New photosensitizers containing the dipyridoquinoxaline moiety and their use in dye-sensitized solar cells

• 6,7-Dicarboxylicdipyrido quinoxaline, dpq(COOH)2, as an anchoring ligand was presented.
• Better photovoltaic performance of two dpq(COOH)2 units than one dpq(COOH)2 unit.
• The key role of HOMO–LUMO alignment of dye with conduction band of semiconductor.

New Ru(II) polypyridyl complexes containing two N, N bidentate ligands, [Ru(phendione)2dpq(COOH)2](BF4)2 (L101), {Ru(phendione)[dpq(COOH)2]2}(BF4)2 (L102) and [Ru(phendione)dpq(COOH)2 (SCN)2](L103), (phendione = 5,6 dione-1,10-phenanthroline and dpq(COOH)2 = 6,7-dicarboxylicdipyrido[2,2-d:2′,3′f]quinoxaline) have been synthesized and characterized, and attached to a TiO2 substrate to be tested as solar cell sensitizers. We found that the solar to electricity conversion efficiency of cell is strongly affected by the type of ancillary ligand, the efficiency of L102 (with one phendione moiety) adsorbed on TiO2 nanocrystalline films being 2.5 times as large as that of L101 (with two phendione moieties) adsorbed on the same films. The effect of ancillary ligand on the photovoltaic performance of the complexes was compared to results of computational methods by ab initio DFT molecular dynamics simulations and quantum dynamics calculations of electronic relaxation to investigate the interfacial electron transfer in L101–103/TiO2-anatase nanostructures. It is found that the primary process in the interfacial electron-transfer dynamics involves an ultrafast (τ1 = 6 fs) electron-injection. The concepts of attribute ancillary ligand substitution and HOMO–LUMO alignment of dye with conduction band of semiconductor and electrolyte redox state are shown very helpful for tuning the photovoltaic properties and the rational architecture of polypyridyl photosensitizer with anticipated good properties.

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