Photoluminescent dopants

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

Mg2+ ion. 49 has been used to deposit MgO by atomic layer epitaxy,222 and is commonly employed as a />-type dopant for semiconductors, particularly GaAs,223 GaN,224,225 and AlGaN.226 In GaN, Mg doping induces a blue 2.8 eV photoluminescence band arising from donor-acceptor (D-A) pair recombination.227 It is likely that isolated Mg... 

One problem with methods that produce polycrystalline or nanocrystalline material is that it is not feasible to characterize electrically dopants in such materials by the traditional four-point-probe contacts needed for Hall measurements. Other characterization methods such as optical absorption, photoluminescence (PL), Raman, X-ray and electron diffraction, X-ray rocking-curve widths to assess crystalline quality, secondary ion mass spectrometry (SIMS), scanning or transmission electron microscopy (SEM and TEM), cathodolumi-nescence (CL), and wet-chemical etching provide valuable information, but do not directly yield carrier concentrations. 

Another class of red dopants, tetraphenylporphyrins (63), offer a direct energy transfer from blue to red [151], The absorption bands comprise the sharp porphyrin Soret band at 418 nm and the weaker Q bands at 512 and 550 nm. The photoluminescence shows two sharp transitions at 653 and 714 nm and can be induced from a blue emitting host by Forster transfer to the Soret band and internal conversion to the Q bands. 

The zone of recombination can be very small as was shown by Aminaka et al. [225] by doping only a thin layer (5 nm) in the device by a red emission material. By observing the ratio of host and dopant emission, the authors were able to show that the recombination zone of the device was as thin as 10 nm. The emitted light is usually coupled out at the substrate side through the transparent anode. As a rule, the electroluminescence spectrum does not differ much from the photoluminescence spectrum. 

Metal complexes with Schiff base ligands have useful applications in organic optoelectronics due to their outstanding photoluminescent (PL) and electroluminescent (EL) properties, and their ease of synthesis, which readily allows structural modification for optimization of material properties.28 Hamada and co-workers pioneered the use of zinc(II) Schiff base complexes as blue to greenish white emitters for EL devices. We have demonstrated Pt(II) Schiff base triplet emitters as yellow dopants for organic light-emitting devices... 

Figure 26. Experimental photoluminescence quantum yields for Mn2+ CdS nanocrystals, plotted versus Mn2+ concentration (a) for the long-lived yellow emission from Figure 25 (b) for the shortlived red emission from Figure 25. The solid lines represent the proportion of particles with (a) 1 and (b) 2-5 Mn2+ dopant ions, as described by Eqs. 1 and 2. [Adapted from (82).]...

State lifetimes and modes of energy transfer within the structure. Examples of this are photoluminescence of ZnS nanoparticles studied by Wu et al. (1994), and Mn doped ZnS nanoparticles by Bhargava et al. (1994). In the latter study, the doped nanocrystals were found to have higher quantum efficiency for fluorescence emission than bulk material, and a substantially smaller excited state lifetime. In the case of environmental nanoparticles of iron and manganese oxides, photoluminescence due to any activator dopant would be quenched by magnetic coupling and lattice vibrations. This reduces the utility of photoluminescence studies to excited state lifetimes due to particle-dopant coupling of various types. The fluorescence of uranyl ion sorbed onto iron oxides has been studied in this way, but not as a function of particle size. 

Fig. 16 Temperature dependent emission intensity from TPP dopants in a PF2/6 host. The host is optically excited and the TPP guest is populated by a combination of Forster transfer and exciton migration. Data is corrected for the temperature dependencies of the host and guest photoluminescence quantum yields...

Fig. 3.24 Photoluminescence (PL) transients of the different samples. The detection energy is 2.53 eY corresponding to the S,—>S0 (0-0) emission band of PPPV [83], The lines are fitting curves calculated from Eq. (3.15) - see text for further details. The inset shows experimental DCM PPPV ratios and the dopant concentrations used in the calculations. Reprinted with permission from [82], Copyright 2004 by the American Physical Society.

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