Optimization of titanium dioxide film prepared by electrophoretic deposition for dye-sensitized solar cell application

Nanocrystalline TiO₂ films were deposited on a conducting glass substrate by the electrophoretic deposition technique. It was found that the thickness of TiO₂ film increased proportionally with an increase in deposition time and deposition voltage. However, as the deposition duration or deposition voltage increased, the film surface was more discontinuous, and microcracks became more evident. The characteristic of the dye-sensitized solar cell using TiO₂ film as a working electrode was analyzed. The results of the energy conversion efficiency and the photocurrent density exhibited a relationship dependent on the TiO₂ thickness. Curve fitting of energy conversion efficiency vs. TiO₂ thickness revealed the optimum solar cell efficiency ~ 2.8% at the film thickness of ~ 14 μm.     

The effect of Li intercalation on different sized TiO2 nanoparticles and the performance of dye-sensitized solar cells

The effect of Li⁺ insertion into different sized TiO₂ nanoparticles and their influences on the photoconversion efficiency of dye-sensitized solar cells (DSSC) were investigated. TiO₂ nanoparticles with different particle sizes (22 nm, 14 nm and 6 nm) doped with Li⁺ were employed to form thin film electrodes and their properties were characterized by X-ray diffraction (XRD) and electrochemical impedance spectroscopy analysis. XRD evidenced the presence of anatase as the main phase. From the XRD analysis, it was observed that the Li⁺ ions could be inserted into both the surface and bulk of the TiO₂ nanoparticles. In the larger particle size, the Li⁺ ions are inserted into the bulk anatase where as Li⁺ ions bounded on the TiO₂ surface for the smaller crystallite size. The photovoltaic properties were measured by a current-voltage meter under AM1.5 simulated light radiation. It exhibited that the overall photoconversion efficiency of DSSC was decreased in the larger particles while it was enhanced in the smaller nanoparticles when Li⁺ was doped into the TiO₂ nanoparticles. A nearly 40% decrease in the efficiency (η) of DSSC was observed upon intercalation of Li⁺ ions into 22 nm sized TiO₂ nanoparticles (P25). The 14 nm sized TiO₂ nanoparticles (P90) showed slightly less efficiency (η) upon Li⁺ doping than that of the undoped sample. However, the smallest sized TiO₂ nanoparticles (6 nm) showed higher efficiency than that of the undoped one. This phenomenon is explained based on electron trapping and charge recombination due to lithium doping.     

Cube-like mixed-phases TiO2 mesocrystalline hollow boxes from in situ topotactic transformation for highly efficient dye-sensitized solar cells

The cube-like TiO₂ mesocrystalline hollow boxes (TiO₂-MHBs) are fabricated by a topotactic transformation method. TiO₂-MHBs consist of one-dimensional nanorod-like rutile TiO₂ combined with anatase TiO₂. The ordering of TiO₂-MHBs is in favor of efficient electron transport. TiO₂-MHBs with mesoporous structure show excellent light scattering performance. Dye-sensitized solar cell with TiO₂-MHBs light scattering overlayer exhibits an 18.3% increment of cell efficiency (9.51%) compared with the TiO₂ nanoparticles film cell (8.04%). The improved photovoltaic performance is attributed to its unique crystallographic property and highly porous structure, which can enhance light scattering capability, quicken electron transport and electrolyte diffusion, and reduce charge recombination.     

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.     

Enhanced performances of dye-sensitized solar cells based on graphite-TiO2 composites

The graphite-incorporated TiO₂ composites for the photoanodes of the Dye-sensitized solar cells were prepared by ultrasound-assisted mixing method. The performances of these solar cells with different graphite additions were investigated by the photocurrent-voltage characteristics, open-circuit voltage decay measurement and electrochemical impedance spectroscopy. The results showed that the addition of graphite had a significant impact on the electron transport and recombination. The photocurrent-voltage results indicated that short-circuit current density (J_sc) and open-circuit voltage (V_oc) enhanced by 40% and 2%, respectively. A 30% improvement in conversion efficiency of dye-sensitized solar cell from 4.44% to 5.76% was achieved using 0.01 wt% graphite-TiO₂ composite electrodes compared to the pure TiO₂ electrode.     

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.     

TiO2 coated SnO2 nanosheet films for dye-sensitized solar cells

TiO₂-coated SnO₂ nanosheet (TiO₂-SnO₂ NS) films about 300 nm in thickness were fabricated on fluorine-doped tin oxide glass by a two-step process with facile solution-grown approach and subsequent hydrolysis of TiCl₄ aqueous solution. The as-prepared TiO₂-SnO₂ NSs were characterized by scanning electron microscopy and X-ray diffraction. The performances of the dye-sensitized solar cells (DSCs) with TiO₂-SnO₂ NSs were analyzed by current-voltage measurements and electrochemical impedance spectroscopy. Experimental results show that the introduction of TiO₂-SnO₂ NSs can provide an efficient electron transition channel along the SnO₂ nanosheets, increase the short current density, and finally improve the conversion efficiency for the DSCs from 4.52 to 5.71%.     

