Co-evaporated Films Research Articles

Resistive switching and artificial synapses performance of co-evaporated Cs3Cu2I5 films

Perovskite memristors have garnered significant interest for their potential simulating artificial synapses; however, the presence of the toxic lead-based perovskites has hindered advancements in this field. In this work, a nontoxic, thickness-controllable Cs3Cu2I5 perovskite functional layer is synthesized through a dual-source vapor deposition for the Ag/Cs3Cu2I5/ITO memristor. The co-evaporation method shows advantages of various element, controllable atomic ratio and thickness, free impurity, and continuously uniform film. This device demonstrates an operating voltage of 1.2 V, a low power consumption of 0.013 W, a retention time exceeding 104 s, and an endurance of over 400 cycles. The synaptic behavior is emulated using the memristor, focusing on phenomena such as short-term potentiation and depression, paired-pulse facilitation, and spike-time-dependent plasticity. The migration of Na+ and Cl− ions, which occurs between the synaptic cleft and the postsynaptic membrane in biological synapses, is analogously represented by the movement of Ag+ ions between functional layer and the bottom electrode of the memristor. This process is further analyzed using the Hodgkin–Huxley neuron model. The Cs3Cu2I5-based memristor shows considerable promise for applications in storage systems and artificial synapses.

In Situ Annealing Effect on Thermally Co-Evaporated CsPbI2Br Thin Films Studied via Spectroscopic Ellipsometry.

All-inorganic cesium lead halide perovskites possess excellent thermal stability, a feature that renders them highly favorable for optoelectronic applications with an elevated thermal budget. Employing a coevaporation approach for their deposition holds promise for manufacturing at an industrial level, owing to improvements in device scalability and reproducibility. For unlocking the full potential of vacuum-evaporated perovskite thin films, it is crucial to delve deeper into their crystallization process, which, as a solid-state reaction, has been less investigated compared to the crystallization process of, most commonly used, solution-based methods. In this work, we employ spectroscopic ellipsometry, a nondestructive, high speed, and high accuracy characterization method, to study the real time annealing effect on thermally coevaporated CsPbI2Br thin films in a temperature range between 25 and 300 °C. We achieve this by developing a singular dynamic model that can be fitted in real time as a function of temperature, providing insights into how thermal annealing influences the perovskite film's morphology and optical constants. Based on the latter, we derive the temperature dependence of the thermo-optic coefficient and Urbach energy as well as analyze the interband transition energies via critical point analysis. We demonstrate that the γ- to β-phase transition can be identified through a pronounced shift in the bandgap energy, whereas the β- to α-phase transition can be discerned by a sharp increase in the film's roughness. We corroborate the obtained fit results with additional in- and ex situ measurements, such as in situ grazing incidence wide-angle X-ray scattering, atomic force microscopy, reflectance/transmittance, and profilometry.

The synergy of pre-frozen substrates and post-annealing for high-efficiency co-evaporated blue perovskite LEDs

The co-evaporation technique shows great potential for producing perovskite films. However, there is a lack of research on the growth process and the film characteristics of co-evaporated perovskite. Herein, we found that pre-frozen substrates and post-annealing can synergically improve the performance of co-evaporated blue perovskite light-emitting diodes (PeLEDs). The optimized blue PeLEDs obtained an impressive peak external quantum efficiency (EQE) of 7.2%, which is the best reported for co-evaporated blue PeLEDs till now. The systematic investigation found that the pre-frozen substrates can facilitate the exotherm of vaporized precursor molecules near the interface, leading to more efficient nucleation and constraining grain enlargement. Furthermore, low-temperature annealing (LTA) can efficiently suppress compositional segregation and release residual compressive strain in co-evaporated perovskite films while retaining the advantages of pre-frozen substrates. This work provided deeper insights into perovskite films prepared by co-evaporation and offered a feasible strategy for enhancing the performance of the co-evaporated PeLEDs.

