Driving Transistor Reset for OLED Display Uniformity
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Solution Overview
Problem
In active-matrix organic electroluminescence (EL) display devices, residual images occur due to hysteresis characteristics of driving transistors, leading to uneven display quality as current values differ from desired levels, affecting light emission.
Innovation Solution
The image display device incorporates a light-emitting element, capacitors, and transistors with a specific circuit configuration that includes switching elements and a driving circuit to control voltage and current, ensuring accurate current flow by resetting the driving transistor during non-light-emitting periods, thereby stabilizing the threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If gate voltage is applied to driving TFT to control current supply, then light emission is maintained, but hysteresis characteristics cause threshold voltage changes leading to display unevenness
Solution Approach 1:
The patent applies a reset voltage to the gate electrode before the light-emitting period to pre-compensate for threshold voltage shifts caused by hysteresis. This preliminary action ensures that the driving TFT starts each frame with a known threshold voltage, preventing display unevenness while maintaining continuous light emission throughout the frame period.
Solution Approach 2:
The patent dynamically changes the gate voltage parameter by applying different voltages during different phases: a reset voltage (Vreset) before light emission to compensate for hysteresis, and a data voltage (Vdata) during light emission to control luminance. This parameter switching resolves the contradiction between maintaining light emission and ensuring display uniformity.
2Manufacturing precision
If reset operations are performed frequently to compensate for hysteresis, then display quality improves, but power consumption increases
Solution Approach 1:
The patent implements periodic reset operations synchronized with the frame period of the display. The reset voltage is applied once per frame to the gate electrode, which is sufficient to compensate for hysteresis effects without requiring continuous or more frequent resetting. This periodic approach maintains display uniformity while minimizing power consumption compared to continuous reset operations.
3Device complexity
If simple circuit configuration is used, then device complexity is reduced, but residual images occur due to hysteresis characteristics
Solution Approach 1:
The patent makes the gate electrode serve multiple functions: it controls the driving TFT during light emission and also receives reset voltage to compensate for hysteresis. This multi-functionality allows the simple circuit configuration to achieve both low complexity and high display uniformity by utilizing the existing gate electrode for dual purposes without adding separate reset circuitry.
Data Source
AI summary
An image display device includes: an organic EL element, a first electrostatic capacitor, a driving transistor having a gate connected to a first electrode of the first electrostatic capacitor and a source connected to an anode of the organic EL element, a second electrostatic capacitor having an electrode connected to a second electrode of the first electrostatic capacitor, a negative power source line which determines the potential of a cathode of the organic EL element, and a scanning line driving circuit which controls a first switching transistor, a second switching transistor, and a third switching transistor. In a non-light-emitting period, the scanning line driving circuit sets, during a period from a start of a reset period to an end of the non-light-emitting period, a fixed voltage corresponding to the potential of the negative power source line to the source electrode of the driving transistor.