Effect of the anchoring group on electron injection : theoretical study of phosphonated dyes for dye-sensitized solar cells

The attachment chemistry of the chromophore onto the semiconductor surface influences the efficiency of electron injection in dye-sensitized solar cells (DSSCs). In this work, we study injection times for dyes that bind to the semiconductor surface via the phosphonic acid anchoring group and the effect on the injection time of different binding modes (molecular or dissociative, monodentate or bidentate) of phosphonic acid for both TiO2 rutile (110) and anatase (101) surfaces. We calculate electron injection times for a large set of organic dyes on TiO2 rutile (110) and anatase (101) surfaces for the most stable adsorption geometries of the phosphonic acid anchoring group, using a model based on partitioning the semiconductor chromophore system into fragments. We analyze the influence of the size and nature of the anchoring group on the injection times, performing calculations with larger models of the anchoring group (e.g., phenyl-phosphonic acid). Through the partitioning procedure, we are able to separate the effect of the binding geometry from other effects influencing the efficiency of the electron injection. The results show that dissociative bidentate adsorption modes generally lead to faster injection, compared to monodentate and molecular ones, similar to the results obtained earlier for analogous carboxylated dyes. Our results are in good agreement with experiments (where available), showing that our model is capable of predicting the effects of the anchoring groups and of different spacer groups on the injection times and is therefore suitable for designing new and more efficient chromophores