Journal article

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 no. of different sensitizing dyes is increasing rapidly. To produce low-cost dyes, much work is being directed toward synthesizing all-org., Ru-free dyes with high extinction coeffs. 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 ∼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 the complex D5L2A3. This gave an IPCE onset at ∼750 nm, but unfortunately, it also decreased the overall efficiency to a modest 1.7%. By femtosecond transient absorption, the electron injection into TiO2 for the 2 dyes are ultrafast and indistinguishable with the time resoln. (<200 fs). However, charge recombination is also ultrafast, with different fractions of a ∼500 fs component for the 2 dyes. Yet, the fraction of the faster decay component is larger for D5L2A3 than for D5L2A1. An interpretation of changing the anchoring group is presented. Probably a lack of electron d. 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.


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