Effects of porphyrinic meso-substituents on the photovoltaic performance of dye-sensitized solar cells: Number and position of p-carboxyphenyl and thienyl groups on zinc porphyrins

In order to understand the effects of meso-substituents of the zinc porphyrins on optical, electrochemical, and photovoltaic properties, a series of porphyrins with different combinations of thienyl (S) and p-carboxyphenyl (A) groups as the meso substituents have been systematically synthesized and studied. The properties of zinc complexes 3S1A, trans-2S2A, cis-2S2A, and 1S3A were fully investigated by absorption and emission spectra, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra, density functional theory (DFT) calculations, electrochemical, photophysical, and photovoltaic measurements. With the increasing number of meso-thienyl groups, slight red-shifts of Soret and Q bands were observed in both absorption and emission spectra. All of the absorption spectra of zinc porphyrins on TiO ₂ film show broadening and splitting of Soret bands because of excitonic coupling of porphyrins. ATR-FTIR spectra revealed likely modes to determine either a single arm (in 3S1A and trans-2S2A) or double arms (in 1S3A and cis-2S2A) attached on TiO ₂. Two factors of p-carboxyphenyl and thienyl groups affecting the devices performance-heavy atom effect and the amount of dye loading on TiO ₂-are concluded. Overall, the power conversion efficiencies (η) of the devices exhibit the following order: 1S3A (3.0%) > cis-2S2A (2.5%) > trans-2S2A (1.8%) ≫ 3S1A (0.2%).