OLED Pixel Circuit for Stable Current and Uniform Luminance
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Solution Overview
Problem
Conventional organic light emitting display devices experience instability in driving current and uneven pixel luminance due to voltage drop in large-scale devices, leading to non-uniform luminance across the display.
Innovation Solution
A novel pixel circuit design that includes specific transistors and a storage capacitor to control driving current, applying bias stress to transistors to mitigate hysteresis effects and maintain consistent luminance, eliminating the need for separate on-bias stress wiring.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the organic light emitting display device is large-scaled, then the display area is increased, but the high-potential voltage drops due to resistance of the high-potential voltage line
Solution Approach 1:
The pixel circuit is segmented into multiple functional blocks with dedicated transistors (first transistor connecting gate and drain, second transistor for data voltage, third transistor for high-potential voltage, fourth transistor for current path, fifth transistor for initial voltage, sixth transistor for reset voltage). This segmentation allows independent control of voltage paths, isolating the high-potential voltage line from other signal lines and reducing the impact of voltage drops on overall pixel performance.
Solution Approach 2:
The patent implements a nested transistor configuration where the first transistor's source is connected to the driving transistor's gate, and its drain is connected to the driving transistor's drain. This nested structure creates a feedback mechanism that compensates for voltage drops by dynamically adjusting the gate voltage based on the actual voltage at the drain, effectively nesting a voltage compensation circuit within the pixel structure.
2Device complexity
If conventional pixel circuits are used, then the device structure is simple, but the driving current becomes unstable due to high-potential voltage drop
Solution Approach 1:
The first transistor creates a feedback loop by connecting the driving transistor's drain back to its gate through the transistor network. This feedback mechanism continuously monitors the voltage at the drain and adjusts the gate voltage accordingly, compensating for voltage drops in the high-potential voltage line and stabilizing the driving current despite variations in supply voltage.
Solution Approach 2:
The pixel circuit performs self-compensation for voltage drops using only the transistors already present in the pixel, without requiring external compensation circuits or additional wiring. The fifth transistor applies an initial voltage to the driving transistor, and the sixth transistor applies a reset voltage to the anode, enabling the circuit to self-correct for voltage variations and maintain stable operation.
3Reliability
If separate on-bias stress wiring is added, then the hysteresis effect is mitigated, but the bezel area increases and resolution decreases
Solution Approach 1:
The patent merges the on-bias stress function with the existing pixel circuit transistors. The first transistor, which connects the gate and drain, simultaneously serves as both a feedback element and an on-bias stress transistor. By applying the high-potential voltage to the gate through this transistor during specific time periods, the circuit achieves hysteresis mitigation without requiring separate wiring, as the same transistor structure serves multiple functions.
Solution Approach 2:
The first transistor is designed with universal functionality, serving multiple purposes: it provides feedback for voltage compensation, acts as a switch for data programming, and functions as an on-bias stress transistor for hysteresis mitigation. This multi-functionality eliminates the need for dedicated separate wiring for each function, reducing the overall circuit area and allowing for higher resolution displays with smaller bezels.
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
The solution stabilizes driving current and ensures uniform pixel luminance, enhancing image quality by maintaining a constant voltage level on the anode electrode and reducing the bezel area, while increasing panel resolution.
Implementation Method 1
When the driving voltage is applied to the anode electrode and the cathode electrode, holes passing through a hole transport layer HTL and electrons passing through an electron transport layer ETL are moved to an emission layer EML to form excitons, and as a result, the emission layer EML generates visible light.
Data Source
AI summary
A pixel for an organic light emitting display device includes an organic light emitting diode that emits light by a driving current, a driving transistor configured to control the driving current, a first transistor to connect the second node and the third node, a second transistor to apply a data voltage to the first node, a third transistor to apply a high-potential driving voltage to the second node, a fourth transistor that forms a current path between the driving transistor and the organic light emitting diode, a fifth transistor to apply an initial voltage to the driving transistor, a sixth transistor configured to apply a reset voltage to a fourth node which is an anode electrode of the organic light emitting diode, a seventh transistor configured to apply the high-potential driving voltage to the fifth node, and an eighth transistor configured to apply a reference voltage to the fifth node.