The Structures Of Metal/Organic Interfaces And Their Correlation With The Performances Of Organic (opto-)Electronic Devices

Carbon-based organic conjugated materials, such as conjugated polymers, graphene, fullerenes are wildly used in the fabrication of organic light-emitting diodes (OLEDs), organic solar cells (OSCs), organic field-effect transistors (OFETs) and lithium sulfur batteries (LSBs), because of their advantages in terms of the tunable band gap, low-cost, simple fabrication, light-weight and flexibility. Electronic and chemical structures at metal/organic and organic/organic interfaces are the key factors that strongly influence the performance of these (opto-)electronic devices. Therefore, it’s urgently needed to deeply understand the relationship between the interfacial structure and the performance of the devices involved. This will guide us to design rational interfaces to fabricate devices with higher efficiency and stablility. In this dissertation, the surface morphologies of the organic materials were characterized using atomic force microscropy (AFM) and scanning electron microscopy (SEM). The interfacial structures and band alignments between the low-work-function metal electrodes (Li and Al) and organic thin films were investigated in-situ/ex-situ through synchrotron radiation photoemission spectroscopy (SRPES), near-edge x-ray absorption fine structure (NEXAFS), x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). These fundamental studies on the electronic/chemical structure evolution during the interface formation provide useful information for optimal design of novel devices. The main achievements in this dissertation are summarized as follows:(1) SRPES and XPS have been applied to in-situ investigate the chemical reactions and electronic structures during the interface formation of Li on the regioregular poly (3-hexylthiophene)(rr-P3HT) thin films. Upon Li adsorption onto P3HT at300K, Li dopes electrons into P3HT, inducing the occurrence of the P3HT band bending. Moreover, Li diffuses into the subsurface and reacts with both S and C atoms in the thiophene rings, leading to the formation of Li2S and Li-C complex. Compared to the interface of Ca/P3HT, the diffusion/reaction depth of Li is much larger at the Li/P3HT interface. Through the investigation of the evolution of core level and valence band spectra together with secondary electron cut-off, an energy level alignment diagram at the Li/rr-P3HT interface is derived. (2) Lithium/poly (9,9-dioctylfluorene-co-benzothiadiazole)(F8BT) interface formation process was investigated in-situ using SRPES, XPS, UPS and NEXAFS, together with DFT calculations and device fabrications. It is found that upon Li deposition onto F8BT strong interfacial chemical reactions occurs. By controlling the amount of lithium deposited, the interface dipole at Li/F8BT interface can be tuned. As a consequence, through choosing an appropriate amount of Li to modify F8BT surface, an improved luminance and power conversion efficiency of F8BT-based OLED device can be achieved. These results demonstrate that the effect of interfacial chemical reactions and diffusion between metal electrode and organic layer in the reacted region on device performance cannot be generalized. Actually, the types and extent of reactions, which play a critical role in altering the static dipole on organic surface, should be taken in account. This new reaction-diffusion mechanism may provide new insights into the fundamental understanding of interface between dissimilar materials and will be helpful for optimal design of organic electronic device structures.(3) In-situ investigation of Li/F8BT interface formation at90K was carried out using XPS and ultraviolet photoelectron spectroscopy (UPS). Comparing with the Li/F8BT interface at300K, our results indicate that both the diffusion and chemical reactions between Li and F8BT are reduced at90K, leading to the formation of a "sharper" interface. In addition, no S segregation or upward electric field was observed. Instead, a downward interface dipole appeared. Moreover, no gap states appears at Li/F8BT interface at90K, implying the possibility of improve device performance by building a sharp interface. Summarize the XPS and UPS data, the distinct energy level alignment of Li/F8BT at90K is given.(4) We developed a new method to transfer chemical vapor deposition (CVD)-grown graphene on organic material surface to reduce the content of contaminations on the sample surface, in which the target organic materials were directly used as the supporting materials. Through this method, the introduced impurities were minimized and high-quality single-layer graphene/organic thin films were achieved. In-situ SRPES results indicate that the unwanted interfacial chemical reactions between reactive metallic electrode and organic materials can be reduced by inserting single-layer graphene as a buffer layer. These results demonstrate the great potential of this new transfer technique for the fabrication of more stable organic (opto-)electronic devices with single-layer graphene. (5) The application of Li/S cells is hampered by short cycle life. Sulfur-graphene oxide (S-GO) nanocomposites have shown promise as cathode materials for long-life Li/S cells because oxygen-containing functional groups on the surface of graphene oxide were successfully used as sulfur immobilizer by forming weak bonds with sulfur and polysulfides. While S-GO showed much improved cycling performance, the capacity decay still needs to be improved for commercially viable cells. In this study, we attempt to understand the capacity fading mechanism by ex-situ studying the structural and chemical evolution of S-GO nanocomposites cathode with various numbers of cycles using scanning electron microscopy (SEM), near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). It is found that both the surface morphologies and chemical structures of the cathode materials change considerably with increasing number of cycles. These changes are attributed to several unexpected chemical reactions of lithium with S-GO nanocomposites occurred during the discharge/charge processes with the formation of Li2CO3, Li2SO3, Li2SO4, and COSO2Li species. These reactions result in the loss of cyclable active sulfur on the surface of the electrode, and thus capacity fades while coulombic efficiency is near100%. Moreover, the reaction products accumulate on the cathode surface, forming a compact blocking insulating layer which may make the diffusion of Li ions into/out of the cathode difficult during the discharge/charge process and thus lead to lower utilization of sulfur at higher rates. These two observations are significant contributors to the capacity and rate capability degradation of the Li/S-GO cells. Therefore, for the rechargeable Li/S-GO cells, the content of oxygen in GO should be optimized and more stable functional groups need to be identified for further improvement of the cycling performance.