Pinhole-free Thin Films Research Articles

Perovskite-Surface-Confined Grain Growth for High-Performance Perovskite Solar Cells.

The conventional post-annealing (CPA) process is frequently employed and regarded a crucial step for high-quality perovskite thin-films. However, most researchers end up with unwanted characteristics because controlling the evaporation rate of perovskite precursor solvents during heat treatment is difficult. Most perovskite thin-films result in rough surfaces with pinholes and small grains with multiple boundaries, if the evaporation of precursor solvents is not controlled in a timely manner, which negatively affects the performance of perovskite solar cells (PSCs). Here, we present a surface-confined post-annealing (SCPA) approach for controlling the evaporation of perovskite precursor solvents and promoting crystallinity, homogeneity, and surface morphology of the resulting perovskites. The SCPA method not only modulates the evaporation of residual solvents, resulting in pinhole-free thin-films with large grains and fewer grain boundaries, but it also reduces recombination sites and facilitates the transport of charges in the resulting perovskite thin-films. When the method is changed from CPA to SCPA, the power conversion efficiency of PSC improves from 18.94% to 21.59%. Furthermore, as compared to their CPA-based counterparts, SCPA-based PSCs have less hysteresis and increased long-term stability. The SCPA is a potentially universal method for improving the performance and stability of PSCs by modulating the quality of perovskite thin-films.

Toward Optimization of the Chemical/Electrochemical Compatibility of Halide Solid Electrolytes in All-Solid-State Batteries

All-solid-state batteries (ASSBs) that rely on the use of solid electrolytes (SEs) with high ionic conductivity are the holy grail for future battery technology, since it could enable both greater energy density and safety. However, practical application of ASSBs is still being plagued by difficulties in mastering the SE–electrode interphases. This calls for a wide exploration of electrolyte candidates, among which halide-based Li+ conductors show promise despite being not stable against Li or LixIny negative electrodes, hence the need to assemble cells with a dual SE design. In the work described herein, we studied the electrochemical/chemical compatibility of Li3InCl6 against layered oxide positive electrode (LiNi0.6Mn0.2Co0.2O2, NMC622), carbon additive, and Li6PS5Cl under both cycling and aging conditions. Combining electroanalytical and spectroscopic techniques, we provide evidence for the onset of electrochemically driven parasitic decomposition reactions between Li3InCl6 and NMC622/carbon at lower potentials (3.3 V vs LiIn/In) than theoretically predicted in the literature. Moreover, to combat chemical incompatibility between dual SEs, we propose a new strategy that consists of depositing a nanometer-thick (1 or 2 nm) surface protective layer of Li3PO4 made by atomic layer deposition between Li3InCl6 and Li6PS5Cl. Through this surface engineering process with highly conformal and pinhole-free thin films, halide-based solid-state cells showing spectacular capacity retention over 400 cycles were successfully assembled. Altogether, these findings position halide SEs as serious contenders for the development of ASSBs.

Chemical Vapor Deposition of Wide-Bandgap p-Type Semiconductor Cuprous Iodide for Optoelectronic Device Applications

