Effect of silver (Ag) ions irradiation on the structural,optical and photovoltaic properties of Mn doped TiO2 thin films based dye sensitized solar cells

Dye sensitized solar cell (DSSC) is an emerging energy harvesting tool which converts direct sunlight into electrical energy. These cells have much better properties in contrast with silicon based solar cells because of their flexible nature, light weight, low cost, environment friendly nature, and involvement of a simple manufacturing process. Since, a photoanode is the backbone of DSSC, we synthesized a pure and 1% manganese (Mn) doped titanium dioxide (TiO₂) films by sol-gel method which are irradiated with silver (Ag) ions at two different concentrations (2 × 10¹⁴ and 4 × 10¹⁴) ions-cm^?2. X-ray diffraction revealed that Mn doping followed by Ag irradiation transformed TiO₂ from pure anatase to rutile phase. Ultraviolet–visible spectroscopy exposed the reduction in band gap of TiO₂ film during this doping and irradiation process. Therefore, absorption is enhanced with red shift in UV-range. When these films are used as a photoanode in DSSC, 1% Mn doped TiO₂ film exposed with Ag at the concentration of (2 × 10¹⁴) ions-cm^?2 exhibited maximum efficiency of 2.40%.     

Enhancement of the photoelectric performance of dye-sensitized solar cells using Ag-doped TiO<Subscript>2</Subscript> nanofibers in a TiO<Subscript>2</Subscript> film as electrode

For high solar conversion efficiency of dye-sensitized solar cells [DSSCs], TiO₂ nanofiber [TN] and Ag-doped TiO₂ nanofiber [ATN] have been extended to be included in TiO₂ films to increase the amount of dye loading for a higher short-circuit current. The ATN was used on affected DSSCs to increase the open circuit voltage. This process had enhanced the exit in dye molecules which were rapidly split into electrons, and the DSSCs with ATN stop the recombination of the electronic process. The conversion efficiency of TiO₂ photoelectrode-based DSSCs was 4.74%; it was increased to 6.13% after adding 5 wt.% ATN into TiO₂ films. The electron lifetime of DSSCs with ATN increased from 0.29 to 0.34 s and that electron recombination was reduced.     

Aqueous Colloidal Stability Evaluated by Zeta Potential Measurement and Resultant TiO2 for Superior Photovoltaic Performance

Controlling the morphological structure of titanium dioxide (TiO₂) is crucial for obtaining superior power conversion efficiency for dye‐sensitized solar cells. Although the sol–gel‐based process has been developed for this purpose, there has been limited success in resisting the aggregation of nanostructured TiO₂, which could act as an obstacle for mass production. Herein, we report a simple approach to improve the efficiency of dye‐sensitized solar cells (DSSC) by controlling the degree of aggregation and particle surface charge through zeta potential analysis. We found that different aqueous colloidal conditions, i.e., potential of hydrogen (pH), water/titanium alkoxide (titanium isopropoxide) ratio, and surface charge, obviously led to different particle sizes in the range of 10–500 nm. We have also shown that particles prepared under acidic conditions are more effective for DSSC application regarding the modification of surface charges to improve dye loading and electron injection rate properties. Power conversion efficiency of 6.54%, open‐circuit voltage of 0.73 V, short‐circuit current density of 15.32 mA/cm², and fill factor of 0.73 were obtained using anatase TiO₂ optimized to 10–20 nm in size, as well as by the use of a compact TiO₂ blocking layer.     

Enhanced photoelectric efficiency by surface modification of TiO<Subscript>2</Subscript> thin film using various acidic species

This study examined the photoelectric conversion efficiency of the dye-sensitized solar cell (DSSC) when the surface of a nanometer-sized TiO₂ film, which was prepared using the solvothermal method, was modified by five acid compounds. The TiO₂ film exhibited an anatase structure with an average particle size in the range of 10–15 nm, and the maximum absorption band was shown in the UV-visible spectrum around 360 nm. The surface colors of the carboxylic acid-modified TiO₂ films were changed to light or dark with differing energy conversion efficiencies. Particularly, the conversion efficiency was considerably enhanced from approximately 6.25% for the non-modified TiO₂ film to approximately 7.50% for the film treated by acetic acid of 1.0 mole, with the N719 dye under 100 mW/cm² of simulated sunlight. FT-IR analysis of the films after N719 dye adsorption confirmed that the IR spectrum of the modified TiO₂ showed a sharp and strong band at 500 cm⁻¹, which was assigned to a metal-O bond, due to the formation of a new Ti-O bond between the O of COO⁻ and the Ti atom, which was relatively weaker in the non-modified TiO₂. Furthermore, these results were in agreement with an electrostatic force microscopy (EFM) study showing that the electrons were transferred rapidly to the surface of the acetic acid-modified TiO₂ film, compared with that on the nonmodified TiO₂ film.     

