OLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Conventional organic light emitting display devices face issues with non-uniform image luminance due to varying threshold voltages of driving transistors, leading to inconsistent light production across pixels.
Innovation Solution
A pixel circuit comprising four transistors (M1 to M4) and a storage capacitor, which controls the current supplied to an organic light emitting diode, compensates for the threshold voltage of the driving transistor by applying off-bias voltage and managing voltage periods, ensuring uniform luminance regardless of the transistor's threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a conventional pixel circuit is used with a driving transistor, then the pixel can control current to the organic light emitting diode, but the luminance becomes non-uniform due to varying threshold voltages of the driving transistor
Solution Approach 1:
The pixel circuit applies off-bias voltage to the driving transistor during an initialization period before the emission period. This preliminary action compensates for the threshold voltage in advance, ensuring that when the transistor drives the OLED during emission, the threshold voltage variation has already been compensated, resulting in uniform luminance across pixels despite manufacturing variations in transistor threshold voltages
2Adaptability or versatility
If the power supply voltage varies, then the pixel circuit can operate with different voltages, but the luminance uniformity deteriorates due to inconsistent voltage levels across pixels
Solution Approach 1:
The pixel circuit uses the storage capacitor to store the compensated voltage (data signal voltage minus threshold voltage) during the initialization period. This stored voltage serves as a feedback mechanism that maintains consistent drive conditions across pixels even when power supply voltages vary, ensuring luminance uniformity is preserved across different operating voltage levels
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 solution effectively compensates for the threshold voltage of the driving transistor, reducing luminance non-uniformity and ensuring consistent image brightness across pixels, even with varying power supply voltages.
Implementation Method 1
The organic light emitting diode produces light by recombining an electron and a hole
Implementation Method 2
a storage capacitor connected between the gate electrode of the first transistor and the second node
Data Source
AI summary
A pixel may include: an OLED; a first transistor having a first electrode connected to a power line receiving a first power, the first power changes to an initial voltage, a reference voltage, and a high voltage, the first transistor controlling the amount of current supplied to the OLED; a second transistor connected between a data line and a second node and turned on when a scan signal is supplied to a scan line; a third transistor connected between a gate electrode and a second electrode of the first transistor and having a turn-on time partially overlapping the turn-on time of the second transistor; a fourth transistor, connected between the second node and the power line, and having a turn-on time not overlapping the turn-on time of the second transistor; and a storage capacitor connected between the gate electrode of the first transistor and the second node.