Early Stages Of Film Formation Research Articles

Iodine Modulates the MACl‐Assisted Growth of FAPbI3 for High Efficiency Perovskite Solar Cells

AbstractThe preferential growth of α‐phase formamidinium perovskite (α‐FAPbI3) at low temperatures can be achieved with the incorporation of chloride‐based additives, with methylammonium chloride (MACl) being the most common example. However, compared to other less‐volatile chloride additives, MACl only remains in the growing perovskite film for a short time before evaporating during annealing, primarily influencing the early stages of film formation. In addition, evaporation of MACl as methylamine (MA0) and HCl can introduce a side reaction between MA0 and formamidinium (FA), undermining the compositional purity and phase stability of α‐FAPbI3. In this study, it is demonstrated that addition of iodine (I2) into the FAPbI3 precursor solution containing MACl suppresses the MA‐FA side reaction during annealing. Additionally, MACl evaporation is delayed owing to strong interaction with triiodide. The added I2 facilitates spontaneous growth of α‐FAPbI3 prior to annealing, with an improved bottom morphology due to the formation of fewer byproducts. Perovskite solar cells derived from an I2‐incorporated solution deliver a champion power conversion efficiency of 25.2% that is attributed to suppressed non‐radiative recombination.

Optimizing the Buried Interface in Flexible Perovskite Solar Cells to Achieve Over 24% Efficiency and Long-Term Stability.

The buried interface of the perovskite layer has a profound influence on its film morphology, defect formation, and aging resistance from the outset, therefore, significantly affects the film quality and device performance of derived perovskite solar cells. Especially for FAPbI3 , although it has excellent optoelectronic properties, the spontaneous transition from the black perovskite phase to nonperovskite phase tends to start from the buried interface at the early stage of film formation then further propagate to degrade the whole perovskite. In this work, by introducing ─NH3 + rich proline hydrochloride (PF) with a conjugated rigid structure as a versatile medium for buried interface, it not only provides a solid α-phase FAPbI3 template, but also prevents the phase transition induced degradation. PF also acts as an effective interfacial stress reliever to enhance both efficiency and stability of flexible solar cells. Consequently, a champion efficiency of 24.61% (certified 23.51%) can be achieved, which is the highest efficiency among all reported values for flexible perovskite solar cells. Besides, devices demonstrate excellent shelf-life/light soaking stability (advanced level of ISOS stability protocols) and mechanical stability.

Fast Optical Reflectance Measurements during Spin Coating and Annealing of Organic–Inorganic Perovskite Precursor Solutions

In article number 2000479, Jörg Rappich and co-workers address early stages of film formation and crystallization of organic lead triiodide perovskites by employing in-situ high-speed spectral reflectance measurements. The time scale ranges from 0.2 milliseconds to seconds. Data were taken during spincoating and subsequent low-temperature annealing. The measurements enable the investigation of changes in the solution of the spin-coating process. The evaporation of the solvent during spin-coating results in the spontaneous crystallization of the perovskite layer. During the subsequent annealing process, the diffuse reflectance measured with high sensitivity provides insight into the transformation and crystallization process of the precursor layer to the final perovskite layer.

The Influence of Slide–Roll Ratio on ZDDP Tribofilm Formation

The anti-wear performance and action mechanisms of zinc dithiophosphate (ZDDP) have been investigated under various test conditions. The Mini Traction Machine–Space Layer Imaging (MTM–SLIM) is a widely used and useful method for monitoring tribofilm formation by ZDDPs. However, tests are generally carried out in mixed sliding–rolling conditions, typically between 50 % SRR (slide–roll ratio) and 100 % SRR. In this paper, the authors describe an investigation of ZDDP film formation at SRRs much higher than 100 % SRR, including pure sliding conditions using a novel MTM–SLIM technique. At high SRRs, ZDDP tribofilms form without damaging the ball surfaces so long as both surfaces move above a threshold speed with respect to the contact, regardless of whether the two surfaces move in the same or opposing directions. In pure sliding conditions, although the worn area expands with time under pure sliding conditions showing that wear takes place, tribofilms are still built up throughout a test and the ZDDP has a beneficial effect on wear rate. The very early stages of film formation are studied to show that a tribofilm with a high concentration of S is formed initially and then replaced with a film having a high concentration of Zn and P.

