Dye‐Sensitized Solar‐Cell Performance of a Cobalt(III/II) Redox Mediator with the 2,6‐Bis(8‐quinolinyl)pyridine Ligand

Abstract

Optical and electrochemical properties of the homoleptic cobalt(III/II) complexes mer,mer-[Co(tpy-κ3N,N′,N″)]n+ and mer,mer-[Co(bqp-κ3N,N′,N″)]n+ (n = 3+ or 2+) are presented [tpy = 2,2′:6′,2″-terpyridine; bqp = 2,6-bis(8′-quinolinyl)pyridine]. Both [Co(tpy)2]3+/2+ and [Co(bqp)2]3+/2+ systems have been used to formulate redox-mediator electrolytes for application in dye-sensitized solar-cell (DSSC) devices with the MK-2 dye TiO2 sensitizer. The [Co(tpy)2]3+/2+-based electrolyte shows a slightly higher percentage power-conversion efficiency (η = 4.47 %) in a DSSC device relative to the [Co(bqp)2]3+/2+-based electrolyte (η = 3.73 %). Although the [Co(bqp)2]3+/2+ electrolyte shows a slightly higher open-circuit potential (VOC = 0.79 V), it suffers from a reduced short-circuit current (JSC = 6.21 mA cm–2) relative to that of the [Co(tpy)2]3+/2+ system (JSC = 10.84 mA cm–2). Tafel and electrochemical impedance analysis (EIS) of symmetric FTO/Pt||Pt/FTO cells, as well as EIS and chronocoulometry of the functional DSSC device are presented. Collectively, this data points toward a decreased turnover of the CoIII/II redox couple at the Pt counter electrode for the [Co(bqp)2]3+/2+ system concomitant with an increased resistance to mass-transfer diffusion.