TiO2-B narrow nanobelt/TiO2 nanoparticle composite photoelectrode for dye-sensitized solar cells

In order to possess the merits of both building blocks, i.e. the rapid interfacial electron transport of TiO₂-B narrow nanobelts (NBs) and the high surface area of TiO₂ nanoparticles (NPs), the TiO₂-B NBs and TiO₂ NPs composites photoelectrodes were prepared with different weight ratios. The dye-sensitized solar cell prototypes were fabricated based on the composite photoelectrodes and the photoelectrical properties have been systematically studied. Although the amount of adsorption dye of composite solar cells decreased, the composite cells could obtain higher power conversion efficiency compared to pure TiO₂ NP solar cell by rational tuning the weight ratio of TiO₂-B NBs and TiO₂ NPs, which was due to the faster electron transfer rate. The dye adsorption amount and interfacial electron transport, which together determined the overall photoelectrical conversion efficiency, were investigated by the UV–vis spectra, the electrochemical impedance spectra (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS).     

Synthesis of Zn-doped TiO2 microspheres with enhanced photovoltaic performance and application for dye-sensitized solar cells

Zn-doped TiO₂ microspheres have been synthesized by introducing a trace amount of zinc nitrate hexahydrate to the reaction system. Scanning electron microscope (SEM), field-emission scanning electron microscope (FESEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) have been utilized to characterize the samples. Both surface photovoltage spectroscopy (SPS) technique based on lock-in amplifier and transient photovoltage (TPV) measurement reveal that the slight doping of Zn can promote the separation of photo-generated charges as well as restrain the recombination due to the strong interface built-in electric field and the decreasing of surface trap states. The photovoltaic parameters of dye-sensitized solar cells (DSSCs) based on Zn-doped TiO₂ are significantly better, compared to that of a cell based on undoped TiO₂. The relation between the performance of DSSCs and their photovoltaic properties is also discussed.     

Conversion enhancement of flexible dye-sensitized solar cells based on TiO2 nanotube arrays with TiO2 nanoparticles by electrophoretic deposition

We prepared highly ordered titanium dioxide nanotube arrays (TNAs) by anodizing Ti foils in F⁻ containing electrolyte. The thickness and dye loading amount of TNAs were 26 μm and 1.06 × 10⁻⁷ mol cm⁻², respectively. TiO₂ nanoparticles (TNPs) were electrophoretically deposited on the inner wall of nanotube to produce coated nanotube arrays (TNAP). The dye loading was increased to 1.56 × 10⁻⁷ mol cm⁻², and the electron transport rate improved. TNAs and TNAP were sensitized with ruthenium dye N3 to yield dye-sensitized TiO₂ nanotube solar cells. The power conversion efficiency of TNA-based dye-sensitized solar cells (DSSCs) was 4.28%, whereas the efficiency of TNAP-based DSSCs increased to 6.28% when illuminated from the counter electrode. The increase of power conversion efficiency of TNAP-based DSSCs is ascribed to the increased surface area of TNAs and the faster electron transport rate.     

Role of surface state on the electron flow in modified TiO2 film incorporating carbon powder for a dye-sensitized solar cell

An investigation of surface-related traps in nanostructured TiO₂ films modified by the incorporation of carbon powder was conducted by the potential-step chronoamperometric method. For the modification of the morphology and surface state of the nanoporous TiO₂ electrode, the incorporation of carbon into the white TiO₂ powder was accomplished. In the chronoamperometric data, all of the transients showed an initial fast phase (<1 s) followed by a slower phase which is related to the trap filling process. The trap-filling period of the carbon incorporated TiO₂ film becomes longer, as the applied negative potential increases, due to the widely distributed traps induced by the increased surface area. Furthermore, the film capacitance was derived as a function of the applied bias by integrating the current to time curves of the chronoamperometric data. The accumulated charge of the carbon incorporated TiO₂ film increases prominently in two regions. The dominant increase shown in the positive region (−0.7 to −0.9 V vs. Ag/AgCl at pH 13) of the flat band potential implies that the electron occupancy in the surface-related traps is increased. At a more negative potential (below −1.2 V vs. Ag/AgCl), electrons from the conduction band of the TiO₂ film substantially influence the total current, thereby inducing an exponential increase in the current. Therefore, it is found that most of the traps are located in the positive region of the flat band potential, since the Fermi level of the nanostructured TiO₂ film is positioned at −1.14 V vs. Ag/AgCl at pH 13. The trap sites in the sub-bandgap region of the TiO₂ film are important in the electron transport of photoinjected electrons from dye molecules and partially charge recombination with redox electrolyte in operating dye-sensitized solar cell. The influence of charge trap formed by increased surface states on the electron transport and electron transfer was investigated by photovoltage and photocurrent transient measurements.     

