Pixel Circuit Sampling Transistor for OLED Threshold Compensation
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Solution Overview
Problem
Existing active matrix OLED displays face issues with variations in drive current due to inconsistencies in threshold voltage, leading to reduced display quality and increased complexity in correcting these variations, particularly with large capacitance requirements and complex scanning line structures.
Innovation Solution
The proposed solution involves a pixel circuit with a sampling transistor, driving transistor, and switching transistors, along with capacitances to manage signal and threshold voltages, using a specific arrangement of scanning lines and drive circuits to control current flow and voltage sampling, thereby simplifying the structure and reducing surface area while stabilizing drive current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large capacitance is used to prevent threshold voltage draining, then threshold voltage stability is improved, but the surface area of the pixel circuit increases and defect incidence rises
Solution Approach 1:
The patent applies preliminary action by performing threshold voltage sampling and compensation before the actual display signal is written to the pixel. The sampling transistor samples the threshold voltage of the driving transistor during a dedicated sampling period, and this sampled voltage is stored in a capacitance. This pre-sampled threshold voltage is then used to generate a compensation signal that corrects for threshold voltage variations before the display signal is applied, thereby maintaining threshold voltage stability without requiring excessively large capacitance values in the pixel circuit.
2Reliability
If multiple scanning lines are added for threshold voltage correction and signal sampling, then threshold voltage control is improved, but the structural complexity increases and defect incidence rises
Solution Approach 1:
The patent applies multi-functionality by designing the sampling transistor to serve multiple purposes: it samples both the threshold voltage of the driving transistor and the display signal voltage using the same transistor and capacitance structure. The sampling transistor is controlled by a sampling signal that enables it to selectively connect the gate of the driving transistor to either the threshold voltage source or the display signal line, thereby eliminating the need for separate dedicated scanning lines for threshold voltage correction and signal sampling.
3Measurement precision
If the sampling transistor is made conductive for signal voltage sampling, then signal voltage is successfully sampled, but the held threshold voltage drains away due to the driving transistor being in ON state
Solution Approach 1:
The patent applies periodic action by dividing the operation into distinct time periods: a sampling period and a display period. During the sampling period, the sampling transistor is made conductive to sample the threshold voltage while the driving transistor is kept in an OFF state by controlling its gate voltage, preventing threshold voltage draining. During the display period, the sampling transistor is made non-conductive to retain the sampled threshold voltage in the capacitance, while the driving transistor is turned ON to drive the display signal. This periodic switching ensures both accurate sampling and reliable threshold voltage retention.
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 configuration effectively suppresses variations in drive current, maintains display quality, and simplifies the structural elements, reducing the risk of defects and power consumption while allowing for stable operation.
Implementation Method 1
a first capacitance for holding a sampled signal voltage and a threshold voltage of the driving transistor across a gate electrode and a source electrode of the driving transistor, during a light emission period of the light emitting element
Implementation Method 2
each pixel including a light emitting element for emitting light as a result of current flow
Data Source
AI summary
To efficiently compensate a threshold value of a driving transistor. In a state where a first switching transistor is non-conductive and a second switching transistor in conductive, a sampling transistor is made conductive and a reference voltage is supplied from a signal line to write a threshold voltage of a driving transistor to a first capacitance. After that, in a state where first and second switching transistors and are non-conductive, the sampling transistor is made conductive and a signal voltage from the signal line is written to the first capacitance. Further, after that the sampling transistor is put into a non-conductive state, and the first and second switching transistors are put in a conductive state, to drive the driving transistor and supply current to a light emitting element.