DFT-INDO/S modeling of new high molar extinction coefficient charge-transfer sensitizers for solar cell applications

A new Ru(II) complex, Bu4N [ruthenium (4-carboxylic acid-4'-carboxylate-2,2'-bipyridine)(4,4'-di(2-(3,6-dimethoxyphenyl)ethenyl)-2,2'-bipyridine)(NCS)2] (N945H), was synthesized and characterized by anal., spectroscopic, and electrochem. techniques. The absorption spectrum of the N945H sensitizer is dominated by metal-to-ligand charge-transfer (MLCT) transitions in the visible region, with the lowest allowed MLCT bands appearing at 25,380 and 18,180 cm-1. The molar absorptivities of these bands are 34,500 and 18,900 M-1 cm-1, resp., and are significantly higher when compared to than those of the std. sensitizer cis-dithiocyanatobis(4,4'-dicarboxylic acid-2,2'-bipyridine)ruthenium(II). An INDO/S and DFT study of the electronic and optical properties of N945H and of N945 adsorbed on TiO2 was performed. The calcns. point out that the top 3 frontier-filled orbitals have a Ru 4d (t2g in the octahedral group) character with a contribution coming from the NCS ligand orbitals. The calcns. also reveal that in the TiO2-bound N945 sensitizer, excitation directs charge into the carboxylbipyridine ligand bound to the TiO2 surface. The photovoltaic data of the N945 sensitizer using an electrolyte contg. 0.60M butylmethylimidazolium iodide, 0.03M I2, 0.10M guanidinium thiocyanate, and 0.50M tert-butylpyridine in a mixt. of MeCN and valeronitrile (vol. ratio = 85:15) had a short-circuit photocurrent d. of 16.50 ± 0.2 mA/cm2, an open-circuit voltage of 790 ± 30 mV, and a fill factor of 0.72 ± 0.03. This corresponds to an overall conversion efficiency of 9.6% under std. AM 1.5 sunlight and stable performance under light and heat soaking at 80° was confirmed.