Dual-Index Overcoating in Displays to Prevent Undercut Defects

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Solution Overview

Problem

Display devices experience reduced light extraction efficiency due to total internal reflection in polarizing layers, leading to increased perished light and decreased brightness, and the formation of undercut sections in low refractive index layers causes reliability issues and defects.

Innovation Solution

A display device configuration with a first overcoating layer having a low refractive index and a second overcoating layer with a high refractive index is employed, where the second layer fills undercut sections and increases the angle of incidence of light to below the total reflection angle, enhancing light extraction efficiency and preventing defects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If a low refractive index layer is disposed between the light source and the polarizing layer to control the angle of incidence, then the light extraction efficiency is improved, but undercut sections occur along the edge portion of the low refractive index layer causing reliability problems

Engineering Contradiction:

Improvelight extraction efficiencyVSAvoidresistance to undercut section defects

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent divides the overcoating layer into two distinct layers: a first overcoating layer with low refractive index (for light extraction) and a second overcoating layer with high refractive index (for preventing undercut sections). This segmentation allows each layer to perform its specific function without compromising the other, resolving the contradiction between light extraction efficiency and reliability.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The second overcoating layer with high refractive index acts as an intermediary layer between the first overcoating layer and the encapsulation layer. This intermediary structure prevents the formation of undercut sections by providing a stable interface, while the first overcoating layer continues to perform the light extraction function.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Productivity

If the angle of incidence of light is reduced below the total reflection angle to reduce perished light, then the light extraction efficiency is improved, but the bonding strength between insulating layers is compromised

Engineering Contradiction:

Improvelight extraction efficiencyVSAvoidbonding strength between insulating layers

Core Design Contradiction:

ProductivityVSStrength

Solution Approach 1:

The patent applies different refractive indices to different regions of the overcoating structure. The first overcoating layer has low refractive index for optimal light extraction, while the second overcoating layer has high refractive index for strong bonding. This local differentiation of material properties allows simultaneous optimization of both light extraction and bonding strength.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent uses a composite structure combining two overcoating layers with different refractive indices. The first layer (low refractive index) and second layer (high refractive index) work together as a composite system, where the first layer optimizes light extraction and the second layer ensures strong bonding, resolving the contradiction between optical performance and mechanical strength.

Inventive Principle:

Principle #40Composite materials

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 improves light extraction efficiency, increases bonding strength between insulating layers, and enhances the reliability of the display device by preventing undercut sections and delamination.

Implementation Method 1

Some of the light incident on the polarizing layer can be perished without being emitted to the outside due to a total internal reflection. As the amount of light undergoing the total internal reflection in the polarizing layer increases, the amount of perished light increases and the light extraction efficiency decreases.

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection

Implementation Method 2

By disposing a low refractive index layer between the light source and the polarizing layer, the angle of incidence of light incident on the polarizing layer can be controlled.

Methodology Applied

Scientific EffectRefraction: Refraction

Implementation Method 3

Methodology Applied

Scientific EffectAdhesion: Adhesive

Data Source

PatentUS20260033097A1Display device

Publication Date: 2026.01.29 LG DISPLAY CO LTD

AI summary

A display device can include a substrate, a plurality of driving chips disposed on the substrate, a plurality of light-emitting elements disposed on one of the plurality of driving chips and being electrically connected to the one driving chip, an optical insulating layer covering the plurality of light-emitting elements, a first overcoating layer disposed on the optical insulating layer, and a second overcoating layer disposed on the first overcoating layer. The refractive index of the first overcoating layer can be lower than the refractive index of the second overcoating layer.