TV screens have evolved from oversized cathode-ray tube technologies into highly efficient, semiconductor-based LED-based ones. Flat LED LCD screens have become ubiquitous, crowding out other display technologies and appearing everywhere from homes, to medical offices and stores, and even gas pumps. Anywhere a consumer might have a moment of downtime, a screen is installed.
The technology has progressed even further and today organic light-emitting diode (OLED) displays are of the highest display order. They are extremely thin, really light and versatile. Promising research indicates they can even be 3D printed onto fabrics and other complex structures.
How do OLEDs work?
LEDs are electronic components that are used as small colored indicators. Compared to outdated incandescent lamps, they are reliable, tiny and energy efficient. Their working principle is also different - they don't illuminate via heating the wire filament. Instead, they produce light when photons of lights are emitted due to electron movement in doped semiconductors. An organic-LED is simply an LED variant, in which light is emitted by organic molecules. Organic molecules are molecules built upon chains of carbon atoms, for example, alcohol, gasoline, sugar, plastics and wood.
Figure 1. Source: Structure of a passive OLED display. Source: Pro_Vector / Adobe Stock
In a standard LED, two semiconductor materials are fused to form a P-N junction layer. N-type materials have more number of electrons and P-type have more number of holes. This junction is also known as the depletion layer, as the holes and electrons cross this junction to cancel out each's effect and develop a neutral charge on this layer. Once the electrical connections are made to such a fused semiconductor material via a battery, the current starts to flow, but like a diode it only permits current in one direction. The light is produced when the electrons cross the P-N junction, recombine with the holes on the P side, and emit extra energy - a photon of light - in doing so. These quick flashes give the constant, efficient, diffused light of LEDs.
Therefore, a diode is simply a P-N junction and an LED is also a diode with an extra attribute: it produces light.
Organic LEDs also work similarly to traditional LEDs and diodes; however, they use organic molecules to produce an excess of holes and electrons rather than using P-type and N-type semiconductors. A basic organic LED would have six distinct layers as shown in Figure 1. Protective plastic or glass materials cover the top and bottom layers. The bottom layer is also referred to as a substrate and the top layer a seal. Immediately inside these layers, there are positive and negative terminals, also called anode and cathode, respectively. Lastly, inside these terminals lie the two organic molecules layers known as the conductive and emissive layers. The conductive layer lies alongside the anode and the emissive layer lies alongside the cathode and is responsible for producing light.
When a certain voltage is applied across the cathode and anode terminals, the current starts to flow. The voltage source supplies electrons to the cathode and holes to the anode. In other words, the anode loses electrons. The emissive layer will now have a negative charge (just like an N-type semiconductor) and the conductive layer will have a positive charge (just like a P-type semiconductor). The holes have higher mobility than electrons. Therefore, they travel from the conductive layer to the emissive layer. A photon of light is produced when a hole recombines with an electron and their effect is canceled out. This phenomenon happens several times every second; an organic LED emits constant light for as long as the voltage source is connected.
Advantages of organic LEDs
The main benefit of an organic LED is that it is much smaller, flexible and lighter than an LCD. The LCDs are generally 10 times thicker than OLEDs, which have around 0.2 to 0.3 mm thickness. OLED displays do not need a backlight and thus use less power than comparable LCD screen. Consequently, portable devices that use OLEDs, like smartphones, benefit from longer serviceable battery life.
When displaying sports or other fast-moving images, the LCDs response is generally slower, whereas OLEDs refresh up to 200 times faster. They also offer a much large viewing angle and display truer colors when compared to LCDs. Since each image pixel produces its own light and is not illuminated from a backlight, OLED TVs have the best contrast of an TV technology.
Disadvantages of organic LEDs
One major drawback of OLED screens is that they are not long-lasting. The organic molecules degrade with time, which means that OLEDs can deteriorate up to four times faster than the standard LED and LCD screens. However, manufacturers are trying hard to find engineering solutions. One more drawback is that organic molecules are water sensitive, which can create problems for portable devices like smartphones that may face much more humidity or challenging environments. Also, currently, OLEDs are expensive but with time they will get cheaper as they become more popular.
Conclusion
The future of OLED technology is likely to rest in its ability to become more accessible for average consumers. It remains at a higher price point than a standard LED display, but delivers a much higher quality.
Perhaps the best asset to OLED growth is its limitless form factor. OLED displays can be rolled and folded. There is also research that promises transparent and unique shapes for OLED displays with 3D printing. It creates a future where almost anything can be a screen.
