Se Vapor Research Articles

Suppressing Se Vacancies in Sb2Se3 Photocathode by In Situ Annealing with Moderate Se Vapor Pressure for Efficient Photoelectrochemical Water Splitting.

Sb2Se3 emerges as a promising material for solar energy conversion devices. Unfortunately, the common deep-level defect VSe (selenium vacancy) in Sb2Se3 results in a low solar conversion efficiency. The post selenization process has been widely adopted for suppressing VSe. However, the effect of selenization on suppressing VSe is often compromised and even more VSe are induced due to defect-correlation. Herein, high-quality Sb2Se3 films are prepared using an unconventional selenization process, with precisely regulating in situ annealing Se vapor pressure. It is found that moderate Se vapor pressure annealing can efficiently suppress VSe by overcoming defect-correlation, as well as promotes grain growth and forms a better heterojunction band alignment. Consequently, the Sb2Se3 photocathode shows a high-level photocurrent of 19.5 mA cm-2 at 0 VRHE, an onset potential of 0.40 VRHE and a half-cell solar-to-hydrogen conversion efficiency of 1.9%, owing to the inhibited charge recombination, excellent charge transport and interface charge extraction. This work provides a significant insight to suppress deep-level defect VSe by adjusting Se vapor pressure for efficient Sb2Se3 photocathode.

Дисперсии оптических констант тонких пленок селенидов меди, полученных методом вакуумно-термического испарения

Thin films of Cu/Se system were obtained by vacuum-thermal evaporation of Se and Cu. The influence of the mass of Cu deposited on the phase and optical characteristics of the synthesized films after their thermal annealing was investigated at a fixed mass of Se deposited. Thin films containing trigonal selenium, various phases of copper selenides, and embedded Cu nanoparticles were obtained by varying the mass of Cu. Transmittance and reflectance spectra, as well as ellipsometric parameters characterizing amplitude and phase changes of reflected light, of the obtained films in a wide range of wavelengths, including UV, visible and near-IR ranges at various angles of light incidence on the film were measured using a spectroscopic ellipsometer SER 850. Using modeling , the dispersions of refractive index, extinction coefficient, absorption coefficient, as well as the spectra of real and imaginary parts of dielectric permittivity were found from the recorded spectra. The spectra of extinction coefficient and absorption coefficient have also been determined from the direct measurements of transmittance and reflectance spectra recorded with the ellipsometer. The findings obtained with the two methods are in good agreement. The presented results show that the synthesized films have unique optical properties suitable for photonics and optoelectronics applications.

Strain-Induced Defect Evolution for the Construction of Porous Cu2-xSe with Enhanced Thermoelectric Performance.

Superionic Cu2-xSe, with disordered and even liquid-like Cu ions, has been extensively studied as a high efficiency thermoelectric material. However, the relationship between lattice stability and microstructure evolution in Cu2-xSe under strain, which is crucial for its application, has seldom been explored in previous research. In this study, we investigate the impacts of hydrostatic compression strain on the microstructural evolution and, consequently, its implications for thermoelectric performance. Molecular dynamics (MD) simulations show that high hydrostatic compression strain could induce local diffusion of Cu ions and Se twin evolution, resulting in the breaking and reforming of Cu-Se dynamic bonds and the unstable Se sublattice. The subsequent annealing process of the destabilized structure promoted Se evaporation from the sublattice and resulted in lotus-seedpod-like pores. The reduced sound velocity and intensified phonon scattering, due to pores, lead to a reduction in the lattice thermal conductivity from 0.44 W m-1 K-1 to 0.24 W m-1 K-1 at 800 K, a decrease of approximately 45%, in the porous Cu1.92Se sample. These findings reveal the relationship between stability and defect evolution in Cu2-xSe under high hydrostatic compression, offering a straightforward strategy of defect engineering for designing unique microstructures by leveraging the instability in superionic conductor materials.

Structural, Optical, Electrical, and Thermoelectric Properties of Bi2Se3 Films Deposited at a High Se/Bi Flow Rate.

