OLED Pixel Circuit Decoupling Current from Voltage Variations
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Solution Overview
Problem
In organic light emitting diode (OLED) displays, non-uniformity of display and material attenuation lead to varying pixel currents and luminance levels due to IR-drops in power supply voltage and differences in threshold voltages of transistors, causing display non-uniformity and reduced luminance over time.
Innovation Solution
A pixel circuit design incorporating additional switching transistors and a specific driving method that decouples pixel current from the power supply voltage and threshold voltage of the driving transistor, ensuring pixel current is related only to data voltage and OLED cross-voltage, thereby stabilizing luminance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional pixel circuit with a driving transistor and storage capacitor is used, then the circuit structure is simple, but the display uniformity deteriorates due to IR-drop and threshold voltage variations
Solution Approach 1:
The pixel circuit is segmented into multiple functional blocks: a first switching transistor for controlling the storage capacitor charging, a second switching transistor for controlling the data voltage input, a third switching transistor for resetting the OLED, and a fourth switching transistor for controlling the emission phase. This segmentation allows independent optimization of each function to achieve uniform display while managing complexity.
Solution Approach 2:
The invention changes the control parameters by using multiple switching transistors with different gate control signals (first gate control signal, second gate control signal, third gate control signal, fourth gate control signal) to precisely control the timing and sequence of operations. This parameter control approach ensures that the pixel current is determined only by the data voltage and OLED characteristics, not by power supply variations or transistor threshold variations.
2Manufacturing precision
If additional switching transistors are added to decouple pixel current from power supply voltage, then display uniformity is improved, but device complexity increases
Solution Approach 1:
The storage capacitor serves multiple functions: it stores the data voltage during the programming phase, maintains the gate-source voltage of the driving transistor during the emission phase, and is controlled by the switching transistors to enable sequential operation modes. This multi-functionality reduces the need for additional dedicated components.
Solution Approach 2:
The first switching transistor is activated in advance to charge the storage capacitor to a predetermined voltage before the data voltage is applied. This preliminary action ensures that the storage capacitor is ready to maintain a stable gate-source voltage for the driving transistor, preventing display uniformity issues before they occur.
3Stability of the object's composition
If the pixel circuit uses material attenuation compensation, then luminance stability is improved, but the circuit operation complexity increases
Solution Approach 1:
The third switching transistor provides a feedback mechanism by resetting the OLED to a predetermined voltage level after emission. This feedback action compensates for material attenuation by ensuring that the OLED starts each emission cycle from a known state, maintaining luminance stability without complex compensation circuits.
Solution Approach 2:
The pixel circuit operates in periodic cycles with distinct phases: programming phase (first switching transistor on), emission phase (second switching transistor on), and reset phase (third switching transistor on). This periodic operation simplifies the control logic by using simple on/off switching sequences rather than continuous complex regulation.
Data Source
AI summary
An exemplary pixel circuit includes an organic light emitting diode (OLED), a storage capacitance, a driving transistor and first through fourth switching transistors. The driving transistor is for generating a pixel current according to a charge amount stored on the storage capacitance to drive the OLED at a predetermined luminance. The on/off states of the first through fourth transistors are controlled by the same control signal. By means of particular electrical connection relationships of the first through fourth transistors in the pixel circuit, the pixel current flowing through the OLED is irrelevant to the power supply voltage and the threshold voltage of the driving transistor but is increased along with the increase of a cross-voltage of the OLED resulting from long-term use. The present invention also provides an active matrix OLED display using the above-mentioned pixel circuit and a driving method for the pixel circuit.