Charge transport in disordered organic semiconductors

Charge-carrier traps can not only be emptied, but also completely filled in the stationary state by high current densities. Then the defects no longer act as traps, but rather as potential scattering centres. 

While in the previous sections of this chapter we treated organic semiconductors which consist of single crystals of molecules with conjugated tt-electron chains, in 

Films of these molecules can in many cases be prepared by controlled evaporation in vacuum onto a substrate. In other cases, they are spun onto the substrate from solution. After evaporation of the solvent, amorphous or partially crystalline or liquid-crystal films are formed, whose thickness can in many cases be varied in a controlled way. 

The lateral homogeneity of the evaporated or spun films is sufficient for experiments on charge transport through the films, if they are carefully prepared. In the case of evaporated films, this is in many cases also true of extremely thin films, e.g. of 20 nm thickness and with an area of 2x 2mm.  

Soluble polymers can likewise be prepared in the form of thin films by spinning the solution onto a substrate and then evaporating the solvent. Insoluble polymers can in suitable cases be spun in the form of their soluble precursors and then after evaporation of the solvent be converted into the insoluble polymer by elimination of the substituents which rendered them soluble. A well-known example of this is poly(para-phenylene-vinylene) (P PV). 

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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... 

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

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... 

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) ... 

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... 

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

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