Active Matrix Display Optical Feedback Ageing Compensation
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Solution Overview
Problem
Active matrix electroluminescent display devices using amorphous silicon transistors face issues with differential ageing due to variations in threshold voltage and LED efficiency, leading to non-uniform light output and reduced display lifetime, as existing voltage-addressed pixel circuits fail to effectively compensate for these changes.
Innovation Solution
An active matrix display device with a current-driven light emitting display element, a drive transistor, and pixel circuitry including an optical feedback element that controls the drive transistor to maintain constant current and adjusts output configuration parameters such as power supply voltages and field period in response to ageing, ensuring prolonged compensation for differential ageing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If voltage-addressed pixel circuits are used to compensate for LED ageing, then light output uniformity is improved, but threshold voltage variations in amorphous silicon transistors cause differential ageing and non-uniformity
Solution Approach 1:
The patent implements optical feedback by incorporating a light sensing element (photodiode) that detects the actual light output of the display element. This feedback signal is used to control the discharge transistor, which adjusts the storage capacitor voltage to compensate for LED efficiency degradation. The feedback mechanism directly addresses the non-uniformity problem by using actual light output measurements rather than relying on unstable transistor threshold voltages.
Solution Approach 2:
The patent dynamically changes the operating parameters of the pixel circuit, specifically the storage capacitor voltage and discharge timing, based on the detected light output. By adjusting these parameters in response to measured performance, the system maintains uniform light output despite LED ageing and transistor threshold variations.
2Ease of manufacture
If amorphous silicon transistors are used for pixel circuits, then manufacturing ease is improved, but differential ageing due to voltage stress causes large threshold voltage changes
Solution Approach 1:
The optical feedback mechanism compensates for the instability of amorphous silicon transistor threshold voltages by using light output measurements to control the discharge transistor timing. This feedback loop effectively bypasses the need for stable transistor characteristics, allowing the use of easily manufactured amorphous silicon transistors while maintaining display uniformity.
Solution Approach 2:
The patent replaces reliance on electrical characteristics (transistor threshold voltages) with optical measurement and control. By using a photodiode to sense light output and controlling the discharge transistor based on optical feedback rather than electrical signals, the system substitutes optical control for electrical control, eliminating the impact of transistor threshold variations.
3Reliability
If optical feedback elements are added to pixel circuits, then ageing compensation is improved, but device complexity increases
Solution Approach 1:
The patent merges the light sensing function with the existing pixel circuit by integrating the photodiode into the pixel structure and using the same discharge transistor for both ageing compensation and normal operation. This consolidation reduces the overall complexity increase compared to adding completely separate compensation circuitry.
Solution Approach 2:
The discharge transistor serves multiple functions: it controls the storage capacitor during normal pixel operation and also acts as the feedback control element for ageing compensation. The photodiode integrates both light sensing for ageing compensation and contributes to the overall pixel functionality, reducing the need for dedicated separate components.
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 extends the display's operational life by maintaining constant brightness and compensating for ageing effects, reducing the impact of threshold voltage variations and LED efficiency degradation, thereby enhancing the overall display performance and longevity.
Implementation Method 1
pixel circuitry including an optical feedback element, for controlling the drive transistor...
Implementation Method 2
active matrix electroluminescent display devices... current-driven light emitting display element
Data Source
AI summary
An active matrix display device has an array of display pixels, each pixel comprising a current-driven light emitting display element (2), a drive transistor (22) for driving a current through the display element and pixel circuitry including an optical feedback element (38) for controlling the drive transistor to drive a substantially constant current through the display element for a duration which depends on the desired display pixel output level and an optical feedback signal of the optical feedback element. An output configuration is applied to the display which includes values for the pixel power supply voltages, the field period and an allowed range of pixel drive levels. The output configuration is varied in response to ageing of the display element. In this device, an output configuration is varied as the device ages, so that the optical feedback system can continue to provide compensation for differential ageing of the display elements for a longer period of use of the display.