Cyclometalated ruthenium complexes with 6-(ortho-methoxyphenyl)-2,2′-bipyridine as panchromatic dyes for dye-sensitized solar cells

Abstract

A cyclometalated ruthenium complex with 6-(ortho-methoxyphenyl)-2,2'-bipyridine (MeO-L) and tricarboxyterpyridine (tpy(CO2H)3) (RuCC·PF6) was prepared, characterized, and evaluated as a dye for dye-sensitized solar cells (DSSCs). The first oxidation and reduction potentials are +0.99 V and −0.81 V vs NHE, respectively. This complex exhibits panchromatic absorption spectrum extending up to 800 nm. The transitions upon visible light excitation were mixtures of metal-to-ligand (Ru → tpy(CO2H)3) and interligand (MeO-L → tpy(CO2H)3) charge transfer transitions. Adsorption of the ruthenium complex on TiO2was carried out from solutions of RuCC·PF6in the free acid form or solutions of RuCC·(NBu4)2in a deprotonated carboxylate form. The DSSC devices employing these complexes as the dye sensitizers generate electricity in response to light over the whole visible range and into the near infrared region over 900 nm. Remarkably, RuCC·(NBu4)2recorded higher short-circuit current density than a benchmark N719, while the open-circuit voltage was lower, resulting in a modest overall photon-to-current conversion efficiency of 6.4%. The transient absorption and electrochemical impedance spectroscopy have been conducted to get insight into the mechanistic details of the DSSC cells, which suggested that the presence of NBu4cation was beneficial in retarding the unwanted recombination reaction. Considering the efficient charge injection and regeneration as manifested by the large short-circuit current, modification based on the present structure would be a promising strategy toward higher efficiency dyes for DSSCs.