Photoluminescence OLEDs

Yam and co-workers reported the first luminescent study of 4-coordinate gold(I) phosphine complexes, [Au(PAP)2]X (PAP = l,8-bis(diphenylphos-phino)naphthalene, 4-methyl-l,8-bis(diphenylphosphino)naphthalene X = Cl, PF6) in 2000 [28]. The complexes were shown to exhibit orange-red photoluminescence both in the solid state and in dichloromethane, and the emission origin was attributed to the triplet state derived from a [a -> 77- (naphthyl)] IL transition. The PF6 salt of the complexes were also employed as an emissive layer in the fabrication of OLEDs. 

The /3-diketonate [Nd(dbm)3bath] (see figs. 41 and 117) has a photoluminescence quantum efficiency of 0.33% in dmso-7r, solution at a 1 mM concentration. It has been introduced as the active 20-nm thick layer into an OLED having an ITO electrode with a sheet resistance of 40 il cm-2, TPD as hole transporting layer with a thickness of 40 nm, and bathocuproine (BCP) (40 nm) as the electron injection and transporting layer (see fig. 117). The electroluminescence spectrum is identical to the photoluminescence emission the luminescence intensity at 1.07 pm versus current density curve deviates from linearity from approximately 10 mA cm-2 on, due to triplet-triplet annihilation. Near-IR electroluminescent efficiency <2el has been determined by comparison with [Eu(dbm)3bath] for which the total photoluminescence quantum yield in dmso-tig at a concentration of 1 mM is Dpi, = 6% upon ligand excitation, while its external electroluminescence efficiency is 0.14% (3.2 cdm-2 at 1 mAcm-2) ... 

A single-layer OLED with [Er(acac)3phen] doped into a 80-nm thick film of PVK (see fig. 117) prepared by spin-coating and deposited on an ITO electrode, and with a 100-nm lithium-doped (0.1%) aluminum cathode has also been tested and shows an onset voltage of about 12 V for electroluminescence (Sun et al., 2000). [Er(dbm)3bath] has a photoluminescence quantum yield of 0.007% in dmso-7fl at 1 mM concentration the OLED based on this compound and similar to the one described above for Ndm has a NIR external electroluminescence efficiency of 1 x 10-6 (Kawamura et al., 2001). 

Dibenzophosphole A was in fact the first type of phosphole to be prepared [9], but it has only very recently been used as a building block for the preparation of jr-conjugated systems. In this regard, polymer 70 is obtained with a high polydis-persity (Mn = 5 X 102 Mw = 6.2 X 103) by Ni-catalyzed homo-coupling of derivative 69 (Scheme 4.20) [52]. The presence of o3-P centers, which are potential donor sites for the Ni catalyst, does not prevent C-C bond formation. This macromolecule is photoluminescent in the solid state (2em = 516 nm), a property of potential interest for the development of OLEDs [52]. 

Spin-orbit coupling not only governs the amount and pattern of ZFS of the emitting triplet state, but it is also of dominant importance for the radiative emission decay rates and thus for the photoluminescence quantum yields. These properties are crucial for the suitability of triplet emitters in OLEDs. In conclusion, detailed spectroscopic studies of compounds triplet state properties in combination with... 

To be a good emitting material for use in OLEDs, firstly the lanthanide complex must have high photoluminescent efficiency, which is one of the essentials for excellent electroluminescence devices. As noted above, the external quantum efficiency hext of an OLED can be expressed as hext = hr other conditions are kept unchanged, the higher the photoluminescent efficiency, the better the electroluminescence performance. 

Similar to the europium complexes, the terbium complexes used as good emitters in OLEDs also need to have high photoluminescent efficiencies, good carrier transporting properties, and sufficiently good thermal stability for small molecule materials to form a film by thermal evaporation in vacuum. 

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. 

As noted above, observations of large enhancements of the photoluminescence are insufficient to guarantee utility for application of plasmon-enhanced emission in OLEDs where the excited state is not photogenerated. In principle, increases in photoluminescence observed exfierimentally could be completely due to absorption enhancement. Even observation of reduced excited state lifetimes in conjunction with increased emission is insufficient to prove radiative rate enhancement since the lifetime reduction could be due to excited state quenching by the metallic surface and compensated by large absorption enhancements. 

FIGURE 1.3. The photoluminescence (PL) and electroluminescence (EL) spectra of some representative 7r-conjugated films and OLEDs, respectively (a) EL of blue aminooxadia-zole fluorene (AODF) and green Alq3 OLEDs,9 (b) PL and EL of PPV films and PLEDs, respectively,10 (c) PL of m-LPPP films, (d) EL of DPVBi (solid line) and DPVBi/Alq3 (dashed line) OLEDs,11 and (e) PL of CBP films and EL of CBP OLEDs.12... 

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