Effect of N719 Dye Dipping Temperature on the Performance of Dye-Sensitized Solar Cell

Preparation parameter of dye plays important role in determining DSSC performance. This paper reports the influence of N719 dye dipping temperature on the optical properties and performance parameters of the DSSC utilizing TiO₂ films prepared via microwave technique. The TiO₂ coated N719 dye films were prepared at various temperatures, namely, 30, 50, 60, 70 and 80°C. It is found that the TiO₂ film dipped into N719 dye solution at 50°C possesses the broadest optical absorption window and the highest dye loading. It is also found that the dye dipping temperature does not affect the leak current in the device. The short-circuit current density (J_SC) and power conversion efficiency (η) are strongly influenced by the dipping temperature. The DSSC utilizing the sample prepared at 50°C demonstrated the highest J_SC and η of 4.06 mA cm^–2 and 1.36%, respectively due to highest dye loading and recombination resistance.     

Preparation and characterization of highly photoactive nanocrystalline TiO<Subscript>2</Subscript> powders by solvent evaporationinduced crystallization method

Highly photoactive bi-phase nanocrystalline TiO₂ photocatalyst was prepared by a solvent evaporation-induced crystallization (SEIC) method, and calcined at different temperatures. The obtained TiO₂ photocatalyst was characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET surface areas. The photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air. The results show that solvent evaporation can promote the crystallization and phase transformation of TiO₂ at 100°C. When calcination temperatures are below 600°C, the prepared TiO₂ powders show bimodal pore size distributions in the mesoporous region. At 700°C, the pore size distributions exhibit monomodal distribution of the inter-aggregated pores due to the collapse of the intra-aggregated pores. At 100°C, the obtained TiO₂ photocatalyst by this method shows good photocatalytic activity, and at 400°C, its photocatalytic activity exceeds that of Degussa P25. This may be attributed to the fact that the prepared TiO₂ photocatalyst has higher specific surface areas, smaller crystallite size and bimodal pore size distribution.     

N-doped TiO<Subscript>2</Subscript> applied in low-temperature-based dye-sensitized solar cells

With urea as nitrogen source, N-doped TiO₂ powders were synthesized and fabricated for low-temperature dye-sensitized solar cells (DSSCs) by the method of doctor-blade, and the highest temperature of the whole process was 120 °C. SEM, TEM, XRD, DRS, and XPS were used to analyze the microstructure of the N-doped TiO₂ powders. EIS, Bode plot, UV–Vis and I–V were employed to measure the photovoltaic performance of the DSSCs. The maximum photoelectric conversion efficiency (η) was 5.18 % when the amount of the doped nitrogen was 4 %, and, when compared with the η of 4.22 % for pure TiO₂, the short circuit current was increased by 22.2 % and the efficiency was increased by 22.7 %. It has been shown that the doped nitrogen could effectively suppress TiO₂ crystal phase transition from anatase to rutile, and decrease the size of particles. Therefore, the increased photoelectric conversion efficiency of the N-doped TiO₂-based DSSC was ascribed to the more suitable crystal phase, sizes and inner structure.     

Dye-Sensitized Solar Cell Utilizing TiO<Subscript>2</Subscript> Nanostructure Films: Effect of Synthesis Temperature

This paper is concerned with a study of the influence of synthesis temperature on the properties of TiO₂ films and the performance of dye-sensitized solar cell (DSSC). The TiO₂ film samples synthesized via liquid phase deposition for 5 h at various temperatures, namely, 40, 50, 60, 70 and 80°C. It was found that the morphological shape of the film changes with growth temperature. The optical absorption increases with growth temperature. However, the photoluminescence decreases with growth temperature. These TiO₂ samples were applied in a DSSC of ITO/TiO₂/electrolyte/platinum. The DSSC utilizing the sample grown at 40°C demonstrated the highest photovoltaic parameters with the J_sc, and η of 1.40 mA cm^–2 and 0.44% respectively. This is due to the smallest grain size of TiO₂ films and the smallest bulk resistance of the device.     

