A Thiophene-Based Anchoring Ligand and Its Heteroleptic Ru(II)-Complex for Efficient Thin-Film Dye-Sensitized Solar Cells

A novel heteroleptic Ru-II complex (BTC-2) employing 5,5'-(2,2'-bipyridine-4,4'-diyl)-bis(thiophene-2-carboxylic acid) (BTC) as the anchoring group and 4,4'-dinonyl-2,2'-bipiridyl and two thiocyanates as ligands is prepared. The photovoltaic performance and device stability achieved with this sensitizer are compared to those of the Z-907 dye, which lacks the thiophene moieties. For thin mesoporous TiO2 films, the devices with BTC-2 achieve higher power conversion efficiencies than those of Z-907 but with a double-layer thicker film the device performance is similar. Using a volatile electrolyte and a double layer 7 + 5 mu m mesoporous TiO2 film, BTC-2 achieves a solar-to-electricity conversion efficiency of 9.1% under standard global AM 1.5 sunlight. Using this sensitizer in combination with a low volatile electrolyte, a photovoltaic efficiency of 8.3% is obtained under standard global AM 1.5 sunlight. These devices show excellent stability when subjected to light soaking at 60 degrees C for 1000 h. Electrochemical impedance spectroscopy and transient photovoltage decay measurements are performed to help understand the changes in the photovoltaic parameters during the aging process. In solid state dye-sensitized solar cells (DSSCs) using an organic hole-transporting material (spiro-MeOTAD, 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene), the BTC-2 sensitizer exhibits an overall power conversion efficiency of 3.6% under AM 1.5 solar (100 mW cm(-2)) irradiation.