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Organic Electroluminescent Devices: Recent Developments

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Organic Electroluminescent devices have recently attracted much interest due to their physical properties and potential applications in light-emitting devices and flat panel displays. The development of new materials displaying proper color with high efficiency and stability is essential for the development of a full color display. This review focuses on the recent progress in development in OLED’s based on polymer blends, charge transfer nanocomposites and various approaches to achieve full color display for lightweight and flat panel display devices.

Nowadays, one of the most important part of optoelectronic devices are light emitting diodes (LEDs), because several factors are driving their development. The most important ones are brightness, efficiency, flexibility, lifetime, rugged construction, low power consumption and suitable driving voltage. Organic light emitting devices (OLEDs) have attracted enormous research interests and activities during the last decade and will continue to do so in the future because of their fascinating properties and promising applications. OLEDs can operate with low power consumption and are highly luminescent, with emission spectra covering the whole range of visible light. OLEDs have undergone dramatic improvements in performance in the last fifteen years. Two main types of OLEDs either based on conjugated polymers, or small-molecular-weight materials have emerged. The organic materials require different technologies such thermal evaporation or spin coating. The electrical, mechanical and thermal properties of organic semiconductor materials are different from those of inorganic semiconductor materials
As device design, fabrication and characterization of organic light-emitting diodes OLEDs [1, 2] are transferred out of the laboratory and into the manufacture of flat-panel displays, it will become increasingly necessary to develop accurate and reliable models of performance. Such models will permit design optimization, integration with existing tools for the design of silicon driver circuitry, the diagnosis of problems in process control, and the understanding of degradation mechanisms in the never ending quest for longer operating lifetimes. In this paper, we summarize efforts, in our own laboratory and elsewhere, to develop and validate a model describing the physics of OLED device operation, including charge injection, transport and recombination. Much of this work is already published, or will shortly appear in print. Hence, this paper includes a relatively extensive list of references. The ultimate goal of any model of OLEDs is to describe the current–voltage–luminance behavior, which includes all the information necessary to obtain quantum and power efficiencies.

Recent Developments
Towards cross-linked polymer blends
A study aimed at evaluating cross-linkable polymer blends for application in OLED devices has been undertaken. Polymers similar to those of various earlier works on multilayering are used, where the insolubility of the cross-linked network was exploited in order to coat successive layers [3, 4]. In the case of cross-linkable polyfluorenes, it was shown that they have additional advantages with respect to non-cross-linked polymers, namely they were found to exhibit less aggregation and excimer formation than regular polymers [5, 6]. After processing, these cross-linkable polymers yield networks with high glass transition temperatures and good photoluminescence (PL) properties. In the case of blends, network formation precludes and prevents the two component systems from undergoing extensive phase segregation and provides a more intimate mixing of the two components than in the absence of cross-linking. Depending on the relative kinetics and the temperature dependencies of the cross-linking reaction and polymer chain mobility, we expect to be able to control the degree of phase segregation. The specific polymers used are poly(9,9-di-n-hexylfluorene) (DHF) and a poly-[4-nhexyltriphenylamine] (HTPA) each terminated with 4-phenylethenyl cross-linking end groups [7] (Fig. 1). HTPA is in the class of triarylamines, which are well known as hole transport materials, and this polymeric form has been shown to form an excellent hole transporting layer [5].
http://www.euroasiasemiconductor.com/03 2006 03 Mar Web/Nilesh/RAW/1.jpg

Fig 1: The chemical structure of specific polymers such as poly(9, 9-di-n-hexylfluorene) (DHF) (a) and of the poly-[(4-n-hexyltriphenyl)amine] (HTPA) (b). The R indicates the termination, in the case of the cross-linkable materials (a) and (b) are styrene terminated while for noncross-linkable the polymers are t-butyl phenyl terminated. The material synthesis and x-HTPA structure are indicated in (c).

The termination of the oligomers with mono-functional end groups plays two key roles in obtaining a processable material. First, the end caps allow control of molecular weight and the synthesis of oligomeric materials that are more soluble in the coating solvents. The second purpose, exemplified by the 4-phenylethenyl termini, is to provide reactive functionality that can be thermally triggered after coating into a thin film, chemically cross-linking the oligomer units and preventing further molecular diffusion which can lead to phase segregation. Designation of the crosslinkable forms of DHF and HTPA as x-DHF and x-HTPA, respectively has been reported. The coupling reagent bis(1,5-Cyclooctadiene)nicke(0) is handled under inert atmosphere [8]. The solvents is dried according to standard procedures. All reactions is carried out under an argon atmosphere [9].
The reaction involved the nickel(0)-mediated polymerization of 2,7-dibromo-9,9,-di-nhexylfluorene in the presence of 4-bromostyrene. The hole transport layer (HTL) oligomer, x-HTPA, is prepared in a similar fashion, except using bis(4-bromophenyl)-(4-nhexylphenyl amine) as the monomer. The amount of 4-bromostyrene is kept constant in all the polymerization reactions (15 mol% based on the dibromomoner). This stoichiometry of terminating reagent would result in a linear oligomer with a molecular weight of approximately 7000 g/mol in the polymerization of 2,7- dibromofluorenes. The molecular weights of all of the oligomers used is determined using gel permeation chromatography (GPC), referred to polystyrene standards, which are well known to lead to an overestimate of the molecular weights of rigid-rod polymers. Oligomeric materials with molecular weights <10 000 g/mol were found to be soluble up to concentrations of at least 10 mg/ml and to yield (homopolymer) films of good optical quality. The non-cross-linkable oligomers for the hole transport Layer (HTL), namely n-HTPA, and emitter layer, n-DHF, is each terminated by reaction with 1-bromo-4-tertbutylbenzene. The blends are prepared by mixing the HTPA oligomers and DHF oligomers in high-purified xylene in nitrogen environment and the various percentages of the materials mixtures are calculated by weight.