Efficiency improvement of dye-sensitized solar cells based on electrodeposited TiO2 films by low temperature post-treatment

Porous crystalline TiO₂ films can be prepared at low temperatures (80 °C) by surfactant-assisted electrodeposition from TiCl₃ solution. Nevertheless, up to now calcination at high temperatures (typically 450 °C) was still necessary to establish a good performance of these films in dye-sensitized solar cells (DSSC). With this study we report that water vapour treatment at much lower temperatures (150 °C) for 1 week improves the performance of the films in DSSC to the same degree as calcination although the overall crystallinity remains lower. Reason for the good efficiency is that the porous structure stays intact and thus the dye molecules can be better adsorbed. Avoiding high temperatures during the preparation process of TiO₂ films for the application in DSSC enables the use of polymer substrates for the fabrication of flexible solar cells.     

Dye-sensitized solar cells based on double-layered TiO2 composite films and enhanced photovoltaic performance

Dye-sensitized solar cells (DSSCs) are fabricated based on double-layered composite films of TiO₂ nanoparticles and hollow spheres. The photoelectric conversion performances of DSSCs based on nanoparticles/nanoparticles (PP), hollow spheres/hollow spheres (HH), hollow spheres/nanoparticles (HP), and nanoparticles/hollow spheres (PH) double-layered films are investigated, and their photo-electric conversion efficiencies are 4.33, 4.72, 4.93 and 5.28%, respectively. The enhanced performance of TiO₂ nanoparticles/hollow spheres double-layered composite film solar cells can be attributed to the combined effect of following factors. The light scattering of overlayer hollow spheres enhances harvesting light of the DSSCs and the underlayer TiO₂ nanoparticle layer ensures good electronic contact between film electrode and the F-doped tin oxide (FTO) glass substrate. Furthermore, the high surface areas and pore volume of TiO₂ hollow spheres are respectively beneficial to adsorption of dye molecules and transfer of electrolyte solution.     

Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells

Spherical pure anatase TiO₂ spheres with a mesoporous structure and high surface area of up to 116.5 m² g⁻¹ were prepared by a simple urea-assisted hydrothermal process and investigated as dye-sensitized solar-cell electrodes. Although the particle diameters of the prepared TiO₂ spheres ranged from 0.4 to 1.3 μm, due to the high specific surface area, mesoporous TiO₂ sphere electrode was obtained with enhanced light harvesting and a larger amount of dye loading. An overall light conversion efficiency of 7.54% under illumination of simulated AM 1.5G solar light (100 mW cm⁻²) was achieved using the mesoporous TiO₂ spheres electrode, which was significantly higher than a commercial Degussa P25 TiO₂ nanocrystals electrode (5.69%).     

Effect of compressed TiO2 nanoparticle thin film thickness on the performance of dye-sensitized solar cells

In this study, dye-sensitized solar cells (DSSCs) were fabricated using nanocrystalline titanium dioxide (TiO₂) nanoparticles as photoanode. Photoanode thin films were prepared by doctor blading method with 420 kg/cm² of mechanical compression process and heat treatment in the air at 500°C for 30 min. The optimal thickness of the TiO₂ NP photoanode is 26.6 μm with an efficiency of 9.01% under AM 1.5G illumination at 100 mW/cm². The efficiency is around two times higher than that of conventional DSSCs with an uncompressed photoanode. The open-circuit voltage of DSSCs decreases as the thickness increases. One DSSC (sample D) has the highest conversion efficiency while it has the maximum short-circuit current density. The results indicate that the short-circuit current density is a compromise between two conflict factors: enlargement of the surface area by increasing photoanode thickness and extension of the electron diffusion length to the electrode as the thickness increases.     