Continuous, pinhole-free thin films of transparent conductive materials (TCMs) with p- and n-type conductivity are critical components of optoelectronic devices including photovoltaics (PV), transparent electronics, and LEDs [1]. TCMs with n-type conductivity are widely available in the form of semiconducting oxides [2]; however, p-type TCMs (hole transport materials, HTMs) with suitable conductivity are comparatively rare [1]. Particularly in the rapidly growing field of thin film perovskite PV, device-quality thin films of HTMs with high stability and conductivity are urgently needed to replace the organic HTMs typically employed in perovskite PV research [3]. As such, techniques to deposit inorganic HTM thin films are desired.With a focus on applying HTMs in optoelectronic devices at the commercial scale, these materials must be deposited by a scalable technique yielding thin, continuous, pinhole-free films. Chemical vapor deposition (CVD) is a highly scalable thin film deposition technique widely used in industry, with a proven ability to yield high-quality thin films on large-area substrates [4]. A variety of n-type TCMs are accessible by CVD techniques, but few CVD methods exist for p-type TCMs; examples include atomic layer deposition of NiOx and VOx [5].The cuprous halides (CuX, X = Cl, Br, I), which are optically transparent p-type semiconductors with bandgaps ~3 eV, are promising inorganic HTM candidates. Cuprous iodide is of particular interest due to its low resistivity (~10-2 Ohm·cm), high hole concentration and mobility (~1019 cm-3 and ~1-10 cm2V-1s-1, respectively), and its valence band maximum position (ionization potential: 5.0-5.4 eV), which is well-aligned with common perovskite absorber materials [6]. While CuI crystallite arrays have been obtained by metal-organic CVD [7], CVD of continuous CuI thin films has only been achieved by a two-step vapor conversion process, requiring initial deposition of a copper chalcogen followed by vapor-phase conversion to the halide [6].Our group has recently published a CVD technique enabling direct deposition of continuous CuBr thin films via CVD reaction between hydrogen bromide and vinyltrimethylsilane(hexafluoroacetylacetonato)copper(I) [Cu(hfac)(vtms)] [8], and this technique has been extended to deposition of CuI. X-ray photoelectron spectroscopy indicates pure CuI films with an atomic ratio of approximately 1:1 Cu:I, and x-ray diffraction confirms deposition of γ-CuI. Similar to our observations for CuBr films, substrate identity has significant effects on CuI film continuity. Progress towards CVD of continuous CuI films on substrates of interest for practical application in optoelectronic devices will be discussed, with a particular focus on perovskite PV.[1] A. N. Fioretti and M. Morales-Masis, "Bridging the p-type transparent conductive materials gap: synthesis approaches for disperse valence band materials," J. Photon. Energy, 10, 042002 (2020).[2] M. Morales-Masis, S. De Wolf, R. Woods-Robinson, J. W. Ager, and C. Ballif, "Transparent Electrodes for Efficient Optoelectronics," Adv. Electron. Mater., 3, 1600529 (2017).[3] B. Gil, A. J. Yun, Y. Lee, J. Kim, B. Lee, and B. Park, "Recent Progress in Inorganic Hole Transport Materials for Efficient and Stable Perovskite Solar Cells," Electron. Mater. Lett., 15, 505 (2019).[4] R. Gordon, "Chemical Vapor Deposition of Coatings on Glass," J. Non-Cryst. Solids, 218, 81 (1997).[5] J. A. Raiford, S. T. Oyakhire, and S. F. Bent, "Applications of atomic layer deposition and chemical vapor deposition for perovskite solar cells," Energy Environ. Sci., 13, 1997 (2020).[6] R. Heasley, L. M. Davis, D. Chua, C. M. Chang, and R. G. Gordon, "Vapor Deposition of Transparent, p-Type Cuprous Iodide Via a Two-Step Conversion Process," ACS Appl. Energy Mater., 1, 6953 (2018).[7] V. Gottschalch, S. Blaurock, G. Benndorf, J. Lenzner, M. Grundmann, and H. Krautscheid, "Copper iodide synthesized by iodization of Cu-films and deposited using MOCVD," J. Cryst. Growth, 471, 21 (2017).[8] C. M. Chang, L. M. Davis, E. K. Spear, and R. G. Gordon, "Chemical Vapor Deposition of Transparent, p-Type Cuprous Bromide Thin Films," Chem. Mater., 33, 1426 (2021).

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.

A simple, low-cost modified drop-casting method to develop high-quality CH3NH3PbI3 perovskite thin films

To make perovskite solar cells competitive and efficient, it is necessary to control the crystallinity along with full coverage of the used substrate. Nowadays, numerous techniques have applied to improve and control the crystallinity of fabricated perovskite films, but still, have the most critical issue for perceiving high-performance perovskite solar cells. Herein, a low-cost yet effective procedure has used to fabricate thin-films of methylamine lead halide (CH3NH3PbI3) perovskite semiconductor. Uniform and high-quality CH3NH3PbI3 perovskite thin films were successfully fabricated on a glass substrate using a modified drop-casting method. Morphological characterization has shown an extremely compact, highly crystalline, and dense microstructure pinhole-free thin-film without any impure phase. Compared to the spin coating method, thin-films processed have been exposed to retain an enhanced film quality with higher crystallinity. Furthermore, a voltage-current characteristic has shown that the photoconductivity significantly enhanced the films processed with this versatile method. These results were also validated by absorbance and photoluminescence measurements. Achieved results highlight the significance of this low-cost method for forthcoming high-performance photovoltaic devices based on CH3NH3PbI3 semiconductor thin films.

