Electroluminescence

Electroluminescence is observed to occur during anodization on both n- and p-type materials. The luminescence onp type is uniform on the sample surface, whereas that on n type is highly nonuniform.It occurs only when the oxide reaches a certain thickness as shown in Fig. 3.14. ° No light emission is observed below a thickness of 15 nm. For Si02 greater than 25 nm thick, the intensity of emitted light increases exponentially, the exponential factor being lOnm as shown in Fig. 3.14. 

FIGURE 3.14. Integrated light intensity as a function of SiOa thickness in KNOs-containing ethylene glycol. After Zhou et al3 (Reproduced by permission of The Electrochemical Society, Inc.) 

Whereas bulk materials caimot respond quickly enough to an alternating current of such a high frequency, AC creates rapid electron-hole recombination within nanoclusters. Similar fluorescence emission phenomena had been observed with a variety of ultra-small metal nano-clusters. 

Apart from efficiency, the power of LEDs is also an important driving force. LEDs with input power of 30 W, with external energy efficiency of about 10%, have been demonstrated in 2003 by the Japanese company Nichia. 

In this section, only luminescent materials that can be used in inorganic low voltage electroluminescent devices will be discussed in some detail. Phosphors are used for two reasons  

Phosphors for LEDs have to fuUiU rather harsh conditions. The Stokes Shift must be small, the absorption must be high and, in addition, as the excitation densities are of the order of 20 W cm (the area which emits Ught is much smaller than e.g. in fluorescent lamps), the luminescent materials must remain efficient up to high temperatures, should not show saturation (meaning a less than Unear increase in output power with input power at high excitation densities) and must be radiation stable. 

Robbins has treated these processes more quantitatively [5.225]. A comparison of predicted and experimentally obtained efficiencies is given in Table 5.20. Quite good agreement is observed. 

The use of light emitting diodes (LED) continues to increase dramatically, for example in color displays. The inorganic semiconductors like GaN, ZnS, ZnSe, 

Since the calcium or other metals used so far as electrodes must be rigorously protected from the atmosphere to prevent degradation further applications have to find stable electrode materials. Also, the attainable electric currents and the efficiency of the polymeric LED depends sensitively on the polymer-electrode interface. 

Due to the large band gap and high triplet energy level of the poly(3, 6-dibenzosilole) 5, the copolymer is an excellent host for the fabrication of blue polymer phosphorescent light-emitting diodes. A high external quantum efficiency (t/el) of 4.8% and a luminance efficiency of 7.2 cd/A at 644 cd/m2 have been achieved for blue phosphorescence devices (emission peak (AEL) at 462 nm, CIE coordinates x = 0.15,y = 0.26). The performances of the devices are much better than those reported for blue phosphorescent devices with poly(A--viny 1 cabarzo 1 e) (PVK) as the host.32 

Copolymers 10 derived from 3,6-dibenzosilole and 2,7-fluorene are blue electroluminescent SCPs.27 When the copolymers are used as the emissive layer in EL devices, highly efficient pure blue emissions with CIE coordinates of (x = 0.16, y = 0.07), a 7EL of 3.34%, and a luminance efficiency of 2.02 cd/A at 326 cd/m2 are achieved from the copolymer with 90% fluorene content. The blue color matches the NTSC blue standard (x = 0.14, y = 0.08) quite well. The EL spectral stability of the devices is quite good, even under operation at elevated temperatures. Copolymer 9 derived from 3,6- and 2,7-dibenzosiloles also exhibits high performance with a jyEE of 1.95%, a luminous efficiency of 1.69 cd/A, and a maximal brightness of 6000 cd/m2, with the CIE coordinates of (x = 0.162, y = 0.084).26 

Copolymers 11 derived from 2,7-fluorene and 2,3,4,5-tetraphenylsilole are green electroluminescent SCPs.28 The EL spectra of the copolymers show exclusive green emissions (AEL 528 nm) from the silole units, which are almost not changed with the copolymer compositions. A maximum 7yEL of 1.51% can be achieved with the copolymers as the emissive layer in EL devices. 

Green electroluminescence is also achieved from the well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2, 5-silole.29 With its neat film as the emissive layer, the EL device shows a maximum //Kr of 0.47%, but the device performance can be largely improved to a maximum //Ki. of 1.99% when using a copolymer/PF8 blend film as the emissive layer. Copolymer 14 derived from 2,7-dibenzosilole and 2,1,3-benzothiadiazole is also an excellent green EL polymer.26 A maximum 7el of 3.81% can be realized in EL devices. 

Copolymers 15 derived from 2,7-fluorene and 2,5-dithienylsilole are red electroluminescent SCPs.31 The EL devices with the copolymers as the emissive layer can display red light emissions with AEL up to 638 nm. The maximum //EL of the devices can reach 0.89%. Copolymer 18 derived from 2,7-fluorene and 

When two radical ions formed by electrolysis recombine to restore the closed-shell neutral species, there is in principle enough energy liberated to produce one excited molecule (and one ground state molecule). 

Thermoluminescence is a related process of light emission when some solids are heated to temperatures close to their melting point. If ions have been trapped in the solid so that they cannot move within the time of observation in the rigid matrix, they will recombine when diffusion can take place. The energetics of this process is similar to that of electroluminescence. 

Orbital diagrams of electroluminescence (or recombination luminescence) of positive and negative radical ions 

Thermal decomposition of cyclic peroxides producing an electronically excited state of a carbonyl group 

The luminescence arises from dye molecules which are sensitized by energy transfer from the excited carbonyl group. 

Other requirements are placed on electroluminescent devices in addition to supporting efficient hole and electron transport (1) the exciton energy in the transport layer has to be higher than the exciton energy in the luminescent layer (2) the formation of molecular complexes between the fluorescent layers should be precluded (3) the luminescent layer has to possess a high fluorescence quantum yield and (4) the film should be processed readily. 

