OLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Existing pixel circuits are unable to effectively compensate for variances in currents caused by non-uniformity and threshold voltage drift in both depletion and enhancement type TFTs, particularly affecting the display performance of OLEDs.
Innovation Solution
A pixel circuit comprising a light-emitting element, a driving thin film transistor, and four additional thin film transistors, along with a capacitor, which operates through precharging, compensating, and keeping light-emitting stages to isolate the gate-source voltage from threshold voltage variations, ensuring current stability independent of threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If existing pixel circuits use diode connection for driving TFT, then enhancement type TFT can be driven, but depletion type TFT cannot be properly compensated due to Vgs=0 turning on state
Solution Approach 1:
The patent employs dynamic control of transistor switching states through scanning signals and control signals. The first TFT is turned on during compensation phase to charge the capacitor, while the fourth TFT switches between on and off states based on control signals. This dynamic switching enables the circuit to adapt its operation mode, allowing proper compensation for both enhancement and depletion type TFTs by controlling the gate-source voltage appropriately for each TFT type.
Solution Approach 2:
The patent changes the gate-source voltage parameter dynamically during different operation phases. During compensation, the gate of the first TFT receives a high level signal to turn it on, charging the capacitor to store threshold voltage information. During light emission, the gate receives a low level signal to turn it off. This parameter change enables the circuit to store and utilize threshold voltage compensation data, achieving current uniformity across different TFT types.
2Ease of manufacture
If simple pixel circuit structure is used, then manufacturing is easier, but threshold voltage drift and non-uniformity cannot be compensated
Solution Approach 1:
The patent implements preliminary compensation action by charging the capacitor during a dedicated compensation phase before the light emission phase. The first TFT is turned on in advance to charge the capacitor with the threshold voltage information, and this stored charge is then used during light emission to maintain accurate current control. This preliminary action ensures that compensation is performed before display operations, achieving current uniformity without complicating the overall circuit structure.
3Reliability
If more TFTs and capacitors are added for compensation, then current variance compensation improves, but device complexity increases
Solution Approach 1:
The patent achieves multi-functionality with a compact circuit design. The capacitor serves multiple purposes: storing threshold voltage compensation data during the compensation phase and maintaining the gate-source voltage during the light emission phase. The first TFT functions as both a switching element and a compensation element. This universal usage of components achieves effective current compensation without proportionally increasing device complexity.
Solution Approach 2:
The patent merges the compensation function and light emission control function into a unified circuit structure. The capacitor is connected to share common nodes with the light emission control circuitry, and the first TFT is integrated into the existing pixel circuit topology. This merging of functions achieves effective compensation while minimizing the increase in device complexity compared to separate compensation circuits.
Data Source
AI summary
The embodiments of the invention disclose a pixel circuit, a display device and a driving method thereof. The pixel circuit comprises a light-emitting element; a driving TFT, its drain is input a power supply voltage signal; a first TFT, its drain is connected with a source of the driving TFT, its source is connected with the light-emitting element, its gate receives a first control signal; a second TFT, its source receives a data signal, its drain is connected with a gate of the driving TFT, its gate receives a scanning signal; a third TFT, its source receives a reference voltage signal, its gate receives the scanning signal; a fourth TFT, its source is connected with a drain of the third TFT, its drain is connected with the gate of the driving TFT and the drain of the second TFT, its gate receives a second control signal; and a capacitor.