Unexpected Effects of Co-adsorption in the Popular Triphenylamine Dyes for Efficient Dye-Sensitized Solar Cells

In this work, two triphenylamine dyes coded XY1b and L1 and used recently in many highly efficient dye-sensitized solar cells (DSSCs) with copper-based electrolyte were studied using stationary and time-resolved spectroscopic techniques. The main focus was put on the effect of their cosensitization as well as adding chenodeoxycholic acid (CDCA) as coadsorbate. Adding CDCA to both dyes as well as L1 to XY1b resulted in an expected improvement in fill factor of the cells. However, in both cases, the addition of another compound caused a surprising decrease of the relative photocurrent of the device. Femtosecond transient absorption (TA) technique applied to the complete cells revealed that the reason for such photocurrent lowering is the increased contribution of unwanted charge recombination. Moreover, TA studies enabled determination of charge transfer rate constants for all studied systems. In particular, it was observed that the lifetimes of excited state of L1 dyes as well as the lifetimes of the oxidized L1 are shortened in the mixtures with XY1b. This is a strong confirmation of the remarkable effects of electron injection and dye regeneration of the wider bandgap dye (L1) mediated by the smaller bandgap one (XY1b). Another important finding observed in the studied cosensitized systems were related to the Stark shift effect: a noteworthy decrease of its contribution in TA dynamics with less stationary dye absorbance was revealed as well as the recently reported “co-Stark shift effect” was confirmed. Finally, the photovoltaic data were used to train a deep learning algorithm which subsequently predicted unexpected best cosensitization ratio for preparation of DSSC, which outperformed the previous entries. This is a significant step toward data-driven optimization of cosensitized systems.