17-01-2013, 03:36 PM
Organic Light Emitting Diodes operation and application in displays
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Introduction
Organic materials are of great interest for electronics applications, as they have many advantages over their inorganic counterparts.They may often be solution-processed, allowing the fabrication of devices such as circuits,displays,and radio-frequency identification deviceson plastic substrates, and deposition by unconventional means, such as screen and inkjet printing.The most attractive prospect, however, is the incorporation of functionality by design. The versatility of organic synthetic techniques and the wide spectrum of commercially available building blocks allow seemingly infinite flexibility in tuning molecular structure, and therefore the corresponding molecular packing and macroscopic properties. Already, organic solids such as pentaceneand rubrenehave surpassed amorphous Si in performance, of thin film transistors (TFT) and light emitting devices creating organic light emitting diodes (OLEDs) which are used by the former. An OLED is a thin-film solid state device, which makes it easier to apply to flexible displays because of its relatively simple fabrication process and reduced distortion according to the geometric form of display.
OLEDs, theirs structure and operation
Structures of OLEDs OLED (organic light emitting diode) is a monolithic, thin-film, semi conductive device that emits light when a voltage is applied to it. Various ways of light are generated by applying an electric eld to organic materials, without involving any intermediate energy forms - the phenomenon known as organic electroluminescence (EL). EL is the result of the electric eld–
imposed formation of emissive states without recourse of any
intermediate energy forms, such as heat.
In its most basic form, an OLED consists of a series of vacuumdeposited,
small-molecule organic thin films that are sandwiched
between two thin-film conductors. The following figures show most
often met constructions of this device. In Fig. 1 is presented one of
the possible simple structures of OLED. Here emission of EL
occurs in the electron and hole transmission layers. However, in
more complicated but also more efficient OLED is shown in Fig. 2,
the emission takes place in a separate layer.
Basic phenomena in OLEDs
OLEDs made of small molecules behave very similarly to conventional inorganic LEDs, but the fundamental difference exists between conventional inorganic semiconductors and the “so-called” molecular semiconductors. This difference originates primarily from two intrinsically different electronic and optical characteristics between conventional inorganic and organic semiconductors.
Model electroluminescence
In order to obtain the EL effect carriers should be injected
from the electrodes into the organic material. Usually diodes are
fabricated on ITO coated glass substrates. The model assumes that
injection of charges from the electrode into the organic
semiconductor is governed by thermionic emission. The
thermionic emission occurs across the energy barrier that is
formed between the work function of the injecting electrode and
the HOMO or LUMO of the organic semiconductor depending on
whether hole or electron injection, respectively, is considered.
Application
The tremendous strides have been made in the science and technology of organic electroluminescence (EL). Most of this progress has been applied in developing at panel displays. If this rate of progress can be sustained into the next decade, organic EL technology has the potential to exert an impact not only on displays, but also on general lighting applications. In particular, a large-area white-light-producing organic light-emitting device could potentially provide a solid state diffuse light source that could compete with conventional lighting technologies in performance and cost. The vision of solid state lighting has largely been driven by the desire to reduce energy consumption.