Origin of Meyer-Neldel type compensation behavior in organic semiconductors at large carrier concentrations: Disorder versus thermodynamic description
Sprache des Titels:
Englisch
Original Kurzfassung:
We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Sitter,
Singh, Sariciftci, and B¨assler, Phys. Rev. B 81, 045202 (2010)] to investigate how polaron formation affects
the Meyer-Neldel (MN) compensation behavior observed for temperature-dependent charge-carrier transport in
disordered organic semiconductors at large carrier concentrations, as realized in organic field-effect transistors
(OFETs). We show that the compensation behavior in organic semiconductor thin films can be consistently
described for both nonpolaronic and polaronic hopping transport in the framework of the disorder formalism
using either Miller-Abrahams or polaron Marcus rates, respectively, provided that the polaron binding energy is
small compared to the width of the density of states (DOS) distribution in the system. We argue that alternative
models based on thermodynamic reasoning, like the multiexcitation entropy (MEE) model, which assumes charge
transport dominated by polarons with multiphonon processes and ignores the energy disorder, are inherently
not applicable to describe adequately the charge-carrier transport in disordered organic semiconductors. We
have suggested and realized a test experiment based on measurements of the compensation behavior for the
temperature-dependent conductivity and mobility in OFET devices to check the applicability of these models.
We point out that the MN behavior observed in thin-film OFETs has nothing to do with the genuine MN rule
predicted by the MEE approach, but rather it is an apparent effect arising as a consequence of the functional
dependence of the partial filling of the DOS in a disordered system with hopping transport. This fact is fully
supported by experimental results. The apparent MN energy was found to depend also on the shape of the DOS
distribution and polaron binding energy.