Polysilicon TFT Pixel Circuit for OLED Luminance Compensation
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Solution Overview
Problem
Conventional active matrix display devices using polysilicon transistors for OLEDs suffer from variations in threshold voltage, leading to inconsistent luminance due to defects in crystal grain boundaries, and storage capacitors formed from MOS transistors fail to accurately hold video signals.
Innovation Solution
The display device incorporates a pixel configuration with multiple transistors and storage capacitors, where one transistor supplies current to the OLED, another sets the first transistor to a diode-connection state, and additional transistors control the storage capacitors to hold a voltage based on the threshold voltage, ensuring consistent luminance by compensating for transistor variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If polysilicon transistors are used for driving OLEDs, then the display device can be manufactured with lower cost and simpler process, but variations in threshold voltage occur due to defects in crystal grain boundaries, leading to inconsistent luminance
Solution Approach 1:
The invention introduces a storage capacitor as an intermediary element between the polysilicon transistor and the OLED. This capacitor compensates for the threshold voltage variations of the transistor, thereby maintaining consistent luminance output despite the inherent defects in polysilicon crystal grain boundaries. The capacitor acts as a mediator that decouples the relationship between transistor parameter variations and luminance output.
Solution Approach 2:
The invention changes the electrical parameters of the pixel circuit by adding a storage capacitor with specific capacitance value. This parameter change allows the circuit to compensate for threshold voltage shifts in the polysilicon transistor, thereby stabilizing the drive current to the OLED and achieving consistent luminance without requiring higher manufacturing precision.
2Device complexity
If conventional pixel configuration with single transistor is used, then the circuit is simpler, but it cannot compensate for transistor threshold voltage variations, resulting in luminance inconsistency
Solution Approach 1:
The invention segments the pixel circuit into distinct functional blocks: a driving transistor for current supply, a storage capacitor for voltage compensation, and an OLED for light emission. This segmentation allows the storage capacitor to specifically address the threshold voltage variation problem without requiring complete redesign of the entire pixel circuit, thus achieving luminance consistency with minimal added complexity.
Solution Approach 2:
The storage capacitor is pre-charged to a voltage that compensates for the threshold voltage of the polysilicon transistor before the OLED driving phase. This preliminary action ensures that when the transistor drives the OLED, the capacitor's stored voltage offsets the threshold voltage drop, maintaining consistent luminance without requiring real-time adjustment mechanisms.
3Ease of manufacture
If MOS transistor-based storage capacitors are used, then the capacitor can be integrated with the transistor制造工艺, but they fail to accurately hold video signals due to leakage currents
Solution Approach 1:
The invention uses a simplified capacitor structure that copies the essential function of signal storage without replicating the complex MOS transistor-based capacitor design. By using a different capacitor implementation that is less sensitive to leakage currents, the invention achieves accurate video signal holding while maintaining compatibility with the existing polysilicon transistor manufacturing process.
Data Source
AI summary
To provide a display device and a driving method thereof, where variations in the threshold voltage of transistors can be compensated and thus variations in luminance of light-emitting elements can be suppressed. In a first period, initialization is performed; in a second period, a voltage based on the threshold voltage of a first transistor is held in first and second storage capacitors; in a third period, a voltage based on a video signal voltage and the threshold voltage of the first transistor is held in the first and second storage capacitors; and in a fourth period, voltages held in the first and second storage capacitors are applied to a gate terminal of the first transistor to supply a current to a light-emitting element, so that the light-emitting element emits light. Through the operation process, a current obtained by compensating variations in the threshold voltage of the first transistor can be supplied to the light-emitting element, thereby variations in luminance can be suppressed.