Multiple adsorption of tributyl phosphate molecule at the dyed-TiO2/electrolyte interface to suppress the charge recombination in dye-sensitized solar cell

Abstract

Electron recombination and dye aggregation at the dyed-TiO₂/electrolyte interface are still problems in dye-sensitized solar cell (DSC) research. In this paper, tributyl phosphate (TBpp) as a special additive to modify the dyed-TiO₂/electrolyte interface was introduced to enhance the photovoltaic performance. The adsorption mode of TBpp and the interaction between cis-dithiocyanate-N,N′-bis-(4-carboxylate-4-tetrabutylammonium carboxylate-2,2′-bi-pyridine) ruthenium(II) (N719) and TBpp were investigated. It was found that one TBpp parent molecule split into several smaller fragments and formed four anchoring modes on the TiO₂ surface. It was very interesting that the molecular cleavage of TBpp and adsorption of N719 assisted each other on the sensitized TiO₂ surface. The fragments distributed around N719 result in steric hindrance, consequently hydrogen-bonding among N719 molecules was decreased. The unstable type N719 transformed into stable type N719 accompanied by molecular cleavage of TBpp and the N719 aggregation was reduced. Furthermore, these new fragments were multiply adsorbed on the non-sensitized TiO₂ surface to form an insulating barrier layer. Therefore, the electron recombination at the dyed-TiO₂/electrolyte interface was restrained. Besides the change of surface configuration, the TiO₂ band edge negatively shifted and the rate of electron transport in the TiO₂ films decreased with the addition of TBpp. As a result, an increase in the photoelectric conversion efficiency (η) was obtained of almost 40%.