ELECTRONIC TRANSPORT PROPERTIES VS STRUCTURAL ORGANIZATION OF THIN LAYERS OF DIFFERENT REGIOREGULAR POLYTHIOPHENE DERIVATIVES: EFFECT OF CHAIN LENGTH AND PROCESSING CONDITIONS
J.-M. Verilhaca, G. LeBlevenneca, A. Pronb, R. Pokropc, M. Zagorskac, J-P. Traversb
a CEA LITEN-DTNM-LCH, CEA-Grenoble, 38054 Grenoble Cedex 9, France
b UMR SPrAM (CEA-CNRS-UJF), DRFMC/SPrAM/LEMOH, CEA-Grenoble,
38054 Grenoble Cedex 9, France(email@example.com)
cFaculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664.
In the framework of the development of organic electronics, one of the major challenges is the improvement in the control of the (opto)electronic properties of the active semiconducting material. It is known, for example, that the performance of polymer based field effect transistors (FETs) is directly related to the charge carrier mobility in the organic thin layer. This parameter is, in turn, strongly dependent on the microstructure of an individual chain as well as on the supramolecular organisation of the polymer.
In this communication, we present the results of a comparative study of the effect of molecular weight, i.e. chain length, on structural, morphological and electrical transport properties, carried out for two polythiophene derivatives which present two different types of chain regioregularity, namely HT-HT coupled poly(3-hexylthiophene) and poly(3,3"-dioctyl-2,2':5',2"-terthiophene).
The relationship between the polymer molecular weight, Mn, the morphology and the microstructure of the films and the charge carriers' mobility has been established from detailed characterization of the semiconducting films by Grazing Incident X-ray Difrraction (GIXD) and Atomic Force Microscopy (AFM).
Based on the preparation of several fractions of distinctly different molecular weight and reduced polydispersity, this study demonstrates that for both polymers, the charge carriers' mobility is increased by more than three orders of magnitude with a 25-fold increase of the molecular weight, Mn. This power law dependence appears to be valid for all processing techniques used for the fabrication of FETs (drop casting, spin and dip coating). These results srongly suggest that in polythiophene derivative based semiconducting thin layers, the charge mobility and therefore the FETs performances are essentially determined by the polymer chain length.