Interfacial electronic structure for high performance organic devices

Organic semiconductors (OSCs) are at the center of attention in a wide range of research fields since their unique advantages meet the requirements for next-generation optoelectronics applications. Since OSCs are lacking intrinsic carriers, charges for device operation have to be injected through organic/electrode and organic/organic interfaces. Therefore, the charge injection efficiency, which is determined by the energy level alignments at those interfaces, governs the device performance. In other words, high performance organic devices cannot be achieved without facilitating proper energy level alignments. Thus, the interfacial electronic structure, which should be determined from accurate measurements of the charge transport level, must be understood to establish the design strategy for high performance organic devices. In this review, various spectroscopic methods to investigate the surface and interface electronic structures, including direct photoelectron spectroscopy, inverse photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray emission spectroscopy, are discussed along with their fundamental principles. Examples of device performance enhancements with modification of the interfacial electronic structure in organic photovoltaics and organic light-emitting diodes are presented.