Towards Charge Transfer Nanocomposite:
Organics are quite good at converting photons into excitons, but it is nearly impossible to remove substantial amounts of these excitations from a polymer layer as a useful current. The difficulty comes from a fundamental mismatch of length scales: migration lengths for the donor-acceptor excitations (50nm) [10-11] are much shorter than the absorption lengths required to create the excitations (500nm). Clearly, it would be useful to engineer optical and electronic phenomena by linking relevant length scales, thus altering the coupling between polymer excitations and photon fields. In fact, this seems to be possible with blends of conjugated polymers and dispersed single-walled carbon nanotubes (SWNTs) [12].
Subtly different in concept from polymer-polymer blends, these ‘charge-transfer nanocomposites’ (CTNs), as they have come to be called, can express remarkable electronic conduction, photo-emissive, and photo-absorption properties. While fullerenes and their derivatives (such as SWNTs) were among the first examples, these concepts have now been extended to include any engineered nanophase dispersed into the electro-active polymer matrix to create a family of CTNs. There have already been a number of surprising demonstrations of technology based on CTNs. Among the most striking is one showing control over the recombination region in an organic light-emitting diode (OLED). By using CTNs, shallow trapping states are introduced into the emissive matrix that exhibit field dependent de-trapping. This results in a narrow recombination zone with a field-dependent position within the structure. When used in multilayered OLED structures, CTNs allow the construction of dynamic, multicolored, pixels.
It has been known for some time that the nanoparticle (C60, SWNT, etc.) dispersant, or the nanophase, introduces a discrete donor-acceptor state within the HOMO-LUMO gap (the gap between the highest occupied molecular orbit and the lowest) of the polymer host. The next most natural step is to engineer the position of this state within the band gap. Then, one might argue that a greater range of properties could be tapped. In fact, this can be seen using standard thin film OLED structures with a fullerene (C60) based CTNs. These have long been studied for their potential in photovoltaic applications. This implies luminescence quenching would be associated with such a CTN, due to dissociation of the excitons through resonant energy transfer of the electron to the nanophase. However, when the emissive polymer is chosen such that the fullerene levels fall outside of the polymer band gap—an unusual enhancement in luminosity of the CTN device is observed (when compared with the pure polymer devices) [13].

Fig 2: Nonocomposite devices are more proficient as compared to pure PPV devices

As seen in Fig. 2, this enhancement can be significant. Further, OLED structures using carbon nanotube-based CTNs have now been shown to be compatible with a number of lithographic techniques. When the nanophase-polymer system is chosen such that the nanoparticle trapping state falls within the HOMO-LUMO gap of the host, then the donor-acceptor behavior of the CTN is ideal for bulk heterojunction formation in photovoltaic applications. C60-bulk-heterojunction organic photovoltaics are currently being investigated by a number of groups [14]. Unfortunately, charge removal requires hopping conduction throughout the nanophase, raising the internal resistance of the device and lowering the overall external efficiency. An interesting solution to this predicament was recently attempted using carbon nanotubes [15]. In this case the nanotubes form high-mobility pathways out of the device. Unfortunately, this approach has also has a difficulty: holes are typically transferred onto the nanotubes and the electrons have low mobility.




Approches to Achieve Full-Color Displays
Side-by-Side Patterning of Red, Blue, and Green OLED’s
The conceptually simple approach to full-color is to use side-by-side R, G and B sub-pixels to make a compound, full-color pixel in much the same way as is achieved in a cathode ray tube. To our knowledge, this technique has only been demonstrated at extremely low resolution, where large and widely separated R, G and B pixels have been produced on the same substrate [16]. The principal limitation of the technique is that each OLED must have a different organic thin film as its light-emitting layer, necessitating separate growth and patterning of arrays of R,G and B OLED’s. Postdeposition processing of OLED’s must be compatible with the relatively low temperatures tolerated by small molecule organic materials, and their solubility or “swelling” in many of the organic solvents used in conventional lithographic patterning processes (for example, acetone used for photoresist removal). However, a process for 10-μm feature patterning of vacuum-deposited crystalline organic semiconductors using reactive ion etching has been published [17].
It is conceivable, therefore, that post deposition patterning of OLED’s can be accomplished at a sufficiently high-resolution for display applications; 100-μm diameter pixels corresponds to a density of 300 dots per inch (dpi), assuming the pixels have a 100% fill factor. The requirement for post-deposition patterning may be eliminated by using a substrate upon which a dielectric is deposited and patterned to form “walls” between adjacent R, G and B sub-pixels [18]. This allows for a particular color (e.g., blue) OLED to be grown at an angle to the substrate, so that the areas reserved for the other colors (e.g., red and green) are shadowed from the deposition. By changing the angle of deposition, R, G and B sub-pixels may be consecutively deposited on separate areas of the substrate. This technique has to our knowledge, not yet been demonstrated.

