Electronic structure calculations for the study of D-π-A organic sensitizers: Exploring polythiophene linkers

Highlights: • We study D-π-A dyes with polythiophene (Tn) or polycyclopentadithiophene (Cn) linkers. • Molecular geometry plays a crucial role in the photophysical properties of organic dyes. • Cn linkers induce lower transition energies and larger oscillator strengths than Tn separators. • We discuss a variety of computational tools to quantify the CT nature of electronic transitions. • We compute ground and excited state oxidation potentials with a long-range corrected functional. - Abstract: In this work we present a detailed study of the atomic and electronic structure of a collection of push–pull organic dyes for high-performance sensitized solar cells (DSSCs). We compare the computed photophysical properties of donor-bridge-acceptor (D-π-A) dyes with polythiophene (Tn) or polycyclopentadithiophene (Cn) conjugated linkers with up to four fused thiophene rings. Excitation energies to lowest excited singlet state have been rationalized by means of fragment and molecular orbitals. Vertical and adiabatic excitation energies are systematically lower for the Cn family and become smaller with the length of the molecular conjugation. We discuss a large variety of computational techniques for the characterization of the charge transfer (CT) nature of the electronic excitation. In addition to standard procedures to quantify CT character, we propose and explain several novel interaction based measures of CT. Finally, we have computed ground and excited state oxidation potentials (GSOP and ESOP) with long-range corrected (LRC) functional