The Effect of Doping on the Energy Distribution of Localized States and Carrier Transport in Disordered Organic Semicond
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The Effect of Doping on the Energy Distribution of Localized States and Carrier Transport in Disordered Organic Semiconductors Vladimir I, Arkhipov,1,2 Paul Heremans,1 Evgenia V. Emelianova,3 Guy J. Adriaenssens,3 and Heinz Bässler Institute of Physical, Nuclear and Macromolecular Chemistry, Philipps University of Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany 1 IMEC, Kapeldreef 75, B-3001 Heverlee-Leuven, Belgium 2 Darmstadt University of Technology, Institute of Material Science, Petersenstrasse 23, D-64281 Darmstadt, Germany 3 Semiconductor Physics Laboratory, University of Leuven, Celestijnenlaan 200D, B-3001 Heverlee-Leuven, Belgium ABSTRACT Doping of a disordered organic semiconductor gives rise to additional energy disorder due to the Coulomb interaction between randomly distributed dopant ions and carriers localized in intrinsic hopping sites. Although the carrier density increases with increasing doping level the additional energy disorder can significantly reduce the carrier hopping mobility. At higher doping levels the filling of deep states takes over, which leads to steeply increasing mobility at high dopant concentrations.
INTRODUCTION Charge carrier hopping within a positionally random and energetically disordered system of localized states was shown to be an adequate model for the description of charge transport in disordered organic semiconductors [1]. In a non-crystalline material, the energy disorder is mostly due to Van der Waals and dipole-dipole interactions within a positionally and orientationally random system of molecules [2]. Doping of such a system, in addition, creates a random distribution of dopant ions that will Coulombically interact with carriers localized in intrinsic hopping sites. This interaction further increases the energy disorder. This effect is especially important in view of a small value of the dielectric constant and, concomitantly, long range of the Coulomb interaction typical for molecular semiconductors. Increasing energy disorder with increasing dopant concentration will lead to broadening of the effective density-ofstates (DOS) distribution. Therefore, doping of a disordered organic semiconductor, on the one hand, increases the concentration of charge carriers and lifts up the Fermi level but, on the other hand, broadens the DOS distribution. While the former effect facilitates conductivity, the latter strongly suppresses the carrier hopping rate. The latter effect can dominate at some dopant concentrations such that doping appears to be even counterproductive as far as the carrier mobility is concerned. In the present paper, an analytic model describing the DOS distribution and the carrier hopping mobility in doped organic materials is formulated. It is shown that doping shifts some intrinsic hopping sites to the deeper tail of the DOS distribution and, thus, creates additional deep traps for charge carriers. This leads to decreasing mobility at low dopant concentrations. At higher
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