OLED Pixel Circuit with Bias Initialization for Brightness Uniformity
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Solution Overview
Problem
Organic light emitting displays face issues with uniform brightness due to shifted threshold voltages of driving transistors and degradation of OLEDs over time, leading to reduced brightness and deteriorated picture quality, especially when displaying gray scales and moving images.
Innovation Solution
The proposed solution involves a pixel structure with multiple transistors and a storage capacitor that applies an off bias voltage to the driving transistor, allowing for initialization before data signal supply, and includes a sensing unit to extract deterioration information, which is used to adjust data signals and maintain uniform brightness by compensating for OLED degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional pixel structure with a single driving transistor is used, then the device complexity is low, but the brightness uniformity deteriorates due to shifted threshold voltages and OLED degradation
Solution Approach 1:
The pixel circuit is divided into multiple functional blocks: a driving transistor for current control, an emission control transistor for timing control, and a storage capacitor for voltage holding. This segmentation allows each component to perform its specific function optimally, with the storage capacitor compensating for threshold voltage shifts and the emission control transistor managing the emission timing, thereby maintaining brightness uniformity while managing complexity through functional specialization
Solution Approach 2:
The storage capacitor is pre-charged to a reference voltage before the data signal is applied to the pixel. This preliminary action ensures that the capacitor is ready to compensate for any threshold voltage shifts in the driving transistor during the emission phase, maintaining consistent brightness levels without requiring complex real-time adjustment circuits
2Reliability
If multiple transistors and a storage capacitor are added to initialize the driving transistor and compensate for degradation, then the brightness uniformity is improved, but the device complexity increases
Solution Approach 1:
The emission control transistor serves multiple functions: it controls the emission timing by enabling/disabling the OLED current path, it manages the charging/discharging of the storage capacitor in coordination with the data signal, and it helps reset the pixel circuit for the next frame. This multi-functionality reduces the need for separate dedicated components, thereby improving brightness uniformity while limiting the increase in device complexity
3Reliability
If the emission control transistor is turned on before the data transistor, then the initialization is improved, but the power consumption increases due to overlapping current paths
Solution Approach 1:
The emission control transistor is activated in a periodic sequence: it turns on before the data transistor to initialize the pixel circuit and charge the storage capacitor, then turns off before the data transistor turns on to prevent current overlap. This periodic switching pattern ensures accurate initialization while minimizing power consumption by avoiding simultaneous conduction of both transistors, thus preventing unwanted current paths
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach ensures consistent brightness across images by initializing the driving transistor with an off bias voltage and compensating for OLED deterioration, thereby improving the display's response characteristics and maintaining desired brightness levels despite transistor voltage shifts and OLED degradation.
Implementation Method 1
OLED displays display images using OLED that generate light by re-combination of electrons and holes
Data Source
AI summary
A pixel capable of displaying an image with uniform brightness is disclosed. In one aspect, the pixel includes an organic light emitting diode (OLED), a first transistor for controlling an amount of current that flows from a first power supply to a second power supply via the OLED in response to a voltage applied to a first node. The pixel also includes a second transistor that is coupled between a bias power supply and the first node and whose gate electrode is coupled to an emission control line. The pixel further includes a third transistor that is coupled between an anode electrode of the OLED and a feedback line and whose gate electrode is coupled to a control line.