The Role of π Bridges in High-Efficiency DSCs Based on Unsymmetrical Squaraines

• Published in: Chemistry : a European journal 2013-01, Vol.19 (5), p.1819-1827
• Main Authors: Delcamp, Jared H., Shi, Yanrong, Yum, Jun-Ho, Sajoto, Tissa, Dell'Orto, Elisa, Barlow, Stephen, Nazeeruddin, Mohammad K., Marder, Seth R., Grätzel, Michael
• Format: Article
• Language: English
• Subjects: Absorptivity; Anchors; chromophores; Dyes; dyes/pigments; Energy conversion efficiency; Molecular orbitals; Photovoltaic cells; sensitizers; Solar cells; Spacers; squaraines
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201202677

A series of squaraine‐based sensitizers with various π bridges and anchors were prepared and examined in dye‐sensitized solar cells. The carboxylic anchor group was attached onto a squaraine dye through π bridges with and without an ethynyl spacer. DFT studies indicate that the LUMO is delocalized throughout the dyes, whilst the HOMO resides on the squaraine core. The dye that incorporates a 4,4‐di‐n‐hexyl‐cyclopentadithiophene group that is directly attached onto the π bridge, JD10, exhibits the highest power conversion efficiency in a DSC; this result is attributed, in part, to the deaggregative properties that are associated with the gem‐di‐n‐hexyl substituents, which extend above and below the π‐conjugated dye plane. Dye JD10 demonstrates a power‐conversion efficiency of 7.3 % for liquid‐electrolyte dye‐sensitized solar cells and 7.9 % for cells that are co‐sensitized by another metal‐free dye, D35, which substantially exceed the performance of any previously tested squaraine sensitizer. A panchromatic incident‐photon‐to‐current‐conversion efficiency curve is realized for this dye with an excellent short‐circuit current of 18.0 mA cm−2. This current is higher than that seen for other squaraine dyes, partially owing to a high molar absorptivity of >5 000 M−1 cm−1 from 400 nm to the long‐wavelength onset of 724 nm for dye JD10. Bridge to success: The squaraine sensitizer (see figure) demonstrates a power conversion efficiency of 7.3 % for liquid‐electrolyte dye‐sensitized solar cells and 7.9 % for co‐sensitized cells with another metal‐free dye. A near‐ideal IPCE curve shape is realized for this dye, with an excellent short circuit current of 18.0 mA cm−2. This strong current is, in part, due to a high molar absorptivity of >5 000 M−1 cm−1 from 400 to 724 nm.