AMOLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Existing active-matrix organic light-emitting diode (AMOLED) displays face issues with image distortion due to variations in the threshold voltage of driving transistors over time, leading to reduced pixel aperture and display lifetime, and current-programmed pixel circuits suffer from overhead associated with low programming current levels.
Innovation Solution
A method and system for programming and driving AMOLED displays that involves a pixel circuit with a driving transistor, a switch transistor, and storage capacitors, where voltages are controlled during programming cycles to store and apply voltages independent of the threshold voltage, ensuring stable current delivery to the OLED, thereby reducing image distortion and extending display lifetime.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If voltage-programmed pixel circuits are used to provide current independent of threshold voltage, then image distortion is reduced, but device complexity increases and pixel aperture is reduced
Solution Approach 1:
The pixel circuit is segmented into two transistors (switch transistor and driving transistor) with distinct functions. The switch transistor controls programming voltage application while the driving transistor provides stable OLED current. This functional segmentation achieves threshold voltage compensation without requiring additional compensation transistors, thereby reducing circuit complexity while maintaining image stability.
Solution Approach 2:
The driving transistor serves dual functions: it acts as the current source for the OLED during the display period and simultaneously provides threshold voltage compensation during the programming period. This multi-functionality eliminates the need for separate compensation circuitry, reducing device complexity while maintaining reliable current delivery independent of threshold voltage variations.
2Duration of action of stationary object
If extra transistors are added to compensate for threshold voltage shifts, then display lifetime is extended, but pixel aperture and yield are reduced
Solution Approach 1:
The programming and driving functions are merged into a single driving transistor that operates in different modes. During programming, it receives threshold voltage compensation; during display, it drives the OLED. This merging eliminates extra transistors, maximizing pixel aperture area while extending display lifetime through threshold voltage compensation.
Solution Approach 2:
The driving transistor is designed to perform multiple functions: threshold voltage compensation during programming cycles and current driving during display cycles. This multi-functionality reduces the transistor count, increasing pixel aperture while maintaining extended display lifetime through proper voltage management.
3Reliability
If current-programmed pixel circuits are used to reduce threshold voltage sensitivity, then image distortion is reduced, but overhead from column line charging time increases
Solution Approach 1:
The pixel circuit operates with periodic programming cycles that apply compensation voltages to the driving transistor gate. This periodic action refreshes the threshold voltage compensation before each display period, ensuring current stability without requiring continuous programming current, thereby reducing column line charging overhead and programming time.
Solution Approach 2:
Threshold voltage compensation is applied in advance during brief programming cycles before the display period begins. This preliminary action pre-charges the storage capacitor and sets the driving transistor gate voltage, eliminating the need for continuous programming current during the display period and reducing overall programming time overhead.
Data Source
AI summary
Method and system for programming and driving active matrix light emitting device pixel is provided. The pixel is a voltage programmed pixel circuit, and has a light emitting device, a driving transistor and a storage capacitor. The pixel has a programming cycle having a plurality of operating cycles, and a driving cycle. During the programming cycle, the voltage of the connection between the OLED and the driving transistor is controlled so that the desired gate-source voltage of a driving transistor is stored in a storage capacitor.