OLED, The television of the future

The growing number of manufacturers developing displays based on OLED technologies shows us that this will be the trend in the near future

OLED stands for Organic Light-Emitting Diode. It works on the basis of a series of dots on a surface that light up by themselves and change color depending on the intensity of the electrical stimulus they receive. They are made up of colored molecules (green, red and blue) that emit light when placed between two electrodes and an electric current is applied to them.

An OLED diode is a solid-state semiconductor device, usually made up of two or three layers of organic molecules (semiconducting polymers). The use of organic molecules instead of inorganic semiconductors (which are used to make conventional LEDs) is a major advance, because the greater complexity of the former results in far superior performance. To achieve this effect, these devices also contain two electrodes, an anode and a cathode, placed just above and below the layers of organic material. This peculiar configuration gives the device a sandwich-like structure, in which the two slices of bread would be the anode and the cathode, and the filling would be the layers of organic plastic molecules.

Electroluminescence in organic materials was produced in the 1950s by Bernanose and his co-workers.

In a 1977 article in the Journal of the Chemical Society, Shirakawa, Heeger and MacDiarmid reported the discovery of high conductivity in iodine-doped polyacetylene. They received the 2000 Nobel Prize in Chemistry for "the discovery and development of conductive organic polymers".

In a 1990 article in the journal Nature, Burroughs and his co-workers reported research into a highly efficient green-light-emitting polymer.

As an application in the field of displays, OLED technology was developed in 1979 by Kodak engineers. The fact that it could not work in color at the time delayed its application until the 1990s, when Pioneer used it in the displays of some car radios. What is new is its application in large-diagonal displays, since until now it has been used quite frequently in small displays. For example, its first use was in the Kodak Easyshare LS633 camera in 2002. Since then, it has been common in numerous devices such as cell phones and MP3 players.

It has also been used fairly regularly as a computer monitor, in both desktops and laptops. The pioneer in applying this type of technology to computing was Sony, which even today is including OLED screens in its new VAIO models.

There are many different OLED technologies, as many as the wide variety of structures and materials that have been devised to contain and support the electroluminescent layer, and also depending on the type of organic compounds used.

The main advantages they offer are: lower cost, greater scalability, a wider color range, more contrast and brightness, wider viewing angles, lower power consumption and, in some technologies, flexibility, which will allow us to have roll-up screens. However, the rapid degradation of the materials used in their manufacture has limited their use for the time being. Research is currently under way to solve the resulting problems, which will make OLEDs a technology capable of replacing the current dominance of LCD (TFT) and plasma displays.

For all these reasons, OLED can and will be used in all kinds of applications: televisions, computers, portable devices (cell phones, PDAs, MP3 players...), information or warning displays, etc., in formats that, in any design, will range from small dimensions (2") to enormous sizes (equivalent to those being achieved with LCD). They can also be used to create large or small advertising signs, as well as light sources for general lighting. In addition, some technologies have the ability to have a flexible structure, which may one day lead to screens on clothing and fabrics and even surgical uses.

As for how it works, an OLED consists of two thin organic layers, an emissive layer and a conductive layer, which in turn are sandwiched between a thin film that acts as the anode terminal and another identical film that acts as the cathode. In general, these layers are made of molecules or polymers that conduct electricity. Their levels of electrical conductivity range from those of insulators to those of conductors, which is why they are called organic semiconductors. The choice of organic materials and the structure of the layers determine the device's operating characteristics: emitted color, lifetime and energy efficiency.

A voltage is applied across the OLED so that the anode is positive with respect to the cathode. This causes a current of electrons to flow in this direction. In this way, the cathode supplies electrons to the emissive layer and the anode withdraws them from the conductive layer.

Next, the emissive layer begins to become negatively charged (through an excess of electrons), while the conductive layer fills up with holes (through a lack of electrons). Electrostatic forces attract the electrons and the holes to each other, and they recombine (if the charge were reversed there would be no recombination and the device would not work). This happens closer to the emissive layer because in organic semiconductors holes are more mobile than electrons (the opposite of what happens with inorganic semiconductors).

Recombination is the phenomenon in which an atom captures an electron. The electron moves from a higher energy level to a lower one, releasing energy equal to the difference between the initial and final energies in the form of a photon.

This process causes radiation to be emitted at a frequency in the visible region, and a point of light of a particular color is observed. The sum of many of these recombinations occurring simultaneously is what we would call an image.

OLED technologies have numerous advantages over the displays most widely used today, namely LCD and plasma:



1. Lower power consumption. They do not need backlight technology; in other words, an element that is switched off really produces no light and consumes no energy, unlike LCDs, which cannot display a true "black" and create it with light, consuming energy continuously. OLEDs therefore display images with less light output, and when powered by a battery they can operate for a long time on the same charge.

2. They will be cheaper in the future. In general, organic elements and plastic substrates will be much cheaper. Assembly-line manufacturing processes can also lower production costs.

3. More brightness and contrast. The pixels emit light directly and autonomously. That is why, compared with LCDs, they allow a wider range of colors and wider viewing angles.

4. They are thinner and more flexible. On the one hand, the organic layers of polymers or molecules are thinner, brighter and much more flexible than the crystalline layers of an LED or LCD. On the other hand, in some technologies the printing substrate can be plastic, which offers flexibility compared with the rigid glass that supports LCDs or plasma screens.

5. Greater scalability and new applications. The future ability to scale screens up to the large sizes already achieved by LCDs and, above all, to roll up and fold screens in some of the OLED technologies that allow it, opens the door to a whole world of new applications yet to come.



Logically, like all new technologies, it also has some disadvantages, which we list below:

1. Water can permanently damage OLED devices.

2. Environmental impact. Organic components (molecules and polymers) have been shown to be difficult to recycle (high cost, complex techniques). This could have a very negative impact on the environment in the future.

3. Short lifespan. Green and red OLED layers have long lifetimes (10,000 to 40,000 hours), but blue ones currently last much less (only 1,000 hours).

4. Expensive manufacturing process. Currently, most OLED technologies are at the research stage, and the manufacturing processes (especially initially) are costly, unless a design is adopted that can be used in economies of scale, something that is not currently planned.



Research is currently under way to develop a new version of the organic LED that not only emits light but also collects solar energy to produce electricity. For the moment there is no date for its commercial launch, but there is already talk of how to manufacture it on a mass scale. With this technology, all kinds of small electrical devices could be built that would be self-sufficient in energy through their own display.

In conclusion, and although in this article we have often spoken in the future tense, we have already seen that OLED is a present reality. Tangible proof of this came at IFA 2008, where manufacturers as important as Sony and Samsung presented televisions with this technology with diagonals of more than 30". The only question that remains to be answered is how long it will be before we have one at home, but it will surely be sooner than we imagine.