Photoelectrodes In Dye-sensitized Solar Cells Research Articles

Nanoporous tin oxide photoelectrode prepared by electrochemical anodization in aqueous ammonia to improve performance of dye sensitized solar cell

In this work, a nanoporous thin film consisting of sieve-like tin oxide (SnO2) sheets was prepared by anodization of tin foil in aqueous ammonia and applied as a photoelectrode for dye-sensitized solar cell (DSSC). Compared to the conventional SnO2 nanoparticle-based photoelectrode, the anodized nanoporous SnO2 electrode offers higher short circuit current (6.85 vs. 3.64 mA/cm2) and open circuit voltage (0.56 vs. 0.51 V) due to better charge transportation and better suppression of charge recombination. Consequently, the power conversion efficiency of the DSSC equipped with the anodized SnO2 electrode is enhanced by 63%.

Preparation of Anatase/Rutile Mixed-Phase Titania Nanoparticles for Dye-Sensitized Solar Cells

Acid-labile high surface mesoporous ZnO/Zn(OH)2 composite material is used as a novel hard template for the preparation of mesoporous amorphous TiO2. The template-free amorphous TiO2 material is then thermally crystallized at suitable temperature to control the relative ratio of anatase and rutile phases in a particle. Four different anatase/rutile (AR) mixed-phase TiO2 nanoparticles (AR-3, AR-15, AR-20, and AR-23 denoted for the samples of 3%, 15%, 20%, and 23% rutile phase, respectively) are prepared and characterized by powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM). The coexistence of anatase and rutile phases in a TiO2 nanoparticle is visually confirmed by HRTEM analysis. These mixed-phase TiO2 nanoparticles are examined as candidates for photoelectrodes of dye-sensitized solar cells (DSSCs). The J-V curves and IPCE spectra for the DSSCs prepared from the mixed-phase TiO2 nanoparticles are obtained, and their photovoltaic properties are investigated. The photo-conversion efficiency (eta) indicates the highest value of 5.07% for AR-20. The synergistic effect of coexisting anatase and rutile phases with an optimal ratio in a TiO2 nanoparticle of AR-20 for an efficient interfacial transfer of photo-generated electrons is likely to lead to the highest efficiency among the AR-n samples.

Double light-scattering layer film based on TiO2 hollow spheres and TiO2 nanosheets: Improved efficiency in dye-sensitized solar cells

A novel TiO2 double light-scattering layer (TiO2-DLL) film consisting of TiO2 hollow spheres (TiO2-HS) as overlayer and TiO2 nanosheets (TiO2-NS) as underlayer was designed as the photoelectrode of dye-sensitized solar cells (DSSCs). The photoelectric conversion performances of DSSCs based on TiO2-HS film, TiO2-NS film, and TiO2-DLL film were investigated. It was found that the TiO2-DLL film cell achieved the highest conversion efficiency and reached up to a maximum value of 5.08%, which is 23.3% higher than that of TiO2-HS film cell (η=3.62%) and 8.3% higher than that of TiO2-NS film cell (η=4.31%) under identical film thickness at a constant irradiation of 100mWcm−2. The enhanced efficiency of TiO2-DLL film can be attributed to its relatively high specific surface area and enhanced light-scattering capability. Furthermore, the TiO2-NS underlayer ensures good electronic contact between TiO2 film and the F-doped tin oxide (FTO) conducting glass.

Annealing effects on self-assembled TiO2 nanotubes and their behavior as photoelectrodes in dye-sensitized solar cells

Self-assembled highly ordered TiO2 nanotube arrays were grown on Ti foils in NH4F ethylene glycol electrolyte under mild anodic oxidation conditions. The effect of annealing post-treatment on their structural properties was systematically investigated with respect to their electrical characteristics and photoelectrochemical performance in back-side illuminated dye-sensitized solar cells (DSCs). A variety of parameters was controlled and optimized including the annealing temperature, the heating rate and duration of the thermal treatment. The obtained results confirmed a correlation between the crystalline/structural properties of the TiO2 nanotubes and their electrical characteristics, thus revealing the close interplay of crystal size, grain boundaries and crystallite interconnectivity with the electron dynamics (transport/recombination) governing the DSC operation and efficiency. The high importance of a barrier layer at the interface between the nanotubes and the Ti foil was also highlighted.

