Enhanced performance of bi-layer Nb2O5 coated TiO2 nanoparticles/nanowires composite photoanode in dye-sensitized solar cells

Dye sensitized solar cells (DSSCs) were fabricated based on coumarin NKX-2700 dye sensitized bi-layer photoanode and quasi-solid state electrolyte sandwiched together with cobalt sulfide coated counter electrode. A novel bi-layer photoanode has been prepared using composite mixtures of 90 wt.% TiO₂ nanoparticles + 10 wt.% TiO₂ nanowires (TNPWs) as active layer and Nb₂O₅ is coated on the active layer, which acts as scattering layer. Hafnium oxide (HfO₂) was applied over the TNPWs/Nb₂O₅ photoanode film, as a blocking layer. TiO₂ nanoparticles (TNPs), TiO₂ nanowires (TNWs) and TNPWs/Nb₂O₅ were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sensitizing organic dye coumarin NKX-2700 displayed maximum absorption wavelength (λ_max) at 525 nm, which could be observed from the UV–vis spectrum. DSSC-1 fabricated with composite bi-layer photoanode revealed enhanced photo-current efficiency (PCE) as compared to other DSSCs and illustrated photovoltaic parameters; short-circuit current J_SC = 18 mA/cm², open circuit voltage (V_OC) = 700 mV, fill factor (FF) = 64% and PCE (η) = 8.06%. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSC-1 showed the lowest charge transport resistance (R_tr) and the longest electron lifetime (τ_eff). Therefore, in the present investigation, it could be concluded that the novel bi-layer photoanode with blocking layer increased the short circuit current, electron transport and suppressed the recombination of charge carriers at the photoanode/dye/electrolyte interface in DSSC-1.     

Dye-sensitized solar cells based on ZnO nanowire array/TiO2 nanoparticle composite photoelectrodes with controllable nanowire aspect ratio

The ZnO nanowire (NW) array/TiO₂ nanoparticle (NP) composite photoelectrode with controllable NW aspect ratio has been grown from aqueous solutions for the fabrication of dye-sensitized solar cells (DSSCs), which combines the advantages of the rapid electron transport in ZnO NW array and the high surface area of TiO₂ NPs. The results indicate that the composite photoelectrode achieves higher overall photoelectrical conversion efficiency (η) than the ZnO NW alone. As a result, DSSCs based on the ZnO NW array/TiO₂ NP composite photoelectrodes get the enhanced photoelectrical conversion efficiency, and the highest η is also achieved by rational tuning the aspect ratio of ZnO NWs. With the proper aspect ratio (ca. 6) of ZnO NW, the ZnO NW array/TiO₂ NP composite DSSC exhibits the highest conversion efficiency (5.5 %). It is elucidated by the dye adsorption amount and interfacial electron transport of DSSCs with the ZnO NW array/TiO₂ NP composite photoelectrode, which is quantitatively characterized using the UV-Vis absorption spectra and electrochemical impedance spectra. It is evident that the DSSC with the proper aspect ratio of ZnO NW displays the high dye adsorption amount and fastest interfacial electron transfer.     

TiO2 Wedgy Nanotubes Array Flims for Photovoltaic Enhancement

In this study, TiO₂ wedgy nanotubes with rectangular cross-sections were fabricated on transparent conductive substrates by using TiO₂ nanorods as the precursor via the anisotropic etching route. TiO2 nanotubes with V-shaped hollow structure and the special crystal plane exposed on the tube wall possess nature of high surface area for more dye molecules absorption, and the strong light scattering effects and dual-channel for effective electron transport of the TiO₂ V-shaped nanotubes based dye-sensitized solar cell exhibit a remarkable photovoltaic enhancement compared with the TiO₂ nanorods. The photoanode based on our V-shaped TiO₂ nanotubes with a length of 1.5 μm show a 123% increase of the dye loading and a 182% improvement in the overall conversion efficiency when compared with 4 μm rutile TiO₂ nanorods photoanode.     

