The Fabrication Of TiO₂ Nanotube Arrays And The Effect Of Blocking Layer Modification On Photovoltaic Performance Of Dye Sensitized Solar Cells

One-dimensional titanium nanotube?TNT?arrays is one of the most widely used photoanodes or conductive materials in the thin film solar cells because of its excellent electron transport properties,suitable energy band position and band gap,physical and chemical stable,and simple preparation.There is a large number of grain boundaries in the traditional TiO₂ nanoparticles photoanode for dye-sensitized solar cells?DSSCs?,which will hinder the diffusion of electrons in the photoanode.One-dimensional TNT are highly ordered and perpendicular to the substrate,which provides the direct transfer route for electrons and has obvious advantages in carriers'collection and transmission.However,one-dimensional nanotube arrays has an insufficient specific surface area,which results in the low adsorption for the dye molecules and hence the low light adsorption for DSSCs.Furthermore,the carriers in the interface may be recombined with the electrolyte species and oxidation state species of the dye molecules,which will negatively affect the photoelectric properties of the DSSCs.Obviously,in order to obtain DSSCs with high efficiency,it is necessary to design the photoanodes:using one-dimensional nanotube arrays to ensure the high-speed charge transmission;improving the specific surface area by introducing TiO₂ nanoparticles into TNT arrays to ensure the large light adsorption;suppressing the recombination in interface to enhance the power conversion efficiency?PCE?.In this paper,the preparation of photoanodes based on one-dimensional TNT arrays,the suppressing of carriers'recombination in the interfaces,and the improvement of specific surface area are investigated in details.?1?The highly ordered one-dimensional TNT nanotube arrays were prepared by the two step anodic oxidation method,and the formation mechanism of some novel nanoporous films and nanotube arrays was analyzed.The TiO₂ nanoporous membrane was prepared by two-step anodic oxidation.During the second anodic oxidation process,a series of novel nanoporous membrane structures appeared on the surface of the titanium with the anodic oxidation time increases.At the initial stage of oxidation,the overlapped porous nanostructure was observed.The structure of the double-layer nanotube array and the monolayer nanotube array were obtained with the progress of oxidation.The formation mechanism of these novel nanoporous membrane structures and stable titanium oxide nanotube arrays was qualitatively analyzed by the oxidation and dissolution etching theory.?2?The TiO₂ nanotube arrays were obtained by two-step anodic oxidation method.They were then separated from the titanium and transferred to FTO conductive glass as photoanodes of DSSCs.The length of nanotubes was optimized by adjusting the anodic oxidation time.As a first try,the TiO₂ particles were introduced into TNT arrays by the TiCl₄hydrolysis method to increase the specific surface area of the phtoanodes.For TNT arrays with 24h anodic oxidation,the assembled DSSC obtains the highest PCE of 5.88%.After the photoanode was further treated by TiCl₄ solution,the short-circuit current density of DSSC increases from 12.95 mA/cm² to 16.56 mA/cm²,PCE increased from 5.88%to 8.47%.?3?The thin blocking layer of SiO₂ with high dielectric constant and high band gap was introduced into TNT arrays as the insulating layer to form the TiO₂/SiO₂ core-shell structured photoanodes of DSSCs to suppress the carrier recombination in the interface of photoanodes.Based on the above preparation of TNT arrays and the obtained optimum tube arrays'length,the thin SiO₂ insulating layer was further coated onto the surface of TNTs by sol-gel treatment.The thickness of SiO₂ shell was controlled by adjusting the sol-treated time to form a proper energy barrier between the dye and TiO₂.The photo-generated electrons can be injected into the TiO₂ semiconductor through the tunneling effect,while the SiO₂ insulating barrier layer on TiO₂ surface can effectively prevent the photo-generated electrons from the TiO₂ to the dye and electrolyte.The DSSC device with TiO₂/SiO₂ core shell photoanode with 6h's sol treatment achieves the optimum photoelectric properties.In comparison to the cell based on photoanode without SiO₂ sol treatment,the open circuit voltage of cell based on photoanode with 6 h's sol treatment increases from 0.73 V to 0.77 V,and the short circuit current increases from 12.06 mA/cm² to 20.13 mA/cm²,and accordingly the PCE increases from 5.14%to8.87%.?4?In order to further improve the photovoltaic performance of the DSSCs based on TiO₂/SiO₂ core-shell arrays photoanode,the Ti O₂ nanoparticles?NP?with average particle size of 20 nm were introduced into the TiO₂ nanotube?NT?arrays,and then the SiO₂ blocking thin layer was further coated onto the arrays to form TiO₂ NT/?TiO₂/SiO₂?NP hierarchical core-shell arrays.The Ti O₂ nanoparticles were prepared by using TiCl₄ solution as precursor and combined with sol treatment and annealing.By control the particle size to induce the necking effect between particles,the specific surface area of photoanodes was enhanced and light adsorption of DSSCs was accordingly improved.After further treating the TiO₂ NT/TiO₂NP arrays with SiO₂ sol to form TiO₂ NT/?TiO₂/SiO₂?NP hierarchical core-shell arrays photoanodes,it is found that the optimum DSSC device achieved open circuit voltage of 0.78V and short-circuit current density of 20.89 mA/cm²,the high fill factor of 64.09%and the highest PCE of 10.41%when the arrays with 6 h's SiO₂ sol treatment.