Vacuum Co-evaporation Research Articles

Photoactive Properties of Transport Sol-Gel Layers Based on Strontium Titanate for Perovskite Solar Cells

In this work, we have investigated the photocurrent and spectral sensitivity of the silicon/SrTiO3:xNb/perovskite structures. The sol–gel method carried out the deposition of undoped SrTiO3 layers as well as niobium-doped (SrTiO3:Nb) layers at atomic concentrations of 3 and 6% Nb. The perovskite layer, CH3NH3PbI3−xClx, has been deposited by the vacuum co-evaporation technique. The layers have been characterized by scanning electron microscopy and X-ray diffraction measurements. The volt–ampere characteristics and spectral sensitivity of the fabricated samples have been measured under illumination with selective wavelengths of 405, 450, 520, 660, 780, 808, 905, 980, and 1064 nm of laser diodes. We have shown that for different configurations of applied voltage between silicon, SrTiO3:xNb, and CH3NH3PbI3−xClx, the structures are photosensitive ones with a variation of photocurrent from microamperes to milliamperes depending on Nb concentration in SrTiO3, and the highest photocurrent and spectral sensitivity values are observed when a SrTiO3:Nb layer with 3 at.% of Nb is used. A possible application of the proposed structure with a SrTiO3:Nb layer for perovskite solar cells and photodetectors is being discussed.

Luminescent and Raman study of nanostruc-tures formed upon annealing of SiOx:Sm films

ncs-Si–SiO x :Sm structures formed by high-temperature air annealing of the SiO x films doped with samarium during thermal co-evaporation in vacuum of silicon monoxide and metallic Sm was studied. By measuring the spectra of photoluminescence (PL), it has been shown that doping of SiO x films with Sm stimulates their decomposition into Si and SiO 2 , and also reduces the transition temperature of silicon nanoparticle from the amorphous state to the crystalline one. With an increase in the impurity content up to 2 wt.%, along with the ncs-Si luminescence, the PL spectrum exhibits emission bands of Sm 3+ and Sm 2+ ions, which do not appear at a lower Sm concentration. The presence of silicon nanocrystals in SiO x films doped with Sm and annealed at 970 °C in air has been confirmed using Raman scattering spectra. A possible mechanism for interaction of samarium ions with the SiO x matrix and ncs-Si has been discussed.

Structural and Optical Properties of Vacuum‐Evaporated Mixed‐Halide Perovskite Layers on Nanotextured Black Silicon

To improve the absorption of light in two‐junction tandem perovskite–silicon solar cells, antireflective textures are often created on the front surface in the form of micronsized pyramids using wet‐chemical etching. It is known that single‐junction silicon solar cells with nanotextured black silicon (BS) formed by dry‐etching techniques are more efficient than pyramidal microtextures. Herein, experimental evidence of enhanced efficiency of structures using BS between the perovskite layer and the silicon substrate for the first time is demonstrated. BS is fabricated using plasma‐based reactive‐ion etching on the front surface of the crystalline silicon. The deposition of mixed‐halide perovskite layers is performed by vacuum coevaporation and sequential evaporation. The structural and optical properties of evaporated perovskite layers with various thicknesses are investigated. It is found that the structural‐phase quality (viz., surface coverage, chemical composition, dimensionality, crystallinity, and phase purity) of perovskite layers on planar and nanotextured substrates is similar. However, the root mean square roughness of the perovskite layers on nanotextured substrates is much higher, which reduces the reflection significantly, especially, in the range of radiation incidence angle up to 60°. These results pave the way for the development of high‐performance perovskite–silicon tandem solar cells for improved efficiency.

