Vapor Phase Deposition Technique Research Articles

Inherently Selective Atomic Layer Deposition for Optical and Sensor Applications: Microreactor Direct Atomic Layer Processing (μDALP™)

In parallel to additive manufacturing leading the revolution in traditional manufacturing, the same principles can revolutionize traditional thin film deposition techniques. Where lithography and vapor phase deposition techniques struggle, for example, with rapid iterations for prototyping or incompatibility with the used chemistry, additive manufacturing can shine. Indeed, several approaches are in development for 3D nanopriting1,2,3.Atomic Layer Deposition, and in more general Atomic Layer Processing, offers a unique opportunity for localized 3D processing/printing due to its two-step process. While simple in theory, due to well-developed examples of Spatial Atomic Layer Deposition (SALD), in practice miniturization of SALD requires substantial effort into the creation of suitable micro-nozzles. Uniquely, ATLANT 3D has developed proprietary Spatial ALD micronozzles, naming the process microreactor Direct Atomic Layer Processing - µDALPTM.The µDALPTM process undergoes the same cyclic ALD process but, thanks to the in-house microreactor development, is only done in a localized area. The microreactor or micronozzle confines the flows of gases used for ALD within a µm-scale area on the substrate, wherein the reactive species adsorb on the surface to deposit one monolayer of the desired material. Similarly, to spatial ALD, the creation of this monolayer then hinges on the movement of the substrate.4 Since the µDALPTM process is based on physical separation, it is theoretically compatible with any ALD process. As such, the material capabilities can match traditional ALD and exceed other special or patterning thin film techniques, such as lithography, which can be costly and time-consuming, especially for rapid prototyping required for innovation.Films deposited with ATLANT 3D technology have been shown to produce high-quality, crystalline, atomically precise thin films.1 It has been used to fabricate temperature (Fig.1) and capacitive sensors with sensitivities that meet or exceed those of devices made using conventional vapor phase deposition techniques.5 Low-cost rapid prototyping facilitated by ATLANT 3D technology of such devices enables design innovation and optimization not possible with other thin film deposition techniques.ATLANT 3D platform provides flexibility in deposition geometry (Fig.2) that enables a range of optical applications that require high-precision fabrication techniques. Optically transparent films can be used as localized thin protective layers for device encapsulation, as well as for atomic corrections of defects. Alternating deposition materials enable multilayer mirror fabrication of custom dimensions. Smooth thickness gradients can be used to manufacture ultrathin optical lenses. Overlapping depositions with discrete steps enable the manufacturing of binary lenses and phase masks. Growth rate enhancement in rastered direct processing mode using ATLANT 3D technology can produce periodic grating structures.1 Conformal coatings on complex surfaces can act as functionalization layers or seed layers for further material deposition through e.g. electrochemical processes.[1] Kundrata I. et al., ALD/ALE 2022 [Int. Conf.], 2022 [2] de la Huerta C. A. M. et al., arXiv, 2020, 0523. [3] Winkler, R. et al., J. Appl. Phys., 2019, 125, 210901[4] Paul Poodt., JVSTA., 2012, 30, 010802[5] Kundrata, I. et al., Small Methods., 2022, 6 (5), 2101546 Figure 1

Significant Progress of Initiated Chemical Vapor Deposition in Manufacturing Soft Non-spherical Nanoparticles: Upgrading to the Condensed Droplet Polymerization Approach and Key Technological Aspects

Initiated chemical vapor deposition is a unique solvent-free and completely dry vapor-phase deposition technique used to synthesize organic polymer films. In this process, an activated initiator, monomer, and carrier gas are introduced into the reaction chamber simultaneously. This technique has been widely adopted. However, if the monomer and initiator are introduced into the chamber in stages—allowing gas-phase monomer deposition and condensation first, followed by initiator introduction and controlling the monomer partial pressure to be higher than the saturated vapor pressure—non-spherical polymer nanoparticles with dome-like shapes can be obtained. This advanced iCVD technique is referred to as the “Condensed Droplet Polymerization Approach”. This high monomer partial pressure gas-phase deposition is not suitable for forming uniformly composed iCVD films; but interestingly, it can rapidly obtain polymer nanodomes (PNDs). Using CDP technology, Franklin polymerized multifunctional nanodomes in less than 45 s, demonstrating a wide range of continuous particle size variations, from sub-20 nanometers to over 1 micron. This rapid synthesis included a variety of functional polymer nanodomes in just a matter of seconds to minutes. This review discusses the crucial process conditions of the Condensed Droplet Polymerization (CDP) Approach for synthesizing PNDs. The main focus of the discussion was on the two-step method for synthesizing PNDs, where the nucleation mechanism of PNDs, factors influencing their size, and the effect of pressure on the distinct condensation of monomer vapor into polymer nanodomes and polymer films were extensively explored.

