Synthesis and modeling of uniform complex metal oxides by close-proximity atmospheric pressure chemical vapor deposition

Can We Rationally Design and Operate Spatial Atomic Layer Deposition Systems for Steering the Growth Regime of Thin Films?

Fine control over the growth of materials is required to precisely tailor their properties. Spatial atomic layer deposition (SALD) is a thin-film deposition technique that has recently attracted attention because it allows producing thin films with a precise number of deposited layers, while being vacuum-free and much faster than conventional atomic layer deposition. SALD can be used to grow films in the atomic layer deposition or chemical vapor deposition regimes, depending on the extent of precursor intermixing. Precursor intermixing is strongly influenced by the SALD head design and operating conditions, both of which affect film growth in complex ways, making it difficult to predict the growth regime prior to depositions. Here, we used numerical simulation to systematically study how to rationally design and operate SALD systems for growing thin films in different growth regimes. We developed design maps and a predictive equation allowing us to predict the growth regime as a function of the design parameters and operation conditions. The predicted growth regimes match those observed in depositions performed for various conditions. The developed design maps and predictive equation empower researchers in designing, operating, and optimizing SALD systems, while offering a convenient way to screen deposition parameters, prior to experimentation.

Silver nanowire electrodes for transparent light emitting devices based on WS2monolayers

Transition metal dichalcogenide (TMDC) monolayers with their direct band gap in the visible to near-infrared spectral range have emerged over the past years as highly promising semiconducting materials for optoelectronic applications. Progress in scalable fabrication methods for TMDCs like metal-organic chemical vapor deposition (MOCVD) and the ambition to exploit specific material properties, such as mechanical flexibility or high transparency, highlight the importance of suitable device concepts and processing techniques. In this work, we make use of the high transparency of TMDC monolayers to fabricate transparent light-emitting devices (LEDs). MOCVD-grown WS₂is embedded as the active material in a scalable vertical device architecture and combined with a silver nanowire (AgNW) network as a transparent top electrode. The AgNW network was deposited onto the device by a spin-coating process, providing contacts with a sheet resistance below 10 Ω sq^-1and a transmittance of nearly 80%. As an electron transport layer we employed a continuous 40 nm thick zinc oxide (ZnO) layer, which was grown by atmospheric pressure spatial atomic layer deposition (AP-SALD), a precise tool for scalable deposition of oxides with defined thickness. With this, LEDs with an average transmittance over 60% in the visible spectral range, emissive areas of several mm²and a turn-on voltage of around 3 V are obtained.

Tuning the band gap and carrier concentration of titania films grown by spatial atomic layer deposition: a precursor comparison

Spatial atomic layer deposition retains the advantages of conventional atomic layer deposition: conformal, pinhole-free films and excellent control over thickness. Additionally, it allows higher deposition rates and is well-adapted to depositing metal oxide nanofilms for photovoltaic cells and other devices. This study compares the morphological, electrical and optical properties of titania thin films deposited by spatial atomic layer deposition from titanium isopropoxide (TTIP) and titanium tetrachloride (TiCl₄) over the temperature range 100-300 °C, using the oxidant H₂O. Amorphous films were deposited at temperatures as low as 100 °C from both precursors: the approach is suitable for applying films to temperature-sensitive devices. An amorphous-to-crystalline transition temperature was observed for both precursors resulting in surface roughening, and agglomerates for TiCl₄. Both precursors formed conformal anatase films at 300 °C, with growth rates of 0.233 and 0.153 nm s^-1 for TiCl₄ and TTIP. A drawback of TiCl₄ use is the HCl by-product, which was blamed for agglomeration in the films. Cl contamination was the likely cause of band gap narrowing and higher defect densities compared to TTIP-grown films. The carrier concentration of the nanofilms was found to increase with deposition temperature. The films were tested in hybrid bilayer solar cells to demonstrate their appropriateness for photovoltaic devices.

In-situ spatial and temporal electrical characterization of ZnO thin films deposited by atmospheric pressure chemical vapour deposition on flexible polymer substrates

A technique is presented for collecting data on both the spatial and temporal variations in the electrical properties of a film as it is deposited on a flexible substrate. A flexible printed circuit board substrate with parallel electrodes distributed across its surface was designed. Zinc oxide films were then deposited on the flexible substrate at different temperatures via atmospheric pressure chemical vapour deposition (AP-CVD) using a spatial atomic layer deposition system. AP-CVD is a promising high-throughput thin film deposition technique with applications in flexible electronics. Collecting data on the film properties in-situ allows us to directly observe how deposition conditions affect the evolution of those properties in real-time. The spatial uniformity of the growing film was monitored, and the various stages of film nucleation and growth on the polymer substrate were observed. The measured resistance of the films was observed to be very high until a critical amount of material has been deposited, consistent with Volmer-Weber growth. Furthermore, monitoring the film resistance during post-deposition cooling enabled immediate identification of metallic or semiconducting behaviour within the conductive ZnO films. This technique allows for a more complete understanding of metal chalcogen film growth and properties, and the high volume of data generated will be useful for future implementations of machine-learning directed materials science.

