Final Report Summary - TRANSSIM (Multiscale charge-transport simulation of organic-based materials and devices) This project aims to develop innovative multiscale computational tools for tackling charge transport in organic materials (organic crystals, organic crystalline thin films, etc.). Based on a combination between first principles calculation and mesoscopic transport approaches, we investigate charge transport in materials of strong technological interest at an advanced realistic level. The method takes into account strong electron-phonon coupling and related polaronic states, as well as material imperfections (disorder) on the same footing. The exploration of temperature dependent charge mobility is endeavoured for bulk as well as for low-dimensional organic compounds. As a first step to describe transport in three-dimensional (3D) models of organic semiconductors, the fellow has developed a numerical transport code to describe electronic transport including a microscopic description of (static) disorder which has been subsequently generalised towards three dimensions. One of the milestones towards the inclusion of electron-phonon coupling for organic materials was to establish a methodology to capture polaron effects and describe polaronic transport in disordered organic materials in the low-temperature transport regime. The first step towards this goal was the development of the disorder generalisation of the coherent part of the mobility taking into account polaron renormalisation. This has been accomplished and, subsequently, semi-classical electronic transport has been studied in 3D models of disordered organic crystals. Afterwards an extensive set of simulations have been carried out to scrutinise localisation effects in organic matter. Localisation at low temperatures has been studied for 3D models of disordered organic crystals. It has been shown that these effects may impact for low temperatures on the carrier mobility if disorder is strong enough even in the 3D case. For weak disorder there is also an influence on disorder but transport remains rather semi-classical. As an important methodological step, the fellow included the incoherent hopping contribution of transport in the developed simulation tools. This finalises the methodological part of the project. Together with the previously achieved goals the implementation of a simulation tool for the (longitudinal) charge transport in organic matter has been achieved. The ab initio simulation of electron-phonon coupling constants and electronic structure parameters are essential for the realistic description of organic materials. At the time of the final report these simulations that aim at computing thousands of material parameters are still on-going. At the same time the transport simulation code has been parallelised to tackle with very large-scale systems. With the speed-up code the fellow studied the influence of disorder (in the Anderson model) on the incoherent high-temperature part of the carrier mobility. It was found that the impact is significantly weaker as for the coherent part which clearly points out the ambivalent nature of the dynamic disorder (phonons) in the transport process but also the nontrivial interaction of static and dynamic disorder both of which can be described now. The fellow wrote a paper on the newly developed approach which has been published [Phys. Rev. B 84, 180302(R) (2011)] and already been cited evidencing its high impact on the community. During the project, the fellow has been invited to various places in Europe and Japan (in total 5 times) to present his work and contributed to 5 conferences. In total the fellow contributed to four scientific articles including a review article on quantum-transport simulations and an additional book contribution. Further manuscripts have been submitted. For the future research based on the developed techniques and tools, we expect a strong improvement of the understanding of basic transport principles of organic materials relevant for their use in electronic and optoelectronic industry.
