Insights into aggregation effects on optical property and electronic coupling of organic dyes in dye sensitized solar cells

Combining density functional theory (DFT), time-dependent DFT, and density functional tight binding (DFTB) methods, we present a computational investigation on the aggregation effects of two organic dyes, WS-2 and WS-6, which are used as sensitizers in dye sensitized solar cells (DSSCs). The calculated absorption and emission spectra in solution and on TiO₂ agree well with the experimental observations, which demonstrate WS-6 is less prone to aggregation due to a hexyl substituent on the thiophene ring compared to WS-2. Importantly, our results predict strong aggregation interactions inducing larger electronic coupling between the stacking dimers, which may be detrimental for electronic injection from dye to TiO₂ and partly responsible for the loss of photo-voltaic efficiency. The deeper understanding of the dye aggregation effects shed lights on a better knowledge about the complex factors determining the function of DSSC and rational design of high efficiency sensitizers.