1 Solar Energy Programme, Risø National Laboratory for Sustainable Energy, Technical University of Denmark2 Risø National Laboratory for Sustainable Energy, Technical University of Denmark3 unknown4 Department of Energy Conversion and Storage, Technical University of Denmark
Improved synthesis, charge carrier mobility, conformation, crystalline structure, and molecular packing of the regiochemically pure 4',3""-dihexyl-2,2';5',2";5",2 "';5"',2"";5"",2""'-sexithiophene (beta,beta'-DH6T) are reported. The sum of charge carrier mobilities of beta,beta'-DH6T measured by the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique was found to be Sigmamu(min) = 3.9 x 10(-3) cm(2) V-1 s(-1), which is comparable with the PR-TRMC mobility found for alpha,omega-DH6T. The field-effect mobility (FEM) of beta,beta'-DH6T was found to be on the order of 10(-5) cm(2) V-1 s(-1), which is considerably less than the FEM of alpha,omega-DH6T. To understand the reason for such poor macroscopic electrical properties, the conformation and the molecular packing of beta,beta'-DH6T were systematically studied by means of UV-vis spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray techniques. Absorption spectra of a beta,beta'-DH6T spin-cast film indicate the planar conformation of the aromatic backbone. SEM and AFM reveal the formation of an ordered lamellar phase. As a single-crystal X-ray study shows, beta,beta'-DH6T exhibits less dense crystalline packing than alpha,omega-DH6T. In contrast to the almost upright orientation of alpha,omega-DH6T molecules against the substrate (tilt angle about 68), the long axis of beta,beta'-DH6T molecules and the surface plane form an angle of similar to20degrees. Thus, the crystalline structure of alpha,omega-DH6T "allows" the current to flow along the molecular stacks; the crystalline structure of beta,beta'-DH6T suppresses the charge transport.
Chemistry of Materials, 2004, Vol 16, Issue 23, p. 4757-4764