Nb2O5 Films Research Articles

High-temperature anodization route to nanoporous niobium oxides with enhanced supercapacitive properties in aqueous electrolytes

Relatively little work has been done on Nb2O5 pseudocapacitive behaviors in aqueous solutions. Here, the galvanostatic anodization of niobium in K2HPO4-containing glycerol electrolyte at high temperature is fully explored. The anodically grown films on niobium have ordered nanoporous morphology and are composed of a low-crystalline orthorhombic Nb2O5, which can be directly employed as supercapacitor electrodes without any posttreatment process. To further improve their supercapacitive performances, a facile technique to generate Fe-doped Nb2O5 films is developed, where the as-anodized films are treated by an electrochemical doping in FeCl3 ethylene glycol solution. The Fe-doped Nb2O5 films show a wider potential window and higher capacitance than that of the as-anodized films in aqueous electrolytes. They present a maximum areal capacitance of 168.1 mF cm−2 at 1 mV s−1 and excellent cycling stability with 85.8 % capacitance retention after 10,000 cycles. The charge storage in the Fe-doped Nb2O5 is believed to occur by the incorporation of Li+ ions with concomitant reduction of Nb5+ to Nb4+ in aqueous lithium-containing solutions. Unlike nonaqueous media, Li+ intercalation/deintercalation in aqueous media is a diffusion-controlled process, leading to the comparatively slower pseudocapacitive processes. Nevertheless, in terms of an industrial applicability the Fe-doped Nb2O5 films deserve special attention.

Wide spectral range optical characterization of niobium pentoxide ([formula omitted]) films by universal dispersion model

In this work, the optical properties of niobium pentoxide (Nb2O5) films were extensively studied across a wide spectral range using heterogeneous data-processing methods, combining ellipsometric and spectrophotometric measurements for five samples with thicknesses between 20 and 250 nm. This study primarily determined the optical constants of Nb2O5 from the far infrared to the vacuum ultraviolet, presenting these constants as dispersion parameters using the universal dispersion model to describe valence electron excitations in ultraviolet region as well as phonon vibrations in infrared region. These comprehensive and reliable data across such a broad spectral range are unprecedented. Secondly, presented optical characterization proofs that Nb2O5 films can be grown without defects such as surface roughness, porosity, or inhomogeneity. This fact, together with its high refractive index, makes Nb2O5 a promising material for optical applications.

A comparative study of TiO2, Ta2O5 and Nb2O5 coated Ti6Al4V titanium alloy for biomedical applications

Refractory metal oxides such as titanium dioxide (TiO2), tantalum oxide (Ta2O5), and niobium oxide (Nb2O5) coated-titanium and its alloys for medical applications have recently gained significant attention due to their remarkable properties, including high hardness, strong chemical inertness, and excellent biocompatibility. However, comparative studies among them remain scarce. In this study, thin films of Ta2O5, Nb2O5, and TiO2 were deposited on Ti6Al4V titanium alloy using radio frequency sputtering technology. Their properties were comparably evaluated through a series of tests, including scratching, nanoindentation, friction and wear, potentiodynamic polarization, and cell cultivation experiments. The results revealed that all three coatings, with a thickness of approximately 2.6–2.8 μm, exhibited an amorphous columnar growth pattern with dense surfaces resembling cauliflower. The bonding force of the TiO2 coating exceeds 50 N, which is 26.2 times and 14.5 times greater than that of Ta2O5 and Nb2O5, respectively. Furthermore, all coated samples demonstrate superior wear resistance, corrosion resistance, and biocompatibility compared to bare Ti6Al4V. Among the coatings, TiO2 showed the best performance, followed by Ta2O5, while Nb2O5 exhibited the least favorable performance. These findings revealed the performance differences among the three ceramic coatings on Ti6Al4V samples and offer valuable insights for further research and the selection of coatings to enhance the properties of Ti6Al4V titanium alloy for biomedical applications.