Co-evaporated oriented DMA1-xCsxPbI3 perovskite films for photovoltaics

Thermally evaporated cesium lead triiodide (CsPbI3) is a frontier research direction for perovskite-silicon tandem solar cells because of the matching bandgap and good conformality with textured silicon. However, the random orientation and small grains for the co-evaporated CsPbI3 thin films fabricated at low temperature, resulting in harmful defects and non-radiative recombination. Here, dimethylammonium iodide (DMAI) is introduced into the co-evaporation system at a substrate temperature of 50 °C to prepare preferentially oriented DMA0.06Cs0.94PbI3 film with less defects and enhanced humidity stability. The bandgap also can be reduced from 1.76 eV (γ-CsPbI3) to 1.73 eV. The p-i-n photovoltaic device based on this film achieves an efficiency of 16.10% with suppressed non-radiative recombination, rivaling with the thermally evaporated wide-bandgap CsPbI3 solar cells prepared at a high temperature (∼ 350 °C). This in-situ evaporated alloying strategy can pave the way for the crystallization and defect control during perovskite co-evaporation.

Annealing Effect on the Properties of Co-evaporated Cu4SnS4 Films

Annealing Effect on the Properties of Co-evaporated Cu4SnS4 Films

Variation of Charge Carrier Dynamics for Polycrystalline Perovskite Films by One-Step Solution and Vapor-Deposition Methods.

To date, solution-processing and vapor-deposition fabrication methods have achieved huge successes in high-efficiency perovskite solar cells (PSCs) and satisfy special demanding requirements for diverse application purposes, respectively. Although people realize that the fabrication procedure is crucial in device performance, insightful studies of charge carrier dynamics in perovskite films by different methods still lack. In this work, we compare the carrier behaviors in one-step spin-coated and dual-source coevaporated MAPbI3 perovskite films by combining time-resolved photoluminescence spectroscopy and carrier dynamics simulation. We suggest that strains, lattice orientations, and defects at buried side of perovskite films, which are associated with different preparation processes, lead to variations in carrier behaviors. Hence fabrication of perovskite layers should be elaborately designed in order to satisfy the needs of different carrier behaviors in specified device configurations of PSCs such as smooth planar or textured monolithic tandem structures.

Charge Carrier Dynamics in Co-evaporated MAPbI3 with a Gradient in Composition

Co-evaporation of metal halide perovskites by thermal evaporation is an attractive method since it does not require harmful solvents and enables precise control of the film thickness. Furthermore, the ability to manipulate the Fermi level allows the formation of a graded homojunction, providing interesting opportunities to improve the charge carrier collection efficiency. However, little is known about how these properties affect the charge carrier dynamics. In this work, the structural and optoelectronic properties of co-evaporated MAPbI3 films varying in thickness (100, 400, and 750 nm) with a gradient in composition are analyzed. The X-ray diffraction patterns show that excess PbI2 is only present in the thick layers. From X-ray photoelectron spectroscopy depth analysis, the I/Pb atomic ratio indicates methylammonium iodide deficiencies that become more prominent with thicker films, resulting in differently n-doped regions across the thick MAPbI3 films. We suggest that due to these differently n-doped regimes, an internal electric field is formed. Side-selective time-resolved microwave photo conductivity measurements show an elongation of the charge carrier lifetimes on increasing thickness. These observations can be explained by the fact that excess carriers separate under the influence of the electric field, preventing rapid decay in the thick films.

Direct synthesis of BaTaO2N nanoparticle film on a conductive substrate for photoelectrochemical water splitting

Perovskite BaTaO2N has been regarded as a promising photoanode material owing to its narrow bandgap of 1.8 eV and appropriate band edge positions for photoelectrochemical (PEC) water splitting. Compared with photoanodes made with pre-synthesized BaTaO2N particles, direct deposition of BaTaO2N thin film on a conductive substrate is a promising way to improve the carrier transport capability. Herein, we propose a co-evaporation and nitridation method for the direct synthesis of BaTaO2N nanoparticle film on metal substrate for PEC water splitting. The crystallinity, morphology, and defects of the synthesized BaTaO2N nanoparticle films are varied by controlling the Ba:Ta ratios in the co-evaporated precursor films. Photoanode based on the optimized BaTaO2N nanoparticle film achieves a photocurrent density of 4.7 mA cm−2 at 1.23 VRHE and a maximum applied bias photon-to-current efficiency of 1.18% at 0.83 VRHE. Our work provides a facile method to fabricate high-crystallinity nanostructured oxynitride thin films for PEC water splitting application.

