Organic EL Pixel Circuit Threshold Compensation
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Solution Overview
Problem
In active matrix organic electroluminescence (EL) display devices, variations in threshold voltages of driver transistors across the display panel lead to inconsistent brightness, degrading display quality due to the difficulty in matching transistor characteristics, and existing compensation circuits reduce the aperture ratio and require changes in peripheral driving circuits.
Innovation Solution
A pixel circuit design that uses a short-circuiting transistor to set the voltage on the driver transistor's control terminal corresponding to the data voltage and threshold voltage, with a potential controlling transistor connected to an emission set line, ensuring stable voltage settings and independent drive current supply to the organic EL element, regardless of threshold voltage variations, employing n-channel transistors for superior characteristics and amorphous silicon active layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a circuit for compensating for threshold value variation is added, then the effects of variation in threshold voltages among driver transistors can be prevented, but the number of elements in the pixel circuit is increased and the aperture ratio is reduced
Solution Approach 1:
The patent merges the threshold voltage compensation function into the existing pixel circuit operations by using the short-circuiting transistor to connect the control terminal to the data line during the writing period. This integration allows compensation without adding separate dedicated compensation circuits, thereby maintaining a high aperture ratio while achieving consistent display quality across pixels with varying transistor thresholds.
2Reliability
If a circuit for compensating for threshold value variation is added, then the effects of variation in threshold voltages among driver transistors can be prevented, but the peripheral circuit for driving the pixel circuit must be changed
Solution Approach 1:
The short-circuiting transistor serves multiple functions: it acts as a switch for threshold voltage compensation during the writing period, and also functions as part of the normal pixel selection and driving operations. This multi-functionality allows the peripheral driving circuit to remain unchanged while achieving compensation, as the same circuit infrastructure supports both compensation and standard display operations.
3Device complexity
If the control terminal of the driver transistor is directly connected to the data line, then the circuit is simple, but the voltage on the control terminal is affected by threshold voltage variations
Solution Approach 1:
The patent introduces dynamic control of the connection between the control terminal and data line through the short-circuiting transistor. During the writing period, the transistor connects them to allow threshold compensation. During the light emission period, the transistor disconnects them to maintain stable voltage. This dynamic switching enables the circuit to achieve both simplicity and voltage stability by adapting the connection state to operational requirements.
Data Source
AI summary
A potential on a control terminal of a driver transistor is controlled and a drive current corresponding to the potential is supplied to an organic electroluminescence element. A drive controlling transistor is inserted between the driver transistor and the organic electroluminescence element and the drive current is switched on and off by the drive controlling transistor. A short-circuiting transistor is provided which controls whether or not the driver transistor is to be diode-connected. A selection transistor controls whether or not a data signal from a data line is to be supplied to the control terminal of the driver transistor. A capacitor is placed between the selection transistor and the control terminal of the driver transistor and a connection between a terminal of the capacitor on the side of the selection transistor and a predetermined power supply is switched on and off by a potential controlling transistor.