Extensive enhancement in power conversion efficiency of dye-sensitized solar cell by using Al-doped TiO<Subscript>2</Subscript> photoanode

In this work, we have prepared Al-doped TiO₂ nanoparticles via a hydrothermal method and used it for making photoanode in dye-sensitized solar cell (DSSC). Material characterizations were done using XRD, AFM, SEM, TEM and EDAX. XPS results reveal that Al is introduced successfully into the structure of TiO₂ creating new impurity energy levels in the forbidden gap. This resulted in tuning of the conduction band of TiO₂ and reduced charge recombination which led to better current conversion efficiency of DSSC. Greater dye loading and enhanced surface area was obtained for Al-doped TiO₂ compared to un-doped TiO₂. I-V analysis, EIS and Bode plots are employed to evaluate photovoltaic performance. The short-circuit current density (J _sc) and efficiency (η) of cell employing Al-doped TiO₂ photoanode were extensively enhanced compared to the cell using un-doped TiO₂. The optical band gap (E _g) for Al-doped and un-doped TiO₂ was obtained as 2.8 and 3.2 eV, respectively. J _sc and η were 13.39 mAcm^?2 and 4.27%, respectively, under illumination of 100 mWcm^?2 light intensity when thin films of 1% Al-doped TiO₂ was employed as photoanode in DSSC using N719 as the sensitizer dye. With the use of un-doped TiO₂ as photoanode under similar conditions, J _sc 5.12 mAcm^?2 and η 1.06% only could be obtained. The maximum IPCE% obtained with Al-doped TiO₂ and un-doped TiO₂ was 67 and 38% respectively at the characteristic wavelength of dye (λ _max = 540 nm). The EIS analyses revealed resistive and capacitive elements that provided an insight into various interfacial processes in terms of the charge transport. It was observed that Al-doping reduced the interfacial resistance leading to better charge transport which has improved both photocurrent density and conversion efficiency. Higher electron mobility and fast diffusion resulting in greater charge collection efficiency was obtained for Al-doped TiO₂ compared to the un-doped TiO₂. Using the Mott–Schottky plot, the donor density was calculated for un-doped and Al-doped TiO₂. The work demonstrated that the Al-doped TiO₂ is potential photoanode material for low-cost and high-efficiency DSSC.     

Low-temperature versus oxygen plasma treatment of water-based TiO2 paste for dye-sensitized solar cells

High-temperature treatment steps in fabrication process of dye sensitized solar cell (DSSC) significantly contribute to the manufacturing costs and limit the use of temperature sensitive substrates. Therefore our aim was to develop a simple method for the preparation of water-based TiO₂ paste. The paste is based on peroxotitanic acid (PTA) sol–gel matrix and commercial TiO₂ nanoparticles (P25). Two fabrication processes to decompose/transform the PTA matrix in the printed TiO₂ layer are explored and combined: annealing at temperatures up to 250 °C and/or oxygen plasma treatment. The results show that the PTA matrix in the paste converts to anatase phase and to some extent also attaches to the TiO₂ nanoparticles P25 acting as an interconnecting network within TiO₂ layer. The transformation of the PTA matrix occurs around 250 °C, but in the presence of TiO₂ nanoparticles P25 it starts already at 120 °C. In addition the results reveal that the crystallization is achievable also solely with the oxygen plasma treatment. The efficiency of the TiO₂ layers, exposed to different post-deposition treatments, is evaluated in DSSCs. The results show that oxygen plasma treatment of the TiO₂ layers could efficiently replace temperature curing at 250 °C. Within this study the DSSCs with the efficiency up to 4.2 % measured under standard test conditions (1,000 W/m², AM1.5, 25 °C) were realized.     

