Structure and properties of (1−x)(0.6Pb(Zn1/3Nb2/3)O3-0.4Pb (Mg1/3Nb2/3)O3)-xPbTiO3 thin films with perovskite phase promoted by polyethylene glycol

The preferential formation of a pyrochlore structure is a knotty problem in the preparation of Pb(Zn_1/3Nb_2/3)O₃ (PZN)-based thin film materials and its presence is significantly detrimental to the dielectric and piezoelectric properties. In this study, 40 mol% of PZN was replaced with Pb(Mg_1/3Nb_2/3)O₃ (PMN) for obtaining a perovskite composition around a morphotropic phase boundary (MPB), (1−x)(0.6PZN-0.4PMN)-xPT ((1−x)PZMN-xPT, PT: PbTiO₃) where x = 0.23. The thin films with this composition were prepared with a polyethylene glycol (PEG) modi-fied sol-gel method on LaAlO₃ substrates. The microstructural evolution of the films on heat treatment was examined with X-ray diffraction. With the aid of PEG, the formation of the pyrochlore phase was suppressed and the perovskite phase formed directly from the amorphous gel film. The multilayer films with a thickness around 0.25 μm showed a single perovskite phase without any detectable pyrochlore structure. Microscopic images showed uniform grain size of a few tens of nanometers. The role of the polymer dramatically promoting the perovskite phase was investigated with the aid of X-ray photoelectron spectroscopy and thermal analysis. The dielectric constant of the obtained film was 4160 at 1 kHz. The film demonstrated typical ferroelectric hysteresis loops and exhibited excellent piezoelectric performance.     

Role of reaction atmosphere in preparation of potassium tantalate through sol–gel method

Potassium tantalate (KT) thin films and powders of both K₂Ta₂O₆ (KT pyrochlore) and KTaO₃ (KT perovskite) structures were prepared by means of chemical solution deposition method using Si(111) with ZnO and MgO buffer layers as a substrate. The influence of reaction atmosphere on reaction pathway and phase composition for both KT powders, and KT thin films has been studied mainly by means of powder diffraction and infrared spectroscopy. When an oxygen flow instead of static air atmosphere has been used the process of pyrolysis in oxygen runs over much narrower temperature interval (200–300 °C), relatively to air atmosphere (200–600 °C) and almost no (in case of powders), or no (in case of thin films) pyrochlore intermediate phase has been detected in comparison with treatment in air, where the pyrochlore phase is stable at temperatures 500–600 °C (powders). KT perovskite phase starts to crystallize at temperatures 50° and 150 °C lower compared to air atmosphere in case of powders and thin films, respectively. Microstructure formed by near-columnar grains and small grains of equiaxed shape was observed in films treated in oxygen and air atmosphere, respectively.     

Chemistry-Crystallization-Microstructure Relations of Sol-Gel Derived Lanthanum Modified Lead Titanate Thin Films

Lanthanum modified lead titanate thin films have been obtained by the deposition of sol-gel solutions onto platinized (100) silicon substrates. Crystallization of perovskite films was achieved by thermal treatments at 650°C with slow or rapid heatings. Lead oxide excesses were used in the precursor solutions to counterbalance the lead losses produced during the thermal treatment. Rapid heatings and large excesses of lead produce a preferred orientation of the films. These films have more homogeneous and denser microstructures than slow heated films without lead excess.     

Bismuth iron tungstate pyrochlore thin films for photovoltaic applications

Thin films based on the ternary complex oxide Bi_0.50Fe_0.4WO_q with a cubic pyrochlore structure were obtained and used for the first time as electron-transport layers in perovskite solar cells. The measured power conversion efficiency was ～ 4 rel% higher than that of state-of-the-art TiO₂-based perovskite solar cells.     