TRIBOLOGICAL PROPERTIES OF SILVER DOPED TITANIUMNITRIDE COATINGS

The properties of titanium nitride (TiN:Ag) coatings applied by physical vapour phase deposition technique(PVD) on Ti13Nb13Zr alloy were subject to evaluation. The study presents the results of surface geometricalstructure, adhesion and tribological tests. The geometric structure of the surface was examined using opticalmicroscopy. A scratch test was used to assess adhesion. The model tribological tests were carried out ina rotary motion under technically dry friction conditions, lubricated with Ringer’s solution. In the case oftechnically dry friction, the analysis of the tribological test results indicated that the TiN:Ag coating wascharacterised by higher resistance to motion and lower wear compared to Ti13Nb13Zr. Friction coefficientsregistered during friction subject to lubrication with Ringer’s solution were compared for both materials;however, the surface wear was significantly lower in titanium alloy. The scratch test pointed towards highadhesion of the TiN:Ag coating. The study results provide insight into Ti13Nb13Zr alloy, titanium nitridecoatings and their potential use for surgical instruments.

Controlled vapor phase growth of germanium selenide and germanium microflakes

Graphene and transition metal dichalcogenides have been intensively investigated in recent years. Germanium selenide (GeSe) has emerged lately as a new low-symmetry two-dimensional material due to its extraordinary in-plane anisotropic electrical, optical, and optoelectronic properties. However, the controlled synthesis of large-scale GeSe flakes is still a significant challenge because of its strong interlayer coupling and low lattice symmetry. In this work, we have achieved the synthesis of rectangular GeSe microflakes (MFs) with lateral sizes up to 0.33 mm on the glass substrate via a vapor phase deposition technique. The lateral size, thickness, and growth mode of GeSe MFs can be effectively tuned based on space-confined and substrate engineering strategies. Germanium (Ge) MFs with triangular and hexagonal morphologies were simultaneously fabricated in the high-temperature zone. Our experimental results demonstrated that the growth temperature plays an important role in the selective growth of GeSe and Ge MFs. A corresponding growth mechanism has been discussed in detail based on systematic characterizations. Our work provides a new route for the controlled growth of large-size GeSe and Ge MFs, which promotes a promising future in the fields of novel optoelectronic devices.

Recent Advances in Theoretical Development of Thermal Atomic Layer Deposition: A Review.

Atomic layer deposition (ALD) is a vapor-phase deposition technique that has attracted increasing attention from both experimentalists and theoreticians in the last few decades. ALD is well-known to produce conformal, uniform, and pinhole-free thin films across the surface of substrates. Due to these advantages, ALD has found many engineering and biomedical applications. However, drawbacks of ALD should be considered. For example, the reaction mechanisms cannot be thoroughly understood through experiments. Moreover, ALD conditions such as materials, pulse and purge durations, and temperature should be optimized for every experiment. It is practically impossible to perform many experiments to find materials and deposition conditions that achieve a thin film with desired applications. Additionally, only existing materials can be tested experimentally, which are often expensive and hazardous, and their use should be minimized. To overcome ALD limitations, theoretical methods are beneficial and essential complements to experimental data. Recently, theoretical approaches have been reported to model, predict, and optimize different ALD aspects, such as materials, mechanisms, and deposition characteristics. Those methods can be validated using a different theoretical approach or a few knowledge-based experiments. This review focuses on recent computational advances in thermal ALD and discusses how theoretical methods can make experiments more efficient.

