Crystallinity improvement of benzodithiophene-dimer films for organic field-effect transistors

We have significantly improved the crystallinity of vacuum-evaporated benzodithiophene (BDT)-dimer films using pentacene crystallinity control layers. The pentacene molecules evaporated on hexamethyldisilazane-treated $SiO2$ surfaces first form many small islands. Then, the islands grow two dimensionally. We used thin pentacene films with surface coverage of approximately 90% as crystallinity control layers, and deposited $30-nm$-thick BDT-dimer films on them. Micron-scale planar grains are observed in atomic force microscopy images of the BDT-dimer films. The highly anisotropic shape of the grains indicates that the BDT-dimer films of good crystallinity have been obtained. The field-effect transistors based on the BDT-dimer films showed excellent hole mobility of $1.2cm2V−1s−1$ at maximum.