Synthesis of nanobranched TiO2 nanotubes and their application to dye-sensitized solar cells

Nanobranched TiO₂ nanotubes (TONTs) were synthesized by a sol–gel dipping method for the formation of seed layer, followed by a solution-phase deposition process. The different concentrations of seed solution influence the density of nanobranches on the top surface of TONT, achieving complete surface coverage of nanobranches in 10 mM TiCl₄ seed solution relative to 5 mM TiCl₄ seed solution. With a control sample of bare TONT, the nanobranched TONTs were explored as a photoanode for dye-sensitized solar cells (DSSCs). In the 5 mM TiCl₄ seed solution, the nanotree-shaped branches were sporadically formed on the top surface of TONT, with little effect on the photocurrent-voltage (J–V) properties, while in the 10 mM TiCl₄ seed solution, J_sc and fill factor increased, which may have been on account of the increased surface area and light scattering effect from rutile nanobranches, whereas the fill factor may be also increased by the electron transport property, leading to the degraded charge recombination. Accordingly, the nanobranched TONT showed 33% improved efficiency compared to bare TONT.     

纳米TiO2颗粒/亚微米球多层结构薄膜内电荷传输性能研究

本文主要利用TiO₂亚微米球较强的光散射特性设计了纳米TiO₂颗粒/亚微米球多层结构光阳极, 并借助强度调制光电流谱(intensity-modulated photocurrent spectroscopy)、电化学阻抗谱(electrochemical impedance spectroscopy)和入射单色光光电转化效率(incident photon-to-current conversion efficiency), 研究亚微米球的引入对多层结构薄膜内缺陷态、电子传输时间、电子收集效率和界面电荷转移性能的影响. 强度调制光电流谱反映出亚微米球表面缺陷态少, 但其颗粒间接触不紧密, 导致在接触部位形成了势垒, 阻碍了电子的传输, 导致电子传输时间增长. 电化学阻抗谱结果表明不同多层结构电池界面复合无明显差别, 同时底层采用纳米TiO₂ 透明薄膜结构的电池, 其光利用率要明显高于底层采用亚微米球薄膜结构的电池, TiO₂费米能级电子填充水平也相对增大, 使得电池的光电转换效率得到提升. 多层结构复合薄膜电荷传输和光伏特性的研究, 为高效染料敏化太阳电池光阳极设计提供了实验基础.     

Oriented ZnO nanotubes arrays decorated with TiO2 nanoparticles for dye-sensitized solar cell applications

Dye-sensitized solar cells (DSSCs) based on a novel composite photoanode of TiO₂ nanoparticles coating on electrodeposited ZnO nanotube arrays are fabricated and characterized. An efficiency of 3.94 % is achieved for the composite cell, increasing 86.7 % than 2.11 % of the ZnO nanotubes cell. The short-circuit current (J _sc) and open-circuit voltage (V _oc) are also enhancing 52.9 % and 25.3 %, respectively. The improvements are because of the high surface area of TiO₂ nanoparticles, as well as fast electron transport and light scattering effect of ZnO nanotubes.     

Dye-sensitized solar cells based on trench structured TiO2 nanotubes in Ti substrate

We have proposed dye-sensitized solar cells (DSSCs) with trench-type TiO₂ nanotube structure to improve the low device efficiency of conventional TiO₂ nanotube DSSCs using Ti substrate. Compared to the conventional standing-type TiO₂ nanotube structure based DSSCs, the trench-type TiO₂ nanotube structure based DSSCs have shown an improvement of device efficiency of approximately 40% due to the large increase of Jsc. In the trench-type TiO₂ nanotube structure, the contact area between the TiO₂ nanotube sidewall and the Ti substrate is significantly increased. This increase of contact area provides more charge transport paths than exist in the conventional standing-type TiO₂ nanotube structure and reduces the electrical resistance between the Ti substrate and the TiO₂ nanotubes. Therefore, the remarkable increase of Jsc is the result of the charge collection efficiency, which is improved due to the increase of contact area between the TiO₂ nanotube sidewall and the Ti substrate in the trench-type TiO₂ nanotube structure. The fabrication of the trench-type TiO₂ nanotube structure is an effective manufacturing process for improving the device efficiency of TiO₂ nanotube based DSSCs using Ti substrate. DSSCs having an 11.9 μm thick trench-type TiO₂ nanotube structure have shown an efficiency of 5.74%.     

