OLED Pixel Circuit Current Measurement for Luminance Uniformity
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Solution Overview
Problem
Active matrix OLED displays face luminance non-uniformity due to variations in drive transistor characteristics and differential aging of OLEDs, requiring compensation of pixel circuit parameters to maintain high image quality.
Innovation Solution
A pixel circuit and method for measuring and adjusting electrical currents in OLED-based displays, involving a source driver, readout circuit, and digital processor to convert measured currents into digital codes for calibrating programming signals, ensuring consistent luminance across pixels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If drive transistor characteristics are used to control OLED luminance, then pixel switching and light emission control are achieved, but luminance non-uniformity occurs due to transistor variations and aging
Solution Approach 1:
The patent implements a feedback mechanism where the measured pixel current is used to adjust the programming voltage applied to the drive transistor. The system measures the actual current flowing through each pixel's OLED and uses this information to compensate for deviations caused by transistor variations and aging, thereby maintaining uniform luminance across the display.
Solution Approach 2:
The patent changes the programming voltage parameter dynamically based on measured pixel current characteristics. By adjusting the voltage applied to the drive transistor gate, the system compensates for threshold voltage shifts and mobility variations in the transistors, ensuring consistent current drive and luminance output despite transistor degradation over time.
2Reliability
If programming voltage is adjusted to compensate for aging, then image quality is maintained, but additional measurement and control circuits are required
Solution Approach 1:
The patent makes the data line serve multiple functions: it is used both for normal programming of pixel circuits during display operation and for measuring pixel currents during calibration modes. This multi-functionality eliminates the need for separate dedicated measurement lines or circuits, reducing overall system complexity while maintaining the ability to perform aging compensation.
Solution Approach 2:
The pixel circuit itself performs the measurement function by allowing current measurement through its existing components (drive transistor, storage capacitor, OLED) without requiring external measurement circuits. The circuit uses its own operational characteristics to provide the necessary data for compensation, making the system self-sufficient and avoiding additional complexity.
3Measurement precision
If pixel current measurement is performed, then aging compensation is enabled, but measurement precision requirements increase system complexity
Solution Approach 1:
The patent performs current measurement at selected intervals (e.g., periodically or at specific display refresh cycles) rather than continuously, and measures a subset of pixels rather than all pixels at all times. This partial measurement approach provides sufficient data for effective aging compensation while significantly reducing the complexity and resource requirements of the readout circuit compared to continuous full-array measurement.
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 effectively compensates for transistor and OLED aging by measuring and adjusting pixel currents, maintaining high image quality and uniformity in active matrix OLED displays.
Implementation Method 1
The OLEDs emit light based on an electrical current supplied through the drive transistors
Data Source
AI summary
A pixel circuit for an active matrix organic light emitting diode (AMOLED) and other active matrix displays is disclosed. The pixel circuit is programmed by the voltage supplied through a data line. An electrical current through a light emitting device for a known LED voltage and a pixel current for a pixel programed with a known data signal can be measured by a readout circuit through the data line. A 7T1C implementation enables to pre-charge the drive transistor to a reference voltage in each drive cycle, and to pre-set the light emitting device to a reference voltage prior to emission in each cycle.