Display Device Off-Current Reduction via Reverse Bias
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Solution Overview
Problem
The grayish black effect occurs in display devices with light emitting elements due to residual current when the thin film transistor is turned off, leading to decreased contrast and luminance control accuracy.
Innovation Solution
Implementing a configuration where the potential of the second power source line is set to ensure the voltage between the electrodes of the light emitting element is below the threshold voltage when the thin film transistor is turned off, and optionally using a second transistor or a capacitor to discharge the threshold voltage held in the light emitting element's capacitance, thereby reducing off-current contributions to light emission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the thin film transistor is turned off by setting the gate-source potential difference to be equal to or lower than the threshold voltage, then the light emitting element should not emit light, but a slight amount of drain current (off current) still flows causing the grayish black effect and decreased display contrast
Solution Approach 1:
A reverse bias voltage is applied to the light emitting element before and during the period when the thin film transistor is turned off. This preliminary action of applying reverse bias prevents the accumulation of charge carriers and reduces the off current that causes the grayish black effect, thereby improving display contrast and luminance control accuracy.
Solution Approach 2:
The patent changes the voltage parameters applied to the light emitting element by implementing a driving method that applies reverse bias voltage (making the anode potential lower than the cathode potential) during specific time periods. This parameter change from forward bias to reverse bias effectively reduces the off current and eliminates the grayish black effect, resolving the contradiction between display contrast and luminance control accuracy.
2Illumination intensity
If a reverse bias voltage is regularly applied to the light emitting element to reduce the grayish black effect, then display contrast is improved, but power consumption increases
Solution Approach 1:
The reverse bias voltage is applied periodically rather than continuously. Specifically, the reverse bias is applied during the period when the thin film transistor is turned off and before the next forward bias cycle begins. This periodic application of reverse bias reduces the grayish black effect and improves display contrast while minimizing the time during which power is consumed, thereby resolving the contradiction between display contrast and power consumption.
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 approach enhances display contrast and reduces power consumption by minimizing the grayish black effect, allowing for clearer and more accurate luminance control.
Implementation Method 1
An electroluminescence element and the like can be used as the light emitting element 105
Implementation Method 2
a value of a current (hereinafter referred to as a drain current) flowing between a source and drain of the thin film transistor 102 is controlled according to a potential G1 applied to a gate of the thin film transistor 102
Implementation Method 3
a voltage approximately the same as the threshold voltage of the light emitting element 105 is constantly held between the first electrode 105a and the second electrode 105b because of capacitance of the light emitting element 105 itself
Data Source
AI summary
A problem in that a light emitting element slightly emits light is solved by an off current of a thin film transistor connected in series to the light emitting element, thereby a display device which can perform a clear display by increasing contrast, and a driving method thereof are provided. When the thin film transistor connected in series to the light emitting element is turned off, a charge held in the capacitance of the light emitting element itself is discharged. Even when an off current is generated at the thin film transistor connected in series to the light emitting element, this off current charges this capacitance until the capacitance of the light emitting element itself holds a predetermined voltage again. Accordingly, the off current of the thin film transistor does not contribute to light emission. In this manner, a slight light emission of the light emitting element can be reduced.