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.     

Solid‐state dye‐sensitized and bulk heterojunction solar cells using TiO2 and ZnO nanostructures: recent progress and new concepts at the borderline

In the field of photovoltaic energy conversion, hybrid inorganic/organic devices represent promising alternatives to standard photovoltaic systems in terms of exploiting the specific features of both organic semiconductors and inorganic nanomaterials. Two main categories of hybrid solar cells coexist today, both of which make much use of metal oxide nanostructures based on titanium dioxide (TiO₂) and zinc oxide (ZnO) as electron transporters. These metal oxides are cheap to synthesise, are non‐toxic, are biocompatible and have suitable charge transport properties, all these features being necessary to demonstrate highly efficient solar cells at low cost. Historically, the first hybrid approach developed was the dye‐sensitized solar cell (DSSC) concept based on a nanostructured porous metal oxide electrode sensitized by a molecular dye. In particular, solid‐state hybrid DSSCs, which reduce the complexity of cell assembly, demonstrate very promising performance today. The second hybrid approach exploits the bulk heterojunction (BHJ) concept, where conjugated polymer/metal oxide interfaces are used to generate photocurrent. In this context, we review the recent progress and new concepts in the field of hybrid solid‐state DSSC and BHJ solar cells based on TiO₂ and ZnO nanostructures, incorporating dyes and conjugated polymers. We point out the specificities in common hybrid device structures and give an overview on new concepts, which couple and exploit the main advantages of both DSSC and BHJ approaches. In particular, we show that there is a trend of convergence between both DSSC and BHJ approaches into mixed concepts at the borderline which may allow in the near future the development of hybrid devices for competitive photovoltaic energy conversion. Copyright © 2011 Society of Chemical Industry     

Fabrication of Three-Dimensionally Ordered Macroporous TiO2 Films with Enhanced Photovoltaic Conversion Efficiency

Negative-charged polystyrene (PS) microspheres were prepared through a soap-free emulsion polymerization method using potassium persulfate as initiator. Three-dimensionally ordered macroporous TiO₂ films were fabricated using the high-quality PS colloidal crystals templates obtained via a horizontal deposition method. The as-prepared macroporous TiO₂ films were applied as the photoanodes in dye-sensitized solar cell (DSSC). The microstructure of the products were characterized by X-ray diffractometer, fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption–desorption analyzer. The results showed that the macroporous TiO₂ films replicated well the 3D ordered structure derived from PS colloidal crystal templates and revealed a relatively large specific surface area (69.99 m²/g), which could increase the capacity of TiO₂ film anode for absorbing dyes and scattering light. The photocurrent–voltage (I–V) characteristics of DSSC were measured by a digital source meter under simulated solar light. The results indicated that the introduction of an ordered macroporous TiO₂ interfacial layer increased the photovoltaic conversion efficiency, which was improved by 68 % from 3.61 to 6.08 %, as compared to a device using a bare P25 TiO₂ photoanode. Our results showed that the hierarchically ordered macroporous TiO₂ bilayer films photoanode for DSSC could be helpful to improve the photovoltaic conversion efficiency.     

Rutile TiO2-modified multi-wall carbon nanotubes in TiO2 film electrodes for dye-sensitized solar cells

The beneficial influence of incorporation of acid-treated and rutile TiO₂ (r-TiO₂)-modified multi-wall carbon nanotubes (MWNTs) in TiO₂ films on photocurrent–voltage characteristics of dye-sensitized solar cells (DSSCs) was studied. Two different routes were adopted for the modification of acid-treated MWNTs (a-MWNTs) with r-TiO₂. The films and MWNTs were characterized by electron microscopy, energy dispersive X-ray spectroscopy, XRD and Raman spectroscopy. In the case of incorporation of a-MWNTs with r-TiO₂ modification, short-circuit photocurrent (J _sc) of the pertinent DSSC increased by 35% compared with that of a cell with bare TiO₂ film. The open-circuit voltage remained almost the same for all cases. The enhanced J _sc is explained by the increased surface area of the film, enhanced cluster formation of TiO₂ particles around a-MWNTs, and improved interconnectivity of TiO₂ particles in the presence of a-MWNTs.     

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.     

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).     