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.

An Analysis and Research on the Transmission Ratio of Dye Sensitized Solar Cell Photoelectrodes by Using Different Etching Process

Classical photoelectrodes for Dye Sensitized Solar Cells (DSSCs) were fabricated by using the electrochemical method on the titanium (Ti) template, for that the fabrication process would influence the characteristics of the DSSCs. In this study, at first three different methods were used to etch Ti templates from 10 to 17 min, (1) polishing-chemical etching: Ti template was annealed at 450°C for 1 h, abraded using number 80 to 1500 SiC sheet, and then etched in a solution of 5% HF + 95% H2O; (2) electrochemical polishing-chemical etching: Ti template was annealed at 450°C for 1 h, electrolytic polishing with 42% CH3OH + 5% HClO4 + 53% HOCH2CH2OC4H9solution, and the chemical-etching in a solution of 5% HF + 95% H2O; (3) chemical etching: Ti template was etched in a solution of 5% HF + 95% H2O and annealed at 450°C for 1 h. When the etching time was changed from 10 to 17 min, the thicknesses of Ti templates decreased from 75.3 μm to 14.8 μm, depending on the etching method. After etching process, the TiO2nanotube arrays were fabricated as the photoelectrode of DSSCs by electrochemical process, in which the Ti as anode and platinum (Pt) as cathode. The electrolyte solution included C2H4(OH)2, NH4F, and deionized water. After annealing the grown TiO2nanotube arrays at 450°C for 3 h, we would show that the etching process had large effect on the structure and transmittance ratio of the TiO2nanotube arrays.

Photovoltaic Characteristics of a Dye-Sensitized Solar Cell (DSSC) Fabricated by a Nano-Particle Deposition System (NPDS)

Nano particle deposition system (NPDS) was evaluated for the potential photoelectrode fabrication method for dye-sensitized solar cell (DSSC). TiO2 powders as a photoelectrode for DSSC, were fabricated via NPDS and screen printing methods to compare its photovoltaic characteristics. The NPDS is a novel system for depositing ceramic and metallic powders on substrates at room temperature without using solvents by accelerating particles to supersonic velocity. The conventional method for preparing working electrodes uses TiO2 paste where poor adhesion to the substrate and numerous steps to properly disperse the powders are the main problems. To overcome these issues, TiO2 powders were directly sprayed onto ITO glass using the NPDS. The nano-sized TiO2 particles were densely deposited, no adhesion problems were noted and the solar-cell properties were improved. The photovoltaic performance of a cell having TiO2 layers first deposited by the NPDS method and then deposited by the paste method was compared to a cell fabricated only using the NPDS and only using the paste. A light conversion efficiency of 3.5% was obtained for the former cell, which indicates synergy effect of using these two methods. The screen-printing step right after NPDS deposition served to densify the TiO2 film, which decreased the series resistance. Therefore, NPDS was found to be a promising approach for improving DSSC properties and efficiencies. [doi:10.2320/matertrans.M2013220]

Efficient ZnO-based counter electrodes for dye-sensitized solar cells

ZnO is usually exploited as a photoelectrode for dye-sensitized solar cells (DSSCs), but it has not been applied as a DSSC counter electrode. Herein, it is reported that ZnO with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) could be an efficient DSSC counter electrode with a maximum power conversion efficiency of 8.17%. Furthermore, it was found that the conversion efficiency of DSSCs with the ZnO/PEDOT:PSS counter electrode strongly depends on the ZnO content. Density functional theory (DFT) calculations, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) measurements revealed that the variation of the conversion efficiency with ZnO content is determined by the fill factor (FF), which is dependent on the combination of the catalytic activity of ZnO and the conductivity of PEDOT:PSS.

