Optimization of performance and long-term stability of p-type dye-sensitized solar cells with a cycloruthenated dye through electrolyte solvent tuning

Abstract

p-Type dye sensitized solar cells (DSCs) have been assembled using the structurally unrefined, zwitterionic cyclometallated ruthenium dye [Ru(bpy)₂(H1)] (H₃1 = (4-(2-phenylpyridin-4-yl)phenyl)phosphonic acid) in combination with FTO/NiO photocathodes and an I₃⁻/I⁻/acetonitrile (AN) electrolyte. Values of the short-circuit current density (J_SC) = 4.06 mA cm⁻², open-circuit voltage (V_OC) = 95 mV and photoconversion efficiency (η) = 0.139% are the highest achieved for state-of-the-art cyclometallated ruthenium dyes in p-type DSCs; data are confirmed using duplicate devices. J_SC values are higher than those observed for the standard dye P1, and electrochemical impedance spectroscopy (EIS) shows that DSCs with [Ru(bpy)₂(H1)] exhibit a both a lower transport resistance (R_t) and recombination resistance (R_rec) than DSCs with P1. Changing the electrolyte solvent from AN to propionitrile (PN), valeronitrile (VN), 3-methoxypropionitrile (MPN) or N-methylpyrrolidone (NMP) confirms a dependence of J_SC on solvent in the order AN > PN > MPN > VN > NMP, whilst V_OC follows the trend VN > PN > MPN > AN > NMP. The opposing trends in J_SC and V_OC lead to only a small drop in the overall η values for PN versus AN. EIS measurements revealed that although PN has a higher resistance to recombination reactions than AN, it also exhibits an increased amount of trapped charge carriers, leading to worsened DSC performance. DSCs based on AN do not remain stable over a 1–2 month period; both J_SC and V_OC decrease significantly. However, DSCs with the less volatile and more viscous PN show enhanced performance upon ageing with a gain in J_SC over the first 33 days. Electrolytes with mixed solvents were investigated; addition of PN, VN or MPN to AN leads to lower J_SC and this is most pronounced for VN and least marked for PN. The optimal solvent is an AN : PN mixture with volume ratios of 3 : 1 or 1 : 1; this mixed solvent results in enhanced long-term stability with respect to DSCs with pure AN and this is at the expense of a only small decrease in photoconversion efficiency.