Combined MD/DFT protocol for the simulation of molecular materials for organic solar cells

In much of the literature about organic photovoltaics, the topic is framed within the current landscape of energy production and the research on these materials is cited as a possible solution to the energy crisis looming ahead. Despite being the most frequent, this is by no means the only perspective that can be offered. Indeed, the same research may also be set within the larger perspective offered by the field of functional materials. These materials are usually exploited for their particular responses to electrical, magnetic and chemical stimuli and are at the basis of many technologies fundamental to our society. The prominent position of functional materials in modern science is due to the emergence of novel technological needs that such materials have been able to satisfy thanks to their peculiar properties. These properties have been rationalised and mastered by expanding the theoretical description of the underlying physical mechanisms. This theoretical body, combined with the growth and diffusion of computational capabilities has fostered a change in the scientific paradigm underpinning the research effort. More and more, the predictive power of numerical approaches is exploited to lead the way in the exploration of the immense chemical space. The ultimate promise is to achieve the purpose-driven design of compounds thanks to which the molecular structure can be engineered before the actual synthesis to meet the demands dictated by a specific application. To fulfil this role, computational approaches need to be able to simulate the solid state properties at the most relevant time and length scales. If this can be accomplished then a reliable prediction of the performance can be achieved. The current work deals with the development and application of one such protocol, for the particular case of organic photovoltaic semiconductors. Given the specific application, the properties targeted are light absorption and charge transport. Particular effort is put in the simulation of local morphologies at scales above the molecular one to describe supramolecular organisation with sufficient resolution. In this thesis, the protocol is applied to two molecular systems employed in solar devices. Both systems have been selected on the basis of data suggesting that a detailed microscopic description of their behaviour could be highly informative about the aspects responsible for their photovoltaic performance. In particular, chapter 3 details the investigation of a small-molecule donor that has been shown in the literature to have a remarkable behaviour in absorption. While chapter 4 reports the study of a donor-acceptor dyad used as active layer in single-component solar devices with relatively high conversion efficiency. In both cases, the computational protocol has proven capable of providing a detailed microscopic description of the systems. The picture drawn has allowed to clarify the plausible mechanisms behind the observations and to rationalise these behaviours in a broader and more general theoretical framework.