Organic field effect transistors (OFETs) are providing exciting prospects for potential applications in electronics. The active elements of these devices use “plastic” semiconductors, based on carbon and hydrogen. Among the advantages compared to classical silicon transistors, this new generation of components should combine mechanical flexibility, low weight, transparency and low cost. Enormous progress has been made to improve the performance of these devices through optimising the synthesis processes, drastically reducing the concentration of impurities present. At this point, a more fundamental understanding of the microscopic mechanisms governing electron transport in the organic materials becomes necessary. Systematic studies of transport properties in organic transistors based on rubrene have been done by A. F. Morpugno’s team at the Institute of Nanosciences of Delft Technical University (Netherlands).
In order to increase the capacitance, that is the maximum density of charge carriers, they use semiconductors having higher and higher dielectric constants for the gate material. We have noticed that the electronic conduction, instead of increasing proportionally to the number of charge carriers, has a tendency to saturate, and even to decrease. Measurements as a function of temperature showed that this phenomenon is associated with a regime where the carrier mobility becomes thermally activated : to travel from one electrode to the other, the electrons must jump from molecule to molecule, crossing a finite energy barrier at each jump. The microscopic origin of this phenomenon is to be found in the interaction of the charge carriers with the ions constituting the dielectric material of the gate. The combined effects of this “electron-phonon” interaction and the small bandwidths observed for molecular crystals (of order 0.5 eV) lead to the formation of new quasi-particles : the polarons. Polaron formation traps the carriers’ wave functions on individual molecules, and their motion is then by jumps, which explains the experimentally measured energy barriers. Also mobility saturation is consistently explained in terms of the long-range Coulomb repulsion between the polarons. Upon increasing the carrier density, these interactions aquire importance leading to an increase of the polaronic activation energy.
People: S. Ciuchi
I. N. HULEA, S. FRATINI, H. XIE, C.L. MULDER, N.N, IOSSAD, G. RASTELLI, S. CIUCHI, AND A. F. MORPURGO. Tunable Fröhlich Polarons in Organic Single-Crystal Transistors. NATURE MATERIALS. vol. 5, pp. 982 (2006)
S. FRATINI, H. XIE, I. N. HULEA, S. CIUCHI, AND A. F. MORPURGO. Current saturation and Coulomb interactions in organic single-crystal transistors NEW JOURNAL OF PHYSICS. vol. 10, pp. 033031 (2008)
S. CIUCHI, AND S. FRATINI. Hopping dynamics of interacting polarons. Phys. Rev. B 79 035113 (2009)