Dye-sensitized solar cell based on an inclusion complex of a cyclic porphyrin dimer bearing four 4-pyridyl groups and fullerene C60

Abstract

Cyclic free-base porphyrin dimers (H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO)) linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes (C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO)) with fullerene C₆₀ have been applied to dye-sensitized solar cells (DSSCs) as a new class of porphyrin dye sensitizers with pyridyl anchoring groups for attachment on a TiO₂ electrode. The FTIR spectra of the porphyrin dimers adsorbed on TiO₂ nanoparticles demonstrated that these porphyrin dimers are adsorbed on the TiO₂ surface through the formation of hydrogen bonding of pyridyl groups and/or pyridinium ions at Brønsted acid sites on the TiO₂ surface. The adsorption amount of the porphyrin dimers adsorbed on the TiO₂ electrode is 2.0 × 10¹⁷ molecules per cm², that is, the adsorption amount of the porphyrin unit is 4.0 × 10¹⁷ cm⁻², which is higher than that of dye sensitizers with pyridyl groups reported so far. The photovoltaic performance of DSSCs based on phenothiazine-bridged cyclic porphyrin dimer H₄-Ptz-CPD_Py(TEO) is higher than that of DSSCs based on butadiyne-linked cyclic porphyrin dimer H₄-C₄-CPD_Py(TEO). Moreover, the photovoltaic performances of DSSCs based on cyclic free-base porphyrin dimers are higher than those of DSSCs based on their C₆₀ inclusion complexes C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO). On the basis of the electrochemical measurements (voltammetry and electrochemical impedance spectroscopy) and the transient absorption spectroscopy, the differences in the photovoltaic performances among these cyclic free-base porphyrin dimers are discussed from kinetic and thermodynamic considerations concerning the electron transfer processes in DSSCs.