Influence of growth time and annealing on rutile TiO2 single-crystal nanorod arrays synthesized by hydrothermal method in dye-sensitized solar cells

Vertically aligned TiO₂ nanorod (NR) arrays have been grown on the fluorine doped tin oxide (FTO) substrates by hydrothermal methods and the structures were employed to fabricate the dye-sensitized solar cells (DSSCs). The charge transport properties were investigated by the current to voltage curves and the electrochemical impedance spectroscopy. It was found that DSSCs containing the as-prepared and 500 °C annealed TiO₂ NRs exhibit different trends with the growth time (t). The DSSCs assembled with the un-sintered NR arrays showed the highest power conversion efficiency (η) of 1.87%. When DSSCs were assembled with the sintered NR arrays, nearly 400% enhanced efficiency were obtained, and the values (η) showed a positive correlation with t. This behavior may be attributed to the improved adhesion and electric contact between TiO₂ and FTO, as well as the reduced number of recombination sites.     

Photoinduced hydrophilicity of titanium dioxide thin films prepared by cathodic electrodeposition

Crystal structure and microstructural properties of titanium dioxide thin films prepared by cathodic electrodeposition on indium-tin-oxide coated glass substrates from aqueous peroxo-titanium complex solutions have been investigated as a function of sintering temperature (25-500 °C) for the first time. We have noticed pronounced photoinduced hydrophilicity for such thin films on exposure to ultraviolet (UV) light illumination. It was observed that all the films, irrespective of their crystalline nature (amorphous and crystalline), display transformation from hydrophobic to super-hydrophilic behavior upon UV illumination. This observation can be correlated with typical nanoporous morphology of electrodeposited TiO₂ films.     

Large area dye-sensitized solar cells with titanium based counter electrode

Construction of dye-sensitized solar cell of large area using platinum sputtered titanium metal counter electrode is demonstrated. An impressive increase in the fill factor and consequently the efficiency compared to the conventional platinized conducting glass based counter electrodes result from very low sheet resistance of the titanium plate and a cell of active area 151 cm² with parallel silver collecting grids delivered an efficiency of 7.4%. The possibility of using this technique for commercial production of dye-sensitized solar cells was discussed giving details of fabrication procedure.     

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.     

An investigation of the electrolytic solution effects on stainless steel electrode for dye-sensitized solar cells

In the field of dye-sensitized solar cells (DSSCs) the tendency is nowadays to replace the classical conducting glass by a metallic substrate (i.e. stainless steel 304L) as the counter-electrode. However the electrolytic solution (iodide/triiodide in an organic solvent) may be aggressive for stainless steel. An electrolytic solution optimization should improve the long-term stability and increase the cell performance. For this purpose, the electrolytic solution was submitted to different treatments (i.e. deoxygenation, dehydration and dehydration–deoxygenation) in order to increase the corrosion resistance of stainless steel (304L) electrode by eliminating the dissolved oxygen and water.     

Non-aqueous preparation of anatase TiO2 hollow microspheres for efficient dye-sensitized solar cells

TiO₂ hollow spheres are fabricated by a facile and template-free approach, which is efficient, cost-saving and favorable for large scale production. The as-prepared TiO₂ hollow spheres with diameters ranging from 1 to 1.5 μm and a shell thickness of 150 nm are formed by the self-assembly of nanoparticles with a size of about 12 nm. The mesoporous TiO₂ hollow spheres possess a high specific surface area up to 166.2 m² g⁻¹. TiO₂ hollow spheres show superior light trapping characteristics and significantly improve the light scattering ability. The formation of hollow structure is interpreted by the Ostwald ripening mechanism. By employing a double-layered photoanode made of the as-prepared TiO₂ hollow spheres as the overlayer and P25 as the bottom layer, the dye-sensitized solar cell achieved a power conversion efficiency of 7.90%, which is ascribed to the enhanced dye loading and light scattering ability of TiO₂ hollow spheres.     