Low-temperature synthesis of Bi2Se3 thin film semiconductor thermoelectric materials is prepared by the plasma-enhanced chemical vapor deposition method. The Bi2Se3 film demonstrated excellent crystallinity due to the Se-rich environment. Experimental results show that the prepared Bi2Se3 film exhibited 90% higher transparency in the mid-IR region, demonstrating its potential as a functional material in the atmospheric window. Excellent mobility of 2094 cm2/V·s at room temperature is attributed to the n-type conductive properties of the film. Thermoelectrical properties indicate that with the increase in Se vapor, a slight decrease in conductivity of the film is observed at room temperature with an obvious increase in the Seebeck coefficient. In addition, Bi2Se3 thin film showed an enhanced power factor of as high as 3.41 μW/cmK2. Therefore, plasma-enhanced chemical vapor deposition (PECVD)-grown Bi2Se3 films on Al2O3 (001) substrates demonstrated promising thermoelectric properties.

Microwave‐Assisted Synthesis of Cobalt‐Based Selenides as Catalyst Precursors for the Alkaline Water Oxidation

When used to promote the oxygen evolution reaction (OER), transition‐metal chalcogenides convert into oxyhydroxide/(hydr)oxide catalysts, the performance of which depends on the properties of the precursor. The present study aims to explore these effects for cobalt and cobalt–iron selenides (CoSe n and Co1Fe1Se n ) prepared using a simple microwave‐assisted method, in comparison to a reference material synthesized by high‐temperature reaction of CoO x H y with Se vapors. Physical characterization of the microwave‐synthesized CoSe n demonstrates their sheet‐like morphology and identifies Co3Se4 as the major phase, which is essentially completely transformed into CoOOH during the OER. The temperature during the microwave‐assisted CoSe n synthesis affects the crystallinity, the electrochemically active surface area, and thereby the performance of the resulting catalysts. Further improvements in the activity are achieved by combining cobalt with iron into a bimetallic Co1Fe1Se n precursor, which transforms in situ into a CoOOH + FeOOH composite and sustains the OER rate of 100 mA cm−2 (33 A g−1) at an overpotential of ≈ 0.31 and 0.26 V at 24 ± 2 and 80 ± 1 °C, respectively. Satisfactory stability of the Co1Fe1Se n ‐derived electrodes is demonstrated through a 4‐day‐long test at 80 ± 1 °C and 100 mA cm−2.

Effect of ball milling time on the formation and thermal properties of Ag2Se and Cu2Se compounds

The thermoelectric materials community has made significant progress on nanostructured and processable materials to improve efficiency and flexibility, reducing manufacturing costs. Selenide compounds, such as Ag2Se and Cu2Se, have received a lot of attention because of their promising capabilities in thermoelectric applications. Additionally, Se is significantly more abundant than Te, with approximately ten times higher availability. High-energy ball milling (HEBM) process is a powerful solid-state synthesis/powder mechanical alloying method. The objective of this work is to form selenide compounds at different milling times (1–20 h) by HEBM process. The structural characterization of the compounds was studied by X-ray Diffraction and X-ray Photoelectron Spectroscopy, while the thermal stability of the prepared samples was examined by Thermogravimetric Analysis. The experimental results show that the Ag2Se sample synthesized at 20 h presents lower thermal stability because of the higher specific surface area and the increased porosity as a result of the hardening effect. Additionally, Cu2Se presents Cu2O(SeO3) as the main phase at low temperatures, while at higher temperatures, CuO is the dominant phase. Therefore, as the heating temperature increases, there is a complete evaporation of Se and a conversion of the remaining copper into copper oxide. The present study demonstrates a simple method for the synthesis of Ag2Se and Cu2Se thermoelectric materials with high oxidation resistance.

