Doubly β-Functionalized Meso−Meso Directly Linked Porphyrin Dimer Sensitizers for Photovoltaics

Novel meso−meso directly linked porphyrin dimers tethered at β-pyrrolic positions of the porphyrin ring with 2-propenoic acids or 2,4-pentadienoic acids were prepared for sensitization of nanocrystalline titanium dioxide solar cells. Most importantly, the absorption spectra of dimeric porphyrins were notably extended into the midvisible region in the solar spectrum while the LUMO levels are sufficiently high to inject electrons to the conduction band of TiO₂. Among these dimers, PEG-2b-bd-Zn2 dimer, bearing a poly(ethylene glycol) end group at one meso position, showed the highest incident photon-to-photocurrent generation with 47% efficiency at the Soret region as well as the power conversion efficiency of 4.2% under standard AM 1.5 solar condition. To elucidate electronic structures and excited-state properties, UV−vis absorption and emission, cyclic voltammetry measurements, and density functional theory calculations were performed. To our best knowledge, the obtained conversion efficiency of 4.2% from PEG-2b-bd-Zn2-sensitized DSSC is the highest photovoltaic performance among DSSCs based on oligomeric porphyrins. Thus, this work suggests that the wide spectral response of directly linked porphyrin dimers due to the excitonic coupling is an intriguing aspect of directly linked pophyrin dimers to be utilized for DSSCs and gives an insight into the design of more efficient light-harvesting molecules.