TiO2 nanocrystals shell layer on highly conducting indium tin oxide nanowire for photovoltaic devices

We demonstrated a highly efficient conducting indium tin oxide (ITO) core-TiO2 nanocrystals shell nanowire array for a photoelectrode in dye-sensitized solar cells with regard to light harvest and charge collection. The TiO2 shell layer, consisting of anatase nanocrystals of ~2 nm, were successfully formed on a single crystalline ITO nanowire prepared via a vapor transport method using repetitive TiCl4 aqueous solution treatments at 50 °C. We found that the nanocrystal size and number of Cl(-) ions remaining on the formed shell layer critically influence the dye loading properties. Moreover, these factors can be controlled by means of a post-annealing process. We also found that the dye loading and the back electron transport from the conductive ITO nanowire to the electrolyte mainly determine the final cell performance. The proposed double-shell layer structure consisting of dense and porous layers showed significantly improved cell performance.

Photovoltaic Performance of Vertically Grown ZnO Nanorods in Dye-sensitized Solar Cells

For an application to the photoelectrode in dye-sensitized solar cells (DSSCs), we fabricated ZnO nanorod arrays on fluorine-doped tin oxide (FTO) coated glass substrate by chemical bath deposition, and modified its surface through sodium silicate coating. Effects of the surface modification on the morphology of ZnO nanorod arrays, as well as the photoelectric conversion properties in DSSCs were investigated. From the field-emission scanning electron microscopy (FE-SEM) images, ZnO nanorods are observed to be vertically grown on FTO substrate. The surface treatment via dip coating within sodium silicate solution gives rise to increase the photocurrent, resulting in the increase of solar-to-electrical power conversion efficiency. This may be originated from that the surface-treated ZnO photoelectrode has higher conduction edge to introduce an energy barrier for increasing charge separation and suppressing recombination rate at the photoelectrode-electrolyte interface.

Enhanced charge collection efficiency of dye-sensitized solar cells based on size-tunable hierarchically structured TiO2beads

Hierarchically structured mesoporous TiO2 beads (HS-TBs), which are used as photoelectrodes in highly efficient dye-sensitized solar cells (DSCs), were prepared by an electrostatic spray (e-spray) technique. To prepare different sized HS-TBs, the electric field and the concentration of TiO2 particles were carefully controlled, because they are critical factors in preparing size-controlled TiO2 beads. Four different HS-TBs were formulated with average diameters of 250, 450, 700, and 1200 nm as high-quality photoelectrodes for use in DSCs. In this study, we found that the zero-dimensional HS-TBs were the most promising photoelectrode for DSCs due to enhanced charge collection efficiency and better penetration of electrolytes through relatively large pores among the HS-TBs. The HS-TBs were characterized by intensity modulated photocurrent spectroscopy (IMPS), the scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) method, Barrett–Joyner–Halenda (BJH) method, and electrochemical analysis. The conversion efficiency of HS-TB photoelectrodes improved with increasing bead size due to the enhanced electron transport through electrodes. The present HS-TB cells exhibit a noticeable improvement in the overall efficiency: maximum 9.54% (1200 nm) versus 5.83% for the reference cell made of a TiO2 nanocrystalline film.

