Energetics of metal–organic interfaces: New experiments and assessment of the field

Considerable research and development means have been focused in the past decade on organic semiconductor thin films and devices with applications to full color displays, flexible electronics and photovoltaics. Critical areas of these thin films are their interfaces with electrodes, with other organic films and with dielectrics, as these interfaces control charge injection and transport through the device. Full understanding of the mechanisms that determine the electronic properties of these interfaces, i.e. the relative position of molecular levels and charge carrier transport states, is an important goal to reach for developing reliable device processing conditions. This report provides an extensive, although probably somewhat biased, review of polymer– and small molecule–metal interface work of the past few years, with emphasis placed specifically on (i) the electronic structure and molecular level alignment at these interfaces, (ii) the perceived differences between small molecule and polymer interfaces, (iii) the difference between organic-on-metal and metal-on-organic interfaces, and (iv) the role played by electrode surface contamination in establishing interface energetics. Environmental conditions, e.g. vacuum vs. ambient, are found to be critical parameters in the processing of polymer and small molecule interfaces with metals. With similar processing conditions, these two types of interfaces are found to obey very similar molecular level alignment rules.