Insertion of a naphthalenediimide unit in a metal-free donor–acceptor organic sensitizer for efficiency enhancement of a dye-sensitized solar cell

• Insertion of a highly conjugated functionality, naphthalenediimide, in the D–π–A framework.
• Extension of central π-system for improved light-harvesting.
• Improved DSSC performance with the incorporation of naphthalenediimide unit.
• New dye with the embodiment of naphthalenediimide provided longer electron lifetime of DSSCs.

Through the use of a planar conjugated naphthalenediimide (NDI) functionality, a novel, metal-free organic chromophore was designed, synthesized and characterized for application in dye-sensitized solar cells (DSSCs). The new sensitizer, 2-cyano-3-(5-(9-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn] [3,8]phenanthrolin-4-yl)thiophen-2-yl)acrylic acid (coded as A1), was based on a donor–π-bridge–acceptor (D–π–A) module where a simple triphenylamine functionality served as an electron donor, a cyanoacrylic acid as an electron acceptor and anchoring group, and a NDI moiety as the π-bridge embedded between the two thiophene units. Because of the extensively conjugated NDI unit, the new dye A1 exhibited high extinction coefficient, tuned energy levels and improved DSSC performance when compared with DSSCs fabricated using the conventional sensitizer, 2-cyano-3-(5’-(4-(diphenylamino)phenyl)-[2,2’-bithiophen]-5-yl)acrylic acid (R1). Furthermore, DSSCs that were constructed using A1 showed an increase in electron lifetime when compared with DSSCs fabricated using R1. A1 demonstrated its suitability with a variety of electrolyte systems and DSSC power conversion efficiencies of 6.24%, 6.05% and 5.17% were recorded using cobalt-, solvent- and ionic liquid-based redox shuttles. To our knowledge, A1 is the first reported example in the literature where NDI unit has been embedded between thiophene functionalities to extend the π-conjugation in a given D–A system for DSSC applications.

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