Ultra-broadband light-emitting diodes from co-evaporated lead-free CsCu2I3

• CsCu 2 I 3 films were prepared by vacuum co-evaporation method. • Co-evaporated CsCu 2 I 3 film exhibits self-trapped exciton behavior. • LEDs using the CsCu 2 I 3 films show red-shifted and ultra-broad EL. As an alternative to the conventional solution-processing, a vacuum co-evaporation method is developed to prepare CsCu 2 I 3 thin film, for fabricating light-emitting diodes (LEDs). LEDs using the co-evaporated lead-free CsCu 2 I 3 films exhibit ultra-broad warm-white emission. We found that the LEDs using the CsCu 2 I 3 thin films have red-shift and broadened electroluminescence (EL) with a full width at half-maximum of 245 nm in contrast to 132 nm from photoluminescence (PL) spectra for the CsCu 2 I 3 films. The current hysteresis of LEDs using the CsCu 2 I 3 films indicates that the migration of ions provides a new radiative-recombination center and further resulting in EL change and broadening. The experimental work presented here gains insight on broadening EL spectra for the LEDs with self-trapped exciton materials.

Efficient hydrogen evolution from water over thin film photocathode composed of solid solutions between ZnSe and Cu(In, Ga)Se2 with composition gradient

Solid solutions between ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) have promising properties as photocathodes for solar hydrogen production from water, such as a long absorption edge of about 900 nm and a large driving force for reaction, >0.9 V, originated by a deep valence band maximum of 1.0–1.1 V vs normal hydrogen electrode (NHE). However, their performance is limited with an incident photon-to-current conversion efficiency (IPCE) of 65% at 400 nm and a half-cell solar-to-hydrogen energy conversion efficiency (HC-STH) of 3.6% so far. Te addition during ZnSe:CIGS thin film deposition by vacuum co-evaporation clearly decreased the optimal deposition temperature from 450 to 380 °C and resulted in columnar shaped grains of submicrometer size in diameter and with almost no grain boundary between the film surface and the backside electrode of Mo, which is significantly larger than the case of without Te addition. Interestingly, a Ga/In compositional ratio gradient was further introduced to the depth profile, which can facilitate charge separation. Structural characterizations using XRD and cross-sectional transmission electron microscopy revealed that the composition gradient was mainly formed by the diffusion of In through grain boundaries in the Ga-rich layer, and thus, the mixing between the Ga-rich and In-rich layers was more significant at a lower deposition temperature owing to the smaller grain size. The photocathode fabricated from the ZnSe:CIGS thin film with the composition gradient showed a very high IPCE of 89% at 540 nm and 0 V vs the reversible hydrogen electrode, and HC-STH of 3.7%, which is higher than values reported thus far. In conclusion, it is clarified that the potential gradient, which can be introduced by composition gradient, is beneficial for photoelectrodes and photocatalysts to achieve higher performance.

Electrical and ion beam analyses of yttrium and yttrium-titanium getter thin films oxidation

Yttrium, titanium, and yttrium-titanium getter thin films were elaborated on silicon by coevaporation in ultrahigh vacuum. Y-Ti films exhibit nanometric crystallites size (18–35 nm) leading to a very high grain boundary density, which is a favorable microstructure for activation at low temperature. The yttrium content in Y-Ti alloys influences grain size, resistance against room temperature oxidation, and gettering performance for oxygen. Y-Ti films with an yttrium content higher than 30% show strong oxygen sorption during annealing at low temperature (<300 °C). After 1 h of annealing at 250 °C, it was estimated that the yttrium-based getter films can trap between 0.2 and 0.5 μmol of oxygen per cm2, while no oxygen sorption was detected for a single metal titanium film. This makes Y-Ti getter alloys attractive candidates for the packaging of MEMS under vacuum with a low bonding temperature.

