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

Abstract

Poly[5,${5}^{\ensuremath{'}}$-bis(3-dodecyl-2-thienyl)-2,${2}^{\ensuremath{'}}$-bithiophene] (PQT-12) is a conjugated polymer that shows promising performance $(\ensuremath{\mu}>0.1\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s})$ as a semiconductor for thin-film electronics. The electrical properties of PQT-12 thin films can vary by over 3 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 ${\text{SiO}}_{2}$ dielectric or on ${\text{SiO}}_{2}$ treated with OTS. The microstructure of the two forms of polymer, as characterized using specular and grazing x-ray diffraction in addition to rocking curves, was compared and correlated with 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, independent 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 significantly 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 $\ensuremath{\pi}\text{\ensuremath{-}}\ensuremath{\pi}$ 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 in the out-of-plane misorientation of neighboring crystallites without any significant grain growth in the plane of the film. The mobility increase in 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.