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.     

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.     

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.     

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%).     

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.     

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.     

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%).     

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.     

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).     

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).     

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%).     

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.     

Carbon nanotube counter electrode for high-efficient fibrous dye-sensitized solar cells

ABSTRACT: High-efficient fibrous dye-sensitized solar cell with carbon nanotube (CNT) thin films as counter electrodes has been reported. The CNT films were fabricated by coating CNT paste or spraying CNT suspension solution on Ti wires. A fluorine tin oxide-coated CNT underlayer was used to improve the adherence of the CNT layer on Ti substrate for sprayed samples. The charge transfer catalytic behavior of fibrous CNT/Ti counter electrodes to the iodide/triiodide redox pair was carefully studied by electrochemical impedance and current-voltage measurement. The catalytic activity can be enhanced by increasing the amount of CNT loading on substrate. Both the efficiencies of fibrous dye-sensitized solar cells using paste coated and sprayed CNT films as counter electrodes are comparative to that using Pt wires, indicating the feasibility of CNT/Ti wires as fibrous counter electrode for superseding Pt wires.     

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.     

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.     

Electro-optical properties of new anthracene based organic dyes for dye-sensitized solar cells

Dipolar compounds containing anthracene-based triarylamine donor and cyanoacrylic acid acceptor were synthesized and characterized by electro-optical measurements. The optical spectra of the dyes are dominated by a charge transfer transition. This band is red-shifted and increased in intensity on elongation of conjugation by the introduction of bithiophene or dithienylbenzothiadiazole moiety. Similarly, the oxidation potentials of the dyes are shifted anodically on extension of the conjugation attributable to the reduction in the interaction between the triarylamine and acceptor segments. Theoretical calculations revealed the charge transfer to occur between anthracene and triarylamine moieties. However, in the benzothiadiazole containing dye the charge transfer is observed between the amine and the acceptor segment. Dye-sensitized solar cells fabricated using these dyes showed moderate efficiency which is highly dependent on the nature of the conjugation bridge.     

Organic dyes incorporating low-band-gap chromophores based on π-extended benzothiadiazole for dye-sensitized solar cells

A series of new π-conjugated organic dyes (HKK-BTZ1, HKK-BTZ2, HKK-BTZ3 and HKK-BTZ4), comprising triphenylamine (TPA) moieties as the electron donor and benzothiadiazole moieties as the electron acceptor/anchoring groups, was synthesized for the use in dye-sensitized solar cells (DSSCs). TPA units are bridged to benzothiadiazole with single(S), double(D) and triple bonds(T) in different derivatives. And HKK-BTZ1 was modified by introducing alkoxy group of TPA unit, because the bulky alkoxy group is a strong donating group for the more red shift and for reducing aggregation of dyes in TiO₂ film. The structure-property relationship was investigated. Under standard global AM 1.5 G illumination, a maximum photo-to-electron conversion efficiency of 7.30% was achieved with the DSSC based on dye HKK-BTZ4 (J_SC = 17.9 mA/cm⁻², V_OC = 0.62 V, FF = 0.66), while the Ru dye N719-sensitized DSSC showed an efficiency of 7.82% with a J_SC of 17.5 mA/cm⁻², a V_OC of 0.62 V, and a FF of 0.72.     

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.     

New photosensitizer with phenylenebisthiophene central unit and cyanovinylene 4-nitrophenyl terminal units for dye-sensitized solar cells

A new low band gap photosensitizer, D, which contains 2,2′-(1,4-phenylene) bisthiophene central unit and cyanovinylene 4-nitrophenyl terminal units at both sides was synthesized. The two carboxyls attached to the 2,5-positions of the phenylene ring act as anchoring groups. Dye D was soluble in common organic solvents, showed long-wavelength absorption maximum at 620–636 nm and optical band gap of 1.72 eV. The electrochemical parameters, i.e. the highest occupied molecular orbital (HOMO) (−5.1 eV) and the lowest unoccupied molecular orbital (LUMO) (−3.3 eV) energy levels of D show that this dye is suitable as molecular sensitizer. The quasi solid state dye-sensitized solar cell (DSSC) based on D shows a short circuit current (J_sc) of 9.95 mA/cm², an open circuit voltage (V_oc) of 0.70 V, and a fill factor (FF) of 0.64 corresponding to an overall power conversion efficiency (PCE) of 4.40% under 100 mW/cm² irradiation. The overall PCE has been further improved to 5.52% when diphenylphosphinic acid (DPPA) coadsorbent is incorporated into the D solution. This increased PCE has been attributed to the enhancement in the electron lifetime and reduced recombination of injected electrons with the iodide ions present in the electrolyte with the use of DPPA as coadsorbant. The electrochemical impedance spectroscopy (EIS) results indicated that the augment ascribes to inhibited interfacial charge recombination between the conduction band electrons and the tri-iodide ions in the electrolyte.     

Variation of carboxylate-functionalized cyanine dyes to produce efficient spectral sensitization of nanocrystalline solar cells

The sensitizing properties of cyanine dyes in dye-sensitized nanocrystalline TiO₂ solar cells are shown to be controlled by the character of the carboxyl functions used to attach the molecules to the surface. These tether functions affect the degree of aggregation of the subject cyanine dyes attached to TiO₂ as well as the short circuit photocurrents they produce in these solar cells. Use of two carbons, acetic acid linkages, on the dye results in performance in a sensitized solar cell comparable with a control ruthenium complex, in contrast to the greatly diminished performance of dyes with longer, methylbenzoic acid linkages.