Dual Transistor OLED Pixel for Threshold Voltage Stability
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Solution Overview
Problem
Organic light emitting displays using amorphous silicon thin film transistors face issues with threshold voltage deterioration, leading to uneven electric current flow and degraded display quality due to shifting threshold voltage under continuous electric current application.
Innovation Solution
A display device with first and second driving transistors connected to a light emitting diode, where control voltages of alternating polarity are applied to the transistors, and capacitors are used to charge and apply data voltages, reducing stress on the transistors and maintaining stable current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If amorphous silicon TFTs are used in organic light emitting displays, then manufacturing complexity is reduced and wide screen production is easier, but threshold voltage deteriorates under continuous electric current application
Solution Approach 1:
The pixel electrode is divided into two separate electrodes (first pixel electrode and second pixel electrode), each connected to separate driving transistors. This segmentation allows independent control and stress distribution, preventing threshold voltage deterioration in a single transistor while maintaining ease of manufacture with amorphous silicon TFTs.
Solution Approach 2:
The display operates by alternately applying driving signals to the first and second driving transistors in periodic frames. This periodic action allows each transistor to rest and recover from electric current stress, preventing cumulative threshold voltage shifts while maintaining continuous display operation.
2Duration of action of moving object
If continuous electric current is applied to OLEDs driven by amorphous silicon TFTs, then display operation is maintained, but threshold voltage shifts and electric current becomes uneven
Solution Approach 1:
By dividing the pixel into two separate OLEDs with independent driving transistors, the electric current path is segmented. This prevents uneven current distribution that would occur in a single transistor system, as each transistor drives its own OLED without interfering with the other's current flow.
Solution Approach 2:
The alternating activation of first and second driving transistors creates a periodic operation pattern. Each transistor operates for one frame then rests, allowing threshold voltage recovery. This maintains uniform electric current through each OLED over time while enabling continuous display operation.
3Reliability
If duality drive method is implemented with separate pixel electrodes, then threshold voltage deterioration is prevented, but device structure becomes more complex
Solution Approach 1:
The first and second pixel electrodes are merged into a single common pixel electrode structure that serves both OLEDs. This merging reduces structural complexity by eliminating the need for completely separate electrode systems, while still maintaining the benefits of duality drive for threshold voltage stability.
Solution Approach 2:
The common pixel electrode serves multiple functions: it acts as the electrode for both first and second OLEDs, provides a shared electrical connection point, and simplifies the overall pixel structure. This multi-functionality reduces device complexity while maintaining threshold voltage stability through the duality drive method.
Data Source
AI summary
A display device includes a light emitting diode, and first and second driving transistors connected between a driving voltage and the light emitting diode to supply driving electric current to the light emitting diode. A control voltage or control voltages differentiated in polarity from each other is/are applied to control terminals of the first and the second driving transistors. The first driving transistor has a control electrode located below a semiconductor layer of the light emitting diode while the second driving transistor has a control electrode located over the semiconductor layer. Two driving transistors are formed at each pixel, and an area occupied thereof within the pixel is reduced. Control voltages differentiated in polarity from each other are applied to the respective driving transistors, substantially preventing deterioration of the driving transistors.