Self-templated synthesis of large-scale hierarchical anatase titania nanotube arrays on transparent conductive substrate for dye-sensitized solar cells

The large-scale hierarchical anatase titania nanotube arrays on transparent conductive substrate are fabricated via in situ conversion from anatase titania nanowire arrays. The first-step hydrothermal reaction is the growth of ultra-long anatase titania nanowire arrays, and the second-step hydrothermal reaction is the conversion of titania nanowire arrays to titania nanotube arrays modified with a large number of nanosheets. The resultant hierarchical titania nanotube array film provides a large surface area and superior light scattering ability. Dye-sensitized solar cell based on the hierarchical titania nanotube array photoanode obtains a power conversion efficiency as high as 5.96% and shows a prominent increase compared to the pristine titania nanowire array photoanode (2.12%). In addition, the most interesting result is that an optimized efficiency of 7.54% is achieved for the cell based on the hierarchical titania nanotube array photoanode with titania sol modification.     

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.     

CuI-Si heterojunction solar cells with carbon nanotube films as flexible top-contact electrodes

We report the fabrication of CuI-Si heterojunction solar cells with carbon nanotubes (CNTs) as a transparent electrode. A flexible CNT network was transferred onto the top of a polycrystalline CuI layer, making a conformal coating with good contact with the underlying CuI. The solar cells showed power conversion efficiencies in the range of 6% to 10.5%, while the efficiency degradation was less than 10% after the device was stored in air for 8 days. Compared with conventional rigid electrodes such as indium tin oxide (ITO) glass, the flexibility of the CNT films ensures better contact with the active layers and removes the need for press-contact electrodes. Degraded cells can recover their original performance by acid doping of the CNT electrode. Our results suggest that CNT films are suitable electrical contacts for rough materials and structures with an uneven surface.      

Controlled synthesis of ZnO branched nanorod arrays by hierarchical solution growth and application in dye-sensitized solar cells

We demonstrate the controlled synthesis of ZnO branched nanorod arrays on fluorine-doped SnO₂-coated glass substrates by the hierarchical solution growth method. In the secondary growth, the concentration of Zn(NO₃)₂/hexamethylenetetramine plays an important role in controlling the morphology of the branched nanorod arrays, besides that of diaminopropane used as a structure-directing agent to induce the growth of branches. The population density and morphology of the branched nanorod arrays depend on those of the nanorod arrays obtained from the primary growth, which can be modulated though the concentration of Zn(NO₃)₂/hexamethylenetetramine in the primary growth solution. The dye-sensitized ZnO branched nanorod arrays exhibit much stronger optical absorption as compared with its corresponding primary nanorod arrays, suggesting that the addition of the branches improves light harvesting. The dye-sensitized solar cell based on the optimized ZnO branched nanorod array reaches a conversion efficiency of 1.66% under the light radiation of 1000 W/m². The branched nanorod arrays can also be applied in other application fields of ZnO.     

Dye-sensitized solar cells with TiO2 nanocrystalline films prepared by conventional and rapid thermal annealing processes

Titanium oxide (TiO₂) thin films are prepared by the sol-gel method and annealed at 600 °C by conventional (CTA) and rapid thermal annealing (RTA) processes on fluorine-doped tin oxide -coated glass substrates for application as the work electrode for the dye-sensitized solar cells (DSSC). TiO₂ thin films are crystallized using a conventional furnace and the proposed RTA process at annealing rates of 5 °Cmin⁻¹ and 600 °Cmin⁻¹, respectively. The TiO₂ thin films are characterized by X-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller analysis. Based on the results, the TiO₂ films crystallized by RTA show better crystallization, higher porosity and larger surface area than those of CTA. The short-circuit photocurrent and open-circuit voltage values increased from 5.2 mAcm⁻² and 0.6 V for the DSSC with the CTA-derived TiO₂ films to 8.3 mAcm⁻² and 0.68 V, respectively, for the DSSC containing RTA-derived TiO₂ films.     

Preparing thick, defect-free films of anatase titania for dye-sensitized solar cells

Anatase titania (TiO₂) nanoparticle films were prepared on fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO) substrates. The films were characterized by X-ray diffraction, scanning electron microscopy, profilometry, Raman spectroscopy, and optical microscopy. The results show that defects are initiated during the sintering step and continue to propagate once the film is cooled. The sintering and annealing steps were controlled by reducing the pressure and the rate of temperature change. These steps reduced the stresses generated during film preparation, allowing thick titania films on both FTO and ITO substrates to be prepared with minimal defects. Using the optimized conditions for film preparation, 20 μm thick films of titania on FTO and ITO substrates were obtained with calculated defect densities of 2.5 and 7.8%. Films as thick as 25 μm were prepared on FTO substrates with a defect density of only 6.0%. Dye-sensitized solar cells (DSSCs) were fabricated using the titania films prepared by both standard and vacuum sintering methods. DSSCs made with 20 μm titania films sintered at intermediate pressures show improvements to short-circuit current, open-circuit voltage, and device efficiency.     