High performance dye-sensitized solar cells (DSSCs) achieved via electrophoretic technique by optimizing of photoelectrode properties

This paper describes a simple method utilizing electrophoretic deposition (EPD) of commercial P25 nanoparticles (NPs) films on fluoride-doped tin oxide (FTO) substrate. In this process, voltage and the number of deposition cycles are well controlled to achieve TiO2 film thickness of around 1.5–26μm, without any mechanical compression processing. The experimental results indicate that the TiO2 film thickness plays an important role as the photoelectrode in DSSCs because it adsorbs a large number of dye molecules which are responsible for electrons supply. Furthermore, it was found that effects of the bulk traps and surface states within the TiO2 films on the recombination of the photo-injected electrons (electron–hole pairs) strongly depend on the TiO2 electrode annealing temperature. Finally, a DSSC with a 24μm thick TiO2 film and annealed at 500°C produced the highest conversion efficiency (η=6.56%, ISC=16.4, VOC=0.72, FF=0.55) with an incident solar energy of 100mW/cm2.

Formation of spherical TiO2 nanorod aggregates with multiple functions for dye-sensitized solar cells

Spherical TiO2 nanorod aggregates were fabricated by a facile hydrothermal method and studied for exploring the formation mechanism and improving energy conversion efficiency in dye-sensitized solar cells (DSSCs). A four-step nucleation and growth process was proposed for the formation of the spherical TiO2 nanorod aggregates. Using the spherical TiO2 nanorod-aggregate photoelectrode for DSSC, an overall energy conversion efficiency of 6.05% and a short-circuit photocurrent density of 17.87mAcm−2 are achieved. These are respectively 46.1% and 44.9% higher than those of the DSSC using the nanoparticle photoelectrode. These improvements are attributed to the multiple functions of spherical TiO2 nanorod aggregates, including large surface area for sufficient dye adsorption, sub-micrometre size for efficient light scattering, open structure for quick electrolyte diffusion, and one-dimensional building units for longer electron lifetime.

Natural dye-based photoelectrode for improvement of solar cell performance

In the present paper, photovoltaic studies of dye-sensitized solar cells (DSSCs) based on betacyanin/TiO2 and betacyanin/WO3–TiO2 have been done. The cell performances were compared through I–V curves and wavelength dependant photocurrent measurements for the two new types of DSSCs. The TiO2-coated DSSC showed the photovoltage and photocurrent of 300 mV and 4.96 mA/cm2, whereas the cell employing WO3–TiO2 photoelectrode showed the values 435 mV and 9.86 mA/cm2, respectively. The conversion efficiency of TiO2 based dye-sensitized solar cell was found to be 0.69 %, while WO3–TiO2-based cell exhibited a higher conversion efficiency of 2.2 %. The better performance of the WO3–TiO2 dye-sensitized solar cell photoelectrode is thought to be due to an inherent energy barrier at the electrode/electrolyte interface leading to the reduced recombination of photoinduced electrons.

Diffusion Length in Nanoporous Photoelectrodes of Dye-Sensitized Solar Cells under Operating Conditions Measured by Photocurrent Microscopy.

We determined the carrier diffusion lengths in nanoporous layers of dye-sensitized solar cells by using scanning photocurrent microscopy. The diffusion lengths were found to be 60-100 μm for the conventional cells. In addition, we found a correlation between the carrier diffusion lengths and the cell efficiency, which proved that improvement in the diffusion length is one of the crucial factors for optimizing device performance. The diffusion length was measured for various operating conditions by varying parameters such as solar light intensity and applied electrical voltage. In particular, we observed electric-field-driven, carrier transport phenomena (i.e., drift current) in modified cells. Fitting with the drift-diffusion model enabled us to extract the electric field strengths present in the TiO2 nanoporous layer.

Effects of heat treatment on the dye adsorption of ZnO nanorods for dye-sensitized solar cells

Well-aligned ZnO nanorods for the photoelectrode of dye-sensitized solar cells (DSSCs) were grown via a sonochemical method, and the heat-treatment effects on the dye adsorption in the DSSCs were studied. The heat treatment of well-aligned ZnO nanorods was performed at 200 ∼ 500 °C for 1 h, which was immediately followed by the dye adsorption. The dye amounts adsorbed in the ZnO nanorods were estimated from the UV-Vis absorbance by using Beer-Lambert’s law. The efficiency of the DSSCs with ZnO nanorods was measured to investigate the heat-treatment effects of ZnO nanorods on the dye adsorption properties. The heat-treatment of ZnO nanorods was found to yield a change in their dye adsorption ability, resulting in a change in the efficiency of the DSSCs.

