TiO2 micro-flowers composed of nanotubes and their application to dye-sensitized solar cells

TiO₂ micro-flowers were made to bloom on Ti foil by the anodic oxidation of Ti-protruding dots with a cylindrical shape. Arrays of the Ti-protruding dots were prepared by photolithography, which consisted of coating the photoresists, attaching a patterned mask, illuminating with UV light, etching the Ti surface by reactive ion etching (RIE), and stripping the photoresist on the Ti foil. The procedure for the blooming of the TiO₂ micro-flowers was analyzed by field emission scanning electron microscopy (FESEM) as the anodizing time was increased. Photoelectrodes of dye-sensitized solar cells (DSCs) were fabricated using TiO₂ micro-flowers. Bare TiO₂ nanotube arrays were used for reference samples. The short-circuit current (J_sc) and the power conversion efficiency of the DSCs based on the TiO₂ micro-flowers were 4.340 mA/cm² and 1.517%, respectively. These values of DSCs based on TiO₂ micro-flowers were higher than those of bare samples. The TiO₂ micro-flowers had a larger surface area for dye adsorption compared to bare TiO₂ nanotube arrays, resulting in improved J_sc characteristics. The structure of the TiO₂ micro-flowers allowed it to adsorb dyes very effectively, also demonstrating the potential to achieve higher power conversion efficiency levels for DSCs compared to a bare TiO₂ nanotube array structure and the conventional TiO₂ nanoparticle structure.     

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.     

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 TiO2 nanotubes with TiCl4 treatment on the photoelectrode of dye-sensitized solar cells

In this study, we used the electrochemical anodization to prepare TiO₂ nanotube arrays and applied them on the photoelectrode of dye-sensitized solar cells. In the field emission scanning electron microscopy analysis, the lengths of TiO₂ nanotube arrays prepared by electrochemical anodization can be obtained with approximately 10 to 30 μm. After titanium tetrachloride (TiCl₄) treatment, the walls of TiO₂ nanotubes were coated with TiO₂ nanoparticles. XRD patterns showed that the oxygen-annealed TiO₂ nanotubes have a better anatase phase. The conversion efficiency with different lengths of TiO₂ nanotube photoelectrodes is 3.21%, 4.35%, and 4.34% with 10, 20, and 30 μm, respectively. After TiCl₄ treatment, the efficiency of TiO₂ nanotube photoelectrode for dye-sensitized solar cell can be improved up to 6.58%. In the analysis of electrochemical impedance spectroscopy, the value of R_k (charge transfer resistance related to recombination of electrons) decreases from 26.1 to 17.4 Ω when TiO₂ nanotubes were treated with TiCl₄. These results indicate that TiO₂ nanotubes treated with TiCl₄ can increase the surface area of TiO₂ nanotubes, resulting in the increase of dye adsorption and have great help for the increase of the conversion efficiency of DSSCs.     

Enhancing the performance of dye-sensitized solar cells based on an organic dye by incorporating TiO2 nanotube in a TiO2 nanoparticle film

The photoelectrochemical properties of a high molar extinction coefficient charge transfer organic dye containing thienylfluorene segment called FL, and the effect of incorporating TiO₂ nanotube (TiNT) in TiO₂ nanoparticle film along with the above dye on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were investigated. The influence of soaking time of the TiO₂ electrode in dye solution and the effect of varying its concentration, on the solar cell efficiency was also studied. Cyclic voltammetric (CV) analysis revealed the linear relationship between the anodic peak current and the scan rate, indicating a surface-confined diffusion process.The surface morphology of TiNT was characterized using SEM, TEM and XRD. The open-circuit voltage (V_OC) of the DSSC increased with the increase in the wt% of TiNT and shows optimal value at about 5 wt%, which is correlated with the suppression of the electron recombination as found out from the electron lifetime studies.The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance (R_ct) and electron lifetime under different ratios of the TiNT/nanoparticle. The electron lifetimes of the DSSCs based on FL and N3 dye were very close to one another and the DSSC based on the FL showed respectable photovoltaic performance of ca. 7.8% under the light intensity of 100 mW cm⁻² (AM 1.5G).     

Improving the performance of dye-sensitized solar cells with TiO2/graphene/TiO2 sandwich structure

This study investigates the extent to which the TiO₂/graphene/TiO₂ sandwich structure improves the performance of dye-sensitized solar cells (DSSCs) over that of DSSCs with the traditional structure. Studies have demonstrated that the TiO₂/graphene/TiO₂ sandwich structure effectively enhances the open circuit voltage (V_oc), short-circuit current density (J_sc), and photoelectrical conversion efficiency (η) of DSSCs. The enhanced performance of DSSCs with the sandwich structure can be attributed to an increase in electron transport efficiency and in the absorption of light in the visible range. The DSSC with the sandwich structure in this study exhibited a V_oc of 0.6 V, a high J_sc of 11.22 mA cm^-2, a fill factor (FF) of 0.58, and a calculated η of 3.93%, which is 60% higher than that of a DSSC with the traditional structure.     

