Preface

In this article taken OLED technology explaining the basic operation and some models currently produced. How to write in this technology is designed more for display and for light production environments. I hope I can go into that with your comments.

Introduction

Everyone knows the well-established LED technology and its applications, especially in the field of lighting and electronics.

The LED is a special suitably doped diode that generates photons subjected to a voltage. The color of the emitted radiation depends on the composition of different semiconductor materials (gallium arsenide, silicon carbide, etc. ..).

There is also another technology, the OLED (organic light emitting diode). Born in 1987 on the basis of research conducted by Kodak, it's special light-emitting diodes where done in the same manner as LEDs, but the light through a layer of organic material that allows the Issue of the entire visible spectrum.

This technology, after a beginning marked by a lack of application in the market, is undergoing several developments recently, with the adoption of new construction techniques and new technologies that help solve some of the problems that afflicted, such as poor life, mechanical fragility, high production costs, both for the production process, which for the right to pay for expensive patent in the hands of Kodak.

Science and construction

I'll try to explain the operation of such devices without getting too specific in the article and what is outside as too long and complex.

Within the limits of the approximation will try to give accurate and concise way that will allow even those unfamiliar chemistry to understand its operation.

Basic Theory

From an energy point of view the atoms of any molecule can be in two states:

• fundamental, namely, when the energy of the atom is as small as possible.
• excited, ie, when an atom is at a higher energy level. This energy difference is due to the shift of the electron to an orbital with a higher energy level (ie more distant from the nucleus), displacement caused by an external energy source which in our case is the potential difference applied all'oled. Each atom is always trying to reach a lower energy level possible, and this happens in the excited state when the electron returns all'orbitale lower level. This generates a release of energy by the electron (quantum of light). As in any diode, we are faced with a form of PN junction.

This junction, as we know, consists of two parts: a layer composed of a material doped so as to have in its atomic structure of the holes (ie elements with fewer electrons), and another specifically for the drugged have an "excess electrons"; this course compared to the base material. For example, silicon diode: p doping (aluminum 3 electrons), n doped (phosphorus 5 electrons). The holes in the p-type doping, can be seen as the positive charges. When the electron jumps in the hole it leaves behind another hole. In this way, the hole acts like a positive charge on the move allowing the run. In the case of n-type dopant instead we are faced with an excess of electrons that can move freely in the structure, similarly to what happens in metals.

When a diode is applied to a voltage, the conduction electrons are forced to move in na-hole. This is the characteristic of the diode to allow the passage of electrons only in one direction and run.

The structure of an OLED today is composed of several layers positioned vertically:

• a glass substrate used as storage for the following items

Injectors charge

• an anode consisting of idiopathic and tin
• a cathode consisting of a metal alloy of Ag-Mg or Li-Al

Conductive layer charge

• hole injecting layer (HTL)
• electron transport layer (ETL)

Organic layer emitter

• emitter layer consists of molecules with luminescent properties.

This layer is the key to the OLED. 
In fact, the color of light emitted depends on the composition of this layer which has three main light-emitting materials, blue, red, green, thus being able to reproduce the entire visible spectrum. 
These substances are contained in a host that provides the film a solid foundation for the fluorescent molecules. In the early devices were small organic fluorescent molecules. 
Then to increase the efficiency of the OLED has received a phosphorescent dopant. 
You can draw up a classification based on the type of OLED emitter layer. 

• OLEDs consist of small molecules in the crystalline phase: SMOLED
• OLEDs consist of polymers, or PLEDs poled
• OLED-based dendrimers

The smoled are currently on the market and the most promising new developments. 
I pled have received significant interest because of the possibility of creating a polymer layer that has the ability to transport charge and emit light. 
The dendrimers used in OLED macrosystems as possible.

Operation

Based on the birth of the electroluminescent light can be divided into three parts.

• formation of excitons
• Energy transfer from excitons to the emitter
• Light emission

Formation of excitons

Between the two electrodes when a potential is applied to the anode pulls electrons htl oxidize and creating holes in it. 
The ETL is filled and is reduced by electrons driven by the applied field (electrons subtracted all'htl) 
Electrons and holes meet in the emitter layer (elm), which recombine to give the exciton. The exciton is nothing but the interaction of electron and hole. This exciton can be in a state of triplietto or singlet. These states depend on the spins of electrons configuarzione. The electron spin is the rotation of the electron around its axis.