Templated synthesis of porous TiO2 thin films using amphiphilic graft copolymer and their use in dye-sensitized solar cells

Porous TiO₂ thin films have been prepared using an amphiphilic graft copolymer, i.e. poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-poly(oxyethylene methacrylate) (P(VDF-co-CTFE)-g-POEM) as a structure-directing agent via the sol-gel process. The graft copolymer was synthesized via atom transfer radical polymerization using CTFE units as an initiating site and designed to have a hydrophobic P(VDF-co-CTFE) domain and a hydrophilic POEM domain. Fourier transform-infra red spectroscopy indicated that a hydrophilic titania precursor was selectively incorporated into hydrophilic POEM domains. In-situ formation and morphologies of porous TiO₂ thin films were confirmed by ultraviolet-visible spectroscopy, X-ray diffraction, transmission electron microscopy and thermogravimetric analysis. The resultant porous TiO₂ films with 10-25 nm in size were used as a photoelectrode for solid-state dye-sensitized solar cells, exhibiting energy conversion efficiency of 2.8% at 100 mW/cm².     

A novel synthesis of the bottom-straight and top-bent dual TiO2 nanowires for dye-sensitized solar cells

The bottom-straight and top-bent dual anatase TiO₂ nanowires directly on flurine-doped tin oxide (FTO) glass substrate are successfully synthesized for the first time via a two-step solvothermal reaction, which obtains single anatase TiO₂ nanowires with the diameter of 250–450?nm and the length of ～49?μm in the first growth step, and the slender TiO₂ nanowires with the diameter of 40–80?nm intertwining larger nanowires to form dual nanowires network in the secondary growth step. Dye-sensitized solar cell (DSSC) based on dual TiO₂ nanowires exhibits the power conversion efficiency of 5.30%, which is much higher than those of DSSCs based on the pristine single TiO₂ nanwires (2.36%) due to the twining network like dual nanowires structure maintaining excellent electron transport and simultaneously enhancing the specific surface area and light scattering ability. Moreover, by TiO₂ sol treatment, the performance of dual TiO₂ nanowires based DSSC can be further enhanced, reaching an impressive power conversion efficiency (PCE) of 7.65%.     

Effect of the molecular weight of a polyethylene glycol on the photoluminescence spectra of porous TiO2 films for dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) are expected to be used for future clean energy. In general, when the titania porous electrode in DSCs is made, a polyethylene glycol (PEG) is added to obtain the porous structure. Although the conversion efficiency of the DSC became high when the high molecular weight of PEG was used, its reason was not clear. In the present study, the photoluminescence spectrum of titania films with the different molecular weight of PEG was measured at room temperature, and we discussed the relation between molecular weight and the conversion efficiency of the DSC.     

Glancing angle deposited titania films for dye-sensitized solar cells

A series of sculptured porous nano-columnar titanium oxide films were prepared by glancing angle deposition (GLAD) method using an electron-beam evaporation system. The films were deposited on ITO glasses at various incident angles from 53° to 86°and used as photoanode in a dye-sensitized solar cell (DSSC). The as-deposited TiO₂ films are comprised of helical nano-columns and assembled in an orderly manner with gaps or pores in between. The porous nanostructured films provide a synergetic 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 DSSCs incorporated with the GLAD films of 4 μm thick exhibited a high fill factor (FF) up to 0.77. The TiO₂ film deposited at an incident angle of 73° provides the largest internal surface area and the largest amount of dye absorption and results in the highest light conversion efficiency of 2.78%.     

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%.     

Effects of mesoscopic poly(3,4-ethylenedioxythiophene) films as counter electrodes for dye-sensitized solar cells

Counter electrode coated with chemically polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) in a dye-sensitized solar cell (DSSC) was studied. The surface morphology and the nature of I⁻/I₃⁻ redox reaction based on PEDOT film were investigated using Atomic Force Microscopy and Cyclic Voltammetry, respectively. The performance of the DSSCs containing the PEDOT coated electrode was compared with sputtered-Pt electrode. We found that the root mean square roughness decreases and conductivity increases as the molar ratio of imidazole (Im)/EDOT in the PEDOT film increases. The DSSC containing the PEDOT coated on fluorine doped tin oxide glass with Im/EDOT molar ratio of 2.0, showed a conversion efficiency of 7.44% compared to that with sputtered-Pt electrode (7.77%). The high photocurrents were attributed to the large effective surface area of the electrode material resulting in good catalytic properties for I₃⁻ reduction. Therefore, the incorporation of a multi-walled carbon nanotube (MWCNT) in the PEDOT film, coated on various substrates was also investigated. The DSSC containing the PEDOT films with 0.6 wt.% of MWCNT on stainless steel as counter electrode had the best cell performance of 8.08% with short-circuit current density, open-circuit voltage and fill factor of 17.00 mA cm⁻², 720 mV and 0.66, respectively.     

Template free synthesis of locally-ordered mesoporous titania and its application in dye-sensitized solar cells

A simple template-free synthesis of unique mesoporous titania materials is reported. The synthesis presented is a good, easy and inexpensive method to obtain both ordered and disordered mesoporous titania with tunable and well defined physicochemical properties such us surface area, porosity and crystallinity. The locally-ordered obtained titania was applied as electrode material in dye-sensitized solar cells. The materials prepared were characterized with both wide and small angle XRD, nitrogen adsorption–desorption isotherms, diffuse reflectance UV-Vis, TEM, HRTEM, FT-IR, TGA and DSC measurements. The presented materials showed good activity in photon-to-current conversion process, with overall photoelectric conversion efficiencies reaching up to 5% with fill factors around 60%.     

