TiO2 Buffer Layer Research Articles

Sb2S3/TiO2 Heterojunction Photocathodes: Band Alignment and Water Splitting Properties

Antimony sulfide (Sb2S3) is a promising light-absorbing semiconductor for photovoltaic applications, though it remains vastly unexplored for photoelectrochemical water splitting. Sb2S3 was synthesized by a simple sulfurization of electrodeposited antimony metal at relatively low temperatures (240–300 °C) with elemental sulfur. Using a TiO2 buffer layer and a platinum co-catalyst, photocurrent densities up to ∼9 mA cm–2 were achieved at −0.4 V vs RHE in 1 M H2SO4 under one sun illumination. Using X-ray photoelectron spectroscopy band alignment studies and potential-dependent incident photon-to-current efficiency measurements, a conduction band offset of 0.7 eV was obtained for the Sb2S3/TiO2 junction as well as an unfavorable band bending at the heterointerface, which explains the low photovoltage that was observed (∼0.1 V). Upon inserting an In2S3 buffer layer, which offers a better band alignment, a 0.15 V increase in photovoltage was obtained. The excellent photoelectrochemical water splitting performance and the identification of the origin of the low photovoltage of the Sb2S3 photocathodes in this work pave the way for the further development of this promising earth-abundant light-absorbing semiconductor for solar fuel generation.

The Effect of the Crystalline Structure Transformation in VO2|Glass by Inserting TiO2 Buffer Layer and Its Application in Smart Windows

Vanadium dioxide (VO2) undergoes a reversible metal-insulator transition at low temperature, which has wide range of applications in smart windows and infrared detectors. However, the preparation of VO2 films with controllable phase on glass substrate is still limited. In this paper, it is shown that B-phase can be transformed into M-phase with monoclinic structure by inserting TiO2 buffer layer on glass substrate at low temperature of 400°C. This crystalline transformation might be attributed to that Ti atoms diffuse and form oxygen-deficient environments. Different thicknesses of buffer layers have different effect on characteristic of VO2 film. With 50-nm TiO2 buffer layer, the VO2|TiO2|glass film showed an abrupt resistance change with more than 2.5-order of magnitude across metal–insulator transition, and the visible-light transmittance value is as high as 55.5% with the solar modulation capability up to 8.6%. The current results are very important for the application in smart windows.

Engineering red phosphorus confined in TiO2-coated ultrathin carbon-bubble foam with enhanced Li+ storage capability

Red phosphorus (RP), as one of the most promising anode materials for lithium-ion batteries, attracts much attention for its high theoretical specific capacity, cost-effective, and commercial availability. However, poor cycling stability and rate capability caused by huge volume expansion and low intrinsic electronic conductivity hinder the commercialization of RP-based anode. Herein, RP confined in ultrathin hollow carbon-bubble-constructed foam (RP-HC) is developed to improve electronic conductivity and mechanical structure stability. Additionally, low-temperature atomic layer deposition (LT-ALD) technology is adopted to further modify the surface of the RP-HC, forming a core–shell RP-HC@TiO2 structure with about 5 nm TiO2 coating layer. Benefiting from the dual confinement of TiO2 buffer layer and conductive porous carbon host with effective POC bond, the RP-HC(70%)@TiO2 electrode delivers an outstanding cycling performance (795.2 mAh g−1 after 200 cycles at 100 mA g−1) and rate capability (459.9 mAh g−1 at 2000 mA g−1, almost 1.6 times and 12 times relative to RP-HC(70%) and RP under the same test condition). In-situ electrochemical impedance spectroscopy analyses are also conducted to investigate the superior Li+ storage properties of the RP-HC@TiO2.

