Mesoporous TiO2 Films Research Articles

Aqueous synthesis of alloyed CdSexTe1-x colloidal quantum dots and their In-situ assembly within mesoporous TiO2 for solar cells

Ternary alloyed colloidal quantum dots (QDs) have attracted more and more attention due to their distinct optoelectronic properties that may not be present in their parent binary systems, and they have shown great potential application in optoelectronic devices. In this work, ternary alloyed CdSexTe1-x colloidal QDs were successfully prepared via a simple and facile one-pot hydrothermal technique, and the composition and bandgap of CdSexTe1-x can be tuned by manipulating the feed molar ratio of Se/Te. During the hydrothermal reaction process, the CdSexTe1-x QDs were simultaneously in-situ assembled onto mesoporous TiO2 film due to the use of bifunctional capping agent (thioglycolic acid), so this strategy combines colloidal QDs synthesis and adsorption of QDs within TiO2 in a single step, and thus high loading and uniform distribution of CdSexTe1-x sensitizer within TiO2 matrix could be achieved. The photovoltaic properties of TiO2/CdSexTe1-x photoanode with different composition were investigated, and the best power conversion efficiency of 3.23% under one sun illumination (AM 1.5G, 100 mW cm−2) was obtained for the CdSexTe1-x sensitizer synthesized with Se:Te = 2:2. These results show the potential application of hydrothermal method and alloyed CdSexTe1-x QDs in preparing high-performance QDSSC in the future.

Real time nanoplasmonic sensing for monitoring CH3NH3PbI3 perovskite formation in mesoporous TiO2 films

The formation of methylammonium lead iodide (CH3NH3PbI3) perovskite into mesoporous titania (TiO2) scaffold via a sequential deposition method is known to offer high quality films for good photovoltaic device performance. The local kinetics at the lower interface between the mesoporous TiO2 film and the collecting electrode govern perovskite growth and formation. Here, we have used a NanoPlasmonic Sensing (NPS) approach with gold (Au) nanosensors to monitor the formation of CH3NH3PbI3 perovskite at the lower interface of up to 650 nm mesoporous TiO2 films. This technique provides time-resolved spectral shifts of the localized surface plasmon resonance at different operating temperatures and methylammonium iodide (CH3NH3I3) concentrations by recording changes in the local vicinity of the Au nanosensors at the mesoporous TiO2 film interface. Analytical studies included ellipsometry, scanning electron microscopy, X-ray diffraction, and photoluminescence spectroscopy. The results show that both the intensity of the NPS response and NPS rate constants are correlated with the operating concentrations and temperatures of CH3NH3I3 as well as CH3NH3PbI3 perovskite growth in mesoporous TiO2.

In Situ Formation of Ag Nanoparticles in Mesoporous TiO2 Films Decorated on Bamboo via Self-Sacrificing Reduction to Synthesize Nanocomposites with Efficient Antifungal Activity.

We developed a novel green approach for the in situ fabrication of Ag NPs in mesoporous TiO2 films via the bamboo self-sacrificing reduction of Ag(NH3)2+ ions, which can inhibit fungal growth on the bamboo surface. Mesoporous anatase TiO2 (MT) films were first synthesized on bamboo via a hydrothermal method. Then, Ag NPs with a 5.3 nm mean diameter were incorporated into the pore channels of optimal MT/bamboo (MTB) samples at room temperature without the addition of reducing agents, such that the Ag NPs were almost entirely embedded into the MT films. Our analysis indicated that the solubilized lignin from bamboo, which is rich in oxygen-containing functional groups, serves as a green reductant for reducing the Ag(NH3)2+ ions to Ag NPs. Antifungal experiments with Trichoderma viride under dark conditions highlighted that the antifungal activity of the Ag/MT/bamboo samples were greater than those of naked bamboo, MTB, and Ag/bamboo, suggesting that these hybrid nanomaterials produce a synergistic antifungal effect that is unrelated to photoactivity. The inhibition of Penicillium citrinum effectively followed a similar trend. This newly developed bamboo protection method may provide a sustainable, eco-friendly, and efficient method for enhancing the antifungal characteristics of traditional bamboo, having the potential to prolong the service life of bamboo materials, particularly under dark conditions.