EIS analysis on low temperature fabrication of TiO2 porous films for dye-sensitized solar cells

A low temperature (<150 °C) fabrication method for preparation of TiO₂ porous films with high efficiency in dye-sensitized solar cells (DSSCs) has been developed. The Ti(IV) tetraisopropoxide (TTIP) was added to the paste of TiO₂ nanoparticles to interconnect the TiO₂ particles. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) and electron lifetime in the TiO₂ film (τ_e) under different molar ratios of TTIP/TiO₂ and also at various TiO₂ thicknesses. It was found that the R_ct2 decreased as the molar ratio increased from 0.02 to 0.08, however, it increased at a molar ratio of 0.2 due to the reduction in surface area for dye adsorption. In addition, the characteristic frequency peak shifted to lower frequency at a molar ratio of 0.08, indicating the longer electron lifetime. As for the thickness effect, TiO₂ film with a thickness around 17 μm achieved the best cell efficiency. EIS study also confirmed that, under illumination, the smallest R_ct2 was associated with a TiO₂ thickness of 17 μm, with the R_ct2 increased as the thickness of TiO₂ film increased. In the Bode plots, the characteristic frequency peaks shifted to higher frequency when the thickness of TiO₂ increased from 17.2 to 48.2 μm, indicating the electron recombination increases as the thickness of the TiO₂ electrode increases.Finally, to make better use of longer wavelength light, 30 wt% of larger TiO₂ particle (300 nm) was mixed with P25 TiO₂ as light scattering particles. It effectively increased the short-circuit current density and cell conversion efficiency from 7.44 to 8.80 mA cm⁻² and 3.75 to 4.20%, respectively.     

Titanium flexible photoanode consisting of an array of TiO2 nanotubes filled with a nanocomposite of TiO2 and graphite for dye-sensitized solar cells

A flexible dye-sensitized solar cell (DSSC) was fabricated using a photoanode consisting of an array of TiO₂ nanotubes (TNT) filled with a nanocomposite of TiO₂ (P90) and nanographite. The array of TNT was obtained by anodic oxidation of Ti foil, and this Ti foil with TNT was used as the photoanode of the DSSC. Each tube in the array has an average diameter of 100 nm. The morphologies of the array of TNT were obtained both after and before filling them with the TiO₂/graphite nanocomposite, using a field-emission scanning electron microscopy (FE-SEM). DSSC with photoanode consisting of the nanocomposite (photoanode designated as Graphite/P90-TNT) rendered a light-to-electricity conversion efficiency (η) of 5.75%. In contrast, the cells with photoanodes consisting of only TNT (photoanode designated as TNT) and TNT filled with P90-TiO₂ (photoanode designated as P90-TNT) exhibited efficiencies (η) of 4.44% and 5.14%, respectively. The enhancements in the η’s in favor of the cells with P90-TNT and Graphite/P90-TNT were attributed to the filled P90 and nanocomposite, respectively. The filled particles were assumed to provide more conductive pathways for electron transfer and prolonged lifetime for electrons in the film of TNT. The results were substantiated by light-absorption values, incident-photo-to-current efficiency (IPCE) curves, Nyquist and Bode plots of electrochemical impedance spectroscopy (EIS), and photopotential transient curves.     

Effect of TiO2 rutile nanorods on the photoelectrodes of dye-sensitized solar cells

In order to enhance the electron transport on the photoelectrodes of dye-sensitized solar cells, one-dimensional rutile nanorods were prepared using electrospun TiO₂ nanofibers. The grain size of the nanorods increased with increasing temperature. Electrochemical impedance spectroscopy measurements revealed reduced interface resistance of the cells with the one-dimensional rutile nanorods due to the improved electron transport and the enhanced electrolyte penetration. Intensity-modulated photocurrent/photovoltage spectroscopy showed that the one-dimensional rutile nanorods provided the electrons with a moving pathway and suppressed the recombination of photogenerated electrons. However, an excessive quantity of rutile nanorods created an obstacle to the electrons moving in the TiO₂ thin film. The photoelectrode with 7 wt.% rutile nanorods optimized the performance of the dye-sensitized solar cells.     

