OLED Drive Transistor Threshold Voltage Stabilization
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Solution Overview
Problem
Conventional organic light emitting displays experience degradation of drive transistors and non-uniform brightness due to threshold voltage changes, leading to decreased current flow and overall brightness, especially over time.
Innovation Solution
The display divides the image display period into two phases: one for emitting light with a positive or negative voltage applied to the drive transistor, and another for turning off the OLED and negatively annealing the transistor, with a capacitive element storing and compensating the threshold voltage to maintain uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a drive transistor is used to maintain voltage in the pixel circuit, then the organic light emitting display can maintain brightness, but the threshold voltage of the transistor increases over time due to voltage application causing brightness degradation
Solution Approach 1:
The patent applies periodic action by alternating between a first period where positive voltage is applied to the control electrode to drive the OLED, and a second period where negative voltage is applied to anneal the transistor. This periodic switching prevents continuous voltage stress on the transistor, thereby stabilizing the threshold voltage while maintaining display brightness.
Solution Approach 2:
The patent converts the harmful effect of voltage application (which causes threshold voltage increase) into a beneficial annealing process. By applying negative voltage in the second period, the transistor structure is annealed, which actually improves its performance and stabilizes the threshold voltage, turning the potential harm into a benefit.
2Productivity
If data voltage is applied to each pixel circuit, then the display can show images, but the threshold voltage degradation becomes non-uniform across different pixels causing brightness non-uniformity
Solution Approach 1:
The patent applies local quality by tailoring the annealing process to each pixel's specific needs. Each pixel receives the same periodic voltage structure, but the actual annealing effect is localized to each pixel's transistor based on its individual threshold voltage characteristics. This ensures uniform brightness across the display while maintaining image display capability.
3Duration of action of moving object
If continuous voltage is applied to the control electrode, then the OLED remains on, but the transistor degrades faster and brightness decreases over time
Solution Approach 1:
The patent divides the display cycle into two periods: the first period when the OLED should be on and the second period for annealing. This periodic action allows the transistor to recover during the second period, extending its lifetime while maintaining the required display on-time during the first period.
Solution Approach 2:
The patent performs preliminary annealing action during the second period before the next display cycle begins. This preliminary action prepares the transistor for the upcoming voltage application, preventing degradation during the next first period and extending the overall transistor lifetime.
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
This approach minimizes threshold voltage changes, enhances brightness uniformity, prevents motion blur, and achieves a high contrast ratio by controlling voltage phases and using capacitive elements to stabilize the drive transistor.
Implementation Method 1
a capacitive element electrically coupled to the drive transistor and storing a voltage corresponding to a threshold voltage of the drive transistor
Implementation Method 2
an organic light emitting display device emitting light by electrically exciting fluorescent or phosphorescent materials
Implementation Method 3
applying a negative (or positive) voltage opposite to the voltage applied to the control electrode of the drive transistor in the first period to turn off the organic light emitting diode, and simultaneously, negatively annealing the drive transistor
Data Source
AI summary
An organic light emitting display that can minimize degradation of a drive transistor comprising a first switching element whose control electrode is electrically coupled to a scan line, being electrically coupled between a data line and a first voltage line for transmitting a data signal; a drive transistor whose control electrode is electrically coupled to the first switching element, being electrically coupled between the first and second voltage lines; an organic light emitting diode electrically coupled to the drive transistor, displaying an image by a current supplied through the drive transistor; a first capacitive element electrically coupled between the control electrode of the drive transistor and the first switching element; a second capacitive element electrically coupled between the first capacitive element and the second voltage line; a second switching element electrically coupled between the first voltage line and the control electrode of the drive transistor; a third switching element electrically coupled between the first switching element and the drive transistor; a fourth switching element electrically coupled between the control electrode of the drive transistor and the second voltage line; and a fifth switching element electrically coupled between the drive transistor and the second voltage line.