Importance of interfacial step alignment in hetero-epitaxy and orientation relationships: the case of Ag equilibrated on Ni substrates. Part 2 experiments

The orientation relationships (ORs) that develop when Ag films are equilibrated on more than 200 different Ni substrate orientations distributed over the standard stereographic triangle have been determined by electron backscatter diffraction. Four different types of OR have been observed, including cube-on-cube, twin-related, oct-cube (that develops only on Ni(100)), as well as a family of more complex ORs (referred to as “special”) that arise during the gradual transition from the oct-cube to the twin OR. The importance of the alignment of step edges on the Ag side of the interface with those of the Ni substrate, in the development of ORs, has been validated by comparison with the results obtained on the large range of substrate orientations investigated in this study. This step alignment occurs in the early stages of film formation, but is also consistent with equilibrium ORs as it tends to produce minimum energy interfaces. This feature of hetero-epitaxy and of the resulting ORs has not previously been emphasized.

Dynamics of polymer film formation during spin coating

Standard models explaining the spin coating of polymer solutions generally fail to describe the early stages of film formation, when hydrodynamic forces control the solution behavior. Using in situ light scattering alongside theoretical and semi-empirical models, it is shown that inertial forces (which initially cause a vertical gradient in the radial solvent velocity within the film) play a significant role in the rate of thinning of the solution. The development of thickness as a function of time of a solute-free liquid (toluene) and a blend of polystyrene and poly(methyl methacrylate) cast from toluene were fitted to different models as a function of toluene partial pressure. In the case of the formation of the polymer blend film, a concentration-dependent (Huggins) viscosity formula was used to account for changes in viscosity during spin coating. A semi-empirical model is introduced, which permits calculation of the solvent evaporation rate and the temporal evolution of the solute volume fraction and solution viscosity.

Transmission electron microscopy on early-stage tin oxide film morphology grown by atmospheric pressure chemical vapor deposition

Nucleation and morphology development during the early stages of chemical vapor deposition (CVD) processes are believed to be of major importance for the overall film properties. Here, the authors have investigated the nucleation of tin oxide films, comparing different tin precursors (tin tetrachloride (TTC) and monobutyl tin trichloride (MBTC)) and focusing on the effect of methanol addition on the film morphology. Employing electron transparent silicon oxide membranes as substrates and combining transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis on the same set of samples, we describe a detailed picture of nucleation behavior and film growth during early stages of film formation. Our main conclusion is that methanol addition during deposition acts as surfactant, lowering the surface energy of the substrate and resulting in a higher nucleation grain density. Based on these results, we propose a film growth model based on surface energy to explain morphology differences in tin oxide films resulting from methanol addition. cop. 2014 Elsevier B.V.

Influence of Aggregation on the Performance of All‐Polymer Solar Cells Containing Low‐Bandgap Naphthalenediimide Copolymers

AbstractThe authors present efficient all‐polymer solar cells comprising two different low‐bandgap naphthalenediimide (NDI)‐based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near‐field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large‐scale phase‐separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donor–acceptor heterojunction.

Understanding the differences in film formation mechanisms of two comparable solvent based and water-borne coatings on bronze substrates by electrochemical impedance spectroscopy

Corrosion protection of bronze, a metal alloy which is commonly used in decorative architectural elements and for statuary, is a challenge particularly as new and previously untested water-borne coatings are supplanting traditionally used solvent based polymers as protective coatings. Studying film formation of coatings to better prevent corrosion of culturally significant works is an important problem in the field of conservation science. This work aims to further understanding of differences between films formed from water-borne lattices versus solvent based polymeric coatings on bronze substrates using electrochemical impedance spectroscopy and gravimetric analysis to investigate the changes of coatings as they formed solid films. Using equivalent electric circuit models, electrochemical snapshots of the developing films were produced. Those models established time points for the transitions between theoretical stages of the nascent films and allowed for a comparison between two major classes of coating types (using the water-borne Rhoplex™ WL-81 and the solvent based Paraloid™ B-44). Electrochemical analysis showed that diffusion occurs during early stages of film formation of both coating types. The prolonged coalescence of coatings produced from the water-borne latex was found to leave those films susceptible to water sorption. Annealing films as a means to drive out remaining hydrophilic additives and to facilitate film formation was investigated.