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.     

The effect of dye-sensitized solar cell based on the composite layer by anodic TiO2 nanotubes

TiO₂ nanotube arrays are very attractive for dye-sensitized solar cells (DSSCs) owing to their superior charge percolation and slower charge recombination. Highly ordered, vertically aligned TiO₂ nanotube arrays have been fabricated by a three-step anodization process. Although the use of a one-dimensional structure provides an enhanced photoelectrical performance, the smaller surface area reduces the adsorption of dye on the TiO₂ surface. To overcome this problem, we investigated the effect of DSSCs constructed with a multilayer photoelectrode made of TiO₂ nanoparticles and TiO₂ nanotube arrays. We fabricated the novel multilayer photoelectrode via a layer-by-layer assembly process and thoroughly investigated the effect of various structures on the sample efficiency. The DSSC with a four-layer photoelectrode exhibited a maximum conversion efficiency of 7.22% because of effective electron transport and enhanced adsorption of dye on the TiO₂ surface.     

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.     

Fabrication of hole-patterned TiO2 photoelectrodes for solid-state dye-sensitized solar cells

We suggest a simple process to fabricate a hole-patterned TiO₂ electrode for a solid-state dye-sensitized solar cell (DSSC) to enhance cell performance through interfacial properties of the electrode with the electrolyte with minimum dye loading. The method involves prepatterning of SU-8 photoresist on a conducting glass, followed by the deposition of a nanocrystalline TiO₂ layer, calcination at 450 °C and characterization using scanning electron microscopy (SEM). Hole-patterned TiO₂ photoelectrodes yielded better solar energy conversion efficiency per dye loading compared to a conventional non-patterned photoelectrode. For example, a 50 μm hole-patterned DSSC exhibited 4.50% conversion efficiency in the solid state, which is comparable to an unpatterned flat TiO₂ photoelectrode (4.57%) however the efficiency per dye loading of the former (0.986%/g) was much greater than that of the latter (0.898%/g). The improvement was attributed to improved transmittance through the electrode as well as better interfacial properties between the electrolyte and electrode, as confirmed by UV-visible spectroscopy and electrochemical impedance (EIS) analysis.     

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).     

Rapid synthesis and dye-sensitized solar cell applications of hexagonal-shaped ZnO nanorods

This paper reports, for the first time, a very rapid and large-scale synthesis and dye-sensitized solar cells (DSSCs) application of well-crystallized hexagonal-shaped ZnO nanorods at very low temperature of about 70 °C in 20 min. The thin films of as-grown nanorods were used as photo-anode materials to fabricate the DSSCs which exhibited an overall light to electricity conversion efficiency (ECE) of 1.86% with a fill factor of 74.4%, short-circuit current of 3.41 mA/cm² and open-circuit voltage of 0.73 V.     

Optimization of the dye-sensitized solar cell performance by mechanical compression

In this study, the P25 titanium dioxide (TiO₂) nanoparticle (NP) thin film was coated on the fluorine-doped tin oxide (FTO) glass substrate by a doctor blade method. The film then compressed mechanically to be the photoanode of dye-sensitized solar cells (DSSCs). Various compression pressures on TiO₂ NP film were tested to optimize the performance of DSSCs. The mechanical compression reduces TiO₂ inter-particle distance improving the electron transport efficiency. The UV–vis spectrophotometer and electrochemical impedance spectroscopy (EIS) were employed to quantify the light-harvesting efficiency and the charge transport impedance at various interfaces in DSSC, respectively. The incident photon-to-current conversion efficiency was also monitored. The results show that when the DSSC fabricated by the TiO₂ NP thin film compressed at pressure of 279 kg/cm², the minimum resistance of 9.38 Ω at dye/TiO₂ NP/electrolyte interfaces, the maximum short-circuit photocurrent density of 15.11 mA/cm², and the photoelectric conversion efficiency of 5.94% were observed. Compared to the DSSC fabricated by the non-compression of TiO₂ NP thin film, the overall conversion efficiency is improved over 19.5%. The study proves that under suitable compression pressure the performance of DSSC can be optimized.     

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.     

A novel hierarchical Pt- and FTO-free counter electrode for dye-sensitized solar cell

A novel hierarchical Pt- and FTO-free counter electrode (CE) for the dye-sensitized solar cell (DSSC) was prepared by spin coating the mixture of TiO₂ nanoparticles and poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) solution onto the glass substrate. Compared with traditional Pt/FTO CE, the cost of the new CE is dramatically reduced by the application of bilayer TiO₂-PEDOT:PSS/PEDOT:PSS film and the glass substrate. The sheet resistance of this composite film is 35 Ω sq⁻¹ and is low enough to be used as an electrode. The surface morphologies of TiO₂-PEDOT:PSS layer and modified PEDOT:PSS layer were characterized by scanning electron microscope, which shows that the former had larger surface areas than the latter. Electrochemical impedance spectra and Tafel polarization curves prove that the catalytic activity of TiO₂-PEDOT:PSS/PEDOT:PSS/glass CE is higher than that of PEDOT:PSS/FTO CE and is similar to Pt/FTO CE''s. This new fabricated device with TiO₂-PEDOT:PSS/PEDOT:PSS/glass CE achieves a high power conversion efficiency (PCE) of 4.67%, reaching 91.39% of DSSC with Pt/FTO CE (5.11%).     