Microstructure and Performance Analysis of Low-Temperature Solid Oxide Fuel Cells Synthesized on Inert Al2O3 Substrates

In order to lower the SOFCs operating temperatures new electrolyte materials should be used instead of yttria-stabilized zirconia (YSZ) and the thickness of electrolyte should be decreased to the low micrometer range. To prepare the thin electrolyte, physical vapour deposition methods, such as magnetron sputtering, leading to dense, pinhole-free thin films, are ideal. If, additionally, also the anode and cathode materials are kept thin, low-temperature cells are obtained, also called micro-SOFCs.To demonstrate the possibility of creating such thin-film layers, model anodes and anode/electrolyte systems were prepared on porous anodic aluminum oxide (AAO) substrates. As electrolyte material, Ce0.8Gd0.2O1.9-x (GDC) was used. The composite consisted of NiO and GDC was used as anode. The main advantage of GDC electrolyte for solid oxide fuel cells is four to five times higher ionic conductivity compared to YSZ in the temperature range 500-800°C.The microstructure of NiO-GDC thin-film anode after reduction in hydrogen at temperatures of 450 and 600°С was studied. It was found that at these temperatures there is also agglomeration of Ni particles and segregation on the film surface in the form of small particles as in [1] at 850°C. It is also shown that annealing of the deposited NiO-GDC anode layer at 1000°С allows stabilizing its structure and excluding subsequent agglomeration of Ni particles during operation in a hydrogen atmosphere. Figure 1 shows the microstructure of NiO anode/GDC electrolyte system deposited on AAO substrate.Button cells with a diameter of 13 mm and with different thicknesses of the anode and electrolyte were fabricated. A La0.6Sr0.4CoO3 cathode (Kceracell Co., Korea) was screen printed on the electrolyte surface and sintered in situ during cell testing in temperature range of 500–600°C. First investigations showed promising results, namely high open-circuit voltage values (0.8 V) and a power density up to 100 mW/cm² at 550°C in pure hydrogen. However, the impedance analysis of the cells points out that further anode and cathode optimization would be beneficial for the overall cell performance.[1] F.J. Garcia-Garcia et al., Int. J. Hydrog. Energy, 40(23), 7382 (2015) Figure 1: The microstructure of anode/electrolyte system deposited on AAO substrate (scanning electron microscopy) Figure 1

Applications of Titania Atomic Layer Deposition in the Biomedical Field and Recent Updates

Atomic Layer Deposition (ALD) is a promising technique to precisely deposit nanometer-thick films on inorganic and organic substrates. ALD is known for the deposition of uniform, conformal, and pinhole-free thin films with a controllable thickness with sequential and self-limiting gaseous reactions on the substrate surface of interest. ALD thin films have a wide range of applications, from semiconductor industries to medical devices. In many studies, ALD thin films are used to modify the biomaterial surface properties for particular biomedical applications. Here, polymethyl methacrylate, titanium-based alloys, and collagen were chosen as substrates for this review; thanks to their biocompatibility, these substrates have several applications in the dental, medical, and biomedical fields. The ALD of Titanium (IV) oxide (TiO2) films has been selected for this review as a biocompatible material which has attracted increasing attention in biomedical applications. Selected publications focusing on the modification of the surface of these biomaterials with TiO2 ALD are discussed.

