Electron behaviour at material interfaces
Organic-based devices such as light-emitting displays, solar cells and biological and chemical sensors rely on the deposition of layers of different materials. Thus, it is critical to characterise the interfaces between the various metals, oxides and insulating or semi-conducting materials used. Although extensive work has led to a deeper understanding of the morphology of such materials and interfaces, detailed characterisation of relevant electrical processes was needed to complete the picture. The EU-funded project 'Modelling of electronic processes at interfaces in organic-based electronic devices' (MINOTOR) made excellent progress in filling this gap. Scientists focused simultaneously on metal/organic interfaces (M/O), organic/organic interfaces (O/O) and inorganic/organic (I/O) interfaces. MINOTOR applied Discrete Fourier Transform (DFT) approaches based on density functionals that mathematically describe the electron density of many-electron systems. Scientists employed these methods to evaluate the so-called work function as well as Fermi level pinning (FLP), related to removing an electron from a surface and prevention of such, respectively. Standard DFT approaches worked well in describing M/O interfaces with strong coupling between the molecules and the surface. Scientists showed that surface electron characteristics of metal electrodes can be intricately tuned by modifying the self-assembled monolayer (SAM)-forming molecules used to coat them. In the case of weak coupling, such as in a metal covered by a thin layer of native oxide, DFT reliably reproduced FLP effects. Long-range corrected DFT functionals were recommended to describe interfacial charge distribution in O/O interfaces with partial charge transfer between donor and acceptor entities. However, microelectrostatic (ME) models were preferred over DFT for O/O systems dominated by polarisation effects. In the case of I/O interfaces, investigators demonstrated tuning of the work function of oxide layers by grafting SAM-forming molecules. Tight-binding DFT methods are necessary to simultaneously describe electron density distributions of the interfaces. Numerous devices including spintronics devices and solar cells were fabricated to connect theory to experiment. They demonstrated the ability of SAMs to tune interface characteristics and the role of interface morphology in device performance. An enhanced understanding of electrical processes at all interfaces provided by MINOTOR will enable future designers to create high-performance products.
