OLED Pixel Circuits Mitigating Degeneration via Dynamic Cathode Biasing
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Solution Overview
Problem
In OLED displays, continuous forward bias leads to impurity ion migration and electric dipole movement, causing a degeneration of the electric field, reduced driving current, and decreased brightness, which existing pixel circuits fail to effectively address.
Innovation Solution
The proposed pixel circuits employ a dynamic power signal configuration, where the second power signal at the cathode varies relative to the first power signal at the anode, allowing for reverse biasing of light emitting elements, reducing degeneration, and improving contrast ratio by modifying the electrical fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If continuous forward bias is applied to OLEDs, then light emission is maintained, but impurity ion migration and electric dipole movement cause degeneration
Solution Approach 1:
The patent applies periodic reverse bias pulses to the OLEDs during non-display periods (e.g., between frames or between refresh cycles). This periodic action temporarily reverses the electric field direction to migrate impurity ions and electric dipoles back to their original positions, counteracting the degradation caused by continuous forward bias while maintaining normal light emission during display periods.
2Reliability
If reverse bias is applied to mitigate degradation, then OLED longevity is improved, but additional circuit complexity is required
Solution Approach 1:
The patent integrates the reverse bias function into the existing pixel circuit structure by sharing common components (transistors, capacitors, and power signal lines) between forward bias and reverse bias operations. The same pixel driver circuitry that controls display signals is repurposed to generate and apply reverse bias pulses, eliminating the need for separate dedicated reverse bias circuitry and minimizing additional complexity.
3Illumination intensity
If dynamic power signal configuration is used, then contrast ratio is enhanced, but power signal control complexity increases
Solution Approach 1:
The patent implements dynamic power signal configuration by adjusting the voltage levels of power signals (e.g., ELVDD and ELVSS) based on the display content and operating conditions. During high-contrast scenes, the power signals are dynamically adjusted to enhance the voltage swing across the OLED, improving contrast ratio. This dynamic adjustment is controlled by the pixel driver circuitry that already manages display signals, integrating the control function without requiring separate dedicated control circuits.
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 effectively reduces or alleviates degeneration in OLEDs, enhances the contrast ratio, and prevents floating anode issues by reversely biasing light emitting elements, while minimizing energy consumption by filtering noise only during forward bias.
Implementation Method 1
when a voltage is applied on the anode and cathode of an OLED, holes injected from the anode and electrons injected from the cathode are combined in a light emitting layer of the OLED, emitting light
Implementation Method 2
These changes in the OLEDs can form an electric field of the opposite direction than that formed by the voltage. The driving current and brightness of the OLEDs can decrease, causing degeneration in the OLEDs
Data Source
AI summary
Embodiments of pixel circuits for light emitting elements are disclosed herein. In one example, a pixel circuit includes a pixel driver configured to receive a data signal and drive a light emitting element based on the data signal, a first power signal coupled to an anode of the light emitting element through the pixel driver, and a second power signal coupled to a cathode of the light emitting element. In some embodiments, the second power signal is configured to vary based on the data signal. In some embodiments, in a first period of the data signal, a value of the second power signal is configured to be lower than a value of first power signal, and in a second period of the data signal, the value of the second power signal is configured to be higher than the value of the first power signal.