Studies Of Metal/Conjugated Polymer Surface And Interface Using Photoemission Spectroscopy

Organic electroluminescence is an advanced technology in the field of flat panel display. Polymer light-emitting diodes (PLEDs) have attracted increasing attention due to their promising vast applications in technology. Improving the efficiency and lifetime of devices is the core of developing PLEDs. However, the properties of the metals and polymers interfaces play crucial roles in the performance of PLEDs. The thesis is devoted to the study of surface and interface of metal/polymer in PLEDs using photoemission spectroscopy (PES), especially synchrotron radiation photoemission spectroscopy (SRPES). The main topics of the thesis can be summarized as follows:1) Conjugated polymer, regioregular poly(3-hexylthiophene) (rr-P3HT) and poly(9,9-di-n-hexylfluorenyl-2,7-vinylene) (PDHFV), films were prepared by spin-coating the chloroform solution onto clean silicon wafer surfaces. The surface morphologies of rr-P3HT and PDHFV films were investigated by atomic force microscopy (AFM). The results indicate that the surfaces of the polymer films are relatively flat and uniform.2) Combining the SRPES and X-ray photoemission spectroscopy (XPS), we investigate in-situ the electronic structure and chemical reaction during step-by-step deposition of calcium on the rr-P3HT surface. It is found that Ca atoms diffuse into the rr-P3HT subsurface and preferentially react with S atoms during the formation of the Ca/rr-P3HT interface. No obvious evidence of Ca reacting with C atoms of rr-P3HT can be found. In addition, Ca-induced band bending of rr-P3HT is observed upon Ca deposition. However, no"gap states"is found during the whole process of Ca deposition. From the evolution of the valence band spectra, we have derived the energy levels alignment at the Ca/rr-P3HT interface.3) The interface formation between lithium and rr-P3HT is studied in situ by SRPES and XPS. But here the rr-P3HT samples contain tiny amount of oxygen impurities on the surface. Upon the Li deposition onto the rr-P3HT surface, Li atoms primarily react with oxygen impurities, and then react with S atoms of rr-P3HT. Similar to the case of Ca/rr-P3HT, Li atoms do not react with C atoms of rr-P3HT either, and the Li-induced band bending occurs during Li deposition. According to the investigation of valence bands, it is deduced that the vacuum level and electrons injection barrier of rr-P3HT are significantly reduced during the deposition of Li onto rr-P3HT films.4) The interface formation between Ca and PDHFV has been also studied in situ by SRPES and XPS. It is indicated that Ca not only reacts with oxygen impurities and C atoms on the PDHFV suface, but also dopes electrons into PDHFV, inducing band bending.