Temperature-dependent photoluminescence of Co-evaporated MAPbI3 ultrathin films

The hybrid organic–inorganic perovskite (PVK) thin films have attracted amounts of interest in the past decade due to the thickness-depended quantum confinement effects and tunable band gap as well as the demands of device miniaturization. Vacuum deposition is a promising method to realize precise control of film thickness and composition, low-temperature processing, the good compatibility with dry processing and compatible with the modern silicon-based semiconductor industry. Here, we report on the temperature-dependent photoluminescence properties of MAPbI3 ultrathin films with long-term stability and high quality fabricated by the co-evaporation. XPS, AFM and XRD results reveal that an excess MAI vapor environment plays a key role in sample preparation. PL is more sensitive than XPS to detect trace MAPbI3-like materials with strong luminescence properties. Temperature- dependent PL measurements show that the bandgap of MAPbI3 films red-shifted with the temperature decreasing in the range from 298 to 78 K, which is opposite to that of the traditional semiconductors and can be attributed to the interaction between the electron–phonon and the thermal expansion induced band gap renormalization. Gradual phase transition from tetragonal one to orthorhombic one is observed to start between 108 and 118 K, which is lower than the reported sudden phase transition temperature of 150 K for bulk MAPbI3 and suggests the important role of quantum confinement in ultrathin films. Our findings provide a way to fabricate PVK ultrathin films with long-term stability for novel light emitting devices.

Can Vacuum Deposition Apply to Bismuth-Doped γ-CsPbI3 Perovskite? Revealing the Role of Bi3+ in the Formation of Black Phase.

The B-site doped CsPbI3 has been demonstrated to be very promising for photovoltaics owing to its low black phase transition temperature. Though B-site doped black-CsPbI3 perovskites have been successfully achieved by solution-processing, it is unclear whether these systems are available by other methods such as vacuum deposition. In this work, heterovalent doped CsPb1-xBixI3 is targeted. To incorporate Bi3+ into the final film via vacuum deposition, the solid solution precursor Pb1-xBixI2 (0.01 ≤ x ≤ 0.04) is developed. However, these coevaporated films not only are dominated by another hexagonal perovskite phase but also fail to decrease the black phase transition temperature. The role of Bi3+ in the formation of the black phase is further studied by solution methods with different types of precursors. It is demonstrated that the key factor in the low-temperature black phase transition is small grain size, as well as the colloid size within the precursor solution, rather than simple substitution of Pb2+ with Bi3+.

Effective permittivity of co-evaporated metal-organic mixed films

The combination of organics and metals in a composite film holds promise for combining plasmonic interaction with gain and for the realization of epsilon-near-zero (ENZ) metamaterials. In particular, fluorescent organic dyes can be used to compensate the plasmonic losses of a homogenized metal-organic material. Here, we fabricate such films through thermal co-evaporation of silver and an organic host:guest system and investigate experimentally the resulting linear optical properties for varying metal concentrations. We extract the effective permittivity of the resulting films with ellipsometry measurements and demonstrate the formation of silver nanoparticles, resulting in strongly localized surface plasmon resonances until a percolation threshold is reached. Through enhanced light-matter interaction, we observe a maximum of the photoluminescence for a concentration of 15% in volume of metal in the composite material. These results showcase a variety of growth parameters and will be useful for the future design of gain-compensated plasmonics and ENZ metamaterials.

Fabrication, micro-structure characteristics and transport properties of co-evaporated thin films of Bi2Te3 on AlN coated stainless steel foils

N-type bismuth telluride (Bi2Te3) thin films were prepared on an aluminum nitride (AlN)-coated stainless steel foil substrate to obtain optimal thermoelectric performance. The thermal co-evaporation method was adopted so that we could vary the thin film composition, enabling us to investigate the relationship between the film composition, microstructure, crystal preferred orientation and thermoelectric properties. The influence of the substrate temperature was also investigated by synthesizing two sets of thin film samples; in one set the substrate was kept at room temperature (RT) while in the other set the substrate was maintained at a high temperature, of 300 °C, during deposition. The samples deposited at RT were amorphous in the as-deposited state and therefore were annealed at 280 °C to promote crystallization and phase development. The electrical resistivity and Seebeck coefficient were measured and the results were interpreted. Both the transport properties and crystal structure were observed to be strongly affected by non-stoichiometry and the choice of substrate temperature. We observed columnar microstructures with hexagonal grains and a multi-oriented crystal structure for the thin films deposited at high substrate temperatures, whereas highly (00 l) textured thin films with columns consisting of in-plane layers were fabricated from the stoichiometric annealed thin film samples originally synthesized at RT. Special emphasis was placed on examining the nature of tellurium (Te) atom based structural defects and their influence on thin film properties. We report maximum power factor (PF) of 1.35 mW/m K2 for near-stoichiometric film deposited at high substrate temperature, which was the highest among all studied cases.

