Preparation Of TiO₂Nanosheets-based Dye-sensitized Solar Cell

Nowadays, the sustainable developments of energy and environment are both of the two important strategies for the development of human society in the world. With the development of the global economy, the demands for energy are growing, and the development of new energy is imperative. The solar energy is one of the most potential because of its continuous irradiation, harmless and inexhaustibility. Dye-sensitized solar cell (DSSC) is one of the effective energy conversion devices. Because of its simple fabrication procedure, low cost, better stability and cleanliness, DSSC has been intensively investigated. The dye-sensitized nanocrystalline porous TiO2film is an important part of the DSSC, and its structure has great impact on cell's photoelectric performance. In this work, nanostructure design and modification of TiO2with carbon namomaterials favor the separation of photogenerated charge carriers and thus enhance the photoelectric performances of DSSC. The point can be summarized as follows:1) Anatase TiO2nanosheets with exposed (001) facets:improved photoelectric conversion efficiency in dye-sensitized solar cells. Very recently, a lot of studies find that the (001) facets of anatase TiO2nanosheets is much more reactive than the thermodynamically stable (101) facets, the obtained nanosheets would offer a new chance to Design highly active photocatalytic materials and devices. Dye-sensitized solar cells (DSSC) are fabricated based on anatase TiO2nanosheets (TiO2NS) with exposed{001} facets. The photoelectric conversion performances of TiO2NS solar cells are also compared with TiO2nanoparticles (TiO2NP) and commercial-grade Degussa P25TiO2nanoparticles (P25) solar cells at the same film thickness, and their photoelectric conversion efficiencies (η) are4.56,4.24and3.64%, respectively. The enhanced performance of TiO2NS solar cell is due to their good crystallization, high pore volume, large particle size and enhanced light scattering. The prepared TiO2nanosheet film electrode should also find its widely potential applications in various fields including photocatalysis, catalysis, electrochemistry, separation, purification and so on.2) Adsorption of N719dye on anatase TiO2nanoparticles and nanosheets with exposed (001) facets:equilibrium, kinetic, and thermodynamic studies. Very recently, DSSCs based on two-dimensional anatase TiO2nanosheets have been proven to be effective in improving photoelectric conversion efficiency, because their two-dimensional (2D) nano-structures can enhance the light-collection efficiency by multiple light scattering. On the other hand, the characterization of the dye complex adsorption on the titania surface is very important for a deep understanding of the sensitization phenomenon and, then, optimize the performance of the sensitized cells. However, to the best of our knowledge, there are few systematic studies on the adsorption properties of N719molecules on TiO2NS with dominant (001) facets. In this work, the equilibrium, kinetic and thermodynamic data of the N719dye adsorption on TiO2NS with dominant (001) facets are studied and compared with TiO2NP with dominant (101) facets. Anatase TiO2nanosheets (TiO2NS) with dominant (001) facets and TiO2nanoparticles (TiO2NP) with dominant (101) facets are fabricated by the hydrothermal hydrolysis of Ti(OC4H9)4in the presence and absence of HF, respectively. Adsorption of N719onto the as-prepared samples from ethanol solutions is investigated and discussed. The adsorption kinetic data are modeled using the pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetics equations, indicating that pseudo-second-order kinetic equation and intra-particle diffusion model can better describe the adsorption kinetics. Furthermore, adsorption equilibrium data of N719on the as-prepared samples are analyzed by Langmuir and Freundlich models, suggesting that the Langmuir model provides the better correlation of the experimental data. The adsorption capacities (qmax) of N719on TiO2NS at various temperatures determined using the Langmuir equation are65.2(30℃),68.2(40℃) and76.6(50℃) mg g-1, which are smaller than that on TiO2NP,92.4(30℃),100.0(40℃), and108.2(50℃) mg g-1, respectively. The larger adsorption capacities of N719for TiO2NP versus NS are attributed to its higher specific surface areas. However, the specific adsorption capacities (qmax/SBET) at various temperatures are1.5(30℃),1.6(40℃) and1.7(50℃) mg m-2for TiO2NS, which are otherwise higher than that for NP,0.9(30℃),1.0(40℃) and1.1(50℃) mg m-2, respectively. The larger