OLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Organic light emitting display devices experience non-uniformity in brightness due to variations in threshold voltage of driving transistors, especially when driven at low frequencies, leading to increased brightness over time with low grayscale signals and reduced brightness with high grayscale signals.
Innovation Solution
A pixel design incorporating multiple transistors and a storage capacitor to control current flow to the OLED, ensuring that the voltage of the common node does not affect the gate electrode of the driving transistor, thereby maintaining uniform brightness by overlapping turn-on periods of transistors and using a storage capacitor to stabilize voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the driving transistor is diode connected to compensate for threshold voltage deviation, then manufacturing precision is improved, but brightness uniformity deteriorates due to current leakage variations
Solution Approach 1:
The pixel circuit is divided into multiple independent transistor components (first transistor for current control, second transistor for node voltage control, third transistor for gate electrode control) rather than using a single diode-connected driving transistor. This segmentation allows each transistor to perform a specific function independently, preventing the brightness uniformity issues caused by current leakage in the conventional diode-connected approach.
Solution Approach 2:
A common node is introduced as an intermediary between the power source and the OLED, with dedicated control transistors (second and third transistors) that mediate the voltage and current flow. This intermediary structure isolates the gate electrode of the first transistor from direct voltage fluctuations at the common node, preventing brightness non-uniformity while maintaining threshold voltage compensation capabilities.
2Use of energy by moving object
If the pixel is driven at low frequency to reduce power consumption, then energy efficiency is improved, but brightness uniformity deteriorates due to increased leakage current effects
Solution Approach 1:
The second transistor is configured to maintain a continuous turn-on state during the emission period, ensuring continuous control of the common node voltage. This continuous action prevents brightness non-uniformity even at low driving frequencies by continuously counteracting leakage current effects, while the overall low frequency operation maintains energy efficiency.
Solution Approach 2:
The pixel circuit uses its own internal transistors (second and third transistors) to automatically compensate for and control leakage current effects without requiring external intervention or additional power consumption. The circuit self-regulates the voltage at the common node to maintain brightness uniformity during low-frequency operation.
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 ensures uniform brightness across the display by preventing leakage current variations, which are typically caused by voltage changes at the common node, thus maintaining consistent image quality even at low frequencies.
Implementation Method 1
the organic light emitting display device displays an image by using organic light emitting diodes (OLEDs) that generate light components by re-combination of electrons and holes
Implementation Method 2
a storage capacitor connected between the gate electrode of the first transistor and the first power source, and configured to store the data signal
Data Source
AI summary
There is provided a pixel including an organic light emitting diode (OLED), a first transistor having a first electrode connected to a first power source and configured to control an amount of current supplied from the first power source to the OLED in response to a data signal, a second transistor and a third transistor connected between a second electrode and a gate electrode of the first transistor, and a fourth transistor connected between an initializing power source and a first node that is a common node of the second transistor and the third transistor.