Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Porphyrins have been demonstrated as one of the most efficient sensitizers in dye-sensitized solar cells (DSSC). Herein, we investigated a series of porphyrin sensitizers functionalized with various π-spacers, such as phenyl for LD14, thiophene for LW4, thiophene-phenyl for LW5, and 2,1,3-benzothiadiazole (BTD)-phenyl for LW24. Photo-physical investigation by means of time-resolved fluorescence and nanosecond transient absorption spectroscopy revealed an accelerated inner charge transfer in porphyrins containing the BTD-phenyl π-spacer. Implementation of an auxiliary electron-deficient BTD unit to the porphyrin spacer also results in a broad light-harvesting ability extending up to 840 nm, contributing to an enhanced charge transfer character from the porphyrin ring to the anchoring group. When utilized as a sensitizer in DSSCs, the LW24 device achieved a power conversion efficiency of 9.2%, higher than those based on LD14 or LW5 porphyrins (PCE 9.0% or 8.2%, respectively) but lower than that of the LW4 device (PCE 9.5%). Measurements of transient photovoltage decays demonstrate that the LW24 device features the up-shifted potential band edge of the conduction band of TiO₂, but involves serious charge recombination in the dye/TiO₂ interface. The findings provide insights into the molecular structure and the charge-transfer characteristics for designing efficient porphyrin sensitizers for DSSC applications.