Filtering of White OLED’s
The single heterostructure OLED may be made to emit “white” light by using two or more sequentially deposited, light-emitting layers. The thickness of each lightemitting layer is optimized to yield the correct ratio of R, G and B intensity from each individual layer to appear white when superposed [19, 20]. To form a full-color display, the white OLED’s are grown on top of pre-patterned color bandpass filters which each transmit only R,G and B , or light. This technique eliminates the necessity for post-deposition patterning since only one, white light-emitting OLED is grown on the prepatterned substrate. The principal drawback of this approach is that much of the white OLED output must be removed by the filter to obtain the required primary colors. For example, up to 90% of the optical power from the white OLED is filtered in order to obtain a sufficiently saturated red pixel, with the result that the OLED must be driven up to ten times brighter than the required R,G and B pixel brightness. Since the rate of OLED degradation is a function of drive current, this can substantially shorten the useable operational lifetime of the display. Degradation is further enhanced as the filtered light generates heat in the substrate. The inefficiency inherent in color subtraction renders this approach impractical for many display applications, where power consumption must be minimized.

Microcavity Filtered OLED’s
A pixel based on microcavity filtering of the inherently broad OLED emission spectrum is illustrated. The filtering effects of various microcavity structures have been used to influence the directionality [21] and color [22] of OLED’s, due to variation of the spontaneous emission rate as a function of wavelength and position of the emitting molecule within the thin film [22]. The broad spectral emission of a conventional or a white light-emitting OLED can therefore be designed to emit primarily R,G or B light in the forward direction by placing it in a microcavity consisting of the reflective top metal electrode and a dielectric, distributed Bragg reflector pre-deposited under the transparent ITO electrode. Although separate R, G and B pixels have been demonstrated using microcavity filtering, the potential of such devices for full-color displays appears to be limited. First, the Fabry–Perot resonance wavelength selected by the microcavity is direction-dependent [23].
Design refinements to reduce this “color directionality” [24, 25] by incorporating a scattering layer outside of the microcavity tend to reduce the OLED efficiency and compromise the performance of the device. Furthermore, the required variations in thickness of a simple dielectric stack in order to separately enhance red, green, and blue light ( 3000 °A for an 8-layer stack) are significantly greater than the typical thickness of an OLED ( 1000 °A ). Thus, it is difficult to deposit a continuous organic thin film along with metal interconnects over the entire area of a substrate pre-patterned with microcavities optimized for three separate wavelengths.

Color-Tunable OLED’s
An array of continuously color-tunable OLED’s removes the requirement that at least three sub-pixels be used to make a single, full-color element. This approach, therefore, results in at least a three-fold improvement in resolution and display fill-factor as compared to the various side-by-side architectures. Since only one OLED structure is grown over the entire area of the display, this also eliminates the need for substrate patterning prior to thin film deposition, yielding advantages in manufacturing simplicity. This simplicity is somewhat offset by additional complexity required in the drive circuit, which must be capable of controlling color while simultaneously controlling brightness and greyscale. Until recently, there is only published examples of color-tunable OLED’s utilized polymer blends [26-29] or polymer electrochemical cells [30].
In the former device, each component of the blend emits at a different energy. The color is tuned by varying the applied voltage; higher voltage results in more emission from the higher excitation energy (toward blue emitting) polymer, while also resulting in higher overall brightness due to increased current injection into the device. Tuning from orange to white has been demonstrated, but incomplete quenching of the low-energy emission has thus far prohibited tuning into the blue. In addition, the emission intensity is controlled by using a pulsed current source at reduced duty cycles. In a color display, therefore, high drive voltages at very low duty cycles may be necessary for blue pixels, which is likely to enhance degradation. Furthermore, in a high resolution display, color tuning the pixels while independently controlling brightness and greyscale appears to be problematic. In the case of two-color polymeric electrochemical cells, the polarity of the drive voltage across the device has been shown to affect its color. However, such devices are far too slow [31] for video applications, and apparently can not be extended beyond two-color operation.

Conclusion:
Organic based LEDs successfully raid the way in full colour display applications for costumer electronics like digital cameras, cell phones, car stereos, personal digital assistants or monitors. The unique properties of organic thin films may be used to realize display devices which are unobtainable with conventional semiconductor technologies. Their great advantage is that they are self-emitters, need less energy, can be formed into different shapes and can be much thinner as LCD devices. If they are grown on a plastic substrate, they are flexible too. Thus future of LEDs is clear: more colours, more brightness and low energy consumption.


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