Comparative study on the interactions of TEMPO and iodine with organic dyes in dye-sensitized solar cells

Intermolecular interactions of two donor-acceptor organic dyes, D131 (1) and 5.13 M (2), with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) species and iodine species were investigated using density functional theory. Population analyses of the optimized dye+-TEMPO0 complexes verified that the regeneration of both 1+ and 2+ by the TEMPO/TEMPO+ redox couple occurs via a one-electron transfer process. The interactions with I− and I2− revealed that the oxidized dyes can be regenerated by two I− ions via a two-step mechanism and/or by I2−via a one-step mechanism rather than by one I− through a one-step mechanism using the iodine/iodide redox couple. The most preferable interactions with I2− observed among the dye+-I2− complexes occurred via the N atom of the cyano group at the dye anchor moiety. The strongest interactions with TEMPO+ were also found at the cyano group with four different configurations forming N⋯H hydrogen bonds, and the distances from TEMPO+ to the TiO2 surface were shorter than those from I2. These findings strongly suggest that the recombination of injected electrons with electron acceptors is more accelerated in the TEMPO/TEMPO+ electrolyte than in the iodine/iodide; this results in a lower increase of open-circuit photovoltage for a dye-sensitized solar cell than expected.