Semiconductors disordered organic

Fig. 19 The effect of doping on the density of states distribution in a disordered organic semiconductor at variable concentration of charged dopants. The energy scale is normalized to the width of the DOS, expressed through a, of the undoped sample. The parameters are the intrinsic site concentration V and the dopant concentration N. From [125] with permission. Copyright (2005) by the American Institute of Physics...

Arkhipov VI, Heremans P, Emelianova EV, Adriaenssens GJ, Bassler H (2002) Weak-field carrier hopping in disordered organic semiconductors the effects of deep traps and partly filled density-of-states distribution. J Phys Condens Matter 14 9899... 

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... 

Craciun NI, Wildeman J, Blom PWM (2008) Universal Arrhenius temperature activated charge transport in diodes from disordered organic semiconductors. Phys Rev Lett 100 056601... 

Van der Holst JJM, van Cost FWA, Coehoom R, Bobbert PA (2009) Electron-hole recombination in disordered organic semiconductors validity of the Langevin formula. Phys Rev B 80 235202... 

Charge transport in disordered organic semiconductors generally occurs by hopping between adjacent conjugated segments. This process has been investigated in detail in molecularly doped polymers and even in doped crystalline materials... 

The mobihty itself also depends in general on the field strength /r = /r(F). This holds in particular for the hopping conductivily in disordered organic semiconductors (see Sect. 8.6), but also for band conductivity in ideal organic molecular crystals (see Sect. 8.5). 

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. 

In addition to the temperature dependence, the disordered organic materials also exhibit a characteristic electric-field dependence pi F). A series of experimental results from a large number of disordered organic semiconductors su ests that the mobility quite generally has the following form ... 

Aside from the temperature and field dependencies of the mobUities in disordered organic semiconductors, the explicit form of the TOF transients was investigated (see e.g. Fig. 8.44). A particularly typical example is shown in Fig. 8.49 the TOF transient of the hole photocurrent of a 10 /xm thick film of the commercially available, purified polymer PVK (poly-(N-vinyl-carbazole)) at T= 279 K after an excitation pulse of width 10 ns and an appHed voltage of V= 400 V [52]. The hopping transport of the holes takes place in this system between the carbazole side... 

Charge Transport in Disordered Organic Semiconductors 297 8.6.5.2 The Theory of Space-Charge Limited Currents in Disordered Films... 

The properties of energetically-distributed charge-carrier traps are above all in disordered organic semiconductors of considerable significance for the analysis of real current-voltage characteristics. We consider the following two energy distributions for shallow traps an exponential distribution, with its maximum at the transport level Ee (compare Fig. 8.6) ... 

In the general case that there is both an energy distribution of traps and a dependence of the mobihty on the electric field, the system of equations (Eqns. (8.48)-(8.56)) could as yet not be analytically solved. This case is however realistic, in particular for disordered organic semiconductors. Therefore, a numerical simulation based on Eq. (8.57) is necessary. It was carried out for the model system Alqs, and successfully tested by comparison with experimental data over a large range of the variable parameters (see Sect. 8.6.5.3). 

Paasch, G., Lindner, T, and Scheinert, S. 2002. Variable range hopping as possible origin of a universal relation between conductivity and mobility in disordered organic semiconductors. Synthetic Metals 132 97-104. 

Monte Carlo approaches are attractive for disordered organic semiconductors because they incorporate the disorder in energy levels in a natural way. This is because the chemical nature can be incorporated explicitly through the energy levels and interactions between different molecules and because the effects of 3D variations in structure (such as in molecular packing and phase segregation) can be... 

Mendels D, Tessler N (2013) Drift and diffusion in disordered organic semiconductors the role of charge density and eharge energy transport. J Phys Chem C 117 3287-3293... 

Ruble V, Lukyanov A, May F, Schrader M, Vehoff T, Kirkpatrick J, Baumeier B, Andrienko D (2011) Mieroscopic simulations of charge transport in disordered organic semiconductors. J Chem Theory Comput 7 3335-3345... 

Arkhipov, V.I., Heremans, P., Emelianova, B.V., and Bassler, H. (2005) Effect of doping on the density-of-states distribution and carrier hopping in disordered organic semiconductors. Phys. Rev. B, 71, 045214-045221. 

Dtfley S et al (2010) Electronic properties of disordered organic semiconductors via QM/MM simulations. Acc Chem Res 43 995-1004... 

Clark J, Silva C, Friend RH, Spano FC (2007) Role of intermolecular coupling in the photophysics of disordered organic semiconductors aggregate emission in regioregular polythiophene. Phys Rev Lett 98 206406... 

---