1. Dye-sensitized Solar Cells (DSSC)

2. Photo-triggers

3. Opto electronic materials

4. Aggregation of Dyes

Highlights: Squaraine based Dyes for Dye Sensitized Solar Cells

Among the phthalocyanine, porphyrin and squaraine chromophores that absorb in the red and NIR regions of the solar spectrum, phthalocyanine and squaraine dyes possess the transitions with high molar extinction coefficient in the far-red, and NIR regions (10⁵ M^-1 cm^-1). Porphyrin and phthalocyanine sensitizers have reached the device efficiency of 13% and 6.4%, respectively by tuning the steric and electronic effect. Squaraines, as a metal free dye which can convert far-red photons to electrons in DSSC, have been an attractive component in this field.  Although squaraine dye design and synthesis are exploited in dye sensitized solar cell research, understanding and controlling the squaraine aggregate formation on TiO₂ surface that helps in achieving high device efficiency, approach towards panchromatic light harvesting with a single dye have remained unexplored.

In the context of photogeneration, the unique aggregation property of squaraine dyes is explored for xerography, organic solar cells, and organic light emitting diodes, field effect transistors in which self-assembly of active layer components is required for an efficient charge transport properties. However in the case of DSSC, aggregation of dyes on TiO₂ surface, provides varied device efficiencies.  On the other hand, aggregation of dyes is good in the sense that it broadens the spectrum as H-aggregation and J-aggregation results in blue and red shifted absorption spectra, respectively. As in the DSSC literature, charge injection from H- and J- aggregated structure has a mixed response; the questions remains to be answered are (i) how to control the aggregation of dyes on TiO₂ surface? and (ii) whether photons can be efficiently harvested from the aggregated state?

 The present dye design is based on controlling the aggregation of dyes on the TiO₂ surface by varying the position of sp³-alkyl group in squaraine dyes. We have observed that the H-aggregate from a squaraine dye SQ5 can also actively inject the charges with the device efficiency of 9.0%, which is highest among the squaraine dyes. By having out-of-plane branching on the sp³-C atom that helps controlling the aggregation of dyes and allowing a better charge injection, which in turn provide the device efficiency. Owing to the aggregation property of red/NIR active squaraine dyes, the present system with the advantage of H-aggregate, and it can respond like panchromatic dyes.

 Thus, understanding the squaraine dye assembly in the TiO₂ interface shed the light on,

(a)           The importance of dye-aggregate in charge injection at the aggregate-TiO₂ interface.

(b)        Design and synthesis of highly efficient panchromatic dyes by taking the advantage of steric, electronic and aggregation effects.