Promoteur de thèse: Monsieur David Beljonne
Résumé de la dissertation
In the quest for new routes towards a replacement of fossil-based energy sources, the last 40 years witnessed an increase in the interest in renewable energies, such as the production of electricity directly from sunlight. In particular, much attention was paid to the use of organic π-conjugated molecules, the building blocks in active layers of emerging photovoltaic thin film technologies, as organic solar cells (OSCs). Yet, OSCs has not largely penetrated the market nowadays, since they still suffer from lower power conversion efficiencies (PCEs) with respect to those of inorganic silicon-based technologies, notwithstanding the multiple advantages OSCs offer: flexibility, lightness, cheapness, semi-transparency, ease of processing and production over large areas. Throughout the years, molecular engineering with the use of small molecule non-fullerene acceptors, local morphology control and optimization of the operating conditions have led to PCEs of above 18%, still far away from the limit set by the Shockley-Queisser law of 33%. However, the bar has been raised and the challenge now is to reach PCEs larger than 20%, provided that all the superfluous loss pathways would be identified and removed. In a bulk heterojunction (BHJ) donor:acceptor OSC, a paramount role is carried out by intermolecular charge-transfer (CT) states, which control the key photophysical phenomena occurring at the heterointerface. Indeed, CT states are intermediates of exciton dissociation, charge separation and recombination process and directly affect the performances of a working device. In addition, several other factors like local morphology at the interface, environmental electrostatic interactions, energetic disorder, delocalization and hybridization, affect dramatically the energetic landscape of CT states, the fate of free charge carriers and their transport properties. Therefore, a fundamental, thorough understanding of such effects, both from an experimental and a theoretical point of view, is mandatory in order to design new organic materials or architectures leading to superior performances. In this respect, modelling plays a major role in the complete rationalization of all the above-mentioned processes which take place at different length- and timescale. Setting up a proper multilevel computational approach combining several techniques, like atomistic molecular dynamics (MD) simulations, density functional theory (DFT) and its time-dependent (TD) version calculations, microelectrostatic (ME) models, is of utmost importance. The leitmotif of this thesis is the application of electronic structure calculations in conjunction with a ME model, specifically designed to take into account solid-state environmental effects, in the modelling of different topics in the organic semiconductor community. In this work, we resort to MD/TDDFT/ME methods in order to provide a fully atomistic modelling study of the broad energetic landscape and dynamics of singlet electronic excitations in amorphous thin films of two small organic π-conjugated donor-bridge-acceptor (push-pull) molecules, slightly different in their chemical structures, but largely different in the transport of singlet excitons. The latter is modelled by performing kinetic Monte Carlo simulations, which yield the macroscopic properties of interest, such as the diffusion coefficients and the exciton diffusion lengths of the two materials. Then, we discuss the existence of multiple electronic CT states in amorphous donor:acceptor blends with large frontier orbital energy offset and their contributions to the photocurrent. We also focus on the energetic and dynamics of interfacial low-lying electronic CT states, by unravelling with a combined MD/(TD)DFT/ME computational protocol the different role exerted by static and dynamic disorder to the density of states of the CT states manifold. At last, we deal with the role of local environmental interactions on the generation of free charge carriers in doped binary and ternary blends. In these systems, both the ionization and the charge dissociation step are controlled by molecular quadrupoles of host and dopant molecules, as found by our theoretical approach based on DFT/ME calculations.