OLED Pixel Circuit Threshold Voltage Compensation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The threshold voltage shift in driving TFTs of organic light emitting diode (OLED) display devices causes variations in drain-source current and luminance across pixels, even when the same data voltage is applied, due to degradation of the TFTs, leading to inconsistent light emission.
Innovation Solution
The OLED display device incorporates a pixel structure with specific TFT configurations and capacitors that allow for internal and external compensation of the threshold voltage, enabling real-time sensing and adjustment of the threshold voltage and electron mobility, thereby stabilizing the current supplied to the OLEDs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a diode-connected 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 a negative voltage because the gate-source voltage difference cannot reach the threshold voltage
Solution Approach 1:
The patent inverts the conventional sensing approach by applying a negative voltage to the source node instead of relying on the gate node to reach the threshold voltage. By making the source node voltage lower than the threshold voltage during the sensing period, the gate-source voltage difference exceeds the threshold voltage even when the threshold voltage has shifted negatively, enabling successful threshold voltage sensing across all threshold voltage conditions.
2Measurement precision
If the gate node and drain node are coupled during the sensing period, then the threshold voltage sensing can be performed, but the sensing fails when the threshold voltage is lower than 0 V because the gate node voltage cannot go below 0 V
Solution Approach 1:
The patent introduces a third node as an intermediary between the source node and the voltage source. This third node allows the source node voltage to be pulled below 0 V through capacitive coupling during the sensing period, enabling the gate-source voltage difference to exceed the threshold voltage even when the threshold voltage has shifted negatively, thus resolving the sensing failure issue.
3Ease of operation
If the same data voltage is supplied to each pixel, then the data transmission is simplified, but the luminance differs across pixels due to threshold voltage variations
Solution Approach 1:
The patent implements threshold voltage compensation by sensing the actual threshold voltage of each pixel's driving TFT and using this information to adjust the gate node voltage. This feedback mechanism ensures that each pixel receives the appropriate compensation voltage to maintain consistent luminance output even when the same data voltage is supplied to all pixels, thereby resolving the luminance inconsistency issue.
Data Source
AI summary
An organic light emitting diode display device comprises: a plurality of pixels, wherein each of the pixels comprising: a driving TFT including 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 including an anode coupled to the second node and a cathode coupled to a low-potential voltage source; a first TFT in response to a first scan signal to connect the first node to a data line; a second TFT in response to a second scan signal to connect the first node to a first reference voltage source; a third TFT in response to an emission signal to connect the second node to the third node; and capacitors.