Influence of TiO2 nanotube morphology and TiCl4 treatment on the charge transfer in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ nanoparticles (NPs), TiO₂ nanotube arrays (NTAs), and surface-modified NTAs with a TiCl₄ treatment. The photovoltaic efficiencies of the DSSCs using TiO₂ NP, NTA, and TiCl₄-treated NTA electrodes are 4.25, 4.74, and 7.47 %, respectively. The highest performance was observed with a TiCl₄-treated TiO₂ NTA photoanode, although in the case of the latter two electrodes, the amounts of N719 dye adsorbed were similar and 68 % of that of the NP electrode. Electrochemical impedance measurements show that the overall resistance, including the charge–transfer resistance, was smaller with NTA morphologies than with NP morphologies. We suggest that a different electron transfer mechanism along the one-dimensional nanostructure of the TiO₂ NTAs contributes to the smaller charge–transfer resistance, resulting in a higher short circuit current (J _sc), even at lower dye adsorption. Furthermore, the TiCl₄-treated NTAs showed even smaller charge–transfer resistance, resulting in the highest J _sc value, because the downward shift in the conduction band edge improves the electron injection efficiency from the excited dye into the TiCl₄-treated TiO₂ electrodes.     

Modification of TiO2 nanotube arrays by solution coating

Packed and sintered titania (TiO₂) nanoparticles are conventionally used as a photoanode for dye-sensitized solar cells. As a result, barriers are created for the electron transport and electron-hole recombination due to the large number of interfacial boundaries and voids in such a mesoporous photoanode. A high aspect ratio of length/diameter of titania nanotube arrays (TNAs) is required to meet the high surface area and the reduction of interfacial boundaries. TNAs prepared by the organic anodic oxidation method in this work possess a vertical nanotube structure that exhibits a high surface area, high degree of crystallinity, geometric pre-orientation and active electro-optical nanograins. Such a synthesized mesoporous structure is typically obtained by 60 V/2 h and 450 °C/30 min post-annealing and has a tube diameter of 110 nm, and a tube length of 15 mm. When the TNAs are modified by a TiO₂ nano-layer (TNL), the photocatalytic efficiency for methylene blue (MB) decomposition is further improved by up to 90%. Photosensitivity shows an obvious improvement when illuminating the UV light for the TNL modified TNAs and the visible light for the CuInS₂ layer modified TNAs.     

Anodic TiO<Subscript>2</Subscript> nanotubes powder and its application in dye-sensitized solar cells

An increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. TiO₂ semiconductor material is expected to play an important role in helping solve the energy crisis through effective utilization of solar energy based on photovoltaic devices. Dye-sensitized solar cells (DSSCs) are potentially lower cost alternative to inorganic silicon-based photovoltaic cells. In this study, we report on the fabrication of DSSCs from anodic TiO₂ nanotubes (NT) powder, produced by rapid breakdown potentiostatic anodization of Ti foil in 0.1 M HClO₄ electrolyte, as photoanode. TiO₂ NT powders with a typical NT outer diameter of approximately 40 nm, wall thickness of approximately 8–15 nm, and length of about 20–25 μm, have been synthesized. The counter electrode was made by electrodeposition of Pt from an aqueous solution of 5 mM H₂PtCl₆ onto fluorine-doped tin oxide (FTO) glass substrate. The above front-side illuminated DSSCs were compared with back-side illuminated DSSCs fabricated from anodic TiO₂ NTs that were grown on the top of Ti foil as photoanode. The highest cell efficiency was 3.54% under 100 mW/cm² light intensity (1 sun AM 1.5G light, Jsc = 14.3 mA/cm², Voc = 0.544 V, FF = 0.455). To the best of our knowledge, this is the first report on the fabrication of DSSC from anodic TiO₂ NTs powder. The TiO₂/FTO photoanodes were characterized by FE-SEM, XRD, and UV–Visible spectroscopy. The catalytic properties of Pt/FTO counter electrodes have been examined by cyclic voltammetry.     