Study of quasi-solid electrolyte in dye-sensitized solar cells using surfactant as pore-forming materials in TiO<Subscript>2</Subscript> photoelectrodes

A novel polymer gel electrolyte was used to improve the performance and long-term stability in dye-sensitized solar cells (DSSCs). The polymer gel electrolyte (PGE) was prepared by mixing 5 wt% poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and 2 % TiO₂ nanoparticles. The conductivity of PGE with P25 reached 9.98 × 10^?3 S/cm, which increased by 34.9 % compared with 7.40 × 10^?3 S/cm of PGE without P25, and the diffusion coefficient was also increased by 19.0 %. Different photoelectrodes were obtained by using three kinds of surfactants (cetylamine, octadecylamine, and P123) as pore-forming materials, and their morphologies were contrasted through scanning electron microscopy (SEM). The results showed that gel electrolyte can increase the short-circuit current density (J _sc) from 11.01 to 12.99 mA/cm² in DSSCs. Moreover, unlike the liquid electrolyte, the gel electrolyte is more conducive to the TiO₂ photoelectrodes with larger pores. In conclusion, the efficiency of DSSC with gel electrolyte and P123 as pore-forming material was 6.73 %, which was 12 % higher than the liquid electrolyte in the same test condition. In addition, the sealed gel electrolyte DSSCs showed better stability than did liquid electrolyte DSSCs during nearly 600 h.     

Structurally Stabilized Organosilane‐Templated Thermostable Mesoporous Titania

Structurally thermostable mesoporous anatase TiO₂ (m‐TiO₂) nanoparticles, uniquely decorated with atomically dispersed SiO₂, is reported for the first time. The inorganic Si portion of the novel organosilane template, used as a mesopores‐directing agent, is found to be incorporated in the pore walls of the titania aggregates, mainly as isolated sites. This is evident by transmission electron microscopy and high‐angle annular dark field scanning transmission electron microscopy, combined with electron dispersive X‐ray spectroscopy. This type of unique structure provides exceptional stability to this new material against thermal collapse of the mesoporous structure, which is reflected in its high surface area (the highest known for anatase titania), even after high‐temperature (550 °C) calcination. Control of crystallite size, pore diameter, and surface area is achieved by varying the molar ratios of the titanium precursor and the template during synthesis. These mesoporous materials retain their porosity and high surface area after template removal and further NaOH/HCl treatment to remove silica. We investigate their performance for dye‐sensitized solar cells (DSSCs) with bilayer TiO₂ electrodes, which are prepared by applying a coating of m‐TiO₂ onto a commercial titania (P25) film. The high surface area of the upper mesoporous layer in the P25–m‐TiO₂ DSSC significantly increases the dye loading ability of the photoanode. The photocurrent and fill factor for the DSSC with the bilayer TiO₂ electrode are greatly improved. The large increase in photocurrent current (ca. 56 %) in the P25–m‐TiO₂ DSSC is believed to play a significant role in achieving a remarkable increase in the photovoltaic efficiency (60 %) of the device, compared to DSSCs with a monolayer of P25 as the electrode.     

The effects of the nanostructure of mesoporous TiO<Subscript>2</Subscript> on optical band gap energy

Mesoporous TiO₂ is prepared by sol–gel process with a triblock copolymer as an organic template and aqueous TiOCl₂ solution as inorganic precursor. The XRD patterns reveal that only the anatase phase can be observed in mesoporous TiO₂, regardless of the different calcining temperatures, and with increasing calcining temperature the grain size gradually increases. The grain sizes of TiO₂ increased from 4.7 to 11.9 nm with calcining temperature increasing from 300 to 400 °C. The pore size and the surface area evaluated from the Barrett–Joyner–Halenda model and Brunauer–Emmett–Teller method indicated that the average pore sizes increased from 87 to 153 Å and specific surface areas decreased from 179.71 to 74.31 m²/g for 300–400 °C calcination. The relationship between the optical band gap (E _g) and microstructure of anatase has been determined and discussed. The quantum confinement effect is observed at grain sizes lower than 10 nm, and the estimated E _g shifts from 3.32 to 3.46 eV. These results suggest that there are potential applications of mesostructured TiO₂ with nanocrystals in the design of optical devices and photocatalysts.     

