Synthesis of TiO2 submicro-rings and their application in dye-sensitized solar cell

In this paper, novel TiO₂ submicro-rings were synthesized via potentiostatic anodization of titanium powder coated on transparent conducting oxide glass. The TiO₂ submicro-rings film was characterized by SEM, XPS and 3D optical profiling. Accordingly, a possible growth mechanism of submicro-rings was discussed. The TiO₂ submicro-rings based dye-sensitized solar cell (DSSC) with the film thickness of ca. 3.1 μm was assembled and a conversion efficiency of 1.36% was achieved under AM 1.5 illumination. The photoelectron transport properties of TiO₂ submicro-rings based DSSC were also discussed according to the electron impedance spectroscopy.     

Columnar rutile TiO2 based dye-sensitized solar cells by radio-frequency magnetron sputtering

Columnar-structured rutile TiO₂ film with a thickness of 1.4 μm is prepared using the radio-frequency (RF) magnetron sputtering technique, for application in dye-sensitized solar cells (DSSCs). Pure rutile TiO₂ films are fabricated by controlling the substrate temperature during sputtering and using a substrate with a rough surface morphology. Successive substrate heating to 623 K induces the growth of a rutile TiO₂ film that has a specific direction in the (1 1 0) plane, which results in a decrease in the average grain size. This causes in an increase of dye uptake and thereby contributes to enhancement of the photocurrent in the DSSC.     

Plastic dye-sensitized photo-supercapacitor using electrophoretic deposition and compression methods

A plastic photo-rechargeable capacitor is studied using a three-electrode configuration, separating a flexible dye-sensitized solar cell (DSSC) and a supercapacitor by sharing a common Pt electrode. The thick and uniform TiO₂ film is formed by using commercially available TiO₂ nanocrystals, which are treated in an isopropyl alcohol without surfactant by the electrophoretic deposition (EPD) to deposit the mesoporous TiO₂ photoanode film with good adherence onto the plastic substrate. Afterward, a static mechanical compression technique as the post-treatment is employed to the electrophoretic deposited film in order to enhance the particles connection. In addition, a supercapacitor using PEDOT (poly(3,4-ethylenedioxythiophene)), which is potentiostatically electropolymerized to form a thick film, is fabricated to store the energy. The flexible DSSC part is fabricated with a TiO₂ film of 10.9 μm thickness and it can provide photoelectric conversion efficiency up to 4.37% under 1 sun illumination. The photocapacitor is made with such a flexible DSSC and a supercapacitor with ca. 0.5 mm thick PEDOT film, which provides a specific capacitance of 0.52 F cm⁻².     

Low-temperature oxygen plasma treatment of TiO2 film for enhanced performance of dye-sensitized solar cells

The effects of low-temperature O₂ plasma treatment of a TiO₂ film are studied with the objective of improving the performance of dye-sensitized solar cells (DSSCs). X-ray photoelectron spectra (XPS) reveal that the ratio of titanium dioxide to titanium sub-oxides is increased in the O₂ plasma-treated TiO₂ film, compared with that of the untreated TiO₂ film. This increase suggests that the oxygen vacancies in the film are effectively reduced. The near-edge X-ray absorption fine structure (NEXAFS) spectra results agree with the XPS result. It is proposed that there is a correlation between the shifts of the peaks in the NEXAFS spectra and the adsorption of N719 dye on the TiO₂ particles. A DSSC having an O₂ plasma-treated, 4 μm thick TiO₂ film electrode renders a short-circuit photocurrent of 7.59 mA cm⁻², compared with 6.53 mA cm⁻² for a reference cell with an untreated TiO₂ electrode of the same thickness. As a result of these changes, the solar-to-electricity conversion efficiency of the O₂ plasma-treated cell is found to be 4.0% as compared with 3.5% for the untreated cell. This improvement in the performance is rationalized on the basis of increased N719 dye adsorption on to the TiO₂, due to the reduction in the number of oxygen vacancies caused by the oxygen plasma treatment.     