Control of TiO2 electron transport layer properties to enhance perovskite photovoltaics performance and stability

Abstract This study demonstrates the effect of electron collection and transportation for TiO2 electron transport layer (ETL) of the mesoscopic perovskite solar cells (PSCs). The influence of compact TiO2 layer (c-TiO2) with various spray cycles, the particle size effect of mesoporous TiO2 (meso-TiO2) film and post-treatment of TiO2 electrode for perovskite solar cells have been studied systematically. We further optimize the meso-TiO2 thickness to enhance the electron collection and transport efficiency and to reduce the anomalous J-V hysteresis phenomenon of PSCs. After adjusting the fabrication process of TiO2 ETL, the highest performance of small cell PSC shows the power conversion efficiency (PCE) of 19.39% in the reverse scan and 19.12% in the forward scan, respectively. A sub-module PSC within an active area of 11.7 cm2 exhibits impressive PCE of 16.03% under the illumination of 100 mW/cm2 (AM1.5G). Moreover, it also shows an outstanding PCE of 25.49% under the illumination of 6000 lx of T5 indoor light source.

Voltage-assisted SILAR deposition of CdSe quantum dots into mesoporous TiO2 film for quantum dot-sensitized solar cells

CdSe quantum dots (QDs) grows slowly in mesoporous TiO2 film deriving from a large lattice mismatch (11%) and poor interaction between the two, which resulting the low power conversion efficiency (PCE) of quantum dot-sensitized solar cells (QDSSCs). An external voltage is applied in the successive ionic layer adsorption reaction (SILAR) process, which drives the Cd2+ and SeSO32− ions quickly diffused into the mesoporous TiO2 film and reacted to form CdSe QDs at room temperature. The CdSe QDSSCs yields a PCE of 3.27%. This work provides an efficient strategy to assemble QDs for enhancing the performance of QDSSCs.

Semi-Transparent and Stable Solar Cells for Building Integrated Photovoltaics: The Confinement Effects of the Polymer Gel Electrolyte inside Mesoporous Films.

The semi-transparent solar cells are promising to be applied in building integrated photovoltaic (BIPV) and tandem solar cells. In this study, we fabricate semi-transparent and stable solar cells for BIPV by utilizing a poly (ethylene oxide) electrolyte and controlling the size of TiO2 nanoparticles and the thickness of the TiO2 film. The power conversion efficiency of the semi-transparent (over 50% transmittance at 620–750 nm) and quasi-solid solar cells is 5.78% under standard AM1.5G, 100 mW cm–2. The higher conductivity and smaller diffusion resistance of the quasi-solid electrolyte inside the mesoporous TiO2 film indicate the confinement effects of the polymer electrolyte inside a mesoporous TiO2 film. The unsealed semi-transparent and quasi-solid solar cell retains its initial efficiency during 1000 h irradiation in humid air.

Interfacial Engineering for High-Efficiency Nanorod Array-Structured Perovskite Solar Cells.

TiO2 nanorod (NR) array for perovskite solar cells (PSCs) has attained great importance due to its superb power conversion efficiency (PCE) compared to that of the traditional mesoporous TiO2 film. A TiO2 compact layer for the growth of TiO2 NR array via spin-coating cannot meet the requirements for efficient NR-based PSCs. Herein, we have developed and demonstrated the insertion of a bifunctional extrathin TiO2 interlayer (5 nm) by atomic layer deposition (ALD) at the interface of the fluorine-doped tin oxide (FTO)/TiO2 compact layer to achieve alleviated electron exchange and a reduced energetic barrier. Thus, an accelerated extraction of electrons from TiO2 NR arrays via the compact layer and their transfer to the FTO substrate can improve the PSC efficiency. The thickness of the spin-coated TiO2 compact layer on the ALD-deposited TiO2 layer is spontaneously optimized. Finally, an outstanding efficiency of 20.28% has been achieved from a champion PSC with negligible hysteresis and high reliability. To the best of our knowledge, this is the first study demonstrating the superiority of TiO2-NR-based PSCs withstanding the dry heat and thermal cycling tests. The results are of great importance for the preparation of efficient and durable PSCs for real-world applications.

