Current-Driven Pixel Circuit for OLED Threshold Voltage Compensation
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Solution Overview
Problem
Existing electric current driving type display devices face challenges in properly compensating for variations in threshold voltage of drive elements, leading to unwanted light emission and reduced contrast in organic EL displays.
Innovation Solution
The implementation of a pixel circuit configuration that includes a drive element, capacitors, and switching elements, where specific switching element control states and potential applications ensure the drive element is set to a conduction state regardless of previous states, allowing for precise compensation of threshold voltage variations and prevention of unwanted light emission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a voltage control type drive scheme is adopted in an organic EL display, then the circuit design is simpler, but it is very difficult to suppress variations in the luminance of the organic EL element
Solution Approach 1:
The patent changes the control parameter from voltage to current. The organic EL element is driven by controlling the current flowing through it rather than the voltage applied, since luminance is substantially proportional to current and this proportional relationship is less susceptible to external factors such as ambient temperature, thereby suppressing luminance variations.
2Reliability
If an electric current program scheme is used to compensate for TFT variations, then threshold voltage and mobility variations are compensated, but it is difficult to design pixel circuits and drive circuits due to handling very small currents
Solution Approach 1:
The patent changes the control parameter from current to voltage. The voltage program scheme is adopted where the amount of current is controlled by a voltage signal instead of directly handling small currents. This makes circuit design simpler while still achieving compensation for threshold voltage variations, and mobility variations can be suppressed to a certain extent in TFT fabrication process.
3Extent of automation
If conventional pixel circuits are used for electric current driving, then the drive scheme is implemented, but variations in threshold voltage of drive elements are not properly compensated, leading to unwanted light emission
Solution Approach 1:
The patent applies preliminary action by setting the drive element to a conduction state before the compensation operation. A potential that brings the drive element into a conduction state is applied to a predetermined power supply wiring line, and a switching element is controlled to a conduction state to provide this potential to the control terminal of the drive element. This ensures the drive element is in a known state regardless of previous pixel circuit states, enabling accurate threshold voltage compensation.
Solution Approach 2:
The patent uses an intermediary potential (a potential that brings the drive element into conduction state) applied through a switching element. This intermediary mechanism ensures the drive element is properly initialized to a conduction state before compensation, acting as a mediator between the power supply and the drive element control terminal.
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 compensates for threshold voltage variations, enhances display quality by preventing unwanted light emission, and extends the lifetime of organic EL elements, improving contrast and overall performance.
Implementation Method 1
An organic EL element included in an organic EL display emits light with higher luminance as the voltage to be applied thereto is higher and the amount of current flowing therethrough is larger
Data Source
AI summary
In a pixel circuit 100, a driving TFT 110, a switching TFT 115, and an organic EL element 130 are provided between a power supply wiring line Vp and a common cathode Vcom and a capacitor 120 and a switching TFT 111 are provided between a gate terminal of the driving TFT 110 and a data line Sj. A switching TFT 112 is provided between a connection point B between the capacitor 120 and the switching TFT 111 and the power supply wiring line Vp, a switching TFT 113 is provided between the gate and drain terminals of the driving TFT 110, and a switching TFT 114 is provided between the gate terminal of the driving TFT 110 and a reference supply wiring line Vs. A potential that brings the driving TFT 110 into a conduction state is applied to the reference supply wiring line Vs. Thus, variations in the threshold voltage of a drive element can be properly compensated for and unwanted light emission from an electro-optical element can be prevented.