Abstract
Dye-Sensitized Solar Cells (DSSCs) are a highly promising alternative to conventional photovoltaic silicon-based devices, due to the potential low cost and the interesting conversion efficiencies. A key-role is played by the dye, and porphyrin sensitizers have drawn great interest because of their excellent light harvesting properties mimicking photosynthesis. Indeed, porphyrins are characterized by strong electronic absorption bands in the visible region up to the near infrared and by long-lived π* singlet excited states. Moreover, the presence of four meso and eight β-pyrrolic positions allows a fine tuning of their photoelectrochemical properties through structural modification. Trans-A2BC push–pull ZnII porphyrins, characterized by a strong and directional electron excitation process along the push–pull system, have been extensively investigated. On the other hand, A4 β-pyrrolic substituted tetraaryl ZnII porphyrins, which incorporate a tetraaryl porphyrinic core as a starting material, have received lower attention, even if they are synthetically more attractive and show several advantages such as a more sterically hindered architecture and enhanced solubility in most common organic solvents. The present contribution intends to review the most prominent A4 β-substituted ZnII porphyrins reported in the literature so far for application in DSSCs, focusing on the strategies employed to enhance the light harvesting capability of the dye and on a comparison with meso-substituted analogs.
Highlights
INTRODUCTIONSince the first appearance of the revolutionary work by Grätzel in 1991 (O’Regan and Grätzel, 1991), Dye Sensitized Solar Cells (DSSCs) emerged as a cutting-edge technology in the field of photovoltaics (PV)
Since the first appearance of the revolutionary work by Grätzel in 1991 (O’Regan and Grätzel, 1991), Dye Sensitized Solar Cells (DSSCs) emerged as a cutting-edge technology in the field of photovoltaics (PV).DSSCs belong to the third generation PV technology and they consist of a nanocrystalline and mesoporous high band-gap semiconductor, an organometallic or organic dye covalently linked to its surface and an electrolyte comprising a redox shuttle
Particular attention is devoted to porphyrin dyes showing A4 β-pyrrolic substitution pattern which have been less extensively studied than the trans-A2BC push-pull counterparts despite their promising results in TiO2 sensitization
Summary
Since the first appearance of the revolutionary work by Grätzel in 1991 (O’Regan and Grätzel, 1991), Dye Sensitized Solar Cells (DSSCs) emerged as a cutting-edge technology in the field of photovoltaics (PV). The most promising approach to enhance the solar light harvesting by DSSCs is the improvement of the lightcapture ability of the dye over the entire visible and near-infrared (NIR) spectrum, by a rational tailoring of the structure, the electronic absorption spectrum, the molar extinction coefficient and the loading onto the semiconductor surface. Porphyrins satisfy the principal requirements for efficient solar energy collection They possess strong electronic absorption bands (the B or Soret band around 400– 450 nm and the Q bands in the range 550–650 nm, with molar extinction coefficients typically over 100,000 M−1 cm−1), and long-lived π∗ excited states of appropriate energy to allow the injection of the photogenerated electron in the CB band of TiO2. The reader is addressed to several comprehensive reviews (Hagfeldt et al, 2010; Clifford et al, 2011; Li and Diau, 2013; Gong et al, 2017; Song et al, 2018) for the description of the approaches that has been exploited for the optimization of the other key variables affecting the performances of a DSSC (electrolyte redox couple, dynamics of charge transfer processes, absorption of the dye on the TiO2 surface)
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