Tech 101: How OLED Displays Work

In our previous article, we explained How Modern LCDs Work. In this article, we’ll be exploring how the new generation of displays based on OLED technology work.

What Exactly are OLEDs?

There are a lot of products in the market today ranging from TV’s to Laptops and even Smart Phones that contain an OLED display. But what are these displays and how do they work? OLED stands for Organic Light Emitting Diode. This technology is named after the organic polymers that are used in its construction. For a long time, organic compounds have been assumed to be electrical insulators but new research and manufacturing techniques have allowed the industry to use these polymers for many applications on a vast scale.

OLEDs are solid-state devices which are composed of thin films of organic molecules which emit light when a current is applied. OLEDs can provide brighter crisper displays that use less power than conventional Light Emitting Diode (LED) and Liquid Crystal Display (LCD) technologies. Another application for OLEDs is in the form of lights to be used in homes and offices. These lights have a long life and are very efficient.

The Manufacturing Process

OLEDs are basically layers of organic material sandwiched between an anode and a cathode and mounted on a substrate. This entire device is usually between 100 to 500 nanometers thick or is about 200 times thinner than a human hair.

The substrate is usually a sheet of glass which is coated with a transparent conductive oxide which works as the anode. The next layer consists of hole injection material, the organic emitters and the electron transport layer which are together referred to as the organic stack. On top of the organic stack is the inorganic cathode. The device operates as follows:

• Cathode – The cathode injects electrons into the system when a current flows through the device. It may or may not be transparent.
• Organic Stack – This layer is made up of different organic polymers that are built up in layers. This is where light is made. The organic stack consists of the following layers:
  
• Electron Transport Layer (ETL) – This layer consists of organic polymers or plastics that transport electrons from the cathode to the Organic Emitters.
• Organic Emitters (OE) – This is a layer that is made up of organic polymers that may or may not be mixed with phosphorescent chemicals.  One polymer used in this layer is polyfluorene.When an electron finds a hole, it falls into the hole and gives up energy in the form of light.
• Hole Injection Layer (HIL) – This layer is made up of polymers that transport electrons to the anode. This leaves ‘holes’ for electrons transported from the ETL to fill.
• Anode – The anode removes electrons from the system when a current flows through the device. It is always transparent.

This is the process by which a single OLED works. To turn them into colour displays, manufacturers place several layers of OEs on a display. There are various methods with which these individual pixels can be turned into a full colour display.

Types of OLED Displays

There are several types of OLED Display

• Passive-matrix OLED
• Active-matrix OLED
• Transparent OLED
• Top-emitting OLED
• Foldable OLED
• White OLED

Passive Matrix OLEDs or PMOLEDs

PMOLEDs are designed in a hash pattern. The anode and cathode are made in strips that are arranged perpendicular to each other. The points of intersection of the strips of anode and cathode make up the individual pixels where light is emitted. External circuitry is used to control which pixels are on and what colour they emit. The brightness of each pixel is dependent on the amount of current in the system.

Active Matrix  OLEDs or AMOLEDs

AMOLEDs have a full layer of anode and cathode materials but they borrow a technology from LCDs to produce a visible display. Sandwiched between the anode and the organic layer is a layer of thin film transistors or TFTs that makes the array. The TFT array is the circuitry that decides which pixel is on and what colour it displays. AMOLEDs are more efficient than PMOLEDs because the external circuitry used to run the PMOLEDs consumes more power. AMOLEDs also have a faster refresh rate which makes them more suitable to larger displays used in televisions and computer monitors.

Transparent OLEDs or TOLEDs

TOLEDs are constructed with only transparent  components, and are upto 85% as transparent as the substrate used when they are turned off. When it is turned on, the display still allows light to pass both ways and remains transparent. It can have either an active matrix or a passive matrix. This technology would be perfect for Heads Up Displays and Medical Equipment.

Top Emitting OLEDs or TEOLEDs

TEOLEDs are constructed with a substrate that is either opaque or reflective. They are perfectly suited for an active matrix design. They are used in smart-cards.

Foldable OLEDs or FOLEDs

FOLEDs are constructed with a highly flexible substrate which could either be a plastic or metallic foil. These types of OLEDs are very lightweight and durable. They are used in cell phones and can reduce breakage. They could also potentially be integrated into fabrics to create smart clothing.

White OLEDs or WOLEDs

WOLEDs  emit pure white light that is brighter and more uniform than the light which is emitted by fluorescent lights. . WOLEDs can be made in large sheets and can reduce energy costs massively if used to light homes and buildings due to their low power consumption.

Advantages and Disadvantages of OLEDs

OLEDs offer many advantages over the current favorite, LCDs and LEDs.

• The organic construction of an OLED is much thinner than the many layers required for an LCD or LED display.
• OLEDs can be built onto a thin plastic substrate which allows them to be flexible instead of the glass used in LCDs and LEDs.
• Due to its design, an OLED display is much brighter than an LED display. This is because the conductive and emissive layers of an OLED can be stacked several times to produce more light than an LED while still remaining extremely thin.
• Since OLEDs emit light themselves, they do not require a back-light. LCDs work by selectively  blocking areas of the back-light to produce individual pixels. OLEDs are also much more energy efficient than LCDs because they lack a back-light.
• As OLEDs are made from plastics, they can be built into large thin sheets which makes them much easier to produce. It is much more difficult to create an LCD display of the same size.
• OLEDs have a much viewing angle than LCDs because they do not block light in any fashion. Since they produce their own light, they can provide a field of view of upto 170^o.

For all their advantages over LCDs and LEDs, OLEDs have a few problems as well.

• Red and green OLED films have a very long life of between 46,000 to 230,000 hours while blue OLED films currently have a much shorter lifespan of around 14,000 hours.
• The manufacturing process for OLEDs is not cheap as of now.
• OLEDs are not at all water resistant and can easily be damaged on contact with even a few drops of water.

While LCD and LED technology remains in the mainstream, OLEDs are making headway into the television, computer and mobile segments. Manufacturers and engineers are realizing the potential benefits of using OLED technology for their devices. In the next few years we should see an increase in OLED sales as well as some novel applications of the technology.

Stay tuned to the Tech 101 segment to learn more about what makes our everyday devices tick.