Mobility of holes

Although MEH-PPV 13 (at the time of discovery) was one of the most efficient soluble polymers for PLEDs application, its performance is not high enough for commercialization as LEP. One of the reasons is unbalanced hole-electron mobility in MEH-PPV (the mobility of holes is 100 times faster than the mobility of electrons) [133]. Copolymerization with other conjugated monomers, to some extent, can improve the electron-transporting properties and increase the EL performance. 

Beside the excellent optical properties and suitable HOMO-LUMO energy levels, the PFs possess great charge-transport properties. Time-of-flight (TOF) measurements of PFO showed nondispersive hole transport with a room temperature mobility of holes of fi+ = 4 x 10-4 cm2/(V s) at a field of li 5 x 105 V/cm that is about one order of magnitude higher than that in PPV [259]. The polymer revealed only a weak-field dependence of the mobility, from /r+ = 3 x 1(U4 cm2/(V s) at E= 4 x 104 V/cm to /r+ = 4.2 x 1(V4 cm2/(V s) at E= 8 x 105 V/cm. 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

Charge transport The holes and electrons must move through the device under the influence of the applied electrical field. The mobility of holes in typical hole transport organic materials is approximately 10 cm2/(V s) [65], For electrons the mobility is usually one or more orders of magnitude lower [66],... 

Studies of the direct effect have been largely confined to DNA samples in the solid state. This is done in order to maximize direct-type damage and minimize indirect-type damage. In addition, low temperatures are often employed both as a means of sequestering the DNA from the bulk water and as a means of stabilizing free radical intermediates. In frozen DNA samples, the mobility of holes and excess electrons differs for the different sample components ice, solvation shell, DNA backbone, and base stacks. We start with the ice phase. 

The mobilities of holes are always less than those of electrons that is fXh < Me- In silicon and germanium, the ratio [ie/[ih is approximately three and two, respectively (see Table 6.2). Since the mobilities change only slightly as compared to the change of the charge carrier densities with temperature, the temperature variation of conductivity for an intrinsic semiconductor is similar to that of charge carrier density. 

The mobility of holes pp and of electrons p depends on the effective mass m of the carriers and on temperature. This is given approximately as... 

The dipoles in a low-7 material can be aligned in situ by an external field during the mobility measurement. The material can become more ordered at a high applied field than at a low one and can undergo a quasimorphological change with the increase of the applied field. The carrier mobility can behave differently in the field ranges studied. For example, Fig. 5 shows the field dependence of the mobility of holes and electrons in materials that consist of compound 1 [27]... 

The usual TFT structure is shown in Fig. 10.7 and comprises the a-Si H channel, a gate dielectric, and source, drain, and gate contacts. N-channel accumulation mode operation using an undoped a-Si H channel is the only structure widely used. Depletion mode devices are prevented by the high defect density of doped material, which makes it difficult to deplete the channel. The much lower mobility of holes compared to electrons gives p channel devices a lower current by about a factor 100, which is undesirable. 

One assumption which is usually made in determining the reaction orders with respect to the various carriers in the semiconductor is that the electrons and holes are essentially in translational equilibrium across the space-charge layer. Their concentrations may be very far from equilibrium with respect to recombination, analogous to having water with an ion product (at room temperature) very different from 10 . However, the very high mobility of holes and electrons tends to make the gradients of their chemical potentials (Fermi levels) quite small. Under such conditions, the hole and electron concentrations at the surface (ps and ns) are related to their concentrations (pi and nj) just to the semiconductor side of the space-charge layer by the equations... 

Figure 33. Dependence of the mobiiity on CTM concentration mobility of holes (/r) in N-isopropylcarbazole (NlPC)-doped bisphenol-A polycarbonate as a function of field-strength ( ), plotted as for a) 9 %, b) 17 %, c) 23 %, d) 29 %, e) 33 %, and f) 38 % NIPC by weight. (Reprinted with permission from Ref. [51].)...

Figure 43. Comparison of the mobilities of holes in polystyrene doped with equivalent molar concentrations of TTA (above, 30 wt.%) and TAPC (below, 33 wt,%) at various field strengths and temperatures. Because the two triarylamine moieties in TAPC are linked, their arrangement should be somewhat different from that in TTA, yet the mobility behavior is nearly identical. At least in this example, differences in molecular packing do not lead to major differences in the mobility. Zero field values are extrapolations. (Reprinted with permission from Ref. [60b].)...

Figure 44. Velocity (o, not mobility) of holes in polystyrene doped with equivalent, low molar concentrations of TTA (6.8 wt.%), TAPC (7.5 wt."/)), and TETRA (7.3 wt.%), all at 335 K. Unlike the situation at high concentrations (Figure 43), linking the triarylamine groups in clusters of two (TAPC) or four (TETRA) dramatically affects the transport behavior. In TETRA, at high field strengths ), the mobility [v/E) drops so rapidly that even the velocity itself decreases with increasing E. (Reprinted with permission from Ref. [69a].)...

The effect of a limited mobility of holes in the material may be accounted for in Eq. (1) by using a parameter called the mobility field, E, which is the electric field necessary for a hole to drift 1 radian across a sinusoidal intensity pattern in one hole recombination lifetime. The assumptions of long lifetime and high mobility make the mobility field small in comparison with the projection of the poling field along... 

The fundamental properties of electronic states include energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as well as mobilities of hole and electron carriers. These... 

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