50% transparent solar cells of CuO /TiO2: Device aspects

Abstract In this study, we have fabricated type-I straddling gap heterojunction solar cells of Ag/CuOx/TiO2/FTO configuration utilizing extremely thin p-n junction of 150 nm. We have studied four unique devices by varying the oxygen vacancies in CuOx and its effect on the overall performance of the cells. The average transmittance of 28–52% made these devices semitransparent for visual perception and transparent for visible spectrum of the Sun. The solar cell characteristics were recorded for 365 nm UV light which showed a linear increase in open circuit voltage (Voc) from 0.266 to 1.01 V while changing CuOx phases from Cu2O to CuO. However, a non-linear trend was observed for the photocurrent density and that has been discussed in detail by considering both material and device aspects. The optimized solar cell with CuO active layer possessed Voc of 1.01 V with 3.48 mA/cm2 current density and fill factor (FF) of 31% which sums up to an overall efficiency of 16.22% for an input power of 7 mW/cm2. The electrical parameters of solar cells were analyzed by using a two-diode model and the influence of TiO2 buffer layer has been discussed by the charge transfer mechanisms using the band diagrams. It was found that CuO/TiO2 device offered highest efficiency due to large Δ E c which increased the tunneling probability of minority charge carriers and reduced the recombination process at the interface. This study is the first experimental report on CuOx/TiO2 heterojunction solar cell which may find applications to design fancy and semitransparent power generating windows with colors yellow to brown by varying CuOx stoichiometry.

Epitaxial growth of SrCaTiO3 films on GaN by molecular beam epitaxy with a TiO2 buffer layer

We demonstrate the epitaxial growth of (111)-oriented Sr1 − xCaxTiO3 (SCTO) thin films on (0002) GaN using a thin (100) TiO2 buffer layer by RF-plasma-assisted oxide molecular beam epitaxy. We explore the growth window of SCTO for both x = 0 (i.e., SrTiO3) and x = 0.24 with various TiO2 buffer layer thicknesses and find that a substrate temperature of 650 °C and TiO2 thickness of 1 nm produce the highest quality films, as determined by reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy. SCTO films deposited without a buffer layer are polycrystalline, confirming the necessity of the TiO2 buffer layer to facilitate (111)-oriented epitaxial growth of SCTO on GaN. Transmission electron microscopy performed on the samples shows that the SCTO films grown with a TiO2 buffer layer are highly textured, and selected-area electron diffraction patterns indicate that some domains in the oxide exhibit cation ordering along the (111) direction. We also show that the growth mode changes from 2D to 3D as the thickness of SCTO increases past 10 nm, which leads to film relaxation and a rougher surface morphology.

Enhanced Photoelectrochemical Performance by Interface Engineering in Ternary g‐C3N4/TiO2/PbTiO3 Films

AbstractPolycrystalline ferroelectric PbTiO3 (PTO) films deposited on transparent fluorine‐doped tin oxide (FTO) glass have been proved to be a photocathode for photoelectrochemical (PEC) water splitting. However, the hitherto reported PEC performances remain inferior to meet the requirements for practical applications. Herein, it is reported that a compact TiO2 (TO) buffer layer, inserted between PTO and g‐C3N4 (CN), improves significantly its PEC performance. The optimal CN/TO/PTO film shows a photocathodic current density of −68.5 µA cm−2 at 0 V versus Ag/AgCl electrode under 100 mW cm−2 Xe‐lamp illumination, which is 4.5 and 2 times higher than that of pristine PTO and CN/PTO films. The performance improvement can be attributed to the following reasons: one is the improved light harvesting ability owing to TO and CN, and the other is that TO functions as a buffer layer to build a type II and a Z‐scheme interfaces between PTO, TO, and CN to promote charge carrier transfer and separation. Last but not the least, the ternary photoelectrode also shows tunable PEC performance by ferroelectric polarization switching.

Unconventional Route to Oxygen‐Vacancy‐Enabled Highly Efficient Electron Extraction and Transport in Perovskite Solar Cells

AbstractThe ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high‐efficiency solar energy conversion. Now, a simple yet robust acid‐treatment strategy is used to judiciously create an amorphous TiO2 buffer layer intimately situated on the anatase TiO2 surface as an electron‐transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO2, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO2 surface. Such amorphous TiO2‐coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO2. Compared to pristine TiO2‐based devices, the perovskite solar cells (PSCs) with acid‐treated TiO2 ETL exhibit an enhanced short‐circuit current and power conversion efficiency.

Unconventional Route to Oxygen-Vacancy-Enabled Highly Efficient Electron Extraction and Transport in Perovskite Solar Cells.