Optimal design of novel precursor materials for the atomic layer deposition using computer-aided molecular design

Nanomaterials and nanostructures with multi-functional properties found widespread applications such as electronics, optics, and coatings that can be fabricated using Atomic Layer Deposition (ALD). ALD is a vapor phase deposition technique to generate thin films of metals and metal oxides on a substrate. In this process, a precursor, which often comprises of organic functional groups that surround the depositing metal, chemisorbs on the substrate or reacts with the surface sites and with each other. Precursor chemisorption on the substrate leads ALD to be a self-limiting process. Thus, the precursor(s) should be chosen in a way to enhance deposition based on the ALD conditions. For a given application, it is practically impossible to carry out a large number of experiments using numerous precursors with varied deposition conditions to find the one that maximizes the growth rate in ALD. In addition, only existing precursors can be tested experimentally. The overall objective of this work is to develop a computational tool for the in-silico design of precursor materials using adsorbate solid solution theory (ASST). In the first part of this paper, we apply the ASST to derive properties of the functional groups present in the precursor using a new Group Contribution Method (GCM). The GCM is successfully derived and compared with the experimental data from ALD. The method shows good agreement and is useful for the design of novel materials. In the second part of this paper, using the thermodynamic properties as obtained from GCM, we develop a computer-aided molecular design (CAMD) framework for the optimal design of novel precursor materials with enhanced deposition properties for the ALD of metal oxides and metals. The metaheuristic efficient ant colony optimization algorithm (EACO) developed in-house is used for both parts. CAMD is a combinatorial optimization methodology, where molecules with optimal desired properties are generated from functional groups. The precursor selection optimization problem is posed as a mixed integer nonlinear programming problem, which is solved using EACO. The ALD growth kinetics is used as an objective of optimization and a solution is validated with thermodynamic constraints. Along with the number of groups, the temperature is also included in the optimization framework as decision variables. Forty-one novel titanium precursor molecular structures for ALD are generated with growth rates ranging from 1.23 Å/cycle to as high as 1.65 Å/cycle. Thus, these precursors have shown growth rates higher than the known titanium precursors. ALD growth rate is found to be a function of the combination of the precursor functional groups as well as temperature with a complex correlation among them.

Ternary-source vapor-phase deposition of CH3NH3PbI3 polycrystalline thin films using CH3NH2 and HI gas sources with PbI2 solid source

We have investigated reaction processes of vacuum deposition of CH3NH3PbI3 using CH3NH3I and PbI2 solid sources. CH3NH3I decomposes to CH3NH2 and HI at around 100 °C under usual vacuum deposition conditions. Therefore, CH3NH3I solid source can be replaced with CH3NH2 and HI gas sources. We have demonstrated that high-quality CH3NH3PbI3 polycrystalline thin films can be fabricated by ternary-source vacuum deposition using CH3NH2 and HI gas sources combined with PbI2 solid source. The newly developed ternary-source vapor-phase deposition technique is useful because of its high stability and controllability in deposition rates.

Uniform Polypyrrole Layer-Coated Sulfur/Graphene Aerogel via the Vapor-Phase Deposition Technique as the Cathode Material for Li-S Batteries.

The practical application of Li-S batteries is hampered because of their poor cycling stability caused by electrolyte-dissolved lithium polysulfides. Dual functionalities such as strong chemical adsorption stability and high conductivity are highly desired for an ideal host material for the sulfur-based cathode. Herein, a uniform polypyrrole layer-coated sulfur/graphene aerogel composite is designed and synthesized using a novel vapor-phase deposition method. The polypyrrole layer simultaneously acts as a host and an adsorbent for efficient suppression of polysulfide dissolution through strong chemical interaction. The density functional theory calculations reveal that the polypyrrole could trap lithium polysulfides through stronger bonding energy. In addition, the deflation of sulfur/graphene hydrogel during the vapor-phase deposition process enhances the contact of sulfur with matrices, resulting in high sulfur utilization and good rate capability. As a result, the synthesized polypyrrole-coated sulfur/graphene aerogel composite delivers specific discharge capacities of 1167 and 409.1 mA h g-1 at 0.2 and 5 C, respectively. Moreover, the composite can maintain a capacity of 698 mA h g-1 at 0.5 C after 500 cycles, showing an ultraslow decay rate of 0.03% per cycle.

