Tuning the Electronic Structure of Organic/Metal Contacts via Additive Interfacial Segregation

All organic-based devices, i.e., “organic electronics”, require good electrical contacts between the organic semiconductor and an electrode, most commonly a metal. It has been established early-on that the nature of the metal/organic electrical contact often determines the overall device performance and stability. Tuning the interfacial energy level alignment can generally be achieved by depositing interlayers between the organic semiconductor and the metal contact. However, understanding and eventually adjusting the energy level alignment at buried interfaces formed by depositing a metal electrode on top of a pre-formed organic semiconductor poses formidable challenges. Very recently, we have discovered that the evaporation of a metal contact onto a polymer blend could induce phase separation and segregation within the polymer film. Aluminum evaporation induced segregation of additives to form (partial) interlayers at the polymer/metal interface and significantly increased the performance of a photovoltaic cell. In the proposed project we aim to establish a new paradigm for optimizing the energy level alignment of buried metal/organic interface in organic electronic devices by inducing additive segregation to the organic/metal interface. The additives will be judiciously selected to segregate to the organic/metal interface and to form the desired interfacial interactions. The presence of the interlayers will be confirmed using mainly X-ray photoelectron spectroscopy (XPS) and electron microscopy. The resulting electronic structure will be measured by ultraviolet photoelectron spectroscopy (UPS), and also high kinetic energy photoemission, which allows probing composition and energy level alignment up to 30 nm below the sample surface. Finally, the positive effect of energy level tuning on device performance will be established through comprehensive characterization of photovoltaic cells and thin film transistors. Revealing the fundamental mechanism(s) allowing this type of interfacial energy level tuning will enable establishing reliable knowledge-based further development of the method for the use in a broad range of organic electronics applications.