TiO2/modified natural clay semiconductor as a potential electrode for natural dye-sensitized solar cell

TiO₂/modified natural bentonite clay semiconductor, as a potential electrode of dye-sensitized solar cell, having a Ti:Si molar ratio of 85:15 was, for the first time, compared with pure TiO₂ (commercial P25) electrode in terms of solar cell efficiency and characteristics. 4-Chloro-2,5-difluorobenzoic acid and 4-(chloromethyl)benzoyl chloride were added to the electrodes to increase light harvesting ability of natural dyes extracted from red cabbage, rosella, and blue pea. The results showed that the TiO₂/clay semiconductor provided a higher surface area but a slightly lower efficiency than the pure TiO₂. The best natural sensitizer was found to be the dye extracted from red cabbage. Besides, the 4-(chloromethyl)benzoyl chloride provided a higher short circuit current for the TiO₂/clay semiconductor.     

Al2O3-coated SnO2/TiO2 composite electrode for the dye-sensitized solar cell

A novel Al₂O₃-coated SnO₂/TiO₂ composite electrode has been applied to the dye-sensitized solar cell. In such an electrode, two kinds of energy barriers (SnO₂/TiO₂ and TiO₂/Al₂O₃) were designed to suppress the recombination processes of the photo-generated electrons and holes. After the SnO₂ was modified by colloid TiO₂, the photoelectric conversion efficiency of the SnO₂/TiO₂ composite cell increased to 2.08% by a factor of 2.8 comparing with that of the SnO₂ cell. The Al₂O₃ layer on the SnO₂/TiO₂ composite electrode further suppressed the generation of the dark current, resulting in 37% improvement in device performance comparing with the SnO₂/TiO₂ cell.     

Quasi solid state dye-sensitized solar cells with modified TiO2 photoelectrodes and triphenylamine-based dye

Quasi solid state dye-sensitized solar cells (DSSCs) have been fabricated with organic sol or TiCl₄ modified TiO₂ and porous TiO₂ photoanode and a triphenylamine-based dye (TPAR3) used as photosensitizer. Dark current measurements suggested that both modified TiO₂ photoelectrodes had significantly reduced the recombination rate of photoelectrons due to the reduced bare FTO surface in comparison to porous photoelectrode. The DSSC based on modified TiO₂ photoelectrodes showed improved photovoltaic parameters compared to the porous TiO₂ photoelectrode. The overall power conversion efficiency (PCE) is 3.27%, 4.73% and 6.8% for porous, TiCl₄ modified and sol modified TiO₂ photoelectrodes, respectively. The improved PCE with modified TiO₂ electrodes was attributed to the formation of a compact layer. This effectively improves adherence of TiO₂ to FTO surface, providing a larger TiO₂/FTO contact area and reducing the electron recombination by blocking the direct contact between redox electrolyte and the conductive FTO surface and enhances the electron collection efficiency.     

Cationic surfactant promoted reductive electrodeposition of nanocrystalline anatase TiO2 for application to dye-sensitized solar cells

A new strategy involving the introduction of the common cationic surfactant cetyltrimethylammonium bromide (CTAB) for the cathodic deposition of titanium dioxide from hydrolyzed TiCl₄ and TiCl₃ solutions by cyclic voltammetry has been developed. Crack-free and non-transparent anatase TiO₂ films were obtained for the first time and characterized with the aid of Raman spectra and SEM. Selection of TiCl₄ as the precursor for the electrodeposition is quite a novel approach for the research in the area of dye-sensitized solar cells (DSSCs). It is noted that NO₃⁻ ion is essential for such a deposition. Under the same conditions, a thicker TiO₂ film was obtained by adding CTAB into KNO₃ electrolyte, compared with the case without it. The CTAB-promoted film led to an increased energy conversion efficiency of the corresponding DSSC. Mechanisms are proposed for the electrochemical deposition and the beneficial role of CTAB.     