Preparation of (1? x %)(Na0.5Bi0.5)TiO3– x %SrTiO3 thin films by a sol–gel method for dielectric tunable applications

(1−x%)(Na_0.5Bi_0.5)TiO₃–x%SrTiO₃ (NBTSx) thin films were deposited on Pt/TiO₂/SiO₂/Si substrates by a spin-on sol–gel method and rapid thermal annealing. The stock solutions were prepared using sodium acetate, bismuth acetate, strontium acetate, and titanium n-butoxide as precursors, ethanol, and acetic acid as solvents and acetylacetone as the chelating agent. The thermal treatment conditions were determined by thermal analyses of the dry gel powders derived from the stock solutions. Structure and dielectric tunable properties of the films were studied as functions of Sr concentration. NBTSx films exhibit the perovskite structure of a pseudo-cubic symmetry with the lattice parameters increasing with increasing Sr concentration. It was found that the substitution of Sr in Na_0.5Bi_0.5TiO₃ may greatly reduce the dielectric loss while decrease the tunability at the same time. The best figure of merit was achieved in NBTS80 films. The results were discussed and compared with related materials.     

Controllable Growth of Perovskite Films by Room‐Temperature Air Exposure for Efficient Planar Heterojunction Photovoltaic Cells

A two‐step solution processing approach has been established to grow void‐free perovskite films for low‐cost high‐performance planar heterojunction photovoltaic devices. A high‐temperature thermal annealing treatment was applied to drive the diffusion of CH₃NH₃I precursor molecules into a compact PbI₂ layer to form perovskite films. However, thermal annealing for extended periods led to degraded device performance owing to the defects generated by decomposition of perovskite into PbI₂. A controllable layer‐by‐layer spin‐coating method was used to grow “bilayer” CH₃NH₃I/PbI₂ films, and then drive the interdiffusion between PbI₂ and CH₃NH₃I layers by a simple air exposure at room temperature for making well‐oriented, highly crystalline perovskite films without thermal annealing. This high degree of crystallinity resulted in a carrier diffusion length of ca. 800 nm and a high device efficiency of 15.6 %, which is comparable to values reported for thermally annealed perovskite films.     

Physicochemical characterisation of lead titanate solutions applied to the processing of thin films

A new sol-gel method, in which dihydroxy alcohols are used as solvents, is described for the preparation of lead titanate thin films. Infrared spectroscopy and measurements of viscosity are used to study the reaction mechanisms involved in the formation of polymerized solutions. Crack-free lead titanate layers with thickness 0.5 µm are obtained from these solutions, on Pt/TiO₂/Si substrates, by spin-coating and subsequent heat treatments. The formation of the perovskite and the microstructure of these films are monitored by X-ray diffraction, thermogravimetry analysis, differential thermal analysis and scanning electron microscopy.     

Preparation and Optical Characterization of Sol-Gel Deposited Pb(Zr0.45Ti0.55)O3 Films

Homogeneous crack-free lead zirconate titanate (Pb(Zr_0.45Ti_0.55)O₃: PZT 45/55) films were prepared by a chemically modified sol-gel process using lead acetate trihydrate, zirconium n-propoxide, and titanium isopropoxide precursors. The coating solutions were modified by the addition of diethanolamine. Single and multilayer films were deposited with a 2000 rpm spin rate on fused silica and MgO(100) substrates. Multiple spin coating with an intermediate heat treatment in air at 400°C for 3 min between coatings was performed to obtain films up to 2 m in thickness. The formation of the tetragonal perovskite structure was found to depend on the intermediate firing temperature, final annealing temperature, and annealing time. A 650°C rapid thermal annealing treatment in oxygen was required to crystallize the PZT film into the perovskite structure. The films were characterized using optical spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and thermo-gravimetry and differential scanning calorimetry (TG-DSC). The optical constants of the PZT films were evaluated from spectral transmittance and reflectance measurements. Optical constants are presented over the visible and near infrared region.     

