Electrospun Nanofibrous Titania Films And Their Application In Dye-sensitized Solar Cells

A solar cell is a photovoltaic device that can convert the energy of light directly into electricity.Solar cells are expected to play an important role in exploiting renewable energy sources.Dye-sensitized solar cells(DSSCs),as a new type of solar cells,have attracted much attention in the past two decades.Due to their relatively low cost and simple fabrication,DSSCs provide us with a more cost-efficient approach to utilize solar energy on a large scale.Hence,DSSCs have great potential to be one of the most promising next generation photovoltaic devices to substitute for current polycrystalline silicon-based solar cells.Despite the highest conversion efficiency(?)of 12.3%,the efficiencies for most DSSCs are far inferior to this record.To further improve the energy conversion efficiency and long-term stability of DSSCs is imperative from the viewpoint of practical application.A thin sheet of nanostructured photoanode film on the conductive glass substrate is the core part of DSSCs,and it plays a key role in dye adsorption and electron transport.Conventional photoanode film is made of anatase titania(TiO2)nanoparticles(NPs).A major bottleneck to yield higher conversion efficiency for NPs-based DSSCs can be ascribed to a fact that the photogenerated electrons will encounter huge amounts of grain boundaries(corresponding to surface state sites)when they travel through the three-dimensional NPs network in a "Random-Walk" way.When the electrons inject into the conduction band of TiO2,a considerable number of them will be trapped by these surface state sites(i.e.,trap sites).The trapping process is accompanied by an opposite detrapping process.Thousands of trapping/detrapping events will occur repeatedly before the electrons reach the conductive glass substrate,which will greatly retard the electron transport.Moreover,the detrapping rate is much slower than the trapping rate,that's to say,the electrons have more probability to stay around the trap sites.In addition,the surface state sites below the conduction band of TiO2 provide shorter pathways for the trapped electrons to recombine with the electron-acceptors I3-in the electrolyte,which will decrease the photocurrent density and conversion efficiency of DSSCs.Hence,how to suppress the charge recombination has become a common concern in the field of DSSCs.Based on the above analysis,we know that the surface state sites arising from grain boundaries are main recombination centers of the photogenerated electrons.An effective strategy to suppress the recombination is the surface modification of conventional TiO2 NPs photoanode film through some methods,such as chemical treatment,doping,and surface coating.As a result,the recombination centers are passivated and the density of the surface state sites is diminished.Another strategy is to develop new types of photoanode film materials.Some photoanode films based on one-dimensional(1D)TiO2 nanotubes,nanorods,and nanowires with less grain boundaries and recombination centers have been investigating extensively in view of their potential advantage in suppressing recombination.Besides,1D TiO2 nanofibers(NFs)film has also been applying in the photoanode of DSSCs in recent years.In the introduction section,the recent progress in these two strategies was reviewed.It is generally accepted that electrospinning is a simple approach to fabricate nanofibers,especially polymeric ones.In the past decade,this technique has also been expanding to fabricate many inorganic ceramic nanofibers.Compared with the methods(e.g.,anodic oxidation)to obtain the 1D nanostructures including nanotubes,electrospinning provides 1D NFs with greater possibility to be fabricated on an industrial scale.At present,many efforts are being made to develop some energy-related devices based on electrospun nanofibrous films.Besides DSSCs,other devices,such as lithium-ion batteries,fuel cells,and supercapacitors,are also included.These researches and applications were also reviewed in the introduction section.When TiO2 NFs film is used as the photoanode of DSSCs,the poor adhesion of ceramic NFs film to its conductive glass substrate is a main drawback for this kind of photoanode.In order to solve this problem,a systematic proposal is put forward.The substrate is carried out a key spin-coating pre-treatment using a viscous solution,and then electrospinning is performed.The spin-coating solution and electrospinning solution have the same components except that the concentration of the former is a little higher than that of the latter.During calcination,the spin-coating layer will be converted into Ti02 NPs layer,which can service as a transition layer between the crisp NFs meshwork and the hard glass substrate.On the basis of this proposal,the electrospun