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How Organic Light Emitting Diodes Work

By Megan Tung

An OLED (Organic Light-Emitting Diode) is similar to an LED; however, an OLED has an emissive electroluminescent layer of film made up of organic molecules. Light is emitted when electrical current travels through the organic molecules. So, why would you choose an OLED over a LCD (liquid crystal display) or VFD (vacuum fluorescent display)? Well, to answer your question, an OLED is thinner and has a better display. The OLED has a brighter, higher contrast display with faster response times, wider viewing angles, and less power consumption.

OLED DisplayOLED Display

What makes up an OLED?

An OLED consists of 6 layers that all play a role in making the OLED well-structured and energy efficient:
  1. Seal: Glass top plate
  2. Cathode: Negatively charged electrode (attracts cations)
  3. Emissive Layer: Made up of organic molecules or polymers that transport electrons from the cathode layer
  4. Conductive Layer: Made up of organic molecules or polymers that transport holes from anode layer
  5. Anode: Positively charged electrode (attracts electrons or anions)
  6. Substrate: Glass bottom plate
How do OLEDs Emit Light?

Electrical current needs to be applied to the anode and cathode layer for the OLED to emit light. Energy travels from the cathode layer, through the organic material (emissive and conductive layer), and to the anode layer. The current gives electrons to the emissive layer and removes electrons from the conductive layer. The removal of the electrons leaves holes in the conductive layer that need to be re-filled with electrons. To recombine with electrons the holes jump from the conductive layer to the electron-filled emissive layer. As the electrons fill these holes extra energy is released, which produces the bright, electroluminescent light that is visible through the outermost layer of glass (substrate and seal).

Instead of having a backlight, like in LEDs, the OLED's display is self-illuminating because of its organic material. Since there is no backlight, the OLEDs are significantly thinner than standard LCD or VFD modules.

Types of OLEDs and Their Uses

There are various types of OLEDs, such as passive-matrix OLEDs, active-matrix OLEDs, transparent OLEDs, top-emitting OLEDs, foldable OLEDs, and white OLEDs.

Passive-matrix OLEDs (PMOLED) have anode strips that are arranged perpendicular to the cathode strips. This specific intersection makes up pixels that emit the light. The brightness of each pixel is proportional to the amount of current applied. However, external circuitry is needed to apply current to the selected strips of anodes and cathodes. This external circuitry makes PMOLEDs consume more power than other types of OLEDs. PMOLEDs are good for text, icons, and smaller screens (such as cell phones).

PMOLED structure
PMOLED Structure

Active-matrix OLEDs (AMOLED) have a thin film transistor (TFT) under the anode layer which forms a matrix. The TFT array determines which pixels get turned on in order to form an image. The TFT arrays require less power than external circuitry, so AMOLEDs use less power than PMOLEDs. AMOLEDs are more effective for larger displays and videos (because they have faster refresh rates). AMOLEDs are typically used for computer monitors, TV screens, and electronic signs/billboards.

AMOLED structureAMOLED Structure

Transparent OLEDs are built with only the transparent materials, such as the substrate, cathode, and anode. When the OLED is turned off it is up to 85% transparent, but once the OLED is turned on light can pass through in both directions. Transparent OLEDs can also be either a passive- or active-matrix. Transparent OLEDs are used for heads-up displays, such as google glass or aircrafts. These displays are called heads-up displays because the user can maintain looking forward, rather than looking repetitively looking up and down between the path ahead and the dashboard.

Heads-up displayHeads-Up Display


Transparent OLED StructureTransparent OLED Structure
Top-Emitting OLED StructureTop-Emitting OLED Structure


Top-emitting OLEDs have a substrate that can either be opaque or reflective. This type of OLED works best with the active-matrix design. The main use for top-emitting OLEDs are smart cards (also known as cards with the microprocessor chip).

Foldable OLEDs are made up of flexible metallic foils or plastics for the substrate. Foldable OLEDs are very lightweight and durable, which can help prevent breakage in the devices they are in. Some cell phone companies have started incorporating foldable OLEDs into their devices.

White OLEDs emit white light that is brighter and more energy efficient than light emitted by fluorescent lights. Using white OLEDs in homes and buildings is a step that could help move towards reducing energy costs.

Pros and Cons of an OLED

Pros
Generates all colors/Bright images
Wide viewing angle (nearly 90°)
Low power consumption (when viewing darker colors)
Fast response time

Cons
Some OLEDs are moisture sensitive
Limited life (mainly due to sensitivity to moisture)
Higher power consumption (when viewing brighter colors)
Prolonged exposure to UV light can damage OLED
Megan Tung is a summer intern at Jameco Electronics and a rising freshman at UC Santa Barbara. Her interests include photography, music, business, and engineering.