Study of the crystallization pathway of Na0.5Bi0.5TiO3 thin films obtained by chemical solution deposition

Na_0.5Bi_0.5TiO₃ (NBT) thin films were fabricated by a chemical solution deposition (CSD) method. A route involving the reaction between sodium and bismuth acetates and titanium n-butoxide was used to synthesise the different precursor solutions. The thermal decomposition and crystallization pathways of different modified precursors have been studied by thermal analysis and X-ray diffraction techniques. As a consequence of the modification of the precursor solutions and their different thermal behaviour, the nucleation of the stable perovskite phase happens at different temperatures depending on each case but is found to be at temperatures as low as 500 °C. For the thin film processing, the drying and pyrolysis temperatures were chosen according to the thermogravimetric data to minimize the strain resulting from the shrinkage of the film during the elimination of solvents and organic ligands. The crystallization process was studied and the experimental results are discussed in terms of structural, microstructural and electrical features investigated by field-emission scanning electron microscopy, atomic force microscopy in tapping and piezo-force modes and X-ray diffraction.     

Effect of vapor thermolysis on microstructure development of sol–gel-derived lead zirconate–titanate (PZT) and PZT-Sr(K<Subscript>0.25</Subscript>Nb<Subscript>0.75</Subscript>)O<Subscript>3</Subscript> solid solutions

Ceramic granules of PZT (Pb(Zr₅₃,Ti₄₇)O₃) and fibers of SKN-doped PZT (1 Mol-% Sr(K_0.25Nb_0.75)O₃ substitution in PZT) were fabricated from sol–gel precursors. The transformation of the metal–organic gel to the oxide and its crystallization behavior was investigated by thermogravitmetry, and XRD. The formation of pyrochlore or perovskite phase was sensitive to the preceeding heat treatment conditions in moist atmosphere (termed vapor thermolysis). Lower thermolysis temperatures resulted in the preferred formation of metastable pyrochlore phase. Higher thermolysis temperatures favored the formation of perovskite. Investigation of sintered materials revealed a superior sintering activity for samples with pronounced intermediate pyrochlore content whereas low-pyrochlore samples remained highly porous after sintering. For constant chemical precursors the vapour thermolysis conditions (220–280 °C) siginificantly affected crystallization and sintering behavior of the PZT and SKN-PZT material.     

Effect of biphase on dielectric properties of Bi-doped lead strontium titanate thin films

Pb_0.4Sr_0.6TiO₃ (PST) thin films doped with various concentration of Bi were prepared by a sol-gel method. The phase status, surface morphology and dielectric properties of these thin films were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance analyzer, respectively. Results showed that the thin films with the maximum dielectric constant and minimum dielectric loss were obtained for x=0.15. For x<0.15, only pure PST perovskite phase were in the thin films. For 0.2<x<0.4, the PST/Bi₂Ti₂O₇ biphase were obtained. The thin films with pure Bi₂Ti₂O₇ pyrochlore phase were obtained for x=0.67. The biphase thin films had high tunability and high figure of merit (FOM). The FOM of PST/Bi₂Ti₂O₇ biphase thin film was about 6 times higher than that thin films formed with pure perovskite phase or pure pyrochlore phase.     

Volatile Perovskite Precursor Ink Enables Window Printing of Phase-Pure FAPbI3 Perovskite Solar Cells and Modules in Ambient Atmosphere

Despite the great success of perovskite photovoltaics in terms of device efficiency and stability using laboratory-scale spin-coating methods, the demand for high-throughput and cost-effective solutions remains unresolved and rarely reported because of the complicated nature of perovskite crystallization. In this work, we propose a stable precursor ink design strategy to control the solvent volatilization and perovskite crystallization to enable the wide speed window printing (0.3 to 18.0 m/min) of phase-pure FAPbI₃ perovskite solar cells (pero-SCs) in ambient atmosphere. The FAPbI₃ perovskite precursor ink uses volatile acetonitrile (ACN) as the main solvent with DMF and DMSO as coordination additives is beneficial to improve the ink stability, inhibit the coffee rings, and the complicated intermediate FAPbI₃ phases, delivering high-quality pin-hole free and phase-pure FAPbI₃ perovskite films with large-scale uniformity. Ultimately, small-area FAPbI₃ pero-SCs (0.062 cm²) and large-area modules (15.64 cm²) achieved remarkable efficiencies of 24.32 % and 21.90 %, respectively, whereas the PCE of the devices can be maintained at 23.76 % when the printing speed increases to 18.0 m/min. Specifically, the unencapsulated device exhibits superior operational stability with T₉₀>1350 h. This work represents a step towards the scalable, cost-effective manufacturing of perovskite photovoltaics with both high performance and high throughput.     