Atomic layer deposition of TiO2 blocking layers for dye-sensitized solar cells

PurposeThe purpose of this paper is to improve the efficiency of dye-sensitized solar cells (DSSCs) which present promising low-cost alternative to the conventional silicon solar cells mainly due to comparatively low manufacturing cost, ease of fabrication and relatively good efficiency. One of the undesirable factor in DSSCs is the electron recombination process that takes place at the transparent conductive oxide/electrolyte interface, on the side of photoelectrode. To reduce this effect in the structure of the solar cell, a TiO2 blocking layer (BL) by atomic layer deposition (ALD) was deposited.Design/methodology/approachScanning electron microscope, Raman and UV-Vis spectroscopy were used to evaluate the influence of BL on the photovoltaic properties. Electrical parameters of manufactured DSSCs with and without BL were characterized by measurements of current-voltage characteristics under standard AM 1.5 radiation.FindingsThe TiO2 BL prevents the physical contact of fluorine-doped tin oxide (FTO) and the electrolyte and leads to increase in the cell’s overall efficiency, from 5.15 to 6.18%. Higher density of the BL, together with larger contact area and improved adherence between the TiO2 layer and FTO surface provide more electron pathways from TiO2 to FTO which facilitates electron transfer.Originality/valueThis paper demonstrates that the introduction of a BL into the photovoltaic device structure is an important step in technology of DSSCs to improve its efficiency. Moreover, the ALD is a powerful technique which allows for the highly reproducible growth of pinhole-free thin films with excellent thickness accuracy and conformality at low temperature.

Characterization of Cobalt Sulfide Thin Films Synthesized from Acidic Chemical Baths

Cobalt sulfide thin films were synthesized from acidic chemical baths by varying the deposition time. The powder X‐ray diffraction studies indicated that there are hexagonal CoS, face‐centered cubic Co3S4, and cubic Co9S8 phases of cobalt sulfide. The crystallite size of the hexagonal CoS phase decreased from 52.8 nm to 22.5 nm and that of the cubic Co9S8 phase increased from 11 nm to 60 nm as the deposition time increased from 2 hrs to 3.5 hrs. The scanning electron microscopic images revealed crack and pinhole free thin films with uniform and smooth background and few large polygonal grains on the surface. The band gap of the cobalt sulfide thin films decreased from 1.75 eV to 1.3 eV as the deposition time increased from 2 hrs to 3.5 hrs. The photoluminescence (PL) spectra of the films confirmed the emission of ultraviolet, violet, and blue lights. The intense PL emission of violet light at 384 nm had red shifted with increasing deposition time that could be resulted from the increase in the average crystallite size. The FTIR spectra of the films indicated the presence of OH, C‐O‐H, C‐O, double sulfide, and Co‐S groups. As the deposition time increased, the electrical resistivity of the cobalt sulfide thin films decreased due to the increase in both the crystallite size and the films’ thickness.

Characterization of Cerium Dioxide Thin Films Prepared By Plasma-Enhanced Atomic Layer Deposition for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are considered to be the next generation of energy conversion devices because of their environmental friendliness and high efficiency. Particularly due to its high operating temperature, it has high fuel flexibility and does not require water management. However, the high operating temperature that causes these advantages accelerates the degradation of fuel cell components, thereby reducing the durability and limiting the usable materials. Therefore, in recent years, a lot of research has been conducted to lower the operating temperature of the SOFCs. One method of lowering the operating temperature of SOFCs is to apply an electrolyte having good ionic conductivity at low temperatures(400-600℃). Potential examples are doped ceria materials. One of the well-known ceria-based electrolytes is Gd-doped ceria (GDC), which exhibits ion conductivity 1-2 times higher than yttria stabilized zirconia, YSZ, which is the most commonly used electrolyte in SOFC. GDC has good ionic conductivity, but when exposed to a reducing environment, Ce4+ is reduced to Ce3+ and begins to have electronic conductivity, which causes a decrease of open circuit voltage (OCV). Therefore, in order to apply GDC as an electrolyte, an additional blocking layer must be added so that it is not exposed to the hydrogen environment of the anode. This additional layer increases not only the thickness of the electrolyte but also the interfacial resistance between the layers, thereby increasing the ohmic resistance. Therefore, the fabrication of GDCs with thinner and better surface properties is critical to improving the performance of low temperature SOFCs. In this study, CeO2 thin film was fabricated using atomic layer deposition (ALD) as a preceding step for the fabrication of GDC thin films. Oxygen plasma was used to oxidize the precursor to widen the ALD window and to produce the highly crystalline CeO2. The growth rate of CeO2 was determined by controlling precursor and oxidizer exposure time, and the thickness of thin film per cycle was confirmed. The surface morphology was observed by scanning electron microscope (SEM), and dense and pinhole-free thin film surface was confirmed. In addition, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed and the characteristic crystalline peaks and chemical compositions were compared with those of the thermal ALD CeO2 thin films.