Improved charge collection efficiency of hollow sphere/nanoparticle composite TiO2 electrodes for solid state dye sensitized solar cells

The photoanodes of solid state dye sensitized solar cells (ss-DSCs) embedded with different contents of TiO₂ hollow spheres (HSs) were prepared and the photovoltaic performances were systematically characterized. TiO₂ hollow spheres were synthesized by a facile sacrificial templating method, grounded and added in different ratios to TiO₂ nanoparticle (NP) paste, from which composite HS/NP electrodes were fabricated. The composite photoanodes include hollow spheres of 300–700 nm with enhanced light scattering characteristics in visible range which leads to improved light absorption in conventional thin film electrodes of ss-DSC. By optimizing the amount of HSs in the paste, 40% improvement in efficiency was obtained in comparison to ss-DSC utilized pure NP electrodes. By increasing the fraction of HSs in the electrode the current density increased by 56% (from 2.5 to 3.9 mA cm^?2). The improved photovoltaic performance of ss-DSC is primarily due to different morphology and altered charged trap distribution in HSs in comparison to NP which leads to significant enhancement in electron transport time and electron lifetime as well as charge collection efficiency and light absorption properties.     

A novel TiO2 nanoparticles/nanowires composite core with ZrO2 nanoparticles shell coating photoanode for high-performance dye-sensitized solar cell based on different electrolytes

The novel TiO₂ nanopartilces/nanowires (TNPWs) composite with ZrO₂ nanoparticles (ZNPs) shell-coated photoanodes were prepared to fabricate high-performance dye-sensitized solar cell (DSSC) based on different types of electrolytes. Hafnium oxide (HfO₂) is a new and efficient blocking layer material applied over the TNPWs-ZNPs core-shell photoanode film. TiO₂ nanoparticles (TNPs) and TiO₂ nanowires (TNWs) were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). DSSCs were fabricated using the novel photoanodes with an organic sensitizer D149 dye and different types of electrolytes namely liquid electrolyte, ionic liquid electrolyte, solid-state electrolyte, and quasi-solid-state electrolyte. The DSSC-4 made through the novel core-shell photoanode using quasi-solid-state electrolyte showed better photocurrent efficiency (PCE) as compared to the other DSSCs. It has such photocurrent-voltage characteristics: short circuit photocurrent (Jsc)?=?19 mA/cm², the open circuit voltage (Voc)?=?650 mV, fill factor (FF)?=?65 %, and PCE (η)?=?8.03 %. The improved performance of DSSC-4 is ascribed to the core-shell with blocking layer photoanode could increased electron transport and suppressed recombination of charge carriers at the TNPWs-ZNPs/dye/electrolyte interface.     

Hydrothermal synthesis of TiO2 nanocrystals in different basic pHs and their applications in dye sensitized solar cells

In this research TiO₂ nanocrystals with sizes about 11–70 nm were grown by hydrothermal method. The process was performed in basic autoclaving pH in the range of 8.0–12.0. The synthesized anatase phase TiO₂ nanocrystals were then applied in the phtoanode of the dye sensitized solar cells. It was shown that the final average size of the nanocrystals was larger when the growth was carried out in higher autoclaving pHs. The photoanodes made of TiO₂ nanocrystals prepared in the pHs of 8.0 and 9.0 represented low amounts of dye adsorption and light scattering. The performance of the corresponding dye sensitized solar cells was also not acceptable. Nevertheless, the energy conversion efficiency was better for the state of pH of 9.0. For the photoanodes made of TiO₂ nanocrystals prepared at autoclaving pH of 10.0, the dye adsorption and light scattering were quite higher. The photovoltaic characteristics of the best cell in this state were 15.25 mA/cm², 740 mV, 0.6 and 6.8% for the short-circuit current density, open-circuit voltage, fill factor and efficiency, respectively. The photoanodes composed of TiO₂ nanocrystals prepared in autoclaving pHs of 11.0 and 12.0 demonstrated lower amount of dye adsorption and higher light scattering. This was quite considerable for the state of pH of 12.0. The energy conversion efficiencies were consequently decreased compared to that of the pH of 10.0. The optimum situation was finally discussed based on the nanocrystals size and its influence on the sensitization and light harvesting efficiency.     

