Mobility temperature-dependent

The mobility of electrons and holes is affected by two main scattering mechanisms chemical impurities and lattice scattering. The mobility temperature dependence due to... 

With the Monte Carlo method, the sample is taken to be a cubic lattice consisting of 70 x 70 x 70 sites with intersite distance of 0.6 nm. By applying a periodic boundary condition, an effective sample size up to 8000 sites (equivalent to 4.8-p.m long) can be generated in the field direction (37,39). Carrier transport is simulated by a random walk in the test system under the action of a bias field. The simulation results successfully explain many of the experimental findings, notably the field and temperature dependence of hole mobilities (37,39). 

Thermal Properties. Before considering conventional thermal properties such as conductivity it is appropriate to consi r briefly the effect of temperature on the mechanical properties of plastics. It was stated earlier that the properties of plastics are markedly temperature dependent. This is as a result of their molecular structure. Consider first an amorphous plastic in which the molecular chains have a random configuration. Inside the material, even though it is not possible to view them, we loiow that the molecules are in a state of continual motion. As the material is heated up the molecules receive more energy and there is an increase in their relative movement. This makes the material more flexible. Conversely if the material is cooled down then molecular mobility decreases and the material becomes stiffer. 

Carriers and channels may be distinguished on the basis of their temperature dependence. Channels are comparatively insensitive to membrane phase transitions and show only a slight dependence of transport rate on temperature. Mobile carriers, on the other hand, function efficiently above a membrane phase transition, but only poorly below it. Consequently, mobile carrier systems often show dramatic increases in transport rate as the system is heated through its phase transition. Figure 10.39 displays the structures of several of these interesting molecules. As might be anticipated from the variety of structures represented here, these molecules associate with membranes and facilitate transport by different means. 

Figures 12-17 and 12-18 show the temperature dependencies of the mobility in a hopping system with a Gaussian DOS of variance <7=0.065 eV as a function of the relative concentration c of traps of average depth ,=0.25 eV and as a function of the trap depth E, at a fixed concentration < =0.03, respectively. For c=0...

Figure 12-18. The temperature dependencies of the mobility simulated for different nap depths. The trap concentrations were 3x1 O 2 and the field 2xl05 V em 1 (Ref. 72J).

Figure 12-27. Temperature dependence of the hole mobility in DPOP-PPV at different electric fields Dale for T= 0 have been obtained by extrapolation. The inset shows the intersection of Arrhenius plots at T()=465 K (Ref. 1831).

Figure 14-25. Arrhenius plot of the temperature-dependent mobility of 8T evaporated lilm. Data were recorded at various gale voltages and corrected for the contact scries resistance (taken from Ref. [ 1241).

Figure 14-26. Temperature dependence of the mobility of pen-lacenc-evaporalcd film. Data are shown for three devices grown under similar conditions (data taken from Ref. [961).

MIM or SIM [82-84] diodes to the PPV/A1 interface provides a good qualitative understanding of the device operation in terms of Schottky diodes for high impurity densities (typically 2> 1017 cm-3) and rigid band diodes for low impurity densities (typically<1017 cm-3). Figure 15-14a and b schematically show the two models for the different impurity concentrations. However, these models do not allow a quantitative description of the open circuit voltage or the spectral resolved photocurrent spectrum. The transport properties of single-layer polymer diodes with asymmetric metal electrodes are well described by the double-carrier current flow equation (Eq. (15.4)) where the holes show a field dependent mobility and the electrons of the holes show a temperature-dependent trap distribution. 

In order to study the chaiged photoexcitalions in conjugated materials in detail their contribution to chaige transport can be measured. One possible experiment is to measure thermally stimulated currents (TSC). Next, we will compare the results of the TSC-expcrimenls, which are sensitive to mobile thermally released charges trapped after photoexcilation, to the temperature dependence of the PIA signal (see Fig. 9-17) which is also due to charged states as discussed previously. 

There is abundant evidence that the above formalism provides a framework for explaining the majority of experimental facts including the temperature and field dependence of mobility albeit not in the entire field regime, notably (i) the temperature dependence of the slope parameter of lnxF172 plots, (ii) the prefaetor mobility, (iii) the influence of randomly positional dipoles on the width of the... 

Figure 12-25. The temperature dependencies of die zero-field mobility for TTA and TTA doped wilh DTA, DAT, and TAA. The TTA concentration was 40%, ihe hinder material was polystyrene. The DTA, DAT and TAA to TTA concemialiinis were 1.11x10 5 niol/niol TIA (Ref. 7(> ).

Figure 12-28. Temperature dependence Figure 12-29. I hotocurrem transient of the 11X3/ of the hole mobility in DPOP-l PV MeLPPP/AI sample after excitation through the ITO ati-...

A celebrated derivation of the temperature dependence of the mobility within the hopping model was made by Miller and Abrahams 22. They first evaluated the hopping rate y,y, that is the probability that an electron at site i jumps to site j. Their evaluation was made in the case of a lightly doped semiconductor at a very low temperature. The localized states are shallow impurity levels their energy stands in a narrow range, so that even at low temperatures, an electron at one site can easily find a phonon to jump to the nearest site. The hopping rate is given by... 

Besides its temperature dependence, hopping transport is also characterized by an electric field-dependent mobility. This dependence becomes measurable at high field (namely, for a field in excess of ca. 10d V/cm). Such a behavior was first reported in 1970 in polyvinylcarbazole (PVK) [48. The phenomenon was explained through a Poole-ITenkel mechanism [49], in which the Coulomb potential near a charged localized level is modified by the applied field in such a way that the tunnel transfer rale between sites increases. The general dependence of the mobility is then given by Eq. (14.69)... 

We note a temperature dependence of the zero field mobility as exp[—( F()/F)2], a behavior which is indeed encountered in real organic semiconductors, and differs from both Millers-Abrahams fixed range and Moll s variable range hopping models. 

More recently, the Thiais group reported on temperature-dependent mobility of 6T and 8T down to 10 K [ 124]. In this case, the mobility was estimated from the linear regime and corrected for the contact resistance. Data for 8Tare shown in Figure 14-25. 

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