Dye-sensitized solar cells (DSCs) stand actually as a promising alternative to conventional silicon photovoltaic devices owing to lower production cost, low environmental footprint and unique properties to convert efficiently low light intensities and diffuse day light.⁽¹⁾ Today more than 14 % power conversion efficiency (PCE) has been achieved by optimizing a cocktail of organic dye in conjunction with a Co^+III/+IItris(1,10-phenanthroline) redox couple.⁽²⁾ So far, the current chemistry developed for DSC still present a lack in absorbing and converting substantially low-lying energy photons greater than λ >750 nm. With the aim to continue improving light harvesting capability of DSC for approaching closer to silicon standards, two important breakthroughs are needed: (i) reducing the internal energy loss arising from energy mismatch for electron injection and for dye regeneration (ii) extending light harvesting towards the NIR by the development of a new class of efficient sensitizers energetically well-coupled with the electronic properties of anatase TiO₂ semi-conductor and electrolyte redox couple.

We have developed a series of new efficient organic sensitizers based on squaraine core-units, croconines or cyanines which are able to shift towards the red the light absorption from ca. 520 nm from the current ruthenium polypyridil-based sensitizers to as high as 830 nm. These new sensitizers not only show NIR absorption but also exhibit significantly higher molar extinction coefficients (100 000 - 320 000 mol^-1cm^-1) while being cost competitive.

This communication will describe these new efficient sensitizers converting up to 20% IPCE until 950 nm. Their light-to-electricity performances have been optimized by using highly diluted dye solution to promote aggregate-free self-assembled monolayer and carefully chosen electrolyte composition. Finally, the relationship between dye structure, electrolyte composition and the kinetic of charge transfers will be discussed on the basis of complementary set of techniques such as electrochemical impedance spectroscopy, time-correlated single photon-counting and ultra-fast pump-probe transient absorption spectroscopy.

1.            B. O'Regan and M. Gratzel, Nature, 1991, 353, 737-740.

2.            K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J.-i. Fujisawa and M. Hanaya, Chem. Commun., 2015, 51, 15894-15897.