The Structures And Properties Of Metal/Polymer Interfaces In Polymer-based Photoelectronic Devices

Nowadays, the n-conjugated polymer-based solar cells (PSCs) and light emitting diodes (PLEDs) have drawn more and more attentions from the scientists and enterprises around the global world, because of the superior features of these polymers, for instance high conductivity, light weight, low-cost, and more importantly, easily solved in organic solution and spin-coated on soft substrate to produce large-scale and flexible photoelectric devices. However, the active metal electrodes deposited on the polymer films during the manufacture process usually diffuse into the subsurface of polymers, followed by distinct changes of chemical and electronic structure of polymers. Furthermore, the polymer/polymer interface in the heterojunction always affects the separation process of exitons. Therefore, the metal/polymer and polymer/polymer interfaces play an critical role on the performce of PSCs and PLEDs.This thesis thus focus on elucidating the chemical reaction and electronic characteristics of such interfaces by means of various surface-sensitive analytical technique, such as atomic force microscope (AFM), synchrotron radiation photoemission spectroscopy (SRPES), X-ray photoemission spectroscopy (XPS), and near-edge x-ray absorption fine structure (NEXAFS). The in-situ analysis of these interfaces help to built up an appropriate interfacial model to analyze, how these characteristics affect device properties of PSCs and PLEDs.All these studies help us to better understand the corelation between the interfaces and the device performance, and provide theoretical supports for further enhancing the efficiencies of PSCs and PLEDs. The following results have been achieved in this thesis:(1) Poly(9,9-dioctylfluorene-co-benzothiodiazole)(F8BT) is one of the most widely used polymers in PSCs and PLEDs. Atomic force microscopy(AFM), synchrotron radiation photoemission spectroscopy(SRPES), and near edge X-ray absorption fine structure(NEXAFS) are applied to in situ investigate the morphology, structure and molecular orientation of spin-coated F8BT films and their interaction with vapor-deposited Al metal. The room temperature spin-coated F8BT film is rather flat, while mild annealing treatments below the glass transition temperature lead to an apparent increase of surface roughness of F8BT film. After70℃annealing in vacuum, the aromatic rings of F8BT preferentially stand more edge-on, with an average tilt angle of approximately49°with respect to the substrate,while the9,9-dioctylfluorene unit (F8) and the benzothiodiazole unit (BT) nearly lie in the same plane. Upon vapor- deposited Al metal onto F8BT at300K, in addition to the strong chemical interaction between Al and F8BT, the doping ofelectrons from Al into F8BT is observed, leading to the downward band bending and partialoccupation of lowest unoccupied molecular orbits (LUMO).(2) The interface between Ca and F8BT was in situ studied by XPS, SRPES and NEXAFS. Upon vapor-depositing Ca metal onto F8BT at room temperature, Ca dopes electrons into F8BT, inducing the occurrence of the F8BT band bending. Moreover, Ca diffuses into F8BT subsurface, leading to strong chemical interactions occur between Ca and F8BT, as evidenced by the significant changes of the S2p, N ls and C ls spectra. What is more important, a distinct model to demonstrate the mechanism of the chemical interaction between Ca and benzothiodiazole unit (BT) is derived. Compared to the interface of Al/F8BT, the diffusion/reaction depth of Ca is much larger, resulting into the occurrence of an unusual interface dipole whose orientation points away from Ca to F8BT while an appropriate dose (1.0-2.5A) of Ca is deposited onto the F8BT specimen. The evolutions of C K-edge and N K-edge NEXAFS spectra indicate electrons transfer into F8BT from Ca lead to partial occupation of LUMO bands of F8BT, as a result, higher barrier should be overcome to inject electrons into the LUMO+1bands in Ca/F8BT-based PLEDs. Most importantly, the efficiency of Al/Ca/F8BT-based PLEDs is to be further enhanced promisingly by managing to control the deposited Ca dose ranging from2.0A to2.5A, due to the dipole occurring in opposite orientation might greatly help to increase the radiative recombination efficiency of excitons at the Ca/F8BT interface.(3)Blend polymers are usually used as heterojunction in PSCs. The interface between Ca and the F8BT:PC6oBM film was studied as a prototype system to investigate the change of chemical and electronic structure occurring at room temperature by XPS and NEXAFS. As indicated by the N ls, S2p, valence band and NEXAFS spectra of pristine blend polymers film, F8BT component is enriched on the outer surface. Upon vapor-depositing Ca metal onto F8BT at room temperature, Ca mainly reacts with the F8BT enriched layer. Electrons transfer from PC60BM to F8BT occures with the formation of F8BT/PC60BM interface, as evidenced by the changes of C ls, N ls and S2p. As compared with the Ca/F8BT interface, the interface of Ca/F8BT:PC6oBM comprises an different electronic energy level structure. An "sandwich"model interface of Ca/F8BT enriched layer/PC60BM enriched layer is built up to clarify the formation of interfacial dipoles in the Ca/F8BT:PC60BM interface.