Investigation on the influence of co-sensitization on semi-transparent DSSCs fabricated using NIR-sensitive squaraine dyes and visible dyes.

Two squaraine dyes ("blue colored" SQ258 and "green colored" SQ259) active in the near-infrared spectral range were co-sensitized with popular commercial dyes (D35 and N719) and their photophysical characterizations were performed using stationary and time-resolved spectroscopic techniques. The dye mixtures, D35:SQ258 and N719:SQ259 demonstrated a better photovoltaic performance when compared with their individual photovoltaic efficiencies. The studied different DSSC configurations using squaraine dyes and mixtures in the devices with thin TiO layer show the potential application in multi-color semi-transparent cells. The D35:SQ258 system was also evaluated in the presence of cobalt-based redox shuttle, revealing improved performance compared to I/I. The stationary, transient absorption and external quantum efficiency spectra indicated the squaraine dye de-aggregating function of D35 and N719 dyes. The fast internal conversion of the excited squaraine dyes which competes with too slow electron injection was observed as the main reason for the limited charge separation efficiency in the studied cells of pure squaraine dyes. However, as a result of the synergetic effects of the co-sensitized dyes, the improvement of the charge separation efficiency was revealed. Finally, transient absorption studies revealed that upon exclusive excitation of squaraine dyes an ultrafast energy transfer between the squaraine monomers and their H-aggregates takes place, and a Stark shift of absorption band of co-sensitized dyes can be observed.