Why Can LCD Background Colors Be Customized While TFT Remains Black?

    From smartphones to TV screens, the background color of liquid crystal displays (LCDs)—such as cool white, warm white, or blue—can be customized, while thin-film transistor (TFT) regions always appear black. This seemingly contradictory phenomenon stems from the distinct physical roles these technologies play at different layers. This article explores the core logic behind this distinction and unveils the "battle between light and dark" within display panels.

一. The "Role Division" Between LCD and TFT

1.1 How LCDs Display Color

The core function of LCDs (Liquid Crystal Displays) is to control light transmission and generate colors through filters. The process involves:

• Backlight: Typically white LED arrays that provide uniform background light.
• Liquid Crystal Layer: Adjusts light transmission via electric field-controlled molecular alignment.
• Color Filters: Split white light into red, green, and blue (RGB) subpixels to mix colors.

In this process, the background color is determined by the backlight spectrum and filter combination. For example:

• Cool White: High-color-temperature LEDs (＞6500K) + transparent filters.
• Warm Yellow: Low-color-temperature LEDs (~3000K) + coated filters.

1.2 The Inherent "Blackness" of TFT

As the driver layer for liquid crystals, TFTs (Thin-Film Transistors) are designed to control pixel switching. Their physical properties mandate a "pure black" appearance:

• Opaque Structure: Made of non-transparent metals (Al/Cu) and semiconductors (a-Si/oxide).
• Circuit Layout: Wires and transistors occupy space (～70-80% aperture ratio), blocking light.
• Light-Blocking Design: Black Matrix (BM) material covers TFT areas to prevent light leakage.

In short, TFTs act as light "valves"—they block light when inactive, hence appearing black.

二. Technical Paths for Customizing LCD Background Colors

2.1 Backlight Adjustments

Modifying the backlight spectrum indirectly alters the LCD’s background tone:

2.2 Filter Optimization

Filter materials and processes directly impact background color:

• Color Resists: Tune RGB filter transmission curves (e.g., Nitto Denko’s wide-gamut filters).
• Black Matrix (BM): Low-reflectivity chromium oxide (CrOₓ) reduces whitish haze.
• Microlens Arrays: Boost light efficiency (Sharp IGZO tech).

2.3 Polarizer Tuning

Polarizer angles and phase-difference films fine-tune color bias:

• Circular Polarizers: Soften metallic reflections (common in OLEDs).
• Retardation Films: Compensate wavelength phase differences (reduce blue tint).

三. Physical Limitations Preventing TFT "Coloring"

3.1 Non-Optical Materials

TFT materials are inherently non-transparent:

Conflict: While ITO is transparent, it coexists with opaque metal traces, preventing overall light transmission.

3.2 Manufacturing Constraints

TFT fabrication locks in opaque structures:

1. Photolithography: Masks define transistor/wire patterns.
2. Etching: No post-etch modifications for transparency.
3. Lamination: TFTs sit beneath liquid crystals, isolated from light.

3.3 Functional Priorities

TFTs prioritize electrical performance, not optics:

• Mobility: Pixel response speed (Oxide TFT > LTPS > a-Si).
• Stability: Threshold voltage drift prevention (critical in high heat).
• Density: High-resolution demands tighter layouts (e.g., 4μm lines for 8K).

四. Future Possibilities

4.1 Transparent TFT Research

Emerging materials aim for transparency:

• ZnO-Based TFTs: 80% transmittance (lower mobility).
• Graphene Electrodes: 97.7% theoretical transmittance (high cost).
• Perovskite Semiconductors: High mobility + partial transparency (lab-stage).

4.2 Structural Innovations

• Light Diffraction: Microstructures redirect light around metal traces (Sony patent).
• Transparent Traces: Silver nanowires/carbon nanotubes replace metal (Samsung R&D).

4.3 Paradigm-Shifting Tech

• Micro LED: Self-emissive pixels (no TFTs; Apple Pro Display XDR).
• Electrochromic Displays: Ion-driven color shifts (BMW iX Flow).

Conclusion: The Necessity of Specialized Roles

    The customizable LCD background and TFT’s "black fate" reflect an optimal division of labor—TFTs focus on control, while color tasks fall to specialized materials. This balance ensures performance and reliability. While future transparent TFTs might merge these roles, today’s "black-and-white synergy" creates the vibrant displays we see.

Question: If TFTs could someday emit light, what new applications would emerge? Share your ideas!