Construction of TiO<Subscript>2</Subscript> NP@TiO<Subscript>2</Subscript> NT double-layered structural photoanode to enhance photovoltaic performance of CdSe/CdS quantum dots sensitized solar cells

TiO₂ nanoparticles (NP) at top of TiO₂ nanotube (TiO₂ NP@TiO₂NT) double-layered architecture is constructed to combine the advantages of TiO₂ NP and TiO₂ NT together. This composite TiO₂ NP@TiO₂NT architecture as photoanode possesses a larger surface area for more QDs loading, and highly tubular structure for electron swift transport. Based on this architecture, CdSe/CdS quantum dots (QDs) have been successfully synthesized by successive ionic layer adsorption reaction (SILAR) method for quantum dots-sensitized solar cell application. The photovoltaic performance of QDSSCs based on TiO₂ NP@TiO₂ NT have been investigated in contrast with bare TiO₂ NP and bare TiO₂ NT architectures with almost the same thickness. The results show that the power conversion efficiency (PCE) of QDSSCs could be enhanced using TiO₂ NP@TiO₂ NT and improved to 3.26%, which is 80% and 38% higher than QDSSCs based on bare TiO₂ NT and bare TiO₂ NP, respectively. The BET surface area, UV–vis absorption spectra, and incident photon to current conversion efficiency (IPCE) measurements results show the evidence that the TiO₂ NP@TiO₂ NT can combine advantages of TiO₂ NP and TiO₂ NT structures together and lead to a higher light harvesting efficiency, electron collecting efficiency, and efficient electron transport.     

Hydrothermal Growth of TiO2 Nanorod Arrays and In Situ Conversion to Nanotube Arrays for Highly Efficient Quantum Dot‐Sensitized Solar Cells

TiO₂ nanorod (NR) and nanotube (NT) arrays grown on transparent conductive substrates are attractive electrode for solar cells. In this paper, TiO₂ NR arrays are hydrothermally grown on FTO substrate, and are in situ converted into NT arrays by hydrothermally etching. The TiO₂ NR arrays are reported as single crystalline, but the TiO₂ NR arrays are demonstrated to be polycrystalline with a bundle of 2–5 nm single crystalline nanocolumns grown along [001] throughout the whole NR from bottom to top. TiO₂ NRs can be converted to NTs by hydrothermal selective etching of the (001) core and remaining the inert sidewall of (110) face. A growth mechanism of the NR and NT arrays is proposed. Quantum dot‐sensitized solar cells (QDSCs) are fabricated by coating CdSe QDs on to the TiO₂ arrays. After conversion from NRs to NTs, more QDs can be filled in the NTs and the energy conversion efficiency of the QDSCs almost double.     

Fabrication of dye-sensitized solar cells with multilayer photoanodes of hydrothermally grown TiO<Subscript>2</Subscript> nanocrystals and P25 TiO<Subscript>2</Subscript> nanoparticles

TiO₂ nanocrystals (NCs) with sizes around 20 nm were synthesized by hydrothermal method in acidic autoclaving pH. The hydrothermally grown TiO₂ NCs and P25 TiO₂ nanoparticles (NPs) were used in the preparation of two different pastes using different procedures. These pastes with different characteristics were separately deposited on FTO glass plates to form multilayer photoanodes of the dye-sensitized solar cells. The aim of this study was to search how a thin sub-layer of the hydrothermally grown TiO₂ NCs in the photoanodes could improve the efficiency of TiO₂ P25-based solar cells. The highest efficiency of 6.5% was achieved for a cell with a photoanode composed of one transparent sub-layer of hydrothermally grown TiO₂ NCs and two over-layers of P25 NPs. Higher energy conversion efficiencies were also attainable using two transparent sub-layers of hydrothermally grown TiO₂ NCs. In this case, an efficiency of 7.2% was achieved for a cell with a photoelectrode made of one over-layer of P25 TiO₂ NPs. This could show an increase of about 30% in the efficiency compared to the similar cell with a photoanode made of two layers of hydrothermally grown TiO₂ NCs.     

Synthesis and characterization of anatase TiO2 nanotubes and their use in dye-sensitized solar cells

Anatase TiO₂ nanotubes (NTs) with the diameter of about 12 nm and the length of several hundreds nanometers were synthesized by hydrothermal method. The samples were characterized by X-ray diffraction (XRD), transmitting electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmet–Teller (BET) measurements. The NTs are used to make composites photoanode with pristine TiO₂ nanoparticles in dye-sensitized solar cells (DSSCs). It was found that the NT/nanoparticle ratio had a pronounced impact on the performance of solar cells. The electrode composite has better photoelectric properties than the full nanoparticle and NTs solar cells. The optimum content NTs, was found to be 5%, at which content the incident photon to current efficiency was about 63.1%, an 13.8% increment compared to that using full P25 under the same condition.     

