Enhanced Dye-Sensitized Solar Cells Based On Titanium Dioxide Nanomaterials

In this research,a comparison study was performed between the properties and photoconversion efficiency of different types of titanium dioxide(TiO₂) nanomaterials(nanoparticles,nanotubes and nanoribbons),synthesized by various synthesis techniques including;sol-gel,hydrothermal and anodic oxidation depending on the required TiO₂ nanomaterial type.Characterization of the synthesized nanomaterials was performed by means of X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM) and selected area electron diffraction (SAED).The synthesized TiO₂ nanomaterials were assembled into dye-sensitized solar cells(DSSC) and their photoelectrochemicai parameters were measured using 1.5 AM solar simulator(Oriel 91192).Titanium dioxide(TiO₂) nanoparticles were synthesized by sol-gel technique with and without the addition of surfactant.Cetyltrimethylammonium bromide(CTAB) and polyvinylpyrolidone(PVP) were used as particle size inhibitors.Characterization results show that the prepared nanoparticles show different particle sizes,particle size distribution and phase composition,where powders prepared without surfactant and with PVP show a bi-phase structure with different rutile concentrations.The photocatalytic activity of the prepared powders in methyl orange was measured and compared to commercial TiO₂ nanopowder(Com.TiO₂).The highest degradation efficiency was obtained by the photocatalyst prepared with PVP as surfactant;while the lowest degradation efficiency was obtained by the photocatalyst prepared using CTAB.The bi-phase powders show higher degradation efficiency compared to pure anatase phase powders,probably due to a synergetic effect between anatase and rutile powders,which enhances the electron-hole separation and thus increases the photocatalytic activity.The overall conversion efficiency of the samples(N-TiO₂), (CTAB-TiO₂),(PVP-TiO₂) and(Com.TiO₂) was calculated to be 2.81,1.57,0.09 and 1.21% respectively.Observations showed that the DSSC assembled with TiO₂ nanoparticles prepared without the introduction of surfactant(N-TiO₂) presented much higher values of short-circuit current and overall conversion efficiency in comparison to other cells,which was attributed to its high surface area and porous structure,allowing more sensitizer dye to be chemically anchored in the electrode and thus increasing the light harvesting.Titanate nanotubes and nanoribbons have been synthesized hydrothermally using commercial anatase nanopowder(particle size≈50-400nm) and NaOH.It has been found that the hydrothermal temperature and the treatment duration have a strong effect on the morphology control of the resulting products.At a fixed temperature of 180℃,the morphology of the products changes with the hydrothermal duration(5-72 h).Hollow nanotubes with an outer diameter of about 10nm,were formed at treatment durations between 5-20 h.Bundles of nanoribbons with widths ranging from 50nm - 500nm and lengths up to several tens of micrometers were produced at treatment duration of 72 h.Experimental results on samples treated at different temperatures(135℃-195℃) for different durations revealed in the transformation of the nanotubes into nanoribbons after specific treatment duration at any selected temperature.The DSSC measurement for the titanate nanotubes nanoribbons, and mixed tubes-ribbons structure,revealed conversion efficiencies of 0.03,0.02 and 0.21% respectively.The mixed structure gave higher conversion efficiency with improved open-circuit voltage(V_OC) and short-circuits current(I_SC) when compared to pure nanotubes or nanoribbons.The effect of ammonium fluoride(NH₄F) concentration on the dimensions(length,diameter,and wall thickness) of the self-organized nanotube arrays has been investigated.Results show that varying the concentration of NH₄F exerts a strong effect on changing the dimensions of the as-grown nanotube arrays.The length of the nanotube arrays increases gradually by increasing the concentration up to a maximum length at a concentration of 1.00 wt%,after which the length decreases slightly with the increase in NH₄F concentration.It was also observed that the diameter and wall thickness of the nanotube arrays vary with the change in concentration of NH₄F,where the diameter was found to alter between 80 and 140 nm,and the wall thickness decreases by increasing the NH₄F concentration.Self-organized nanotube arrays were grown on titanium thin film coated on conducting glass (400 nm thick) and Ti foil(6μm thick) and their DSSC measurements were performed.The overall conversion efficiency of the nanotube arrays grown on Ti foil electrode and titanium film coated on glass was calculated to be 0.89 and 0.43%respectively.It was found that to obtain highly efficient cell,the thickness of nanotube array electrode must be increased to attain high amount of dye.