Influence of intermolecular interactions on the reorganization energy of charge transfer between surface-attached dye molecules

The parameters controlling the kinetics of intermolecular charge transfer are traditionally estimated from electronic structure calculations on the charge donor and charge acceptor in isolation. Here, we show that this procedure results in inaccuracies for hole transfer between a pair of organic dye molecules by comparing charge-constrained density functional theory (DFT) calculations on a dye cation/neutral dye pair to the conventional DFT calculations on the isolated molecules. We quantify the error made in the reorganization energy of hole exchange between dye molecules (λ_i). We choose three indolene-based organic dyes with application to dye-sensitized solar cells, namely, D149, D102, and D131, for which experimental values of λ are available. We find that, although highly system dependent, the intermolecular interaction between the charge donor and acceptor can lead to a 0.25 eV change in λ_i, illustrating the limitations of the widely used original method in predicting the rate of charge transfer.