Review of Atomic Layer Deposition of Nanostructured Solar Cells 4

This study reviews atomic layer deposition technique with a special interest on solar cells applications. Atomic layer deposition is a vapour phase deposition technique used for producing thin films for several applications. This review focuses on the chemistry of Atomic Layer Deposition of solar cells, merits and demerits of ALD on thin film solar cells. Solar cells have attracted a lot of interest due to their potential for affordable, clean and sustainable energy. Solar cells can be deposited using different deposition techniques but Atomic layer deposition currently attracts attention owing to the merits. ALD has functional merit to bulk materials, great processing flexibility and affordability. The review examined the merits of ALD and solar cells and areas for future study. It offers affordability, ease of control of film growth, conformal and improvement on the deposition of solar cells. Despite few demerits, ALD is poised to be the deposition technique of choice for modifying interfaces of the film for improved performance.

Construction of hierarchical functional nanomaterials for energy conversion and storage

Hierarchical functional nanomaterials have important applications in the fields of new energy and the environment because of performance advantages based on the coupling and collaboration of multi-level structures. The key to application is the design and control of the mesoscale structures, such as the crystal-facet structure, lattice orientation, defects, and dislocations. In this review, we present the controlled synthesis of novel semiconductor oxide and carbon-based nanocomposites with three-dimensional hierarchical structures by the multiphase-reaction process. We describe several typical reaction processes, such as the gas-phase flame-combustion method, the vapor-phase deposition technique, and the solid–liquid interface reaction process, to investigate the properties of fluid flow, mixing, transport, and chemical reaction. We also study the formation mechanism, structural evolution and control methods for hierarchical nanomaterials by using combined experimental and simulation methods. These well-designed and prepared hierarchical functional materials can be used in energy storage and conversion fields. Valuable information is provided for the structural design and performance control of new functional nanocomposites.

Effect of Cl doping amount on the microstructure, photovoltaic properties and ferroelectric properties of Bi-based lead-free perovskite

Organic–inorganic hybrid perovskites with 3D perovskite structure have gained much attention as light harvesting materials in thin-film photovoltaics. This is because of their outstanding light-absorption characteristics, charge-transport dynamics and their simple processability using lab-scale solution and vapor phase deposition techniques. However, the inherent instability and lead toxicity of lead-based PSCs are the major problems at present. Recent studies have shown that the (CH3NH[Formula: see text]Bi2I9 (MBI) 0D bismuth-based compound can be used as an optical absorption layer in solar cells. In this paper, the (CH3NH[Formula: see text]Bi2I9 was doped with Cl− and a series of (CH3NH[Formula: see text]Bi2I[Formula: see text]Clx films were prepared. The effects of different doping amounts on the microstructure, photovoltaic properties and ferroelectric properties were systematically investigated. Scanning electron microscope (SEM) and Atomic force microscope (AFM) analysis showed that with the increase of doping content, the density of the films increased and the roughness decreased. The photoelectric conversion efficiency of (CH3NH[Formula: see text]Bi2I[Formula: see text]Clx raises with the increase of doping content. For example, the photoelectric conversion efficiency of (CH3NH[Formula: see text]Bi2I3Cl6 is 0.473%. We find that the leakage current descends into the increase in doping content, which may be due to the increase in the film density and the decrease of porosity. These research results have a positive effect on the development of Bi-based lead-free perovskite.

Tuning morphological features of lead iodide by low pressure vapor phase deposition

Lead iodide (PbI2) is a semiconductor with extensive use as an active layer for X-ray detectors and as a precursor for perovskite solar cells. Here we present a low vacuum method to obtain very uniform PbI2 films with full substrate coverage. This method consists in the sublimation of PbI2 inside a hot zone and its transport by an Ar flow to a substrate held at a controlled temperature. Using scanning electron microscopy combined with focused ion beam and X-ray diffraction we studied the morphology and crystallographic structure of the PbI2 films with different deposition parameters: substrate and source evaporation temperature, deposition time and substrate material. At high substrate temperature (80 °C) and low evaporation temperature (310 °C) onto a glass sample, we obtained dense and smooth PbI2 films showing hexagonal crystals, or platelets, stacked parallel to the substrate. The choice of the substrate material has a significant impact on the film morphology yielding porous-like structures with voids within the films for some substrates. A bandgap Eg = 2.42 eV and Urbach energy EU = 34.7 meV were obtained by absorbance measurements, which are comparable to films evaporated in high vacuum. Photoluminescence studies showed a dependence of the emission energies on the crystal orientation of the platelets which grow differently depending on the deposition conditions. The results show the ability of the low pressure vapor phase deposition technique to obtain good film properties, suitable for sensors and optoelectronic devices.

