Evaluating Charge Recombination Rate in Dye-Sensitized Solar Cells from Electronic Structure Calculations

The process of electron recombination from semiconductor (TiO2) particles to oxidized dyes in dye-sensitized solar cells is investigated theoretically. The recombination rate is evaluated using nonadiabatic electron transfer theory with system parameters computed using ab initio density functional theory (DFT) calculations and derived from experimental sources. Our model for the recombination rate includes three contributions: the semiconductor-dye coupling term (calculated by partitioning the semiconductor-dye system into the semiconductor + anchoring group and the isolated dye), the Fermi-Dirac distribution of electrons in the semiconductor's conduction band, and the Franck-Condon term (with the reorganization energy and the driving force evaluated for the isolated dye within an implicit solvent model, and the energy of the TiO2 conduction band edge taken from experimental reports). Recombination lifetimes for several organic dyes are evaluated for a realistic range of conduction band energies. The results are in good agreement with experiment for the NKX family of dyes with a systematic variation in the dyes' structure; however, in a second considered family of dyes, complex adsorption and conformation flexibility of the molecules make quantitative prediction of recombination times more difficult. For all considered dyes, the range of the computed recombination lifetimes is in agreement with experimental data, and the relative ordering can be reproduced for dyes with predictable adsorption chemistry.