Light emission electroluminescence

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. 

Y. Ohmori, H. Kajii, T. Sawatani, H. Ueta, and K. Yoshino, Enhancement of electroluminescence utilizing confined energy transfer for red light emission, Thin Solid Films, 393 407-411 (2001). 

Figure 7.5 shows a schematic example of the electroluminescent process in a typical two-layer OLED device architecture. When a voltage is applied to the device, five key processes must take place for light emission to occur from the device. 

FIGURE 9.14 (continued) (c) The effective light emission luminance (solid square, /.emission) and effective light emission efficiency (open square) versus effective current density of 4-a-Si H TFTs 200 dpi AM-PLED are shown. The evolution of luminance (solid circle, Z-pled) and light emission efficiency (open circle) versus effective current density of the red PLED are also shown, (d) Electroluminescent (EL) spectra and CIE color coordinates of 4-a-Si H TFTs 200 dpi AM-PLED (solid line) and PLED (dashed line) are shown. (From Hong, Y., Nahm, J.-Y., and Kanicki, J., Appl. Phys. Lett., 83, 3233, 2003. With permission.)... 

A blue light-emitting electroluminescent device was claimed with an emission layer comprising a polymer matrix and a chromophoric component, which was either blended with or covalently attached to the polymer matrix. The chromophoric components were... 

Polymer light-emissive device Photovoltaic device Electroluminescent materials... 

Electroluminescent polymers, (III) and (IV), both having improved emission and oxidative stability properties were prepared by Son et al. (3) and Park et al. (4), respectively, which offered high charge mobility and blue light emission. 

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. 

Electroluminescence in anthracene as a result of the injection of holes by a field-enhanced tunnelling through a surface barrier when a gold electrode is employed has been investigated (Sworakowski et al., 1974), and recombination has also been reported following injection from carbon fibre electrodes (Williams et al., 1972). In all cases, because of the difference in hole and electron drift mobilities, maximum light emission occurs near the positive electrode. 

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... 

Keywords Silicon, germanium, carbon, alloys, nanostructures, optoelectronics, light emission, photoluminescence, electroluminescence, quantum well, quantum wire, quantum dot, superlattices, quantum confinement. 

Copolymers containing alternating l,4-bis(phenylethenyl)benzene, l,4-bis(phenylethenyl)-2,5-dimethoxybenzene or l,5-bis(phenylethenyl)naphthalene chromophores, and dibenzo-24-crown-8 spacers within the polymer backbone, best represented by 87, showed blue light emission in solution, and tunable photoluminescence and electroluminescence depending on the structure of the chromophore. Blends of these copolymers with a small amount of poly(ethylene oxide), and lithium salt as active layers, form efficient light-emitting electrochemical cells <2003JMC800>. 

The thulium (III) ion exhibits spectrally narrow light emission at about 480 nm. Li and coworkers were the first to use the Tm + ion in OLEDs [65]. They prepared a Tm complex Tm(acac)3(phen) and constructed double-layer cells with structure ITO/PVK/Tm complex/Al. The electroluminescence spectrum of the OLED with drive voltage 10 V and the photoluminescence spectrum with excitation wavelength at 350 nm are shown in Figure 11.29. The emitting intensity of 6.0cdm was achieved when a 16 V forward bias voltage was applied. 

Iridium(in) /8-diketonates show a relatively week absorption in the 350-400 nm range, that can be ascribed to spin-aUowed MLCT transitions. Electroluminescent spectra (EL) show green light emission. In electronic spectra of Ir(III)), Pt(II), Ru(II) and Os(II) /3-diketonates " weak absorption bands between 330 and 560 nm can be assigned to singlet and triplet MLCT transitions. The n n transition from the complexes are blue-shifted for about 20 mn compared with the free ligands. 

On semiconductors light emission is induced by injection of electrons into the conduction band and subsequent band-to-band radiative recombination with holes (Fig. 38a). The process is reminiscent of electroluminescence or cathodolumines-cence and works with p-type substrates only (at n-type specimens no hole is available at the surface). Tunnel biases of 1.5-2 V are necessary in the case of GaAs, for instance. Figure 38b is a photon map of a GaAlAs/GaAs multiquantum well obtained by Alvarado et al. [140], The white stripes are regions where photons are emitted and correspond to the GaAs layers. The lateral resolution is about 1 nm and is limited by the diffusion distance of minority carriers. In Sec. 5.1 we have seen an example of the application of this technique in the case of porous silicon layers. 

Sawada, M., Oobayashi, S., Yamaguchi, K., Takemura, H., Nakamura, M., Momose, K., and Saka, H., Characteristics of light emission lifetime of electroluminescent phosphor encapsulated by titanium-silicon-oxide film, Jpn. J. Appl. Phys., 41, 3885, 2002. 

---