High-Throughput Atomic Layer Deposition for Surface Coating Materials of Lithium-Ion Batteries

Li-ion batteries have demonstrated a great capability for meeting the needs of an ever-growing market of mobile technology. However, as these applications become more demanding more energy and power will be required from rechargeable Li-ion batteries. To meet these needs there are many R&D efforts ongoing to design and generate new materials capable of higher performance. In contrast to this approach, higher performance electrode materials could be achieved through surface-engineering of existing materials. Atomic Layer Deposition (ALD), a vapor phase deposition technique used to apply Angstrom thick layers of material on the surface of powders and objects, has been demonstrated to enable higher voltage cycling, longer cycle life, and increased safety, as well as improvements to the manufacturing process of Li-ion batteries. Historically ALD has been regarded as slow and expensive, however Forge Nano has developed a method for applying ALD coatings onto materials at low cost and high volume. In this work, Forge Nano demonstrates the ability to utilize ALD to enable significant improvements to Li-ion battery performance as well as reduce costs to produce batteries at scale.

Orientation and Ordering of Organic and Hybrid Inorganic–Organic Polyurea Films Using Molecular Layer Deposition

The molecular orientation and ordering of films produced by a vapor-phase deposition technique known as molecular layer deposition (MLD) is investigated. Specifically, the influence of incorporating an inorganic component into MLD films is determined. Film growth properties and surface saturating conditions of a purely organic polyurea film and a mixed inorganic–organic polyurea film are investigated by in situ quartz crystal microbalance. Polarization-dependent X-ray absorption spectroscopy was employed to determine the orientation and ordering of these films. Both films demonstrate preferential orientation parallel to the substrate surface. However, the degree of ordering between the films is found to be starkly different. In-depth analysis revealed that the hybrid inorganic–organic films display a high degree of ordering compared to purely organic polyurea films. This study sheds light on the ability of MLD to finely tune oligomer packing density, growth rate, molecular orientation, and film ordering.

Direct Chemical Vapor Phase Deposition of Organometal Halide Perovskite Layers

ABSTRACTRecently, organometal halide perovskite solar cells have passed the threshold of 20 % power conversion efficiency (PCE). While such PCE values of perovskite solar cells are already competitive to those of other photovoltaic technologies, processing of large-area devices is still a challenge. Most of the devices reported in literature are prepared by small-scale solution-based processing techniques (e.g. spin-coating). Perovskite solar cells processed by vacuum thermal evaporation (VTE), which show uniform layers and achieve higher PCE and better reproducibility, have also been presented. Regarding the co-evaporation of the perovskite constituents, this technology suffers from large differences in the thermodynamic characteristics of the two species. While the organic components evaporate instantaneously at room temperature at pressures in the range of 10−6 hPa, significantly higher temperatures are needed for reasonable deposition rates of the metal halide compound. In addition, hybrid vapor phase deposition techniques have been developed employing a carrier gas to deposit the organic compound on the previously solution-processed metal halide compound. Generally, vapor phase processes have proven to be a desirable choice for industrial large-area production. In this work, we present a setup for the direct chemical vapor phase deposition (CVD) of methylammonium lead iodide (MAPbI3) employing nitrogen as carrier gas. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements are carried out to investigate the crystal quality and structural properties of the resulting perovskite. By optimizing the deposition parameters, we have produced perovskite films with a deposition rate of 30 nm/h which are comparable to those fabricated by solution processing. Furthermore, the developed CVD process can be easily scaled up to higher deposition rates and larger substrates sizes, thus rendering this technique a promising candidate for manufacturing large-area devices. Moreover, CVD of perovskite solar cells can overcome most of the limitations of liquid processing, e.g. the need for appropriate and orthogonal solvents.

