OLED Pixel Bias Control for Driving Transistor Stability
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Solution Overview
Problem
Display devices with organic light emitting diodes (OLEDs) face degradation in pixel characteristics due to shifted threshold voltages of driving transistors, leading to inadequate luminance and image quality, especially when expressing grayscale levels, as they fail to generate desired light levels during frame periods.
Innovation Solution
A display device and driving method that apply a bias voltage during a bias period at low frequency operation, using a data driver to supply a bias signal during specific periods and utilizing a source capacitor to store and discharge the bias signal, thereby maintaining the on-bias state of driving transistors and ensuring consistent luminance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bias voltage is continuously applied to maintain driving transistor characteristics, then the threshold voltage shift is reduced, but power consumption increases
Solution Approach 1:
The patent applies bias voltage periodically rather than continuously. A bias voltage is applied during specific periods (when the emission control signal is not supplied to pixels) to maintain driving transistor characteristics, and then turned off to reduce power consumption. This periodic application resolves the contradiction by providing maintenance only when necessary.
Solution Approach 2:
The bias voltage is applied in advance during periods when pixels are not emitting light, before the actual display period begins. This preliminary action maintains the driving transistor characteristics in preparation for the next display period, ensuring stable operation without requiring continuous voltage application.
2Use of energy by moving object
If the display device operates at low frequency to reduce power consumption, then energy usage decreases, but pixel characteristic degradation increases due to threshold voltage shift
Solution Approach 1:
The patent introduces a bias period during low-frequency operation where bias voltage is applied to pixels. This periodic intervention compensates for the threshold voltage shift that occurs during extended idle periods between frames, maintaining pixel characteristics without requiring high-frequency operation.
Solution Approach 2:
The bias voltage acts as an intermediary mechanism that indirectly maintains pixel characteristics. Instead of directly addressing the threshold voltage shift through high-frequency operation, the bias voltage mediates by keeping driving transistors in an on-bias state during idle periods, preventing characteristic degradation.
3Reliability
If the bias signal is supplied continuously from the data driver, then driving transistor characteristics are maintained, but device complexity and power consumption increase
Solution Approach 1:
The data driver alternates between supplying bias signal and not supplying it, based on the emission control signal state. During bias periods (when emission control signal is inactive), the bias signal is supplied to maintain characteristics. During display periods, normal operation resumes. This periodic switching reduces complexity compared to continuous supply control.
Solution Approach 2:
The system uses the existing emission control signal timing to automatically determine when to apply bias voltage, without requiring a separate complex control mechanism. The bias supply is self-regulated based on the display device's own operating cycle, reducing additional control complexity.
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 reduces the degradation of driving transistor characteristics, maintains continuous emission with stored voltage, and reduces power consumption by controlling the bias signal's on/off states, enhancing image quality and luminance uniformity.
Implementation Method 1
a source capacitor coupled to each of the data lines, and a data driver configured to supply the data signal during a display period, to supply a bias signal during a first period in the bias period, and to not supply the bias signal during a second period
Data Source
AI summary
In a display device including pixels that are coupled to data lines, that are supplied with a data signal during a display period, and that are configured to emit light corresponding to the data signal during a bias period, the display device includes a source capacitor coupled to each of the data lines, and a data driver configured to supply the data signal during the display period, to supply a bias signal during a first period in the bias period, and to not supply the bias signal during a second period.