Highly efficient and stable inorganic CsPbBr3 perovskite solar cells via vacuum co-evaporation

High power conversion efficiency (PCE) and good long-term stability are everlasting pursuit for perovskite solar cells. We present here CsPbBr3 perovskite film deposited by vacuum co-evaporation of CsBr and PbBr2. By optimizing evaporation rate ratio of the two precursor and annealing temperature, perovskite films with large grain and high crystallinity are obtained. Furthermore, by regulating spinning speed of TiO2 precursor solution and thickness of CsPbBr3 film, a best PCE of 9.43% is recorded under 100 mW cm−2 illumination, providing a significant enhancement in contrast with 6.26% of device based on spin-coating method. Additionally, the PCE of resulting perovskite solar cell without encapsulation remains at 96% of initial value after storing for 480 h under humidity of 40% in air. Our findings in this work deliver an effective strategy to fabricate high-performance inorganic perovskite solar cells.

Optical and dispersion parameters of co-evaporated SnS0.7Se0.3 thin films

Tin sulfoselenide (SnS 1-x Se x ) is a novel ternary semiconductor that belongs to the IV – VI group of compounds. It possesses the properties of both SnS and SnSe, which are proved as good absorber materials for solar cell fabrication. In the present work, SnS 0.7 Se 0.3 thin films were deposited by vacuum co-evaporation technique at four different substrate temperatures ( T s ), 200 °C, 250 °C, 300 °C and 350 °C. The optical properties of as-deposited SnS 0.7 Se 0.3 films were analyzed using optical measurements. From the optical transmittance and reflectance data, the band gap energy of the layers was determined that varied in the range, 1.59–1.46 eV. The other optical parameters such as skin depth, refractive index, extinction coefficient, optical conductivity and dielectric constant of SnS 0.7 Se 0.3 layers were determined and discussed as a function of substrate temperature. Moreover, the dispersion parameters of the layers were also evaluated using Wemple – DiDomenico (WDD) single oscillator model. The dispersion energy ( E d ) and oscillator energy ( Ε o ) of the films were evaluated and varied in the range, 2.19–2.38 eV and 7.55–9.52 eV respectively. Moreover, the optical dispersion moments ( M −1 and M −3 ) were also calculated that were varied from 0.25 to 0.29 and from 2.76 × 10 -3 to 5.08 × 10 -3 respectively.

Enhanced current collection of CdTe solar cells in the long wavelength region by co-evaporation deposition CdSexTe1-x films

Using a gradient band gap absorption layer to ensure high short-circuit current density is one of the key factors to prepare high-efficiency CdTe solar cells. CdSexTe1-x films with different selenium compositions were prepared by vacuum co-evaporation deposition technology. The effects of composition changes on the structure, morphology and properties of the films were studied. CdSexTe1-x films were incorporated into the CdTe solar cells with the structure of Glass/SnO2:F/CdS/CdSexTe1-x/CdTe/ZnTe:Cu/Au. The results show that the structure of CdSexTe1-x films is cubic when x is 0, 0.23 or 0.41, and while x is 0.61, 0.78 or 1 the structure is hexagonal. The minimum optical band gap is 1.40 eV with x = 0.41. It is found that in the long wavelength region, the cut-off edge of quantum efficiency curve depends on not only the thickness of CdSexTe1-x film with specific Se composition, but also the whole preparation process of solar cells. After using a 70 nm CdSe0.41Te0.59 film and 70 nm ZnTe:Cu back contact buffer layer, the cut-off edge of the long wavelength region can be reduced to 1.40 eV and the short-circuit current density of the device without antireflection layer reaches up to 28.8 mA/cm2.

All-vacuum-deposited inorganic cesium lead halide perovskite light-emitting diodes

Polycrystalline CsPbBr3 thin films are deposited by vacuum co-evaporation of cesium halide and lead halide precursors, leading to uniform pinhole-free morphology and precise control over the film thickness and precursor stoichiometry. By utilizing the organic hole and electron transport layers, all-vacuum-deposited perovskite LEDs are fabricated. The resulting devices exhibit a maximum luminance of 1800 cd/m2, a 531 nm emission wavelength peak with a spectral linewidth of 21 nm, and an external quantum efficiency of 1.1%.

