There is a future for N-heterocyclic carbene iron(II) dyes in dye-sensitized solar cells: improving performance through changes in the electrolyte

By systematic tuning of the components of the electrolyte, the performances of dye-sensitized solar cells (DSCs) with an N -heterocyclic carbene iron(II) dye have been significantly improved. The beneficial e ects of an increased Li+ ion concentration in the electrolyte lead to photoconversion e ciencies (PCEs) up to 0.66% for fully masked cells (representing 11.8% relative to 100% set for N719) and an external quantum e ciency maximum (EQEmax ) up to approximately 25% due to an increased short-circuit current density (JSC ). A study of the effects of varying the length of the alkyl chain in 1-alkyl-3-methylimidazolium iodide ionic liquids (ILs) shows that a longer chain results in an increase in JSC with an overall e ciency up to 0.61% (10.9% relative to N719 set at 100%) on going from n -methyl to n -butyl chain, although an n -hexyl chain leads to no further gain in PCE. The results of electrochemical impedance spectroscopy (EIS) support the trends in JSC and open-circuit voltage (VOC ) parameters. A change in the counterion from I- to [BF4 ]- for 1-propyl-3-methylimidazolium iodide ionic liquid leads to DSCs with a remarkably high JSC value for an N -heterocyclic carbene iron(II) dye of 4.90 mA cm-2 , but a low VOC of 244 mV. Our investigations have shown that an increased concentration of Li+ in combination with an optimized alkyl chain length in the 1-alkyl-3-methylimidazolium iodide IL in the electrolyte leads to iron(II)-sensitized DSC performances comparable with those of containing some copper(I)-based dyes