High-Quality CH3NH3PbI3 Films Obtained via a Pressure-Assisted Space-Confined Solvent-Engineering Strategy for Ultrasensitive Photodetectors.

High-quality organic-inorganic hybrid perovskite films are crucial for excellent performance of photoelectric devices. Herein, we demonstrate a pressure-assisted space-confined solvent-engineering strategy to grow highly oriented, pinhole-free thin films of CH3NH3PbI3 with large-scale crystalline grains, high smoothness, and crystalline fusion on grain boundaries. These single-crystalline grains vertically span the entire film thickness. Such a film feature dramatically reduces recombination loss and then improves the transport property of charge carriers in the films. Consequently, the photodetector devices, based on the high-quality CH3NH3PbI3 films, exhibit high photocurrent (105 μA under 671 nm laser with a power density of 20.6 mW/cm2 at 10 V), good stability, and, especially, an ultrahigh on/off ratio (Ilight/Idark > 2.2 × 104 under an incident light of 20.6 mW/cm2). These excellent performances indicate that the high-quality films will be potential candidates in other CH3NH3PbI3-based photoelectric devices.

Low-temperature amorphous boron nitride on Si0.7Ge0.3(001), Cu, and HOPG from sequential exposures of N2H4 and BCl3

Low-temperature sequential exposures of N2H4 and BCl3 have been performed on Si0.3Ge0.7(001), Cu, and HOPG surfaces at 350 °C. A novel BN ALD process has been achieved on Si0.3Ge0.7(001) with 60 cycles of BN ALD producing a uniform, pinhole-free thin film with low contamination, as characterized with XPS and AFM. On Cu and Si0.3Ge0.7(001), XPS spectra indicated a near stoichiometric BN film. While AFM imaging indicated the deposition on Cu yielded nanometer-scale etching, conformal deposition was observed on Si0.3Ge0.7(001). The BN ALD also nucleated on inert HOPG via step edges. In situ STM imaging showed that cyclic exposures at 350 °C were able to decorate step edges with features ∼2 nm tall and ∼200 nm wide, indicating the propensity for BN to grow in the planar direction. The N2H4 and BCl3 ALD allows for the deposition of low oxygen, low carbon films, but to avoid etching, the growth should be nucleated by N2H4, since exposure to BCl3 can result in the formation of volatile Cl-containing surface species on many substrates. Therefore, the formation of a stable surface nitride prior to BCl3 exposure is necessary to prevent formation and desorption of volatile species from the substrate.

High Quality Oxide Films Deposited at Room Temperature by Ion Beam Sputtering

ABSTRACTWe have deposited dense and pinhole-free thin films of SiO2, Al2O3 and ITO at room temperature via ion beam sputtering. The SiO2 films were found to be of similar quality as thermal oxide with a resistivity greater than 1015 Ω·cm and breakdown field in excess of 7 MV/cm. The Al2O3 films were part of a Pt- Al2O3-Pt vertical tunnel junction and were kept extremely thin, from 2 nm to 4 nm. The current-voltage characteristics of these junctions indicated a breakdown field in excess of 20 MV/cm, roughly twice that achieved by ALD films. This breakdown voltage was found to be independent of junction area, strongly suggesting the absence of pinholes in the film. The ITO films were 50 nm to 100 nm thick. As deposited, they are fully transparent with an electrical resistivity of 5x10-4 Ω·cm.

Gallium platinum alloys – a new material system for UV plasmonics

We describe a new material system based on alloys of gallium and platinum that is well-suited for ultraviolet (UV) plasmonics. Although gallium has previously been shown to be useful for such studies, creating a continuous, pinhole-free thin film has been technically challenging. For example, when vacuum deposition techniques are used, gallium forms as isolated spherical nanoparticles on a wide variety of substrates. We demonstrate that when a platinum wetting layer is deposited first on a substrate followed by a thick gallium layer, a Ga-Pt alloy thin film is formed near the interface. The excess surface gallium can then be removed using a focused ion beam (FIB), exposing the alloy film. Ellipsometry measurements show that the alloy largely retains the dielectric properties of solid gallium throughout the UV, although the properties of the two diverge somewhat in the visible. We fabricate periodic subwavelength aperture arrays in the alloy thin film and observe enhanced optical transmission resonances that are sharper in the UV than in the visible. The patterned films appear to be stable over time periods exceeding six months based on optical measurements.

