Monodentate pyrazole as a replacement of labile NCS for Ru (II) photosensitizers: Minimum electron injection free energy for dye-sensitized solar cells

Abstract A mono-dentate ligand of 3-(trifluoromethyl) pyrazole, instead of the labile isothiocyanate ligand for Ru (II) photosensitizers, was developed. This molecular modulation resulted in a new structure motif ( HD-11 ) with 50 nm red shift in the low energy metal-to-ligand charge transfer (MLCT) absorption peak. However, electrochemical measurements revealed that both ground and excited state oxidation potentials were stabilized, we attributed this to the presence of strong electron withdrawing group CF 3 tethered to pyrazole, which resulted in more free energy for dye regeneration (−ΔG regenration ), but less driving force for electron injection (less −ΔG injection ) into the conduction band of TiO 2 . Hence HD-11 , resulted in lower J sc of 12.89 mA cm −2 compared to 17.07 mA cm −2 of N719 under similar conditions. Additionally, V oc of HD-11 was lower than N719 , owing to decreased eTiO 2 life time of 0.674 ms compared to 8.8 ms of N719 for injected electrons as found by impedance measurements.