In-situ observation of nucleation and property evolution in films grown with an atmospheric pressure spatial atomic layer deposition system

Atmospheric pressure—spatial atomic layer deposition (AP-SALD) is a promising open-air deposition technique for high-throughput manufacturing of nanoscale films, yet the nucleation and property evolution in these films has not been studied in detail. In this work, in situ reflectance spectroscopy was implemented in an AP-SALD system to measure the properties of Zinc oxide (ZnO) and Aluminum oxide (Al2O3) films during their deposition. For the first time, this revealed a substrate nucleation period for this technique, where the length of the nucleation time was sensitive to the deposition parameters. The in situ characterization of thickness showed that varying the deposition parameters can achieve a wide range of growth rates (0.1–3 nm/cycle), and the evolution of optical properties throughout film growth was observed. For ZnO, the initial bandgap increased when deposited at lower temperatures and subsequently decreased as the film thickness increased. Similarly, for Al2O3 the refractive index was lower for films deposited at a lower temperature and subsequently increased as the film thickness increased. Notably, where other implementations of reflectance spectroscopy require previous knowledge of the film’s optical properties to fit the spectra to optical dispersion models, the approach developed here utilizes a large range of initial guesses that are inputted into a Levenberg-Marquardt fitting algorithm in parallel to accurately determine both the film thickness and complex refractive index.

Over 20% Efficiency in Methylammonium Lead Iodide Perovskite Solar Cells with Enhanced Stability via "in Situ Solidification" of the TiO2 Compact Layer

In methylammonium lead iodide (MAPbI₃) perovskite solar cells (PSCs), the device performance is strongly influenced by the TiO₂ electron transport layer (ETL). Typically, the ETL needs to simultaneously be thin and pinhole-free to have high transmittance and avoid shunting. In this work, we develop an "in situ solidification" process following spin coating in which the titanium-based precursor (titanium(diisopropoxide) bis(2,4-pentanedionate)) is dried under vacuum to rapidly achieve continuous TiO₂ layers. We refer to this as "gas-phase quenching". This results in thin (60 ± 10 nm), uniform, and pinhole-free TiO₂ films. The PSCs based on the gas-phase quenched TiO₂ exhibits improved power conversion efficiency, with a median value of 18.23% (champion value of 20.43%), compared to 9.03 and 14.09% for the untreated devices. Gas-phase quenching is further shown to be effective in enabling efficient charge transfer at the MAPbI₃/TiO₂ heterointerface. Furthermore, the stability of the gas-phase quenched devices is enhanced in ambient air as well as under 1 sun illumination. In addition, we achieve 12.1% efficiency in upscaled devices (1.1 cm² active area).

The Physics of Light Emission in Halide Perovskite Devices

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light-emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state-of-the-art metal-halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

Influence of the Geometric Parameters on the Deposition Mode in Spatial Atomic Layer Deposition: A Novel Approach to Area-Selective Deposition

Within the materials deposition techniques, Spatial Atomic Layer Deposition (SALD) is gaining momentum since it is a high throughput and low-cost alternative to conventional atomic layer deposition (ALD). SALD relies on a physical separation (rather than temporal separation, as is the case in conventional ALD) of gas-diluted reactants over the surface of the substrate by a region containing an inert gas. Thus, fluid dynamics play a role in SALD since precursor intermixing must be avoided in order to have surface-limited reactions leading to ALD growth, as opposed to chemical vapor deposition growth (CVD). Fluid dynamics in SALD mainly depends on the geometry of the reactor and its components. To quantify and understand the parameters that may influence the deposition of films in SALD, the present contribution describes a Computational Fluid Dynamics simulation that was coupled, using Comsol Multiphysics®, with concentration diffusion and temperature-based surface chemical reactions to evaluate how different parameters influence precursor spatial separation. In particular, we have used the simulation of a close-proximity SALD reactor based on an injector manifold head. We show the effect of certain parameters in our system on the efficiency of the gas separation. Our results show that the injector head-substrate distance (also called deposition gap) needs to be carefully adjusted to prevent precursor intermixing and thus CVD growth. We also demonstrate that hindered flow due to a non-efficient evacuation of the flows through the head leads to precursor intermixing. Finally, we show that precursor intermixing can be used to perform area-selective deposition.

