Engineering of thiocyanate-free Ru(II) sensitizers for high efficiency dye-sensitized solar cells

We synthesized a series of Ru(II) metal complexes TFRS-1, -2, -4, -21, -22 and -24 with a single 4,4′-dicarboxylic acid-2,2′-bipyridine together with two functionalized pyridyl azolate ancillary ligands consisting of pyrazolate or triazolate groups. Both photophysical measurements and DFT/TDDFT calculations were conducted to gain insight into their electronic and optical properties. The triazolate series of sensitizers TFRS-21, -22 and -24 showed an enlarged optical band gap with respect to their pyrazolate counterparts TFRS-1, -2 and -4. When employed in dye sensitized solar cells (DSCs), the triazolate sensitizers show slightly inferior J_SC values due to the poor incident photon-to-current conversion efficiencies recorded compared to the pyrazolate series. Moreover, the endowed 5-(hexylthio)thiophen-2-yl substituents exert a notable hyperchromic effect and bathochromic shift in the absorption spectra, which then improves the short circuit current J_SC to 18.7 and 15.5 mA cm⁻² and the overall conversion efficiency to η = 10.2% and 8.25% for TFRS-4 and TFRS-24, respectively. For the evaluation of V_OC, transient photocurrent and photovoltage decay measurements were carried out to compare the rates of interfacial recombination of electrons from the TiO₂ conduction band to electrolyte.