OLED Pixel Circuit with Dual Gate Electrodes for Threshold Compensation
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Solution Overview
Problem
In organic light emitting display devices, non-uniform threshold voltages of thin film transistors across pixels lead to reliability issues and picture quality degradation due to manufacturing variations and time-dependent shifts.
Innovation Solution
The implementation of a pixel structure with first and second gate electrodes overlapping a semiconductor layer, along with capacitors to store threshold voltages and difference voltages, allows for compensation of driving transistor characteristics, enhancing reliability and picture quality by adjusting the driving transistor's operation through stored voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional pixel structure with single gate electrode is used, then the device structure is simple, but the threshold voltage non-uniformity across pixels deteriorates reliability and picture quality
Solution Approach 1:
The driving transistor gate is segmented into first and second gate electrodes that can be independently controlled. This segmentation allows separate sensing and driving operations, enabling threshold voltage compensation while maintaining a manageable structure within the pixel
Solution Approach 2:
The sensing transistor measures the threshold voltage of the driving transistor before the actual driving operation. This preliminary sensing action allows the system to compensate for threshold voltage variations in advance, improving reliability without requiring complex real-time adjustment mechanisms
2Manufacturing precision
If manufacturing processes are simplified, then production cost decreases, but threshold voltage non-uniformity across pixels increases
Solution Approach 1:
The display device performs self-diagnosis and self-compensation through the sensing transistor and capacitor mechanism. Each pixel autonomously measures and compensates for its own threshold voltage variations, eliminating the need for external calibration processes or higher precision manufacturing equipment
Solution Approach 2:
The sensing transistor and storage capacitor create a feedback mechanism that measures threshold voltage variations and compensates for them. This feedback loop automatically corrects manufacturing variations, allowing standard manufacturing processes to produce uniform display performance
3Measurement precision
If threshold voltage compensation is implemented, then picture quality improves, but pixel circuit complexity increases
Solution Approach 1:
The sensing transistor, storage capacitor, and driving transistor are integrated within a single pixel circuit. This merging of functions into a compact arrangement achieves threshold voltage compensation and high picture quality without requiring additional external circuitry or increasing overall pixel complexity significantly
Data Source
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Figure 4A
AI summary
An organic light emitting display device includes a display panel having a plurality of pixels (P), each pixel (P) connected to a data line (DL), a gate line group (GLG) and a reference line (RL), each pixel (P) further including: an organic light emitting device (OLED); a driving transistor (Tdr) controlling a current flowing in the organic light emitting device (OLED) and including first and second gate electrodes (gl, g2) overlapped with each other, with a semiconductor layer (14) provided therebetween; a first switching transistor (Tsw1) selectively supplying a data voltage supplied to the data line (DL) to a first node (n1) connected to the first gate electrode (g1); a second switching transistor (Tsw2) selectively supplying a sensing voltage to the second gate electrode (g2); a third switching transistor (Tsw3) selectively connecting a second node (n2) connected to a source electrode (s) of the driving transistor (Tdr) to the first node (n1); a fourth switching transistor (Tsw4) selectively connecting the reference line (RL) to the second node (n2); a first capacitor (C1) connected between the second gate electrode (g2) and the second node (n2), the first capacitor (C1) storing a threshold voltage of the driving transistor (Tdr); and a second capacitor (C2) connected between the first and second nodes (nl, n2), the second capacitor (C2) storing a difference voltage of the first and second nodes (nl, n2).