Electrochemical properties of TiO2 nanoparticle/nanorod composite photoanode for dye-sensitized solar cells

A unique composite of TiO₂ nanoparticles (NPs) and nanorods (NRs) has been used to fabricate a photoelectrode for developing dye-sensitized solar cells (DSSCs) with higher sensitivity. The TiO₂ nanorods were synthesized using a mechanical process, in which electrospun TiO₂ nanofibers was grinded in a controlled way to obtain uniform size distribution. The characteristics of electron transport, recombination lifetime and charge collection were investigated by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). Photoelectrodes prepared with the composites of NRs and NPs showed significant improvements in electron transportation compared to only NP photoelectrodes, which would enhance the photovoltaic performance of DSSCs. IMPS and IMVS measurements show that fast electron transport and slightly decreased recombination lifetime resulted in the improvement of efficiency. The highest energy conversion efficiency obtained from the photoelectrodes fabricated with the as-prepared rutile TiO₂ nanofibers at 5 wt% NR content was up to 6.1% under AM1.5G solar illumination. The results demonstrate that the composite nanostructure can take advantage of both the fast electron transport of the nanorods and the high surface area of the nanoparticles.     

Synthesis of two-dimensional nanowall of Cu-Doped TiO2 and its application as photoanode in DSSCs

Two-dimensional nanowall of Cu-doped TiO₂ (CuTNW) has been prepared in this work to study the role of Cu doping on its photoactivity properties and its photovoltaic performance as photoanode in a dye-sensitized solar cell (DSSC). TiO₂ nanowall with five Cu ion doping, i.e. 6.25, 12.5, 25.0, 50.0 and 100.0 mM, were prepared via a liquid-phase deposition method using ammoniumhexafluorotitanate and hexamethylenetetramine as the reagents with a growth temperature of 90 °C. The X-Ray Diffraction (XRD), X-ray energy dispersion (EDX) and diffuse optical reflectance spectroscopy analysis results confirmed the successfulness of the Cu doping process in the TiO₂ nanowall and effective modification on the photoactivity of the TiO₂ nanowall. We found that the power conversion efficiency of the DSSC utilizing TiO₂ nanowall as photoanode can be enhanced up to 2 times, i.e. from 0.2% to 0.44%, when the TiO₂ nanowall doped with Cu ion. The nanostructure preparation, device fabrication and the mechanism for the device performance enhancement will be discussed.     

Dependence of photovoltaic parameters on the size of cations adsorbed by TiO2 photoanode in dye-sensitized solar cells

The effects of “pre-adsorbed cations” in photoanodes on the performances of dye-sensitized solar cells (DSSCs) were studied using two different size cations (K⁺ and guanidine cation (G⁺)). While the DSSCs with optimized K⁺ ions pre-adsorbed photoanodes showed a maximum efficiency of 7.04%, the DSSCs with G⁺ ions pre-adsorbed photoanodes showed an efficiency of 6.73%. DSSCs fabricated with conventional photanodes (without pre-cation adsorption) showed an efficiency of 6.21%. Differences in efficiencies are very likely due to the cation pre-adsorption effects and could be due to a higher number of K⁺ cation adsorption per unit area of TiO₂ surface of the photoanode compared to a smaller number of G⁺ cation adsorption in TiO₂, due to their difference in sizes. This pre-cation adsorption technique can be used to improve the overall efficiency of a DSSC by about 14% fold over the conventional photoanodes use in DSSCs, specially using smaller cations.

