Undoped emission spectra

We have indicated how a modest (1.3) enhancement in angular intensity can be obtained in cavity devices with Alq emissive layers. Further enhancements in angular intensity are possible by choosing emissive layers with narrower free-space emission spectra than Alq. Alq doped with small quantities of the laser dye pyrromethene 580 (PM) results in the emission spectrum of the system becoming narrower than that of Alq. This is result of resonance energy transfer33 from the excited states of Alq to the excited states of PM580. The full width at half-maximum of the luminescence drops from 100 nm to 45 nm. The spectra are shown in Fig. 4.12. The external quantum efficiency of noncavity devices is enhanced in comparison with devices with an undoped Alq emissive layer. For a device with a ITO/TAD/Alq+0.5%PM (20 nm)/Alq/Li (1 nm)/Al(200 m) structure, an external quantum efficiency of 1.8-2% photons/electron was measured. For comparison, equivalent LEDs without the pyromethene dye had an external quantum efficiency (with Li/Al cathodes) of 0.8%. 

The effects produced by a planar microcavity on the electroluminescence characteristics of organic materials have been described. A number of organic and polymeric semiconductors have been employed by various groups in studies on microcavity LEDs. However, for detailed descriptions, three categories of emissive materials have been considered undoped Alq, Alq doped with 0.5% pyrromethene, and Alq+NAPOXA. Alq has a broad free-space emission spectrum spanning the... 

Figure 58 shows the photoluminescence spectrum of the undoped MgO degassed at the same temperature as the methane oxidative coupling reaction together with the photoluminescence speclrmn of the 3 mol% Li-doped MgO (Fig. 58, 2) and its deconvoluted curves (Fig. 58, 2-a and 2-b). In addition to a characteristic photolumincscence spectrum at around 370 nm, attributed to the surface sites in low coordination on MgO, the Li-doped MgO exhibits a new photoluminescence band at about 350-550 nm with a at about 450 nm (Fig. 58, 2-b). The intensity of this new emission depends on the amount of Li doped. The excitation spectr um corresponding to this new emission is evident at about 260-290 nm 100, 240), which suggests that surface sites with a coordination number of four may be associated with this new photoluminescence. 

Donor-acceptor absorption can also be observed in semiconductors, but this process is weak because of the small overlap of the wavefunctions (like an n -> n transition). Donor-acceptor absorption is best monitored through the emission, that is by excitation spectra. In the normal situation, the donor-acceptor absorption can be observed but the valence band-to-donor and the acceptor-to-conduction band transitions can also be seen, as they also contribute to the luminescence. All three of these transitions are weak but of similar strengths [6]. In undoped AgCl and AgBr, only a very weak excitation spectrum is seen, which consists of a relatively sharp line near the band edge. In Cd2 + doped AgBr both the sharper line, whose onset is about... 

Figure 8.12 (a) Electroluminescence spectra of all ZnO p-n junction recorded at 293 K for DC forward-bias currents of 8, 12, and 16 mA. The arrow indicates the absorption edge of p-type ZnO top layer, (b) PL spectra at 293 K of undoped and p-type ZnO layers (dotted lines) and transmission spectrum (broken line) of p-type ZnO layer. The EL emission is redshifted... 

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