Electric-field assisted spray technique for controlled pore filling of nanostructured films: device applications

A novel pore-filling system was prepared using spray as the key-technique, for solar cells and various other applications. High performance in perovskite, dye-sensitized and eta cells is often achieved using metal oxide layers or their mesoporous analogues. One dimensional scaffold materials such as nanorods or nanotubes are also employed in order to improve charge collection. Herein, we introduce a method to more efficiently fill the pores in the most common nanostructure architecture namely mesoporous, nanorods or nanotubes. The method employs the use of spray technique with applied DC voltage (through two different voltage supplies) as the cost-effective technology for time efficient pore filling. SnS nanoparticles and N719 dye have been pore-filled onto a previously deposited ZnO nanorods and mesoporous TiO2 film, respectively, using the novel setup. Scanning electron microscopy images revealed an improved pore-filling, the complementary enhancement of ~ 24% in the DSSC efficiency and ~ 16% and 7% in terms of current density and fill factor, respectively has been found in comparison to the reference standard device. It is attributed to an increase in the concentration of dye molecules into the pores of TiO2 nanostructures due to better dye loading and hence observed an improvement in light absorption, electron transportation and charge collection. This pioneer pore filling technique exponentially reduced the dye loading time duration from overnight immersion of photo anodes in dye solution to 15–20 min.

Introducing of MnS passivation layer on TiO2 mesoporous film for improving performance of quantum dot sensitized solar cells

Despite great efforts to improve the power conversion efficiency of quantum dot-sensitized solar cells, the cell performances are still far from the maximum theoretical value. Coating a passivation layer on the surface of quantum dot sensitizers has been proven an effective method to enhance cell performance. Herein, a MnS passivation layer is introduced between mesoporous TiO2 and quantum dot sensitizers, rather than being coated on the surface of the quantum dot sensitizers as is generally done. Two kinds of photoanodes, based on TiO2 nanoparticles and {001}-faceted TiO2 nanosheets, are investigated in this work. The results show that introducing a MnS layer induces larger charge transfer resistance and longer carrier lifetime, thus improving cell performance. The photoanode based on {001}-faceted TiO2 nanosheets achieves better power conversion efficiency than that of the TiO2 nanoparticle-based photoanode. A maximum efficiency of 5.01% is acquired, representing a 27.8% relative increase compared with that of the cell without a deposited passivation layer (3.92%). This improvement is attributed to the formation of a cascaded band structure among quantum dots/MnS/TiO2 nanosheets. The present MnS passivation layer achieves the highest improvement in cell efficiency among the limited reports thus far on the surface passivation of TiO2.

Pulsed Laser Fabrication of TiO2 Buffer Layers for Dye Sensitized Solar Cells.

We report on the fabrication of dye-sensitized solar cells with a TiO2 buffer layer between the transparent conductive oxide substrate and the mesoporous TiO2 film, in order to improve the photovoltaic conversion efficiency of the device. The buffer layer was fabricated by pulsed laser deposition whereas the mesoporous film by the doctor blade method, using TiO2 paste obtained by the sol–gel technique. The buffer layer was deposited in either oxygen (10 Pa and 50 Pa) or argon (10 Pa and 50 Pa) onto transparent conducting oxide glass kept at room temperature. The cross-section scanning electron microscopy image showed differences in layer morphology and thickness, depending on the deposition conditions. Transmission electron microscopy studies of the TiO2 buffer layers indicated that films consisted of grains with typical diameters of 10 nm to 30 nm. We found that the photovoltaic conversion efficiencies, determined under standard air mass 1.5 global (AM 1.5G) conditions, of the solar cells with a buffer layer are more than two times larger than those of the standard cells. The best performance was reached for buffer layers deposited at 10 Pa O2. We discuss the processes that take place in the device and emphasize the role of the brush-like buffer layer in the performance increase.

