Amphiphilic Ruthenium Sensitizers and Their Applications in Dye-Sensitized Solar Cells

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Abstract
Amphiphilic ligands 4,4‘-bis(1-adamantyl-aminocarbonyl)-2,2‘-bipyridine (L 1), 4,4‘-bis{5-{N-[2-(3β-cholest-5-en-3-ylcarbamate-N-yl)ethyl]aminocarbonyl}}-2,2‘-bipyridine (L 2), 4,4‘-bis{5-{N-[2-(3β-cholest-5-en-3-ylcarbamate-N-yl)propyl]aminocarbonyl}}-2,2‘-bipyridine (L 3), and 4,4‘-bis(dodecan-12-ol)-2,2‘-bipyridine (L 4) and their heteroleptic ruthenium(II) complexes of the type [Ru(II)LL1(NCS)2] (5), [Ru(II)LL2(NCS)2] (6), [Ru(II)LL3(NCS)2] (7), and [Ru(II)LL4(NCS)2] (8) (where L = 4,4‘-bis(carboxylic acid)-2,2‘-bipyridine) have been synthesized starting from dichloro(p-cymene)ruthenium(II) dimer. All the ligands and the complexes were characterized by analytical, spectroscopic, and electrochemical techniques. The performance of these complexes as charge-transfer photosensitizers in nanocrystalline TiO2-based solar cells was studied. When complexes 5−8 anchored onto a 12 + 4 μm thick nanocrystalline TiO2 films, very efficient sensitization was achieved (85 ± 5% incident photon-to-current efficiencies in the visible region, using an electrolyte consisting of 0.6 M butylmethylimidazolium iodide, 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butyl pyridine in 1:1 acetonitrile + valeronitrile). Under standard AM 1.5 sunlight, the complex 8 yielded a short-circuit photocurrent density of 17 ± 0.5 mA/cm2, the open-circuit voltage was 720 ± 50 mV, and the fill factor was 0.72 ± 0.05, corresponding to an overall conversion efficiency of 8.8 ± 0.5%.

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