Shield Electrode Reduces Parasitic Capacitance in Active Matrix Displays
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Solution Overview
Problem
In organic electroluminescence (EL) displays, the increase in parasitic capacitance due to the overlap between the gate of the drive transistor and the anode of the OLED leads to a decrease in brightness and a loss of dynamic range, making it difficult to achieve desired white brightness levels.
Innovation Solution
The introduction of a shield electrode between the display electrode and the storage capacitor and the gate electrode of the drive transistor, set at a constant potential, reduces parasitic capacitance by shielding these electrodes, thereby maintaining a stable current supply to the OLED.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the OLED area is increased to decrease current density and moderate brightness degradation, then the parasitic capacitance increases due to overlap with the gate electrode, but the brightness level and dynamic range decrease
Solution Approach 1:
A shield electrode is introduced as an intermediary component between the OLED anode and the drive transistor gate electrode. This shield electrode, connected to a constant potential (typically ground or power supply potential), acts as an electrostatic shield that reduces the parasitic capacitance between the overlapping electrodes. By placing this intermediate conductive layer with insulating layers on both sides, the patent successfully decouples the electrostatic interaction while maintaining the structural overlap needed for space efficiency and large OLED area.
Solution Approach 2:
The patent addresses the parasitic capacitance problem by adding a vertical dimension to the structure. Instead of simply increasing the OLED area in the planar dimension, the solution introduces a multi-layer vertical stack with the shield electrode positioned in between the anode and gate. This vertical arrangement with insulating layers creates an electrostatic shield that reduces capacitance while allowing the OLED to maintain its large horizontal area for low current density operation.
2Illumination intensity
If the OLED area is increased to achieve desired white brightness, then the overlap with the gate electrode increases, but the parasitic capacitance causes potential propagation and brightness reduction
Solution Approach 1:
The shield electrode serves as an electrostatic intermediary that blocks the propagation of potential changes from the OLED anode to the drive transistor gate. When the OLED anode potential changes during operation, the shield electrode, being held at a constant potential by its connection to the power supply or ground, prevents these changes from coupling to the gate electrode through capacitive coupling. This ensures stable gate potential and reliable transistor operation despite large OLED areas.
3Area of stationary object
If elements are arranged to increase OLED area, then the parasitic capacitance increases, but the dynamic range of video signal voltage is lost
Solution Approach 1:
The shield electrode introduces an intermediate electrostatic barrier that reduces the coupling between the OLED anode and the drive transistor gate. By holding the shield at a constant potential, it creates a capacitive divider effect that attenuates the propagation of anode potential changes to the gate. This reduction in parasitic capacitance preserves the full dynamic range of the video signal voltage, allowing the system to maintain both large OLED area and complete signal information.
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 configuration minimizes parasitic capacitance, ensuring consistent brightness and reducing dynamic range loss, resulting in improved display quality by maintaining the desired current supply to the OLED.
Implementation Method 1
a shield electrode arranged between the display electrode connected to the drive transistor and at least one of the first electrode of the storage capacitor and the gate electrode of the drive transistor with interposing an insulating layer, and set at a constant potential
Data Source
AI summary
An active matrix display includes a substrate, a video signal line on the substrate, first and second power lines on the substrate, a self-emitting element having two display electrodes facing each other and connected to one of the first and second power lines, a drive transistor connected between the first power line and one of the display electrodes, a storage capacitor having a first electrode connected to a gate electrode of the drive transistor and a second electrode facing the first electrode with interposing an insulating layer, the storage capacitor and the gate electrode of the drive transistor being arranged under the display electrode, and a shield electrode arranged between the display electrode connected to the drive transistor and at least one of the first electrode of the storage capacitor and the gate electrode with interposing an insulating layer, and set at a constant potential.