Poly-Silicon OLED Pixel Circuit Threshold Compensation
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Solution Overview
Problem
In organic light emitting displays, non-uniform grain size of poly-silicon thin film transistors due to non-uniform pulse amplitude during crystallization leads to varying brightness across pixels at the same gray level, affecting display uniformity.
Innovation Solution
A pixel circuit design incorporating multiple transistors and capacitors to store and combine data signals and threshold voltages, optimizing the driving current for the organic light emitting diode, with specific configurations of transistors and capacitors to ensure uniformity and improved luminance control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If poly-silicon thin film transistors are used in organic light emitting displays, then field-effect mobility is significantly higher than amorphous silicon (100 to 200 times larger), but non-uniform grain size due to non-uniform pulse amplitude during crystallization causes varying threshold voltages and mobility across pixels, resulting in non-uniform brightness
Solution Approach 1:
The patent introduces a compensation circuit that dynamically adjusts the threshold voltage parameter of the driving transistor to compensate for variations caused by non-uniform grain size. By changing the effective threshold voltage parameter through the compensation circuit, the display uniformity is improved despite manufacturing variations in grain size
Solution Approach 2:
The patent employs a feedback mechanism where the compensation circuit monitors and adjusts for threshold voltage variations in the driving transistor. This feedback loop compensates for the non-uniformity caused by grain size variations, ensuring more uniform brightness across all pixels
2Reliability
If a simple pixel circuit with one transistor and one capacitor is used, then device complexity is low, but luminance uniformity and image quality are insufficient due to threshold voltage and mobility variations
Solution Approach 1:
The patent segments the pixel circuit into multiple functional blocks: a driving transistor for current control, a compensation circuit for threshold voltage compensation, and storage capacitors for signal retention. This segmentation allows each component to perform a specific function, improving luminance uniformity while keeping the overall design systematic
Solution Approach 2:
The compensation circuit serves multiple functions: it compensates for threshold voltage variations, stores compensation signals, and adjusts for mobility differences. By making the compensation circuit multi-functional, the patent improves luminance uniformity without proportionally increasing device complexity
Data Source
AI summary
The pixel circuit of an organic light emitting display includes a first transistor to a seventh transistor, a first capacitor, a second transistor and an organic light emitting diode. The first capacitor stores the data signal from the first, second and third transistors, and the second capacitor stores the threshold voltage of the fourth transistor from the fifth transistor. Voltages stored in the first and second capacitors are combined by the sixth transistor, and the fourth transistor generates a driving current corresponding to a combined voltage of the voltages stored in the first and second capacitor. The seventh transistor transmits the driving current and the organic light emitting diode emits light corresponding to the driving current.