Evaporated Molybdenum Oxide Research Articles

Impact of annealing on charge storage capability of thermally evaporated molybdenum oxide thin films

Rapid technological advancements in recent years have necessitated the creation of energy-related devices. Due to their unique properties, including an exceptional cycling life, safe operation, low processing cost, and a higher power density than batteries, supercapacitors (SCs) have been identified as one of the most promising candidates to meet the demands of human sustainability. To increase the energy density of SCs, several advanced electrode materials and cell designs have been researched during the past few years. Utilizing the Faradaic charge storage process of transition metal cations, transition metal compounds have recently received attention as prospective electrode materials for SCs with high energy densities. In this work, the structure, morphology and electrochemical properties of molybdenum oxide (MoO3) films deposited via thermal evaporation technique and annealed at various temperatures were systematically investigated in order to examine the potential use of MoO3 for supercapacitors. Electrochemical analysis confirmed the pseudocapacitance characteristics of the synthesized films. The annealing temperature affects the oxidation and reduction observed in the cyclic voltammetry (CV) plots. Areal capacitance of thin films annealed at 150°C was found to be maximum and this could be attributed to the formation of hollow tube-like nanostructures which provided more active sites, than films annealed at higher temperatures. This also influences the charge storage ability of the synthesized films. It would be logical to assume that additional research in this area will result in more interesting discoveries and, eventually, the vi-ability of those promising Mo-based compounds in high-tech energy storage systems.

Structural, electrical, and linear/nonlinear optical characteristics of thermally evaporated molybdenum oxide thin films

Homogeneous thin films of Molybdenum oxide (MoO3) were grown on quartz and glass substrates using the thermal evaporation method. XRD results showed that the MoO3 powder has a polycrystalline structure with an orthorhombic crystal system whereas the MoO3 thin films have amorphous nature. SEM images showed that the MoO3 thin films have a nearly uniform surfaces with worm-like shape grains. The film thickness influences on the linear and nonlinear optical characteristics of MoO3 thin films that were examined using spectrophotometric measurements and from which, the linear optical constants of the MoO3 thin films were estimated. The electronic transition type was determined as a direct allowed one. The values of the optical band gap were obtained to be in the range of 3.88–3.72 eV. The dispersion parameters, third-order nonlinear optical susceptibility, and the nonlinear refractive index of the MoO3 thin films were determined and interpreted in the light of the single oscillator model. The temperature dependence of the DC electrical conductivity and the corresponding conduction mechanism for the MoO3 films were investigated at temperatures ranging from 303 to 463 K.

Performance Analysis of Silicon Carrier Selective Contact Solar Cells With ALD MoOx as Hole Selective Layer

Carrier selective contact (CSC) architecture is one of several promising candidates for high-efficiency silicon solar cells operating close to the thermodynamic limit. However, an industrially feasible process is lacking for manufacturable CSC cells. Specifically, although evaporated molybdenum oxide (MoOx) has been actively researched as a hole selective layer, a more industry-compatible deposition process is critically needed. Atomic layer deposition (ALD) has been widely employed in the industry for material stacks requiring precise thickness control and uniformity. In this paper, we present a performance analysis of CSC solar cells fabricated using ALD MoOx as the hole selective layer. As a first step, the standard ALD recipe was modified to obtain sub-stoichiometric MoOx with a higher work function due to increased oxygen vacancy concentration. The modified MoOx recipe resulted in low open-circuit voltage values in fabricated not-passivated CSC solar cells. A 4 nm thick amorphous silicon layer inserted in the hole selective stack showed significant improvement in minority lifetime but worse solar cell performance due to increased series resistance. Besides indicating a strong trade-off between passivation and series resistance, the solar cell data highlights series resistance as a key performance limiter in CSC solar cells.