A natural tomato slurry as a photosensitizer for dye-sensitized solar cells with TiO2/CuO composite thin films

We have studied the performance of dye-sensitized solar cells by employing natural dye “anthocyanins” extracted from the tomato slurry as a sensitizer for the TiO₂/CuO photoanode. The extracts were anchored on TiO₂/CuO films deposited on an ITO substrate which was used as a photoanode. The dye adsorbed TiO₂/CuO films electrode, the copper plate as a counter electrode, and iodolyte as an electrolyte were assembled into DSSCs. The conversion efficiency of the DSSCs was found to be 2.96% with a V_OC of 0.615 V, J_SC of 6.6 mA/cm², and an FF of 0.73. This work highlights the use of contribution of the tomato slurry as a natural sensitizer to enhance the efficiency of DSSCs.     

Photovoltaic performance improvement of dye-sensitized solar cells based on tantalum-doped TiO2 thin films

Dye-sensitized solar cells based on a tantalum (Ta)-doped TiO₂ thin film prepared by the hydrothermal method show a photovoltaic efficiency of 8.18%, which is higher than that of the undoped TiO₂ thin film (7.40%). The Mott-Schottky plot indicates that the Ta-doped TiO₂ photoanode shifts the flat band potential positively and increases the electron density. The positive shift of the flat band potential improves the driving force of injected electrons from the LUMO of the dye to the conduction band of TiO₂. Furthermore, the increased electron density caused by the Ta-doped TiO₂ improves the fill factor of the solar cell. The increased electron density accelerates the transfer rate of electrons in the Ta-doped TiO₂ thin films by comparison to undoped films, which is confirmed by intensity-modulated photocurrent spectroscopy measurements.     

Performance enhancement of dye-sensitized solar cells using nanostructural TiO2 films prepared by a graft polymerization and sol–gel process

Nanostructural TiO₂ films with large surface areas were prepared by the combined process of graft polymerization and sol–gel for use in dye-sensitized solar cells (DSSCs). The surface of the TiO₂ nanoparticles was first graft polymerized with photodegradable poly(methyl methacrylate) (PMMA) via atom transfer radical polymerization (ATRP), after which the particles were deposited onto a conducting glass. The PMMA chains were removed from the TiO₂ films by UV irradiation to generate secondary pores, into which titanium isopropoxide (TTIP) was infiltrated. The TTIP was then converted into small TiO₂ particles by calcination at 450 °C, as characterized by energy-filtering transmission electron microscopy (EF-TEM) and field emission scanning electron microscopy (FE-SEM). The nanostructural TiO₂ films were used as a photoelectrode in solid-state DSSCs; the energy conversion efficiency was 5.1% at 100 mW/cm², which was higher than the values achieved by the pristine TiO₂ (3.8%) and nongrafted TiO₂/TTIP photoelectrodes (3.3%). This performance enhancement is primarily due to the increased surface area and pore volume of TiO₂ films, as revealed by the N₂ adsorption–desorption isotherm.     

Enhanced power conversion efficiency in dye-sensitized solar cells with TiO2 aggregates/nanocrystallites mixed photoelectrodes

Photoelectrodes of mixed microsized TiO₂ aggregates and individually dispersed TiO₂ nanocrystallites with different ratios were fabricated and studied for improved power conversion efficiency in dye-sensitized solar cells (DSCs). TiO₂ aggregates/nanocrystallites composites possess several advantages for high performance of DSCs, including the light scattering by the microsized TiO₂ aggregates and the high surface area of nanocrystallites both in aggregates and individually dispersed. A high power conversion efficiency of 7.59% was achieved with mixed TiO₂ aggregates/nanocrystallites photoelectrode using conventional dye N3, without applying anti-reflection coating, back-scattering layer, or chemical treatment. The electron transport properties of DSCs with mixed photoelectrodes were investigated by electrochemical impedance spectra, and the results showed that such a photoelectrode with mixed aggregates and nanocrystallites possess better connectivity for efficient electron transport.