Vapor deposited sculptured nano-porous titania films by glancing angle deposition for efficiency enhancement in dye-sensitized solar cells

Sculptured porous titania films as photoanode in dye-sensitized solar cell (DSSC) were prepared using an electron-beam evaporation system with glancing angle deposition (GLAD) method. By controlling the substrate rotation rate and the incident angle of evaporant, titania films of various thicknesses were prepared on ITO glasses. The as-deposited nano-porous films are comprised of helical nano-columns and assembled in an orderly manner with gaps or pores in between, which offer large internal surface area for dye adsorption and direct electron transfer path. There is a positive correlation between the film thickness, film effective surface area, amount of absorbed dye and cell efficiency. The nano-porous films provide a synergistic effect of high surface area, effective route for electron transfer, tight interfaces, and enhanced light trapping, which are all beneficial for higher cell efficiency. The DSSC consisting of a 6 μm titania film, deposited at substrate rotating rate 0.17 rpm and incident angle 73°, gave a cell efficiency of 6.1%.     

Sputter deposition and surface treatment of TiO2 films for dye-sensitized solar cells using reactive RF plasma

Sputter deposition followed by surface treatment was studied using reactive RF plasma as a method for preparing titanium oxide (TiO₂) films on indium tin oxide (ITO) coated glass substrate for dye-sensitized solar cells (DSCs). Anatase structure TiO₂ films deposited by reactive RF magnetron sputtering under the conditions of Ar/O₂(5%) mixtures, RF power of 600 W and substrate temperature of 400 °C were surface-treated by inductive coupled plasma (ICP) with Ar/O₂ mixtures at substrate temperature of 400 °C, and thus the films were applied to the DSCs. The TiO₂ films made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼ 25 nm. The DSCs made of this TiO₂ material exhibited an energy conversion efficiency of about 2.25% at 100 mW/cm² light intensity.     

Fabrication and properties of meso-macroporous electrodes screen-printed from mesoporous titania nanoparticles for dye-sensitized solar cells

A meso-macroporous TiO₂ film electrode was fabricated by using mesoporous TiO₂ (m-TiO₂) nanoparticles through a screen-printing technique in order to efficiently control the main fabrication step of dye-sensitized solar cells (DSSCs). The qualities of the screen-printed m-TiO₂ films were characterized by means of spectroscopy, electron microscopy, nitrogen adsorption–desorption and photoelectrochemical measurements. Under the optimal paste composition and printing conditions, the DSSC based on the meso-macroporous m-TiO₂ film electrode exhibits an energy conversion efficiency of 4.14%, which is improved by 1.70% in comparison with DSSC made with commercially available nonporous TiO₂ nanoparticles (P25, Degussa) electrode printed with a similar paste composition. The meso-macroporous structure within the m-TiO₂ film is of great benefit to the dye adsorption, light absorption and the electrolyte transportation, and then to the improvement of the overall energy conversion efficiency of DSSC.     

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

Nanocrystalline anatase TiO₂ thin films with different thicknesses (0.5-2.0 μm) have been deposited on ITO-coated glass substrates by a sol-gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO₂ films can increase adsorption of the N3 dye through TiO₂ layers to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc of DSSC with a TiO₂ film thickness of 2.0 μm (8.5 mA/cm² and 0.61 V) are smaller than those of DSSC with a TiO₂ film thickness of 1.5 μm (9.2 mA/cm² and 0.62 V). It could be due to the fact that the increased thickness of TiO₂ thin films also resulted in a decrease in the transmittance of TiO₂ thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO₂ film thickness of 1.5 μm.     

Charge recombination reduction in dye-sensitized solar cells by depositing ultrapure TiO2 nanoparticles on “inert” BaTiO3 films

Ultrapure TiO₂ nanoparticles (∼5 nm in size) were supported on “inert” BaTiO₃ films by TiCl₄ treatment, which was used to fabricate dye-sensitized solar cells (DSSCs). The optimized electrode, designated as BaTiO₃/TiO₂(4), was obtained upon four cycles of TiCl₄ treatment. DSSC with BaTiO₃/TiO₂(4) electrode exhibits superior power conversion efficiency (PCE) compared to that with conventional anatase TiO₂ (∼25 nm in size) electrode. The interfacial charge recombination kinetics was investigated by electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). In contrast to DSSC with anatase TiO₂ electrode, the dramatically enhanced electron lifetime for DSSC with BaTiO₃/TiO₂(4) electrode could be attributed to the decrease of recombination reaction at the TiO₂ photoelectrode/electrolyte interface. It is proposed that the lower interfacial charge recombination can be related to the relatively shallower trap distributions in DSSC with BaTiO₃/TiO₂(4) electrode.     