CdS quantum dot-sensitized solar cells based on nano-branched TiO2 arrays

Nano-branched rutile TiO₂ nanorod arrays were grown on F:SnO₂ conductive glass (FTO) by a facile, two-step wet chemical synthesis process at low temperature. The length of the nanobranches was tailored by controlling the growth time, after which CdS quantum dots were deposited on the nano-branched TiO₂ arrays using the successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The photovoltaic properties of the CdS-sensitized nano-branched TiO₂ solar cells were studied systematically. A short-circuit current intensity of approximately 7 mA/cm² and a light-to-electricity conversion efficiency of 0.95% were recorded for cells based on optimized nano-branched TiO₂ arrays, indicating an increase of 138% compared to those based on unbranched TiO₂ nanorod arrays. The improved performance is attributed to a markedly enlarged surface area provided by the nanobranches and better electron conductivity in the one-dimensional, well-aligned TiO₂ nanorod trunks.     

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.     

Surface plasma resonant effect of gold nanoparticles on the photoelectrodes of dye-sensitized solar cells

In this study, we prepared different shapes of gold nanoparticles by seed-mediated growth method and applied them on the photoelectrodes of dye-sensitized solar cells (DSSCs) to study the surface plasma resonant (SPR) effect of gold nanoparticles on the photoelectrodes of dye-sensitized solar cells. The analyses of field emission scanning electron microscopy show that the average diameter of the spherical gold nanoparticles is 45 nm, the average length and width of the short gold nanorods were 55 and 22 nm, respectively, and the average length and width of the long gold nanorods were 55 and 14 nm, respectively. The aspect ratio of the short and long gold nanorods was about 2.5 and 4, respectively. The results of ultraviolet–visible absorption spectra show that the absorption wavelength is about 540 nm for spherical gold nanoparticles, and the absorption of the gold nanorods reveals two peaks. One is about 510 to 520 nm, and the other is about 670 and 710 nm for the short and long gold nanorods, respectively. The best conversion efficiency of the dye-sensitized solar cells with spherical gold nanoparticles and short and long gold nanorods added in is 6.77%, 7.08%, and 7.29%, respectively, and is higher than that of the cells without gold nanoparticles, which is 6.21%. This result indicates that the effect of gold nanoparticles on the photoelectrodes can increase the conductivity and reduce the recombination of charges in the photoelectrodes, resulting in the increase of conversion efficiency for DSSCs. In addition, the long gold nanorods have stronger SPR effect than the spherical gold nanoparticles and short gold nanorods at long wavelength. This may be the reason for the higher conversion efficiency of DSSCs with long gold nanorods than those of the cells with spherical gold nanoparticles and short gold nanorods.     

Stability study of carbon-based counter electrodes in dye-sensitized solar cells

This study describes a systematic investigation of the stability of a carbon/TiO₂ counter electrode for use in dye-sensitized solar cells (DSSCs). In this system, nanoparticle additives were introduced by adding Ti-hydrogel. The additives then bound carbon particles and enhanced the adhesion of carbon materials to the conductive substrate. After introducing the Ti-hydrogel into the carbon paste, the carbon/Ti-hydrogel composited counter electrode (HC-CE) showed a better conductivity and stability compared with that of the carbon counter electrode (C-CE), while the catalytic activity was not influenced. The device based on the HC-CE showed superior power conversion efficiency (6.3%) and long-term stability over the device based on the C-CE (5.8%).     

Effect of pyridine in electrolyte on the current-voltage characteristics in dye-sensitized solar cells

This study aimed to investigate the effect of pyridine in electrolyte on the photocurrent density-photovoltage (J-V) characteristics in dye-sensitized solar cells. When the concentration of pyridine (c_p) in an electrolyte (0.1 M LiI and 0.05 M I₂ in acetonitrile) is increased from 0 to 0.4 M, the open-circuit voltage (V_oc) increased by 0.14 V, whereas the short-circuit photocurrent density (J_sc) decreased by 3.0 mA cm⁻². The optimum c_p for the maximum cell performance was around 0.1-0.2 M. The model calculation predicted that by increasing c_p from 0 to 0.4 M, the electron diffusion coefficient (D) decreases by more than one order of magnitude and the lifetime of conduction band-free electrons (τ) increases by more than one order of magnitude, implying that the electron diffusion length () increases by 31%, whereas the electron injection flux from dye molecules to TiO₂ nanoporous films (Φ) decreases by 28%.