Morphology and photoelectrochemical properties of TiO2 electrodes prepared using functionalized plant oil binders

Chemically functionalized plant oils, viz. acrylated epoxidized soybean oil (AESO) and maleinized acrylated epoxidized soybean oil (MAESO), were used as bio-based binders for the TiO₂ electrodes of dye-sensitized solar cells (DSSC). The surface roughness and number of appropriate pores were increased in the TiO₂ films prepared using the plant oil binders in comparison with the film prepared using polyethylene glycol (PEG), due to the larger number of functionalities. The short circuit photocurrent (I_SC) and open circuit photovoltage (V_OC) were increased, and the conversion efficiency was significantly improved, in the cell using the plant oil binders.     

Preparation of large-area dye-sensitized solar cells based on hydrothermally synthesized nitrogen-doped TiO2 powders

Low-cost, yellowish, nanocrystalline nitrogen-doped titanium dioxide (N-doped TiO₂) powder was synthesized by a hydrothermal method. The as-prepared N-doped TiO₂ powder was characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–Vis absorption spectra, X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller analysis techniques. The grain size of the prepared powder was around 13 nm as estimated by both Scherrer’s method and TEM images. The effect of the ratio of N-doped TiO₂ particles to Degussa P25 on the photovoltaic performance of large-area dye-sensitized solar cells (DSSCs) was also investigated. The N-doped TiO₂ electrode showed higher photovoltaic performance compared with that of pure P25 at constant irradiation of 100 mW cm^?2, which is attributed to the large pore size and high surface area of N-doped TiO₂ resulting in the introduction of extra charge carrier pathways that could be beneficial for overall charge transportation. Energy conversion efficiency of 5.12 % was achieved in a DSSC device with active area of 51.19 cm².     

TiO2 Thin Film with Varied Porous Structure Applied on the Degradation of Methylene Blue

In this study, porous TiO₂ thin films were prepared by the sol‐gel method employing polyethylene glycol 1000 (PEG 1000) as an organic template. Pore sizes were adjusted by varying the concentration of PEG 1000. The optimal PEG concentration range required to form TiO₂ films with a regular porous structure was investigated and was found to be 0.01–0.015 M. As the PEG 1000 concentration increased, the surface of these films became rougher because of larger pores. Degradation of methylene blue (MB) under UV irradiation was used to determine the photocatalytic activity of the films. In addition, the effect of the pH value of the MB solution on the films was evaluated by controlling its pH value at 5, 7, and 9. The results showed that the photocatalytic activity was correlated to the pore size and pore density of the thin films. TiO2 thin films possessing pore sizes in the diameter range of 35–85 nm exhibited the best conversion of 98% after 8 h of UV irradiation when the pH value was 7.     

Dye sensitization of a large band gap semiconductor by an iron(III) complex

The Fe(III) complex, [Fe^III(HQS)₃] (HQS = 8-hydroxyquinoline-5-sulfonic acid), is found to effect sensitization of the large band gap semiconductor, TiO₂. The role of interfacial electron transfer in sensitization of TiO₂ nanoparticles by surface adsorbed [Fe^III(HQS)₃] was studied using femtosecond time scale transient absorption spectroscopy. Electron injection has been confirmed by direct detection of the electron in the conduction band. A TiO₂-based dye-sensitized solar cell (DSSC) was fabricated using [Fe^III(HQS)₃] as a sensitizer, and the resulting DSSC exhibited an open-circuit voltage value of 425 mV. The value of the short-circuit photocurrent was found to be 2.5 mA/cm². The solar to electric power conversion efficiency of the [Fe^III(HQS)₃] sensitized TiO₂-based DSSC device was 0.75 %. The results are discussed in the context of sensitization of TiO₂ by other Fe(II)-dye complexes.     