Determination of the complete set of optical parameters of micron-sized polycrystalline CH3NH3PbI3−xClx films from the oscillating transmittance and reflectance spectra

The complete set of optical parameters of micron-sized polycrystalline CH3NH3PbI3−xClx films deposited by the vacuum co-evaporation of lead iodide and methylammonium chloride is determined by analysis of oscillating optical transmittance and reflectance spectra in the wavelength range 400–1000 nm. It is shown that for a medium and weak absorption region the envelope method is valid for the extraction of refractive index, extinction and absorption coefficient when is using only transmittance spectra. As well thickness of the film is determined from transmittance and reflectance spectra with interference-effect. The absorption coefficient for the strong absorption region and optical band gap (direct transition at Eopt = 1.62 eV) are calculated based on transmittance and reflectance spectra by using conventional approximated formulas.

Resolving the detrimental interface in co-evaporated MAPbI3 perovskite solar cells by hybrid growth method

Abstract Among various processing methods for growing perovskite thin layers, dual-source vacuum deposition was once regarded as the most economical and high throughput method. However, co-evaporation of MAI and PbI2 often leads to an inevitable PbI2 interfacial layer formed by the initial growth of evaporation resulting in a compromised efficiency of vacuum deposited MAPbI3 solar cell. Here, two modified growth methods based on vacuum co-evaporation, namely solution-vacuum hybrid method (SVHM) and soaking-assistant fully-vacuum method (SAFVM), are developed to grow a completely converted MAPbI3 perovskite layer at the interface prior to the co-evaporation of MAPbI3 perovskite layer. Upon these two methods, the unwanted PbI2 layer can be removed from the interface resulting in much improved power conversion efficiency up to 14.35%. Our result shows that, while unwanted interfacial PbI2 layer is the main cause of low performing co-evaporated perovskite solar cell devices, by removing the PbI2 interfacial layer we can unlock the full potential of vacuum-based perovskite solar cells to industrial mass production.

Effect of Ion Beam Assisted Deposition technology on the cathode of solar cells

We realized organic solar cells on ITO anode based on small – molecular materials (CUPC:C60) to be easy to fabricate, cost price, production on large surfaces, efficiency, flexibility and large market of solar panels, but the organic materials are limited by their lifetime. Then, on the ITO we deposited the organic materials by using the vacuum co-evaporation deposition. Cathode is a silver layer to protect the organic layer from the humidity. The penetrations of humidity from the cathode to the organic layer give to organic electronic components a short lifetime, and limit the fabrication of organic solar panels. In order to optimize the lifetime of organic solar cells, we use in this study the IBAD’s technology to deposit the cathode instead of vacuum co-evaporation deposition. The goal of this technology is to reduce the permeation of H2O and O2 from the cathode to the organic layer, by using the argon ions to assist the aluminium deposition of cathode. We realized two series of cells under vacuum and do not exposed to the air. The first, with cathode deposit by the vacuum co-evaporate deposition, but the second we use the IBAD technology to deposit the cathode (60nm unassisted and 40nm deposited with ionic assistance), after we compare their lifetime in the ambient air and under continuous illumination.

Deviations from bulk behavior in TbFe(Co) thin films: Interfaces contribution in the biased composition

Ferrimagnetic TbFe or TbFeCo amorphous alloy thin films have been grown by co-evaporation in ultra-high vacuum. They exhibit an out-of-plane magnetic anisotropy up to their Curie temperature with a nucleation and propagation reversal mechanism suitable for current induced domain wall motion. Rutherford back scattering experiments confirmed a fine control of the Tb depth-integrated composition within the evaporation process. However, a large set of experimental techniques were used to evidence an interface related contribution in such thin films as compared to much thicker samples. In particular, scanning transmission electron microscopy experiments evidence a depth dependent composition and perturbed top and bottom interfaces with preferential oxidation and diffusion of terbium. Despite of that, amorphous and homogeneous alloy film remains in a bulk-like part. The composition of that bulk-like part of the magnetic layer, labeled as effective composition, is biased when compared with the depth-integrated composition. The magnetic properties of the film are mostly dictated by this effective composition, which we show changes with different top and bottom interfaces.