Ga-doped ZnO transparent electrodes with TiO<Subscript>2</Subscript> blocking layer/nanoparticles for dye-sensitized solar cells

Ga-doped ZnO [GZO] thin films were employed for the transparent electrodes in dye-sensitized solar cells [DSSCs]. The electrical property of the deposited GZO films was as good as that of commercially used fluorine-doped tin oxide [FTO]. In order to protect the GZO and enhance the photovoltaic properties, a TiO₂ blocking layer was deposited on the GZO surface. Then, TiO₂ nanoparticles were coated on the blocking layer, and dye was attached for the fabrication of DSSCs. The fabricated DSSCs with the GZO/TiO₂ glasses showed an enhanced conversion efficiency of 4.02% compared to the devices with the normal GZO glasses (3.36%). Furthermore, they showed better characteristics even than those using the FTO glasses, which can be attributed to the reduced charge recombination and series resistance.     

A photocatalytic performance of TiO<Subscript>2</Subscript> photocatalyst prepared by the hydrothermal method

Anatase phase nanocrystalline TiO₂ powders were prepared by hydrothermal method with the TTIP (titanium tetra isopropoxide) at 200 oC in a stirred autoclave system. The effects of synthesis conditions on the physical properties of catalyst were investigated by using XRD, SEM, DLS, DSC and BET. The TiO₂ powders obtained from the optimum condition showed uniform spherical shape, crystalline structure, submicron size with a sharp size distribution and few agglomerates. The optimum synthesis conditions were obtained within the covered experimental ranges. The photocatalytic activity of TiO₂ powders prepared by the hydrothermal method was tested for photooxidation of methyl orange.     

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.     

Mesoporous TiO<Subscript>2</Subscript> microspheres synthesized via a facile hydrothermal method for dye sensitized solar cell applications

Mesoporous TiO₂ microspheres were successfully synthesized by a facile hydrothermal process and the obtained product was sintered at 450 °C. The sintered TiO₂ powder was characterised by powder X-ray diffraction pattern and the result shows pure anatase phase with good crystalline nature. The morphological image of field emission scanning electron microscopy and high resolution transmission electron microscopy shows spherical shape and size of the particles is around 100 to 300 nm. The Brunauer–Emmett–Teller surface area of synthesized TiO₂ material was 56.32 m² g^?1 and average pore width of synthesized materials was 7.1 and 9.3 nm. Bimodal pore structure of TiO₂ microspheres has been very effective for electrolyte diffusion into photoanode in dye sensitized solar cells. The synthesized anatase TiO₂ microsphere based dye sensitized solar cells have high surface area with light scattering effect to enhance the photocurrent and conversion efficiency than the commercial P25 photoanode material. The power conversion efficiency of synthesized mesoporous TiO₂ microspheres and commercial P25 material is 4.2 and 2.7 % respectively. Therefore bimodal mesoporous anatase TiO₂ microsphere appears to be a promising and potential candidate for dye sensitized solar cells (DSSC) application.     

Photovoltaic Properties and Negative Capacitance Spectroscopy of PCBM:P3HT/FTO Nanostructured Counter Electrode for TiO2-Based DSSC

Nano-clusters blind films of phenyl C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT) were deposited on fluorine doped tin-oxide (FTO) substrate by spin coating and applied as counter electrodes instead of platinum for a new FTO/TiO₂?+?K30 dye-sensitized solar cell. The photovoltaic parameters of the fabricated solar cell; open circuit voltage, short circuit current, output power and fill factor, were studied under various light intensities in the range 20:110?mW?cm^?2. An impedance spectroscopy study was also performed in a wide frequency range (5?kHz–1?MHz) to study the electron transport properties of the solar cells. The capacitance–voltage of the prepared DSSC is characterized by two parts: positive values of capacitance at low frequency range, f?≤?100?kHz and negative capacitance i.e., an inductive behavior, in higher frequency range f?≥ 300 kHz Conducting polymer electrode based on PCBM:P3HT/FTO can be used as a counter electrode in a DSSC.     

Uniform color coating of multilayered TiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> films by atomic layer deposition

Thin film optics, based on light interference characteristics, are attracting increasing interest because of their ability to enable a functional color coating for various applications in optical, electronic, and solar industries. Here, we report on the dependence of coloring characteristics on single-layer TiO₂ thicknesses and alternating TiO₂/Al₂O₃ multilayer structures prepared by atomic layer deposition (ALD) at a low growth temperature. The ALD TiO₂ and Al₂O₃ thin films were studied at a low growth temperature of 80°C. Then, the coloring features in the single-layer TiO₂ and alternating TiO₂/Al₂O₃ multilayers using both the ALD processes were experimentally examined on a TiN/cut stainless steel sheet. The Essential Macleod software was used to estimate and compare the color coating results. The simulation results revealed that five different colors of the single TiO₂ layers were shown experimentally, depending on the film thickness. For the purpose of highly uniform pink color coating, the film structures of TiO₂/Al₂O₃ multilayers were designed in advance. It was experimentally demonstrated that the evaluated colors corresponded well with the simulated color spectrum results, exhibiting a uniform pink color with wide incident angles ranging from 0° to 75°. This article advances practical applications requiring highly uniform color coatings of surfaces in a variety of optical coating areas with complex topographical structures.     

