TFT-LCD Afterimage: In-Depth Analysis of Afterimage Formation Mechanisms and Improvement Measures

The core principle of LCD display is the “orderly deflection of liquid crystal molecules under an electric field,” and the stability of this process entirely depends on the matching characteristics of two key materials: the polarizing layer (PI) and the liquid crystal (LC). PI determines the initial alignment direction (pre-tilt angle) and retention capability of liquid crystal molecules, while LC dictates the response speed and interference resistance of molecular deflection. Failure to meet specifications in either parameter can trigger image retention.

PI Material: Insufficient Alignment Force as the “Hidden Culprit” of Image Retention

As the core material of the alignment layer, PI (polyimide) functions by creating microscopic grooves through the “rubbing process.” This guides liquid crystal molecules to align in a fixed direction, while intermolecular forces maintain the stability of this alignment—a force termed “alignment force.” The strength of this alignment force and the stability of the pre-tilt angle directly correlate with the probability of AC ghosting and long-term image retention.

Core Mechanism of Afterimage Induction in PI Materials

Insufficient alignment force causes AC afterimages: When PI alignment force is weak, the bonding strength between liquid crystal molecules and the PI surface layer is inadequate. During prolonged display of static patterns (e.g., checkerboards, horizontal stripes), liquid crystal molecules in white pixel areas undergo “pre-tilt displacement” due to sustained electric field effects— — increasing the original tilt angle along the channel direction. This shift propagates to surrounding areas via van der Waals forces between liquid crystal molecules. When the display switches to a grayscale background, the light transmittance of pre-tilted areas differs from normal regions, forming visible planar afterimages (AC afterimages). These persist for up to 24 hours under low grayscale conditions.

Poor pre-tilt stability causes long-term afterimages: High-quality PI materials should maintain stable pre-tilt angles under high-temperature and high-humidity conditions. If PI molecular chains are loosely structured (e.g., due to incomplete curing) or contain easily hydrolyzed functional groups, the chains may undergo slight contraction at temperatures above 65°C, leading to pre-tilt fluctuations. This fluctuation shifts the pixel voltage-transmittance relationship (V-T curve) to the left, causing transmittance deviations at the same gray level. Prolonged use results in “long-term afterimage” formation, with recovery times ranging from several hours to tens of hours.
Low Voltage Hold Ratio (VHR) Exacerbates DC Persistence: PI’s VHR denotes its ability to retain charge under an electric field. Standards require VHR ≥ 95% at room temperature and ≥ 90% at high temperatures (65°C). Insufficient material purity in PI (e.g., residual unreacted monomers or impurity ions) increases leakage current and reduces VHR. When the TFT drive circuit outputs AC voltage, inadequate VHR prevents complete charge retention on pixel electrodes, effectively creating a “residual DC voltage (Offset DC)” that induces DC afterimages.

Improvement Strategies for PI Materials: Dual Optimization from Material Selection to Process

Material Selection: Prioritize high orientation force and high stability parameters
<1> Prioritize PI materials with fluorine-containing side chains: The high electronegativity of fluorine atoms enhances polar interactions between PI and liquid crystal molecules (often containing cyano or ester groups), boosting alignment force. For example, a Japanese PI material demonstrated a 25% increase in alignment force compared to standard PI, reducing AC ghosting occurrence by 40%.
<2> Verify Pre-Tilt Stability: Conduct “high-temperature aging tests (65°C, 90% RH, 1000h)” to measure PI’s pre-tilt change rate, requiring ≤0.5° variation. Simultaneously test V-T curve drift to ensure post-high-temperature deviation ≤5%.
<3> Strictly control PI purity: Require suppliers to provide “ion content test reports” to ensure metal ion (Na+, K+, Ca2+) and organic impurity levels meet specifications (lower is better), preventing VHR degradation. (This is particularly critical during material changes at stable production facilities, explaining why ghosting tests are mandatory for PI material changes.)

Process Optimization: Enhancing PI Film Formation and Curing Effectiveness
<1> Optimize Pre-cure Temperature Parameters: After PI coating, a Pre-cure step is required to remove solvents. Insufficient temperature or duration may result in residual solvents, causing uneven film density and reduced alignment force. During validation, increase the Pre-cure temperature and extend the curing time to ensure residual solvent content ≤0.5%.
<2> Extend Main Cure Duration: Main cure is critical for PI molecular chain crosslinking. To address AC ghosting issues, extend this curing time from the standard 40 minutes to 50 minutes. This allows full crosslinking of PI molecular chains, enhancing alignment force and hydrolysis resistance.
<3> Control rubbing process parameters: Rubbing intensity must align with PI orientation strength—PI with strong orientation can tolerate lower rubbing intensity (to prevent molecular chain breakage), while PI with weak orientation requires appropriately increased rubbing intensity (to enhance groove depth).

