Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK.
J Am Chem Soc. 2013 Jan 23;135(3):1092-101. doi: 10.1021/ja310240q. Epub 2013 Jan 11.
The molecular orientation and microstructure of films of the high-mobility semiconducting polymer poly(N,N-bis-2-octyldodecylnaphthalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene) (P(NDI2OD-T2)) are probed using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. In particular a novel approach is used whereby the bulk molecular orientation and surface molecular orientation are simultaneously measured on the same sample using NEXAFS spectroscopy in an angle-resolved transmission experiment. Furthermore, the acquisition of bulk-sensitive NEXAFS data enables a direct comparison of the information provided by GIWAXS and NEXAFS. By comparison of the bulk-sensitive and surface-sensitive NEXAFS data, a distinctly different molecular orientation is observed at the surface of the film compared to the bulk. While a more "face-on" orientation of the conjugated backbone is observed in the bulk of the film, consistent with the lamella orientation observed by GIWAXS, a more "edge-on" orientation is observed at the surface of the film with surface-sensitive NEXAFS spectroscopy. This distinct edge-on surface orientation explains the high in-plane mobility that is achieved in top-gate P(NDI2OD-T2) field-effect transistors (FETs), while the bulk face-on texture explains the high out-of-plane mobilities that are observed in time-of-flight and diode measurements. These results also stress that GIWAXS lacks the surface sensitivity required to probe the microstructure of the accumulation layer that supports charge transport in organic FETs and hence may not necessarily be appropriate for correlating film microstructure and FET charge transport.
使用掠入射广角 X 射线散射（GIWAXS）和近边 X 射线吸收精细结构（NEXAFS）光谱技术研究了高分子半导体聚合物聚（N,N-双-2-辛基二萘并[1,2-b:5,6-b']二噻吩-5,8-二羧酸-2,6-二基-alt-5,5-二噻吩-2,2-二基）（P（NDI2OD-T2））薄膜的分子取向和微结构。特别是采用了一种新颖的方法，即在角度分辨透射实验中，使用 NEXAFS 光谱技术同时测量同一样品的体分子取向和表面分子取向。此外，采集体敏感 NEXAFS 数据可直接比较 GIWAXS 和 NEXAFS 提供的信息。通过比较体敏感和表面敏感的 NEXAFS 数据，在薄膜表面观察到与体相明显不同的分子取向。虽然在薄膜体相中观察到共轭主链的更“面对面”取向，与 GIWAXS 观察到的层片取向一致，但在薄膜表面使用表面敏感的 NEXAFS 光谱技术观察到更“边缘向上”的取向。这种明显的边缘向上的表面取向解释了在顶栅 P（NDI2OD-T2）场效应晶体管（FET）中实现的高面内迁移率，而体相中面向上的织构解释了在飞行时间和二极管测量中观察到的高面外迁移率。这些结果还强调，GIWAXS 缺乏探测在有机 FET 中支持电荷输运的积累层微观结构所需的表面灵敏度，因此可能不适合关联薄膜微观结构和 FET 电荷输运。