Improved Hybrid Solar Cells via in situ UV Polymerization

One approach for making inexpensive inorganic–organic hybrid photovoltaic (PV) cells is to fill highly ordered TiO₂ nanotube (NT) arrays with solid organic hole conductors such as conjugated polymers. Here, a new in situ UV polymerization method for growing polythiophene (UV‐PT) inside TiO₂ NTs is presented and compared to the conventional approach of infiltrating NTs with pre‐synthesized polymer. A nanotubular TiO₂ substrate is immersed in a 2,5‐diiodothiophene (DIT) monomer precursor solution and then irradiated with UV light. The selective UV photodissociation of the C? I bond produces monomer radicals with intact π‐ring structure that further produce longer oligothiophene/PT molecules. Complete photoluminescence quenching upon UV irradiation suggests coupling between radicals created from DIT and at the TiO₂ surface via a charge transfer complex. Coupling with the TiO₂ surface improves UV‐PT crystallinity and π–π stacking; flat photocurrent values show that charge recombination during hole transport through the polymer is negligible. A non‐ideal, backside‐illuminated setup under illumination of 620‐nm light yields a photocurrent density of ≈5 µA cm²—surprisingly much stronger than with comparable devices fabricated with polymer synthesized ex situ. Since in this backside architecture setup we illuminate the cell through the Ag top electrode, there is a possibility for Ag plasmon‐enhanced solar energy conversion. By using this simple in situ UV polymerization method that couples the conjugated polymer to the TiO₂ surface, the absorption of sunlight can be improved and the charge carrier mobility of the photoactive layer can be enhanced.     

Dye-sensitized solar cells using TiO<Subscript>2</Subscript> nanoparticles transformed from nanotube arrays

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ mesoporous layers obtained by very simple method—transformation of TiO₂ nanotube (NT) films grown by electrochemical oxidation to nanoparticle (NP) films. This transformation is based on thermal annealing of TiO₂ NT arrays formed by anodization of titanium foil in fluorine ambient. Performance of DSSCs fabricated with different size NPs was studied in the range from 35 to 350 nm. Highest nominal efficiency (9.05%) was achieved for DSSC with NP size 65 nm while the lowest nominal efficiency (1.48%) was observed for DSSCs with NP size 350 nm. The dependence of the solar cell parameters with NP size is discussed.     

Enhanced photovoltaic performance of solar cell based on front-side illuminated CdSe/CdS double-sensitized TiO2 nanotube arrays electrode

Free-standing TiO₂ nanotube (NT) arrays have been prepared by a two-step anodization method. These translucent TiO₂ NT arrays can be transferred to the fluorine-doped tin oxide glass substrates to form front-side illuminated TiO₂ NT electrodes. The TiO₂ NT electrodes were double-sensitized by CdSe/CdS quantum dots (QDs) through successive ionic layer adsorption and reaction (SILAR) process. The absorption range of the TiO₂ NT electrode was extended from ~380 to 700 nm after sensitization with CdSe/CdS QDs. The SILAR cycles were investigated to find out the best combination of CdS and CdSe QDs for photovoltaic performance. The power conversion efficiency of 2.42 % was achieved by the CdSe(10)/CdS(8)/TiO₂ NT solar cell. A further improved efficiency of 2.57 % was obtained with two cycles of ZnS overlayer on the CdSe(10)/CdS(8)/TiO₂ NT electrode, which is 45.19 % higher than that of back-side illuminated solar cell. Furthermore, the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell possesses a higher stability than CdSe(10)/CdS(8)/TiO₂ NT solar cell during the same period. The better photovoltaic performance of the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell has demonstrated the promising value to design quantum dots-sensitized solar cells with double-sensitized front-side illuminated TiO₂ NT arrays strategy.     

Dye-sensitized solar cell based on nanocrystalline TiO<Subscript>2</Subscript> with 3–10 nm in diameter

Nanocrystalline TiO₂ with 3–10 nm in diameter was prepared with a surfactant-template method. Dye-sensitized solar cells were assembled using the prepared nanocrystalline TiO₂ with large surface area and high crystallinity, which achieved significant higher Jsc when compared to cells fabricated with bigger particles of 25 nm in diameter. In the cells with nanocrystalline TiO₂, the sintering temperature drastically affected the conversion performance of the cells.     