Investigation of silicon surface passivation by sputtered amorphous silicon and thermally evaporated molybdenum oxide films using temperature- and injection-dependent lifetime spectroscopy

Crystalline silicon (c-Si) surface passivation has been investigated by sputtered hydrogenated intrinsic amorphous silicon (S-i-a-Si:H) and thermally evaporated molybdenum oxide (MoOx) thin films. The temperature- and injection-dependent lifetime spectroscopy technique has been adopted for analyzing the passivation quality of the c-Si surface, using parameters such as the minority carrier effective lifetime (τeff), the activation energy of surface/interface defect states (ΔE), and the electron to hole carrier capture cross-section ratio (k) at the interface. With S-i-a-Si:H films, a τeff of ∼70 µs and ΔE of ∼51 meV have been observed in comparison to a τeff of ∼110 µs and ΔE of ∼109 meV from the MoOx films. These entirely different parameters are an indication of the relatively strong carrier recombination with dense interface/surface states from the S-i-a-Si:H passivation layers. The S-i-a-Si:H layers are unable to minimize the c-Si surface trap states with the chemical passivation for reducing carrier recombination due to the generation of additional surface defect states by the sputtering damage. However, the MoOx layers show better c-Si surface passivation due to the reduction of majority carriers by the carrier inversion (field-effect passivation) and chemical passivation. This effect is clearly reflected with the opposite trend in the carrier capture analysis from S-i-a-Si:H and MoOx layers.

Solution‐Processable Anode Double Buffer Layers for Inverted Polymer Solar Cells

Although organic solar cells have surpassed the 17% power conversion efficiency threshold, commercial modules efficiencies are only around 4–5%. One of the reason is the lack of effective solution‐processable hole transport materials that are a key element for the scale‐up on roll‐to‐roll printing equipments and the commercial development. Herein, a class of novel vanadium and molybdenum polyoxometallate salts are developed that, alone or in combination with a traditional poly(ethylene‐3,4‐dioxytiophene):poly(styrene sulfonate) (PEDOT:PSS) layer, can be used as anodic buffer layer in inverted polymer solar cells. These materials exhibit work function values around 5.8 eV that match well with highest occupied molecular orbital energies of typical polymer donors. They are tested with different widely used active systems, including PTB7:PC71BM, PV2000:PCBM, and PffBT4T:PC71BM. Vanadium and molybdenum polyoxometallate can be deposited from solutions and, contrary to PEDOT:PSS used alone, do not cause a drop of performances compared with evaporated molybdenum oxide (e‐MoOx); on the contrary, in the best cases they achieve similar performances to e‐MoOx. Slot‐die‐coated PV2000:PCBM solar cells on flexible substrate achieve a remarkable power conversion efficiency of almost 7.6%.

Effect of thickness and post deposition annealing temperature on the structural and optical properties of thermally evaporated molybdenum oxide films

In this paper, the influence of thickness and post deposition annealing temperature on thermally evaporated molybdenum oxide films has been studied. The X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) are used for crystal structural and surface morphological characterization of the films, respectively. The XRD analysis showed that the presence of α-MoO3 and MoO2 crystalline phase of the films annealed at elevated temperature ~ 250 °C after deposition. The optical constants are determined from UV–Vis transmission spectra. The optical band gap and Urbach energy is found to be temperature dependent. The refractive index of the films is estimated by the optical method as well as cross-sectional SEM image analysis. It is found that the refractive index of the films increases from ~ 1.70 to 2.03 with the decrease in film thickness from ~ 2.9 to 1.7 µm. It is also observed that the refractive index decreased from ~ 2.03 to 1.61 with the increase in post deposition annealing temperature from room temperature (RT) to ~ 250 °C. Moreover, extinction coefficient, optical conductivity, porosity, and film density are investigated as a function of source-substrate distance and post deposition annealing temperature. The Photoluminescence (PL) properties of the films are also investigated by recording spectra under the excitation wavelength at 250 nm.

Combining solvents and surfactants for inkjet printing PEDOT:PSS on P3HT/PCBM in organic solar cells

Abstract A commonly used strategy in the fabrication of organic electronics involves the use of orthogonal solvents, i.e. the alternating use of organic solvents and water for the application of consecutive layers to prevent dissolution of previous layers. This strategy therefore requires the deposition of sequential layers with a large mismatch in surface energy. In case of organic photovoltaics (OPV) a particularly challenging deposition is that of the aqueous Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS) on the hydrophobic Poly(3-hexylthiophene-2,5-diyl/Phenyl-C61-butyric acid methyl ester (P3HT/PCBM) layer. The mismatch in surface energy causes dewetting and inhomogeneous layer formation. In inkjet printing individually placed droplets should spread sufficiently far to form a homogeneous closed layer before the solvents in the droplet evaporate. A formulation for the aqueous PEDOT:PSS layer has been developed in which the combination of solvents and surfactants is essential for achieving a homogenous layer on the timescales needed for inkjet printing. Homogenous layers of PEDOT:PSS on P3HT/PCBM were achieved for layer thicknesses from 400 nm to 50 nm. The efficiency of the OPV's fabricated with the new formulation were comparable with reference devices, where evaporated molybdenum oxide (MoOx) was used as a topelectrode. This shows that by the addition of solvents and surfactants a hydrophilic solution can be inkjet printed successfully to form homogenous layers on hydrophobic surfaces and achieve good efficiencies in an inverted organic solar cell.