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high-efficiency solar energy conversion. Now, a simple yet robust acid-treatment strategy is used to judiciously create an amorphous TiO2 buffer layer intimately situated on the anatase TiO2 surface as an electron-transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO2 , thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO2 surface. Such amorphous TiO2 -coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO2 . Compared to pristine TiO2 -based devices, the perovskite solar cells (PSCs) with acid-treated TiO2 ETL exhibit an enhanced short-circuit current and power conversion efficiency.

Optimizing the Performance of CsPbI3-Based Perovskite Solar Cells via Doping a ZnO Electron Transport Layer Coupled with Interface Engineering

HighlightsDevice simulations and first-principle calculations are employed to derive a guideline for the optimization of CsPbI3-based perovskite solar cells (PSCs).The open voltage and power conversion efficiency of the PSCs are, respectively, improved to 1.31 V and 21.06% by simultaneously introducing an ultra-thin TiO2 buffer layer and increasing the doping concentration of the ZnO electron transport layer.The influence of the interfacial buffer layer and doping of the CsPbI3/ZnO interface on PSC performance is discussed.

Quasiepitaxy Strategy for Efficient Full‐Inorganic Sb2S3 Solar Cells

AbstractAntimony sulfide (Sb2S3) as a wide‐bandgap, nontoxic, and stable photovoltaic material reveals great potential for the uppermost cells in Si‐based tandem cell stacks. Sb2S3 solar cells with a compatible process, acceptable cost, and high efficiency therefore become the mandatory prerequisites to match silicon bottom cells. The performance of vacuum processed Sb2S3 device is pinned by bulk and interfacial recombination. Herein, a thermally treated TiO2 buffer layer induces quasiepitaxial growth of vertical orientation Sb2S3 absorber overcoming interface defects and absorber transport loss. Such novel growth could pronouncedly improve the open‐circuit voltage (Voc) due to the superior interface quality and intraribbon transport. The epitaxial rough Sb2S3 surface shows a texturized‐like morphology. It is optimized by tuning the grain sizes to form strong light trapping effect, which further enhances the short‐circuit current density (Jsc) with a 16% improvement. The final optimal device with high stability obtains a power conversion efficiency of 5.4%, which is the best efficiency for full‐inorganic Sb2S3 solar cells. The present developed quasiepitaxy strategy supports a superior interface, vertical orientation, and surface light trapping effect, which provides a new perspective for efficient noncubic material thin film solar cells.

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.

Optimization of absorber layer for band gap energy moderation of nanostructured SnS thin films

In the present work, band gap energy moderated nanostructured Sn1−xZnxS thin film solar cell devices are investigated in details in order to optimize the number of their layers, which can enhance their efficiency. Sn1−xZnxS nanostructures were synthesized by simple and cost-effective co-precipitation method and were primarily characterized for the study of their structural and phase purification as well as morphological and optical properties. Structural results confirmed the formation of polycrystalline orthorhombic and hexagonal phase of SnS and ZnS nanostructures, respectively. Morphological studies showed that increasing the Zn concentrations changed the morphology of samples from the rod- to spherical- and hexagonal-like particles. Electrical characterization also presented the highest carrier concentration and conductivity conversion in the Sn1/2Zn1/2S sample. Photovoltaic devices were deposited using the ethyl cellulose as a green binder on the transparent substrates and TiO2 buffer layers. Photovoltaic characterization showed that a sample moderated from low-to-high values of band gap energy and with four layers has better efficiency (3.17%) because of factors such as having a wide broadband range of band gap energy in absorber layer, increase in carrier lifetimes, presence of minor carriers in output current, and an increase in carrier concentration of the moderated layers. This paper also investigates and compares our results with the literature and gives some suggestions for coping with the problems involved in having a high number of layers in the fabricated devices.

Silver Nanowire-Based Stretchable Transparent Electrode for Flexible Organic Light-Emitting Diode.