Nanoporous piezo- and ferroelectric thin films

Nanoporous barium titanate and lead titanate thin films (～100 nm calculated from ellipsometric data) are prepared starting from sol-gel solutions modified with a commercially available block-copolymer and evaporation-induced self-assembly methodology. The tuning of the thermal treatment followed by in situ ellipsometry allows the decomposition of the organic components and of the structuring agent leading to the formation of porous tetragonal crystalline perovskite structures as observed by XRD, HRTEM, SEM, and ellipsoporosimetry. Both nanoporous barium titanate and lead titanate thin films present local piezoelectric and ferroelectric behavior measured by piezoresponse force microscopy (PFM), being promising platforms for the preparation of the generation of new multifunctional systems.     

Preparation of lead and tin oxide thin films by spin coating and their application on the electrodegradation of organic pollutants

In this work, lead and tin oxide films (Pb _x O _y /SnO₂) were prepared, using the spin coating technique. The influence of the temperature and duration of the thermal treatment on the final film composition were analysed. The metallic oxide films that were prepared, Pb _x O _y /SnO₂, were characterized by means of XRD, SEM/EDS and cyclic voltammetry. When different experimental preparation conditions were used different lead oxide phases were obtained. The electrochemical studies show that the films are stable and can be used as electrodes. Finally the films were tested as electrodes for the electrochemical degradation of a CHCl₃ aqueous solution.     

PMN-PT thin films grown by sputtering on silicon substrate: influence of the annealing temperature on the physico-chemical and electrical properties of the films

Studies of piezoelectric and electrostrictive properties of (1?x)PMN-xPT thin films were carried out. We have chosen the compositions 90/10 and 70/30, which exhibit, respectively, mostly electrostrictive and piezoelectric behaviour in bulk material. Annealing temperature effects on PMN-PT structural, dielectric, ferroelectric and electromechanical properties have been investigated. We demonstrate that with conventional annealing the pure perovskite phase can be obtained at very low temperature (400°C) without any pyrochlore phase for the two compositions. We show that electromechanical response is a mix between electrostrictive and piezoelectric response for the two compositions. However, as can be easily understood, piezoelectric contribution is larger for 70/30. It is shown that electrical responses of the films obtained at 400°C are largely satisfied for many applications; for higher annealing temperature we observe an enhance of the electrical properties due to an improvement of the material quality in terms of crystalline structure.     

Film Composites of polyimide with polyaniline and poly(aniline-<Emphasis Type="Italic">co</Emphasis>-anthranilic acid)

New two-component composite polymer films are prepared and studied in terms of thermal stability and stress-strain properties. The matrix component of the composite is polyimide based on 3,3′,4,4′-(1,3-diphenoxybenzene)tetracarboxylic dianhydride and 4,4′-bis(4″-aminophenoxy)diphenyl sulfone. Another component is polyaniline or the copolymer poly(aniline-co-2-aminobenzoic acid). Composite films are cast from mixed solutions of individual polymers in N-methyl-2-pyrrolidone. Interpolymer interactions in polyimide composites with polyaniline or the copolymer of aniline and 2-aminobenzoic acid are investigated via viscometry and IR spectroscopy. The thermal treatment of composite films with poly(aniline-co-2-aminobenzoic acid) results in decarboxylation of the copolymer and formation of polyimide-polyaniline composite films. The morphology, microphase structure, and porosity of the composite films are different from those of the films cast from solutions of polyimide-polyaniline mixtures.     