Novel construction of CdS-encapsulated TiO2 nano test tubes corked with ZnO nanorods

A novel TiO₂ nanotube array/CdS nanoparticle/ZnO nanorod (TiO₂ NT/CdS/ZnO NR) photocatalyst was constructed by chemical assembling CdS into the TiO₂ NTs, and then laying ZnO NRs on the surface. The SEM results showed that the TiO₂ NTs looked like many “nano test tubes” and the ZnO NRs served as the corks to seal the nozzle. This photocatalyst exhibited a wide absorption range (200-535 nm) in both ultraviolet and visible regions (UV-vis region), and maintained very high photoelectrocatalytic (PEC) activities. The maximum photoelectric conversion efficiencies (η) of TiO₂ NT/CdS/ZnO NRs are 31.8 and 5.98% under UV light (365 nm) and visible light (420-800 nm), respectively.     

Preparation of sub-micron size anatase TiO<Subscript>2</Subscript> particles for use as light-scattering centers in dye-sensitized solar cell

The sub-micron size anatase TiO₂ particles with size about 0.2-0.3 μm were synthesized with basic peptizing agent and hydrothermal method and added into TiO₂ film as light scattering center. The addition of the sub-micron size anatase TiO₂ particles enhanced light scattering and dye adsorption abilities of the TiO₂ film. When the weight proportion of the sub-micron size/nano-size TiO₂ particles in the TiO₂ film attained to 1.25/10, the highest energy conversion efficiency about 7.41% was obtained, which was 23% enhancement comparing with the TiO₂ film containing pure nano-size TiO₂ particles. It presented an efficient way for improving the photovoltaic performance of dye-sensitized solar cell.     

Tuning the dye aerosol impaction and TiO2 nanoparticle stacking structures for High-Efficiency Dye-Sensitized solar cells

The rapid manufacturing of high-efficiency dye-sensitized solar cells (DSSCs) is limited by the slow dye adsorption on TiO₂ nanoparticles (NPs)-accumulated photoelectrode using conventional dip-coating process. Therefore, we aim to accelerate the adsorption of dyes that are attached on TiO₂ NPs by employing an aerosol impactor. Herein the aerosolized dyes are designed to get deposited rapidly on the TiO₂ NPs-accumulated photoelectrode. In addition, to effectively trap the irradiated sunlight in DSSCs, we assemble the photoelectrodes incorporated with bilayered TiO₂ thin films comprising small TiO₂ NPs-based underlayer and large TiO₂ NPs-based overlayer as dye-supporting and light-scattering mediums, respectively. Furthermore, the effects of dye aerosol impaction and TiO₂ stacking structures on the efficiency of DSSCs are examined. The power conversion efficiency (PCE) of DSSCs comprising a N719 dye-supporting layer treated with dip-coating process was determined as ～ 5.67%; however, when the bilayered TiO₂ thin films with an optimized thickness ratio of light-scattering overlayer and dye-supporting underlayer were coated with N719 dyes using dye aerosol impactor, the resulting PCE increased to ～ 7.46%. This suggests that the photovoltaic characteristics of DSSCs can be enhanced considerably using the effective TiO₂ NP stacking structures coated with rapid, uniform, and strong aerosol dye adsorption throughout the TiO₂-based photoelectrodes.     

Enhancing the photovoltaic performances of dye-sensitized solar-cells by modifying the TiO2 electrode-sensitized dye interface

Enhanced photovoltaic performances of N719 dye-sensitized solar-cells were achieved by modifying the titanium oxide (TiO₂) electrode-sensitized dye interface. Surface of TiO₂ thin film electrode was coated with a calcium oxide (CaO) or lithium fluoride (LiF) thin layer, respectively, in a thermal deposition chamber. As compared to a cell using a bare TiO₂ nanoparticle (NTP) electrode, the solar energy conversion efficiency (η)? was enhanced by 15.1% and 12.8% for the surface of a NTP electrode coated with CaO and LiF, respectively. Moreover, for the surface of a TiO₂ nanotube electrode respectively coated with CaO and LiF, the efficiency was enhanced by 4.8% and 11.6%. This increase in efficiency is attributed to an increase in the adsorption of N719 dye on the CaO or LiF coated TiO₂ thin film electrodes, and the formation of a potential barrier by a CaO or LiF interlayer at the TiO₂ electrode-sensitized dye interface.     

