Surface analytical studies of interfaces in organic semiconductor devices

Abstract

Surface and interface analytical studies have generated critical insight of the fundamental processes at interfaces involving organic semiconductors. I will review surface analytical studies of interface formation of organic semiconductors with different materials. Metal/organic interface is a focus of both device engineering and basic science, since it is a key factor in nearly all important aspects of device performances, including operation voltages, degradation, and efficiency. I will discuss metal–organic interface dipole formation, charge transfer, chemical reaction, energy level alignment, in-diffusion, quenching of luminescence and possible recovery of it. The effect of the insertion of ultra-thin interlayers such as LiF and doping by alkali metals will also be discussed. In organic/organic interface, the energy offset between the two dissimilar organic materials is vitally important to efficient device operation of organic light emitting diodes (OLED), as well as change separation at donor–acceptor interface in organic photovoltaic devices (OPV). I will discuss the interface energy level alignment, band bending, Debye screening, and charge separation dynamics as observed in surface analytical studies, and the implications to OLED and OPV. The interfaces of OSCs with other inorganic materials are also important. For organic thin film transistors (OTFT), the electronic properties of the interface formed between the organic and the dielectric strongly influences the current–voltage characteristics, as the electronic activity has been shown to occur primarily at the interface between the dielectric and the organic materials. I will review the interface formation of OSCs with dielectric materials and with indium-tin-oxide (ITO), a material whose transparency and conductivity make it indispensable for a number of opto-electronic applications and whose electronic properties and energy level alignment with organics have proven dramatically altered by surface treatments.