In Situ Atmospheric Deposition of Ultrasmooth Nickel Oxide for Efficient Perovskite Solar Cells

Organic-inorganic perovskite solar cells have attracted significant attention due to their remarkable performance. The use of alternative metal-oxide charge-transport layers is a strategy to improving device reliability for large-scale fabrication and long-term applications. Here, we report solution-processed perovskite solar cells employing nickel oxide hole-extraction layers produced in situ using an atmospheric pressure spatial atomic-layer deposition system, which is compatible with high-throughput processing of electronic devices from solution. Our sub-nanometer smooth (average roughness of ≤0.6 nm) oxide films enable the efficient collection of holes and the formation of perovskite absorbers with high electronic quality. Initial solar-cell experiments show a power-conversion efficiency of 17.1%, near-unity ideality factors, and a fill factor of >80% with negligible hysteresis. Transient measurements reveal that a key contributor to this performance is the reduced luminescence quenching trap density in the perovskite/nickel oxide structure.

Oxygen plasmas: a sharp chisel and handy trowel for nanofabrication

Although extremely chemically reactive, oxygen plasmas feature certain properties that make them attractive not only for material removal via etching and sputtering, but also for driving and sustaining nucleation and growth of various nanostructures in plasma bulk and on plasma-exposed surfaces. In this minireview, a number of representative examples is used to demonstrate key mechanisms and unique capabilities of oxygen plasmas and how these can be used in present-day nano-fabrication. In addition to modification and functionalisation processes typical for oxygen plasmas, their ability to catalyse the growth of complex nanoarchitectures is emphasized. Two types of technologies based on oxygen plasmas, namely surface treatment without a change in the size and shape of surface features, as well as direct growth of oxide structures, are used to better illustrate the capabilities of oxygen plasmas as a powerful process environment. Future applications and possible challenges for the use of oxygen plasmas in nanofabrication are discussed.

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI)

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiO_x |BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.

Rapid open-air deposition of uniform, nanoscale, functional coatings on nanorod arrays

Coating of high-aspect-ratio nanostructures has previously been achieved using batch processes poorly suited for high-throughput manufacturing. It is demonstrated that uniform, nanoscale coatings can be rapidly deposited on zinc oxide nanorod arrays in open-air using an atmospheric pressure spatial deposition system. The morphology of the metal oxide coatings is examined and good electrical contact with the underlying nanorods is observed. The functionality of the coatings is demonstrated in colloidal quantum dot and hybrid solar cells.

Effect of Injection Layer Sub-Bandgap States on Electron Injection in Organic Light-Emitting Diodes

It is generally considered that the injection of charges into an active layer of an organic light-emitting diode (OLED) is solely determined by the energetic injection barrier formed at the device interfaces. Here, we demonstrate that the density of surface states of the electron-injecting ZnO layer has a profound effect on both the charge injection and the overall performance of the OLED device. Introducing a dopant into ZnO reduces both the energy depth and density of surface states without altering the position of the energy levels-thus, the magnitude of the injection barrier formed at the organic/ZnO interface remains unchanged. Changes observed in the density of surface states result in an improved electron injection and enhanced luminescence of the device. We implemented a numerical simulation, modeling the effects of energetics and the density of surface states on the electron injection, demonstrating that both contributions should be considered when choosing the appropriate injection layer.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn_1-xMg_xO thin films onto p-type thermally oxidized Cu₂O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu₂O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu₂O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu₂O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu₂O interface. Optimization of AP-SALD conditions as well as keeping Cu₂O away from air and moisture in order to minimize Cu₂O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn_1-xMg_xO/Cu₂O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu₂O-based photovoltaics.

Photoelectrochemical water splitting strongly enhanced in fast-grown ZnO nanotree and nanocluster structures

We demonstrate selective growth of ZnO branched nanostructures: from nanorod clusters (with branches parallel to parent rods) to nanotrees (with branches perpendicular to parent rods). The growth of these structures was realized using a three-step approach: electrodeposition of nanorods (NRs), followed by the sputtering of ZnO seed layers, followed by the growth of branched arms using hydrothermal growth. The density, size and direction of the branches were tailored by tuning the deposition parameters. To our knowledge, this is the first report of control of branch direction. The photoelectrochemical (PEC) performance of the ZnO nanostructures follows the order: nanotrees (NTs) > nanorod clusters (NCs) > parent NRs. The NT structure with the best PEC performance also possesses the shortest fabrication period which had never been reported before. The photocurrent of the NT and NC photoelectrodes is 0.67 and 0.56 mA cm^-2 at 1 V vs. Ag/AgCl, respectively, an enhancement of 139% and 100% when compared to the ZnO NR structures. The key reason for the improved performance is shown to be the very large surface-to-volume ratios in the branched nanostructures, which gives rise to enhanced light absorption, improved charge transfer across the nanostructure/electrolyte interfaces to the electrolyte and efficient charge transport within the material.

Blue-Green Color Tunable Solution Processable Organolead Chloride-Bromide Mixed Halide Perovskites for Optoelectronic Applications

Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425-570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1-x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride-bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.