Charge recombination reduction in dye-sensitized solar cells by depositing ultrapure TiO2 nanoparticles on “inert” BaTiO3 films

Ultrapure TiO₂ nanoparticles (∼5 nm in size) were supported on “inert” BaTiO₃ films by TiCl₄ treatment, which was used to fabricate dye-sensitized solar cells (DSSCs). The optimized electrode, designated as BaTiO₃/TiO₂(4), was obtained upon four cycles of TiCl₄ treatment. DSSC with BaTiO₃/TiO₂(4) electrode exhibits superior power conversion efficiency (PCE) compared to that with conventional anatase TiO₂ (∼25 nm in size) electrode. The interfacial charge recombination kinetics was investigated by electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). In contrast to DSSC with anatase TiO₂ electrode, the dramatically enhanced electron lifetime for DSSC with BaTiO₃/TiO₂(4) electrode could be attributed to the decrease of recombination reaction at the TiO₂ photoelectrode/electrolyte interface. It is proposed that the lower interfacial charge recombination can be related to the relatively shallower trap distributions in DSSC with BaTiO₃/TiO₂(4) electrode.     

Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%

Techniques of TiO₂ film fabrication for dye-sensitized solar cells having a conversion efficiency of global air mass 1.5 (AM 1.5, 1000 W/m²) solar light to electric power over 10% are reported. Newly implemented fabrication technologies consist of pre-treatment of the working photoelectrode by TiCl₄, variations in layer thickness of the transparent nanocrystalline-TiO₂ and applying a topcoat light-scattering layer as well as the adhesion of an anti-reflecting film to the electrode's surface. TiCl₄ treatments induce improvements in the adhesion and mechanical strength of the nanocrystalline TiO₂ layer. Optimization of the thickness of the TiO₂ layer, acting as the working electrode, affects both the photocurrent and the photovoltage of the devices. Covering of the TiO₂ photoanode by an anti-reflecting film results in enhancement of the photocurrent. Each of these components of film fabrication exerts a significant influence on the overall photovoltaic parameters of the devices resulting in improvements in the net energy conversion performance.     

Multi-layered TiO2 nanostructured films for dye-sensitized solar cells

Multi-layered TiO₂ nanostructured films were fabricated to improve the light harvest efficiency of the dye-adsorbed TiO₂ electrode in dye-sensitized solar cells (DSSCs) by light scattering. Three different structures of TiO₂ electrodes, with layers consisting of TiO₂ pastes with average diameters of 9, 20, and 300 nm, respectively, were fabricated and their photovoltaic effects on the DSSC devices were investigated. By utilizing the multi-layered TiO₂ electrode constructed using the three different TiO₂ pastes, the overall power conversion efficiency of the DSSC devices in the PEG-based electrolyte was increased to 5.24% under irradiation of 100 mW/cm² at AM 1.5.     

Synthesis and characterization of polythiophene-modified TiO<Subscript>2</Subscript> nanotube arrays

The highly ordered and uniform TiO₂ nanotube arrays were fabricated by anodic oxidation method and PTh(polythiophene)/TiO₂ nanotube arrays electrode were obtained by electrochemical polymerization. X-ray powder diffraction (XRD) analysis confirmed the formation of TiO₂ phase. The morphologies and optical characteristics of the TiO₂ nanotube arrays were studied by scanning electron microscope (SEM), UV-Vis absorption spectra and Raman spectra. The results demonstrate that the PTh/TiO₂ electrode could enlarge the visible light absorption region and increase the photocurrent in visible region. The modified TiO₂ electrode with light-to-electric energy conversion efficiency of 1·46%, the short-circuit current density of 4·52 mAcm^− 2, open-circuit voltage of 0·74 V and fill factor of 0·44, were obtained.     

Enhanced efficiency and improved photocatalytic activity of 1 : 1 composite mixture of TiO<Subscript>2</Subscript> nanoparticles and nanotubes in dye-sensitized solar cell

TiO₂-based nanotubes (NTs), nanoparticles (NPs) and composite structural film (50% NP + 50% NT film) were synthesized by sol-gel hydrothermal process. Synthetic indigo dye was used as a sensitizer with the unique combination of electrolyte (EMII + BMII + PMII) and with cobalt sulphide as counter electrode. The structure and morphology of the three films, namely, NP, NT and NPNT is studied through X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The absorption spectra and incident photon-to-current conversion efficiency (IPCE) of the three films were compared and found to be higher for NPNT film. The efficiency and photocatalytic activity of three films were evaluated. The composite structure showed improved efficiency (1.72%) than NP (1%) and NT films (0.78%). The photocatalytic activity of the three films were measured using organic dye, methylene blue under UV light radiation. The composite structure showed higher dye absorption and higher rate of reaction with time. This paper certainly proves that there are many rooms to focus on the photoanode configuration, which plays a key role to improve the efficiency of dye-sensitized solar cell (DSSC).     

