Electroluminescence efficiency effect

The applications of the soluble polythiophene derivatives to the PLED are advantageous in that their polymer blends or composites can be easily formed [147]. Berggren et al. [126] have recently reported that these polymer blends (e.g. a mixture of different kinds of polythiophene derivatives) exhibit voltage controlled colors in electroluminescence. This effect was explained by the assumption that a number of nano-PLEDs of 50-200 nm in size yielded by micro phase separation are coupled parallel and operate in a specific voltage range depending upon the polymer species [126,130,131]. In addition, these nano-PLEDs dispersed in an insulator matrix such as poly(methyl methacrylate) display white hght emission with quantum efficiency ofO.4-0.6% [133]. 

Zinc sulfide, with its wide band gap of 3.66 eV, has been considered as an excellent electroluminescent (EL) material. The electroluminescence of ZnS has been used as a probe for unraveling the energetics at the ZnS/electrolyte interface and for possible application to display devices. Fan and Bard [127] examined the effect of temperature on EL of Al-doped self-activated ZnS single crystals in a persulfate-butyronitrile solution, as well as the time-resolved photoluminescence (PL) of the compound. Further [128], they investigated the PL and EL from single-crystal Mn-doped ZnS (ZnS Mn) centered at 580 nm. The PL was quenched by surface modification with U-treated poly(vinylferrocene). The effect of pH and temperature on the EL of ZnS Mn in aqueous and butyronitrile solutions upon reduction of per-oxydisulfate ion was also studied. EL of polycrystalline chemical vapor deposited (CVD) ZnS doped with Al, Cu-Al, and Mn was also observed with peaks at 430, 475, and 565 nm, respectively. High EL efficiency, comparable to that of singlecrystal ZnS, was found for the doped CVD polycrystalline ZnS. In all cases, the EL efficiency was about 0.2-0.3%. 

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.)... 

An important conclusion which can be drawn from these results is the strong increase of the PL intensity with decreasing particle size. The lower limit of crystallite size which can be currently prepared in the nc-Si/silica films is about 2 nm. For an efficient PL and EL one would like to prepare films with the maximum possible content of the smallest Si nanocrystals. For an efficient electroluminescence a thin silica interface between the crystallites is required in order to ensure good electric transport properties (tunneling of electrons and holes through the oxide). Therefore we have investigated the effect of the average thickness of the silica interface (or of the fraction of nc-Si in the films) on the intensity of the PL. The results are shown in Fig. 6 [40]. 

Deep-level states play an important role in solid-state devices through their behavior as recombination centers. For example, deep-level states are tmdesirable when they facilitate electronic transitions that reduce the efficiency of photovoltaic cells. In other cases, the added reaction pathways for electrons result in desired effects. Electroluminescent panels, for example, rely on electronic transitions that result in emission of photons. The energy level of the states caused by introduction of dopants determines the color of the emitted light. Interfacial states are believed to play a key role in electroluminescence, and commercieil development of this technology will hinge on understanding the relationship between fabrication techniques and tile formation of deep-level states. Deep-level states also influence the performance of solid-state varistors. 

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