What is AMOLED? Active Matrix OLED Technology

The acronym AMOLED stands for active-matrix organic light-emitting diode. It is a type of OLED technology commonly used in large and advanced display systems. The term "active matrix" refers to the arrangement of light-emitting pixels, each controlled by its own dedicated thin-film transistor (TFT) and storage capacitor. This design ensures that each pixel is "actively" managed, resulting in high-quality displays with a long lifespan.

AMOLED Structure

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An AMOLED structure can vary between different systems. The key components of a AMOLED are:

• Emissive OLED Layer
• Electrodes
• Substrates
• Thin-film Transistor (TFT)
• Storage Capacitor

The individual AMOLED pixels have a basic structure consisting of an offset thin-film transistor connected to a transparent anode with a slight overlap. The organic emissive layer is coated on top of the anode, followed by a common cathode applied over the emissive layer. Additional material layers may be added to enhance connectivity and efficiency.

As a larger system of multiple light-emitting pixels there are more electrodes to ensure connectivity. Signal and control electrodes are used to indicate which pixels should be emitting light at a given time. The transistor allows for precise control over the state of each pixel. The different component of AMOLEDs have different roles within the system:

• Control Electrode: Connected to the gate electrode of the TFT, it acts as a switch for the pixel as it blocks or allows the current flow from the signal electrode.
• Signal Electrode: Connected to the source electrode of the TFT, they ensure the appropriate voltage is applied to achieve the desired brightness and color when the transistor is activated by the control electrode.
• Common Cathode/Electrode: This electrode is shared by all pixels and works in conjunction with the pixel anode/electrode allow current to flow for light emission.

Within each pixel circuit:

• Thin-Film Transistor (TFT): The TFT has a source, drain and gate electrode which allows the precise control of current to a the pixel electrode (anode) via the control and signal electrodes. In advanced systems a pixel can have multiple transistors in it.
• Pixel Anode/Electrode: This is the anode that connects the TFT to the organic light-emitting layer to facilitate light emission with the help of the common electrode.
• Storage Capacitor: Used to store the signal voltage sent to the transistor to maintain brightness until a refresh and the next signal is sent.

How does an AMOLED work?

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AMOLEDs consist of a grid of OLED pixels, each controlled by dedicated circuitry that includes transistors and capacitors. Each OLED pixel is paired with at least one transistor and a corresponding capacitor, which precisely manage its on/off state and brightness. When a voltage is applied to the transistor, it charges the capacitor, maintaining the pixel's state until the next refresh cycle.

When a voltage is applied across the AMOLED device, the control and signal electrodes determine which pixels light up, their brightness, and timing. Only selected pixels receive voltage at any given moment to produce the desired image or light output. Within each pixel, the organic light-emitting diode (OLED) component is composed of specific materials that emit light at the required wavelength.

Materials used in the OLED layer of AMOLED devices include polymers and small molecules, each serving specific roles, such as efficiently harnessing electric charge and converting it into light. Typically, these layers are composed of multiple materials working in harmony. However, a notable drawback is that some materials in the OLED layer are highly sensitive to water and oxygen, necessitating careful handling during fabrication. This sensitivity increases both manufacturing time and costs.

AMOLED vs PMOLED

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A contrasting set up to AMOLEDs are passive-matrix organic light-emitting diodes (PMOLEDs). They have a different set up to AMOLEDs giving differing properties and features. The table below summarizes a comparison of AMOLED and PMOLED devices:

	AMOLED		PMOLED
Control	Active Circuitry Individual Pixels		No active circuitry Sequential
Power Consumption	Higher		Lower
Resolution	Higher		Lower
Cost	Higher		Lower
Response Time	Higher		Lower
Image Stability	Higher		Lower
Lifetime	Higher		Lower
Pixel State	Pixels can hold state until next refresh due to storage capacitor		Pixels can’t hold state for long
Components	Control Electrode Signal Electrode Common Electrode	Thin-Film Transistor Pixel Electrode Storage Capacitor	Row Electrode Column Electrodes	Pixel Electrode OLED Pixel
Applications	Smartphones Tablets Laptops TVs	Virtual Reality Devices Flexible Devices Public Display Boards	Basic Wearable Trackers Health Monitors Calculators Basic Readout Screens	Smart Home Devices Toys Electronic Shelf Labels

AMOLED (Active-Matrix OLED) and PMOLED (Passive-Matrix OLED) differ in structure, functionality, and applications. AMOLED uses active circuitry to allow individual pixels to hold their state until the next refresh. This results in higher resolution, better image stability, faster response times, and longer lifespans. However, it comes at the cost of higher power consumption and manufacturing expenses. Whilst power consumption is higher than PMOLEDs, for dark or low-content screens the power consumption is more efficient as they turn off unused pixels entirely. AMOLED is ideal for high-end applications like smartphones, tablets, TVs, VR devices, and flexible displays.

In contrast, PMOLED relies on simpler passive circuitry with row and column electrodes, where pixels cannot hold their state for long. This technology is more cost-effective and consumes less power but offers lower resolution, image stability, and shorter lifespans. PMOLED is suitable for basic and low-power applications such as wearable trackers, calculators, health monitors, smart home devices, and electronic shelf labels.

OLED Materials

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