Bassler model

Fig. 8.45 Schematic of hopping transport in a disordered organic semiconductor. The energy distribution of the states (DOS) is assumed in the Bassler model to be a Gaussian distribution function C(E) with a width cr (compare Eq. (8.77)).

The Bassler model [47] for hopping transport in disordered organic solids is based on a few plausible hypotheses ... 

The overlap parameter 2yAf y is likewise not sharp, but rather statistically distributed. The distribution is assumed to be a Gauss function with a width S. The distribution of the overlap parameters is termed the non-diagonal disorder, a and S are the two important materials parameters in the Bassler model for hopping transport in disordered semiconductors. 

One result of the Bassler model is the relaxation of the excess charge carriers towards thermal equiUbrium after their production by photoexcitation (Fig. 8.46). The equilibrium energy (Eoo) of the charge carriers which were generated with... 

However, the Bassler model likewise gives a exp(VF) dependence. The reason for this is the field dependence of the hopping rate (Eq. (8.78)). The overall field and temperature dependence according to this model for the hopping conductivity in disordered materials at high fields (F> 10 V/cm) is given by ... 

S thus increases Hnearly as This dependence can be tested directly by experiment and used as an indicator for the applicabihty of the Bassler model. Figure 8.48 shows S(d ) for MPMP. Together with the temperature dependence with F = const and the electric-field dependence with T = const, the values quoted above for the two physically well-defined materials constants a and as well as for the empirical constant C were obtained. A further example of the Vf dependence will be treated in Sect 8.6.3. 

The interpretation of these experimental data for the TOF transients in PVK within the Bassler model [53] explains the temperature and time dependence of the hole transport quantitatively in terms of an intrinsic DOS of width a = 0.080 eV and additional traps with a molar concentration of 0.1% and a depth of 0.4eV. 

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

Eishchuk II, Arkhipov VI, Kadashchuk A, Heremans P, Bassler H (2007) Analytic model of hopping mobility at large charge carrier concentrations in disordered organic semiconductors polarons versus bare charge carriers. Phys Rev B 76 045210... 

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

Bassler, H. In Primary Photoexcitations in Conjugated Polymers Molecular Exciton versus Semiconductor Band Model, Sariciftci, N.S., Ed. World Sci. Singapore, 1997. 

In addition to three-dimensional models, there have been several onedimensional models based on the Onsager theory (Holroyd et al., 1972 Haberkom and Michel-Beyerle, 1973 Smetjek et al., 1973 Blossey, 1974 Singh and Bassler, 1975 Charle and Willig, 1978 Hong and Noolandi, 1978b Siddiqui, 1983, 1984). For the one-dimensional case, a field-independent slope-to-intercept ratio cannot be defined. One-dimensional models have been seldom used for organic materials. 

Scher (1984, 1988), Silinsh and Jurgis (1985), Ries and Bassler (1987), Berlin et al. (1990), Rackovsky (1991), Arkhipov and Nikitenko (1993), and Scher (1993). Thus far, these models have not been widely used. 

Yuh and Pai argued that the role of the polymer was related to the activation energy. Borsenberger and Bassler explained their results on a model based on dipolar disorder. According to the model, a is determined by the dipole moment of both the dopant molecule and the polymer repeat unit. The effect of the polymer host is then related to the difference in dipole moments of the dopant molecule and the polymer repeat unit as well as the dopant concentration. Most recent studies have been described by dipolar disorder arguments. 

An alternative framework for interpreting the temperature dependence of drift mobility is provided by the model suggested by Bassler and co-... 

A simple model, the Gaussian Disorder Model of Bassler and co-workers, has been very useful in rationalizing charge transport data on many amorphous molecular solids [59]. Its present version consists of the following assumptions. 

H. Bassler in Primary Photoexdtations in Conjugated Polymers Molecular Exdton versus Semiconductor Band Model,... 

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