OLED Pixel Circuit Compensation for Luminance Uniformity
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Solution Overview
Problem
In organic light emitting displays, non-uniformity in luminance between pixels occurs due to variations in voltage drop across the first power supply line and non-uniform characteristics of thin film transistors, particularly threshold voltages and mobility, leading to inconsistent image quality.
Innovation Solution
A pixel circuit design incorporating first to fifth transistors, first and second capacitors, and an organic light emitting diode, where a reference current is input to compensate for data signals, allowing capacitors to store voltage reflecting the driving transistor's characteristics, and a driving current is generated to emit light, independent of channel width and length ratios and threshold voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a simple pixel circuit with basic transistors and capacitors is used, then device complexity is reduced, but luminance uniformity deteriorates due to voltage drop variations and transistor characteristic non-uniformity
Solution Approach 1:
The pixel circuit performs preliminary compensation actions by storing characteristic information of the driving transistor in capacitors before the actual display operation. The first capacitor stores compensation voltage based on transistor characteristics, and the second capacitor stores corrected data signals, preparing the circuit to eliminate luminance non-uniformity before it occurs during normal operation.
Solution Approach 2:
The pixel circuit implements feedback mechanisms where the driving transistor's own characteristic information (threshold voltage, mobility) is sensed and stored in capacitors, then fed back to correct the data signals. This feedback loop compensates for transistor variations, ensuring uniform luminance output despite manufacturing non-uniformity in transistor parameters.
2Manufacturing precision
If voltage compensation mechanisms are added to improve luminance uniformity, then manufacturing precision improves, but device complexity increases due to additional transistors and capacitors
Solution Approach 1:
The pixel circuit design makes the driving transistor serve multiple functions: it acts as both the light-emitting current source and the object whose characteristic information is compensated. The same driving transistor that controls OLED current also provides the characteristic data (via capacitors) for compensation, eliminating the need for separate compensation transistors and reducing overall circuit complexity.
Solution Approach 2:
The invention changes the operational parameters of the pixel circuit by introducing multi-stage voltage storage and signal correction. Instead of directly applying data signals to the driving transistor, the circuit stores compensation voltages in capacitors, corrects data signal levels, and applies corrected signals, thereby changing the voltage and current parameters to achieve uniform luminance output despite transistor parameter variations.
3Manufacturing precision
If reference current input is used to compensate data signals, then luminance uniformity improves, but power consumption increases
Solution Approach 1:
The pixel circuit employs periodic action by using distinct time periods for different operations: a first period for inputting reference current and storing compensation voltage in capacitors, and a second period for inputting corrected data signals and driving the OLED. This periodic operation allows compensation to occur during specific time windows, minimizing continuous power consumption while achieving luminance uniformity.
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 design ensures uniformity in pixel luminance by compensating for voltage drops and transistor characteristics, improving image quality by making the current flowing into the OLED proportional to the data and reference signals, thus addressing non-uniformity issues.
Implementation Method 1
the organic light emitting display has self-emission structure... the organic light emitting diode emits light in response to the data signal applied between the cathode electrode and the anode electrode
Data Source
AI summary
A pixel circuit includes a first transistor transmitting a reference signal or a data signal in response to a selection signal applied through a scan line, second and third transistors inputting a reference current in response to a control signal applied through a control line, a first capacitor storing a voltage compensated by the input reference current to compensate for the data signal received from the first transistor, a second capacitor storing the compensated data signal by the first capacitor, a fourth transistor receiving the compensated data signal to generate a driving current, a fifth transistor transmitting the driving current in response to the control signal applied through the control line, and an organic light emitting diode receiving the driving current from the fifth transistor to emit light.