OLED Pixel Circuit Compensation for Threshold Shift
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Solution Overview
Problem
In organic light emitting diode (OLED) displays, threshold voltage shifts in thin film transistors (TFTs) and material decay in OLEDs lead to non-uniform frame luminance, reducing the overall display quality due to varying driving currents and increasing voltage drops over time.
Innovation Solution
A method and circuit configuration using multiple TFTs to manage the storage capacitor and OLED, involving a reset process to discharge the capacitor until nearly zero current, storing charges, and then generating a pixel current to drive the OLED, compensating for threshold voltage shifts and material decay by adjusting voltages across the capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional pixel circuit with a single TFT and storage capacitor is used, then the device complexity is low, but the frame luminance uniformity deteriorates due to TFT threshold voltage shift and OLED material decay
Solution Approach 1:
The pixel circuit is divided into multiple functional blocks: a first TFT (T1) for driving the OLED, a second TFT (T2) for resetting the storage capacitor, and a third TFT (T3) for discharging the storage capacitor. This segmentation allows each TFT to perform a specific function, enabling compensation for threshold voltage shifts and maintaining luminance uniformity across frames.
Solution Approach 2:
The storage capacitor is reset before the OLED is driven to emit light. By performing the reset operation in advance (setting the capacitor voltage to a known reference level before driving), the circuit compensates for any voltage drops or threshold shifts that may have occurred during the previous frame, ensuring consistent luminance output.
2Reliability
If the storage capacitor is not reset before driving the OLED, then the operation time is reduced, but the luminance consistency deteriorates due to accumulated voltage drops from TFT threshold voltage shift
Solution Approach 1:
The storage capacitor undergoes periodic reset operations at the beginning of each frame cycle. This periodic action ensures that any accumulated voltage errors from TFT threshold shifts are cleared regularly, maintaining luminance consistency across frames while enabling continuous operation at standard frame rates.
3Reliability
If multiple TFTs are added to the pixel circuit for compensation, then the luminance uniformity improves, but the manufacturing precision requirements increase
Solution Approach 1:
The pixel circuit uses itself to compensate for manufacturing variations. By incorporating reset and discharge operations within the pixel, the circuit automatically compensates for TFT threshold voltage shifts without requiring external calibration or highly precise manufacturing. The self-service mechanism reduces the stringency of manufacturing precision requirements while maintaining luminance uniformity.
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 approach maintains consistent and high image quality over time by stabilizing the relationship between pixel voltage and OLED luminance, regardless of TFT threshold voltage shifts and OLED material decay, ensuring uniform frame luminance and compensating for reduced currents due to material degradation.
Implementation Method 1
charges are stored in a storage capacitor for controlling the luminance of an OLED via a thin film transistor (TFT)
Implementation Method 2
an organic light emitting diode (OLED) display
Data Source
AI summary
A light emitting diode (OLED) display and pixel driving method thereof are disclosed. Each storage capacitor is discharged via the driving TFT and OLED until conductive current of each OLED is almost zero so as to record a sum of a voltage drop across each OLED under a specific condition and the threshold voltage of the driving TFT. By using the sum of the voltage drop across each OLED and the threshold voltage of the corresponding TFT in the subsequent OLED driving process, the luminance reduction issue caused by TFT threshold-voltage shift and OLED material decay can be solved.