Lateral Photodiode Placement for EL Display Ageing Compensation
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Solution Overview
Problem
Existing active matrix electroluminescent display devices face challenges with differential ageing and non-uniformity due to variability in LED material and transistor characteristics, particularly in top-emitting structures where optical feedback systems are difficult to implement effectively, and conventional light-sensitive devices suffer from low efficiency and step coverage issues.
Innovation Solution
The solution involves locating a light-dependent device, such as a photodiode, laterally alongside the light emitting material in the pixel, allowing for improved light collection and reduced step coverage problems, with a configuration that includes a top transparent electrode and a bottom reflective electrode, and optional reflecting layers to direct light to the photodiode, enhancing efficiency and integration within the pixel layout without affecting the aperture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a light-sensitive device is placed under the pixel aperture to enable optical feedback, then ageing compensation is achieved, but step coverage problems and low efficiency occur
Solution Approach 1:
The light-sensitive device is repositioned from a vertical placement under the pixel aperture to a lateral placement at the side of the pixel aperture. This dimensional change eliminates step coverage problems while maintaining optical feedback functionality through side-illuminated light collection
2Reliability
If conventional light-sensitive devices are used under the pixel aperture, then optical feedback is achieved, but light collection efficiency is low
Solution Approach 1:
The light-sensitive device collects light laterally from the side of the pixel aperture rather than from below, increasing the effective light collection area and efficiency while maintaining the optical feedback function for ageing compensation
Solution Approach 2:
A reflective layer is introduced as an intermediary element to redirect light from the pixel aperture to the laterally positioned light-sensitive device, enhancing light collection efficiency through optical redirection
3Reliability
If top-emitting structure is used, then display performance is improved, but implementation of optical feedback systems becomes difficult
Solution Approach 1:
The light-sensitive device is positioned laterally at the side of the pixel aperture rather than vertically underneath it, enabling optical feedback in top-emitting displays by collecting light from the emission direction without interfering with the transparent electrode structure
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
This configuration improves light collection efficiency, reduces non-uniformity, and allows for effective ageing compensation without the need for high instantaneous brightness, maintaining image quality and extending the lifespan of the display elements.
Implementation Method 1
a light-dependent device for detecting the brightness of the display element
Implementation Method 2
a bottom reflective electrode, and optional reflecting layers to direct light to the photodiode
Data Source
AI summary
An active matrix display device comprises an array of display pixels, with each pixel comprising an EL display element, a light-dependent device for detecting the brightness of the display element and a drive transistor circuit for driving a current through the display element. The drive transistor is controlled in response to the light-dependent device output so that ageing compensation can be implemented. The light-dependent device is located laterally of the area of light emitting material of the EL display element. In this way, the light-dependent device does not cause step coverage problems and can be integrated into the pixel layout without affecting the pixel aperture. Furthermore, the light dependent device can extend alongside the full length of the area of light emitting material so that it receives light input from a large part of the display element area.