Direct experimental evidence for the adsorption of 4-tert-butylpyridine and 2,2′-bipyridine on TiO₂ surface and their influence on dye-sensitized solar cells’ performance

We have investigated the binding of two electrolyte additives, 4-tert-butylpyridine (4-TBP) and 2,2′-bipyridine (bipy), to the surface of TiO2 photo-anode by attenuated total reflection – Fourier transform infrared spectroscopy (ATR-FTIR); X-ray photoelectron spectroscopy (XPS); zeta potential; cyclic voltammetry analyses, and explained how the adsorption affects photovoltaic performance of dye-sensitized solar cell (DSC). To clarify whether the adsorption of 4-TBP/bipy on TiO2 surface affects the DSC performance, DSC devices were fabricated with non-grafted and 4-TBP or bipy grafted TiO2 photo-anodes, with application of two types of electrolytes with and without the nitrogen additives. Furthermore, the interaction between 4-TBP or bipy with the electrolyte mediator I−/I3− was investigated by cyclic voltammetry. The results showed that 4-TBP and bipy were able to bind chemically to TiO2 surface at elevated temperatures. DSC prepared with N-additive grafted TiO2 photo-anodes had lower short current and efficiencies than non-grafted cells. The open circuit voltage (Voc) was, however, increased 110 mV for 4-TBP grafted cells with 0.5 M 4-TBP in the electrolyte. One third of the overall increase in Voc was estimated to be due to the adsorption of the 4-TBP to the TiO2 surface. The remaining Voc increase was due to effects of 4-TBP on the electrolyte mediator.