Optical and photo catalytic characteristics of Ag<Subscript>2</Subscript>S/TiO<Subscript>2</Subscript> nanocomposite films prepared by electrochemical anodizing and SILAR approach

Ag₂S decorated titanium oxide nanotubes (Ag₂S/NTs) were prepared by electrochemical anodizing and successive ionic layer adsorption and reaction (SILAR) approach. The prepared samples were characterized by X-ray diffraction, field emission scanning electron microscopy coupled with energy dispersive X-ray analysis and diffuse reflectance spectroscopy. SEM results indicate titanium oxide nanotubes (NTs) with 90–220 nm in diameter and 15–30 nm in wall thickness were prepared by one-step anodizing method on the surface of titanium foils. Characterization of the Ag₂S/NTs samples indicated that the number of SILAR cycles influenced the morphology of fabricated films. The degradation of rhodamine B was used as a model reaction to evaluate the photo catalytic activity of the obtained samples. Results showed that the photo catalytic activity of Ag₂S/NTs nanocomposite samples is higher than that with bare NTs sample. The incorporation of Ag₂S on NTs improves the photo catalytic activity due to the synergetic effect. Ag₂S/NTs nanocomposite sample prepared by SILAR deposition with 2 cycles gives the highest degrading rate, which can be attributed to appropriate Ag₂S content and high surface area of this sample. Ag₂S/NTs nanocomposites are easy to be recycled and have good stability for repeated use. With the improved visible light degradation performance, Ag₂S/NTs samples would be expected to be used in water purification. Since these prepared electrodes can be easily removed and replaced after the photo catalytic reaction, avoiding the filtration step after photoreaction or the immobilizing process required for photo catalyst particles, the operation in the photo-reactor becomes much easier from an engineering point of view.     

Functionalization of electrospun magnetically separable TiO<Subscript>2</Subscript>-coated SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers: strongly effective photocatalyst and magnetic separation

Magnetically separable TiO₂-coated SrFe₁₂O₁₉ electrospun nanofibers were obtained successfully by means of sol–gel, electrospinning, and coating technology, followed by heat treatment at 550–650 °C for 3 h. The average diameter of the electrospun fibers was 500–600 nm. The fibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). The optimized calcining temperature was determined by XRD and the analysis of decolorizing efficiency of methylene blue (MB) under UV–vis irradiation. The photocatalytic activity of the TiO₂-coated SrFe₁₂O₁₉ fibers was investigated using ultraviolet–visible absorbance by following the photooxidative decomposition of a model pollutant dye solution, MB in a photochemical reactor. In contrast to pure TiO₂ fibers, the TiO₂-coated SrFe₁₂O₁₉ fibers have higher absorption in 250–750 nm wavelength regions. The presence of SrFe₁₂O₁₉ not only broadened the response region of visible-light, but also enhanced the absorbance for UV light. The decolorizing efficiency of MB under UV–vis irradiation was up to 98.19%, which was a little higher than that of Degussa P25 (97.68%). Furthermore, these fibers could be recollected easily with a magnet in a photocatalytic process and had effectively avoided secondary pollution of treated water.     

A novel synthesis of the bottom-straight and top-bent dual TiO2 nanowires for dye-sensitized solar cells

The bottom-straight and top-bent dual anatase TiO₂ nanowires directly on flurine-doped tin oxide (FTO) glass substrate are successfully synthesized for the first time via a two-step solvothermal reaction, which obtains single anatase TiO₂ nanowires with the diameter of 250–450?nm and the length of ～49?μm in the first growth step, and the slender TiO₂ nanowires with the diameter of 40–80?nm intertwining larger nanowires to form dual nanowires network in the secondary growth step. Dye-sensitized solar cell (DSSC) based on dual TiO₂ nanowires exhibits the power conversion efficiency of 5.30%, which is much higher than those of DSSCs based on the pristine single TiO₂ nanwires (2.36%) due to the twining network like dual nanowires structure maintaining excellent electron transport and simultaneously enhancing the specific surface area and light scattering ability. Moreover, by TiO₂ sol treatment, the performance of dual TiO₂ nanowires based DSSC can be further enhanced, reaching an impressive power conversion efficiency (PCE) of 7.65%.