OLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
In OLED displays, the variation in threshold voltages of driving TFTs due to device aging and process procedures affects the uniformity of display brightness, as the same gate-source voltage applied to driving transistors results in different operating currents.
Innovation Solution
A pixel circuit comprising a driving transistor, precharge, reset, data writing, and light emission control sub-circuits, where the precharge sub-circuit writes a supply voltage, the reset sub-circuit decreases the node potential, the data writing sub-circuit sets the node potential to the sum of the data voltage and threshold voltage, and the light emission control sub-circuit connects the power supply to the light-emitting unit, ensuring the current through the OLED is independent of the threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional 2T1C pixel circuit is used, then the circuit structure is simple, but the threshold voltage variation of driving TFTs causes non-uniform display brightness
Solution Approach 1:
The pixel circuit is divided into multiple functional modules: precharge module (T1, T2), reset module (T3, C1), data writing module (T4, T5), and light emission module (T6). Each module performs a specific function in the driving process, allowing independent optimization of each function while maintaining overall simplicity.
Solution Approach 2:
The precharge module charges the storage capacitor Cs to the power supply voltage Vdd before data writing. This preliminary action ensures that the capacitor is ready for the subsequent reset and data writing operations, and helps establish a known initial state that compensates for threshold voltage variations.
2Ease of operation
If the same gate-source voltage is applied to driving transistors, then the control is simple, but the operating current varies due to threshold voltage differences
Solution Approach 1:
The reset module uses a feedback mechanism where the voltage on the storage capacitor Cs is compared against a reference voltage through transistor T3. When the capacitor voltage exceeds the reference, T3 conducts to discharge the capacitor, creating a self-regulating system that compensates for threshold voltage variations and maintains stable OLED current.
Solution Approach 2:
The circuit dynamically adjusts the gate-source voltage of the driving transistor by controlling the charge state of the storage capacitor Cs. Through the coordinated operation of precharge, reset, and data writing modules, the voltage parameter is optimized to compensate for threshold voltage variations, ensuring consistent OLED current despite manufacturing variations.
3Manufacturing precision
If device aging and process variations are considered, then the design becomes more complex, but the threshold voltage compensation improves display uniformity
Solution Approach 1:
The pixel circuit performs self-compensation for threshold voltage variations through its internal reset mechanism. The reset transistor T3 automatically detects when the storage capacitor voltage exceeds the reference voltage and discharges it accordingly, eliminating the need for external compensation circuits or complex calibration processes.
Solution Approach 2:
The storage capacitor Cs serves multiple functions: storing data voltage, providing feedback signal for reset control, and participating in threshold voltage compensation. This multi-functionality reduces the need for additional dedicated compensation components, maintaining circuit simplicity while achieving improved display uniformity.
Data Source
AI summary
The present disclosure provides a pixel circuit, a driving method thereof, an array substrate and a display device. The pixel circuit comprises: a driving transistor; a precharge sub-circuit configured to write a supply voltage into a first node under the control of a scan signal and a light emission control signal in a precharge phase; a reset sub-circuit; a data writing sub-circuit configured to write a data voltage into the first node under the control of the scan signal in a data writing phase, so that the potential of the first node is equal to a sum of the data voltage and a threshold voltage of the driving transistor; a light emission control sub-circuit configured to connect a power supply with a light-emitting unit through the driving transistor under the control of the light emission control signal in a light-emitting phase.