PVD-Deposited Micro-SOFC Model Systems

In order to lower the operating temperatures, an important branch of SOFC-related research is dedicated to finding new electrolyte materials to use instead of yttria-stabilized zirconia (YSZ), such as doped ceria or perovskites.[1] In conventional cells, these are around 1073–1273 K,[2] which is too high for mobile applications. Another option is to reduce the electrolyte thickness to the low micrometer range – creating so-called anode-supported SOFCs.[3] However, this temperature could be lowered even further by reducing the thickness of the electrolyte layer to the 100 nm range. To prepare the latter, physical vapor deposition (PVD) methods, such as magnetron sputtering, leading to dense, pinhole-free thin films, are ideal. If, additionally, also the anode and cathode materials are kept thin, lower masses of the whole cells result, creating so-called micro-SOFCs. Thus, less energy would be required for operation, rendering Micro-SOFCs economically efficient as they could also exhibit increased lifetimes. As preliminary tests of the feasibility of such thin film layers, model anodes and anode/electrolyte systems were prepared. As electrolyte material, YSZ was used, with the anode component consisting of a 1:1 nickel/copper alloy. The latter allows to potentially substitute the conventionally used nickel, which is an ideal catalyst for graphite growth.[4] The presence of copper has already been shown to inhibit carbon formation during operation.[5] The YSZ and NiCu thin films, as well as the NiCu/YSZ bilayer, were deposited via magnetron sputtering on heated silicon and sodium chloride single crystals (873 K) to allow for the creation of well-crystallized and -ordered specimens. The latter are ideally suited for various spectroscopic and microscopic experiments. The stability of the samples in a methane atmosphere at elevated temperatures was further investigated.[6] X-ray photoelectron spectroscopy (XPS) indicates that minor carbon deposition occurs on YSZ (primarily consisting of sp3 species) upon treatment in methane at 1073 K. Both NiCu and NiCu/YSZ, in contrast, featured a very thick, strongly graphitic carbon overlayer. Elemental maps (STEM-EDX) reveal that, while the NiCu particles are rather homogeneous in the as-grown state, they form a core-shell structure upon reductive treatment in CH4- the particles consist of a nickel-rich core and a copper-rich outer layer. This shell also features variations in composition. While on those parts of the shell where the Ni/Cu ratio is about 3.5 to 5, carbon is deposited (also featuring carbon nanotubes), those with Ni/Cu ratios of about 0.5 do not exhibit any carbon overlayer at all. This suggests that the copper alloy indeed inhibits carbon growth. However, Ni/Cu ratios lower than 1:1 are required. According to DFT calculations, copper segregation is energetically favored and has been shown to occur in hydrogen.[5,8] Hence, it should be possible to generate the carbon-inhibiting core/shell structure starting from the 1:1 alloy by means of a pre-treatment in hydrogen at higher temperatures. [1] Marinha, D.; Dessemond, L.; Djurado, E. Curr. Inorg. Chem. 2013, 3,2–22 [2] Atkinson, A.; Barnett, S.; Gorte, R. J.; Irvine, J. T. S.; McEvoy, A. J.; Mogensen, M.; Singhal, S. C.; Vohs, J. Nat. Mater. 2004, 3,17–27 [3] Verbraeken, M. Master’s Thesis, University of Twente, 2005 [4] Toebe, M. L.; Bitter, J. H.; van Dillen, A. J.; de Jong, K. P. Catal. Today 2002, 76,33–42 [5] Kim, H.; Lu, C.; Worrell, W. L.; Vohs, J. M.; Gorte, R. J. J. Electrochem. Soc. 2002, 149,A247–A250 [6] Götsch, T.; Schachinger, T.; Stöger-Pollach, M.; Kaindl, R.; Penner, S. Appl. Surf. Sci. 2017, 402,1–11 [7] Götsch, T.; Wallisch, W.; Stöger-Pollach, M.; Klötzer, B.; Penner, S. AIP Adv. 2016, 6 (2),025119 [8] Wolfbeisser, A.; Kovács, G.; Kozlov, S. M.; Föttinger, K.; Bernardi, J.; Klötzer, B.; Neymann, K. M.; Rupprechter, G. Catal. Today 2017,283, 134–143 Figure 1:STEM-EDX map of the NiCu/YSZ sample after methane treatment shows that the carbon deposition does not occur for those surfaces of the core/shell particles that have a Ni/Cu ratio of about 0.5 (marked by blue arrows). Figure 1

