Optimization Of Dye-sensitized Solar Cells By Ferroelectric Dipole

Dye-sensitized solar cells（DSCs）have received great attention due to their facile fabrication,potentially low-cost and competitive power conversion efficiency.A typically DSC consists of a transparent fluorine doped tin oxide（FTO）glass substrate as a collector electrode,a dye-sensitized nanocrystalline titanium dioxide（TiO₂）film as a photoanode,a Pt-coated glass substrate as a counter electrode,and electrolyte containing iodide（I-）/tri-iodide（I3-）redox couple filled between the photoanode and the counter electrode.DSC is the only solar cell that separates two functions of light harvesting and electron transport,where light is absorbed by a monolayer of dye adsorbed on the interfaces of TiO₂,and electrons are injected from the dye into the TiO₂ upon photo-excitation.Once the electrons are injected into the TiO₂ nanoparticles,they experience two types of processes: transport and recombination.Transport is a forward movement of electrons to the collector electrode,whereas recombination is a back flow of electrons to oxidized dyes and tri-iodide ions.Competition between transport and recombination can be denoted as a charge collection efficiency（ηc）,indicating the degree of electrons reaching the collector electrode before getting recombined.Since there is no significant electric field present in the TiO₂ film,transport of injected electrons to the collector electrode occurs mainly through diffusion.In the DSC,the photoanode consisting of mesoporous TiO₂ nanoparticles provides a large surface area for the adsorption of dye molecules and pathways for electron transport.However,the mesoporous structure of TiO₂ nanoparticle film with high surface area has been also considered to escalate charge recombination by reducing electron diffusion length and impeding charge transport in view of the random and highly grained surface and boundaries.Improving electron mobility and thus suppressing charge recombination in the TiO₂ nanoparticle film has been recognized to be one of the key strategies to enhance the power conversion efficiency of practical DSCs.Ferroelectric materials have special structural characteristics which the positive and negative charges in the lattice structure do not coincide,so this characteristics can result in a certain spontaneous polarization electric field below the Curie temperature.The spontaneous polarization electric field is enough to separate the electron holes.In recently years,ferroelectric materials were introduced into organic solar cells（OPV）,and they showed good impact on the performance of OPV.So we introduce ferroelectric materials into DSC,hoping to use its spontaneous polarization electric field to promote the movement of electrons and reduce the charge recombination ratio.In this study,TiO₂ was used as the photoanode material.BaTiO₃/TiO₂ and LiNbO₃/TiO₂ photoanodes were prepared and used for DSCs.The optimum mass ratio of ethyl cellulose（EC）in the preparation slurry and the optimum immersion time and concentration of the photoanode in the dye were studied.The composition and morphology of the photoanode films were characterized by XRD,SEM,AFM and SSPFM.The electrochemical impedance spectroscopy（EIS）and the photocurrent density-voltage（J-V）were used to study BaTiO₃ and LiNbO₃ in composite photoanode for the photovoltaic performance and transport process of DSCs and the optimum fabrication process.The main results obtained are as follows（1）.The DSC has the highest energy conversion efficiency（5.28%）and the highest dye adsorption capacity（74.0 nmol/cm2）when the EC ratio is 9%.EC acts as a pore former and binder in the DSC.（2）.The immersion time of the maximum efficiency of DSCs was increased with the increase of the concentration of dye.We got the highest efficiency at the concentration of 0.5 mM and 0.6 mM with immersion time of 22 h and 18 h,respectively.（3）.With a small amount of BaTiO₃（< 1.0 wt%）,ferroelectric BaTiO₃ dipole induced electric field would increase electron mobility and reduce charge recombination.The photoanode with 1.0 wt% BaTiO₃ addition provided the best device performance（5.39%）,and the power conversion efficiency increased by 11% comparing with the pure TiO₂ photoanode.（4）.The mixture structure of DSC have higher efficiency than layer structure of DSC.In the mixture structure,LiNbO₃ improve the performance of DSC by suppressing the interfacial charge recombination at TiO₂/dye/electrolyte and improving the electron transport in the film,while LiNbO₃ in the layer structure only suppressing the interfacial charge recombination.