OLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
The threshold voltage shift in driving TFTs of organic light emitting diode (OLED) displays can lead to variations in drain-source current and luminance across pixels, even when the same data voltage is applied, due to degradation of the TFTs, making it challenging to maintain consistent image display.
Innovation Solution
The OLED display device incorporates a pixel structure with a driving TFT, a sensing TFT, capacitors, and a reference voltage switching circuit that allows for internal and external compensation of the threshold voltage and electron mobility, enabling real-time sensing and adjustment of the threshold voltage within each pixel, thereby ensuring consistent current supply to the organic light emitting diode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a diode-coupled threshold voltage compensation structure is used, then the threshold voltage can be sensed during the sensing period, but the threshold voltage cannot be sensed when it shifts to negative voltage
Solution Approach 1:
The patent introduces a dual-mode sensing mechanism that dynamically switches between diode coupling and transistor coupling modes. The sensing TFT can operate in different coupling configurations depending on the threshold voltage state, enabling adaptation to both positive and negative threshold voltage shifts. This dynamic reconfiguration allows the system to maintain sensing capability across varying threshold voltage conditions.
Solution Approach 2:
The patent changes the coupling parameter between the sensing TFT and driving TFT from fixed diode coupling to variable transistor coupling. By controlling the gate voltage of the sensing TFT, the coupling strength can be adjusted, enabling the system to sense threshold voltages in different ranges including negative values. This parameter change allows flexible adaptation to different threshold voltage states.
2Ease of operation
If the gate node and drain node are coupled during sensing period, then the threshold voltage can be sensed, but the sensing fails when threshold voltage is lower than 0 V
Solution Approach 1:
The patent introduces an intermediate coupling mechanism through the sensing TFT that mediates between the gate node and drain node. Instead of direct coupling, the sensing TFT acts as an intermediary that can be controlled to enable or disable the coupling path. This intermediary approach allows for more flexible and reliable sensing operation, particularly when threshold voltage shifts to negative values.
Solution Approach 2:
The coupling between gate node and drain node is made dynamic through the controlled operation of the sensing TFT. The coupling is not fixed but can be adjusted based on the sensing requirements and threshold voltage state. This dynamic coupling enables reliable sensing operation across different threshold voltage conditions while maintaining operational simplicity.
3Ease of manufacture
If oxide semiconductor layer is used for driving TFT, then the device can be manufactured with specific properties, but negative threshold voltage shift occurs
Solution Approach 1:
The patent implements a feedback-based threshold voltage sensing and compensation mechanism. The sensing TFT continuously monitors the threshold voltage of the driving TFT and provides feedback information. This feedback enables real-time detection of threshold voltage shifts including negative shifts, allowing the system to compensate for manufacturing variations and maintain stable operation despite using oxide semiconductor materials.
Data Source
AI summary
An organic light emitting diode display device comprises: a display panel having a plurality of pixels, each of the pixels comprising: a driving TFT comprising a gate electrode coupled to a first node, a source electrode coupled to a second node, and a drain electrode coupled to a high-potential voltage source; an organic light emitting diode comprising an anode coupled to the second node and a cathode coupled to a low-potential voltage source; a first TFT in response to a scan signal having a first logic level voltage to connect the first node to a data line; a second TFT in response to an emission signal having the first logic level voltage to connect the second node to the third node; a first capacitor coupled between the first node and the third node; and a second capacitor coupled between the third node and a reference voltage source.