Novel synthetic route for the synthesis of ternary Cd(SSe) photoelectrode and their photoelectrochemical application

In the present investigation, we have synthesized nanocrystalline cadmium sulphoselenide Cd(SSe) thin film photoelectrodes on ultrasonically cleaned bare and fluorine doped tin oxide coated glass substrate using facile, cost effective and extremely convenient arrested precipitation technique at different time intervals (1, 1.5, 2 and 2.5 h). Optical, structural, morphological, compositional and photoelectrochemical (PEC) properties were investigated as function of deposition time. The synthesized films were characterized by UV–Vis-NIR spectrophotometer, X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Optical absorption spectrum shows a strong absorption in the wavelength range 300–1100 nm with band gap variation in the range of 2.05–1.87 eV. The XRD patterns shows nanocrystalline nature with crystallite size 58–76 nm and exhibits pure hexagonal crystal structure. The SEM micrograph demonstrates formation of well adherent, pinhole free thin films with nanosphere, nanopetal like morphology which is finally modified into flowerlike morphology. Transmission electron microscopy (TEM) and HRTEM results confirm the nanocrystalline nature and selected area electron diffraction pattern reveals hexagonal crystal structure of deposited Cd(SSe) thin films. EDS and XPS showed that the composition of Cd(SSe) thin films was of good stoichiometry. Finally these prepared photoelectrodes were tested for their PEC performance in sulphide/polysulphide electrolyte and promising results were found.

Opto-Electronic Properties of Hybrid Perovskites

The photovoltaic efficiency utilizing hybrid perovskites as an absorber layer have reached 22% and offers extraordinary potential for clean sustainable energy technologies and also for low-cost optoelectronic devices. In spite of the unprecedented progress, there still exists tremendous variability in the structural and physical properties of these hybrid perovskite based thin-films across the globe. Crystallinity, defect density and impurities are in general determining factors for these physical properties, which are also highly dependent on the materials formation processes. In addition to this, the stability and reliability of perovskite based devices remains open questions and perhaps will determine the fate of this remarkable technology in the longer run. In this talk, I will describe our recently discovered hot-casting approach for thin-film crystal growth as a general strategy for growing highly crystalline, bulk homogenous pin-hole free thin-films of both 3D and layered 2D organometallic perovskites that yield reproducible and stable high efficiency devices and allow access to the intrinsic photo-physical properties. These results are briefly described below. Photovoltaic devices fabricated using 3D hybrid perovskites show hysteresis-free response, with high degree of reproducibility, with an average efficiency of planar devices approaching ~20%. Photo-physical, electrical characterization and theoretical modeling attribute the improved performance to reduced bulk defects and improved charge-carrier mobility in large-grain devices. Furthermore, our most recent efforts on understanding and controlling photo-degradation in these systems demonstrate that the large grain-size perovskite thin-films are not limited by detrimental effects such as ion migration or defect assisted trapping generally reported in perovskite thin-film devices allowing us to probe intrinsic photo-physical processes that lead to the degradation of PCE in perovskite solar cells and control the light-induced degradation in these materials.