specific adsorption capacities of N719for TiO2NS versus NP are because the (001) surface is more reactive for dissociative adsorption of reactant molecules compared with (101) facets. Notably, the qmax and qmax/SBET for both TiO2samples increase with increasing temperature, suggesting that adsorption of N719on TiO2surface is endothermic process, which is further confirmed by the calculated thermodynamic parameters including free energy, enthalpy and entropy of adsorption process. The present work will provide new understanding on the adsorption process and mechanism of N719molecules onto TiO2NS and NP, which should be of great importance for enhancing the performance of dye-sensitized solar cells.3) Enhanced photovoltaic performance of dye-sensitized solar cells based on TiO2nanosheets/graphene composite films. Very recently, DSSCs based on two-dimensional anatase TiO2nanosheets have been proven to be effective in improving photoelectric conversion efficiency, because their two-dimensional (2D) nano-structures can enhance the light-collection efficiency by multiple light scattering. Owing to the2D nano-structures and excellent electronic conductivity of graphene, numerous attempts have been made to combine graphene with TiO2to enhance their photocatalytic and photoelectric performance. In this work, DSSCs based on TiO2NS/graphene nanocomposite films were for the first time fabricated and the effects of graphene on the microstructures and photoelectric conversion performance of the as-fabricated DSSC were investigated. The graphene loading clearly influences the textural properties and the optical absorption properties. Moreover, the charge transfer and transport versus the charge trapping and recombination is also affected by the graphene loading. As a consequence, the photoelectric conversion efficiency of the TiO2NS/graphene nanocomposite film electrodes can be improved to great extent upon graphene loading, which is dependent on the loading amount of graphene. Moderate amount of graphene (<0.75wt.%) obviously enhanced the DSSC efficiency. Graphene not only reduced the electrolyte/electrode interfacial resistance and the charge recombination rate, but also enhanced the transport of electrons from the films to fluorine doped tin oxide (FTO) substrates. Furthermore, the incorporated graphene improved the light harvesting and thus increased the number of the photoinduced electrons. Besides, the modified porous structures of the composite photoanode facilitated the diffusion of electrolyte in the cell, which in turn helped to regenerate the dye. being important to the photovoltatic response of the solar cells. However, excessive graphene loading (>0.75wt.%) largely lowered the DSSC performance. Higher graphene loading not only impaired the crystallinity of the TiO2NS, but also shielded the light adsorption of the dyes and reduced the number of the photogenerated electrons. This study will provide new insight into fabrication and structural design of highly efficient dye-sensitized solar cells.4) Dye-sensitized solar cells based on anatase TiO2hollow spheres/carbon nanotube composite films. Resently, TiO2hollow structured materials have received extensive attention owing to high specific surface areas, hierarchically nanoporous structures, good photocatalytic activity and enhanced photoelectric conversion efficiency. Furthermore, taking account of the1D nano-structures and good electrical conductivity of carbon nanotubes (CNT), it is reasonable to conclude that CNT/TiO2composites are beneficial to transport the electrons within TiO2films and enhance their photocatalytic and photoelectric conversion efficiencies. In this work, the TiO2HS/CNTs composite films are for the first time applied to prepare the photoanodes of DSSCs. The photoelectric conversion performances of the DSSCs based on TiO2HS/CNT composite film electrodes are also compared with commercial-grade Degussa P25TiO2nanoparticles (P25)/CNT composite solar cells at the same film thickness. The results indicate that the photoelectric conversion efficiencies (η) of the TiO2HS/CNT composite DSSCs are dependent on the amount of CNT loading in the electrodes. A small amount of CNT clearly enhances DSSC efficiency, while excessive CNT loading significantly lowers their performance. The former is because CNT enhance the transport of electrons from the films to FTO substrates. The latter is due to high CNT loading shielding the visible light from being adsorbed by dyes.This study will provide new insight into fabrication and structural design of highly efficient dye-sensitized solar cells.