Photooxidation of acetaldehyde over TiO2 film: Impact of the substrate glassy structure on the photonic efficiency

Mesoporous TiO2 films were dip-coated onto diverse conductive glass substrates such as Indium Tin Oxide (ITO) and F-doped Tin Oxide (FTO) as well soda-lime glass (SL). The mesoporous TiO2 films were assessed for their photodegradation of CH3CHO in gas phase as a probe molecule and compared with both nonporous TiO2 film deposited on FTO and commercial float glass. The thickness and surface areas of the mesoporous TiO2 film were determined to be around 228 ± 10, 184 ± 15 and 205 ± 15 nm and 215, 352 and 295 m2/cm3 deposition on SL, FTO and ITO, respectively. The prepared mesoporous TiO2 film deposited on SL, FTO and ITO exhibited higher photonic efficiency than nonporous TiO2 film and float glass. The photonic efficiency of mesoporous TiO2 film is the following order FTO> ITO > SL. The photonic efficiency of TiO2/FTO is greater 21 and 3 times than that float glass and nonporous TiO2/FTO, respectively. In addition, the outstanding photocatalytic activity of mesoporous TiO2/FTO substrate is shown in recycling tests in which no significant reduce in the photonic efficiency was recognized after five cycles for 10 h continuously, indicating a promising materials for potential applications.

Mg nanoparticles core-CdS QDs shell heterostructures with ZnS passivation layer for efficient quantum dot sensitized solar cell

A core-shell structure of Mg@CdS quantum dots (QDs) with ZnS passivation layer deposited on TiO2 mesoporous film, denoted as TiO2/Mg@CdS/ZnS, is fabricated as photoanode for enhancement of quantum dot sensitized solar cells efficiency. High resolution transmission electron microscope images and X-Ray photoelectron spectroscopy analysis confirm the core sell structure of Mg@CdS. Introduction of Mg nanoparticles in the photoanode indicates a rise in photovoltage and photocurrent density by decreasing the charge recombination and increasing the photon capturing ability indicated by photoluminescence and diffuse reflectance spectroscopy, respectively. TiO2/Mg@CdS signifies excellent charge injection and transfer efficiency in photoanode. Moreover, Zinc sulfide is utilized as the passivation layer to inhibit electron recombination between photoanode and electrolyte interface. The achieved power conversation efficiency of the TiO2/Mg@CdS/3ZnS is 7.47% which is significantly higher than TiO2/CdS (2.87%) and TiO2/Mg@CdS (5.37%) photoanodes. Electrochemical impedance spectroscopy implies the higher charge recombination resistance between photoanode and electrolyte interface in solar cell fabricated using TiO2/Mg@CdS/3ZnS photoanode.

Facile Secondary Deposition for Improving Quantum Dot Loading in Fabricating Quantum Dot Solar Cells.

Sufficient loading of presynthesized quantum dots (QDs) on mesoporous TiO2 electrodes is the prerequisite for the fabrication of high-performance QD-sensitized solar cells (QDSCs). Here, we provide a general approach for increasing QD loading on mesoporous TiO2 films by surface engineering. It was found that the zeta potential of presensitized TiO2 can be effectively adjusted by surfactant treatment, on the basis of which additional QDs are successfully introduced onto photoanodes during secondary deposition. The strategy developed, that is, the secondary deposition incorporating surfactant treatment, makes it possible to load various QDs onto photoanodes regardless of the nature of QDs. In standard AM 1.5G sunlight, a certified efficiency of 10.26% for the QDSC with Cu2S/brass counter electrodes was achieved by the secondary deposition of Zn-Cu-In-Se QDs.