Structure of superconducting MgB2 thin films prepared by vacuum evaporation and ex-situ annealing in Ar and O2 atmospheres

Superconducting MgB2 thin films were fabricated on c-cut sapphire substrates by vacuum co-evaporation of magnesium and boron followed by an ex situ post-annealing. We show that annealing in oxygen atmosphere can significantly improve the superconducting properties of the MgB2 thin films compared to the typical annealing in argon atmosphere. In this work, influence of the ex-situ annealing atmosphere on the structure, texture and morphology of the superconducting MgB2 thin films has been studied by transmission electron microscopy, X-ray diffraction and pole figure measurements. Samples annealed at 800 °C in Ar, at 800 °C in O2 and at 500 °C in O2 have been compared. The annealing in O2 at 800 °C produces MgB2 thin films with the highest superconducting transition temperature and critical current density. We show that this is thanks to the thickest MgO layer at the surface produced in this case, which acts as a protecting barrier against out-diffusion of Mg during the annealing and leads to better stoichiometry and larger MgB2 grains compared to the samples annealed in Ar. Our method can be alternative to the customary ex-situ post annealing of Mg-B precursor in sealed vapor cell. In all samples, the MgB2 phase showed single axis texture with the (0 0 0 1) planes slightly inclined by 0° to 15° with respect to the Al2O3(0 0 0 1) substrate surface incurred with the initial precursor evaporation geometry. During the annealing, the excess Mg also reacts with the Al2O3 substrate and minor MgAl2O4 and MgO phases are produced at the substrate-layer interface.

A Semitransparent Inorganic Perovskite Film for Overcoming Ultraviolet Light Instability of Organic Solar Cells and Achieving 14.03% Efficiency.

Organic solar cells (OSCs) can be unstable under ultraviolet (UV) irradiation. To address this issue and enhance the power conversion efficiency (PCE), an inorganic-perovskite/organic four-terminal tandem solar cell (TSC) based on a semitransparent inorganic CsPbBr3 perovskite solar cell (pero-SC) as the top cell and an OSC as bottom cell is constructed. The high-quality CsPbBr3 photoactive layer of the planar pero-SC is prepared with a dual-source vacuum coevaporation method, using stoichiometric precursors of CsBr and PbBr2 with a low evaporation rate. The resultant opaque planar pero-SC exhibits an ultrahigh open-circuit voltage of 1.44 V and the highest reported PCE of 7.78% for a CsPbBr3 -based planar pero-SC. Importantly, the devices show no degradation after 120 h UV light illumination. The related semitransparent pero-SC can almost completely filter UV light and well maintain photovoltaic performance; it additionally shows an extremely high average visible transmittance. When it is used to construct a TSC, the top pero-SC acting as a UV filter can utilize UV light for photoelectric conversion, avoiding the instability problem of UV light on the bottom OSC that can meet the industrial standards of UV-light stability for solar cells, and leading to the highest reported PCE of 14.03% for the inorganic-perovskite/organic TSC.

High-Efficiency Nanoparticle Solution-Processed Cu(In,Ga)(S,Se)2Solar Cells

Thin film Cu(In,Ga)(S,Se) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based (generally referred to as CIGS) solar cells represent a promising alternative to conventional crystalline silicon solar cells due to their high efficiencies, reduced cost, and better material utilization. In recent years, it has been demonstrated that it is possible to form thin films by annealing nanoparticulate material such that the nanoparticles coalesce to form large grained thin films. In this paper, we present a 13.8% efficient CIGS solar cell derived from printed nanoparticle inks. The approach was successfully extended to fabricate monolithic devices on larger substrates. These results demonstrate that low-cost, nonvacuum printing of CIGS nanoparticles has great potential to achieve high efficiencies and reduce the performance gap with the more traditional vacuum co-evaporation and sputtering techniques.