Effect of post-annealing on the properties of thermally evaporated molybdenum oxide films: Interdependence of work function and oxygen to molybdenum ratio

Molybdenum oxide (MoOx, x < 3) thin films were deposited on glass and (100) silicon substrates by thermal evaporation technique. Post-annealing of the samples was performed in O2 by rapid thermal process at 373–673K. The surface chemical, structural，morphological and optical properties of the samples were investigated. X-ray photoelectron spectra characterization verified the growth of sub-stoichiometric MoOx films as the coexistence of oxidation states of Mo6+ and Mo5+. The O/Mo ratio of MoOx films increases from 2.58 to 2.75 when the annealing temperature increases to 473K, but this ratio rapidly decreases at higher temperatures. The work function of MoOx films is proportional to O/Mo ratio and within the range of 5.40–5.95eV. This correlation can be ascribed to the variation of Fermi level and surface dipole with oxygen vacancy. The MoOx films are polycrystalline with nanoparticles, and both α- and β-phase MoO3 are co-existed with the annealing temperature from 373K to 673K. The optical band gap of MoOx films is in the scope of 2.72–2.95eV. The three critical properties of MoOx films, such as work function, optical band gap and transmittance, are well-correlated to the variation of chemical configuration with the annealing temperature.

Prospects of e-beam evaporated molybdenum oxide as a hole transport layer for perovskite solar cells

Perovskite solar cells have emerged as one of the most efficient and low cost technologies for delivering of solar electricity due to their exceptional optical and electrical properties. Commercialization of the perovskite solar cells is, however, limited because of the higher cost and environmentally sensitive organic hole transport materials such as spiro-OMETAD and PEDOT:PSS. In this study, an empirical simulation was performed using the Solar Cell Capacitance Simulator software to explore the MoOx thin film as an alternative hole transport material for perovskite solar cells. In the simulation, properties of MoOx thin films deposited by the electron beam evaporation technique from high purity (99.99%) MoO3 pellets at different substrate temperatures (room temperature, 100 °C and 200 °C) were used as input parameters. The films were highly transparent (&amp;gt;80%) and have low surface roughness (≤2 nm) with bandgap energy ranging between 3.75 eV and 3.45 eV. Device simulation has shown that the MoOx deposited at room temperature can work in both the regular and inverted structures of the perovskite solar cell with a promising efficiency of 18.25%. Manufacturing of the full device is planned in order to utilize the MoOx as an alternative hole transport material for improved performance, good stability, and low cost of the perovskite solar cell.

Annealing effect on physical properties of evaporated molybdenum oxide thin films for ethanol sensing

This paper deals with some physical investigations on molybdenum oxide thin films growing on glass substrates by the thermal evaporation method. These films have been subjected to an annealing process under vacuum, air and oxygen at various temperatures 673, 723 and 773K. First, the physical properties of these layers were analyzed by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM) and optical measurements. These techniques have been used to investigate the oxygen index in MoOx properties during the heat treatment. Second, from the reflectance and transmittance optical measurements, it was found that the direct band gap energy value increased from 3.16 to 3.90eV. Finally, the heat treatments reveal that the oxygen index varies in such molybdenum oxides showing noticeably sensitivity toward ethanol gas.

Fabrication of poly(methyl methacrylate)-MoS2/graphene heterostructure for memory device application

Combination of two dimensional graphene and semi-conducting molybdenum disulfide (MoS2) is of great interest for various electronic device applications. Here, we demonstrate fabrication of a hybridized structure with the chemical vapor deposited graphene and MoS2 crystals to configure a memory device. Elongated hexagonal and rhombus shaped MoS2 crystals are synthesized by sulfurization of thermally evaporated molybdenum oxide (MoO3) thin film. Scanning transmission electron microscope studies reveal atomic level structure of the synthesized high quality MoS2 crystals. In the prospect of a memory device fabrication, poly(methyl methacrylate) (PMMA) is used as an insulating dielectric material as well as a supporting layer to transfer the MoS2 crystals. In the fabricated device, PMMA-MoS2 and graphene layers act as the functional and electrode materials, respectively. Distinctive bistable electrical switching and nonvolatile rewritable memory effect is observed in the fabricated PMMA-MoS2/graphene heterostructure. The developed material system and demonstrated memory device fabrication can be significant for next generation data storage applications.

