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Correlating the microstructure of thin films of Poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’-bithiophene] with charge transport: effect of dielectric surface energy and thermal annealing.

 

Poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene] (PQT-12) is a conjugated polymer that shows promising performance (? > 0.1 cm2/Vs) as a semiconductor for thin film electronics. The electrical properties of PQT-12 thin films can vary by over three orders of magnitude depending on the chemistry of the substrate onto which they are deposited and on annealing conditions. The highest mobility is obtained in films annealed on a dielectric treated with a self-assembled monolayer of octadecyltrichlorosilane (OTS).

Polymeric thin films were processed from either a solution of dissolved PQT-12 molecules in 1,2-dichlorobenzene or from a nanoparticle dispersion of the polymer in the same solvent (nPQT-12). In addition, the substrate surface chemistry was altered by spin-coating on a bare SiO2 dieletric or on SiO2 ¬ treated with OTS. The microstructure of the polymeric thin films was characterized using specular and grazing incidence diffraction in addition to rocking curves. The microstructure of the two forms of polymer was compared and was correlated to the electrical performance of the films as active layers in thin-film transistors. As-spun films of nPQT-12 are always more crystalline than those of PQT-12, independently of substrate chemistry. Consequently, carrier mobility in as-spun films is higher in nPQT-12 than in PQT-12. The presence of the OTS monolayer at the polymer/dielectric interface increases crystallinity of both PQT-12 and nPQT-12, without affecting their texture. After annealing, the mobility in PQT-12 films and nPQT-12 films is comparable. Annealing causes the polymer films on OTS to undergo crystallite growth in the direction normal to the substrate. In nPQT-12, growth of the crystalline coherence length in the ?-? stacking direction (i.e. parallel to the substrate and in the direction of charge transport) occurs as well. The mobility increase in nPQT-12 on OTS upon annealing is thus attributed to the higher crystallinity of the film. In PQT-12 films deposited on OTS on the other hand, annealing causes a decrease of the out-of-plane misorientation of neighboring crystallites without any significant grain growth in the plane of the film. The mobility increase of PQT-12 on OTS upon annealing is attributed to a better intergrain connectivity, in agreement with electrical modeling of the transistor characteristics using a mobility edge model.

 
citation

Jimison, L. H.; Salleo, A. ; Chabinyc, M. ; Toney, M. F. Correlating the microstructure of thin films of Poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’-bithiophene] with charge transport: effect of dielectric surface energy and thermal annealing. Physical Review B. 2008 September; 78 (12): 125319.