OLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Conventional active matrix organic electro-luminescent display devices face issues with image quality due to continuous rise in threshold voltage of NMOS transistors, leading to reduced current supply to OLEDs and subsequent degradation in brightness.
Innovation Solution
The proposed solution involves storing and offsetting the threshold voltage of the switching device during the write period, using a data voltage and a ramp voltage in the display period to control the light emission of the OLED, thereby maintaining reliability and reducing the impact of threshold voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a passive matrix driving system is used, then device complexity is reduced, but reliability deteriorates due to threshold voltage variations
Solution Approach 1:
The pixel circuit is divided into multiple transistors (first NMOS transistor for data writing, second NMOS transistor for light emission control) and a capacitor, separating the functions of data storage and light emission control to improve reliability while maintaining manageable complexity
Solution Approach 2:
The capacitor stores the data voltage in advance during the write period before the display period begins, preparing the circuit for subsequent light emission control and compensating for threshold voltage variations before they affect display quality
2Duration of action of stationary object
If the threshold voltage of NMOS transistors increases continuously, then device operation continues, but brightness and image quality deteriorate
Solution Approach 1:
The capacitor provides feedback by maintaining the data voltage level despite threshold voltage drift, creating a closed-loop effect that compensates for transistor degradation and maintains consistent brightness over the device lifetime
Solution Approach 2:
The circuit changes the operating parameters by using voltage storage and ramp voltage generation to adapt to threshold voltage variations, maintaining proper current drive to the OLED even as transistor characteristics change over time
3Ease of operation
If a ramp voltage is used in the display period, then light emission time control is improved, but device complexity increases
Solution Approach 1:
The circuit generates a dynamic ramp voltage that changes over time during the display period, enabling precise control of light emission duration and intensity without requiring complex external control circuits
Solution Approach 2:
The pixel circuit generates its own ramp voltage internally using the stored data voltage and transistor characteristics, eliminating the need for external voltage control circuits and reducing overall system complexity while maintaining precise light emission 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
This approach ensures consistent image quality by minimizing the effect of threshold voltage variations, allowing for high reliability and efficient light emission control, while also reducing manufacturing costs and pixel area.
Implementation Method 1
The organic electro-luminescent display device emits light by itself. This electro-luminescent display displays a video image by electrically exciting fluorescent material using carriers such as electrons and holes.
Data Source
AI summary
An organic electro-luminescent display device according to an embodiment includes a light emitting device in a pixel to emit light in response to a current applied thereto; a data line for providing a data voltage in a write period and a ramp voltage in a display period; and a first switching device connected to the light emitting device, the first switching device being selectively turned on depending on a voltage difference between the ramp voltage and the data voltage so as to drive the light emitting device.