Superior stability for perovskite solar cells with 20% efficiency using vacuum co-evaporation.

Chemical composition and film quality are two key figures of merit for large-area high-efficiency perovskite solar cells. To date, all studies on mixed perovskites have used solution-processing, which results in imperfect surface coverage and pin-holes generated during solvent evaporation, execrably influencing the stability and efficiency of perovskite solar cells. Herein, we report our development using a vacuum co-evaporation deposition method to fabricate pin-hole-free cesium (Cs)-substituted perovskite films with complete surface coverage. Apart from the simplified procedure, the present method also promises tunable band gap, reduced trap-state density and longer carrier lifetime, leading to solar cell efficiency as high as 20.13%, which is among the highest reported for planar perovskite solar cells. The splendid performance is attributed to superior merits of the Cs-substituted perovskite film including tunable band gap, reduced trap-state density and longer carrier lifetime. Moreover, the Cs-substituted perovskite device without encapsulation exhibits significantly higher stability in ambient air compared with the single-component counterpart. When the Cs-substituted perovskite solar cells are stored in dark for one year, the PCE remains at 19.25%, degrading only 4.37% of the initial efficiency. The excellent stability originates from reduced lattice constant and relaxed strain in perovskite lattice by incorporating Cs cations into the crystal lattice, as demonstrated by the positive peak shifts and reduced peak width in X-ray diffraction analysis.

Low temperature electrical conductivity and seebeck coefficient of nanostructured CuSe thin films with Ga addition

Electrical conductivity and Seebeck coefficient of nanostructured Ga added p-type CuSe thin films are investigated in the temperature range 6–300 K. The films prepared by vacuum co-evaporation method and characterized by various tools like X-ray diffraction, scanning electron microscopy and energy dispersive analysis of X-rays, for their structural, morphological and compositional studies, have agglomerated grains forming particles of size ranging from about 24 nm to 121 nm and lattice constants a = 4.046 Å and c = 1.717 Å. The analysis of conductivity data suggests Mott's variable range hopping in the range 10–120 K, Seto's grain boundary effect in 120–195 K and Arrenhius' thermal activation in 195–300 K, while Seebeck coefficient data corroborates well with Hall and hot probe observations of p-type conductivity. These films have high conductivity of 903 ± 3% S/m at 300 K with the Fermi level lying ∼6 meV above valence band maximum (VBM) within a direct band gap of 1.99 ± 0.03 eV.

Growth and characterizations of tin doped zinc-phthalocyanine prepared by thermal co-evaporation in high vacuum as a nanomaterial

This research is related to the growth and characterizations of the novel hybrid nanomaterial, tin doped zinc-phthalocyanine thin films (Sn-doped ZnPC), grown by thermal co-evaporation. The concentration of Sn in hybrid films was controlled by adjusting the deposition rate between Sn and ZnPc. The hybrid films were characterized by atomic force microscopy and UV–visible spectroscopy to reveal the physical and optical properties of hybrid films. Moreover, the electrical properties, e.g., carrier mobility and carrier concentration, of the indium tin oxide (ITO)/Sn-doped ZnPc/aluminium (Al) devices were extracted from the current–voltage and capacitance–voltage characteristics. Furthermore, X-ray photoelectron spectroscopy and Raman spectroscopy was employed to explore the chemical interaction taking place in doped films. Sn doping into ZnPc changes the film’s specific properties, e.g., morphology, crystalline packing, absorption spectra, and conductivity. Moreover, no chemical bond is formed between Sn and ZnPc, and Sn dopants are formed as metal clusters covered by derivative oxide (SnOx) embedded in the Sn-doped ZnPc film.