Overcoming cap layer cracking for glancing-angle deposited films

Porous thin films deposited by glancing-angle deposition (GLAD) have found application as sensor, micro-electrical mechanical systems and microfluidic devices. However, conventional micro-fabrication techniques can damage the very properties which make GLAD films attractive for these applications. To facilitate integration of GLAD films with these processes, a capping layer may be used. Such capping layers must be as free of defects as possible to ensure that the GLAD film is well protected. Here, the cracking properties of evaporated TiO₂ caps deposited on GLAD films have been investigated as a function of substrate temperature. Our films are a porous vertical post layer 2.7 μm thick capped with a solid 400 nm layer. This material system experiences tensile stress from two sources: thermal mismatch between the film and substrate, and intrinsic stress in the cap from Volmer-Weber coalescence. Crack properties such as crack length distribution, crack density and branch number were quantified. In general, higher substrate temperatures reduced crack density, branch number and preferentially eliminated longer cracks. The onset of crystallinity at substrate temperatures around 300 °C briefly increases crack area and branch number, but a further reduction can be achieved by depositing above this temperature. Applications of films grown by GLAD requiring high-quality capping layers will benefit from this study.     

Optimization of the cutting process of multi-wall carbon nanotubes for enhanced dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated based on multi-wall carbon nanotube (MWCNT)-TiO₂ photo-anodes, which were prepared by the procedures of cutting MWCNTs and subsequent immobilization TiO₂ on MWCNTs. Through a detailed study, we found that cut-MWCNTs with proper ultrasonication time (2 h) and proper content (0.075%) resulted in 58 and 40% increase in short-circuit photocurrent and overall energy conversion efficiency, respectively, compared with that of a DSSC using only TiO₂ photo-anode. The enhancement of cut-MWCNTs for DSSC was attributed to the introduction of percolative conductive paths which facilitate the rapid electron transfer.     

Improving conversion efficiency of dye-sensitized solar cells by metal plasma ion implantation of ruthenium ions

Dye-sensitized solar cells (DSSC) are based on the concept of photosensitization of wide-band-gap mesoporous oxide semiconductors. At present, DSSC have ventured into advanced development and pilot production. Our current research emphasizes on improvements on titanium dioxide (TiO₂) photosensitivity under visible light irradiation by using metal plasma ion implantation (MPII). The anatase TiO₂ electrode was prepared via a sol-gel process and deposited onto indium-tin oxide glass substrates. Subsequently, the as-deposited TiO₂ films were subjected to MPII at 20 keV in order to incorporate ruthenium (Ru) atoms onto the TiO₂ surface layer. The Ru-implanted TiO₂ thin film possessed nanocrystalline Ru clusters of 20 nm in diameter and distributed in near surface layer of TiO₂ films. The Ru clusters showed effective in both prohibiting electron-hole recombination and generating additional Ru-O impurity levels for the TiO₂ band gap structure. A significant reduction of TiO₂ band gap energy from 3.22 to 3.11 eV was achieved, which resulted in the extension of photocatalysis of TiO₂ from UV to Vis regime. A small drop of photoelectric performance of 8% was obtained due to the incorporation of Ru atoms in the surface layer of TiO₂, a similar side effect as observed in the Fe-implanted TiO₂. However, the overall retention of the photocatalysis capability is as high as 92% when switch from UV to Vis irradiation. The improvement of the photosensitivity of TiO₂ DSSC by means of metal plasma ion implantation is promising.     

Surface treatment of TiO2 films for dye-sensitized solar cells using atmospheric-pressure non-equilibrium DC pulse discharge plasma jet

In this paper, we report the utilization of the DC pulse discharge plasma jet technique as a means for the preparation of titanium oxide (TiO₂) films on fluorine dope tin oxide (FTO) coated glass substrates used for dye-sensitized solar cells (DSCs). The TiO₂ film made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼25 nm. The DSCs made by the TiO₂ film exhibited an energy conversion efficiency of 5.7% at 100 mW/cm² light intensity. Consequently, we believe that the optimization between the specific surface area and photocurrent density of TiO₂ film was achieved by the plasma surface treatment which also contributed to the improvement of energy conversion efficiency of DSCs.     

The role of the TiO2 nanotube array morphologies in the dye-sensitized solar cells

Arrays of TiO₂ nanotubes were fabricated by the anodization of Ti foils and then used in assembling dye-sensitized solar cells (DSSCs). The role of the morphologies of the TiO₂ nanotubes in the photovoltaic performances of the DSSCs was studied in terms of the surface topography and the tube length. The necessity of removing the nanoporous films from the surface of the nanotube arrays for good DSSC performance has been demonstrated. Also, it has been shown that appropriately increasing the tube length was an effective measure for enhancing both the short-circuit current density and the conversion efficiency of the DSSCs.     