LC Materials: Ion Contamination and Dielectric Property Imbalance Are the “Direct Causes” of Afterimages

Liquid crystal (LC) serves as the “display medium” in LCDs. Parameters such as its ion content, dielectric constant (Δε), and viscosity (η) directly influence the occurrence of line persistence and DC persistence. In particular, impurity ions within the LC migrate and aggregate under the influence of an electric field, forming additional electric fields that interfere with the deflection of liquid crystal molecules.

Core Mechanism of Afterimage Induced by LC Materials

Ion aggregation causes line afterimages: During LC synthesis and purification, improper process control may leave trace ions (e.g., metal ions from raw materials, organic acid anions from encapsulation). When displaying static images (e.g., black-white boundaries), a voltage difference (typically 5V-10V) exists between pixel electrodes. Ions migrate along the electric field toward the voltage boundary—positive ions toward the negative electrode and negative ions toward the positive electrode. These accumulated ions create a “localized additional electric field,” causing abnormal deflection angles of liquid crystal molecules in the boundary region. This results in differing light transmittance compared to surrounding areas, forming linear afterimages (line ghosting). Under high-temperature conditions (65°C), accelerated ion movement makes the afterimage more pronounced.

Dielectric constant (Δε) imbalance intensifies afterimages: The dielectric constant of liquid crystals, Δε = ε∥ – ε⊥ (where ε∥ is the dielectric constant parallel to the molecular long axis and ε⊥ is perpendicular), determines their deflection sensitivity in an electric field. If Δε is excessively large, liquid crystal molecules become overly sensitive to electric field changes, leading to excessive deflection during voltage fluctuations. If Δε is excessively small, molecular deflection slows, making rapid recovery difficult after prolonged display of a static image. Both scenarios cause afterimages—excessively large Δε tends to cause short-term afterimages, while excessively small Δε tends to cause long-term afterimages.
Charge trapping induces DC afterimages: Certain functional groups within LC molecules (e.g., cyano groups -CN, ester groups -COO-) possess charge trapping capabilities. When the AC voltage output by the TFT is asymmetrical (e.g., due to feed-through voltage deviation), LC molecules capture and retain a portion of the charge over extended periods. These trapped charges act as an equivalent “DC bias voltage,” forcing the liquid crystal molecules to remain in a fixed deflection state. After switching the display, this creates a DC afterimage, which may persist for several hours at low grayscale levels before gradually fading as the trapped charges are released.

Improvement Measures for LC Materials

Enhanced LC Purification Process: Reducing Ion Content

<1> Implement a “multi-layer filtration + ion exchange” combined process: After LC synthesis, particles are first removed using a 0.1μm PTFE filter membrane. The solution then flows through an ion exchange resin column (e.g., styrene-divinylbenzene resin) to adsorb metal ions and organic acid anions. Final ion content must meet specifications (verified via ion chromatography), reducing line ghosting incidence by 60%.

<2> Introducing a “vacuum degassing” step: Prior to LC injection, vacuum degassing removes dissolved oxygen and carbon dioxide from the LC—these gas molecules react with LC to form impurity ions, exacerbating ghosting. Data from a panel manufacturer shows that after vacuum degassing, LC ion content stabilizes below 30 ppb, with high-temperature line ghosting pass rate increasing to 98%.

Optimizing LC Formulation: Balancing Dielectric and Viscosity Parameters

<1> Adjusting Δε to an optimal range;

<2> Controlling viscosity (η): LC viscosity directly impacts response speed. At room temperature, η should be maintained between 20 mPa・s and 30 mPa・s. Excessively high viscosity slows molecular deflection, hindering recovery after prolonged display; excessively low viscosity makes molecular alignment susceptible to disruption. Viscosity can be adjusted by adding “low-viscosity monomers (e.g., cyclohexane derivatives)” while ensuring viscosity ≤200 mPa·s at low temperatures (-20°C) to prevent cold-temperature afterimages.

Addition of Anti-Charge Trapping Agents: Suppressing DC Afterimages

Incorporating 0.1%-0.3% “anti-charge trapping agents” (e.g., hindered amine compounds) into LC formulations allows these substances to compete with LC molecules for charge capture, thereby reducing residual charge levels. Laboratory testing indicates that adding 0.2% charge capture inhibitor reduces DC afterimage recovery time from 4 hours to 1 hour without affecting LC transmittance or response speed.