White OLED with High Triplet Energy Hole Transport Layer

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Solution Overview

Problem

Conventional white organic light emitting devices with a laminate structure including a fluorescent stack and a phosphorescent stack exhibit low efficiency due to exciton diffusion from the fluorescent stack into the hole transport layer, resulting in significant color differences and reduced image quality, especially at low currents.

Innovation Solution

A white organic light emitting device is designed with a first stack containing a blue fluorescent light emitting layer and a second stack with a phosphorescent light emitting layer, where the triplet energy level of the hole transport layers is higher than the light emitting layers, and blocking layers are used to prevent exciton diffusion, enhancing hole mobility and reducing color variation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If a conventional laminate structure with fluorescent stack and phosphorescent stack is used, then the device structure is simple and easy to manufacture, but the fluorescent stack exhibits low efficiency and great color difference at low current

Engineering Contradiction:

Improveease of manufactureVSAvoidefficiency of blue fluorescent stack

Core Design Contradiction:

Ease of manufactureVSProductivity

Solution Approach 1:

The patent changes the energy level parameters of the hole transport layer by selecting materials with triplet energy levels higher than the fluorescent light emitting layer (e.g., TCTA with 2.7 eV or TAPC with 2.4 eV). This parameter change prevents exciton diffusion from the fluorescent stack into the hole transport layer, thereby improving the efficiency of the blue fluorescent stack while maintaining the simple laminate structure that is easy to manufacture.

Inventive Principle:

Principle #35Parameter changes

2Productivity

If the triplet energy level of the hole transport layer is lower than the light emitting layer, then exciton diffusion occurs from the fluorescent stack into the hole transport layer, but this causes low efficiency and warm white color rendering

Engineering Contradiction:

ImproveefficiencyVSAvoidcolor intensity and image quality

Core Design Contradiction:

ProductivityVSIllumination intensity

Solution Approach 1:

The patent applies preliminary anti-action by designing the hole transport layer with a triplet energy level higher than the fluorescent light emitting layer before exciton diffusion can occur. This energy level barrier prevents excitons from diffusing into the hole transport layer, thereby maintaining high efficiency and preventing warm white color rendering that would otherwise occur due to exciton loss.

Inventive Principle:

Principle #9Preliminary anti-action

3Device complexity

If a blocking layer with insufficient thickness is used, then the device structure remains simple, but exciton diffusion distance exceeds the blocking layer thickness causing efficiency deterioration

Engineering Contradiction:

Improvedevice complexityVSAvoidefficiency of fluorescent stack

Core Design Contradiction:

Device complexityVSProductivity

Solution Approach 1:

Instead of increasing the thickness of the blocking layer, the patent changes the energy level parameter of the hole transport layer itself by selecting materials with sufficiently high triplet energy levels. This parameter change creates an energy barrier that prevents exciton diffusion regardless of the layer thickness, thereby maintaining simple device structure while improving fluorescent stack efficiency.

Inventive Principle:

Principle #35Parameter changes

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 the efficiency of the blue fluorescent stack, reduces color differences between fluorescent and phosphorescent emissions, and decreases power consumption, resulting in improved image quality and reduced color variation across different brightness levels.

Implementation Method 1

a triplet energy level of the hole transport layer is higher than a triplet energy level of the fluorescent light emitting layer

Methodology Applied

Scientific EffectTriplet energy level barrier:

Implementation Method 2

a first light emitting layer emitting blue fluorescent light

Methodology Applied

Scientific EffectFluorescence: Fluorescence

Implementation Method 3

a second light emitting layer formed by doping one host with at least one of phosphorescent dopant

Methodology Applied

Scientific EffectPhosphorescence: Phosphorescence

Data Source

PatentUS9997739B2White organic light emitting device

Publication Date: 2018.06.12 LG DISPLAY CO LTD

AI summary

Disclosed white organic light emitting device includes an anode and a cathode opposing each other; a charge generation layer interposed between the anode and the cathode; a first stack interposed between the anode and the charge generation layer, the first stack including a first hole transport layer and a first light emitting layer emitting blue fluorescent light; and a second stack interposed between the charge generation layer and the cathode, the second stack including a second hole transport layer and a second light emitting layer formed by doping one host with at least one of phosphorescent dopant, wherein a triplet energy level of the first hole transport layer is higher than a triplet energy level of the first light emitting layer, and a hole mobility of the first hole transport layer is 5.0×10−4 cm2/s·V to 9.9×10−3 cm2/s·V.