Thiocyanate-Free Ruthenium(II) Sensitizers for Dye-Sensitized Solar Cells Based on the Cobalt Redox Couple

Two thiocyanate-free ruthenium(II) sensitizers, TFRS-41 and TFRS-42, with distinctive dialkoxyphenyl thienyl substituents were successfully prepared and tested for potential applications in making dye-sensitized solar cells (DSCs). Subsequent device fabrication was conducted by using a [Co(bpy)₃]^2+/3+-based (bpy=2,2′-bipyridine) electrolyte, for which the best performance data, namely, J_SC=13.11 mA cm⁻², V_OC=862 mV, fill factor=0.771, and η=8.71 %, were recorded for the sensitizer TFRS-42 with a 2,6-dialkoxyphenyl substituent under AM 1.5G irradiation. The markedly higher V_oc value was confirmed by the longer electron lifetime revealed in transient photovoltage (TPV) measurements versus the TFRS-1 sensitizer. In addition, DFT calculation and detailed first-principles computational analysis were conducted to provide a rationale for the observed trends in their photovoltaic performances and electron lifetimes, with reference to different performances exhibited by three thiocyanate-free sensitizers, TFRS-1, TFRS-41 and TFRS-42, versus Z907 reference. Through the proper control of peripheral substituents, the thiocyanate-free ruthenium(II)-based DSC sensitizers can positively influence the performances of DSCs, with better light-harvesting capability and suppressed charge recombination, for DSC cells fabricated by using a [Co(bpy)₃]^2+/3+-based electrolyte.