OLED Pixel Circuit With Threshold Sensing for Luminance Uniformity
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Solution Overview
Problem
Display devices with organic light emitting diodes (OLEDs) experience luminance deviations due to varying threshold voltages of driving transistors, leading to image quality degradation, especially in high-resolution displays where threshold voltage compensation periods are shortened.
Innovation Solution
A pixel circuit design incorporating n-channel and p-channel MOS transistors, with separate threshold voltage sensing and data writing periods, allowing for a longer threshold voltage sensing period and reduced image quality degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the resolution of the display device is increased, then the display quality is improved, but the threshold voltage compensation period is shortened, resulting in degraded image quality
Solution Approach 1:
The pixel circuit is divided into multiple functional units: a driving transistor for current control, a switching transistor for data input, a compensation transistor for threshold voltage compensation, and storage capacitors for voltage retention. This segmentation allows each unit to perform its function efficiently, enabling adequate compensation time even in high-resolution displays with shortened frame periods.
Solution Approach 2:
The compensation transistor performs threshold voltage compensation in advance during a dedicated compensation period before the emission period. By measuring and compensating the threshold voltage of the driving transistor beforehand, the circuit ensures accurate driving current regardless of transistor variations, maintaining image quality in high-resolution displays where time is constrained.
2Productivity
If the threshold voltage compensation period is shortened, then the productivity is improved, but the measurement precision of threshold voltage is reduced, leading to luminance deviations
Solution Approach 1:
The compensation transistor forms a feedback loop with the driving transistor and storage capacitor. During the compensation period, the compensation transistor measures the threshold voltage of the driving transistor by comparing voltages across the storage capacitor, and automatically adjusts the stored voltage to compensate for the threshold variation. This feedback mechanism ensures accurate compensation even with shortened compensation periods.
Solution Approach 2:
The pixel circuit performs self-compensation using its own internal components without requiring external intervention. The compensation transistor, storage capacitor, and driving transistor work together in an integrated manner where the circuit automatically measures and corrects its own threshold voltage variations, achieving both speed and accuracy in the compensation process.
Data Source
AI summary
A pixel, wherein: gates of second and fifth transistors receive a first gate signal; gates of third and fourth transistors respectively receive second and third gate signals; first terminals (FTs) of the second to fifth transistors respectively receive a data voltage, reference voltage, initialization voltage, and first power supply voltage (PSV); a second electrode of a second capacitor receives the first PSV; a second terminal (ST) of a light emitting element (LEE) receives a second PSV; a gate of a first transistor, STs of the second and third transistors, and a first electrode of a first capacitor are connected to a first node; STs of the first and fourth transistors, a FT of the LEE, and second and first electrodes respectively of the first and second capacitors are connected to a second node; and a ST of the fifth transistor is connected to a FT of the first transistor.