Substrate-dependent Growth of CH3NH3PbI3 Films Deposited by Vacuum Evaporation

CH3NH3PbI3 (MAPbI3) films were prepared by dual-source vacuum evaporation on Au, poly(3,4-ethylenedioxythiophene) (PEDOT), and indium-tin-oxide(ITO) with the similar deposition parameters. The films were characterized with X-ray diffraction (XRD), steady-state photoluminescence (PL) and Raman spectroscopy. The interface electronic structures of co-evaporated MAPbI3 films on different substrates were studied with ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). The research indicates that Au is more suitable for the film growth than the other substrates, especially when the film is very thin. The poor adsorption of the precursors may make it difficult to form MAPbI3 thin film on ITO. Furthermore, it is found that the charge transfer efficiency at the interface between PEDOT and MAPbI3 is relatively high, which indicates that PEDOT can act as an effective hole transport layer for MAPbI3-based devices.

Effect of substrate temperature on properties of co-evaporated copper antimony sulfide thin films

In recent years, a wide variety of copper based chalcogenides have been investigated to address the need of sustainable photovoltaic materials. The ternary chalcostibite copper antimony sulfide (CuSbS2) is an emerging semiconductor in photovoltaics, optoelectronic devices and energy storage applications due its suitable band gap, high absorption coefficient and environment friendly nature. CuSbS2 thin films were deposited on glass substrates using the co-evaporation method at various substrate temperatures. The films are characterized by X-ray diffractometer, UV–Vis-NIR spectroscopy and Hall-effect measurement to study the impact of substrate temperature on structural, optical and electrical properties. The Photoresponse measurements were carried out for all films under the solar simulator. The composition of the films was confirmed using Energy dispersive X-ray spectroscopy analysis. The valance states of the constituent elements were found by X-ray photoelectron spectroscopy analysis. The energy band gap values of CuSbS2 films are in the range of 1.58–1.70 eV. All films are photoconductive and show p-type conductivity. The present studies confirm that CuSbS2 can be a potential absorber layer in thin film solar cells.

Vertical Self-Defined Thin-Film Thermoelectric Thermocouples by Angled Co-Evaporation for Use in μTEGs

This paper reports the development of a novel thin-film thermocouple structure and associated thermoelectric films for use in micro-thermoelectric generators. The proposed structure enables thermocouple length to be independent of film thickness in vertical thin-film μTEGs, allowing greater thermal resistances with thin-film designs while maintaining fill factor. In the presented structure, thermoelectric thin-films are evaporated over the sidewalls of high aspect-ratio columns. Thermocouple length is thus set by the height of the columns rather than the thickness of the deposited film. The process takes advantage of the shadowing of these columns to form N & P thermocouples on the left and right side of the columns. Also reported is development of thermally co-evaporated thin films Bi2Te3/Sb2Te3 onto vertical surfaces for use. Integration into proof of concept design TEG structure is briefly detailed.

(Invited) Tunable and Efficient Hot Carrier Generation from Plasmonic Au-Pd Nanoalloy Photocatalyst

