Computational Investigation of Dye-Iodine Interactions in Organic Dye-Sensitized Solar Cells

We present a computational investigation, based on DFT, MP2, and Car-Parrinello molecular dynamics simulations, aimed to model the dye iodine interactions in organic dye-sensitized solar cells. We evaluate the binding of I-2 and I-3(-) and Li+ to the various electron-donating sites in organic sensitizers considering two coumarin dyes differing by the number of thiophene rings in the bridge (NKX2587 vs NKX2697) and one carbazole dye (MK3), which have been the subject of a detailed experimental investigation (J. Am. Chem Soc. 2008, 130, 17874-17881). We find that oxygen atoms are the preferred binding sites for I-2, while I-3(-) tends to interact with the it system of the coumarin donor unit. Our results suggest that the increase of the distance of the carbonyl oxygen from the titania going from NKX2S87 to NKX2697 could explain the differences in the lifetime values measured for NKX2587-sensitized solar cells compared to those employing the longer NKX2697 homologues. The shorter lifetime measured for MK3 compared to NKX2687, which have the same pi-spacer and acceptor units, is instead attributed to the twisting of the dye structure in MK3, which possibly forms less compact dye layers on titania. The effect of the dye structure and Li+ coordination to the dyes on the titania conduction band is also examined.