Probing Recombination Mechanism and Realization of Marcus Normal Region Behavior in DSSCs Employing Cobalt Electrolytes and Triphenylamine Dyes

Abstract

Cobalt based, outer-sphere, one-electron redox shuttles represents an exciting class of alternative electrolyte to be used in dye-sensitized solar cells. The flexibility of redox potential tuning by varying the substituents on peripheral organic ligands renders them the advantage of achieving higher photovoltage. However, higher recombination experienced in these systems by employing diffusion-limited cobalt species serves as a bottleneck which significantly limits attaining higher performance. The focus of the present contribution is to systematically investigate in detail the effect of structural variations and steric hindrance of organic triphenylamine dyes (TPAA4 and TPAA5) which differs in the number and nature of binding groups and peripheral hole accepting units on the recombination reactions and mass transport variations employing two different cobalt electrolytes, [Co3]3+/2+ and [Co(phen)3]3+/2+, having variable driving force for recombination. The detailed photovoltaic analysis provides us the information that modification of the architecture of organic dyes plays a decisive role in determining the performance, in particular, employing alternate one-electron outer-sphere redox systems. From our analysis, for both the dyes the charge recombination with the oxidized cobalt species was found to happen in the Marcus normal region which is attributed to the shift in conduction band (CB) that influenced the driving force for recombination. The current observation was quite exciting since the redox systems employed in the present study were previously documented to exhibit Marcus inverted recombination behavior. The impact of structural variations of dyes, change in conduction band, effect of nature of electrolyte species, and its interaction with the semiconductor on the recombination reactions was explored in detail using a range of small and large perturbation techniques.