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

• Replacement of labile NCS with 3-(trifluoromethyl) pyrazole resulted in red shift.
• HD-11 exhibited more stabilized ground and excited state potentials compared to N719.
• Impedance measurements showed decreased eTiO2 life time of HD-11 compared to N719.

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 CF3 tethered to pyrazole, which resulted in more free energy for dye regeneration (−ΔGregenration), but less driving force for electron injection (less −ΔGinjection) into the conduction band of TiO2. Hence HD-11, resulted in lower Jsc of 12.89 mA cm−2 compared to 17.07 mA cm−2 of N719 under similar conditions. Additionally, Voc of HD-11 was lower than N719, owing to decreased eTiO2 life time of 0.674 ms compared to 8.8 ms of N719 for injected electrons as found by impedance measurements.

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