Surface morphology of F8BT films and interface structures and reactions of Al on F8BT films

Abstract

The surface morphology and molecular orientation of -conjugated polymers, along with the chemical interaction and electronic structure at the interface between metals and these polymers, strongly affect the performance of the polymer-based organic electronic and optoelectronic devices. In this study, atomic force microscopy (AFM), synchrotron radiation photoemission spectroscopy (SRPES), and near edge X-ray absorption fine structure (NEXAFS) have been used to in situ investigate the morphology, structure, and molecular orientation of spin-coated poly(9,9-dioctylfluorene-co-benzothiodiazole) (F8BT) films and their interaction with the vapor-deposited Al metal. F8BT films were prepared by spin-coating the F8BT chloroform solution onto clean gold-coated silicon wafer surfaces. The room temperature spin-coated F8BT film is rather flat, while mild annealing treatments (120 ℃) below the glass transition temperature (Tg=130 ℃) lead to an apparent increase of surface roughness of F8BT film, which is helpful to effectively increase the contact areas between metals and F8BT. After 70 ℃ annealing in vacuum, the aromatic rings of F8BT preferentially stand more edge-on, making an average tilt angle of approximately 49 with the substrate, while the 9,9-dioctylfluorene unit (F8) and the benzothiodiazole unit (BT) nearly lie in the same plane. Upon vapor-depositing Al metal onto F8BT at room temperature, strong chemical interactions occur between Al and F8BT, as evidenced by the distinct changes of the S 2p, N 1s and C 1s spectra. Al reacts with S atoms more strongly than with N and C atoms in F8BT. In addition, obvious structural changes in valence band of F8BT are also observed during the Al deposition. Furthermore, Al dopes electrons into F8BT, leading to downward band bending, formation of interfacial dipole at the Al/F8BT interface, and partial occupation of lowest unoccupied molecular orbits (LUMO). However, no doping-induced gap states can be observed during the formation of Al/F8BT interface. Through the investigation of the core-level and valence band spectra evolution of F8BT together with the shifts of secondary electron cutoff during Al deposition, an energy level alignment diagram at the Al/F8BT interface is derived. The information gained through this study will help better understand the correlation between the interface structures of metal electrodes on semiconducting, -conjugated polymer materials and the performances of real polymer-based electronic and optoelectronic devices, which will in turn help develop the more efficient polymer-based organic devices.