Sponge-like TiO2 layers for dye-sensitized solar cells

A titanium oxide layer used in a dye-sensitized solar cell (DSSC) has to meet two opponent properties to enable high conversion efficiency: a large surface area (for high dye loading) and good connection between TiO₂ grains (for efficient extraction of electrons toward the front contact). In order to meet a trade-off between these criteria a preparation method for TiO₂ paste formulation based on Pechini sol–gel method and commercial nanocrystalline TiO₂ powder has been developed. A series of TiO₂ pastes with different molar ratios between titanium isopropoxide, citric acid and ethylene glycol (1:X:4X) in the paste have been examined. The structure and morphology as well as cross-cut tests of deposited and sintered TiO₂ layers have been analyzed. Results reveal that the paste with X = 8 exhibits the best properties, resulting in an overall conversion efficiency of DSSC under standard test conditions (100 mW/cm², 25 °C, AM 1.5G) up to 6.6% for ionic liquid based electrolyte.     

Rubbery copolymer electrolytes containing polymerized ionic liquid for dye-sensitized solar cells

A novel type of random copolymer comprised of a polymerized ionic liquid, poly(1-((4-ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII), and amorphous rubbery poly(oxyethylene methacrylate) (POEM) was synthesized and employed as a solid electrolyte in an I₂-free dye-sensitized solar cell (DSSC). The copolymer electrolytes deeply infiltrated into the nanopores of mesoporous TiO2 films, resulting in improved interfacial contact of electrode/electrolyte. The glass transition temperature (T _g) of the PEBII–POEM (?23 °C) was lower than that of PEBII homopolymer (?4 °C), indicating greater chain flexibility in the former. However, the DSSC efficiency of PEBII–POEM (4.5 % at 100 mW/cm²) was lower than that of PEBII (5.9 %), indicating that ion concentration is more important than chain flexibility. Interestingly, upon the introduction of ionic liquid, i.e., 1-methyl-3 propylimidazolium iodide, the efficiency of PEBII remained almost constant (5.8 %), whereas that of PEBII–POEM was significantly improved up to 7.0 % due to increased I^? ion concentration, which is one of the highest values for I₂-free DSSCs.     

Flexible fiber-type dye-sensitized solar cells based on highly ordered TiO2 nanotube arrays

A double-sided, transparent conducting and flexible dye-sensitized solar cell (DSSC) was developed. The device comprised two metal electrodes whereby the working electrode consisted of highly ordered titania (TiO₂) nanotube arrays. The maximum conversion efficiency of the DSSC was 5.1% and decreased by 6% under a 90° bending. Surface treatment of the TiO₂ nanotube arrays in niobium isopropoxide solution lifted the conversion efficiency to 6.8%.     

Erratum to: Sol–gel synthesis of mesoporous WO3–TiO2 composite thin films for photochromic devices

Mesoporous WO₃–TiO₂ composite films were prepared by a sol gel based two stage dip coating method and subsequent annealing at 450, 500 and 600 °C. An organically modified silicate based templating strategy was adopted in order to obtain a mesoporous structure. The composite films were prepared on ITO coated glass substrates. The porosity, morphology, and microstructures of the resultant products were characterized by scanning electron microscopy, N₂ adsorption–desorption measurements, μ-Raman spectroscopy and X-ray diffraction. Calcination of the films at 450, and 500 °C resulted in mixed hexagonal (h) plus monoclinic phases, and pure monoclinic (m) phase of WO₃, respectively. The degree of crystallization of TiO₂ present in these composite films was not evident. The composite films annealed at 600 °C, however, consist of orthorhombic (o) WO₃ and anatase TiO₂. It was found that the o-WO₃ phase was stabilized by nanocrystalline anatase TiO₂. The thus obtained mesoporous WO₃–TiO₂ composite films were dye sensitized and applied for the construction of photochromic devices. The device constructed using dye sensitized WO₃–TiO₂ composite layer heat treated at 600 °C showed an optical modulation of 51 % in the NIR region, whereas the devices based on the composite layers heat treated at 450, and 500 °C showed only a moderate optical modulation of 24.9, and 38 %, respectively. This remarkable difference in the transmittance response is attributed to nanocrystalline anatase TiO₂ embedded in the orthorhombic WO₃ matrix of the WO₃–TiO₂ composite layer annealed at 600 °C.     

