Lyu Lu, Niu Dongmei, Xie Haipeng, Zhao Yuan, Cao Ningtong, Zhang Hong, Zhang Yuhe, Liu Peng, Gao Yongli
Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, No. 605 Lushan South Road, Changsha, Hunan 410012, P. R. China.
Phys Chem Chem Phys. 2017 Jan 4;19(2):1669-1676. doi: 10.1039/c6cp06919c.
Combining ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM) and small angle X-ray diffraction (SAXD) measurements, we perform a systematic investigation on the correlations of the electronic structure, film growth and molecular orientation of 2,7-diocty[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on silicon oxide (SiO). AFM analysis reveals a phase transition of disorderedly oriented molecules in clusters in thinner films to highly ordered standing-up molecules in islands in thicker films. SAXD peaks consistently support the standing-up configuration in islands. The increasing ordering of the molecular orientation with film thickness contributes to the changing of the shape and lowering of the leading edge of the highest occupied molecular orbital (HOMO). The end methyl of the highly ordered standing molecules forms an outward pointing dipole layer which makes the work function (WF) decrease with increasing thickness. The downward shift of the HOMO and a decrease of WF result in unconventional downward band bending and decreased ionization potential (IP). The correlations of the orientation ordering of molecules, film growth and interface electronic structures provide a useful design strategy to improve the performance of C8-BTBT thin film based field effect transistors.
结合紫外光电子能谱（UPS）、X射线光电子能谱（XPS）、原子力显微镜（AFM）和小角X射线衍射（SAXD）测量，我们对2,7 - 二辛基[1]苯并噻吩并[3,2 - b]苯并噻吩（C8 - BTBT）在氧化硅（SiO）上的电子结构、薄膜生长和分子取向之间的相关性进行了系统研究。AFM分析表明，较薄薄膜中簇状无序取向分子发生相变，变为较厚薄膜中岛状高度有序的直立分子。SAXD峰始终支持岛状中的直立构型。分子取向随薄膜厚度增加而有序化，导致最高占据分子轨道（HOMO）的形状变化和前沿降低。高度有序直立分子的末端甲基形成向外指向的偶极层，使功函数（WF）随厚度增加而降低。HOMO的向下移动和WF的降低导致非常规的向下能带弯曲和电离势（IP）降低。分子取向有序化、薄膜生长和界面电子结构之间的相关性为改善基于C8 - BTBT薄膜的场效应晶体管性能提供了有用的设计策略。