Influences of different TiO2 morphologies and solvents on the photovoltaic performance of dye-sensitized solar cells

The effects of TiO₂ photoelectrode's surface morphology and different solvents on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were studied. By successive coating of TiO₂ suspension, composed of low and high molecular weight poly(ethylene)glycol (PEG) as a binder, double layered TiO₂ photoelectrodes with four different structures were obtained. Among the DSSCs with different TiO₂ electrodes, DSSC with P2P1 electrode (P2 and P1 correspond to PEG molecular weights of 20,000 and 200,000, respectively) showed higher performance under identical film thickness at a constant irradiation of 100 mW cm⁻², which may be correlated with large pore size and high surface area of the corresponding TiO₂ electrode. This was confirmed by electrochemical impedance spectroscopy (EIS) analysis of the DSSC and the transient photovoltage measurement of electrons in the TiO₂ electrode. Among the different solvents investigated here, the DSSC containing acetonitrile showed high conversion efficiency and the order of performance of the DSSCs with different solvents were AN > MPN > PC > GBL > DMA > DMF > DMSO. Better correlation was observed between the donor number of solvents and photoelectrochemical parameters of the DSSCs containing different solvents rather than the measured viscosity and dielectric constant of solvents. The reasons for the low performance of the DSSCs containing DMA, DMSO and DMF, respectively, were due to the negative shift of TiO₂ conduction band and the desorption of dye molecules from the TiO₂ photoelectrode by those solvents.     

Evaluation on over photocurrents measured from unmasked dye-sensitized solar cells

We have investigated the change in photocurrent density (J_SC) of dye-sensitized solar cell (DSSC) before and after covering an aperture mask on the cell, especially its dependence on solar absorption range in dye. Four different dyes having absorption threshold at 460 nm (P5), 520 nm (TA-St-CA), 680 nm (N719) and 820 nm (N749) are tested. J_SC of the DSSC without mask decreases after mask, where the decreasing rate (△J_SC = J_SC (no mask) − J_SC (with mask)/J_SC (no mask)) becomes larger when dye absorption threshold decreases. △J_SC at the given TiO₂ film thickness of 10 μm is determined to be about 20%, 15% and 13% for P5, TA-St-CA and N719-N749, respectively, which is reduced to 14% (TA-St-CA), 11.3% (N719) and 10.5% (N749) after increasing the thickness to 20 μm, except for P5 dye remaining unchanged. According to the analysis based on IPCE and photon flux data, the over photocurrents observed for the unmasked dye-sensitized solar cells and their dependence on dye absorption range are found to be attributed to diffuse light leaving the dye-adsorbed TiO₂ active area.     

The effects of hydrothermal temperature and thickness of TiO2 film on the performance of a dye-sensitized solar cell

The effects of hydrothermal temperature on the preparation of TiO₂ colloids, and their film thickness on fluorine-doped tin oxide (FTO) glass, toward the performance of a dye-sensitized solar cell (DSSC) were investigated. Pore diameter and surface area of the TiO₂ are of paramount importance in determining the cell efficiency. With the increase of hydrothermal temperature, the pore diameter increases linearly; however, the surface area shows the reverse effect. It is found that the DSSC assembled with the TiO₂ films prepared under the hydrothermal temperature of 240 °C, and the film thickness larger than 10 μm gives optimal performance. The effect of film thickness of TiO₂ on the performance of the DSSC can be explained by the relative size of reactive species diffusing into the thin film and the lifetime of injected electrons. Electrochemical impedance spectroscopy (EIS) was also used to analyze the resistance of the cell, developed as a result of the change in the thickness of the TiO₂ thin film. The at-rest stability for over 200 days was monitored and the results show that the solar energy conversion efficiency was found to decrease from 5.0% of initial value to 3.0% at the end.     

Characteristics of triphenylamine-based dyes with multiple acceptors in application of dye-sensitized solar cells

We report the synthesis and photophysical/electrochemical properties of triphenylamine (TPA)-based multiple electron acceptor dyes (TPAR1, TPAR2, and TPAR3) as well as their applications in dye-sensitized solar cells (DSSCs). In these dyes, the TPA group and the rhodanine-3-acetic acid play the role of the basic electron donor unit and the electron acceptor, respectively. It was found that introduction of two rhodanine-3-acetic acid groups into the TPA unit (TPAR2) exhibited better photovoltaic performance due to the increase with a red shift and broadening of the absorption spectrum. The monolayer of these TPA-based dyes was adsorbed on the surface of nanocrystalline TiO₂ mesoporous electrode with the thickness of ∼6 μm, polyethylene oxide (PEO) used as the matrix of gel electrolyte, and 4-nm thick Pt used as a counter-electrode. Photovoltaic device can be realized in a single quasi-solid-state DSSC. TPAR2-based gel DSSC had an open circuit voltage and short circuit current density of about 541 and 10.7 mA cm⁻², respectively, at 1-sun.     

