Pixel Circuit Threshold Voltage Compensation for OLED Luminance Uniformity
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Solution Overview
Problem
In organic light-emitting display devices, differences in electrical characteristics of driving elements across pixels can lead to luminance unevenness, especially at low luminance, due to process deviations during manufacturing.
Innovation Solution
A pixel circuit is designed with an internal compensation circuit that compensates the threshold voltage of the driving element in real time by using a second capacitor to store the threshold voltage and a first capacitor to store the data voltage, thereby improving luminance uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If an internal compensation circuit is added to compensate for threshold voltage differences, then manufacturing precision is improved, but device complexity increases
Solution Approach 1:
The pixel circuit is divided into distinct functional modules: a driving element for current control, a first capacitor for data voltage storage, a second capacitor for threshold voltage storage, and switch elements for timing control. This segmentation allows each component to perform its specific function efficiently, enabling threshold voltage compensation without excessive overall complexity
Solution Approach 2:
The compensation operation is performed in advance during a dedicated time period before the light emitting period. The switch elements transfer the threshold voltage to the second capacitor during the initialization or data writing phase, so that compensation is already in place before luminance display begins, ensuring uniformity without interfering with the main display function
2Device complexity
If threshold voltage compensation is performed during the same time period as data writing, then device complexity is reduced, but measurement precision deteriorates
Solution Approach 1:
The pixel circuit operation is divided into distinct time periods: an initialization period for threshold voltage sampling, a data writing period, and a light emitting period. During the initialization period, switch elements are controlled to transfer threshold voltage to the second capacitor. During the data writing period, data voltage is written to the first capacitor. This periodic separation ensures that threshold voltage measurement and data writing do not interfere with each other, maintaining measurement precision while using a relatively simple circuit structure
3Productivity
If the pixel circuit operates at high driving speeds, then productivity is improved, but luminance uniformity deteriorates
Solution Approach 1:
The compensation voltage stored in the second capacitor is continuously maintained throughout the light emitting period, ensuring that threshold voltage differences are compensated for the entire duration of each frame. This continuous compensation action maintains luminance uniformity even at high refresh rates where the time available for each operation is reduced
Solution Approach 2:
The threshold voltage is sampled and stored in the second capacitor in advance during the initialization period, before the high-speed data writing and light emitting phases begin. This preliminary compensation setup ensures that the compensation is already in place to handle the high driving speed requirements without compromising luminance uniformity
Data Source
AI summary
Disclosed is a pixel circuit and a display device including the pixel circuit. The pixel circuit includes a driving element connected to a first node, a second node, and a third node; a first capacitor connected between the second node and a fourth node; a second capacitor connected between the third node and the fourth node; a light emitting element; a first switch element supplying an initialization voltage to the fourth node; a second switch element supplying the initialization voltage to the second node; a third switch element supplying a data voltage to the second node; and a fourth switch element supplying a reference voltage to the third node or to a fifth node connected to an anode electrode of the light emitting element.