     

Flexible TiO<Subscript>2</Subscript> nanotube-based dye-sensitized solar cells using laser-drilled microhole array electrodes

Here we report on the growth of TiO₂ nanotube arrays (TNAs) on Ti foil with laser-drilled microhole arrays (MHAs). The MHAs promoted the adhesion of the TNA film to Ti substrate, which is well suited for flexible dye-sensitized solar cells (DSSCs). The MHA photoanode and TNAs were characterized by SEM, 3D optical profiling, XRD and TEM. For such a flexible MHA photoanode, the TNA-based DSSC was assembled using a platinized conductive glass counter electrode, and a conversion efficiency of 3.45% was achieved under AM 1.5 condition. A flexible TNA-based DSSC was also fabricated using a flexible MHA photoanode combined with a platinized indium tin oxide-polyethylene naphthalate counter electrode, which achieved 2.67% photovoltaic conversion efficiency under simulated AM 1.5 sunlight.     

Interface modification of MoS2 counter electrode/electrolyte in dye-sensitized solar cells by incorporating TiO2 nanoparticles

To achieve the high efficiency in dye-sensitized solar cells (DSSCs), the interface modification of MoS₂ counter electrode (CE)/electrolyte should be carried out. Making the modified MoS₂ CE by incorporating TiO₂ nanoparticles provides possibilities to enhance electrocatalytic activity. The DSSCs with the MoS₂/TiO₂ CE show enhanced performance compared with DSSCs with the MoS₂ CE. The experimental results revealed that the MoS₂/TiO₂ nanocomposite influences on the power conversion efficiency by enhancing electrocatalytic activity and increasing the active surface area that serve to increase the short circuit current. This understanding can provide guidance for the development of highly efficient DSSCs with platinum-free CEs.     

Dynamics of TiO2-based photoelectrochemical cell

The present work reports the effect of light on the open-circuit voltage of a photoelectrochemical cell (PEC) formed of TiO₂ photoanode, Pt cathode and Na₂SO₄ (0.35 M) aqueous solution as electrolyte. The studies included the measurements of the electromotive force (EMF) during the light-off and light-on cycles for the PEC involving photoanode that was made of both oxidised and reduced TiO₂ thin films. These specimens were formed by oxidation of the titanium metal at high and low oxygen activities. This was achieved by the imposition of the gas phase of two different compositions, including air, p(O₂) = 21 kPa, and the hydrogen–water vapour mixture, p(O₂) = 10^-10p({\rm O}_2) = 10^{-10} Pa, at 1,123 K and subsequent cooling to room temperature. The determined data indicate that the PEC formed of the oxidised specimen exhibits larger EMF and a substantially better stability in time. It is, therefore, concluded that the TiO₂ obtained in air exhibits superior performance-related properties compared to the reduced specimen. The obtained experimental EMF data are considered in terms of the effect of light on the reactivity of TiO₂ with oxygen and water and the related charge transfer.     

Effects of ZnO coating on the performance of TiO2 nanostructured thin films for dye-sensitized solar cells

ZnO-coated TiO₂ (ZTO) thin films were deposited on ITO substrates by a sol–gel method for application as the work electrode for dye-sensitized solar cells (DSSCs). The I–V curve and the incident photon-to-current conversion efficiency (IPCE) value of DSSCs for ZTO thin films were studied and compared with single TiO₂ films. The results show that the short-circuit photocurrent (J _sc) and open-circuit voltage (V _oc) values increased from 3.7 mA/cm² and 0.68 V for the DSSCs with a single TiO₂ film to 4.5 mA/cm² and 0.72 V, respectively, for the DSSCs with a ZTO thin film. It indicated that the DSSCs with a ZTO thin film contributed to provide an inherent energy barrier that suppressed charge recombination significantly. In addition, the higher IPCE value in the ZTO thin film is attributed to the better charge separation by a fast electron transfer process using two semiconductors with different conduction band edges and energy positions.