A simple method to prepare uniform-size nanoparticle TiO2 electrodes for dye-sensitized solar cells

Nanoparticle TiO₂ electrodes are fabricated using an improved electrostatic spray coating (ESC) method which is more simple, low cost and well reproducible comparing with the conventional method of preparing electrode for dye-sensitized solar cells (DSSC) by introducing monoethanolamine (MEA) into precursor solution. It is surprised to find that high transparency of films and good adhesion between film and substrate achieve and that particles size can be easily controlled by adjusting the proportion of MEA. The relationship between particles size and proportion of MEA added is presented in our work. After samples with various particle sizes are applied in DSSC, an increase of open-circuit voltage (V_oc) from 620 mV to 765 mV is observed with the increase of particle size from 8 nm to 48 nm. Associated with photoluminescence results, we ascribe the change of V_oc to the different dominative states of films: surface defects and oxygen vacancies in 8 nm films, oxygen vacancy defects in 25 nm films and higher crystal quality with little of both defects in 48 nm films. In addition, different thickness films with optimized proportion of MEA is applied in DSSC, an overall light to electricity conversion efficiency (η) of 2.91% is obtained with a thickness of 2.0 μm.     

Fabrication of multilayer TiO2 thin films for dye-sensitized solar cells with high conversion efficiency by electrophoresis deposition

This research coats a commercial TiO₂ nanoparticle Degussa P25 with good roundness and size uniformity on an indium tin oxide (ITO) glass substrate and to be photoelectrical electrode by electrophoresis deposition. It combined with dye N719, electrolyte I^-/ and counter-electrode of Pt layer to produce dye-sensitized solar cells (DSSCs). Through the electrophoretic technique, a multilayer film of an appropriate thickness is deposited in the suspension containing TiO₂ nanoparticles and isopropanol. In this process, electric current, voltage, and the number of deposition cycles are well controlled to obtain a single TiO₂ film of around 3.3 μm thick. Stacking is then performed to obtain a multilayer-typed TiO₂ film of around 12 μm thick. As the sintering temperature reaches 400 °C, the prepared multilayer TiO₂ film with a good compactness can increase the dye adsorption capability of the thin film and enhance its adsorption percentage. In addition, the heat treatment will transfer a portion of the rutile crystalline into the anatase crystalline, resulting in better material properties for DSSCs application. DSSCs produced are exposed to metal halide lamp and their energy conversion efficiency is measured. The I-V curve of the produced DSSCs shows that it has an excellent energy conversion efficiency of 6.9%.     

Cubic titanium dioxide photoanode for dye-sensitized solar cells

Following from the recently evolved concept of significantly improving the photovoltaic efficiency in dye-sensitized solar cells (DSSCs) by reducing the loss of electrons on the spherical surface of titanium dioxide, this study examines the synthesis of cubic TiO₂ with a special morphology to overcome this electron loss and investigates its application to DSSCs. Cubic TiO₂ is synthesized by an advanced rapid hydrothermal method, with the addition of an amine species additive. Transmission electron microscopy (TEM) images confirm the cubic shape of the TiO₂ particles with a diameter less than 5-10 nm. Using N719 dye under illumination with 100 mW cm⁻² simulated sunlight, the application of cubic TiO₂ to DSSCs affords an energy conversion efficiency of approximately 9.77% (4.0-μm thick TiO₂ film), which is considerably enhanced compared with that achieved using a commercial, spherical TiO₂. Electrostatic force microscopy (EFM) and impedance analyses reveal that the electrons are transferred more rapidly to the surface of a cubic TiO₂ film than on a spherical TiO₂ film.     

Influence of electrolyte on the photovoltaic performance of a dye-sensitized TiO2 solar cell based on a Ru(II) terpyridyl complex photosensitizer

We have investigated the influence of electrolyte composition on the photovoltaic performance of a dye-sensitized nanocrystalline TiO₂ solar cell (DSSC) based on a Ru(II) terpyridyl complex photosensitizer (the black dye). We have also spectroscopically investigated the interaction between the electrolyte components and the adsorbed dye. The absorption peaks attributed to the metal-to-ligand charge transfer transitions of the black dye in solution and adsorbed on a TiO₂ film, were red-shifted in the presence of Li cations, which led to an expansion of the spectral response of the solar cell toward the near-IR region. The photovoltaic performance of the DSSC based on the black dye depended remarkably on the electrolyte composition. We developed a novel efficient organic liquid electrolyte containing an imidazolium iodide such as 1,2-dimethyl-3-n-propylimidazolium iodide or 1-ethyl-3-methylimidazolium iodide (EMImI) for a DSSC based on the black dye. A high solar energy-to-electricity conversion efficiency of 9.2% (J_sc=19.0 mA cm⁻², V_oc=0.67 V, and FF=0.72) was attained under AM 1.5 irradiation (100 mW cm⁻²) using a novel electrolyte consisting of 1.5 M EMImI, 0.05 M iodine, and acetonitrile as a solvent with an antireflection film.     