Au/Nb2O5 terahertz-gigahertz electro-optical filters designed for high frequency applications

Herein enhanced broad band filters are fabricated by deposing thin films of Nb2O5 onto glass and semitransparent Au nanosheets using the ion coating technique. Remarkable enhancement in the surface roughness of the films and in the visible and infrared light absorption by more than 270% and 750%, respectively, is achieved by coating the films onto Au nanosheets. Au nanosheets redshifted the energy band gap and initiated the free carrier absorption in the Nb2O5 films. As terahertz band filters, Au/Nb2O5 interfaces exhibited higher dielectric constant, higher optical conductivity and higher terahertz cutoff frequency values. Au/Nb2O5 optical filters showed terahertz resonator characteristics displaying six resonance modes two, three and one of which are dominant in the infrared, visible and ultraviolet ranges, respectively. On the other hand the impedance spectroscopy analyses for Au/Nb2O5/Au (ANA) filter showed microwave resonator characteristics with cutoff frequency values reaching 700 GHz for signal carriers of driving frequency of 1.65 GHz. ANA devices exhibited negative capacitance effect in a wide range of driving frequency domain. The features of the Au/Nb2O5 films demonstrate their potential for use as gigahertz-terahertz broad band electro-optical resonators suitable for high frequency applications.

Understanding the Kinetics of Cation Insertion-Coupled Electron Transfer in Thin Film Electrodes from Cyclic Voltammetry

Cyclic voltammetry is a powerful technique for probing the electrochemical kinetics of energy storage materials. However, interpretation of the cyclic voltammetry kinetics of such materials has been limited by the application of analytical solutions initially developed for ion blocking electrodes, whereby mass transport is limited to semi-infinite diffusion in the electrolyte medium. Here, we utilize established numerical simulation models for finite diffusion coupled with experimental results of a model insertion electrode to demonstrate the relationship between solid-state diffusion and cyclic voltammetry current, peak potential, and capacity. This simulation is publicly available.1 We show that the cyclic voltammetry response of thin film electrode materials undergoing solid-solution ion insertion without significant Ohmic polarization can be explained with finite diffusion. Specifically, we investigate the kinetic behavior of Li+ insertion-coupled electron transfer into thin films of orthorhombic Nb2O5. We observe a general trend in the b-value, a descriptive parameter derived from the relation ip=avb between peak current ip and scan rate v. We find that both in simulation and experiment, we can tune the b-value response between the theoretically limiting values of 0.5 and 1 based on the solid-state diffusion characteristics of the thin film and the experimental timescale. We also show that b-values below 0.5 are possible, and arise due to a combined effect of the potential window of the experiment and confined diffusion. Our findings show the power of a simple 1D finite-diffusion model for predicting the kinetic response of an electrode undergoing ion-coupled electron transfer, and help to inform on how the scan rate selection in voltammetry experiments affects the observed kinetic behavior. https://github.com/mchagnot/Numerical_CV_Sim_1D_Diffusion

Interplay between the heat treatment and structural and optical properties of Nb2O5 thin films

Thin films of Nb2O5 were synthesized by RF sputtering. The effect of annealing, at different temperatures (100, 150, 200, 250, 300, 350, and 400 °C), on the structural and optical properties was studied. The crystallinity and structural characteristics of the Nb2O5 thin films were examined employing the X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD and SEM analyses showed an amorphous nature for the as-deposited and annealed samples from 100 to 350 °C and crystalline structure for the annealed sample at 400 °C. The high values of the transmittance and low reflection make these films used as a window layer in solar cell systems. Upon increasing annealing temperature, the energy band gap Eg increased from 4.039 to 4.175 eV. Eg high values again make these films suitable to be used as a window layer in solar cell system. Urbach tail, EU, decreased with increasing annealing temperature. The refractive index showed the normal dispersion. The dispersion energies were found to have the same behavior with respect to annealing temperature, besides, the single oscillator energy Eₒ had a lower value compared to the dispersion energy Ed. The plasma frequency ωp\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\omega }_{p}$$\\end{document} was found to increase with annealing temperature. The first and third optical moments had peak values depending on the incident photon energy.