(Invited) Plasma Deposition of Functional Nanocomposites

Nanocomposite films consisting of metal nanoparticles in a dielectric organic or ceramic matrix have unique functional properties with hosts of applications [1]. The present talk demonstrates how plasma and other vapor phase deposition techniques can be employed for tailoring the nanostructure and the resulting properties. Vapor phase deposition, inter alia, allows excellent control of the metallic filling factor and its depth profile as well as the incorporation of alloy nanoparticles with well-defined composition. The metallic nanoparticles typically form via self-organization during co-deposition of the metallic and matrix components due to the high cohesive energy of the metals and the low metal-matrix interaction energy. Various methods such as sputtering, plasma polymerization, and evaporation have been applied for the deposition of the matrix component, while the metallic component has mostly been sputter-deposited or evaporated. Moreover, gas aggregation cluster sources were utilized to obtain independent control of filling factor and size of the embedded nanoparticles [2,3]. Nanostructure formation was also studied by computer simulations [4]. In most applications, a high filling factor close to the percolation threshold is essential because the functional properties often require short range interaction between nanoparticles. Examples range from plasmonic composites [5,6] through high frequency magnetic cores [7] and sensors 8] to biocompatible antibacterial coatings with tailored release rate [9]. [1] F. Faupel, V. Zaporojtchenko, T. Strunskus, M. Elbahri, Adv. Eng. Mater. 12, 1177 (2010). [2] O. Polonsky, T. Peter, V. Zaporojtchenko, H. Biedermann, F, Faupel, Appl. Phys. Lett. 103 (2013) 033118. [3] T. Peter, M. Wegner, V. Zaporojtchenko, T. Strunskus, S. Bornholdt, H. Kersten, F. Faupel, Surface & Coatings Technology 205, 38 (2011). [4] J. W. Abraham, N. Kongsuwan, T. Strunskus, F. Faupel, and M. Bonitz, J. Appl. Phys. 117 , 014305 (2015). [5] M. Elbahri, M. K. Hedayati, V. S. K. Chakravadhanula, M. Jamali, T. Strunkus, V. Zaporochentko, F. Faupel, Adv. Mater. 23, 1993 (2011). [6] M. K. Hedayati, M. Javaherirahim, B. Mozooni, R. Abdelaziz, A. Tavassolizadeh, V. S. K. Chakravadhanula, V. Zaporojtchenko, T. Strunkus, F. Faupel and M. Elbahri, Adv. Mater. 23, 5410 (2011) [7] H. Greve, C. Pochstein, H. Takele, V. Zaporojtchenko, F. Faupel, A. Gerber, M. Frommberger, E. Quandt, Appl. Phys. Lett. 89, 242501 (2006). [8] C. Hanisch, N. Ni, A. Kulkarni, V. Zaporojtchenko, T. Strunskus, F. Faupel, J. Mater. Sci. 46, 438 (2010). [9] T. Hrkac, C. Röhl, R. Podschun, V. Zaporojtchenko, T. Strunskus, H. Papavlassopoulos, D. Garbe-Schönberg, F. Faupel, Mater. Sci. Eng. C 33, 2367 (2013).

Atomic Layer Deposition for Sensitized Solar Cells: Recent Progress and Prospects

Atomic layer deposition (ALD) and molecular layer deposition (MLD) are vapor phase deposition techniques used to create conformal coatings with molecular‐level control of thickness and composition. Recently, ALD and MLD have been extensively exploited to engineer the complex interfaces of dye‐sensitized solar cells (DSSCs) and other molecularly functionalized photoelectrochemical devices to improve the performance and long‐term stability. This progress report describes the recent advances in the applications of ALD and MLD for sensitized solar cells including DSSCs then discusses current challenges and future opportunities of ALD.

Stabilizing Hybrid Perovskites Via Non-Hydrolytic Atomic Layer Deposited Overlayers

