OLED structure principle and light-emitting process-EEWORLD

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OLEDs are solid-state devices made of thin sheets of organic molecules that emit light when electricity is applied. OLEDs allow electronic devices to produce brighter, clearer images while consuming less power than traditional light-emitting diodes (LEDs) or liquid crystal displays (LCDs) used today.

In this article, you'll learn how OLED technology works, what types of OLEDs there are, the advantages and disadvantages of OLED compared to other light-emitting technologies, and some of the problems that OLED needs to overcome.

Similar to LEDs, OLEDs are solid-state semiconductor devices that are 100-500 nanometers thick, 200 times thinner than a human hair. OLEDs are made of two or three layers of organic materials; in the latest OLED designs, the third layer helps transfer electrons from the cathode to the emission layer. This article focuses on the two-layer design model.

1. The structure of OLED

OLED consists of the following parts:

The structure of OLED

Substrate (transparent plastic, glass, metal foil) - The substrate is used to support the entire OLED.

Anode (transparent) - The anode removes electrons (creates electron "holes") when current flows through the device.

Organic layer - The organic layer is composed of organic molecules or organic polymers.

Conductive layer - This layer is made up of organic plastic molecules that transport "holes" from the anode. Polyaniline can be used as the conductive polymer for OLEDs.

Emission layer - This layer is composed of organic plastic molecules (different from the conductive layer) that transport electrons from the cathode; the light emission process occurs in this layer. Polyfluorene can be used as the emissive layer polymer.

Cathode (can be transparent or opaque, depending on the type of OLED) - When current flows through the device, the cathode injects electrons into the circuit.

2. OLED Manufacturing

The most important step in the OLED production process is to apply the organic layer to the substrate. There are three ways to accomplish this:

1. Vacuum deposition or vacuum thermal evaporation (VTE)

The organic molecules in the vacuum chamber are slightly heated (evaporated) and then condensed in the form of a thin film on a cooler substrate. This method is very expensive and inefficient.

2. Organic vapor phase deposition (OVPD)

In a low-pressure hot-wall reaction chamber, the carrier gas transports the evaporated organic molecules to the low-temperature substrate, where they condense into a thin film. Using carrier gas can improve efficiency and reduce the cost of OLEDs.

3. Inkjet printing

Inkjet technology can be used to spray OLED onto a substrate, just like ink is sprayed onto paper when printing. Inkjet technology greatly reduces the production cost of OLED and can also print OLED onto a film with a very large surface area to produce large displays, such as 80-inch large-screen TVs or electronic billboards.

OLED Manufacturing

3. The light-emitting process of OLED

OLEDs emit light similarly to LEDs, through a process called electrophosphorescence.

OLED light-emitting process

The specific process is as follows:

1. The battery or power source of the OLED device will apply a voltage across the OLED.

2. Electric current flows from the cathode to the anode and through the organic layer (electric current refers to the flow of electrons).

3. The cathode outputs electrons to the organic molecular emission layer.

4. The anode absorbs electrons from the organic molecular conduction layer. (This can be regarded as the anode outputting holes to the conduction layer, and the two effects are equal.

5. At the junction of the emission layer and the conduction layer, electrons combine with holes.

6. When an electron encounters a hole, it fills the hole (it falls into an energy level in the atom that is missing the electron).

7. When this process occurs, the electrons release energy in the form of photons.

8. OLED emits light.

9. The color of light depends on the type of organic molecules in the emission layer. Manufacturers place several organic films on the same OLED to form a color display.

10. The brightness or intensity of light depends on the amount of current applied. The greater the current, the brighter the light.

4. Classification of OLED

Here are several types of OLED: passive matrix OLED, active matrix OLED, transparent OLED, top-emitting OLED, foldable OLED, white light OLED, etc.

Each type of OLED has its own unique purpose. Next, we will discuss each type of OLED one by one. First, passive matrix and active matrix OLED.

Passive Matrix OLED (PMOLED)

Passive matrix OLED structure

PMOLED has a cathode strip, an organic layer, and an anode strip. The anode strip and the cathode strip are perpendicular to each other. The intersection of the cathode and the anode forms a pixel, which is the part that emits light. An external circuit applies current to the selected cathode and anode strips to determine which pixels emit light and which do not. In addition, the brightness of each pixel is proportional to the amount of current applied.

PMOLED is easy to manufacture, but uses more power than other types of OLED, primarily because it requires external circuitry. PMOLED is most efficient when used to display text and icons, and is suitable for small screens (2-3 inches diagonal), such as those often found on mobile phones, PDAs, and MP3 players. Even with an external circuit, passive-matrix OLEDs use less power than the LCDs currently used in these devices.

Active Matrix OLED (AMOLED)