Role of alkyl groups regulating recombination and mass transport at cobalt electrolyte-dye interface in dye sensitized solar cells

Abstract

Cobalt-based one electron outer-sphere redox mediators are considered as promising alternatives to conventional I3−/I− redox electrolyte in dye sensitized solar cells (DSSC). So far, [Co(bpy)3]3+/2+ is one among the most extensively studied alternate cobalt electrolyte used in DSSC. The performance of these alternate cobalt redox shuttles varies extensively with respect to the organic sensitizers. Hence, cobalt redox mediators' selectivity as a function of structural variations in dyes needs to be studied to understand the intermolecular interactions and their effect on various charge transfer processes. In the present work, we systematically investigated the structure-property correlations of two cobalt redox mediators, [Co(bpy)3]3+/2+ and [Co(tbpy)3]3+/2+ with variable alkyl functionality (with and without t-butyl substituent) at the periphery with two carbazole based D-π-A sensitizers, CCN and TCCN having similar peripheral substituents and its role in regulating recombination and mass transport. Photovoltaic measurements reveal that even with mass transport limitation, bulky t-butyl groups effectively suppressed recombination between TiO2 and redox mediator, leading to a longer lifetime and higher open-circuit potential. Our results provide insight into the need for custom dye design, which extends the possibility of using non-conventional cobalt electrolytes like [Co(tbpy)3]3+/2+ offering better charge transfer properties.