Computational Predictions of Conjugated Polymer Properties for Photovoltaic Applications

Sammanfattning: Organic solar cells employing fullerenes blended with conjugated polymers as the main light-absorbing material have achieved power conversion efficiencies exceeding 10%. They hold promise as an alternative energy source with many advantages in terms of long-term sustainability and reduced greenhouse gas emissions. Detailed information on the electronic and geometric structure of the molecules involved is generally not accessible through experimental means, as the typically amorphous polymer films are not readily studied with e.g. X-ray crystallography. Computational chemistry, and in particular quantum chemistry as used for the research presented in this thesis, can however provide molecular level insight into the properties of these conjugated polymers. (Time-dependent) density functional theory calculations are here employed on various polymers, mainly of donor–acceptor (D–A) and D–A1–D–A2 types. Systematic studies demonstrate how the energy levels and optical properties relate to each other, as well as to the chemical composition of the polymers. In particular focus are the traits that are important for efficient solar cells: strong absorption, suitably narrow band gap, appropriate LUMO energy vs. the fullerene LUMO, and extended conjugation promoting high charge carrier mobilities. Several polymers with high-performance solar cells are studied, including TQ1 where a computationally revealed unique helical geometry is used to partially rationalize its 7.08% efficiency, and the D–A1–D–A2 polymer P3TQTIF whose two distinct acceptor units allow two strong low-energy electronic transitions, greatly enhancing its spectral coverage. Size-converged optical properties are obtained through a scheme based on extrapolations from oligomer calculations, and a detailed comparison to experiments has facilitated the development of an empirical correction for absorption energies and strengths. These corrections are subsequently used for a priori predictions of polymer absorption spectra with good agreement to experiments. Finally, a strategy is presented that includes the effect of temperature, in form of thermally populated conformations with reduced conjugation and weaker and blue-shifted absorption, yielding trends in excellent agreement with experimental optical properties. Calculations are in summary able to provide deeper insights into the fundamental properties of conjugated polymers, constituting a valuable tool for the ongoing development of materials for application in high-performance organic solar cells.