Polarization effects in the channel of an organic field-effect transistor

Abstract

We present the results of our calculation of the effects of dynamical coupling of a charge-carrier to the electronic polarization and the field-induced lattice displacements at the gate-interface of an organic field-effect transistor (OFET). We find that these interactions reduce the effective bandwidth of the charge-carrier in the quasi-two dimensional channel of a pentacene transistor by a factor of two from its bulk value when the gate is a high-permittivity dielectric such as (Ta 2 O 5 ) while this reduction essentially vanishes using a polymer gate-insulator. These results demonstrate that carrier mass renormalization triggers the dielectric effects on the mobility reported recently in OFETs.

Quantum coherence and carriers mobility in organic semiconductors

“…Its main effect is to renormalize the transfer integral as outlined in our calculation presented previously in Appendix C of Ref. 23. That calculation yielded a further reduction of the bare transfer integral by a factor of 0.75.…”

“…Fast interactions can be averaged; they just renormalize the parameters of the hamiltonian. 23 On the contrary, slow or static interactions have to be included explicitly in the hamiltonian. This is the case for intermolecular phonons, which as we mentioned earlier, form a band around 50cm −1 in acenes.…”

“…The reduction factors of the bare integral J have been calculated in Ref. 23 for pentacene to be 0.79 for polarization and 0.75 for intramolecular phonons.…”

“…The measurements of Morpurgo et al 9 have shown unambiguously that the hole field effect mobilities in single crystals of rubrene or tetracene depend strongly on the dielectric permittivity of the gate. We have recently shown 23 that such dependence can be understood only if polarization effects are taken into account. Our recent thermopower measurements 41 on high quality pentacene films evaporated on different substrates revealed a large intrinsic temperature independent, contribution to the Seebeck coefficient of 265µV/K.…”

Temperature dependence of the charge carrier mobility in disordered organic semiconductors at large carrier concentrations

“…It worth noting that a model based on Fröhlich surface polaron 36 was recently suggested to rationalize strong influence of polar gate dielectrics on charge mobility in OFETs. 37 The Fröhlich surface polaron might be formed due to strong coupling of a charge carrier at the interface to an ionic polarization cloud of sufficiently polar gate dielectric ͑high-permittivity dielectric͒ even in organic materials with moderate or weak electron-phonon coupling. 37 Although this model looks interesting and is worth further development, for instance, by accounting also for some disorder effects, its consideration is beyond the scope of the present study.…”