A significant challenge to the practical application of hybrid perovskites remains for photovoltaic application. Specifically, the presence of hygroscopic ionic small molecules – alkylammonium cations – has been identified as a primary source of the absorber's sensitivity towards any moisture and any temperatures greater than 150 °C. CH3NH3PbI3 , for instance, rapidly degrades upon exposure to modest relative humidity even at room temperature via: CH3NH3PbI3(s) → PbI2(s) + CH3NH3I(aq) A related, but distinct, decomposition pathway is induced by heating above 150 °C even under an inert and dry environment, where methylamine and hydroiodic acid have measurable vapor pressure: CH3NH3PbI3(s) → PbI2(s) + CH3NH2↑ + HI↑ Various attempts have been made to improve system stability including modification of chemistry, two dimensional (2D) hybrid perovskites, and macroscopic encapsulation. While these measures may slow degradation the effect is often modest, transient, and adds significant device complexity. Synthetically-compatible, pinhole-free functional overlayers, in contrast, would provide a simple route to surface chemical passivation as well as a physical barrier to diffusion in or out of the active layer. Atomic Layer Deposition (ALD) is a vapor-phase deposition technique with the capability to produce ultra-thin and pinhole-free films with well-defined surface chemical control and precise physical thickness, providing a good match to these challenges. However, the vast majority of oxide ALD processes invoke water (H2O), hydrogen peroxide (H2O2) or ozone (O3) as oxygen sources, often at temperatures greater than 150 °C. Unfortunately, these chemicals and temperatures are predicted to be in direct conflict with most hybrid perovskite halides and therefore may preclude an integrated overlayer approach. However, we report low temperature non-hydrolytic (i.e. waterless) ALD to establish stabilizing overlayers of Al2O3 or TiO2 directly on CH3NH3PbI3−xClx films. We demonstrate not only that oxide ALD conditions exist that are compatible with hybrid perovskite halides, but that the resulting systems exhibit dramatically improved stability against moisture (85% RH) and enhanced resistance to considerably higher temperatures (up to 250 °C) and even liquid water. We further demonstrate the capability of nh-TiO2 to serve a dual role – stabilizing the absorber underlayer and accepting electrons in future inverted photovoltaics.The figure shows the effect of a liquid water drop after two minutes directly on a perovskite halide thin film, without and with an ultrathin hybrid ALD passivation layer. Figure 1

Low Temperature Atomic Layer Deposited TiO2 Compact Layers for Planar Perovskite Solar Cells

Titanium dioxide (TiO2) thin films have been utilized in a wide range of applications in optoelectronics and solar energy conversion. In particular, TiO2 has been exclusively used in photovoltaic (PV) devices and photoelectrochemical (PEC) cells owing to its ability to efficiently transport electrons and passivate photoactive materials against aqueous media. However, TiO2 exhibits optimal transport properties in the crystalline anatase phase, and thus requires a high temperature annealing step at 500 °C, which significantly limits its application in temperature sensitive substrates (e.g. PET) or active materials (e.g. organic-inorganic hybrid perovskites). Moreover, while high resistance of TiO2 films necessitates the need for ultrathin films to reduce series resistance in PV devices, the synthesis of ultrathin pinhole-free TiO2 films remains a challenge through conventional deposition methods. To address these issues, TiO2 compact layers for planar perovskite PV devices were deposited via atomic layer deposition (ALD) – a chemically diverse vapor phase deposition technique with the capability to synthesize ultrathin pinhole-free films. We have investigated the influence of low temperature atomic layer deposited (ALD) compact TiO2 layers as a function of post-treatment (i.e. O3, UV-O3, 500 °C) on the performance of planar perovskite PV cells. The performance of devices indicated variations primarily in open circuit voltage (VOC) and fill factor (FF) upon various forms of post-treatment. Changes in the VOC was mainly associated with the density of states within the bandgap of TiO2 as confirmed by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Also, low temperature post-treatment (O3 and UV-O3) resulted in reduced FF compared to the conventional high temperature annealing treatment as a consequence of decreased shunt resistance. We attribute this phenomena to the amorphous nature of the low temperature post-treated compact TiO2 layers providing channels for hole conduction. However, the devices fabricated with UV-O3 treated TiO2 compact layers resulted in a comparable overall performance compared to those fabricated with the conventional post-annealing process at 500 °C, opening possibilities for its application in temperature sensitive substrates as well as active materials.

Low Temperature Atomic Layer Deposited TiO2 Compact Layers for Planar Perovskite Solar Cells

Titanium dioxide (TiO2) thin films have been utilized in a wide range of applications in optoelectronics and solar energy conversion. In particular, TiO2 has been exclusively used in photovoltaic devices and photoelectrochemical cells owing to its ability to efficiently transport electrons and passivate photoactive materials against aqueous media. However, TiO2 exhibits optimal transport properties in the crystalline anatase phase, and thus requires a high temperature annealing, which significantly limits its application in temperature sensitive substrates or active materials (e.g. organic-inorganic hybrid perovskites). Moreover, while high resistance of TiO2 films necessitates the need for ultrathin films to reduce series resistance in PV devices, the synthesis of ultrathin pinhole-free TiO2 films remains a challenge through conventional deposition methods. To address these issues, TiO2 compact layers for planar perovskite PV devices were deposited via atomic layer deposition (ALD) – a chemically diverse vapor phase deposition technique with the capability to synthesize ultrathin pinhole-free films.