Effect of an electrodeposited TiO<Subscript>2</Subscript> blocking layer on efficiency improvement of dye-sensitized solar cell

A TiO₂ blocking layer in DSSC provides good adhesion between the fluorinated tin oxide (FTO) and an active TiO₂ layer, and represses the electron back transport between electrolyte and FTO by blocking direct contact. In addition, it offers a more uniform layer than bare FTO glass. In this study, a dense TiO₂ layer is prepared by electrodeposition technique onto an FTO substrate, and it is further used for efficiency measurement of dye-sensitized solar cell (DSSC). The thickness of TiO₂ blocking layers is controlled by applied voltage and deposition time. The morphology and crystalline structure of TiO₂ blocking layers are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The effect of thickness of TiO₂ blocking layers on transmittance is also examined by UV-vis spectrophotometer. For the best performance of the cell efficiency, the optimum blocking layer thickness is about 450 nm fabricated at 0.7 V for 20 min. The conversion efficiency from the DSSC including the optimum blocking layer is 59.34% improved compared to the reference cell from 2.41% to 3.84%. It demonstrates that the electrodeposition is a useful method to produce TiO₂ blocking layer for DSSC applications.     

Poly(lactic acid)/titanium dioxide nanocomposite films: Influence of processing procedure on dispersion of titanium dioxide and photocatalytic activity

Poly(lactic acid)/titanium dioxide (TiO₂) composite films were prepared by direct melt processing using three different procedures (i.e., compression molding, twin‐screw melt extrusion, and melt extrusion and thermoforming). The effect of TiO₂ loading and processing procedures on the phase morphology and on the thermal, mechanical, and barrier properties of the obtained nanocomposites were analyzed respectively by field‐emission scanning electron microscopy‐energy dispersive spectrometry, differential scanning calorimetry, universal testing machine, and water vapor and oxygen permeability measurements. The incorporation of TiO₂ nanoparticles into the poly(lactic acid) matrix increased the crystallinity and improved the barrier properties of the composites. The maximum tensile strength was achieved at the 2% content of TiO₂ for the films produced by compression molding and twin‐screw melt extrusion, whereas the tensile strength for films produced by melt extrusion and thermoforming decreases markedly with an increasing TiO₂ content. The photocatalytic activities of the obtained nanocomposites were investigated by analyzing the degradation of methyl orange. Results confirmed that the processing procedures and the distribution of TiO₂ in the polymer matrix are the key parameters, which rule the photocatalytic behavior of composite films. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Spectral Selective Solar Light Enhanced Photocatalysis: TiO<Subscript>2</Subscript>/TiAlN Bilayer Films

We demonstrate that spectral selective photocatalytic multilayer films can be tailored such that they can harness the full solar spectrum for enhanced photocatalytic gas-phase oxidation of acetaldehyde. Thin films of anatase TiO₂ were deposited on a thin solar absorber TiAlN film to fabricate bilayer TiO₂/TiAlN films by dc magnetron sputtering on aluminium substrates. The structural and optical properties of the films were characterized by X-ray diffraction and Raman spectroscopy. The reaction rate and quantum yield for acetaldehyde removal was measured and an almost tenfold enhancement of the quantum yield was observed for the TiO₂/TiAlN films compared with the single TiO₂ film, on par with enhancements achieved with new heterojunction photocatalysts. The results were interpreted by a temperature-induced change of the reaction kinetics. Absorption of simulated solar light illumination resulted in a temperature increase of the TIAlN film that was estimated to be at most 126 K. We show that a concomitant temperature increase of the top layer TiO₂ by 100 K shifts the water gas-surface equilibrium from multilayer to submonolayer coverage. We propose that this is the main reason for the observed enhancement of the photocatalytic activity, whereby gas phase molecules may come in direct contact with free surface sites instead of having to diffuse through a thin water film. The implications of the results for judicious control of temperature and relative humidity for efficient gas-phase photocatalysis and exploitation of selective solar absorbing films are discussed.     

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

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