Doping disordered

Arkhipov VI, Emelianova EV, Heremans P, Bassler H (2005) Analytic model of carrier mobility in doped disordered organic semiconductors. Phys Rev B 72 235202... 

Solbrand and coworkers [74] is that the apparent diffusion coefficient is dependent on the conductivity and composition of the electrolyte. This is consistent with an activation energy for electron hopping which depends on the energy required to polarize its environment, a phenomenon which is familiar from conduction in doped disordered materials [110]. It is also a clear signal that the electrochemical interface is very important to transport, and consistent with the idea that elections move along or close to the surface. 

Lanthanide ion doped disordered systems such as glasses have played an important role in the evolution of laser technology and thus there has been a concentration of efforts in those systems which have application to stimulated devices, see Stakowski (1982) and Weber (1982). In large part, lanthanide ions in glasses behave much as their similarly coordinated crystals and various laser spectroscopic studies of glasses have already been alluded to in the preceding... 

In recent years, also percolation models have been successfiiUy applied to experimental data [55] as an alternative approach to model the transport properties of doped disordered systems. For polycarbonate doped with a derivative of benztriazole (BTA) (Fig. 1.2) it has been shown that the concentration dependence of the effective mobility ean be described at low concentrations by [55]... 

Figure 6 shows the field dependence of hole mobiUty for TAPC-doped bisphenol A polycarbonate at various temperatures (37). The mobilities decrease with increasing field at low fields. At high fields, a log oc relationship is observed. The experimental results can be reproduced by Monte Carlo simulation, shown by soHd lines in Figure 6. The model predicts that the high field mobiUty follows the following equation (37) where d = a/kT (p is the width of the Gaussian distribution density of states), Z is a parameter that characterizes the degree of positional disorder, E is the electric field, is a prefactor mobihty, and Cis an empirical constant given as 2.9 X lO " (cm/V). ...

N,N-bis(4-methylphenyl)-N,iV-bis(4-ethylphenyl)-[l,l -(3,3 -dimethyl)biphenyl]-4,4 -diamine [115310-63-9] (ETPD) (8) -doped PMPS (Table 1, entry 22), hole mobihty approaching 10 cm /Vs at 2.5 x 10 V/cm was observed (48). This is the highest recorded hole mobihty for disordered organic systems. From this perceptive, it is very interesting to study the carrier mobihty of polymers heavily doped with semiconductor nanoclusters. 

In molecular doped polymers the variance of the disorder potential that follows from a plot of In p versus T 2 is typically 0.1 eV, comprising contributions from the interaction of a charge carrier with induced as well as with permanent dipoles [64-66]. In molecules that suffer a major structural relaxation after removal or addition of an electron, the polaron contribution to the activation energy has to be taken into account in addition to the (temperature-dependent) disorder effect. In the weak-field limit it gives rise to an extra Boltzmann factor in the expression for p(T). More generally, Marcus-type rates may have to be invoked for the elementary jump process [67]. 

This work is based on the doctoral thesis of Prasad Rao [8] it stemmed from the early work of Santiago, Mulay et al. (Cf. ref. 2a). Amorphous (disordered) carbons (Cabot Co. s Monarch 700, CSX-203, etc.) were used after appropriate desulfurization. Some of these carbons were graphitlzed at high temperatures (2773 K). The above CMC samples were doped with boron in the range from 170 to 260 ppm. 

The charge transport and optical properties of the [Si(Pc)0]-(tos)y)n materials as y=0 -+ 0.67 are reminiscent of the [Si(Pc)0]-(BF4)y)n system, but with some noteworthy differences. Again there is an insulator-to-metal transition in the thermoelectric power near y 0.15-0.20. Beyond this doping stoichiometry, the tosylates also show a continuous evolution through a metallic phase with decreasing band-filling. However, the transition seems somewhat smoother than in the BF4 system for y)>0.40, possibly a consequence of a more disordered tosylate crystal structure. Both [Si(Pc)0]-(tos)y)n optical reflectance spectra and four-probe conductivities are also consistent with a transition to a metal at y 0.15-0.20. Repeated electrochemical cycling leads to considerably more decomposition than in the tetrafluoroborate system. 

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