TiO2 NFs photoanode film with a good adhesion was prepared successfully.In fact,this kind of photoanode film is a bilayered film that consists of an overlaid NFs layer and an underlaid NPs layer.Because the NFs layer is dominant for the performance of DSSCs,the actual NPs/NFs film is called directly as NFs film.In the assembled DSSCs,the NPs and NFs layers have complementary roles.The NPs layer provides the photoanode film with an improved adhesion to the substrate.Meanwhile,the NFs layer endows the photoanode film with better dye-loading and light-harvesting.And the NFs layer also offers higher pore volume to facilitate the electrolyte diffusion and the activity regeneration of dye due to its porous meshwork structure.Consequently,the electron transport is accelerated while the charge recombination is suppressed.The TiO2 NFs-based DSSCs yield a conversion efficiency of 4.46%.This efficiency value is about 14%higher than that(3.92%)of the cells based on TiO2 NPs photoanode film,which was prepared via a simple spin-coating method.Subsequently,the TiO2 NFs photoanode film was further optimized through three modification methods.The TiO2 NFs photoanode films were post-treated by four different molar concentrations of TiCl4 aqueous solutions.And the effect of TiCl4 concentrations on the photoelectric properties of DSSCs was investigated.When the TiCl4 concentration is 0.1M,the corresponding cells yield the highest efficiency of 5.40%,which is nearly 21%higher than that(4.46%)of DSSCs without TiCl4 treatment.The enhancement in performance is mainly ascribed to the increase in light-harvesting efficiency and electron collection efficiency.The joints among the nanofibers are significantly reinforced by the agglomerated TiO2 crystallites arising from TiCl4 hydrolysis,which can further improve the strength as well as the adhesion of the NFs film.Since multi-walled carbon nanotubes(MWCNTs)possess excellent electrical conductivity and electron-storage capacity,the performance of DSSCs can be enhanced when MWCNTs are moderately incorporated into the photoanode film.TiO2-MWCNTs composite NFs photoanode films with four different MWCNTs contents were prepared.The conversion efficiencies of the resulting DSSCs are greatly dependent on the content of MWCNTs.When the composite photoanode film contains 0.3wt.%MWCNTs,the corresponding solar cells yield the highest efficiency of 5.63%.This efficiency is about 26%larger than that(4.46%)of the unmodified counterparts.The purpose of surface coating modification is to construct an energy barrier at the interface between photoanode and electrolyte to suppress the charge recombination.Novel TiO2/Nb2O5 core-sheath(TiO2 is core,and Nb2O5 is sheath)NFs photoanode film was prepared via coaxial electrospinning.Through such a unique technique,two effective strategies to weaken charge recombination,namely,the formation of an interfacial energy barrier and the synthesis of a 1D nanostructure are achieved in one step.Owing to the simultaneous increase in short-circuit current density and open-circuit voltage,the solar cells based on the TiO2/Nb2O5NFs photoanode yield an overall energy conversion efficiency of 5.80%,which shows a nearly 30%enhancement compared with that(4.46%)of the bare TiO2 NFs-based cells.Meanwhile,TiO2/MgO core-sheath NFs were also fabricated via coaxial electrospinning,and then the long TiO2/MgO NFs were converted into short TiO2/MgO nanorods(NRs)through ultrasonic post-treatment.Based on the TiO2/MgO NRs,some flexible DSSCs were prepared.Compared with the bare TiO2 NRs counterparts,the TiO2/MgO NRs-based flexible DSSCs exhibits better performance.The conversion efficiency is enhanced from 3.28%to 3.93%,which corresponds to an increase of approximately 20%.Despite the increase in efficiency,the absolute conversion efficiency should also be further improved.A photovoltaic smart textile is developed by integrating the TiO2/MgO NRs-based flexible DSSCs onto fabric substrates such as clothing,tent,and sunshade umbrellas.In the future,various photovoltaic textiles are expected to provide a mobile and sustainable energy supply for some portable electronic devices including mobile phones,digital cameras,music players,and notebook computers during the daytime outdoors,or service as a power source to charge for their batteries.Through the above three modifications,the increase in the conversion efficiency of DSSCs is achieved under certain conditions.But the absolute efficiency values are still lower than the current average level(?6%).It is likely that the photoanode films are too thinner(?6?m)in contrast with the ideal thickness of 10?20?m.The insufficient film thickness leads to relatively low dye loading and conversion efficiency.Further work should be done to achieve the preparation of thicker photoanode films for DSSCs with higher photoelectric performance.