Theory of the Charge Recombination Reaction at the Semiconductor–Adsorbate Interface in the Presence of Defects

A phenomenological model for the study of the charge recombination process in dye-sensitized solar cells is presented. The model combines a tight binding description of the system with a rate expression for the charge recombination, and it is used to describe the role of defects of different energies and in different positions. We find that electron trap defects in realistic concentration and with realistic energy parameters increase the charge recombination rate and also increase the range of observed charge recombination rates. Changing the energy parameters of the model, it is not possible to reduce at the same time the charge recombination for a dye (or a redox couple) adsorbed close to a defect and for the same dye far from a defect. We observe that the phenomenology is not much affected by the energy characteristics of the dye (reorganization energy, free energy difference between neutral and oxidized dye), while it depends very strongly on the distance between the dye and the defect. On the basis of the results presented, we discuss viable strategies to reduce charge recombination losses in dye-sensitized solar cells.