The new stretchable transparent electrode was proposed and fabricated based on a process which utilizes silver nanowires (AgNWs) on a Polyurethane (PU) substrate. In order to overcome the rough surface nature of the silver nanowire electrode, a titanium oxide (TiO₂) buffer layer was over-coated and then followed by a heat treatment of organo-metalic sol-gel solution. The fabricated stretchable electrodes exhibit electrical sheet resistance of 24 Ω/□, transmittance of 78% at 550 nm wavelength and average surface roughness of less than 5 nm. In addition, without adding additional conductive polymer layers, the fabricated AgNW-based electrode can maintain its initial electrical resistance even if nearly 130% strain is applied. In this letter, the critical role of a TiO₂ buffer layer in achieving the high performance of the AgNW stretchable electrode is discussed.

Full-stainless steel mesh dye-sensitized solar cells based on core-shell ZnO/TiO2 nanorods

We report fabrication and photovoltaic properties of flexible dye-sensitized solar cells (FDSSCs) based on stainless steel mesh (SSM)-supported core-shell ZnO/TiO2 nanorods photoanode and Pt nanoparticles- coated SSM counter electrode (Pt/SSM CE). The core-shell ZnO/TiO2 structures were prepared via simple chemical bath deposition, and optimal conversion efficiency of 2.84% was achieved. When aluminum foil membrane was introduced on the back of the FDSSCs as reflecting film (Rf) for improving utilization of sunlight, the conversion efficiency of FDSSCs was increased from 2.84% to 3.12%. The relative improvement of 218% was achieved in conversion efficiency compared with similar device without TiO2 buffer layer and reflecting film (0.98%). Meanwhile, the Pt CE based on SSM possessed of higher catalytic ability and longer stability compared with ITO/PET-based Pt CE.

Transformation of crystalline structure and photoelectric properties in VO2/glass thin films by inserting TiO2 buffer layers

Vanadium dioxide (VO2) with reversible metal–insulator transition (MIT) is a promising energy-saving material for next-generation smart windows and infrared devices. However, the specific applications are largely limited by controllable preparation with different polymorphs on common glass substrates. Herein, VO2 thin films were prepared on transparent amorphous glass substrates by inserting TiO2 buffer layers using pulsed laser deposition (PLD) technique. The results showed that B-VO2 film would be formed on the amorphous glass substrate at 400 °C. While after inserting TiO2 buffer layer, pure M-VO2 films with clear phase transition properties were obtained. Based on the analysis of X-ray Diffraction (XRD) and Energy Dispersive Spectrometer (EDS) characterization, the crystalline transformation was attributed to the template effect of TiO2 layer and Ti ions diffusion. The characteristics of VO2/TiO2/glass thin films were closely associated with the thickness of TiO2 buffer layer. With 60 nm TiO2 buffer layer, the VO2/TiO2/glass film showed the sharpest resistance change with more than 2.5-order of magnitude across the MIT, and the T-vis value is as high as 53% with the ΔTsol up to 5.2%. Our current results demonstrated the importance of TiO2 buffer layer to the formation of M-VO2 film on amorphous glass substrate, which was very meaningful for the production of energy efficient smart windows in the future.

Roles of TiO2 buffer layer for preparation of high performance VO2 thin films with Monoclinic polymorph

Abstract Vanadium dioxide (VO2) thin films with pure Monoclinic phase could be transformed from B-phase by inserting TiO2 buffer layers on transparent amorphous glass substrates at the low temperature of 400 °C. This crystalline transformation might be ascribed to oxygen-deficient environment induced by the TiO2 buffer layer and the template effect of TiO2 layer for M-VO2 phase. The simplicity in phase regulation of the present method and the superior optical and electrical properties of the films may allow its wide applications in thermo-opto-electro sensing devices.

Strong impact of SrTiO3/TiO2 buffer layer on epitaxial growth and dielectric response of Ba0.7Sr0.3TiO3 thin films on MgO

We demonstrate a significant improvement in the epitaxial growth of perovskite dielectric films on MgO by introducing a SrTiO3 (STO)/TiO2 buffer layer, which enhances their dielectric response. 270-nm-thick (001)-epitaxial (Ba,Sr)TiO3 (BST) films were deposited by pulsed laser deposition on STO/TiO2-buffered MgO with a SrRuO3 (SRO) bottom electrode. The film directly deposited on SRO/MgO grew in a three-dimensional mode, resulting in a rough and poorly crystalline film with an almost relaxed strain. On the other hand, the film with a buffer layer grew in a two-dimensional mode, resulting in a flat and highly crystalline film with a large compressive strain (−0.80%). As a result, the paraelectric-to-ferroelectric phase transition temperature increased by 220 °C and an out-of-plane dielectric constant exceeding 1000 was achieved.

