Doping phosphorescence devices

F Shen, H Xia, C Zhang, D Lin, X Liu, and Y Ma, Spectral investigation for phosphorescent polymer light-emitting devices with doubly doped phosphorescent dyes, Appl. Phys. Lett., 84 55-57,... 

As an extension of the fluorescent sensitizer concept, Forrest et al. have applied this approach to phosphorescent OLEDs, in which the sensitizer is a phosphorescent molecule such as Ir(ppy)3 [342]. In their system, CBP was used as the host, the green phosphor Ir(ppy)3 as the sensitizer, and the red fluorescent dye DCM2 as the acceptor. Due to the triplet and the singlet state energy transfer processes, the efficiency of such devices is three times higher than that of fluorescent sensitizer-only doped device. The energy transfer processes are shown in Figure 3.21. 

In their follow-up paper, they also demonstrated 100% efficient energy transfer of both singlet and triplet excited states. The device exhibits peak external efficiency and power efficiency of 25 cd/A and 17 lm/W at 0.01 mA/cm2, respectively [343]. Liu demonstrated a high-efficiency red OLED employing DCJTB as a fluorescent dye doped in TPBI with a green phosphorescent Ir(ppy)3 as a sensitizer. A maximum brightness and luminescent efficiency of... 

FIGURE 3.21 (a) Energy transfer mechanisms of phosphorescent dye as a sensitizer and (b) the EL external efficiency of the DCM2 doped devices. (From Baldo, M.A., Thompson, M.E., and Forrest, S.R., Nature, 403, 750, 2000. With permission.)... 

Most reported PPLEDs were fabricated by doping a polymer with a phosphorescent dye. However, aggregation and phase separation effects may cause serious problems for device performance and aging. In this section, we describe the very recent progress in intrinsically electrophosphorescent polymers containing triplet-emitting complexes either as pendant substituents or as a part of a backbone. 

C Lee, KB Lee, and J Kim, Polymer phosphorescent light-emitting devices doped with tris (2-phenylpyridine) iridium as a triplet emitter, Appl. Phys. Lett., 77 2280-2282, 2000. 

S Kan, X Liu, F Shen, J Zhang, Y Ma, Y Wang, and J Shen, Improved efficiency of single-layer polymer light-emitting devices with poly(vinylcarbazole) doubly doped with phosphorescent and fluorescent dyes as the emitting layer, Adv. Funct. Mater., 13 603-608, 2003. 

Y Noh, C Lee, J Kim, and K Yase, Energy transfer and device performance in phosphorescent dye doped polymer light emitting diodes, J. Chem. Phys., 118 2853-2864, 2003. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

A first demonstration of phosphorescence-doped OVPD-OLEDs with identical VTE performance was achieved at Universal Display Corporation (UDC) and at TU Braunschweig [48] by use of PtOEP. The electroluminescence spectrum, with emission at 651.1 nm, and the structure of the device are shown in Fig. 9.10. The external quantum efficiency of this OVPD device reached 3.88% at 3.7 V forward potential (Fig. 9.11) this is identical to the external quantum efficiency of the nearly identical VTE device reported by Baldo and Forrest [49]. 

Figure 120 The spectra of the two electroluminescent devices (I and II) containing organic phosphors, Ir(ppy)3 (a) (adapted from Ref. 304), and PtOEP (b) (see Ref. 493a, reprinted from Ref. 493a, Copyright 1998 Macmillan Publishers Ltd. [http //www.nature. com/]). The latter is compared with the EL spectrum of a device with no phosphor inside (III). For the chemical structures of the phosphors, see Fig. 31. The spectra from device I and II are characteristic of molecular phosphorescence as clearly seen from their comparison at different voltages with the PL spectrum (a). The DCM2-doped Alq3 layer of device III becomes dominated by their phosphorescene from the PtOEP-doped Alq3 layer in device II.

The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent Ir + complex and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescence OLEDs . Red-emitting phosphorescence has been described based on the [Ir(btp)2(acac)j fragment attached either directly (spacerless) or through a —(CH2)8— chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. Xu and coworkers reported that an efficient red EP with CIE chromaticity coordinates X = 0.69, y = 0.29, independent on current density, was obtained from [Ir(acac)(btfmp)2] doped devices. The EL spectrum has a maximum at 648 nm. A maximum external quantum efficiency of 9.6%, at current density of 0.125 mAcm, and a maximum luminance of 4200 cdm , at 7 = 552 mAcm , have been obtained. 

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