Research On Photovoltaic Properties Of Dye And Quantum Dots-sensitized Titania-based Solar Cells

A sandwich single-walled TiO2nanotube (STNT) has been proposed as a photoanode material in dye and quatum dot-sensitized solar cells (DSSCs and QDSSCs) in our work. On the basis of first-principles simulations, elementary photoelectronic processes occuring at sensitizer/STNT heterointerface are elucidated. In addtion, quasi-one-dimensional (1D) titania nanobundles were successfully synthesized via a hydrothermal method and used to print random network nanostructured films.By means of density funtional theory (DFT) and non-equilibrium Green funtion calculations, elementary photoelectronic processes occuring at ZnPVB/STNT heterointerface are elucidated, involving electron transfer and electron transport. The calculated results reveal that a groundstate electron of dye molecule is first promoted to the excited state upon light illumination, and ultrafast injects into the conduction band of STNT. An electron injection process was observed from excited porphyrin dye to the conduction band of STNT using both nonadiabatic (kET(ZnPVB)=1.28×1015s-1) and adiabatic methods (τ=7.18fs). Afterwards, the injected electron transports in STNT CB via Ti3dx2-y2orbitals, providing a unidirectional pathway for photogenerated electron toward electrode, unlike nanocluster model in which electrons have a random walk toward electrode.A novel strategy is proposed for exploring the nature of the built-in electric field by using externally applied electric field as perturbation to couple to the built-in electric field. On the basis of theoretical calculations, we found that Fdipole couples to external electric field through the electric dipole moment change (ΔμET) at dye/semiconductor heterointerface to affect the photoelectronic properties in DSSCs. In essence, the influence of gap states and conduction bands is determined by the overlap between the applied electric field and intrinsic dipole moment change of chargeseparated dye/semiconductor. Furthermore, the electron-transfer rate (kET) is also intrinsically related to the coupling of Fdipole with external electric field in the form of electric dipole moment at dye/semiconductor interface. It is found that these properties are intrinsically connected to the electric dipole moment at the dye/semiconductor interface, which are attributed to the overlap between external electric field and the change of intrinsic electric dipole moment.A general strategy is proposed to examine the nature of built-in interfacial electric field (microscopic nature) by means of absorption spectra of adsorbed dye molecule (macroscopic observation). Modulated external electric fields are applied to thesensitized system to couple with built-in heterointerfacial electric field to induce the absorption band shift, by which the nature of built-in interfacial electric field (e.g. direction vector) would be investigated quantitatively through Stark effect equation.The photoelectric properties of Cd2S2sensitized STNT (Cd2S2@STNT) have been investigated under nonperiodic and periodic boundary condition. The calculated results show an electron in the ground state of Cd2S2cluster is firstly promoted to excited states upon light adsorption, which then injects into the conduction band of STNT. The sandwich-like STNT functions as a hollow coaxial nanocable for electron transport in the photoelectric system where the injected electron transports along the unidirectional conducting layer of STNT through Ti3d orbital. The biexponential fitting curve to P(t) indicates that a population decay in adsorbed Cd2S2reaches to about0.2during the early dynamics time (~0-5fs) and the electron injection time is slowing down at longer times (25fs). Through Newns-Anderson modeling, the estimated electron injection time is in the sub-10femtosencond regime (τ=4.8fs). For Cd4S4/Graphene hybrid sensitizer, upon visible light adsorption, the ground electron of Cd4S4cluster in nanomaterials is first promoted to the excitated state, which then injects into the conduction band of graphene and transports along graphene layer through π*orbitals to achieve interfacial electron-hole separation.A quasi-one-dimensional TiO2nanobundle network has been developed for application as a hierarchical nanostructure assembly in dye-sensitized solar cells. For the MK2dye and cobalt redox mediator-based electrolyte, the efficiency achieved (7.7%) was much higher than that of P25(only6.3%). Detailed analysis of the films and devices through diffused reflectance spectroscopy, IPCE, EIS and IMVS measurements led to the conclusion that the excellent performance of the TiO2NN-cobalt electrolyte based devices can be attributed to the large specific surface area, increased scattering ability, the suppression of interfacial electron recombination and effective mass transport of the redox couples through the films. The slow electron recombination rate is demonstrated to result from the reduced number of grain boundaries. In addition, for the iodide electrolytes, TiO2network-based show comparable to photovoltaic performance to corresponding DSCs based on P25.