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

• Published in: European journal of inorganic chemistry Vol. 2015; no. 23; pp. 3843 - 3849
• Main Authors: Lee, Nicholas A., Frenzel, Blaise A., Rochford, Jonathan, Hightower, Sean E.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.08.2015; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Cobalt; Computing time; Data points; Devices; Dye-sensitized solar cells; Dyes; Electrochemical analysis; Electrodes; Electrolytes; Quinolines; Redox mediators; Solar energy; Titanium dioxide
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201500461

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. Electrochemical properties of the [Co(bqp)2]3+/2+ redox mediator is evaluated in a dye‐sensitized solar‐cell (DSSC) device with the MK‐2 dye photosensitizer. A comparable photovoltaic efficiency is observed for [Co(bqp)2]3+/2+ (η = 3.73 %) vs. the established [Co(tpy)2]3+/2+ mediator (η = 4.47 %).