Improvement of photovoltaic performance of DSSCs by modifying panchromatic zinc porphyrin dyes with heterocyclic units

Abstract

A series of novel panchromatic D–D–π–A porphyrin dyes have been synthesized and applied to dye-sensitized solar cells. Three porphyrin dyes named JP1, JP2 and JP3, and their photophysical and electrochemical properties and photovoltaic performance were investigated and compared with reference dye YD2-O-C8. 2-Hexylthiophene chromophores were introduced to the donor groups, which extended the π-conjugation system effectively, then broadened the range of spectral response and improved the charge separation between the donor and acceptor moieties in the excited state. Moreover, this paper used thiophene-2-carboxylic acid instead of the traditional benzoic acid as an anchor group, which can make the molecules arrange to tilted orientation when adsorbed on the TiO₂ surface, and this may effectively suppress the dye aggregation and prevent charge recombination. These dyes were clearly red-shifted when compared with dye YD2-O-C8. Especially for dye JP3, its maximum absorption peak was red shifted 20 nm with respect to dye YD2-O-C8 from 645 to 665 nm, and the molar extinction coefficient (6.2 × 10⁴ M⁻¹ cm⁻¹) of JP3 is double that of YD2-O-C8 (3.1 × 10⁴ M⁻¹ cm⁻¹) at the Q band. Dye JP3 extended the spectral response to 750 nm. The density functional theory (DFT) calculations indicated that the electronic density of the HOMO was increased by the additional thiophene units in these dyes when compared with YD2-O-C8, and this will improve the conjugation and electron donating ability. The power conversion efficiencies of JP1, JP2 and JP3 are 5.09%, 5.62% and 6.40% respectively under AM 1.5G irradiation, which are 74.5%, 82.3% and 93.7% of the YD2-O-C8 based-device (6.83%) under the same conditions.