Transient absorption spectroscopy was employed to study charge-transfer dyn
amics 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 l
ight-to-electricity power conversion efficiencies of up to 5.2%. This rival
s the performance of the tri-iodide/iodide couple that is currently almost
exclusively used in DSSC. Interception of the dye oxidized state by electro
n transfer from the Co(Il) complex in diluted electrolyte was found to foll
ow a first-order kinetics with a rate constant of k(m) = 5 x 10(5) s(-1). A
bove a threshold of 10(-2) M, under which the cationic relay is essentially
adsorbed onto the negatively charged particle surface, larger concentratio
ns of the reduced mediator resulted in a linear increase of the apparent ra
te, yielding a second-order rate constant of k(m)" = 2.9 x 10(6) M-1 s(-1).
Dynamics of the recombination reaction between injected conduction band el
ectrons and the oxidized relay species was monitored spectroscopically, and
a first-order rate constant of k(r) = 3 x 10(3) s(-1) was determined direc
tly. This kinetic behavior compares approximately with that of the tri-iodi
de/iodide redox couple. Practical advantages of the Co-III/Co-II complex me
diator, however, make it a promising candidate to replace this widely used
system in DSSC.