Crystal Growth in Mesoporous TiO2 Optical Thin Films

Synthesizing mesoporous TiO2 films with tunable crystallized content, using rapid and facile hydrolytic sol–gel routes, is of interest for developing optical and photocatalytic applications on different substrates. Here, the crystallinity and the porosity of TiO2 is fine-tuned, starting from low annealing temperatures. We further investigate the role of crystal seeds in temperature-induced crystalline phase changes in mesoporous TiO2 thin films by Raman spectroscopy, which is interpreted in the framework of a phonon quantum confinement model. The presence of crystal seeds in the films and the high atomic diffusion in the porous structure lead to the lower activation energy of crystal growth and stabilization of the anatase phase above 700 °C. The film nanostructural and optical properties emphasize the role of complexation by acetylacetone in the formation of nanocrystals and pores. Such fine-tuned optical thin films with varying crystallite and porosity content offer promising perspectives of application...

Nanoscale Lead(II) Iodide-sensitized Solar Cell

Nanometer-sized PbI2 particles were deposited over mesoporous TiO2 film by spin-coating and tested as a photosensitizer. The as-prepared PbI2-sensitized solar cell gave overall power conversion eff...

Mesoporous Titania-Coated Biosensor and FEM Model for Highly Sensitive Detection of Low Molecular Weight Targets

This paper presents the interest of a highly sensitive biosensor coated with a TiO2 mesoporous film as a sensitive layer. The main novelty is related to the modeling of the device and simulation by using the finite element method with a COMSOL multiphysics software as a good way to take into account the physical properties of porous 3-D layers. The strategy of using such Love wave devices, with 3-D porous layers, offering further easy functionalization, aims not only to increase the amount of targets caught on the sensor surface but also to enhance the detection mechanism by a higher perturbation of the Love wave acoustic energy, which could be trapped inside the 3-D sensitive layer. First, as a proof of concept, experimental devices with a 3-D titania mesoporous layer were realized, and they have shown a good agreement with simulated results. Furthermore, experimental tests with several Newtonian liquids are investigated, in a range of viscosities from 1 to 7 cP, which are typical of those that concern our biochemical applications. The sensitivity with a 300 nm thick porous sensing layer was ten times that of the bare device, with interesting dynamical issues to be further studied, giving rise to the great potentialities of such architectures for biological detection of low weight biochemical targets.

Template deposition of Sb2S3 for solid-state sensitized solar cells

Unavoidable impurities form in Sb2S3 absorber fabricated by conventional chemical bath deposition method which is harmful to the photovoltaic performance of Sb2S3-sensitized solar cells. To obtain high quality Sb2S3 absorber, a two-step template method is introduced to deposit Sb2S3 on mesoporous TiO2 film. This template method is performed by dipping ZnS-coated TiO2 films in a Sb source solution for several minutes. This low temperature method with short operation time allows large area deposition of Sb2S3. Properties of ZnS and Sb2S3 are systematically studied. X-ray photoelectron spectroscopy confirms the absence of impurities. The optimized Sb2S3 device exhibits a power conversion efficiency (PCE) of 3.69%, with an open-circuit voltage of 0.514 V, a short-circuit current-density of 12.14 mA/cm2, and a fill factor of 59.1% under AM 1.5G illumination which is better than the device fabricated by chemical bath deposition. The PCE of our Sb2S3 devices showed an obvious increase after 1–2 months storage which may be caused by reconstruction of the ZnS/Sb2S3 coating fabricated by partial cation exchange.

A colloidal heterostructured quantum dot sensitized carbon nanotube-TiO2 hybrid photoanode for high efficiency hydrogen generation.