Growth and characterization of Al2O3 films on fluorine functionalized epitaxial graphene

Intelligent engineering of graphene-based electronic devices on SiC(0001) requires a better understanding of processes used to deposit gate-dielectric materials on graphene. Recently, Al2O3 dielectrics have been shown to form conformal, pinhole-free thin films by functionalizing the top surface of the graphene with fluorine prior to atomic layer deposition (ALD) of the Al2O3 using a trimethylaluminum (TMA) precursor. In this work, the functionalization and ALD-precursor adsorption processes have been studied with angle-resolved photoelectron spectroscopy, low energy electron diffraction, and X-ray photoelectron spectroscopy. It has been found that the functionalization process has a negligible effect on the electronic structure of the graphene, and that it results in a twofold increase in the adsorption of the ALD-precursor. In situ TMA-dosing and XPS studies were also performed on three different Si(100) substrates that were terminated with H, OH, or dangling Si-bonds. This dosing experiment revealed that OH is required for TMA adsorption. Based on those data along with supportive in situ measurements that showed F-functionalization increases the amount of oxygen (in the form of adsorbed H2O) on the surface of the graphene, a model for TMA-adsorption on graphene is proposed that is based on a reaction of a TMA molecule with OH.

Solution processed perovskite solar cells using highly conductive PEDOT:PSS interfacial layer

We have developed a simple and robust process to prepare efficient perovskite solar cells. Pinhole free thin films of CH3NH3PbI3−xClx can be coated on high conductivity poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS), (Clevios PH1000) when the PEDOT:PSS is deposited together with dimethyl sulfoxide (DMSO) and Zonyl as additives. This process enables the fabrication of perovskite solar cells using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron transport layer with >12% power conversion efficiency, low hysteresis and excellent operational stability. We have performed a detailed opto-electronic characterization of these solar cells and identified the main loss mechanism limiting the device performance.

Atomic Layer Deposition and in-Situ Characterization of Yttrium Oxide and Yttria-Stabilized Zirconia

Y2O3 is an attractive rear-earth oxide due to its large permittivity (~14-18), high refractive index above 1.9, optical transparency over a wide spectral range (0.2-8µm), large bandgap of 5.8eV and excellent thermodynamic stability. Additionally, when co-deposited with ZrO2, it leads to yttria-stabilized zirconia (YSZ), a dense ionic conductor widely used as an SOFC electrolyte. The depositions of Y2O3and YSZ by thermal ALD are therefore important in the context of achieving dense pinhole-free thin films (<50nm) that can potentially be applied with excellent conformallity over high aspect ratio 3D nanostructures either for electronic, optical or energy applications. In this paper, Y2O3 and YSZ thin films are thermally deposited by ALD in a commercial Ultratech Savannah reactor. Tris(ethylcyclopentadienyl)yttrium -Y(EtCp)3 - heated at 120˚C (0.15 Torr vapor pressure) - is used as a precursor with N2 carrier bubbled through via a Low Vapor Pressure Delivery system. Additionally, tetrakis(dimethylamido)zirconium TDMAZr is selected for ZrO2 deposition while H2O is the co-reactant. A quartz crystal microbalance has been implemented in order to rapidly characterize the ALD process space from 150 to 300˚C as a function of Y(EtCp)3 and H2O doses. In-situ spectroscopic ellipsometry data are also collected in order to measure the evolution of the optical film properties with temperature. In the case of Y2O3, a soft saturation of the growth per cycle vs. Y(EtCp)3 bubbling time is observed with a GPC of 1.6Å/cycle at 250˚C. The non-uniformity across 150 mm wafers decreases to ~2% at an optimal Y(EtCp)3 dose. The GPC increases monotonically with temperature with no apparent plateau between 150 to 300˚C. The optical index remains stable at 1.92 above 200˚C. The Y2O3film stoichiometry is confirmed by RBS while SIMS measurements indicate a carbon content below 0.1%. Uniform YSZ thin films were deposited and the yttria to zirconia composition is accurately controlled by adjusting the ratio of TDMAZr to Y(EtCp)3 cycles. In-situ measurements confirm the growth of Y2O3 on ZrO2, and vice-versa, without evidence of nucleation retardation when alternating the chemistries. The Y:Zr mass ratio monitored by QCM appears consistent with the Y2O3 to ZrO2 ALD cycle ratio and is verified with ex-situ XPS. XRD measurements were done on Y2O3 and YSZ samples to characterize the film crystallinity. Overall both Y2O3 and YSZ ALD processes prove robust and reproducible with negligible contamination and no apparent sign of thermal decomposition of the precursor.