Fluorescent dopants

Even in the case of simple aromatic dopants (naphthalene, anthracene, pyrene), fluorescence intensity is found to decrease with successive laser pulses and/or increasing laser fluence. The decrease may be ascribed to thermal desorption of the dopant, as indicated by Fukumura et al. [62], and/or to photooxidation/degradation of the dopants. In the present case, film transmission decreases in parallel with the dopant fluorescence, thus indicating the formation of photoproducts that absorb stronger than the precursor. (The photoproduct(s) are not detected in the fluorescence experiments, evidently because of its (their) low emission yield). NapH and related compounds are known to generate (Ag) efficiently [81]. This has a long lifetime within polymers and it can attack the aromatic to form the corresponding endoperoxide (Nap-02). Such processes have been shown to occur in related polymer systems. 

It is generally found in these experiments that in the presence of the dopant or impurity, the dopant fluorescence is much stronger than would be expected in the absence of migration [24,62-64] ... 

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. 

These materials have large band gaps and are thermally stable with high Tg. Details describing the use of these materials will be presented in the section on fluorescent blue dopants. 

The second class of green dopants is the family of quinacridone (137-139) (Scheme 3.42) fluorescent dyes. Patents on derivatives of these compounds have been filed by Kodak for use in OLEDs [191]. 

A series of red fluorescent dyes has been recently reviewed by Chen [204], The red fluorescent dopants, according to their structural characteristics, are summarized as ... 

The merit of these chromene dopants is their relatively long emission wavelength peaks compared to DCM or DCJTB materials due to the more conjugated chromene moiety, and this contributes to the more saturated red emission. In fact the EL spectra of OLED devices of ITO/TPD/Alq3 chromene-dopants/Alq3/Mg Ag exhibited satisfactory red emission color, especially for Chromene-1 and Chromene-2 dopants. However, these chromene-based red emitters showed lower fluorescent quantum yield (18%, 15%, and 54% for Chro-... 

A higher efficiency, yet simpler structure PPLED device fabricated with the same dopant and host materials was almost simultaneously reported by Yang and Tsutsui [35]. The highest EQE of their device ITO/PVK 5%Ir(ppy)3 /OXD-7/Mg Ag (where ITO is indium tin oxide) (using OXD-7 (7) as an electron-transporting layer (ETL), Chart 4.3) reached the value of 7.5%, which was the first reported PLED with external efficiency above 5%, an upper limit of the fluorescent PLEDs. The power efficiency was 5.8 lm/W at the luminance of 106 cd/m2. 

Another important early advance made by Tang et al. [7] is the use of fluorescent doping, i.e., the addition of a small percentage of an emissive fluorescent material into a host matrix. This can be used to alter the color of emission, in addition to improving the efficiency and the lifetime of devices. The technique of simultaneously vapor depositing the host and the fluorescent dopant material is now widely used in the field of OLEDs. 

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