optical path condition represented by the following equation with respect to emissions of the first and second stacks
Data Source
AI summary
A white organic light emitting device, with improved color shift characteristics and improved efficiency according to viewing angle changes by controlling conditions for designing an optical path in organic material layers between a cathode and an anode or adjusting interior or exterior thicknesses of the organic material layers, has a structure including a first electrode and layers between the first electrode and a second electrode satisfies an optical path condition represented by the following equationnadaλ+∑jnjwdjwλ=1.85∼2.15with respect to emissions of the first and second stacks, where λ is an emission peak wavelength of the first stack or the second stack, na and da are a refractive index and a thickness of a transparent electrode selected from the first and second electrode, and nw and dw are a refractive index and a thickness of any one of the layers disposed between the first electrode and the second electrode, respectively.