Nb2O5 film optical properties and laser-induced damage by phase-change-driven tuning

Niobium oxide (Nb2O5) is widely used in optical and electrical fields because of its high refractive index, wide band gap, high laser-induced damage threshold (LIDT) and high transparency in the ultraviolet to middle-infrared-wavelength range (0.35–7 μm). In this study, Nb2O5 films with different crystalline phases were successfully prepared on BK7 and Si substrates through electron beam evaporation (EB) and post-annealing. Based on the results of X-ray diffraction (XRD) and Raman spectroscopy analysis, the impact of the post-annealing technique on the structure, and optical and physical properties of Nb2O5 films was thoroughly discussed at the molecular level. The crystal phase of an Nb2O5 film and its molecular bonds considerably affect its properties. The Nb2O5(-12 1 2) film prepared in this study exhibited a higher LIDT (17.72 J/cm2) and a higher RMS (0.447 nm) than the corresponding films prepared in previous studies. Following the post-annealing process, the band gap and refractive index of the prepared films were significantly higher than those of the as-deposited film. Thus, this study has significant guiding implications for Nb2O5 film applications and the development costs of the films.

Radiation-enhanced crystallization of amorphous Nb2O5 films according to TEM with in situ video recording

Amorphous films are formed on substrates at room temperature in the process of anodic oxidation of niobium foil. The studies were carried out using methods of electron diffraction and “in situ” transmission electron microscopy (TEM) with video recording of structural changes. It was found that electron beam irradiation of the film with a dose rate (4–7)·104 e−/Å2·s inside the column of the electron microscope causes its layer polymorphous crystallization (LPC) with the formation of Nb2O5 crystals with hexagonal lattice. The dimensionless parameter of the relative crystallization length δ0≈ 5000 − 7000. The dependence on time of the crystal diameter is described by a power function with exponents of 1.55, 1.1 and 1.3. According to this the dependence of the fraction of the crystalline phase on time is described by a power function with exponents of 3.1, 2.2 and 2.6. This non-quadratic nature is explained with the formation of a domain superstructure in the low-temperature modification of α-Nb2O5 and phenomenon of polyamorphism in amorphous films.

Ultra-high ON/OFF ratio with low set voltage for a Pt-Nb2O5-Pt resistivity switching device

The limited ON/OFF ratio constrains the quantity of available resistance states and the multi-level storage capability, a high ON/OFF ratio (>103) is essential for implementing multi-level storage in a resistive device. Through annealing processes for Nb2O5 films prepared by magnetron sputtering, we successfully achieved an ultra-high ON/OFF ratio of 106 and a four-order higher resistance compared to conventional binary oxide resistive random-access memory. Results confirm that annealing can effectively modulate the content of oxygen vacancies within the resistance-switching layer and the formation path of oxygen vacancies. At the same time, we observed a gradual decrease in the set voltage of the device with increasing annealing temperature, which is beneficial for the low power consumption characteristics. Therefore, appropriate annealing is a novel and effective approach to optimize device performances for better information storage applications, and its core mechanism is to modulate the concentration of Vo for constructing fixed conductive paths.

Enhancing Thickness Uniformity of Nb2O5/SiO2 Multilayers Using Shadow Masks for Flexible Color-Filtering Applications.

Using a stainless shadow mask combined with a magnetron-ion-assisted deposition (IAD) sputtering system, we investigate the surface morphologies and optical properties of microfilms. Optimal color-filter (CF) coating microfilms with niobium pent-oxide (Nb2O5)/silicon dioxide (SiO2) multilayers on a hard polycarbonate (HPC) substrate, grown at 85 °C and 50 SCCM oxygen flow, can obtain a fairly uniform thickness (with an average roughness of 0.083 and 0.106 nm respectively for Nb2O5 and SiO2 films) through all positions. On a flexible HPC substrate with the Nb2O5/SiO2 microfilms, meanwhile, the peak transmittances measured in the visible range are 95.70% and 91.47%, respectively, for coatings with and without a shadow mask for this new-tech system. For the optimal CF application with a shadow mask, transmittance on each 100 nm band-pass wavelength is enhanced by 4.04% absolute (blue), 2.96% absolute (green), and 2.12% absolute (red). Moreover, the developed new-tech system not only enhances the quality of the films by achieving smoother and uniform surfaces but also reduces deposition time, thereby improving overall process efficiency. For the with-shadow-mask condition, there is little shift at 50% transmittance (T50%), and high transmittance (~97%) is maintained after high-temperature (200 °C) baking for 12 h. These results are well above the commercial CF standard (larger than 90%) and demonstrate reliability and good durability for flexible optical applications.