Effects of the aspect ratio on the dye adsorption of ZnO nanorods grown by using a sonochemical method for dye-sensitized solar cells

Well-aligned ZnO nanorods for the photoelectrode of dye-sensitized solar cells (DSSCs) were grown via a sonochemical method, and the effects of their aspect ratios on the dye adsorption in DSSCs were studied. The control of the aspect ratio of well-aligned ZnO nanorods was performed by tuning the mole concentration of zinc acetate dehydrate in the range of 0.04–0.06M. The dye amounts adsorbed in the ZnO nanorods were estimated from the UV-Visible absorbance by using the Beer-Lambert law. The efficiency of DSSCs with ZnO nanorods was measured to investigate the effects of the aspect ratio of the ZnO nanorods on the dye adsorption properties. A change in the aspect ratio of the ZnO nanorods was founded to yield a change in their dye adsorption ability, resulting in a change in the efficiency of the DSSCs.

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 Rk (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.

TiO2 flowers and spheres for ionic liquid electrolytes based dye-sensitized solar cells

Micro-sized 3D branched flower like and sphere TiO2 have been synthesized through a controlled hydrolysis followed by hydrothermal process. The resultant flower and sphere TiO2 were characterized by SEM, XRD, BET and light scattering measurements. Overwhelming anatase and strong light scattering were exhibited for the flower sample. Pure sphere and flower and sphere-mixed (flower/sphere) films were formed and applied as photoelectrodes for dye-sensitized solar cells (DSCs) with both ionic liquid and gelled ionic liquid electrolytes. The devices exhibited considerably enhanced overall energy conversion efficiency in comparison with that of those derived from TiO2 nanoparticles. The superior performances were attributed to their fast electrolyte diffusion and good interfacial contact of TiO2/electrolyte as validated by EIS investigations as well as light scattering. The participation of flowers with the sphere films further improved the electrolyte penetration and light scattering, making the flower/sphere device the most efficient with a low dye loading.

Solution-growth and optoelectronic performance of ZnO : Cl/TiO2 and ZnO : Cl/ZnxTiOy/TiO2 core–shell nanowires with tunable shell thickness

Arrays of vertically aligned ZnO : Cl/TiO2 and ZnO : Cl/ZnxTiOy/TiO2 core–shell nanowires (NWs) were prepared by means of the combination of two solution-growth processes. First, single-crystal ZnO NWs with controlled n-type doping were grown on conducting substrates by a low-cost, high-yield and seed-free electrochemical route. These NWs were covered by a titanium oxide shell of tunable thickness mediating successive adsorption-hydrolysis-condensation steps. Using this atomic-layer growth procedure, titania shells with controlled thickness and the anatase TiO2 phase were obtained after sintering at 450 °C. Higher sintering temperatures resulted in the formation of ZnO : Cl/ZnxTiOy/TiO2 core–shell NWs by the interdiffusion of Zn and Ti ions at the ZnO–TiO2 interface. The performance of ZnO : Cl/TiO2 and ZnO : Cl/ZnxTiOy/TiO2 core–shell NWs towards photoelectrochemical (PEC) water splitting was investigated as a function of the titania shell thickness. Furthermore, the performance of such core–shell NWs as photoelectrodes in dye-sensitized solar cells was also characterized. The TiO2 presence at the ZnO : Cl surface promoted a two-fold increase on the produced photocurrent densities, probing their potential for PEC and optoelectronic applications. Electrochemical impedance spectroscopy was used to corroborate the lower resistance for charge transfer between the NWs and the electrolyte in the presence of the TiO2 shell.