TFT Current Determining Unit for OLED Luminance Stability
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Solution Overview
Problem
Conventional image display apparatuses using organic LEDs face challenges in maintaining consistent current flow due to threshold voltage fluctuations in driver elements, leading to luminance instability over time, particularly due to the long time required for voltage writing in capacitors, which is exacerbated by parasitic capacitance and physical structure differences in channel forming areas.
Innovation Solution
The image display apparatus incorporates a current determining unit that controls current flow based on a voltage applied during the writing phase, using a capacitor between the gate and source of a thin film transistor, and operates the current determining unit in the saturation region to minimize threshold voltage fluctuations, along with a reverse voltage applying step to stabilize IV characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a conventional current source is used to supply current to the driver element, then the current can be supplied continuously, but the writing time becomes excessively long due to parasitic capacitance
Solution Approach 1:
The patent changes the operating parameters of the driver element by controlling it to operate in the saturation region rather than the linear region. This parameter change enables the driver element to function as a current source with much faster response characteristics, reducing the writing time from milliseconds to microseconds while maintaining accurate current control for the organic LED
Solution Approach 2:
The patent introduces dynamic control of the driver element's operating region. By dynamically switching between different operating modes (saturation region for fast writing, other regions for steady-state operation), the system achieves both fast writing speed and stable current control, resolving the contradiction between speed and accuracy
2Reliability
If the driver element operates in the linear region, then the circuit is simple, but threshold voltage fluctuations cause current instability and luminance deterioration
Solution Approach 1:
The patent changes the operating parameter of the driver element from linear region operation to saturation region operation. In the saturation region, the current becomes independent of drain-source voltage and primarily controlled by gate-source voltage, making the current much less sensitive to threshold voltage fluctuations and thereby improving current stability and luminance consistency
Solution Approach 2:
The patent implements a feedback mechanism where the gate-source voltage of the driver element is controlled based on the desired current level. By adjusting the gate voltage in response to current requirements, the system compensates for threshold voltage variations and maintains stable current flow through the organic LED, ensuring consistent luminance over time
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 allows for faster voltage writing, reduces threshold voltage fluctuations, and maintains stable current flow, thereby enhancing the longevity and luminance consistency of the organic LED display, even after 20,000 hours of operation, with reduced parasitic capacitance effects and increased material choices for improved light emitting efficiency.
Implementation Method 1
a capacitor (3) arranged between the gate electrode and the source electrode of the thin film transistor (2)
Implementation Method 2
an organic light emitting diode (hereinafter, 'organic LED') display apparatus using an organic electroluminescent (EL) device that emits light itself
Data Source
AI summary
An image display apparatus includes a current-controlled light emitting; a transistor which controls a first current flowing through the current-controlled light emitting diode, based on a first voltage applied between the gate and the source of the transistor at a light emitting phase; a capacitor arranged between the gate and the source. The image display apparatus also includes a current determining unit which controls a second current flowing between the source and the drain, based on a third voltage applied thereto at the writing phase. The second voltage is written in the capacitor at a writing phase and depends on the second current.