Kinetics Study on Initial Growth Stage in Vapor Deposition of Organic Thin Film Using Quartz Crystal Microbalance

The deposition behavior of stearic acid evaporated in vacuum was observed using a quartz crystal microbalance technique, and the time evolution curves of the amount of admolecules in the initial growth were compared with a theoretical curve calculated using a rate equation proposed on the basis of physisorption. From the results, it was demonstrated that the proposed rate equation for the early stage of film formation well described the experimental results and the growth kinetics of organic thin films was dependent on the substrate temperature. Basic parameters for thin-film growth, such as the mean stay time on the substrate of deposited molecules, can be estimated from the fitting of the theoretical equation to the experimental results at each substrate temperature. It should be noted that the basic parameters were sensitive to a small change in substrate temperature.

Morphology and magnetic properties of size-selected Ni nanoparticle films

We present the results of a study on the morphology and magnetic properties of size-selected Ni nanoparticles films grown on Si/SiOx substrates. The films were produced by deposition of preformed Ni nanoparticles, using a gas aggregation nanocluster source and an electric quadrupole mass filter. The diameter d of the produced particles ranged between 3 and 10 nm. The morphology of the films, with average thickness t varying from t=0.5 up to t=7 nm, was studied with atomic force microscopy and scanning electron microscopy, combining in this way information about height and lateral topography. We observed the presence of some small aggregates made of two or three particles at the early stage of film formation, probably due to some degree of cluster diffusion on the substrate, and particle average flattening. For increasing values of t, large agglomerates are formed in the films, resulting in a porous structure. Information about the magnetic properties was obtained with field cooled-zero field cooled (FC/ZFC) magnetization curves. We observed a reversibility-irreversibility transition at temperatures 70 K<TI<80 K, and a significant deviation from the superparamagnetic behavior at T>TI, even for the lowest coverage studied (t=2 nm for ZFC/FC measurements, ⟨d⟩=5.5 nm). A comparison with Monte Carlo simulations of the FC/ZFC curves reveals the concurrence between interparticle exchange interaction and single particle random anisotropy.

Motion of agglomerates on the surface in the formation of methane hydrate crystals

A study of the formation of a crystal hydrate film CH4 · 6H2O on the surface of an immobile solution of methane in water have shown that, at early stages of film formation, hydrate crystals and their agglomerates are in motion at a velocity of 2–5 mm/s caused by crystal growth. Collisions of agglomerates are accompanied by their rotations at a Ω ∼ 0.2–2 rad/s rate. The motion of crystals and agglomerates is explained using the hydrodynamic model that takes into account the surface tension and the difference in the pressure exerted by the medium on the growing and nongrowing faces of crystals, which sets them in chemoreactive motion.

Tribofilms generated from ZDDP and DDP on steel surfaces: Part 2, chemistry

The chemical constitution of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and ashless dialkyldithiophosphate (DDP), has been examined by X-ray Absorption Near Edge Structure (XANES) spectroscopy. The identification of spectral features and interpretation of the results for P, O, Fe, and S species are given, allowing an overall mechanism to be deduced. The role of Fe in these films was investigated in some detail using P L-edge, O K-edge and Fe L-edge XANES spectra. From the P L-edge XANES spectra, the DDP films are uniformly very short chain iron polyphosphates. In contrast, the ZDDP films are formed initially as short chain polyphosphates; but after more rubbing, a bilayer phosphate film is formed with long chain Zn polyphosphates on the surface and shorter chain in the bulk of the film. The O K-edge XANES spectra show that there is, as expected, more Fe in the DDP phosphate films than in the ZDDP phosphate films. The S K-edge spectra of ZDDP films show the presence not only of ZnS as previously observed, but also the presence of FeS for the first time in the early stages of film formation. The predominant S species in the DDP films is FeS.