Effect of solvent on phase composition and particle morphology of lanthanum niobates prepared by polymeric complex sol–gel method

Lanthanum niobates were prepared by a new polymeric complex sol–gel method using Nb-citrate or -tartrate complexes in different solvent (ethanol or methanol) and calcination at 750–1,050 °C. The perovskite La_1/3NbO₃ and pyrochlore LaNb₅O₁₄ phases were formed after calcination at 900 and 1,050 °C from gels synthesized from ethanol and methanol solvents respectively. The very similar xerogel thermal decomposition processes were observed independently on applied solvents, where the pyrochlore monoclinic LaNbO₄ and Nb₂O₅ phases were intermediate products at lower calcination temperatures during transformation. The particle morphologies changed from spherical 20–50 nm particles at 750 °C to granular LN particles (ethanol) or rectangular (methanol) at 1,050 °C. HRTEM images and SAED verified the coexistence of minority monoclinic LaNbO₄ phase with majority phases in individual LN particles after annealing. The strong effect of alcohol solvent on phase formation was shown, while the effect of chelating agent was insignificant.     

A chemical route for the synthesis of cubic bismuth zinc niobate pyrochlore nanopowders

Cubic bismuth zinc niobate pyrochlore (base composition (Bi_1.5Zn_0.5)(Zn_0.5Nb_1.5)O₇) powders were successfully prepared by a chemical method. The formation mechanism of the pyrochlore phase was investigated by TG-DSC, FT-IR, Raman, and X-ray diffraction (XRD). The optical bandgap for the powders treated at temperatures ranging from 500 to 700 °C is 3.0-3.1 eV, indicating low crystallization temperature for the pyrochlore phase. No detectable intermediary phases as BiNbO₄ or a pseudo-orthorhombic pyrochlore were observed at any time and the cubic-BZN phase was already formed after thermal treatment at temperatures as low as 500 °C. The phase formation study reveals that a well-crystallized single-phased nanopowder is obtained after calcination at 700 °C, indicating that the chemical synthesis conferred a higher chemical homogeneity and reactivity on the powder, modifying the crystallization mechanism.     

Ferroelectric Properties of Modified Lead Titanate Thin Films Obtained by a Sol-Gel Method

Calcium substituted lead titanate thin films have been prepared by a sol-gel method, deposited by spin-coating, and thermally treated so as to obtain a perovskite structure. Two types of thermal treatments were given, differing in the heating rate, which in conventional treatments was of some 10°C/min, and in rapid treatments >500°C/min. With rapid heating, materials are obtained in which greater polarizations are commuted and bias fields are smaller. This, together with the fact that additional (undesired) phases are not observed by X-ray diffractometry when rapid treatments are used, shows that these are to be preferred over conventional treatments.     

Phase Formation Study of The Substituted Lanthanum Manganites Solid Solutions

It is well known that the manganites-based solid solutions are interesting for their electric and magnetic properties. LaMnO₃ exhibits a distorted perovskite structure due to Mn³⁺ ion, which determines the presence of the Jahn–Teller effect. Replacing La³⁺ host ions by cations of lower valence leads to the disappearance of this effect and changes the characteristics of these materials. Although the formation of manganites-based solid solutions has been intensively studied, there are some unelucidated aspects concerning their formation mechanism, depending both on the precursors used and on the thermal treatment applied in order to obtain suitable properties. In this work the formation mechanism of La_0.7M_0.3MnO₃ (M=Ca, Sr, Ca+Sr in equimolecular mixture) solid solutions, in isothermal and non-isothermal conditions, was studied. For this purpose XRD, DTA/TG and spectral techniques were used. The solid solutions formation was found to be more dependent on the Mn-precursors type than the thermal treatment conditions. This revised version was published online in July 2006 with corrections to the Cover Date.