Kuhn and D. Mobius, Angew. Chem. Internal. Ed. Engl., 10, 620 (1971). 

Photophysical and Photochemical Tools in Polymer Sciences, (M. A. Winnik ed.), NATO AS1 Ser. Vol. 182, Reidel (1986). 

Adachi, T. Tsutsui and S. Saito, Appl. Phvs. Lett., 55, 1489 (1989) T. Tsutsui and S. Saito, in Intrinsically Conducting Polymers An Emerging Technology, (M. Aldissi ed.), p. 123-134, Kluwer, Amsterdam (1993). 

Cg or Ch denote the number densities of electrons and holes. The variables i/ei represent electric mobilities. They give the ratio between the mean velocity u of charged particle and the acting electric field 

Compared to anorganic semiconductors electric mobilities in conjugated polymers are generally low. 

As has already been mentioned, the existence of the triplet excitons is a disadvantage of polymer LEDs. They do not contribute to the luminescence 

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Other glass-ceramics may find potential use in Hquid crystal and electroluminescent displays. 

In contrast to the LEDs, electroluminescent phosphors operate under high voltage and very high electric fields. The Sharp devices are ac and purely... 

A light-emitting diode (LED) is a forward-biasedp—n junction in which the appHed bias enables the recombination of electrons and holes at the junction, resulting in the emission of photons. This type of light emission resulting from the injection of charged carriers is referred to as electroluminescence. A direct band gap semiconductor is optimal for efficient light emission and thus the majority of the compound semiconductors are potential candidates for efficient LEDs. 

Other sohd-state apphcations of sihcon carbide include its use as an electroluminescent diode for use in sound recording equipment and photomultipliers and controllers. It has been studied as a reflective surface for lasers. By combining its excellent thermal conductivity and high electrical resistance, sihcon carbide has also found apphcation as an insulating material for integrated circuit substrates. 

Donor and acceptor levels are the active centers in most phosphors, as in zinc sulfide [1314-98-3] ZnS, containing an activator such as Cu and various co-activators. Phosphors are coated onto the inside of fluorescent lamps to convert the intense ultraviolet and blue from the mercury emissions into lower energy light to provide a color balance closer to daylight as in Figure 11. Phosphors can also be stimulated directly by electricity as in the Destriau effect in electroluminescent panels and by an electron beam as in the cathodoluminescence used in television and cathode ray display tubes and in (usually blue) vacuum-fluorescence alphanumeric displays. 

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

Benchtop X-ray energy dispersive analyzer BRA-17-02 based on a gas-filled electroluminescent detector with an x-ray tube excitation and range of the elements to be determined from K (Z=19) to U (Z=92) an electroluminescent detector ensures two times better resolution compared with traditional proportional counters and possesses 20 times greater x-ray efficiency compared with semiconductor detectors. The device is used usually for grits concentration determination when analysing of aviation oils (certified analysis procedures are available) and in mining industry. 

The results of simulation have been confirmed by determination of Fe traces in quai tz sand, Cu and Mo in flotation tails and Ag in waste fixing waters on BRA-17-02 analyzer based on X-ray gas-filled electroluminescent detector and on BRA-18 analyzer based on Si-drift detector. The results of the simulation conform satisfactory with the experimental data in the mentioned cases the optimum filtration results in 2 to 5 times lowering of the detection limit. 

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

Another recently discovered form of epitaxy is graphoepitaxy (Geis et al. 1979). Here a non-crystalline substrate (often the heat-resistant polymer polyi-mide, with or without a very thin metallic coating) is scored with grooves or pyramidal depressions the crystalline film deposited on such a substrate can have a sharp texture induced by the geometrical patterns. More recently, this has been tried out as an inexpensive way (because there is no need for a monocrystalline substrate) of preparing oriented ZnS films for electroluminescent devices (Kanata et al. 1988). 

The use of 1 l//-pyrido[2,l-Z)]quinazolin-l 1-ones in an organic electroluminescent device was patented (99JAP(K)99/74080). 2//-Pyrimido[2,l-n]isoquinolin-7-ols were patented as multi-functional fuel and lube additives (97USP5646098). 

Ordered dialkoxy PPV derivative has been prepared by Yoshino et al. [491. oly(2 -nonoyloxy-1,4-phenylene vinylene) 27a forms a nematic liquid-crystalline phase upon melting. The material retains its order upon cooling to room temperature, and its band gap (2.08 eV) is measurably smaller than in an unoricnted sample. Oriented electroluminescence may be achieved by rubbing a thin fdin of the material to induce molecular orientation [50],... 

The synthesis-driven approach towards material science can be applied to create oligomers and polymers with optimized properties, e.g. maximized carrier mobilities and electrical conductivities or high photo- and electroluminescence quantum yields. It becomes obvious, however, that the ability to synthesize structurally defined -architectures is the key to these high performance materials. 

Experiments on transport, injection, electroluminescence, and fluorescence probe the spatial correlation within the film, therefore we expect that their response will be sensitive to the self-affinity of the film. This approach, which we proved useful in the analysis of AFM data of conjugated molecular thin films grown in high vacuum, has never been applied to optical and electrical techniques on these systems and might be an interesting route to explore. We have started to assess the influence of different spatial correlations in thin films on the optical and the electro-optical properties, as it will be described in the next section. 

Figure 6-23. Sketch of the experimental arrangement tor measuring the angular dependence or lluores-ccncc and electroluminescence. The viewing angle 0 is varied between -90 " and +90 .

Figure 9-12. Absorption (Abs), photoluminescence excitation spectrum (PLCX), pholo-lumincscence (PL), and electroluminescence (EL) emission of mLPPP.

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