A Step Toward Efficient Panchromatic Multi-Chromophoric Sensitizers for Dye Sensitized Solar Cells

Panchromatic dyes with absorption profiles extending into the near-infrared are of interest to researchers in the field of dye sensitized solar cells (DSSCs), as they offer potential access to a wide energy range of photons necessary to enhance solar to electric power conversion efficiencies (PCEs). In this report, a porphyrin with a Soret band absorbing at high energy is combined with a squaraine absorbing at low energy via an acetylene linker to form a bichromophoric sensitizer with molar extinctions on the order of 10⁵ M^–1 cm^–1 and an incident photon-to-current efficiency (IPCE) onset of ∼850 nm. Various bulky substituents were installed on both the porphyrin and squaraine moieties, and conjugation was increased with π-bridge spacers to achieve a PCE of 7.6%, which is up to 15% higher than a comparable squaraine-only dye. For the most part, charge injection dynamics indicate slower charge injection rates and lower injection quantum yields for these bichromophoric sensitizers compared to nonporphyrin squaraine-based DSSC sensitizers. Nevertheless, higher PCE was observed for most porphyrin-containing dyes due largely to increased panchromaticity

Near-Infrared Asymmetrical Squaraine Sensitizers for Highly Efficient Dye Sensitized Solar Cells: The Effect of π‑Bridges and Anchoring Groups on Solar Cell Performance

Conventional squaraine dyes exhibit an intense absorption band in the red region of the solar spectrum and with appropriate design can also have high energy absorption as well, making them interesting building blocks toward achieving panchromatic dyes for dye sensitized solar cell (DSSC) applications. In this report, eight squaraine dyes with thiophene, 4-hexyl-4<i>H</i>-dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole, dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­thiophene, and 4,4-bis­(2-ethylhexyl)-4<i>H</i>-silolo­[3,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (DTS) π-bridges with cyanoacetic acid (CA) and cyanophosphonic acid (PA) acceptor/anchoring groups are synthesized to extend the squaraine absorption into the 450–550 nm region and to provide different spatial arrangements of solubilizing groups. Squaraines with CA anchoring groups have higher power conversion efficiencies compared to their PA analogs, with the highest being 8.9% for the DTS-based dye, which is among the highest reported in the literature for squaraine dyes. This is due to high short circuit currents (<i>J</i>_SC) and increased open circuit voltages (<i>V</i>_OC). Dyes with PA anchoring groups exhibited lower <i>J</i>_SC resulting from decreased charge injection efficiency, as determined by femtosecond transient absorption spectroscopy. This study suggests that out-of-plane bulky substituents may increase DSSC performance not only by increasing <i>J</i>_SC through decreased aggregation but also by increasing <i>V</i>_OC through decreased TiO₂/electrolyte recombination