OLED Pixel Circuit Leakage Current Reduction
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Solution Overview
Problem
Organic light emitting displays face challenges in maintaining desired brightness due to increased power consumption and reduced aperture ratios, primarily caused by leakage current issues in pixel circuits.
Innovation Solution
A pixel circuit design that minimizes leakage current by using a specific configuration of transistors, including a fifth transistor formed by serially coupling multiple transistors, which reduces the number of transistors in the leakage current path and ensures only one current leakage path from the gate electrode, thereby minimizing leakage current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If conventional pixel circuit designs are used to control OLED current, then the display can operate, but leakage current increases causing reduced brightness and increased power consumption
Solution Approach 1:
The pixel circuit is divided into multiple functional blocks with dedicated transistors for specific functions: driving transistor (M1) for OLED current control, switching transistors (M2, M3) for signal routing, compensation transistors (M4, M5) for threshold voltage compensation, and emission control transistors (M6, M7) for timing control. This segmentation allows each transistor to be optimized for its specific function, minimizing overall leakage current while maintaining brightness control.
Solution Approach 2:
The compensation transistors (M4, M5) and storage capacitor (Cst) are configured to pre-compensate for threshold voltage variations and leakage currents before they affect the OLED driving current. The emission control transistors (M6, M7) are positioned to control the timing of current flow to the OLED, preventing leakage during non-emission periods. This preliminary action reduces the impact of leakage current on brightness and power consumption.
2Loss of energy
If more transistors are added to the pixel circuit to control leakage current, then leakage current decreases, but device complexity increases
Solution Approach 1:
Several transistors in the circuit serve multiple functions: the storage capacitor Cst both stores the driving voltage and provides a reference for compensation; the emission control transistors M6 and M7 both control timing and prevent leakage paths; the compensation transistors M4 and M5 work together to compensate for threshold voltage variations. This multi-functionality reduces the need for additional dedicated transistors, controlling complexity while effectively managing leakage current.
3Loss of energy
If the aperture ratio is reduced to accommodate more transistors, then leakage current control improves, but the area available for light emission decreases
Solution Approach 1:
The compensation and storage functions are merged into a shared circuit block using transistors M4, M5, and capacitor Cst, which serves both compensation and voltage storage purposes. The emission control functions are merged by using transistors M6 and M7 to simultaneously control timing and block leakage paths. This merging reduces the total transistor count and associated area, maintaining a high aperture ratio while achieving effective leakage current control.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design effectively reduces leakage current and maintains desired brightness while minimizing the complexity of the pixel circuit, thereby improving the aperture ratio and power efficiency of the organic light emitting display.
Implementation Method 1
the organic light emitting display displays images using organic light emitting diodes (OLEDs) that generate light by re-combination of electrons and holes
Data Source
AI summary
A pixel capable of reducing leakage current (to display an image with desired brightness) is provided. The pixel includes: an organic light emitting diode (OLED) coupled to a second power source; a first transistor for controlling an amount of current that flows from a first power source to the second power source via the OLED; a second transistor coupled between a data line and the first transistor, and configured to turn on when a scan signal is supplied to a scan line; a third transistor and a fourth transistor serially coupled between the first transistor and an initializing power source; and a fifth transistor coupled between a first node coupled to a gate electrode of the first transistor, and a second node that is a common node between the third transistor and the fourth transistor, and configured to turn off in a period where current is supplied to the OLED.