Structure and tribological properties of MoSe2 films prepared by two-step process

In order to extend the application range of solid lubricant MoSe2, a two-step process was adopted to prepare the MoSe2 film. Namely, the MoSex precursor layer was firstly deposited on the substrate by magnetron sputtering method, and then selenized in a Se vapor atmosphere to obtain MoSe2 film. The effects of the sputtering and selenization process on the structure and tribological properties of the films were investigated. The results show that the two-step prepared MoSe2 films exhibit a preferential orientation with (002) basal plane parallel to the substrate and an improved crystallinity. The prepared MoSe2 film has good wear resistance and lubricating performance in an ambient air environment, with the lowest average friction coefficient of 0.0443 and a specific wear rate of 1.03×10−5 mm3/(N·m). In addition, the lubrication effects of MoSe2 films through the adhesion mechanism and fill in-repair mechanism were further discussed.

Efficiency Boost of (Ag0.5,Cu0.5)(In1‐x,Gax)Se2 Thin Film Solar Cells by Using a Sequential Process: Effects of Ag‐Front Grading and Surface Phase Engineering

AbstractPost‐selenization‐fabricated (using elemental Se vapor) Cu(In,Ga)Se2 solar cell efficiency is limited by a low open‐circuit voltage, which is attributable to the Ga‐deficient surface and insufficient grain growth. In this study, a band‐grading structure is demonstrated by combining Ag‐front and Ga‐back grading in selenized (Ag,Cu)(In,Ga)Se2 (ACIGSe) absorbers with a properly designed precursor structure (Mo/CuGa/In/AgGa) and high Ag content ([Ag]/([Ag]+[Cu]) = 0.5). The phase evolution during post‐selenization reveals that the precursor structure suppresses Ag2Se formation and promotes the ACIGSe phase formed at a low temperature with enhanced grain growth. A widened surface bandgap by Ag‐front grading substantially increases the open‐circuit voltage. Furthermore, Ag addition promotes ordered vacancy compound (OVC) formation on the front surface to enlarge the valence band offset, which in turn reduces interface recombination. Furthermore, the OVC phase also assists interface passivation. Promoting surface OVC phase by Ag addition is also validated by first‐principles calculations. Furthermore, the K‐doped CuGa precursor is used for a ACIGSe absorber to address the significantly reduced carrier density by the Ag addition. With a band‐grading structure and surface OVC phase, the superior device achieves an efficiency of > 19%, the highest efficiency by post‐selenization with an elemental Se source.

Vertically Aligned One-Dimensional Crystal-Structured Sb2Se3 for High-Efficiency Flexible Solar Cells via Regulating Selenization Kinetics

Recently, antimony selenide (Sb2Se3) has exhibited an exciting potential for flexible photoelectric applications due to its unique one-dimensional (1D) chain-type crystal structure, low-cost constituents, and superior optoelectronic properties. The 1D structure endows Sb2Se3 with a strong anisotropy in carrier transport and a lasting mechanical deformation tolerance. The control of the crystalline orientation of the Sb2Se3 film is an essential requirement for its device performance optimization. However, the current state-of-the-art Sb2Se3 devices suffer from unsatisfactory orientation control, especially for the (001) orientation, in which the chains stand vertically. Herein, we achieved an unprecedented control of the (001) orientation for the growth of the Sb2Se3 film on a flexible Mo-coated mica substrate by balancing the collision rate and kinetic energy of Se vapor particles with the surface of Sb film by regulating the selenization kinetics. Based on this (001)-oriented Sb2Se3 film, a high efficiency of 8.42% with a record open-circuit voltage (VOC) of 0.47 V is obtained for flexible Sb2Se3 solar cells. The vertical van der Waals gaps in the (001) orientation provide favorable diffusion paths for Se atoms, which results in a Se-rich state at the bottom of the Sb2Se3 film and promotes the in situ formation of the MoSe2 interlayer between Mo and Sb2Se3. These phenomena contribute to a back-surface field enhanced absorber layer and a quasi-Ohmic back contact, improving the device's VOC and the collection of carriers. This method provides an effective strategy for the orientation control of 1D materials for efficient photoelectric devices.

