Polyimide Alignment Film Monomolecular Layer for VHR Stability
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Solution Overview
Problem
Liquid crystal display devices face issues with decreased voltage holding ratio (VHR) and image sticking due to residual DC voltage, particularly in vertical alignment films, where existing solutions like epoxy-based additives lead to misalignment and impurities from resist films reduce reliability.
Innovation Solution
A polyimide-based alignment film with a monomolecular film adsorbed on residual carboxyl groups, using a silane-based surfactant to reduce carboxyl group concentration and prevent crosslinking, maintaining high VHR while reducing residual DC voltage, applicable to various display modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If epoxy-based additives are added to the liquid crystal alignment agent to suppress residual DC voltage, then the voltage holding ratio is improved, but unreacted components remain in the alignment film causing misalignment and image sticking
Solution Approach 1:
The patent extracts and removes the harmful epoxy-based additives from the alignment film composition. Instead of using epoxy groups that cause crosslinking and residual DC voltage, the invention employs a polyimide-based alignment agent without epoxy groups, thereby eliminating the source of misalignment and image sticking while maintaining voltage holding ratio through alternative mechanisms.
Solution Approach 2:
The patent changes the chemical composition parameters of the alignment agent by replacing epoxy group-containing compounds with polyimide-based compounds having specific molecular structures. This parameter change eliminates the crosslinking reaction that causes harmful effects while preserving the voltage holding capability through the polyimide's inherent properties.
2Manufacturing precision
If a positive resist is used to chemically adsorb silane-based surfactant for forming a thin uniform alignment film, then alignment uniformity is improved, but impurities from the resist reduce device reliability
Solution Approach 1:
The patent removes the positive resist layer from the manufacturing process entirely. By using a polyimide-based alignment agent that can be directly applied and cured without requiring a resist substrate, the invention eliminates impurities from resist materials while still achieving uniform alignment film formation through the alignment agent's inherent properties and controlled application methods.
Solution Approach 2:
The patent uses the polyimide-based alignment agent itself as the intermediary medium, replacing the traditional resist film. The alignment agent serves both as the adhesive layer and the alignment-inducing layer, eliminating the need for separate resist materials and their associated impurities while maintaining film uniformity.
3Reliability
If the alignment film is made with polyimide and imidization is increased to decrease residual carboxyl groups, then voltage holding ratio is improved, but residual DC voltage increases causing image sticking
Solution Approach 1:
The patent optimizes the imidization ratio parameter to a specific range (30-70%) rather than maximizing it. This parameter change balances the voltage holding ratio improvement from reduced carboxyl groups with the prevention of excessive residual DC voltage, thereby avoiding image sticking while maintaining high voltage holding performance.
Solution Approach 2:
The patent introduces compound polyimide structures with different functional groups at different positions within the polymer chain. This creates local quality variations where some regions provide high voltage holding ratio while other regions minimize residual DC voltage, achieving both goals simultaneously through molecular-level design.
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 prevents VHR decrease and image sticking, ensuring favorable alignment conditions and long-term reliability across different liquid crystal display modes by managing residual DC voltage and impurity issues.
Implementation Method 1
a silane-based surfactant chemically adsorbed via a resin film (positive resist)
Implementation Method 2
an alignment film for controlling liquid crystal molecule alignment
Data Source
AI summary
The present invention provides a liquid crystal display device that can prevent a decrease in the VHR and image sticking on the display screen because of residual DC voltage, and also can produce favorable alignment conditions; and a production method thereof. The liquid crystal display device of the present invention includes: a pair of substrates; a liquid crystal layer that contains liquid crystal molecules and is disposed between the pair of substrates; and an alignment film that is disposed on a liquid crystal layer side of at least one of the pair of substrates, the alignment film containing a polyamic acid or a polyimide with an imidization ratio of less than 100%, the liquid crystal display device including a monomolecular film on the alignment film.