Solar-driven photoelectrochemical (PEC) hydrogen (H2) generation is a promising approach to harvest solar energy for the production of a clean chemical fuel. However, the low photon-to-fuel conversion efficiency and long-term stability of PEC devices are major challenges to be addressed to enable large-scale commercialization. Here we report a simple, fast and cost-effective approach to fabricate high efficiency and stable PEC devices for H2 generation, by fabricating a hybrid photoanode obtained by incorporating small amounts of multiwall carbon nanotubes (MWCNTs) into a TiO2 mesoporous film and sensitizing with colloidal heterostructured CdSe/(CdSexS1-x)5/(CdS)2 quantum dots (QDs). The latter were specially designed to accelerate the exciton separation through a band engineering approach. The PEC devices based on the TiO2/QD-MWCNT (T/Q-M) hybrid photoanode with an optimized amount of MWCNTs (0.015 wt%) yield a saturated photocurrent density of 15.90 mA cm-2 (at 1.0 VRHE) under one sun illumination (AM 1.5G, 100 mW cm-2), which is 40% higher than that of the reference device based on TiO2/QD (T/Q) photoanodes. This is attributed to a synergistic effect of the promising optoelectronic properties of the colloidal heterostructured QDs and improved electron transport (reduced charge transfer resistance) within the TiO2-MWCNT hybrid anodes enabled by the directional path of MWCNTs for the photo-injected electrons towards FTO. Furthermore, the PEC device based on the T/Q-M hybrid photoanode is more stable (∼19% loss of its initial photocurrent density) when compared with the T/Q photoanode (∼35% loss) after two hours of continuous one sun illumination. Our results provide fundamental insights and a different approach to improve the efficiency and long-term stability of PEC devices and represent an essential step towards the commercialization of this emerging solar-to-fuel conversion technology.

Optical properties of the perovskite films deposited on meso-porous TiO2 by one step and hot casting techniques

In this work, CH3NH3PbI3 is coated on meso-porous TiO2 films via one step method (OSM) and hot casting technique (HCT). The optical properties of the solution-processed organic inorganic halide perovskite layers deposited on meso-structured TiO2 scaffolds are investigated using UV–Vis reflectance and transmittance spectroscopies at the wavelength range of 400–900 nm. Optical parameters including refractive index, extinction coefficient, real and imaginary parts of the dielectric constants, optical conductivity and direct and indirect optical band gaps are calculated and compared for the perovskite films created by above mentioned methods. Optical band gaps are determined from the absorption coefficient values using Tauc's power law. The obtained values of direct band gap for two perovskite films are close to each other (1.56 and 1.58 eV for perovskite layer deposited by OSM and HCT respectively). For both prepared films, the difference between direct and indirect band gaps is 60 meV which is in very good agreement with that reported in literature. In addition to the difference between the perovskite morphology, the optical absorption of perovskite layers created by the HCT is less than that of layers prepared by OSM and therefore the amount of generated photo current is less. This is the main reason that causes the efficiency of perovskite solar cells prepared by HCT (3.0%) is ~38% less than that of cells created by OSM (4.15%).

A comparative study of planar and mesoporous perovskite solar cells with printable carbon electrodes

Devices with printable carbon electrodes are promising directions for the commercialization of perovskite solar cells. Most of perovskite solar cells employ mesoporous device structures when using printable carbon as the counter electrodes. Here, we carry a comparative study of planar and mesoporous perovskite solar cells with carbon electrodes. The device efficiency is significantly reduced from 11.37% to 5.27% when the mesoporous TiO2 film is removed from the device structure. Compared with the planar device, smaller carrier transport resistance and bigger carrier recombination resistance are demonstrated for the mesoporous device. Results suggest that the presence of mesoporous TiO2 enables an efficient electron extraction from the perovskite absorber, which remits the serious carrier recombination in the hole transport layer free device due to the hole accumulation. Therefore, the electron extraction efficiency is crucial in these hole transport layer free devices with carbon electrodes. This study helps to develop further optimization of low temperature carbon-based perovskite solar cells for higher reproducibility and higher device performance.