Study of Nb2O5 High-k Dielectric Material Deposited By Atomic Layer Deposition for Metal-Insulator-Metal Capacitor

Metal-Insulator-Metal (MIM) capacitors are important components in many electronic devices, including memory devices, filters, and oscillators. The performance of these devices depends strongly on the electrical properties of the dielectric layer. The application of new high-k materials can improve the properties of modern capacitors such as leakage current and capacitance density. However, it is necessary to understand the deposition conditions of both electrode and dielectric materials in order to master the dielectric properties in future MIM capacitors.One potentially interesting high-k material is niobium pentoxide (Nb2O5) with a wide band gap (3.6 eV) and high refractive index (n=2.3 @633 nm) deposited by Atomic Layer Deposition (ALD) using niobium (V) ethoxide as niobium precursor at 180°C and H2O as reaction agent and oxygen source. Nb2O5 films were deposited directly on a silicon (Si 100) substrate or on a 10nm of titanium nitride (deposited by ALD on Si substrate, n=1.4@633 nm). The Nb2O5 films were deposited with a good film thickness uniformity on the 6-inches of the wafers with a thickness ranging from 20 to 60 nm on each sample. The deposition was done under a suitable ALD temperature of 200-300 °C. In order to investigate the film growth, structure, morphology and optical properties, the samples were characterized by spectroscopic ellipsometry (SE), scanning electron microscopy (SEM), X-Ray diffraction (XRD) and atomic force microscopy (AFM). The as-deposited films were then annealed in a furnace at temperature ranging from 500 to 650°C under N2 atmosphere.Subsequently, a MIM structure (Al/Nb2O5/TiN) was produced by adding a second aluminum layer deposited at 250°C on a Nb2O5/TiN/Si stack by the sputtering method. The surface of the second electrodes was aimed to have 0.1-1.0 mm2 to confirm the surface dependency of the capacitance value. The electrical properties of Nb2O5 dielectric layer such as dielectric constant, breakdown voltage and leakage current at possible user voltage were measured on this last structure. The low leakage current density is desirable for MIM capacitors as it helps to minimize energy loss and increase the energy storage capabilities of the capacitors. The obtained J-E curve was compared to some electrical conduction mechanisms to identify its electrical conduction behaviors. The barrier height of the F-N tunneling mechanism was determined by fitting the experimental data to the F-N equation. These measurements indicate that the electrical properties of the Nb2O5 material produced by ALD with specific settings are promising for the application in capacitors. These results provide new insight into our understanding of the application of Nb2O5 realized by ALD for the MIM capacitor.

Effect of Interdiffusion and Crystallization on Threshold Switching Characteristics of Nb/Nb2O5/Pt Memristors.

The resistive switching response of two terminal metal/oxide/metal devices depends on the stoichiometry of the oxide film, and this is commonly controlled by using a reactive metal electrode to reduce the oxide layer. Here, we investigate compositional and structural changes induced in Nb/Nb2O5 bilayers by thermal annealing at temperatures in the range of 573-973 K and its effect on the volatile threshold switching characteristics of Nb/Nb2O5/Pt devices. Changes in the stoichiometry of the Nb and Nb2O5 films are determined by Rutherford backscattering spectrometry and energy-dispersive X-ray (EDX) mapping of sample cross sections, while the structure of the films is determined by X-ray diffraction, Raman spectroscopy, and transmission electron microscopy (TEM). Such analysis shows that the composition of the Nb and Nb2O5 layers is homogenized by interdiffusion at temperatures less than the crystallization temperature (i.e., >773 K) but that this effectively ceases once the films crystallize. This is explained by comparison with the predictions of a simple diffusion model which shows that the compositional changes are dominated by oxygen diffusion in the amorphous oxide, which is much faster than that in the crystalline phases. We further show that these compositional and structural changes have a significant effect on the electroforming and threshold switching characteristics of the devices, the most significant being a marked increase in their reliability and endurance after crystallization of the oxide films. Finally, we examine the effect of annealing on the quasistatic negative differential resistance characteristics and oscillator dynamics of devices and use a lumped element model to show that this is dominated by changes in the device capacitance resulting from interdiffusion.

