Pixel Circuit Compensation for Drive Transistor Threshold Voltage Variation
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Solution Overview
Problem
In organic EL display devices, variations in threshold voltage of drive transistors lead to inconsistent luminance due to changes in high-level power supply voltage and wiring line resistance, affecting display quality.
Innovation Solution
A display device with a pixel circuit configuration that includes a drive transistor connected in series between power source wiring lines, a capacitance element, and transistors for initialization and writing control, where data is written by applying a data voltage to the capacitance element with an initialization voltage applied via the drive transistor, ensuring stable data writing and compensating for threshold voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If internal compensation method is used with data writing performed while high-level power supply voltage is applied to data-holding capacitor, then threshold voltage variation is compensated, but luminance variation occurs due to voltage drop differences
Solution Approach 1:
The patent applies initialization voltage to the data-holding capacitor before applying the high-level power supply voltage. This preliminary action ensures that the capacitor is pre-charged to a known potential, allowing subsequent data writing to occur without being affected by voltage drops in the power supply wiring. The initialization voltage is applied through a dedicated initialization wiring line that is separate from the power supply lines, thereby eliminating the luminance variation caused by IR drops while maintaining threshold voltage compensation functionality.
2Ease of manufacture
If high-level power supply voltage is applied to data-holding capacitor during data writing, then data can be written to pixel circuit, but voltage drop varies with display pattern and pixel position causing luminance non-uniformity
Solution Approach 1:
The patent segments the voltage supply paths by providing separate wiring lines for different purposes: a first power supply wiring line for supplying high-level power supply voltage to the light-emitting element, a second power supply wiring line for supplying low-level power supply voltage, and a dedicated initialization wiring line for applying initialization voltage to the data-holding capacitor. This segmentation isolates the data writing operation from the power supply voltage drops, ensuring that voltage variations due to display patterns and pixel positions do not affect data writing accuracy or resulting luminance uniformity.
3Reliability
If initialization voltage is applied to data-holding capacitor before data writing, then stable data writing is achieved, but additional wiring line and control are required
Solution Approach 1:
The initialization transistor is designed to serve multiple functions: it controls the application of initialization voltage to the data-holding capacitor, and also functions as a switch to disconnect the power supply control transistor from the capacitor during the initialization phase. This multi-functionality reduces the need for additional dedicated components while achieving stable data writing. The initialization wiring line, while additional, is integrated into the existing pixel circuit architecture and controlled by existing control signals, thereby minimizing the increase in overall device complexity.
Data Source
AI summary
An object of the disclosure is to achieve a current-driven display device able to compensate for a variation in threshold voltage of a drive transistor without causing a variation in luminance.A pixel circuit includes a light-emitting element, a drive transistor, a power supply control transistor, a light emission control transistor, a first writing control transistor, a threshold voltage compensation transistor, a second writing control transistor, an initialization transistor, and a data-holding capacitor. During a data writing period, a scanning signal is active, an initialization voltage is supplied to a first electrode of the data-holding capacitor via the drive transistor, and a data voltage is supplied to a second electrode of the data-holding capacitor.