Large-scale quantum chemistry simulations of organic photovoltaics

Abstract

Organic photovoltaic (OPV) devices rely on the mixture between a conjugated copolymer (electron donor) and an electron acceptor material (typically, but not necessarily, functionalised fullerenes): this active layer is known as the bulk heterojunction, and it is crucial for the device operation, as this is where excitons are split into free electrons and holes to produce current. A deep understanding of the role of the molecular structure of materials on the device physics is necessary to achieve better performances, and to this end, computer simulations are undoubtedly a powerful tool. However, the bulk heterojunction is a complex system, and for theoretical models to be representative, these should be composed of thousands of atoms, a size out of the reach of atomistic quantum mechanics simulations. To overcome this limitation, this project involved the use of the ONETEP code, which, thanks to its linear-scaling computational cost with respect to the system size, allowed to carry out ab initio calculations, within the density functional theory (DFT) framework, on OPV materials and models of bulk heterojunctions on a far larger scale than possible before. Regardless of the device morphology and architecture, fundamental for the exciton splitting is the energy level alignment of the donor and the acceptor components, and several ways exist to fine-tune the electronic properties of these materials. Nevertheless, is it possible to find novel and alternative routes to the well-known strategies currently employed in the laboratory? As for the donor polymer, the results here presented suggest that acting on the polymerisation statistics, that is, the ratio of different blocks in the polymer chain, significantly affects the electronic structure of such materials, with changes in the band gap of the same order of magnitude of those induced by the widely used functionalisation approach. The acceptor fullerenes, on the iii other hand, are generally more challenging to functionalise, and consequently their electronic structure cannot be trivially tuned. However, one could ideally circumvent the issue via the intercalation of different solvent molecules in the crystal phase of fullerenes, in order to attempt to indirectly modify the energy of the frontier orbitals and the band gap. Indeed, results highlighted the crucial role of the solvent in modifying both the electronic and the optical properties of solid-state fullerenes through the formation of fullerene-solvent ⇡-⇡ interactions, which disrupt the close packing of solvent-free fullerenes. Interestingly, more appreciable changes were observed in the properties of pure rather than functionalised fullerenes. Another, yet fundamental, aspect of OPV explored here is the polymeracceptor interaction in the bulk heterojunction, with the acceptor material consisting of fullerene and the more recently introduced non-fullerene acceptors (NFAs). Although attempts to model the polymer-fullerene interface to investigate its excited-state properties are numerous, these have been limited to, within the framework of ab initio atomistic simulations, small 1-to-1 short-oligomer-fullerene pair models. For the first time, DFT calculations for ground and excited state were performed on model interfaces of realistic size composed of more than a single polymer chain and dozens of fullerene molecules, allowing to gain new insights into the physics of exciton generation and splitting. For instance, it was observed that the probability of charge transfer to occur is deeply influenced by the polymer block statistics, and that exciton dissociation is favoured by large polymer phases rather than large fullerene phases, although these are still beneficial. Evidence of long-range charge-transfer states in the low-energy part of the excited-state spectrum was also observed. On the other hand, models of polymer-NFA interfaces are still scarce in the literature, as NFA phases are more intrinsically complex to model than fullerene phases. Critical for both charge mobility and device performance is the NFA solid-state arrangement with respect to the polymer. By constructing large polymer-NFA model interfaces it was possible to highlight and confirm the importance of intermolecular ⇡-⇡ stacking interactions in the NFA phase, as it was found that these deeply influence the exciton delocalisation, the exciton splitting rate, and the mobility anisotropy. This work, which is the outcome of collaboration with Merck, was enabled by the linear-scaling capabilities of the ONETEP code, which allowed to study large-scale realistic models of OPV. This thesis provided novel and important insights into different aspects of organic photovoltaics, both in terms of material design and device physics.

More information

Identifiers

ORCID for Gabriele Boschetto: orcid.org/0000-0001-8830-3572

ORCID for Chris-Kriton Skylaris: orcid.org/0000-0003-0258-3433

Catalogue record

Export record