A simple electrophoretic deposition method to prepare TiO2-B nanoribbon thin films for dye-sensitized solar cells

This paper describes a simple method utilizing electrophoretic deposition (EPD) to quickly synthesize hydrogen titanate nanoribbon films. The subsequent heating of the hydrogen titanate nanoribbon films causes the dehydration of interlayered OH groups, thereby leading to TiO₂-B nanoribbon films. Thick, uniform TiO₂-B nanoribbon films were obtained from prepared alkali suspensions. The crystal structure of the hydrogen titanate and TiO₂-B nanoribbon films obtained from EPD underwent analysis by X-ray diffraction and high-resolution transmission electron microscope. EPD controlled the thickness of TiO₂-B nanoribbons films. TiO₂-B-coated fluorine-doped tin oxide films were dye-sensitized with N3 and used as a photoanode in an electrochemical solar cell. The solar cell yielded conversion efficiencies of 0.87% for an incident solar energy of 100 mW/cm².     

The optical properties of nanoporous structured titanium dioxide and the photovoltaic efficiency on DSSC

This study examined the characterization of nanoporous structured titanium dioxide and its application to dye-sensitized solar cells (DSSCs). TEM revealed nanopore sizes of 10.0 nm with a regular hexagonal form. When nanoporous structured TiO₂ was applied to DSSC, the energy conversion efficiency was enhanced considerably compared with that using nanometer sized TiO₂ prepared using a hydrothermal method. The energy conversion efficiency of the DSSC prepared from nanoporous structured TiO₂ was approximately 8.71% with the N719 dye under 100 mW cm⁻² simulated light. FT-IR spectroscopy showed that the dye molecules were attached perfectly to the surface and more dye molecules were absorbed on the nanoporous structured TiO₂ than on the nano-sized TiO₂ particles prepared using a conventional hydrothermal method. Electrostatic force microscopy (EFM) showed that the electrons were transferred rapidly to the surface of the nanoporous structured TiO₂ film.     

Progress in nanostructured photoanodes for dye-sensitized solar cells

Solar cells represent a principal energy technology to convert light into electricity. Commercial solar cells are at present predominately produced by single- or multi-crystalline silicon wafers. The main drawback to silicon-based solar cells, however, is high material and manufacturing costs. Dye-sensitized solar cells (DSSCs) have attracted much attention during recent years because of the low production cost and other advantages. The photoanode (working electrode) plays a key role in determining the performance of DSSCs. In particular, nanostructured photoanodes with a large surface area, high electron transfer efficiency, and low electron recombination facilitate to prepare DSSCs with high energy conversion efficiency. In this review article, we summarize recent progress in the development of novel photoanodes for DSSCs. Effect of semiconductor material (e.g. TiO₂, ZnO, SnO₂, N₂O₅, and nano carbon), preparation, morphology and structure (e.g. nanoparticles, nanorods, nanofibers, nanotubes, fiber/particle composites, and hierarchical structure) on photovoltaic performance of DSSCs is described. The possibility of replacing silicon-based solar cells with DSSCs is discussed.     

Probing mechanism of dye double layer formation from dye-cocktail solution for dye-sensitized solar cells

Absorption of photon in wide wavelength region is an important requirement for the enhancement of photoconversion efficiency of dye-sensitized solar cells (DSSC). Lack of photon absorption from visible to NIR wavelength region by a single dye requires the use of plural dyes for the panchromatic sensitization of nanoporous TiO₂. To our incredible surprise, when a dye cocktail of organic dye NK3705 and inorganic ruthenium based dye Z907 was implied for the dye adsorption, it led to the formation of dye double layer in spite of random arrangement of two dyes as evidenced from confocal laser microscopic investigations. Investigation pertaining to the evaluation of rate of dye adsorption and dye desorption for different organic and inorganic sensitizing dyes suggests that a combination of one dye with faster diffusion along with weak binding on TiO₂ surface and another dye with slow diffusion along with strong binding leads to the formation of dye double layer from a dye mixture by a simple dipping process.     

Fabrication of dye-sensitized solar cells using TiO2-nanotube arrays on Ti-grid substrates

Self-aligned TiO₂ nanotube arrays (20 μm in length) were fabricated by anodic oxidation of Ti-grid with a thickness of 100 μm in an ethylene glycol electrolyte with an addition of H₂O (1.5 vol.%) and NH₄F (0.2 wt.%). Voltage applied between Ti and Pt cathodes is 60 V at ~ 22 °C. Dye-sensitized solar cell utilizing photoanode structure of TiO₂-nanotube/Ti-grid was fabricated with no transparent conducting oxide (TCO) layer, in which Ti-grid replaces TCO. Overall photoconversion efficiency is very low (< 0.5%) due to the large pore size (100 nm in diameter) of the nanotubes, which may cause insufficient dye molecules to be attached, thus limiting light harvesting.     