Electrochemical corrosion behavior and in vitro biocompatibility of Ti-Nb-Sn alloy for dental applications

Ti-Nb-Sn alloy was prepared by mechanical alloying and mold sintering process, and the effects of Nb on the microstructure, corrosion behavior and in vitro biocompatibility were investigated. With the increased Nb content, the microstructure was transformed from Widmannstatten to needle-like structure. Moreover, the stable passivation films of TiO2, Nb2O5 and SnO2 were formed on the surface of the alloy in artificial saliva, and the surface wettability was changed from hydrophilic to hydrophobic, which made the corrosion resistance of the alloy was intensified. The increase of Nb content can promote adhesion and proliferation of mouse osteoblasts, and accordingly improve cell vitality.

Memristive effect in niobium oxide thin films obtained by the pulsed laser deposition

NbOx thin films demonstrating the analog resistive switching characteristics in planar geometry were fabricated by the droplet-free pulsed laser deposition from Nb metal targets. The memristive properties of the films were controlled by tuning the oxygen pressure from 0.7 to 8 Pa and the wavelength of the ablative laser (λ = 248 nm or λ = 532 nm). The stoichiometry of the NbOx films was controlled by the X-ray photoelectron spectroscopy, optical and electrical investigations. Variation of the film thickness from 20 to 50 nm affects the resistive switching characteristics. The 20 nm NbO2 films obtained at the oxygen pressure of 1.3 Pa show a volatile memristive switching with a memory window of ON/OFF resistances ∼ 101 after 500 direct current cycles at an operating voltage of 4.5 V. The 30 nm Nb2O5 films fabricated at the oxygen pressure of 4 Pa, showed a gradual increase in device conductivity with successive positive voltage sweeps with an amplitude of +5 V, which is characteristic of non-volatile memristive switching. Thus, the use of NbOx memristive films of different compositions and crystallinity degree will allow creating neuromorphic systems based on the same material by controlling the memristive behavior by the laser synthesis conditions.

Hard-Templated Porous Niobia Films for Optical Sensing Applications

Porous Nb2O5 films obtained by a modified hard-template method were studied and their optical and sensing properties were optimized in order to find applications in chemo-optical sensing. Porous films were prepared by following three steps: liquid mixing of niobium sol and SiO2 colloids in different volume fractions, thermal annealing of spin-coated films for formation of a rigid niobia matrix, and selective removal of silica phase by wet etching thus generating free volume in the films. The morphology and structure of the films were studied using transmission electron microscopy and selected area electron diffraction, while their optical and sensing properties were estimated using UV-VIS-NIR reflectance measurements in different ambiences such as air, argon and acetone vapors and nonlinear curve fitting of the measured reflectance spectra. Bruggeman effective medium approximation was applied for determination of the volume fraction of silica and air in the films, thus revealing the formation of porosity inside the films. For further characterization of composite films, their water contact angles were measured and finally conclusions about the impact of initial chemical composition and etching duration on properties of the films were drawn.

A new precursor route for the growth of NbO2 thin films by chemical vapor deposition

Niobium dioxide (NbO2) exhibits metal-insulator transition (Mott transition) and shows the potential for application in memristors and neuromorphic devices. Presently growth of NbO2 thin films requires high-temperature reduction of Nb2O5 films using H2 or sophisticated techniques such as molecular beam epitaxy and pulsed laser deposition. The present study demonstrates a simple chemical route of the direct growth of crystalline NbO2 films by chemical vapor deposition using a freshly prepared Nb-hexadecylamine (Nb-HDA) complex. X-ray diffraction studies confirm the NbO2 phase with a distorted rutile body-centered-tetragonal structure and the film grown with a highly preferred orientation on c-sapphire. X-ray photoelectron spectroscopy confirms the +4 oxidation state. The present method offers facile growth of NbO2 films without post-reduction steps which will be assumed to be a cost-effective process for NbO2 based devices.

