Multichromophoric sensitizers based on squaraine for NiO based dye-sensitized solar cells

Three sensitizers were synthesized and utilized as panchromatic dyes for nanocrystalline NiO films: an iodo-squaraine (SQ), a squaraine-perylene monoimide (SQ-PMI) dyad, and a squaraine-perylene monoimide-naphthalene diimide (SQ-PMI-NDI) triad. Photophysical and photovoltaic studies showed that hole injection into the NiO valence band from the SQ excited state is ultrafast, but also that subsequent recombination is very rapid, preventing SQ from being an efficient sensitizer for photovoltaic purposes. The introduction of a second light harvesting unit (PMI) and a terminal electron acceptor (NDI) significantly improved the photovoltaic performances of the system. Irrespective of which light harvesting unit was photoexcited in NiO/SQ-PMI and NiO/SQ-PMI-NDI, intramolecular charge separation leading to SQ⁺ and PMI^- or NDI^- is the main excited state deactivation process. Intramolecular charge separation occurred in spite of the favorable conditions for energy transfer to the SQ unit. Subsequent hole injection from SQ ⁺ into NiO was in competition with intramolecular recombination, which may have significantly decreased the overall photovoltaic performances. The control of this side-reaction is therefore crucial for the successful design of multichromophoric systems for dye-sensitized solar cells (DSSCs). The quantum yield of NiO⁽⁺⁾/SQ-PMI-NDI^- after 50 ns is higher than that of NiO⁽⁺⁾/SQ-PMI^-, and much higher than that of NiO⁽⁺⁾/SQ^-; intramolecular recombination was slowed down by the localization of the electron further away from the SQ⁺ hole and consequently from NiO⁺. The three sensitizers were tested in NiO based DSSC devices using either the conventional triiodide/iodide electrolyte or a cobalt^III/II(4,4′-di-tert-butyl-2,2′-bipyridine) ₃ electrolyte. The photoconversion efficiencies steadily increased in the following order: SQ < SQ-PMI < PMI-NDI ≠SQ-PMI-NDI. The multichromophoric sensitizers had broader absorption spectra, more long-lived charge separation, and better photovoltaic performance than single unit chromophores. This indicates that bichromophoric systems, ones in which the antenna serves both as electron acceptor and photon harvester, are realistic sensitizers to boost photovoltaic performances. These findings are important for engineering new panchromatic and more efficient sensitizers for p-type DSSCs.