Polaron Arguments

This chapter reviews theories proposed to describe charge transport in materials of potential relevance to xerography. The emphasis is on the disorder formalism, polaron arguments, and the Scher-Montroll formalism. These have been the most widely used during the past decade. For reviews, see Silinsh (1980), Movaghar (1987, 1991), Bassler (1993), Silinsh and Capek (1994), and Silinsh and Nespurek (1996). Experimental results are described in the following chapters. 

Schein et al. (1990) measured hole mobilities of TTA doped PC. The temperature dependence was described by an Arrhenius relationship. The results were described by a small-polaron argument, as proposed earlier by Schein and Mack (1988). The dependence of the activation energy on intersite distance is illustrated in Fig. 54. The authors argued that for p < 15 A the results are consistent with adiabatic small-polaron theory while for p > 15 A the results can be described by a nonadiabatic small-polaron argument. Schein et al. derived an expression for the zero-field polaron mobility as... 

Arrhenius relationship. The activation energies were between 0.52 and 0.45 eV for concentrations between 15 and 60%. The concentration dependence of the zero-field mobility was described by a wavefunction decay constant of 1.3 A. The results were described by a small-polaron argument. Based on the concen-... 

Abkowitz and Stolka (1990, 1991) compared hole mobilities of poly-silylanes and polygermylenes containing aliphatic pendants with compounds that contain only aromatic side groups. While transport occurred via states associated with the backbone chain in both compounds, the nature of the side groups was shown to influence the temperature dependence of the mobility. The results were described by a small-polaron argument, based on the assumption that the polarizability increases when an aromatic pendant group was substituted... 

It is of course possible that a carrier in the conduction band or a hole in the valence band will form a spin polaron, giving considerable mass enhancement. The arguments of Chapter 3, Section 4 suggest that the effective mass of a spin polaron will depend little on whether the spins are ordered or disordered (as they are above the Neel temperature TN). This may perhaps be a clue to why the gap is little affected when T passes through TN. If the gap is U —%Bt -f B2 and Bt and B2 are small because of polaron formation and little affected by spin disorder, the insensitivity of the gap to spin disorder is to be expected. 

Santos-Lemus and Hirsch (1986) measured hole mobilities of NIPC doped PC. Over a range of concentrations, fields, and temperatures, the transport was nondispersive. The field and temperature dependencies followed logn / El/2 and -(T0IT)2 relationships. For concentrations of less than 40%, a power-law concentration dependence was reported. The concentration dependence was described by a wavefunction decay constant of 1.6 A. To explain a mobility that shows features expected for trap-free transport with a field dependence predicted from the Poole-Frenkel effect, the authors proposed a model based on field-enhanced polaron tunneling. The model is based on an earlier argument of Mott (1971). 

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