Pyrrolidine-based dye-sensitized solar cells: A time-dependent density functional theory investigation of the excited state electronic properties

This work reports a theoretical study of the excited state properties of a series of original conjugated metal-free organic dyes containing the pyrrolidine (PYR) moiety. These compounds have recently been developed for dye-sensitized solar cells (DSSCs). Our polarizable continuum model time-dependent density functional theory-based procedure made it possible to efficiently and accurately evaluate (i) the vertical excitation energies, (ii) the related redox potentials, and (iii) the free enthalpies of injection. It turns out that the Becke-Half-and-Half-Lee-Yang-Parr functional, combined to the 6-311+G(2d,2p) basis set, gives reliable auxochromic shifts when the bulk solvation effects are included in the model. Indeed, the theoretical procedure provides excitation energies with a mean absolute deviation limited to ∼0.10 eV only. In addition, we give insights into the geometrical and electronic structures of the dyes, and we unravel the structural modifications that optimize the properties of PYR-based DSSCs. This investigation aims at improving the electron injection process, as well as the light harvesting efficiency (LHE) of the dyes. For this purpose, we considered a set of 17 new dyes, and starting from the basic five-block [PYR]-[phenyl]-[ethylene]-[thiophene]-[cyano acrylic acid] system (PYR-m′ structure), several modifications leading to better electron injection and comparable LHE properties have been proposed.