Phosphorescent OLED device triplet energy

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. 

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. 

Various high triplet energy ETM such as phenanthrolines and benzimidazoles have been tested in the HBL in CBP + Ir(ppy)3 green phosphorescent OLEDs. 5 However, use of these materials usually resulted in poor device stability. In contrast, green triplet OLEDs with BAlq in the HBL demonstrated long lifetimes vide infra). Similar results were reported for red phosphorescent OLEDs (see Section 14.4.2.1). 

Polysilanes are also applicable as matrix materials in phosphorescent OLEDs. Mixtures of polysilanes and triplet emitters are sufficient to effect an energy transfer from polysilane triplet states to emitter triplet states, thus amplifying the luminescence of the device. It has been shown that if polysilanes have electrophosphorescent side chains consisting of triplet emitters, the energy transfer from polysilane to emitter is most effective [124]. Thus the beneficial electronic properties of polysilanes are perfectly combined with the spectroscopic properties of transition metal based triplet emitters. The compounds described are derivatives of polymethylphenylsi-lanes, (Fig. 24) which are covalently attached to triplet emitters with iridium as metal centre. The polymers were applied in OLEDs with an ITO/active layer/Ca/Ag layer sequence. The active layer contained a fraction of 70% by weight of the... 

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... 

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