Low temperature growth of amorphous VO2 films on flexible polyimide substrates with a TiO2 buffer layer

Amorphous VO2 thin films were grown on anatase TiO2-buffered polyimide (PI) films using radio-frequency magnetron sputtering deposition with a VO2 target as low as at 175 °C. For comparison, the authors grew VO2 films on TiO2-buffered SiO2/Si substrates. The structural and morphological properties of the VO2 films were evaluated by x-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. VO2 films grown on TiO2/SiO2/Si were crystalline at 200 and 250 °C and were amorphous at 175 °C. VO2 films grown on TiO2/PI were amorphous. No peak corresponding to the monoclinic phase of VO2 appeared in the Raman spectra of VO2/TiO2/PI films grown at 175 or 200 °C. The chemical compositions of VO2 and the binding energy spectra of V and O atoms were probed by x-ray photoelectron spectroscopy. The authors discussed the multivalence states of V atoms and oxygen vacancies based on the x-ray photoemission spectroscopy of crystalline and amorphous VO2 films. The authors obtained the hysteresis curves of the resistivity as a function of temperature for both VO2/TiO2/SiO2/Si and VO2/TiO2/PI films. In addition, the authors measured the reflectivity of VO2/TiO2/PI films below and above the metal-insulator transition temperature using spectroscopic ellipsometry. The reflectivity changed substantially and was comparable to the literature values of well-crystallized VO2 films, even though the ratio of the switching resistivity values was as low as sixty. This work demonstrates that VO2 films grown on plastic films grown at temperatures as low as 175 °C can be applicable as flexible thermochromic films for use in energy-saving windows.

High dielectric constant and energy density induced by the tunable TiO2 interfacial buffer layer in PVDF nanocomposite contained with core–shell structured TiO2@BaTiO3 nanoparticles

To realize application in high-capacity capacitors and portable electric devices, large energy density is eagerly desired for polymer-based nanocomposite. The core–shell structured nanofillers with inorganic buffer layer are recently supposed to be promising in improving the dielectric property of polymer nanocomposite. In this work, core–shell structured TO@BT nanoparticles with crystalline TiO2 buffer layer coated on BaTiO3 nanoparticle were fabricated via solution method and heat treatment. The thickness of the TO buffer layer can be tailored by modulating the additive amount of the titanate coupling agent in preparation process, and the apparent dielectric properties of nanocomposite are much related to the thickness of the TO layer. The relatively thin TO layer prefer to generate high polarization to increase dielectric constant while the relatively thick TO layer would rather to homogenize field to maintain breakdown strength. Simulation of electric field distribution in the interfacial region reveals the improving effect of the TO buffer layer on the dielectric properties of nanocomposite which accords with the experimental results well. The optimized nanoparticle TO@BT-2 with a mean thickness of 3–5 nm buffer layer of TO is effective in increasing both the ε and Eb in the PVDF composite film. The maximal discharged energy density of 8.78 J/cm3 with high energy efficiency above 0.6 is obtained in TO@BT-2/PVDF nanocomposite with 2.5 vol% loading close to the breakdown strength of 380 kV/mm. The present study demonstrates the approach to optimize the structure of core–shell nanoparticles by modulating buffer layer and provides a new way to further enlarge energy density in polymer nanocomposite.

Ru Alloy-Oxide Buffer Layer for Intergranular Exchange Decoupling of CoPt–B2O3 Granular Media

Investigation of magnetic properties and microstructure for the Co80Pt20–30vol% B2O3 granular medium to clarify the effect of utilizing buffer layer (BL) on intergranular exchange coupling was conducted. It was found that the employment of a Ru50Co25Cr25–30vol% TiO2 BL with thickness of around 2 nm will avoid physical contact between magnetic grains in the initial growth region, which is effective to intergranular exchange decouple the magnetic grains. On the other hand, for deposition of BL thicker than 2 nm, underlayer will be flatten that brings the possibility of physical contact of magnetic grains and forms a granular film with strong intergranular exchange coupling.