Early Stages of Film Formation and Surface Growth on Tin Electrodes at Bicarbonate Medium: a Modelistic Approach

Abstract A modelistic approach of the early stages of film formation and surface growth on tin electrodes at bicarbonate medium under dynamic conditions is presented. The modelistic treatment was applied to both anodic and cathodic processes. In order to organise the paper, the potential interval of interest was divided into two potential windows, each one involving one of the anodic peaks and their corresponding cathodic counterparts. The Müller–Calandra model (also known as the layer-pore resistance model) was considered suitable to interpret the film growth processes occurring during the positive scan. A derivation of the former, developed by Devilliers et al., which take into account the conductive electrical nature of the surface compound, was applied to explain the reduction of the film formed within the first potential window. On the other hand, it was verified that the use of a simple model to interpret the cathodic process occurring at the second potential window was not possible due to the complex nature of the process. In this case, a mixed control kinetics was proposed and the current was considered as the sum of two contributions. In order to complement the description and specially to determine the role of diffusion and mass transport in film formation, rotating disk measurements were also carried out.

Hrem Observation of the Relaxation of Grain Boundaries in Au At Their Intersection with Free Surfaces

Grain boundaries in fee metals with low stacking fault energy are known to undergo extended relaxations that can at times lead to a thin layer of a different structure. in Cu, for example, it has been found that Σ3{112}} boundaries relax into a 9R phase or even a bec phase. in this work, we have used high resolution electron microscopy to investigate the atomic structure of Σ99 grain boundaries in mazed bicrystal films of Au. On {001} Ge single crystal substrates, Au bicrystals can be grown in two equivalent {110} orientation variants, related to each other by a 90° rotation about the surface normal. There is equal probability to nucleate either orientation variant in early stages of film formation and since there is no reason for preferential growth, the morphology of the bicrystal film consists of many grains in only two orientations. All grain boundaries are of the type Σ99 and display tilt character.Thin films 10-200nm thick were grown by physical vapor deposition on {001} Ge substrate at 280°C and followed by annealing at 200°C to relieve stresses.

The structure at the interface of Ni/Pd superlattices with different crystal orientations estimated from molecular dynamics simulation

Molecular dynamics simulations of the formation process of an Ni/Pd film were performed by employing the embedded atom method with high (10 eV) and low (0.1 eV) kinetic energies of incident atoms for two different crystal orientations: fcc (1 1 1) and fcc (1 0 0). The tendencies of the film formation process and the interdiffusion at the interface were similar in both the (1 1 1) and (1 0 0) cases. However, the strain at the interface of films on the (1 0 0) substrate was larger than that on the (1 1 1) substrate. In the case of (1 1 1), lattice relaxation is found at the interface and the strain distribution of Ni layers is not uniform over the plane. From the observation of the traces of the atoms in early stage of film formation, it was found that the range of migration of adatoms on (1 1 1) surface is longer than that on (1 0 0) and the Ni nuclei as large as three atoms are mobile on Pd (1 1 1) surface.

Correlation between the early stage of copper metal organic chemical vapor deposition and the material properties of thin film

Cu metal organic chemical vapor deposition has been performed on TiN/Si(100) substrates using Cu(I)(hfac)(vtms) at various substrate (110⩽Ts⩽300 °C) and bubbler temperature (0⩽Tb⩽30 °C) conditions, in which the early stage of film formation characterized by scanning electron microscopy and in situ laser beam (λ=6328 Å) reflectance measurement could be well correlated with the surface roughness and the electrical resistivity of the copper films developed in a later stage. All the reflectance versus deposition time curves showed a qualitatively similar shape consisting of three sequential parts: a rapid initial decrease to a minimum, an increase to a maximum, and a gradual decay to almost zero, in which the minimum and maximum roughly correspond to the onset and completion of island coalescence, respectively. As the Ts is raised and/or the Tb is lowered, larger islands were formed in a smaller density at the onset of island coalescence due to a lower nucleation rate. The surface roughness gradually develops with increasing film thickness after forming a continuous film, but it does not saturate at a thickness of ∼2 μm and shows a correlation with the average island size in the earlier coalescence stage, i.e., the smaller islands evolves to a smoother surface. Above the threshold temperature (Ts∼230 for Tb=30 °C) the copper islands begin to show facets and at even higher Ts large truncated polyhedron-shaped copper islands are formed, which upon coalescing leave many interfacial voids to result in a film with a very rough surface and a high electrical resistivity. The copper films deposited at 150⩽Ts⩽200 °C have a very low value of ρ≈2 μΩ cm, which increases by a factor of ∼2 when Ts is lowered to 110 °C. The latter was accompanied by the formation of smaller islands at the onset of coalescence, and therefore the increase in ρ at Ts⩽150 °C was attributed to the electron scattering at the grain boundaries.

