N-type Pixel Circuit Stabilizes OLED Drive Current
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Solution Overview
Problem
Organic electroluminescent display apparatuses face issues with non-uniform image quality due to variations in threshold voltage of driving transistors and sensitivity to second power voltage applied at the cathode electrode of OLEDs, leading to unstable brightness and image deterioration as the display size increases.
Innovation Solution
A pixel circuit utilizing N-type transistors, including a driving transistor, capacitors, and additional transistors to manage scan and emission control signals, which compensates for threshold voltage deviations and reduces the influence of second power voltage on drive current, ensuring consistent image brightness across pixels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If N-type transistors are used in the pixel circuit, then the circuit can be implemented with specific transistor characteristics, but the drive current becomes sensitive to threshold voltage variations and second power voltage changes
Solution Approach 1:
The pixel circuit employs a feedback mechanism where the drive current is adjusted based on the voltage stored in the capacitor connected to the gate of the N-type driving transistor. This feedback loop compensates for threshold voltage variations and second power voltage changes, maintaining stable drive current despite transistor parameter fluctuations
Solution Approach 2:
The invention changes the operating parameters of the N-type transistor circuit by introducing a capacitor to store gate voltage and using a specific transistor configuration that transforms the sensitivity to threshold voltage and second power voltage into a more stable drive current output through parameter transformation
2Area of stationary object
If display size increases, then the display area expands, but image quality deteriorates due to threshold voltage variations and power voltage sensitivity
Solution Approach 1:
The pixel circuit is segmented into distinct functional blocks: a driving transistor for current control, a capacitor for voltage storage and threshold compensation, switching transistors for signal control, and a light emitting device. This segmentation allows each component to independently address specific issues, with the capacitor specifically handling threshold voltage compensation to maintain uniformity across large display areas
Solution Approach 2:
The capacitor acts as an intermediary element between the gate of the driving transistor and the data input, storing the voltage information and providing a stable reference that compensates for threshold voltage variations. This intermediary component enables large-scale displays to maintain image quality uniformity despite manufacturing variations
3Reliability
If threshold voltage compensation is implemented, then drive current stability improves, but circuit complexity increases
Solution Approach 1:
The capacitor in the pixel circuit serves multiple functions: it stores gate voltage, compensates for threshold voltage variations, and maintains stable drive current. By making this single component multi-functional, the circuit achieves threshold voltage compensation without adding excessive complexity, as the same component handles multiple critical tasks
Data Source
AI summary
An improved pixel circuit is provided. The pixel circuit includes a light emitting device driven by a drive current according to a voltage applied to a gate electrode of a driving transistor. The pixel circuit also includes a first capacitor, a second transistor for transferring a data signal to a first terminal of the first capacitor in response to a scan control signal applied to a gate electrode of the second transistor, a third transistor for diode-connecting the driving transistor in response to a scan control signal applied to a gate electrode of the third transistor, a fourth transistor for applying a first power voltage to the first electrode of the driving transistor in response to an emission control signal, and a fifth transistor for applying a sustain voltage to a first terminal of the first capacitor in response to the emission control signal. The driving transistor and the second to fifth transistors are N-type transistors.