Panchromatic sensitizers with absorption profiles extending to the near infrared region of the solar spectrum are of interest to researchers in the field of dye sensitized solar cells (DSSCs) as they make it possible to harvest photons across a wide energy range, which may be an effective strategy to enhance solar cells’ power conversion efficiencies (PCEs). Belonging to the family of polymethine dyes, squaraines are characterized by an intense absorption in the red region of the solar spectrum, which with appropriate design can also have high energy absorption, making them interesting building blocks towards achieving the panchromatic absorption for solar cell applications.
In the first part, we have systematically modified the structure of asymmetrical squaraine sensitizers with a “donor-π-acceptor arrangement” by introducing π-bridges that are known to absorb high energy photons and that are equipped with alkyl chains with different chain lengths and spatial orientations. In addition, different anchoring groups were explored in an attempt to further understand their effect on overall dye performance. In the second part, we covalently linked a porphyrin to the squaraine to create a dual chromophoric sensitizer with a “chromophore-chromophore-π-acceptor” arrangement. This panchromatic material had an IPCE onset at around 850 nm.
PCEs up to 9.6% were achieved with squaraine dyes that utilized a ditheinosilole π-bridge with 2-ethylhexyl out of plane chains on its silicon atom and a cyanoacetic acid anchoring group. These efficiencies, which are among the highest reported for near infrared sensitizers are a result of high short circuit currents (JSC) due to reduced intermolecular aggregation, and enhanced open circuit voltages (VOC) due to suppressed electron recombination. Dyes with phosphonic acid anchoring groups demonstrated lower JSC and thus PCEs as an outcome of less efficient charge injection, as determined by femtosecond transient absorption measurements. On the other hand, the porhyrin-squaraine dual chromophoric sensitizers exhibited PCEs up to 7.6% with JSC values that did not match the absorption profile of these dyes, which is hypothesized to be due to the high affinity of these extended aromatic systems to aggregate and lower dye regeneration rates compared to squaraines. Future design strategies for squaraine based DSSCs will be discussed.