Mykhaylo Myahkostupov

Dye-sensitized photovoltaic properties of hydrothermally prepared TiO2 nanotubes

Hydrothermal synthesis utilizing aqueous alkaline reaction conditions was employed for the preparation of TiO2 nanotubes. The ultimate material morphology was found to be extremely sensitive to the nature of the reaction conditions including the chemical nature of the reactor itself—well-defined TiO2 nanotubes were obtained only when the reaction was carried out in a Teflon reactor/autoclave assembly, as ascertained by electron microscopy imaging. In addition, it was shown that the nanomaterial morphology heavily impacted the performance of dye-sensitized photovoltaic devices based on these materials; devices assembled from well-defined TiO2 nanotubes exhibited marked overall improvement in the power conversion efficiency. Several individual experimental parameters including hydrothermal synthesis temperature, film thickness, TiO2 paste composition, and sintering temperature were shown to affect the photovoltaic properties of the resultant solar cells. Upon sensitization with Ru(dcbpyH2)2(NCS)2, optimized devices showed an average power conversion efficiency of 3.76 ± 0.25% under AM1.5G one sun illumination, seemingly limited by low dye surface coverages.

Charge Localization after Ultrafast Photoexcitation of a Rigid Perylene Perylenediimide Dyad Visualized by Transient Stark Effect

The intramolecular charge-transfer (CT) dynamics of a rigid and strongly conjugated perylenediimide-bridge-perylene dyad (PDIPe) has been investigated in dichloromethane using ultrafast transient electronic absorption spectroscopy and quantum chemical calculations. The strong electronic coupling between the dyad units gives rise to a CT band. Its photoexcitation forms a delocalized CT state with well-preserved ion bands despite the strong coupling. In the dyad, the electronic transition dipole moment of the electron donor perylene is aligned along the axis of the electric field vector with respect to the CT species. This alignment makes the donor sensitive to the Stark effect and thus charge density fluctuations in the CT state. Charge localization on the picosecond time scale manifests as a time-dependent Stark shift in the visible region. Quantum chemical calculations reveal a twist around the acetylene bridging unit to be the responsible mechanism generating a partial to an almost complete CT state. An estimate of the electric field strength in the CT state yields approximately 25 MV/cm, which increases to around 31 MV/cm during charge localization. Furthermore, the calculations illustrate the complexity of electronic structure in this strongly delocalized superchromophore and reflect the complications in the interpretation of transient absorption results when compared to steady-state approaches such as spectroelectrochemistry and model chromophore experiments such as photoinduced bimolecular charge transfer.