Molecular Level Understanding of Conjugated Organic Solar Cell Materials: A Computational Study

Among different organic photovoltaic technologies, polymer solar cells (PSCs) have potential as light-weight, cost-effective, flexible, and transparent devices. A great amount of work has been carried out in developing and understanding the factors governing the efficiencies of PSCs. In the electronic structure studies of PSC systems, the careful selection of models and methods employed in theoretical calculations is essential for an accurate description of the π-conjugated PSC materials.

In this thesis, the efficient PSC systems have been examined with the density functional theory (DFT) methods. The studied compounds include fullerene-based PSCs, which combine donor–acceptor (D–A) copolymers as electron donor (eD) and fullerene derivatives as electron acceptor (eA) materials. Emerging non-fullerene (NF) devices, which make use of D–A copolymers or small-molecule acceptors, have been also studied. Both oligomeric and periodic copolymer models have been considered and the performance of global hybrid, non-tuned long-range corrected (LRC), and optimally tuned (OT) LRC functionals have been investigated. The aim has been to find suitable tools for modeling PSC systems, while gaining more understanding of the characteristics of PSC compounds and their interactions that impact the performance of PSCs.

The periodic DFT method is found to be a useful tool in modeling of the D–A copolymers. Both the functional and dispersion corrections impact greatly the results predicted for the studied PSC systems. The dispersion corrected OT-LRC functional gives the best description of the individual PSC compounds and their local interfaces. Both the multi-state treatment and dispersion corrected OT-LRC functional result in more constant electronic coupling values. The charge transfer rates are found to be strongly dependent on both the functional and relative orientation of the eD and eA compounds. These findings serve as guidelines for the selection of methods in future theoretical studies of PSC systems, but also provide beneficial information for designing and developing new materials for emerging NF PSCs.