Thickness effect of RF sputtered TiO2 passivating layer on the performance of dye-sensitized solar cells

We report on the characteristics of a TiO₂ passivating layer grown by radio frequency (RF) magnetron sputtering on F-doped SnO₂ (FTO) electrodes as a function of its thickness. The optical transparency, surface roughness and passivation properties of the TiO₂ layer passivating the FTO electrode depend on the thickness of the TiO₂ passivating layer. In addition, it was found that the power conversion efficiency of the dye-sensitized solar cells (DSSCs) is critically dependent on the thickness of RF sputtered TiO₂ layer inserted between FTO electrode and nanoporous TiO₂ layer. The DSSC fabricated on 50 nm thick TiO₂ passivating FTO electrode showed the maximum power conversion efficiency of 4.42% due to effective prevention of the electron transfer to electrolyte. This indicates that the thickness optimization of the TiO₂ passivating layer is one of the important parameter to obtain high performance DSSCs.     

A dye-sensitized photo-supercapacitor based on PProDOT-Et2 thick films

A photo-rechargeable supercapacitor (photo-supercapacitor, or PSC) is studied using a N3-dye adsorbed TiO₂ photoelectrode and PProDOT-Et₂ poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) polymer films as supercapacitor materials for electron storage. The PSC device, comprising a dye-sensitized solar cell (DSSC) and a supercapacitor (SC), can store the photo-to-electric energy. The PProDOT-Et₂ films are potentiostatically electropolymerized to form thick films (ca. 0.5 mm) with a specific capacitance of ca. 6.5 F cm⁻². A symmetrical (p/p) supercapacitor, with PProDOT-Et₂ coated on both electrodes, is also characterized before fabricating the three-electrode PSC. The PSC is tested under light illumination of 100 mW cm⁻², and attaining a photocharged voltage of 0.75 V and a discharged energy density of 21.3 μWh cm⁻².     

Electrophoretic deposition of ZnO film and its compression for a plastic based flexible dye-sensitized solar cell

A room temperature fabrication method is developed for the preparation of a ZnO porous film on a plastic substrate, involving an electrophoresis deposition (EPD) process, followed by the compression of the film as the post-treatment. The thus prepared ZnO film is used for the photoanode of a dye-sensitized solar cell (DSSC). Besides, an indoline dye is employed as the sensitizer (referred to as D149) for the ZnO semiconductor. Performances of such DSSCs are studied at various compression pressures used for their ZnO films. Electrochemical impedance spectroscopy (EIS) is employed to quantify the charge transport resistance at the ZnO/dye/electrolyte interface (R_ct2) and the electron lifetime (τ_e) in the ZnO film. As for the thickness effect, ZnO film with a thickness of about 22 μm renders the best efficiency for the ZnO based DSSC. In addition, UV-O₃ is applied in two ways; in one way only the compressed ZnO film is treated in one step, and in the second way both the substrate and the compressed ZnO film are treated separately in two steps. The adherence of the ZnO film is shown by a photograph. Scanning electron microscopy is used to characterize the morphologies of the ZnO films.     

On the I–V measurement of dye-sensitized solar cell: Effect of cell geometry on photovoltaic parameters

The effect of mask aperture size with respect to dye-adsorbed TiO₂ area on the response of photocurrent, voltage, fill factor and efficiency of dye-sensitized solar cell (DSSC) was recently studied by Gratzel's research group [S. Ito, Md. K. Nazeeruddin, P. Liska, P. Comte, R. Charvet, P. Pechy, M. Jirousek, A. Kay, S.M. Zakeeruddin, M. Grätzel, Prog. Photovolt. Res. Appl. 14 (2006) 589], where it was proposed that overall efficiency could be overestimated when measuring a DSSC without mask having adequate aperture size. In this report, beside the aperture size, we have studied effects of glass substrate thickness and geometry, thickness and layer structure of TiO₂ film on photovoltaic parameters. Photovoltaic parameters, mostly photocurrent density, were found to be significantly influenced by the glass substrate thickness and the TiO₂ layer structure. Data analysis suggests that photovoltaic characteristics before and after mask are dependent not only on measuring condition such as mask aperture size but also on substrate thickness and TiO₂ layer structure.     