Noble metals are the most studied plasmonic materials due to their chemical stability, small ohmic losses and high DC conductivity. However, the majority of electronic states in the valence band of these metals require at least 2 eV to excite. Therefore, even Au, which is often used for near-infrared hot carrier applications, has a band structure that is not ideal for this low energy wavelength regime. One possible route to improving the efficiency of hot carrier generation in noble metals at near-infrared wavelengths, is to shift their electronic density of states (EDOS) closer to the Fermi level via alloying with a transition metal. Here, we present the tuning of optical and electronic properties of co-evaporated Au-Pd alloy thin films at different compositions by taking ultraviolet photoelectron spectroscopy (UPS) measurements to represent the EDOS, ellipsometry measurements to show the modulated dielectric function and the calculated plasmonic quality factors, and infrared-pump/THz-probe spectroscopy measurements for studying the dynamics of the hot carrier generation in the near-infrared regime. For comparison, calculated EDOS and hot carrier distributions for these alloys are also shown. Furthermore, we characterized the alloy thin film structural properties by grazing incidence x-ray diffraction (GIXRD). Our GIRXD results show that all Au-Pd alloy thin films are true alloys (single phase). UPS data supports our initial hypothesis that adding Pd to Au considerably increases the EDOS in the near-infrared region. The calculated hot carrier distribution show that adding Pd to Au increases the number of carriers generated, and that the Au-Pd alloys thin films generate hotter holes than electrons. Finally, time-dependent changes in ∆E/E from 1550 nm-pump/ THz-probe spectroscopy illustrate that for Pd and Au-Pd alloys there is a change in the conductivity attributed to contributions arising from both thermal and nonthermal electron distribution, while for Au there is no evidence of transitions under 1550 nm pump-excitation. We hypothesize this is due to a lower generation rate of hot carriers in Au than for Pd and Au-Pd alloys in the near-infrared regime.

Study on thermoelectric properties of co-evaporated Sn-Se films with different phase formations

In this study, polycrystalline thin films of 330 nm in thickness with different SnSe and SnSe2 contents were prepared by co-evaporation of Sn and Se through adjusting source temperatures and substrate temperature. The result illustrates that the film with pure SnSe phase is p-type and consists by bulk-like nanoparticles, its resistivity is high up to 139 mΩ·m, which results in a low thermoelectric performance. The films with the coexistence of SnSe and SnSe2 phases are comprised of nanosheets. The existence of SnSe2 phase changes the conductivity type of the films to n-type and reduces resistivity. At the same time, the nanosheets perpendicular to the substrate can improve the Seebeck coefficient by filtering the low energy carrier. When the SnSe2 phase content is 34%, the Seebeck coefficient and the power factor of the film can reach the maximum, 255 μV/K and 155 μW/(m·K2) at 250 °C, respectively.

Growth, morphology and structure of mixed pentacene films

This work investigates the evolution of structure and morphology of pure and co-evaporated thin films, as a function of pentacene concentration in a solid host of p-terphenyl.

Voids and compositional inhomogeneities in Cu(In,Ga)Se2 thin films: evolution during growth and impact on solar cell performance

ABSTRACTStructural defects such as voids and compositional inhomogeneities may affect the performance of Cu(In,Ga)Se2 (CIGS) solar cells. We analyzed the morphology and elemental distributions in co-evaporated CIGS thin films at the different stages of the CIGS growth by energy-dispersive x-ray spectroscopy in a transmission electron microscope. Accumulation of Cu-Se phases was found at crevices and at grain boundaries after the Cu-rich intermediate stage of the CIGS deposition sequence. It was found, that voids are caused by Cu out-diffusion from crevices and GBs during the final deposition stage. The Cu inhomogeneities lead to non-uniform diffusivities of In and Ga, resulting in lateral inhomogeneities of the In and Ga distribution. Two and three-dimensional simulations were used to investigate the impact of the inhomogeneities and voids on the solar cell performance. A significant impact of voids was found, indicating that the unpassivated voids reduce the open-circuit voltage and fill factor due to the introduction of free surfaces with high recombination velocities close to the CIGS/CdS junction. We thus suggest that voids, and possibly inhomogeneities, limit the efficiency of solar cells based on three-stage co-evaporated CIGS thin films. Passivation of the voids’ internal surface may reduce their detrimental effects.

Thickness effects on the microstructure and electrical/thermoelectric properties of co-evaporated Bi-Te thin films

In this study, we investigated the effect of film thickness on the electrical and thermoelectric properties of bismuth-tellurium (Bi-Te) films. Bi-Te films of 1-, 4-, 10-, and 18-μm thicknesses were deposited via co-evaporation. Microstructural analyses using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy indicated columnar film growth, with a highly porous structure that increased with the film thickness. The electron mobility of the films decreased significantly as the film thickness increased, which may be explained by the film porosity. Given a fairly constant Seebeck coefficient, the power factor decreased significantly with film thickness: 2.8 mW/mK2 for the 1-μm-thick film and 1.5 mW/mK2 for the 18-μm-thick Bi-Te film.