Bassler disorder model

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. 

Reversible energy transfer between monomeric and dimeric forms of rhoda-mine 6G in ethylene glycol has been observed" and the concentration dependence of the overall fluorescence quantum yield has been modelled by Monte-Carlo simulations. Triplet energy transfer in disordered polymers has been analyzed on the basis of Bassler s model in which the trap energies have a Gaussian distribution." Energy transfer has also been observed in mono-layers and for photoswitchable molecular triads." The structural requirements for efficient energy transfer from a carotenoid to chlorophyll have been... 

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... 

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. 

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)).

Numerous models have been proposed for hopping transport (see e.g. [Ml], [M2]). Conceptionally the simplest and physically most well-founded is the model of Bassler [47], which we will outline in the next section. In the sections thereafter, we will present typical experimental results for the temperature and electric-field dependencies of the mobility and for the temperature, field and thickness dependence of the dark current I(V) as a function of the applied voltage in disordered organic semiconductors. 

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. 

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 ... 

A prominent and widely appUed model of electron transfer in organic molecules is Marcus theory. It is thoroughly introduced in Section 4.3.1.1 and hence we concentrate here on its applicabiUty to triplet energy transfer supported by recently reported experimental studies of the temperature dependence of the triplet diffu-sivity and of the effect of disorder on triplet transfer in organic molecules. We would also like to refer the interested reader to a recent overview on experimental and theoretical work concerning a unified description of triplet energy transfer by Kohler and Bassler [30]. 

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