Synthesis of porous Cu–Sn using freeze-drying process of CuO–SnO2/camphene slurries

Porous Cu–Sn with controlled pore characteristics was synthesized by a freeze-drying and sintering process. CuO and SnO₂ powders were selected as the source material, which are hydrogen-reduced to metallic Cu–Sn in the sintering stage. Camphene-based CuO–SnO₂ slurries were prepared by milling at 60 °C with a small amount of dispersant. Freezing of a slurry was done at ?40 °C with unidirectional control of the growth direction of the camphene. Pores were generated by sublimation of the camphene. The green bodies were sintered at 650 °C under a hydrogen atmosphere. The sintered bodies with Cu₃Sn, Cu₆Sn₅ and β-Sn phases showed macroscopic aligned pores with an average size of 200 μm. The internal wall of the macroscopic pores is also porous, and there are a number of many small pores in it. The formation of macroscopic and microscopic pores was discussed in terms of solidification behavior of the liquid with foreign particles.     

Sol–gel synthesis of mesoporous WO3–TiO2 composite thin films for photochromic devices

Mesoporous WO₃–TiO₂ composite films were prepared by a sol gel based two stage dip coating method and subsequent annealing at 450, 500 and 600 °C. An organically modified silicate based templating strategy was adopted in order to obtain a mesoporous structure. The composite films were prepared on ITO coated glass substrates. The porosity, morphology, and microstructures of the resultant products were characterized by scanning electron microscopy, N₂ adsorption–desorption measurements, μ-Raman spectroscopy and X-ray diffraction. Calcination of the films at 450, and 500 °C resulted in mixed hexagonal (h) plus monoclinic phases, and pure monoclinic (m) phase of WO₃, respectively. The degree of crystallization of TiO₂ present in these composite films was not evident. The composite films annealed at 600 °C, however, consist of orthorhombic (o) WO₃ and anatase TiO₂. It was found that the o-WO₃ phase was stabilized by nanocrystalline anatase TiO₂. The thus obtained mesoporous WO₃–TiO₂ composite films were dye sensitized and applied for the construction of photochromic devices. The device constructed using dye sensitized WO₃–TiO₂ composite layer heat treated at 600 °C showed an optical modulation of 51 % in the NIR region, whereas the devices based on the composite layers heat treated at 450, and 500 °C showed only a moderate optical modulation of 24.9, and 38 %, respectively. This remarkable difference in the transmittance response is attributed to nanocrystalline anatase TiO₂ embedded in the orthorhombic WO₃ matrix of the WO₃–TiO₂ composite layer annealed at 600 °C.     

Au/TiO2 nanotube catalysts prepared by combining sol–gel method with hydrothermal treatment and their catalytic properties for CO oxidation

A new preparation method for Au/TiO₂ nanotubes (NTs) by combing sol–gel with hydrothermal treatment technique was developed. The TiO₂ NTs calcined at 300 °C were nearly uniform, and the gold particles were distributed homogeneously. The possible formation mechanism was suggested. The 5 % Au/TiO₂ NTs calcined at 300 °C had the best catalytic activity for CO oxidation, and their conversion of CO remained at 100 % during 60 h on stream. This preparation method could improve the thermal stability of Au/TiO₂ nanotube catalysts.