Pixel Circuit Threshold Voltage Compensation for AMOLED Uniformity
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Solution Overview
Problem
Existing AMOLED display technologies face issues with nonuniformity in electrical parameters of TFTs, leading to threshold voltage shifts and brightness differences, resulting in afterimage phenomena due to the limitations of crystallization processes and fabrication levels on large area glass substrates.
Innovation Solution
A pixel circuit comprising a first transistor, a second transistor, a third transistor, a storage capacitor, and a parasitic capacitor, where the current flowing through the transistors is controlled to be independent of the threshold voltage, using a specific switching and charging/discharging method to compensate for threshold voltage deviations and improve light emitting brightness uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If LTPS TFT or Oxide TFT is used to improve mobility and stability, then the display performance is improved, but the manufacturing complexity and cost increase due to crystallization process limitations
Solution Approach 1:
The patent uses conventional amorphous-Si TFTs instead of requiring LTPS or Oxide TFTs with complex crystallization processes. By accepting the shorter operational lifetime and lower mobility of amorphous-Si TFTs, the manufacturing process becomes simpler and more suitable for large area production, effectively applying the trade-off principle of using less durable but easier-to-manufacture components.
2Illumination intensity
If TFT threshold voltage is increased to improve current driving capability, then the brightness is improved, but the afterimage phenomenon worsens due to threshold voltage shifts under long time pressure and high temperature
Solution Approach 1:
The patent introduces a compensation transistor and capacitor that perform preliminary compensation for threshold voltage shifts before they affect the display. By pre-compensating for the expected threshold voltage drift under temperature and pressure, the circuit maintains stable brightness and prevents afterimage phenomena without requiring higher initial threshold voltages.
Solution Approach 2:
The patent implements a feedback mechanism where the compensation transistor continuously monitors and adjusts for threshold voltage shifts in the driving TFT. This closed-loop compensation ensures that brightness remains stable even as the TFT threshold voltage drifts due to temperature and pressure changes, effectively eliminating the trade-off between brightness and stability.
3Area of stationary object
If the number of TFTs in pixel circuit is reduced to simplify structure, then the aperture ratio is improved, but the current control precision worsens
Solution Approach 1:
The patent designs a compact pixel circuit where each transistor performs multiple functions. The compensation transistor serves both as a switching element and as a threshold voltage compensation device, while the capacitor serves dual purposes in charge storage and compensation. This multi-functionality allows adequate current control precision with fewer transistors, thereby maintaining high aperture ratio.
Data Source
AI summary
Provided are a pixel circuit and driving method thereof and a display apparatus. The pixel circuit comprises a first transistor (T1), a second transistor (T2), a third transistor (T3), a storage capacitor (C1), a parasitic capacitor (C2) and a light emitting device (L). A first electrode of the first transistor (T1) is connected to a first power source signal terminal, and its second electrode is connected to a first electrode of the third transistor (T3); the gate of the second transistor (T2) is connected to a first control signal terminal (S1), its first electrode is connected to a data signal terminal (DATA), and its second electrode is connected to the gate of the first transistor (T1); the gate of the third transistor (T3) is connected to a second control signal terminal (S2), and its second electrode is connected to one terminal of the light emitting device (L); one terminal of the storage capacitor (C1) is connected to the gate of the first transistor (T1), and the other terminal of the storage capacitor is connected to one terminal of the light emitting device (L); one terminal of the parasitic capacitor (C2) is connected to one terminal of the light emitting device (L), and the other terminal of the parasitic capacitor (C2) is connected to the other terminal of the light emitting device (L); and the other terminal of the light emitting device (L) is also connected to a second power source signal terminal (ELVSS). The pixel circuit can effectively compensate for the threshold voltage shift of the TFTs and improve the display effect.