Ultrathin ammonium heptamolybdate films as efficient room-temperature hole transport layers for organic solar cells.

Ammonium heptamolybdate (NH4)6Mo7O24·4H2O (AHM) and its peroxo derivatives are analyzed as solution-processed room temperature hole transport layer (HTL) in organic solar cells. Such AHM based HTLs are investigated in devices with three different types of active layers, i.e., solution-processed poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester(P3HT/PC60BM), poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]/[6,6]-phenyl C70-butyric acid methyl ester(PCDTBT/PC70BM) and evaporated small molecule chloro(subphthalocyaninato)boron(III) (SubPc)/C60. By virtue of their high work functions, AHM based HTLs outperform the commonly used poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) HTL for devices employing deep HOMO level active materials. Moreover, devices using AHM based HTLs can achieve higher short circuit current (Jsc) than the ones with evaporated molybdenum oxide(eMoO3), and thus better power conversion efficiency (PCE). In addition, P3HT/PC60BM devices with AHM based HTLs show air stability comparable to those with eMoO3, and much better than the ones with PEDOT:PSS.

Optimization of a High Work Function Solution Processed Vanadium Oxide Hole-Extracting Layer for Small Molecule and Polymer Organic Photovoltaic Cells

We report a method of fabricating a high work function, solution processable vanadium oxide (V2Ox(sol)) hole-extracting layer. The atmospheric processing conditions of film preparation have a critical influence on the electronic structure and stoichiometry of the V2Ox(sol), with a direct impact on organic photovoltaic (OPV) cell performance. Combined Kelvin probe (KP) and ultraviolet photoemission spectroscopy (UPS) measurements reveal a high work function, n-type character for the thin films, analogous to previously reported thermally evaporated transition metal oxides. Additional states within the band gap of V2Ox(sol) are observed in the UPS spectra and are demonstrated using X-ray photoelectron spectroscopy (XPS) to be due to the substoichiometric nature of V2Ox(sol). The optimized V2Ox(sol) layer performance is compared directly to bare indium–tin oxide (ITO), poly(ethyleneoxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and thermally evaporated molybdenum oxide (MoOx) interfaces in both small molec...

Wettability and photochromic behaviour of Molybdenum oxide thin films

The temporal wettability (hydrophobic–hydrophilic) behaviour of thermally evaporated Molybdenum oxide thin films is examined via water droplet contact angle and water droplet lifetime measurements. Super-hydrophilic state is quickly achieved in these films and the rapidity of this conversion depends strongly on the film preparation conditions. It is found that film microstructure, humidity content and UV irradiation have a profound influence on the wettability behaviour of these films. The ensuing photochromic effect under UV irradiation is also followed through the optical changes occurring in the film. Films deposited at higher chamber pressure have a generally smaller initial contact angle and lifetime.

Towards a high efficiency amorphous silicon solar cell using molybdenum oxide as a window layer instead of conventional p-type amorphous silicon carbide

A thermally evaporated molybdenum oxide (MoO3) film was used as a window layer of a hydrogenated amorphous silicon (a-Si:H) solar cell instead of the conventional p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) film. The short circuit current density (JSC) and fill factor were increased due to the wide optical band gap and high conductivity of the MoO3 film. As a result, the conversion efficiency of the fabricated MoO3 solar cell was increased to 6.21% compared to the typical a-Si:H solar cell (5.97%).

