Spectroscopy of Donor−π−Acceptor Porphyrins for Dye-Sensitized Solar Cells

A recent improvement in the design of dyesensitized solar cells has been the combination of lightabsorbing, electron-donating, and electron-withdrawing groups within the same sensitizer molecule. This dye architecture has proven to increase the energy conversion efficiency of the cells, leading to record efficiency values. Here we investigate a zinc(II)-porphyrin-based dye with triphenylamine donor groups and carboxyl linkers for the attachment to an oxide acceptor. The unoccupied energy levels of these three moieties are probed selectively by element-sensitive X-ray absorption spectroscopy at the K-edges of nitrogen and carbon. These results are complemented by ultraviolet/visible spectroscopy to obtain the optical band gap and the occupied molecular levels. Density functional theory and time-dependent density functional theory are employed to obtain a detailed understanding of the X-ray and optical absorption spectra. The attachment of electron-donating groups to the porphyrin ring significantly delocalizes the highest occupied molecular orbital (HOMO) of the molecule. This leads to a spatial separation between the HOMO and the lowest unoccupied molecular orbital (LUMO), with the HOMO having significant weight in the amine donors, while the LUMO remains localized in the porphyrin ring and the acceptor group. Such spatial separation of the frontier orbitals reduces the recombination rate of photoinduced electrons and holes, thus enhancing the energy conversion efficiency.