OLED Threshold Voltage Compensation via Extended Scanning Periods
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Solution Overview
Problem
Conventional organic light emitting display devices face issues with brightness uniformity due to deviations in threshold voltage of thin-film transistors (TFTs), which affect the current flow and result in non-uniform screen brightness, as the conventional method of storing threshold voltage in a capacitor is insufficient for full charging or discharging within the given time frame.
Innovation Solution
The proposed solution involves a display device with a storage capacitor that stores a control voltage based on the threshold voltage of the driving transistor, using additional switching transistors to diode-connect the driving transistor and supply driving voltage, allowing for a longer compensation period to ensure accurate voltage storage and transmission to the control terminal of the driving transistor, thereby compensating for threshold voltage deviations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the conventional method of storing threshold voltage in a capacitor is used within one horizontal period, then the device complexity is reduced, but the threshold voltage cannot be fully stored due to insufficient charging/discharging time, resulting in deteriorated brightness uniformity
Solution Approach 1:
The patent introduces a dual-mode scanning signal (first scanning signal and second scanning signal) that dynamically changes the compensation time duration. The first scanning signal provides a longer compensation period for complete capacitor charging/discharging, while the second scanning signal maintains normal operation. This dynamic time extension resolves the contradiction between limited compensation time and the need for complete voltage storage.
Solution Approach 2:
The patent performs threshold voltage compensation in advance during the first scanning signal period before the actual display operation. By completing the capacitor charging/discharging process during the extended compensation time of the first scanning signal, the system ensures that the storage capacitor is fully charged with the threshold voltage information before normal operation begins, thereby achieving complete compensation without affecting subsequent display performance.
2Reliability
If the scanning signal period is extended to allow full capacitor charging and discharging, then the threshold voltage is fully stored improving brightness uniformity, but the scanning speed decreases reducing productivity
Solution Approach 1:
The patent segments the scanning process into two distinct phases: a first scanning period dedicated to compensation with extended duration for complete capacitor charging/discharging, and a second scanning period for normal display operation with standard speed. This segmentation allows the system to achieve complete threshold voltage compensation without permanently reducing the overall scanning speed, as the extended duration is confined to the compensation phase only.
Solution Approach 2:
The patent implements periodic alternating use of first and second scanning signals. The first scanning signal with extended period is used periodically to perform complete threshold voltage compensation, while the second scanning signal with normal period is used for regular display operations. This periodic action ensures that complete compensation is achieved at regular intervals without continuously reducing the scanning speed, thus maintaining both brightness uniformity and productivity.
Data Source
AI summary
A display device having a plurality of pixels, each pixel includes a light-emitting element, a storage capacitor, a driving transistor, a first switching transistor which supplies a data voltage to the storage capacitor in response to an on-voltage of a scanning signal, a second switching transistor which diode-connects the driving transistor in response to an on-voltage of a compensation signal, and a third switching transistor which supplies a driving voltage to the driving transistor in response to an on-voltage of a light emitting signal. The storage capacitor stores a control voltage depending on a threshold voltage of the driving transistor when the driving transistor is diode-connected, transmits the control voltage and the data voltage to the control terminal of the driving transistor, and a period in which the compensation signal is in an on-voltage state is longer than a period in which the scanning signal is in an on-voltage state.