Highly Efficient Organic Light Emitting Diodes with Hole Injection Layer of Thermally Evaporated Molybdenum Oxide

LiNi1−yM{y}O{2} specimens with compositions of LiNiO2, LiNi0.975Ga0.025O{2}, LiNi0.975Al0.025O2, LiNi0.995Ti0.005O2 and LiNi0.990Al0.005Ti0.005O2 were synthesized by wet milling and solid-state reaction method. All the synthesized samples possessed the α-NaFeO structure of the rhombohedral system (space group; \(R\bar 3m\)) with no evidence of any impurities. Among the α-LiNiO2 cathodes prepared with the weight ratios of LiNiO2: acetylene black: binder = 85≠10≠5, 85≠12≠3 and 90≠7≠3, the cathode with the weight ratio of 85≠10≠5 had the best cycling performance, with a discharge capacity degradation rate of 1.06 mAh/g/cycle and a discharge capacity at n=20 of 143.5 mAh/g. Among all the samples, LiNi0.990Al0.005Ti0.005O2 had the highest first discharge capacities at 0.1 C, 0.2 C and 0.5 C rates. That sample had the smallest R-factor value, indicating that it had the lowest degree of cation mixing. Among all the samples, LiNi0.975Al0.025O2 showed the lowest rate of decrease in the first discharge capacity with C rate. The first discharge capacities at 0.1 C, 0.2 C and 0.5 C rates were 170.5 mAh/g, 155.0 mAh/g and 124.2 mAh/g, respectively.

Post-annealing effect upon optical properties of electron beam evaporated molybdenum oxide thin films

Molybdenum oxide (MoO3) thin films were deposited by electron beam evaporation. The chemical composition, microstructure, optical and electrical properties of MoO3 thin films depend on the annealing temperature and ambient atmosphere. X-ray diffraction (XRD) shows that crystalline MoO3 films can be obtained at various post-annealing temperatures from 200 to 500°C in N2 and O2. X-ray photoelectron spectroscopy (XPS) results reveal that the O-1s emission peak was shifted slightly toward lower binding energies as the annealing temperature in N2 was increased. The oxygen vacancies and conductivity of MoO3 film increased with the annealing temperature. However, when the MoO3 films were annealed in an atmosphere of O2, the optical transmission, the O/Mo ratio and the photon energy increased with the annealing temperature. The results differ from those for films annealed in a N2 atmosphere.

Electron beam evaporated molybdenum oxide films: a study of elemental and surfacemorphological properties

Thin films of transition metal oxides are very important in electrochromic applications. Molybdenumoxide (MoO3) thin films were prepared by using one of the physical vapour depositionsof the electron beam evaporation technique in a vacuum of the order of10−6 mbar. The detailed elemental analysis of the films was performed by a particle-inducedx-ray emission (PIXE) study and the nanosurface nature of the films was studied by usingan atomic force microscopy (AFM) analysis. To the best of our knowledge this is the firsttime PIXE analysis has been used for the elemental studies of electron beam evaporatedMoO3 films.

Physical properties of evaporated molybdenum oxide films

The physical properties of molybdenum oxide films for use in electrochromic display devices were investigated. This paper mainly concerns the dependence of the properties on the preparation conditions. The oxide films were prepared by thermal evaporation of MoO 3 powder. The physical properties of the films depend heavily on the substrate temperature during deposition. The resistivity decreases with increasing substrate temperature, in the range 5×10 4–1×10 10 Ω cm. The optical bandgap and the refractive index of these films are 2.75–2.95 eV and 1.9–2.0, respectively. The structure of the films was examined by X-ray diffraction. The films prepared in the range 50–200°C were amorphous, and the films formed in the range 320–400°C were polycrystalline. The electrochemichromic properties of the films were also studied using asymmetric cells, and it was found that a good electrochromic performance was obtained for the films with ~ 1×10 10 Ω cm.

Electrical Properties of Evaporated Molybdenum Oxide Films

Thin film capacitors of MoO3 are found to be extremely temperature and frequency dependent. Changes in capacitance are reported as high as 60:1 over a temperature range of 100°C (at constant frequency) and over a two-decade frequency range (at constant temperature). At lower temperatures and higher frequencies the capacitance corresponds to the geometric capacitance, but at higher temperatures and lower frequencies the capacitance is independent of the film thickness. Conductance and quality factor of the films are also observed to be extremely frequency and temperature sensitive. The results are explained in terms of Schottky barriers existing at the metal-insulator interfaces, which arise due to the autodoping of the insulator, occurring during deposition of the films, by excess molybdenum. Excellent agreement is found to exist between the experimental data and the theory developed in the previous paper, and this correlation permits determination of the doping density (≃1018 cm−3), the donor depth (≃0.27 eV), and the width of the Schottky barriers (≃160 Å), among other properties of the films.