Organic light-emitting diodes (OLEDs)

The most promising organic-based electro-optic devices are OLEDs. They have recently received a great deal of attention because of their application for a wide range of display applications as well as from the standpoint of scientific interest. They are attractive because of low voltage driving, high brightness, capability of multicolour emission by the selection of emitting materials and easy fabrication of large-area and thin-film devices.¹⁶ Following the reports on OLEDs using single crystals of anthracene¹⁷, recent pioneering works on OLEDs using low molecular-weight organic materials and a conjugated polymer have triggered extensive research and development within this field. Recent years have witnessed significant progress with regard to brightness, multi- or full-colour emission, and durability and thermal stability of OLEDs.

OLEDs fall into two competing technologies based on the materials used:

• polymers; solution processing methods

• low molecular weight materials; vapour deposition methods

Small molecule devices are fabricated using vacuum evaporation techniques, whereas polymer structures can be applied using spin-casting or ink-jet techniques. The screen-printing technique has recently been introduced and is presumed to be applicable to both

The typical structure of molecule OLEDs consists of single or multiple layers of organic thin films sandwiched normally between the transparent indium tin oxide (ITO) coated glass and vacuum-evaporated metals with low work function such as magnesium/silver (Mg/Ag) or aluminium (Al) as represented in Figure 2. The operation of OLEDs involves injection of holes and electrons from the ITO and metal electrodes respectively as well as transport of injected charge carriers. Finally, a recombination of holes and electrons in the emission layer generates an electronically excited state in the molecule, followed by luminescent emission.

Generally, layered devices consisting of charge transport and emitting layers can more readily achieve charge balance than single-layer devices. A suitable combination of charge transporting and emitting materials in layered devices reduces the energy barrier for the injection of charge carriers from the electrodes. The charge transport layer also acts as a blocking layer against the injection of either holes or electrons from the adjoining layer and their subsequent escape from the device. In order to achieve high performance in OLEDs, it is necessary to attain charge balance. In an ideal case, there should only be a negligible energy barrier for charge injection at each interface leading to balanced charge transport and efficient recombination in the emitter layer.

Energy

hν

h⁺

.e

-HTL

ITO ETL

Figure 2. Schematic representation of an OLED and the energy level diagram of the materials involved in it. (reference: www.fotoline.ch/FOTOintern/02-19/oled.jpg)

Due to spin statistics, theoretically only a quarter of the excitons produced by electrical charge injection are singlet excitons and, therefore, by using fluorescent dyes (singlet emitter), the maximum efficiency of OLEDs is limited to 25 %. Recently reports have shown that this rule can not be strictly applicable for conjugated polymers in which efficiency higher than 25 % may be attainable.¹⁸

For more detailed information about the principles, mechanisms and physics of OLEDs, excellent and comprehensive discussion can be found in the monographs by Greenham and Friend¹⁹ as well as Ishii et al.²⁰

Thus, the performance of OLEDs is related to three main technological issues:

• colour range

• electroluminescence efficiency

• reliability or stability

The colour of OLEDs depends on the molecular compositions of the organic interface. The luminance efficiency can be improved by incorporating phosphorescent dyes as dopants in an emitting layer and, thus, exploiting the energy transfer from the singlet state of the host to the triplet state of the phosphorescent emitter to obtain electrophosphorescence.²¹ But this concept demands the use of additional layers/components of host and blocking materials and the realisation of an almost perfect match of energy levels in the various materials involved in order to guarantee a high degree of energy transfer to the triplet emitter. Moreover, phosphorescent emitter has to be doped into a host material to avoid any triplet-triplet annihilation and the recombination zone has to be confined to the doped layer. This has been successfully demonstrated in devices prepared by vapour deposition of low molar mass compounds as well as doped systems in polymer blends.^22,23 This is, at present, one of the most attractive strategies to obtain highly efficient devices with emission in the green to red region. The reliability of OLEDs and other electro-optical devices, is a key source for the sceptical approach in the photonics community. The main cause of the reliability problem is the degradation of organic molecules. Many groups are addressing this issue and one solution is to introduce stabilising agents and efficient sealing methods. Another reliability problem relates to deterioration of the active cathode. Engineering tools have been used to deal with this issue.

For the fabrication of high-performance OLEDs, an understanding of basic processes, such as charge injection from the electrodes, charge transport, recombination of charge carriers to generate the electronically excited-state molecule as well as development of new materials with high performance and judicious choice of the combination of emitting and charge transporting materials and the combination of emitting and luminescent dopant molecules, are of vital importance. For this purpose, not only emitting materials but also charge transporting materials are also required. Both polymers and small molecules are candidates for materials in OLEDs.

The materials for OLEDs should meet the following requirements²⁴:

• possess a suitable ionisation potential and electron affinity in order to match energy levels for the injection of charge carriers at electrode/organic material and organic material/organic material interfaces

• permit the formation of a uniform film without pinholes

• morphological stability

• thermal stability

• electrochemical stability

• high luminescence for emitting materials.

In addition, doping of luminescent compounds has been shown to be an effective method for attaining high brightness and desirable emission colour.

The most important materials required for OLED technology are red, green and blue emitters.

It is possible to get any other colour by using a combination of red, green and blue (RGB).

Concerning the class of emitter materials,

metal chelates like 8-quinolinolato aluminium (III) (Alq3), porphyrine metal complexes, rare earth metal-organic complexes of Eu, Ru, Tb etc.^25,26,

small organic molecules like triphenyldiamines (TPDs), oligothiophenes, rubrene, oxidiazoles, triazoles, porphyrines, perylenes, coumarines, nile red etc.,

polymers like polyfluorenes, poly(vinyl carbazole), poly(TPD)s, poly(phenylene vinylene)s (PPVs), derivatives of poly(thiophene), etc.

and different combinations of these molecules are used in OLEDs^{27, , ,4528 29} . Chemical structures of some materials showing good performance in OLEDs are represented in

Figure 3. Emitting colour in organic materials can be tuned from blue to green, yellow, orange and even red by incorporation of different substituents as it was shown for the case of PPV and thiophene derivatives^30,31.

O

Alq₃ Coumarin-6 Tb(acac)₃

Carbazole derivative Perylene Polyfluorenes Eu(acac)₃ DCM dye

Nile red

Figure 3. Chemical structures of some emitters used in OLEDs.