Modeling interfaces in organic semiconductor devices

Organic electronics is a continuously growing field and in the last decades it has been recognized that organic semiconductors can have an important role in the fabrication of a wide variety of electronic devices as organic LEDs and FETs, photovoltaic cells and memory elements.
From a theoretical point of view, all these devices require a correct understanding of several aspects: charge injection and transport, energy structure of the organic semiconductors, energy levels alignment at the interface between different materials, the role of trap states, charge recombination and generation.
In particular charge injection in the organic is a critical issue. Organic semiconductors have large band gaps (from 1.5 to 3.0 eV) - they are almost insulators - and low mobility, thus current in organic devices is essentially injection controlled. It is then very important to correctly model contact interfaces especially the formation of trap states that can modify the injection barrier. The effect induced by trap states can become more relevant in devices like solid state sensitized solar cells (ssDSC) where the active layer which absorbs light is fabricated mixing two different materials for electron and hole collection and where the interface is ubiquitous inside the cell. The entire cell is usually modeled using drift-diffusion equations for both electrons and holes in an effective medium framework. By effective medium it is meant that the real structure of the active layer is not explicitly taken into account in the model, and an effective material is used instead. However, the real structure of the electron- and hole-conductor materials is far more complex. Being able to introduce the information about the morphology of the two materials allows to discuss the strengths and limitations of the widely-used effective medium model, but also to describe the transport properties of an ssDSC with unprecedented accuracy.