Enhancing efficiency of dye-sensitized solar cells by combining use of TiO2 nanotubes and nanoparticles

Titanium dioxide nanotubes (TiNTs) were fabricated from commercial P25 TiO₂ powders via alkali hydrothermal transformation. Dye-sensitized solar cells (DSCs) were constructed by application of TiNTs and P25 nanoparticles with various weight percentages. The influence of the TiNT concentration on the performance of DSCs was investigated systematically. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the recombination resistance, electron lifetime and time constant in DSCs both under illumination and in the dark. The DSC based on TiNT/P25 hybrids showed a better photovoltaic performance than the cell purely made of TiO₂ nanoparticles. The open-voltage (V_oc), fill factor (FF) and efficiency (η) continuously increased with the TiO₂ nanotube concentration from 0 to 50 wt%, which was correlated with the suppression of the electron recombination as found out from EIS studies. Respectable photovoltaic performance of ca. 7.41% under the light intensity of 100 mW cm⁻² (AM 1.5G) was achieved for DSCs using 90 wt% TiO₂ nanotubes incorporated in TiO₂ electrodes.     

Template- and surfactant-free synthesis of mesoporous TiO2 spheres with hollow core-shell structure for dye-sensitized solar cells

We have demonstrated a simple and effective hydrothermal route to synthesize titania mesoporous spheres with hollow core-shell structure. The synthesis is free of any surfactants or templates. The formation mechanism is investigated on the basis of the results of time-dependent experiments. The as-obtained mesoporous titania spheres with a specific surface area of 21.5?m²?g^?1 and diameters of 1.2–2.3?μm are composed of anatase titania nanocrystals. The excellent light scattering property of mesoporous titania spheres with hollow core-shell structure is proved. A higher cell efficiency of 8.27% is achieved with mesoporous titania spheres with hollow core-shell structure as a light scattering layer, compared with a cell efficiency of 6.63% for the P25 film electrode with the similar thickness. The higher cell efficiency is attributed to the hollow core-shell structure scattering layer, resulting in excellent pore fitting for electrolyte diffusion, enhanced light scattering ability, and reduced charge recombination.     

Titanium dioxide thin films prepared by electrolysis from aqueous solution of titanium-lactic acid complex for dye-sensitized solar cells

Titanium oxide (TiO_x) thin films were prepared on transparent conducting substrate (fluorine-doped tin oxide) by cathodic electrolysis of a solution containing a titanium bis(ammonium lactato)dihydroxide and an ammonium nitrate at 323 K. Post-deposition treatment: calcination at 723 K or hot-water treatment at > 363 K promoted the growth of an anatase type crystalline phase in the TiO₂ thin film, as evidenced by X-ray diffraction and X-ray photoelectron spectroscopy. The calcined films were used as electrodes of a dye-sensitized solar cells and the cells' energy conversion efficiency was comparable to that obtained with commercially available TiO₂ nanoparticle electrodes.     

Single-graded CIGS with narrow bandgap for tandem solar cells

Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se₂ with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe₂ base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.     

Kinetics of titania nanotube formation by anodization of titanium films

We report the kinetics of titania nanotube length evolution during anodization of titanium films. Our results show that the nanotube length increase is thermally activated, and governed by voltage-dependent activation energy 0.6 eV ≤ E^eff ≤ 1.1 eV expressed by E^eff = E₀-αV_anod where α is a constant and E₀ = 1.6 eV is a voltage-independent term. The proximity of E₀ to that of oxygen diffusion in titania suggests that oxygen transport across the titania walls at the pore bottoms is the rate-limiting step. These results provide insights into the mechanism of titania nanotube formation and a framework for their rational synthesis for applications.     

Characteristics of dye-sensitized solar cells using natural dye

Dye-sensitized solar cells are expected to be used for future clean energy. Recently, most of the researchers in this field use Ruthenium complex as dye in the dye-sensitized solar cells. However, Ruthenium is a rare metal, so the cost of the Ruthenium complex is very high. In this paper, various dye-sensitized solar cells have been fabricated using natural dye, such as the dye of red-cabbage, curcumin, and red-perilla. As a result, it was found that the conversion efficiency of the solar cell fabricated using the mixture of red-cabbage and curcumin was about 0.6% (light source: halogen lamp), which was larger than that of the solar cells using one kind of dye. It was also found that the conversion efficiency was about 1.0% for the solar cell with the oxide semiconductor film fabricated using polyethylene glycol (PEG) whose molecular weight was 2,000,000 and red-cabbage dye. This indicates that the cost performance (defined by [conversion efficiency]/[cost of dye]) of the latter solar cell (dye: red-cabbage) is larger by more than 50 times than that of the solar cell using Ruthenium complex, even if the effect of the difference between the halogen lamp and the standard light source is taken into account.