Confined Photodynamics of an Organic Dye for Solar Cells Encapsulated in Titanium-Doped Mesoporous Molecular Materials

Abstract

A triphenylamine dye (TPC1) encapsulated in titanium-doped mesoporous silica structures as alternative materials for dye-sensitized solar cells has been studied by means of stationary absorption and emission as well as ultrafast emission spectroscopy. For the samples prepared by a grafting method, a TPC1 complex with titanium atoms within the mesoporous silica in dichloromethane (DCM) solution is formed, having a red shift of the visible absorption band by about 1300 cm−1 with respect to that of the TPC1 in DCM (from 455 to 485 nm). For the complexes, multi- exponential emission quenching of the relaxed singlet excited state occurs with time constants from 300 fs to 30 ps and is assigned to the confined electron injection process into the Ti–O chromophore. The averaged electron injection rate from the higher energy levels gets smaller values for less energetic probing, from 2.7 × 1012 s–1 at 600 nm to 1.5 × 1012 s–1 at 700 nm. However, in the titanium-doped samples prepared by an impregnation method, we observed about 2–3 times slower injection. The difference is explained by different coupling between TPC1 and titania domains. As a reference to the confining effect on the dynamics, we also studied the behavior of TPC1 when interacting with amorphous silica and purely siliceous MCM-41 material in the same solvent. In amorphous silica, an equilibrium between neutral and anion structures of TPC1 is found to be shifted toward the anion form. For the MCM-41 material, the presence of a new absorption band at around 690 nm is revealed, assigned to the spontaneously created and remarkable stable TPC1 radical cation. The lifetimes of the normal and anion forms in both materials were found to be similar to those in solution. The femtosecond relaxation dynamics in these materials is also similar to that in solution (dominated by the solvation), but additional emission quenching in the TPC1/MCM-41 sample is observed, probably due to intermolecular energy transfer. The rate of energy transfer was estimated to decrease gradually when increasing the observation wavelength, from 1.11 × 1012 s–1 at 500 nm to 0.13 × 1012 s–1 at 700 nm. We believe that our results interrogating ultrafast dynamics of an efficient dye interacting with titania within the mesoporous materials will help in a better understanding and improvement of dye-sensitized solar cells.