Co^II(dbbip)₂²⁺Complex Rivals Tri-iodide/Iodide Redox Mediator in Dye-Sensitized Photovoltaic Cells

Abstract

Transient absorption spectroscopy was employed to study charge-transfer dynamics in dye-sensitized nanocrystalline solar cells (DSSC) containing a new one-electron redox mediator, cobalt(II)-bis[2,6-bis(1‘-butylbenzimidazol-2‘-yl)pyridine]. Photovoltaic cells incorporating this relay have yielded light-to-electricity power conversion efficiencies of up to 5.2%. This rivals the performance of the tri-iodide/iodide couple that is currently almost exclusively used in DSSC. Interception of the dye oxidized state by electron transfer from the Co(II) complex in diluted electrolyte was found to follow a first-order kinetics with a rate constant of km = 5 × 105 s-1. Above a threshold of 10-2 M, under which the cationic relay is essentially adsorbed onto the negatively charged particle surface, larger concentrations of the reduced mediator resulted in a linear increase of the apparent rate, yielding a second-order rate constant of km‘‘ = 2.9 × 106 M-1 s-1. Dynamics of the recombination reaction between injected conduction band electrons and the oxidized relay species was monitored spectroscopically, and a first-order rate constant of kr = 3 × 103 s-1 was determined directly. This kinetic behavior compares approximately with that of the tri-iodide/iodide redox couple. Practical advantages of the CoIII/CoII complex mediator, however, make it a promising candidate to replace this widely used system in DSSC.