Hole drift mobilities

A new branched carbazole derivative with phenyl ethylene moieties attached, l,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB, 41) (Scheme 3.15), was prepared as a HTM for OLEDs [86], TECEB has a HOMO energy level of —5.2 eV and hole-drift mobility of 1(T 4 cm2/(V s), comparable to NPD. The device performance (maximum luminance of about 10,000 cd/m2 and current efficiency of 3.27 cd/A) in a standard HTL/tris-(8-hydroxyquino-line) aluminum double-layer device is also comparable to NPD, but TECEB has a higher Tg (130°C) and its ease of synthesis is superior to NPD. Distyryl units linked to a TPD derivative, A, A"-bis(4-(2,2-diphenylethenyl)-phenyl)-jY,jV -di(p-tolyl)-bendidine (DPS, 42) (Scheme 3.15), reported by Yamashita and coworkers, showed good hole transport properties and improved thermal stability compared with the parent TPD [87]. 

The hole drift mobility can be conveniently obtained from the slope of the curve tj versus Vq (see Fig. 4.13) [15]. 

Let us first consider the carrier drift in pure amorphous selenium. Both the electron and hole drift mobility can be measured in a-Se by the TOF technique outhned earlier. At temperatures above 200 K, a well-defined transit pulse is observed. The transient... 

Fig. 52. Hole drift mobility in tri-phenylamine-Lexan films as function of temperature, x - mass ratio of the dopant to polymer A - activation energy of the hole mobility (290)...

Fig. 5. Field dependence of hole drift mobilities for a range of TNF PVK molar ratios. Data taken at T = 24 °C17 ...

Fig. 3.13. Temperature dependence of the (a) electron and (b) hole drift mobility at different applied fields ranging from 5 x 10 V cm" to 5 X 10 V cm". The field dependence of is caused by the dispersion (Marshall et al. 1986, Nebel et al. 1989).

Fig. 7.8. (a) Electron and (b) hole drift mobility data (points) fitted to the multiple trapping dispersive transport mechanism, assuming an exponential band tail (lines) (Tiedje 1984). 

Strong Field Dependence of Drift Mobility. Hole drift mobility increases sharply with field, and with PVK, a characteristic E dependence of log (X is exhibited, as originally reported by Pai (36), whose data are displayed in Figure 5. Drift mobility values are low and fall into the range cm /V-s at room temperature. 

Strong Temperature Dependence of Drift Mobility. Figure 6 is an Arrhenius representation of hole drift mobility data on a 0.2 1 TNF-PVK film (iO). The apparent activation in this representation is field dependent,... 

Mobility Scales with Average Inter site Hopping Distance, Gill identified an exponential dependence of electron drift mobility on average TNF intersite hopping distance and noted an associated decrease in hole drift mobility (35). Gilfs subsidiary observation that TNF addition also decreased hole mobility precisely because complexed carbazole is removed as a hole transport site provided evidence for the key role of the discrete carbazole groups in hole transport. 

The hole drift mobility was determined with the aid of the time-of-ftight method (TOF) [8] in conjunction with a frequency doubled ruby laser (A k= 347 nm, flash length 20 ns). 

When apolar electron acceptors (p = 0) were examined the hole drift mobility increased with increasing electron affinity (EA) as can be seen from Fig. 4 and Table 1. This trend applies to all compounds listed in Table 1 However, compounds having EA exceeding 1.4 eV turned out to be chemically unstable at the high electric fields afforded for these measurements. TCNQ is a typical compound behaving in that way. 

Table 7.6 Hole drift mobilities of amorphous molecular materials [a].

Numerous studies of charge transport in amorphous molecular materials have shown that hole drift mobilities of amorphous molecular materials vary widely from 10 6 to 10 2 cm2 NT1 s 1 at an electric field of 1.0 X 105 V cm-1 at room temperature, greatly depending upon their molecular structures. Table 7.6 lists hole drift mobilities of some amorphous molecular materials that function as holetransporting materials in OLEDs. 

Fig. 12. Temperature dependence of the hole drift mobility. Solid Unes are theoretical fits as discussed in the text. A, 17 V , 4 V O, 1 V. [Reprinted with permission from Solid State Communications, Vol. 47, T. Tiedje, B. Abeles, and J. M. Cebulka, Urbach edge and the density of states of hydrogenated amorphous silicon, Copyright 1983, Pergamon Press, Ltd.]...

Cody et al, 1981b). If for the valence-band tail is related to the exponential tail on the absorption edge, as has been proposed by Tiedje et al. (1983), then there should be large variations in the room-temperature hole drift mobility with different preparation conditions since is an exponential function of T. No systematic study of this relationship has yet been undertaken, however. 

Hole-transporting materials (HTM) have relatively low ionization potentials (1P). 2 The IP is defined as the energy required to remove an electron from the highest occupied molecular orbital (HOMO) of a substance. It can be measured, for instance, by photoelectron spectroscopy or obtained from electrochemical oxidation potentials in solution. It is also preferable that the HTM have sufficiently high hole drift mobilities. Various classes of materials have been used in the HTL, for example, starburst amorphous molecules, spirocompounds, triarylamines, and tetraarylbenzidines are representative classes of well-known HTMs. The structure of the most commonly used HTM, 4,4 -bis[N-(l-naphthyl)-N-phenylamino]biphenyl (NPB), is shown in Figure 14.2. 

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