Quantum phosphorescence devices

Due to the large band gap and high triplet energy level of the poly(3, 6-dibenzosilole) 5, the copolymer is an excellent host for the fabrication of blue polymer phosphorescent light-emitting diodes. A high external quantum efficiency (t/el) of 4.8% and a luminance efficiency of 7.2 cd/A at 644 cd/m2 have been achieved for blue phosphorescence devices (emission peak (AEL) at 462 nm, CIE coordinates x = 0.15,y = 0.26). The performances of the devices are much better than those reported for blue phosphorescent devices with poly(A--viny 1 cabarzo 1 e) (PVK) as the host.32... 

Fig. 11.25 Summary of best values of the external quantum efficiency and power conversion efficiency, reported for polymer-based electro-phosphorescent devices over the past 3 years.

There is no reason why the same principle cannot be applied for light-emitting polymers as host materials to pave a way to high-efficiency solution-processible LEDs. In fact, polymer-based electrophosphorescent LEDs (PPLEDs) based on polymer fluorescent hosts and lanthanide organic complexes have been reported only a year after the phosphorescent OLED was reported [8]. In spite of a relatively limited research activity in PPLEDs, as compared with phosphorescent OLEDs, it is hoped that 100% internal quantum efficiency can also be achieved for polymer LEDs. In this chapter, we will give a brief description of the photophysics beyond the operation of electrophosphorescent devices, followed by the examples of the materials, devices, and processes, experimentally studied in the field till the beginning of 2005. 

M Suzuki, T Hatekayama, S Tokito, and F Sato, High-efficiency white phosphorescent polymer light-emitting devices, IEEE J. Selected Top. Quantum Electron., 10 115-120, 2004. 

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 complementary application to the use of Os complexes in photovoltaic cells is the use of luminescent Os complexes in electroluminescent devices. There has been a significant amount of work in this area, particularly as it applies to the development of Os complexes with high quantum yields for phosphorescence. A review of transition metal complexes used in OLED development was published in 2006 by Evans et al. [126]. Another very recent review discusses various Os(II) carbonyl complexes with diketonate, hydroxyquinolate, bipyridine, and phenanthroline ligands as emitters in OLED devices [127]. A few select examples of Os complexes in OLEDs are presented here. 

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 181 The external quantum efficiency of devices using [Ir(ppy)3] phosphorescent compound, as a function of the driving current. The data for 6% [Ir(ppy)3] CPB (circles) are taken from Ref. 43. The squares show the data for the 6% [Ir(ppy)3] in (TPDiPC) system for the first run, the diamonds are the same system for the second run, the down triangles are the data for the (TPD PC)/[Ir(ppy)3]/PBD system, and the up triangles are the data for the (TPD PC)/[Ir(ppy)3] system. The intersection between the current-independent segment of the (p (j) plot and its falling part is indicated as Pc- After Ref. 304. Copyright 2002 American Physical Society, with permission.

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. 

Subsequently, Cao et al. [30,31] designed and synthesized polymer 20-22 by similar method and the highly efficient saturated red-phosphorescent polymer light-emitting diodes (PLEDs) were achieved on the basis of copolymer 20. The best device performances are observed with an external quantum efficiency of 6.5% photon/electron (ph/el) at the current density of 38 mA/cm2, with the emission peak at 630 nm (x = 0.65, y = 0.31) and the luminance of 926 cd/m2. 

Some of these carriers may recombine within the emissive layer yielding excited electron-hole pairs, termed excitons. These excitons may be produced in either the singlet or triplet states and may radiatively decay to the ground state by phosphorescence (PL) or fluorescence (FL) pathways (Fig. 1-2). An important figure of merit for electroluminescent materials is the number of photons emitted per electron injected and this is termed the internal quantum efficiency. It is clear, therefore, that the statistical maximum internal efficiency for an EL device is 25% as only one quarter of the excitons are produced in the singlet state. In practice, this maximum value is diminished further because not all of the light generated is visi-... 

In most of the electrophosphorescence based OLEDs the device quantum efficiencies drop rapidly with increasing current density and consequently with the brightness due to triplet-triplet annihilation at high current densities. WOLED based on phosphorescent material had a maximum forward viewing power efficiency of 26 + 3 Im at low luminosity, decreasing to 11 1 Im W-1 at 1000 cd m 2 (Kamata et al 2002, D Andrade et al 2004). 

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