Effect of Anchoring Group on Electron Injection and Recombination Dynamics in Organic Dye-Sensitized Solar Cells

In the field of dye-sensitized solar cells, the number of different sensitizing dyes is increasing rapidly. To produce low-cost dyes, much work is being directed toward synthesizing all-organic, ruthenium-free dyes with high extinction coefficients and broad absorption bands with large solar spectrum overlap. One of the best dyes, the polyene−diphenylaniline dye D5L2A1, has a rather blue absorption with an IPCE onset at around 650 nm, but it still has an energy conversion efficiency of almost 6%. To increase the overlap with the solar spectrum, the cyanoacrylic acid anchoring group was changed to rhodanine-3-acetic acid in complex D5L2A3 . This gave an IPCE onset at around 750 nm, but unfortunately, it also decreased the overall efficiency to a modest 1.7%. By femtosecond transient absorption, we show that the electron injections into TiO 2 for the two dyes are ultrafast and indistinguishable with our time resolution (<200 fs). However, charge recombination is also ultrafast, with different fractions of a ca. 500-fs component for the two dyes. Yet, the fraction of the faster decay component is larger for D5L2A3 than for D5L2A1 . Here, we provide an interpretation of the implications of changing anchoring group. We believe that a lack of electron density on the binding oxygens of the D5L2A3 LUMO, due to the rhodanine group, promotes a higher probability for electron injection to short-lived surface trap states compared to the situation for the fully conjugated D5L2A1 .