Pomegranate leaves and mulberry fruit as natural sensitizers for dye-sensitized solar cells

This study employs chlorophyll extract from pomegranate leaf and anthocyanin extract from mulberry fruit as the natural dyes for a dye-sensitized solar cell (DSSC). A self-developed nanofluid synthesis system is employed to prepare TiO₂ nanofluid with an average particle size of 25 nm. Electrophoresis deposition was performed to deposit TiO₂ nanoparticles on the indium tin oxide (ITO) conductive glass, forming a TiO₂ thin film with the thickness of 11 μm. Furthermore, this TiO₂ thin film was sintered at 450 °C to enhance the thin film compactness. Sputtering was used to prepare counter electrode by depositing Pt thin film on FTO glass at a thickness of 20 nm. The electrodes, electrolyte (), and dyes were assembled into a cell module and illuminated by a light source simulating AM 1.5 with a light strength of 100 mW/cm² to measure the photoelectric conversion efficiency of the prepared DSSCs. According to experimental results, the conversion efficiency of the DSSCs prepared by chlorophyll dyes from pomegranate leaf extract is 0.597%, with open-circuit voltage (V_OC) of 0.56 V, short-circuit current density (J_SC) of 2.05 mA/cm², and fill factor (FF) of 0.52. The conversion efficiency of the DSSCs prepared by anthocyanin dyes from mulberry extract is 0.548%, with V_OC of 0.555 V and J_SC of 1.89 mA/cm² and FF of 0.53. The conversion efficiency is 0.722% for chlorophyll and anthocyanin as the dye mixture, with V_OC of 0.53 V, J_SC of 2.8 mA/cm², and FF of 0.49.     

Fabrication of dye-sensitized solar cells by transplanting highly ordered TiO2 nanotube arrays

Highly ordered TiO₂ nanotube arrays fabricated by anodization are very attractive to dye-sensitized solar cells (DSCs) due to their superior charge percolation and slower charge recombination. However, the efficiency of TiO₂-nanotube-based DSCs is 6.89%, which is still lower than that of TiO₂-nanoparticle-based DSCs. We have suggested the transplanting the highly ordered TiO₂ nanotube arrays to FTO glass to improve the performance of TiO₂-nanotube-based DSCs. DSCs based on transplanted TiO₂ nanotube arrays and TiO₂ nanoparticles were fabricated by same process and materials to exclude the unexpected factors. In TiO₂ thickness of ca. 15 μm, the efficiency of 2.91% in front-side illuminated DSCs based on TiO₂ nanotube arrays was higher than those in back-side illuminated DSCs based on TiO₂ nanotube arrays and in front-side illuminated DSCs based on TiO₂ nanoparticle. Front-side illuminated DSCs based on TiO₂ nanotube arrays having various thicknesses were successfully fabricated. The efficiency in DSCs having 20.0 μm thick TiO₂ nanotube arrays was improved to 5.36% by TiCl₄ treatment.     

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cells

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cell (DSSC) has been investigated to optimize the device geometry for reliable energy conversion efficiency assessment. Mesoporous TiO₂ layers with an identical active area (0.40 cm²) and different dimension are prepared on FTO glass substrate by the screen printing method and used as photoanodes for DSSCs. Under 1 sun illumination (AM 1.5G, 100 mW cm⁻²), both the open-circuit voltage and the short-circuit current density are independent of electrode geometry whereas the fill factor and hence energy conversion efficiency show strong dependency. Electrochemical impedance spectroscopy analysis indicates that the distance between active layer and ohmic contact directly contributes to internal series resistance and influence photovoltaic performance.     

High efficiency flexible dye-sensitized solar cells by multiple electrophoretic depositions

A multiple electrophoretic deposition (EPD) of binder-free TiO₂ photoanode has been developed to successfully fill the crack occurring after air-drying on the first EPD-TiO₂ film surface. With the slow 2nd EPD, high quality TiO₂ thin films are acquired on flexible ITO/PEN substrates at room temperature and the device efficiency of the dye-sensitized solar cell achieved 5.54% with a high fill factor of 0.721. Electrochemical impedance spectroscopy measurements analyze the great enhancement of the photovoltaic performance through multiple EPD. The electron diffusion coefficient improved by about 1 order of magnitude in crack-less multiple-EPD TiO₂ films. With the scattering layer, the device reveals a high conversion efficiency of up to 6.63% under AM 1.5 G one sun irradiation, having a short circuit current density, open circuit voltage, and filling factor of 12.06 mA cm⁻², 0.763 V and 0.72, respectively.