Multi-Layer Alignment Film for LCD Image Sticking
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Solution Overview
Problem
In IPS and FFS mode LCD devices, the use of positive dielectric anisotropy liquid crystals results in insufficient phase retardation and asymmetrical brightness due to strong vertical electric fields, while negative dielectric anisotropy LCs offer advantages but face challenges in achieving high performance and reliability, particularly with issues like AC image sticking and DC image sticking caused by insufficient anchoring force in alignment films.
Innovation Solution
A multi-layered alignment film is developed using a precursor mixture with soluble polyimide (sPI) that includes a photo-decomposable material like cyclobutane, where the first portion near the liquid crystal layer has high molecular weight polyimide for strong anchoring force and the second portion, further away, has lower volume resistance for charge discharge, formed through a chemical imidization process and UV irradiation, and impurity removal processes to maintain molecular weight and prevent image sticking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If contact-type alignment process is used to provide anchoring force, then alignment capability is achieved, but scratches and declination occur causing image blurring and sticking
Solution Approach 1:
The patent replaces the mechanical rubbing process with a photo-alignment process using UV irradiation. The alignment film is treated with polarized UV light to create molecular orientation without physical contact, thereby achieving alignment capability while eliminating scratches and image defects associated with mechanical rubbing
2Object-affected harmful factors
If photo-alignment process is used to eliminate contact-type alignment issues, then image quality improves, but anchoring force becomes insufficient causing image sticking
Solution Approach 1:
The patent modifies the chemical composition parameters of the alignment film by incorporating specific compounds (polyamic acid, polyimide, polyamic acid ester) with controlled molecular weights and ratios. This chemical parameter optimization enhances the anchoring force of the photo-aligned film while maintaining image quality, preventing image sticking through molecular-level interactions rather than mechanical means
3Ease of operation
If conventional photo-alignment polymer is used, then alignment is achieved, but anchoring force is insufficient leading to image sticking
Solution Approach 1:
The patent creates a composite alignment film system combining multiple polymer components (polyamic acid, polyimide, polyamic acid ester) with specific molecular weight ranges. This composite material approach provides synergistic effects where the combination of materials delivers both the alignment properties of photo-alignment polymers and the enhanced anchoring force needed to prevent image sticking
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 effectively suppresses AC and DC image sticking problems, maintaining high anchoring force and reducing optical defects, thereby enhancing the performance and reliability of LCD devices using negative dielectric anisotropy LCs by optimizing the alignment film's molecular weight and structure.
Implementation Method 1
an alignment film made of a synthetic polymer (e.g., polyimide) is irradiated by polarized ultraviolet (UV) light to provide uniaxial anisotropy in a direction perpendicular to the polarization direction. By irradiating the polymer, the main chain of the polymer is cleaved in a direction parallel to the polarization direction.
Implementation Method 2
Positive dielectric anisotropy LC has been commonly used in both IPS mode and FFS mode LCD devices. However, most IPS mode and FFS mode LCD devices have a structure in which the distance between the pixel electrode and common electrode is greater than the cell gap. In such a configuration, strong vertical electric fields tend to form above the electrode surfaces.
Data Source
Figure 1a~1c
Figure 1d~2a(e)
Figure 2b(a)~2c
AI summary
An alignment film for liquid crystal display device includes a first portion positioned towards to the liquid crystal layer and a second portion positioned away from the liquid crystal layer. The first portion provides improved anchoring force while the second portion exhibits a lower volume resistance than the first portion. Thus, AC image sticking and DC image sticking can be minimized at the same time.