Inner-Sphere Electron-Transfer Single Iodide Mechanism for Dye Regeneration in Dye-Sensitized Solar Cells

Abstract

During the regeneration of the oxidized dye in dye-sensitized solar cells, the redox couple of I^–/I3^– reduces the photo-oxidized dye. The simplest mechanism would be a direct charge-transfer mechanism from I^– to D^+ [D^+ + I^– → D0 + I], called the single iodide process (SIP). However, this is an unfavorable equilibrium because the redox potential of I^•/I^– is 1.224 V vs SHE, which is 0.13 V higher than that of the dye. This led to the postulation of the two iodide process (TIP) [(D^+···I^–) + I^– → (D···I^(-)_(2)) → D^0 + I^(-)_(2))] for a sufficiently high reducing power, but TIP is not consistent with either the recent experimental data suggesting the first-order kinetics or recent time-resolved spectroscopic measurements. To resolve this conundrum, we used quantum mechanics including Poisson–Boltzmann solvation to examine the electron-transfer process between I^– and D^+ for the Ru(dcb)_(2)NCS_2 or N3 dye. We find that I^– is attracted to the oxidized dye, positioning I^– next to the NCS. At this equilibrium position, the I^– electron is already 40% transferred to the NCS, showing that the redox potential of I^– is well matched with the dye. This matching of the redox potential occurs because I^– is partially desolvated as it positions itself for the inner-sphere electron transfer (ISET). The previous analyses all assumed an outer-sphere electron-transfer process. Thus our ISET-SIP model is consistent with the known redox potentials and with recent experimental reports. With the ISET-SIP mechanism, one can start to consider how to enhance the dye regeneration kinetics by redesigning ligands to maximize the interaction with iodide.

Group Members

William A. Goddard III

Charles and Mary Ferkel Professor