All-solid,flexible solar textiles based on dye-sensitized solar cells with ZnO nanorod arrays on stainless steel wires

An all-solid, flexible solar textile fabricated with dye-sensitized solar cells (DSSCs) woven into a satin structure and transparent poly(ethylene terephthalate) (PET) film was demonstrated. A ZnO nanorod (NR) vertically grown from fiber-type conductive stainless steel (SS) wire was utilized as a photoelectrode, and a Pt-coated SS wire was used as a counter electrode. A graft copolymer, i.e. poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM) was synthesized via atom transfer radical polymerization (ATRP) and used as a solid electrolyte. The conditions for the growth of ZnO NR and sufficient dye loading were investigated to improve cell performance. The adhesion of PET films to DSSCs resulted in physical stability improvements without cell performance loss. The solar textile with 10 × 10 wires exhibited an energy conversion efficiency of 2.57% with a short circuit current density of 20.2 mA/cm² at 100 mW/cm² illumination, which is the greatest account of an all-solid, ZnO-based flexible solar textile. DSSC textiles with woven structures are applicable to large-area, roll-to-roll processes.     

Structures and properties of titania thin films annealed under different atmosphere

The effects of crystallinity, phase and oxygen vacancies on optical and photocatalytic properties of titania (TiO₂) thin films were systematically studied. The as-deposited amorphous titania films were prepared by reactive sputtering titanium metal targets in argon–oxygen plasma at 100 °C and subsequently annealed at various temperatures of 400–800 °C in air, vacuum and H₂ atmosphere. The results indicate that in general the crystallinity of the annealed films is enhanced with the increasing annealing temperature. At the same temperature, the H₂ annealed films achieve better crystallinity but containing more oxygen vacancies than the films annealed in air and in vacuum. In H₂ or in vacuum, the concentration of oxygen vacancies in the annealed films increases with increasing temperature, while in air it remains constant. Oxygen vacancies in titania film not only facilitate phase transformation but also lower the band gap of titania, and make the film visible-light responsive. Photocatalytic properties of the TiO₂ films were characterized in UV and visible light irradiation by following the Ag reduction and degradation of methylene blue. The films annealed at 600–700 °C in H₂ possess the best film crystallinity and the proper concentration of oxygen vacancies and exhibit the best photocatalytic performance under both UV and visible light.     

Influence of gas flow during vacuum cold spraying of nano-porous TiO2 film by using strengthened nanostructured powder on performance of dye-sensitized solar cell

Nano-porous TiO₂ films, which can be applied to the flexible dye-sensitized solar cell (DSC), were deposited by vacuum cold spraying at room temperature with the strengthened nanostructured TiO₂ powder as feedstock. The spraying was conducted under different accelerating gas flows resulting in various particle velocities. Results show that the short-circuit photocurrent density of the cell (N719 dye) increases from 8.3 to 9.8 mA/cm² with the increase in gas flow from 3 to 7.5 L/min. A maximum overall energy conversion efficiency of 4.2% was obtained for the DSC with the TiO₂ film deposited at the gas flow of 7.5 L/min. The influence of particle velocity on the electron transport parameters and cell performance was discussed to reveal the important role of particle velocity in the formation of particle connection through high impact pressure during vacuum cold spraying.     

Tunable synthesis of mesoporous titania with different morphologies for dye-sensitized solar cells

Different TiO₂ mesoporous structures, including core-shell spheres (CCSs) and micro-tubes (MTs), are synthesized through adjusting the pH of the solution using TiOSO₄ as titanium source in a hydrothermal route. TiO₂ CSSs with an average diameter of 1.3–3.5 μm exhibit excellent light scattering property and high specific surface area (177.63 m² g^?1). TiO₂ MTs show ultrahigh specific surface area of 276.03 m² g^?1. Dye-sensitized solar cell is fabricated using TiO₂ CSSs as the light scattering layer and TiO₂ nanoparticles (NPs) layer as the bottom layer. The efficiency of Cell-NPs + CSSs is up to 9.24% due to the good light scattering effect and excellent dye loading capacity. Furthermore, TiO₂ MTs are introduced to form the NPs/MTs bottom layer. The Cell-NPs/MTs + CSSs achieves an outstanding efficiency of 9.60% due to the further optimized electron transport path.