Film Formation of Acrylic Latices with Varying Concentrations of Non-Film-Forming Latex Particles

We have employed ellipsometry and environmental SEM (ESEM) to determine the kinetics of film formation in mixtures of film-forming (FF) and non-film-forming (NFF) acrylic latices. We find that an increasing concentration of NFF latex leads to progressively larger voids. We have also found that minimizing the number of NFF−NFF particle contacts within the mixture results in a denser material. The rate of void closure during the early stages of film formation is enhanced by the presence of NFF latex particles, because the particles lead to a higher surface area that provides a stronger driving force for void closure. Despite this effect, a latex with a higher concentration of NFF particles takes longer to reach full density. We have applied the Mackenzie−Shuttleworth theory to our densification data and found a value for the viscosity of the FF latex that is consistent with the literature. In some cases, we see evidence for polymer viscosity changing with time, indicative of hydroplasticization. We find that our calculated viscosity of the FF latex decreases with increasing temperature and increases with the addition of 40 wt % NFF particles. We comment on the implications that this work has for the measurement of the critical pigment volume concentration.

A study of the electrochemical formation of Cu(I)-BTA films on copper electrodes and the mechanism of copper corrosion inhibition in aqueous chloride/benzotriazole solutions

Cu electrode behaviour in Cl − and benzotriazole (H-BTA) containing acidic or neutral solutions was studied by voluntary, ammetry, photocurrent response, impedance measurements and chemical analysis of BTA consent in passive layers. It was shown that the rate of formation of Cu-BTA films in acidic solutions is controlled by the transport of CuCl 2 ions through pores in the film at an early stage of film formation and later by the volume diffusion rate in the film. The photocurrent measurements have revealed that Cu-BTA formation is accompanied by the simultaneous deposition of a Cu 2O underlayer. The main portion of the polarisation resistance of the passive layer was ascribed to the Cu 2O underlayer. It is proposed that the role of the Cu-BTA overlayer is to stabilize the Cu 2O underlayer and maintain its high resistance by preventing it from being doped by Cl − ions resulting in the formation of solid CuCl on tope of the Cu 2O.

Early stages of heteroepitaxial growth of ZnSe on GaAs by MOVPE, MOASE and MBE

We have used cross-sectional transmission electron microscopy (XTEM) X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED) to study the early stages of heteroepitaxial growth of ZnSe on GaAs grown by metal-organic vapor phase epitaxy (MOVPE), metal-organic alternating source epitaxy (MOASE) and molecular beam epitaxy (MBE). In MOVPE growth of ZnSe on GaAs, we have observed 3D islands in the early stages of growth. We observe no difference when using nominally (100) oriented substrates and substrates tilted 2° toward [011]. Changing the carrier gas from conventionally used H 2 to Ar affects the initial sticking of deposited atoms to the substrate, but does not seem to affect the growth mode. In contrast, XTEM observations show that the islands that form in the early stages of MOASE growth are anisotropic; that is, islands are elongated along the [011] direction. In MBE, we find that the growth mode is 3D in the very early stages of growth, with no strong sign of anisotropy in the shape of the islands as seen in XTEM observations for both (NH 4) 2S and Se pretreated substrates. However, we do find that the sticking coefficient of the deposited atoms during the early stages of film formation are affected by the substrate pretreatment process. Among the substrate pretreatment processes studied to date, ex situ ammonium sulfide pretreatment is associated with a lower initial sticking coefficient compared to pretreatments which involve an in situ anneal with a Se flux, with or without an ex situ (NH 4) 2S pretreatment.