Influences of deposition and post-annealing temperatures on properties of TiO2 blocking layer prepared by spray pyrolysis for solid-state dye-sensitized solar cells

Influences of the temperature (T_s) for spray pyrolysis deposition of TiO₂ blocking layer (BL) using titanium diisopropoxide bis(acetylacetonate) (TAA) as a precursor and the temperature (T_p) for post-annealing of the BL films on the resulting BL film morphology and photovoltaic performance of solid-state dye-sensitized solar cells (SDSC) are investigated. A T_s ranging from 300 to 400 °C is found preferable for the formation of BL films with smooth surface and dense grain packing. A T_s lower than 300 °C results in insufficient decomposition of the TAA precursor and is unable to form smooth BL films, while a T_s over 400 °C leads to loosely packed grain in BL films. Power conversion efficiency (PCE) of ~ 4.0% is obtained for SDSC devices with BL films prepared at a T_s in the range of 300-450 °C. High temperature post-annealing (T_p = 500-550 °C) of the BL films prepared at a low T_s, such as 300 °C, can improve the PCE up to 4.6%. The improvements are considered due to the higher purity, increased crystallinity, and retained high grain packing density of the post-annealed BL films, which facilitate charge transport and suppress charge recombination.     

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.     

Callindra haematocephata and Peltophorum pterocarpum flowers as natural sensitizers for TiO2 thin film based dye-sensitized solar cells

We have studied the performance of dye-sensitized solar cells employing natural dye extracted from the flowers Callindra haematocephata and Peltophorum pterocarpum as sensitizers for TiO₂ photoanode. The extracts have shown appreciable absorption in the visible region. FTIR studies indicated the presence of anthocyanins and β-carotene in the flowers of C. haematocephata and P. pterocarpum respectively. The extracts were anchored on TiO₂ film deposited on transparent conductive glass (FTO) which were used as photoanode. The dye coated TiO₂ film electrode, Pt counter electrode and electrolyte (I⁻₃) assembled into a cell module was illuminated by a light source with intensity 100 mW/cm² to measure the photoelectric conversion efficiency of the DSSCs. From the J-V characteristic curves of cells, the parameters related to the solar cell performance were determined. The conversion efficiency of the DSSC employing natural dye extract from the flower C. haematocephata and P. pterocarpumwere was found as 0.06% and 0.04%, with open-circuit voltage (V_OC) of 370 mV & 400 mV, short-circuit current density (J_SC) of 0.25 mA/cm² & 0.15 mA/cm², fill factor (FF) of 0.70 & 0.71 and P_max of 65 & 45 μW cm⁻² respectively. The extract of the flower C. haematocephata exhibited better photosensitization action compared to the flower of P. pterocarpum.     

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.     

Enhancement of the Photoelectric Performance of Dye-sensitized Solar Cells by Sol-gel Modified TiO2 Films

Modified TiO2 films have been prepared by combining commercial titania powders (Degussa P25) with sol-gel made by titanium chloride (Ti-sol). The result shows that clusters are formed by nanoparticles and large pores can be seen on the surface of the TiO2 films. The short circuit photocurrent density and photoelectric conversion efficiency of the solar cells are obviously enhanced compared with those without modification. The relationship between the photoelectric conversion efficiency and the amount of Ti-sol was investigated. With the addition of 30 wt% Ti-sol, the photoelectric conversion efficiency as high as 9.75% is achieved, increasing by 28.3% compared with the solar cells without modification.     

Electroless deposition of platinum on indium tin oxide glass as the counterelectrode for dye-sensitized solar cells

A platinum (Pt) layer is electroless-deposited on indium tin oxide (ITO) glass substrate as the counterelectrode for dye-sensitized solar cells (DSSCs). Compared with other methods of depositing Pt layer, electroless deposition is simple, low-temperature, and easy to scale-up for industrial application. The Pt concentration of the electroless plating solution is found to play an important role in the cell performance. With increasing the Pt concentration, i.e. the Pt loading on the ITO surface, the resultant Pt layer exhibits a porous structure. Owing to the porous structure, the Pt layer can provide more active surface area for triiodide reduction and thus reduces the charge-transfer resistance. The cell performance is promoted accordingly with increasing the Pt concentration. Energy conversion efficiency of 6.46%, short-circuit current density of 15.04 mA cm⁻², open-circuit voltage of 0.68, and fill factor of 0.63 can be achieved for the DSSC employing electroless-Pt counterelectrode.     

Titanium dioxide thin films for high temperature gas sensors

Titanium dioxide (TiO₂) thin film gas sensors were fabricated via the sol-gel method from a starting solution of titanium isopropoxide dissolved in methoxyethanol. Spin coating was used to deposit the sol on electroded aluminum oxide (Al₂O₃) substrates forming a film 1 μm thick. The influence of crystallization temperature and operating temperature on crystalline phase, grain size, electronic conduction activation energy, and gas sensing response toward carbon monoxide (CO) and methane (CH₄) was studied. Pure anatase phase was found with crystallization temperatures up to 800 °C, however, rutile began to form by 900 °C. Grain size increased with increasing calcination temperature. Activation energy was dependent on crystallite size and phase. Sensing response toward CO and CH₄ was dependent on both calcination and operating temperatures. Films crystallized at 650 °C and operated at 450 °C showed the best selectivity toward CO.