Threshold switching stabilization of NbO2 films via nanoscale devices

The stabilization of the threshold switching characteristics of memristive NbOx is examined as a function of sample growth and device characteristics. Sub-stoichiometric Nb2O5 was deposited via magnetron sputtering and patterned in nanoscale (50×50–170×170nm2) W/Ir/NbOx/TiN devices and microscale (2×2–15×15μm2) crossbar Au/Ru/NbOx/Pt devices. Annealing the nanoscale devices at 700 °C removed the need for electroforming the devices. The smallest nanoscale devices showed a large asymmetry in the IV curves for positive and negative bias that switched to symmetric behavior for the larger and microscale devices. Electroforming the microscale crossbar devices created conducting NbO2 filaments with symmetric IV curves whose behavior did not change as the device area increased. The smallest devices showed the largest threshold voltages and most stable threshold switching. As the nanoscale device area increased, the resistance of the devices scaled with the area as R∝A−1, indicating a crystallized bulk NbO2 device. When the nanoscale device size was comparable to the size of the filaments, the annealed nanoscale devices showed similar electrical responses as the electroformed microscale crossbar devices, indicating filament-like behavior in even annealed devices without electroforming. Finally, the addition of up to 1.8% Ti dopant into the films did not improve or stabilize the threshold switching in the microscale crossbar devices.

Niobium Pentoxide Films with High Laser-Induced Damage Threshold under High Temperature Environment

Laser damage resistance of films is the key to the performance and durability of high-power laser systems. High temperature, however, often leads to a certain degree of decline in film properties. Here, aiming to explore the mechanism of laser damage under high temperatures, Nb2O5 films were prepared using sol–gel method with NbCl5 as precursor and citric acid as additive. The effect of annealing temperatures in optical performance, microstructure, surface morphologies, and chemical composition of films were studied. Further investigation was carried out of the laser-induced damage threshold (LIDT) under different in situ high temperatures of the as-deposited films. The results showed that the films had prominent optical transmittance and high LIDT. Under 293 K, the LIDT of the film was the highest of 24.8 J/cm2. The increase of temperature brought down the LIDT of the films. It was attributed to the occurrence of oxygen vacancies, the sharp increase of temperature, and rise of defects and destruction of network structure. In this study, even though the LIDT of the film decreased a lot at 523 K, it was still high up to 14.1 J/cm2, representing the great potential for applications in authentic high-temperature environments.

Anomalous thermal conductivity in amorphous niobium pentoxide thin films: A correlation study between structure and thermal properties

Niobium pentoxide (Nb2O5) based thin films are predominantly used in optical filters, solar cells, electrochromic devices, sensors and microelectronic devices. The temperature-dependent thermophysical properties of Nb2O5 films are crucial for the performance and reliability of such devices. Within this work, for the first time, the thermal properties of sputter deposited Nb2O5 films are correlated with their structural properties at different length scales. Thermal measurements were carried out by time-domain thermoreflectance, yielding a thermal conductivity of 3.0±0.3 W/mK at 25 °C for crystalline Nb2O5 films, which decreases to 2.6±0.2 W/mK at 450 °C. In contrast, amorphous Nb2O5 films had a thermal conductivity of 2.2±0.2 W/mK below 275 °C. Above 275 °C, an abrupt increase in thermal conductivity up to a maximum value of 2.8±0.2 W/mK at 325 °C was recorded. The average and local structure are determined by in-situ high-temperature X-ray diffraction and in-situ high-temperature Raman spectroscopy, respectively. These characterization techniques together enable to cross-correlate structural and thermal properties of Nb2O5 thin films highlighting the observed peculiarity of the thermal conductivity. This substantial increase in thermal conductivity cannot be linked to any macroscopic phase change but rather to a local phase rearrangement near the crystallization temperature, evidenced by temperature-dependent Raman spectra analysis. This study serves as a guide to engineering future Nb2O5 based thin film devices and for their reliability optimization.

The role of Nb2O5 deposition process on perovskite solar cells

Two different methods are used to deposit Nb2O5 as compact electron transport layers in n-i-p double cation mixed-halide perovskite Cs0.17FA0.83Pb(I0.83Br0.17)3 solar cells: reactive sputtering and spin coating. These different Nb2O5 films influenced perovskite growth and the charge transport in the cells. Photovoltaic parameters were obtained with an average power conversion efficiency of 17.0% and 15.7% for the devices based on sputtered and spin-coated Nb2O5, respectively. The mobility and the extracted charges were higher in sputtered Nb2O5-based devices than in the spin-coated ones. This effect is attributed to the larger grain sizes observed in the perovskite films when deposited onto the sputtered Nb2O5 layers. The higher densities of grain boundaries in the spin-coated Nb2O5-based devices increase ion diffusion and are expected to decrease efficiency.