Energy transfer from excitons to the emitter

To induce elettrochemiluminescenza hosted in fluorescent molecule, there must be a transfer of energy from the electron donor molecule, the molecule accettrice that can occur in three ways:

• Förster transfer.
• Dexter transfer,
• entrapment charge.

In Förster donor acceptor interaction are talking about a long distance (this process affects the exciton in the singlet state), while in the dexter short distance (exciton in the triplet state). The competition between the three mechanisms for the transfer of energy depends on the exciton lifetime and the emitter layer thickness.

Light emission

The excitons interact with the molecules of the emitter layer by taking them to an excited state. 
Through the processes of radiative and non radiative decay (the initial step of the electron orbit excited more stable orbit) generates a quantum of light to allow the electron light output. The internal and then the efficiency of electroluminescence Issued by the possibility, however, is limited to use only in the state of singlet excitons and because of that you can reach a maximum of about 25% of efficiency. 
What you try to do in recent years is to look for materials that allow doping to exploit even the excitons in the triplet state.

Osram Lamps

Osram has introduced one of the first application of OLED for lighting environments. It's called ORBEOS and looks like a luminous surface of 80mm round and just 2.1mm thick weighing 24g. It gives a warm light without glare with a color temperature of 2,800 K (similar to an incandescent lamp and then with a good color rendering index equal to 75) and light an efficiency of 25 lm / W, which places it above the normal halogen lamps.

Osram ensures a luminance of 1000 cd / m 2 obtained with a specific power of 0.65 W at a voltage of 3.4 volts continuously.

Under these conditions the device has a lifespan of 5000 hours. This equipment is intended for a medium-high market and allows the use of both functional and scenic as light, is dimmable, unlike LEDs, thermal management is easier. The panel contains no mercury and is emission-free UV / IR.

Panels Verbatin

In April 2011 the Verbatin Velvia with the project has officially entered the world OLED lighting. These particular panels with an area of 150 cm 2 mudulabili.

Verbatim will enter the market at the end of 2011 one of the largest OLED panels available. Size 14cm x 14cm soft light and ultra saturated with 1000 cd / m 2 with an energy that ensures consuno 28 lm / W. They are also equipped with a system that allows adjustment of light from cool white 6500 K to 2500 K warmer white

The future: Olet

The olet represent the most promising technology for 'and for the manufacture of electroluminescent displays at low cost.

Born in Italy and made by the research group headed by Michele Muccini, researcher at the Institute of Bologna ISMN for nanostructured materials, components are planar structure (not vertical like the oled)-shaped transistor.

They consist of three layers: a glass substrate, followed by a layer of indium tin oxide and another metal pole for a total thickness of 62 nm.

These clusters have been carefully chosen based on the properties to give the greatest possible efficiency. The advantage of this technology is that it allows the creation of more efficient and less expensive equipment for OLEDs, precisely due to their different structure no longer in vertical layers.

Display

The other scope in some ways most important, but do not want to investigate is the use of OLED screens in the production of high quality computer or smartfone characterized by a thick virtually nonexistent, the most vibrant and bright colors, a viewing angle illimitato.A this is combined with extremely low power consumption, which opens the door for countless applications.

Philips, Samsung etc.. are investing in this technology and its puntanto on the characteristics of OLED. Thanks to their screens was eliminated in the wake of this effect in LCD TVs, thanks to a very short response time (very appreciated for watching movies).

The first TV commercial is Xel-1, 11-inch Sony, but many other ones are born.

Conclusions

The idea for this article came about when, lighting studying at university, I came across the LED. Too bad that in my book led and oled were explained in a rough and incomplete because the text is focused more on standard lamps and lighting calculations.

From what little I read I realized I was faced with a superlative technology that eliminates some of the defects of LED lighting.

The information in this article are taken from my book of lighting and chemistry and a master's thesis on the OLED, which enabled me to understand this technology. In my research then I came across on olet representing another step forward for the Italian engineering and science.

In the world of the future maybe we will live in homes where the lighting will be much more efficient and not limited by a single lamp but rather spread over the entire surface with a very low energy costs. I hope you enjoyed the article.