Defects and Degradation Chemistry at Organic-Inorganic Interfaces

Elucidating the Impact of Interfacial Chemistry on Defects and Degradation at Organic-Inorganic Interfaces Kenneth Graham, University of Kentucky (Principal Investigator) Chad Risko, University of Kentucky (Co-Investigator) Interfaces significantly impact the performance and stability of electronic and electrochemical devices. Charges typically must transfer across several different interfaces in a working device, with interfacial energetics and charge traps significantly influencing these charge transfer processes. Additionally, interfaces often contain high concentrations of defect states that trap charges and limit device performance or include sites that catalyze degradation reactions. Interfacial chemistry is thus a central consideration in the development and optimization of nearly every electronic material and device, from well-established inorganic semiconductors to more recently commercialized organic light emitting diodes to emerging devices still in the research and development phase, such as organic electrochemical transistors, organic and perovskite solar cells, and spintronic devices for quantum computing applications. While decades of research on interfacial chemistry and associated interfacial defect states have led to the optimized inorganic semiconducting devices available today, interfaces and associated defect states in emerging materials and devices are not well understood. Many of these emerging devices include organic semiconductors (OSCs), with the OSC often in contact with an inorganic oxide or metal. This research focuses on understanding interfacial chemistry at organic-oxide interfaces and the impact of this interfacial chemistry on defects and degradation. Establishing this fundamental understanding will enable rapid advances in the development of emerging electronic devices that rely on these critical interfaces Application of self-assembled monolayers (SAMs) is emerging as a key approach to modify organic-inorganic interfaces in devices containing OSCs or halide perovskites. While these SAMs are leading to performance and stability gains in electronic devices, there is a lack of understanding of why some of these SAMs yield such gains and how SAM chemistry can be tuned to enable further advances. Particularly, there is a dearth of knowledge on how SAMs impact interfacial defect states and degradation processes, which impedes their further development. This research centers on the hypothesis that effective SAMs will prevent atomic scale contact between OSCs and oxides, and it is these atomic contact sites that lead to defect states and degradation reactions. In a potential paradigm shift, we propose that the success of SAMs containing organic semiconducting moieties (OSC-SAMs) is not related to the ability of the OSC-SAM to accept and transfer charge-carriers, rather, it is related to OSC-SAMs being more sterically bulky than non-OSC-SAMs and therefore more effectively preventing atomic scale contact between the OSC and oxide. The major objectives include 1) determining the impact of atomic scale contacts between OSC and oxide on degradation mechanisms and stability, 2) determining the impact of the work function (WF) and ionization energy (IE) of the SAM-modified electrode on degradation mechanisms and stability, and 3) determining why degradation is dependent on the nature of the oxide at SAM-modified organic- inorganic interfaces. These objectives will be accomplished through investigating systematic families of SAMs with varying structure and surface coverage. Experimentally, defect states will be probed using variable energy ultraviolet photoemission spectroscopy and thermal admittance spectroscopy in combination with optical spectroscopies and transient electronic spectroscopies. Density functional theory calculations and molecular dynamics simulations will be used to analyze the impact of SAM chemistry on interfacial interactions, defect states, and possible degradation reactions. This combination of experiment and theory will provide unprecedented insight into the interrelationships between interfacial chemistry, defect states, and degradation mechanisms at organic-inorganic interfaces. The research will establish guidelines for SAM design, including a potential paradigm shift in SAM design if indeed sterics are responsible for the success of OSC-SAMs, and provide critical insight into how molecular level interactions influence defects and degradations at organic-inorganic interfaces as a function of both SAM and oxide chemistry.