Grain Growth Mechanism of CZTSSe Films Controlled by the Evaporation Area of Se

Thermally evaporated Se, including the molecules of Se2, Se5, Se6, Se7, and Se8 in the vapor at 550–600 °C, is widely used for the selenization of CZTSSe films. However, active small‐molecule Se2 tends to form large clusters of Se atoms at saturation vapor pressure, resulting in the large Se deficiency and poor crystallization of CZTSSe films. To regulate the time for Se to reach saturation vapor pressure and promote the role of active small‐molecule Se, the evaporation area of Se is controlled. Then the corresponding grain growth mechanism of CZTSSe films and device efficiency are systematically investigated. The results demonstrate that the appropriate evaporation area of Se can not only optimize the time for Se vapor to reach saturation, promote the rapid reaction crystallization between the precursor films and active Se2, but also inhibit the nucleation at the CZTSSe/Mo interface, thus making the CZTSSe films show a perfect top‐down grain growth mode. By preliminary optimization, when the evaporation area of Se is 245 mm2, a large‐grain spanning monolayer CZTSSe thin film is obtained; meanwhile, a device with a high efficiency of 12.39% is achieved. This study provides a new selenization strategy for improving the crystallinity of CZTSSe thin films.

Optical study of oriented double-Se8-ring clusters and luminescent Se2− anions in LTA at extremely high selenium loading density

Recently, LTA-Se(1–8) samples with 1–8 Se atoms per cavity (simplified unit cell, large cavity + sodalite cage) obtained via adsorption at the temperature of ∼450 °C were reported. It was shown that single Se8 or single Se12 ring are formed in the large LTA cavities, Se8/Se12 ring concentration ratio decreasing with an increase in the Se loading density. Contrary, in the present work, using Se vapour adsorption at ∼550 °C, we succeeded in encapsulation of ∼17 Se atoms per cavity (LTA-Se(17)) with a significant increase in the Se8/Se12 concentration ratio manifesting double Se8-ring cluster formation in the most of the LTA large cavities, which is a step towards cluster crystal fabrication. According to our polarization/orientation Raman spectroscopic study of LTA-Se(17) single crystals, the orientations of the Se8 and Se12 appeared to be similar to those in previously investigated LTA-Se(1–8). Importantly, luminescent Se2− anions, oriented along the LTA 4-fold axes and located in the sodalite cages, are detected via Raman polarization/orientation dependencies of LTA-Se(17). Bright Se2− light emission with a maximum at ∼1.56 eV and vibronic structure is observed in the 1.3–1.8 eV spectral range. We show that the anions experience a compression in LTA which is slightly relaxing with a decrease in temperature producing an anomalous Raman band downshift. The compression of Se2− in LTA is weaker/stronger than that in sodalite/cancrinite, luminescence band photon energy depending on its strength. High concentration of regularly arranged Se2− in LTA suggests considering LTA-Se(17) as an important novel light-emitting material.

Cloning the Dirac cones of bilayer graphene to the zone center by selenium adsorption

Dirac cones can foster extraordinary electronic effects, as exemplified by the case of graphene layers. Angle-resolved photoemission reveals that adsorption of selenium (Se) vapor on bilayer graphene creates a symmetric hybrid clone of the Dirac cones at the zone center. A detailed analysis aided by first-principles calculations shows that the adsorbed layer consists of an ordered array of Se8 molecules. The uncovered cloning mechanism illustrates a method to generate electronic features of scientific and technological interests by gentle surface modification via van der Waals adsorption.

Shape-controlled monolayer MoSe2 flakes by chemical vapor deposition towards tuning the photoluminescence emission

In this study, we report on the controllable chemical vapor deposition (CVD) synthesis of monolayer MoSe2 flakes with different shapes such as hexagons, triangles, sawtooth hexagons, dendrites and fractals deposited on SiO2/Si substrates. This broad range of morphologies is due to the change in the vapor composition resulting from the confinement of the MoO3 and Se vapor and to the variation in the growth rate of the variable shaped MoSe2 crystals. It is also revealed that the photoluminescence (PL) response of the MoSe2 flakes is strongly affected by their shape and size. Our findings will open new avenues towards achieving morphology-controlled monolayer MoSe2 flakes for optoelectronic and energy harvesting systems.

Raman and optical absorption spectra of oriented Se8 and Se12 rings formed in zeolites: Dependence on the Se loading density

We encapsulated selenium (Se) into AFI and LTA zeolites via Se vapour adsorption at ∼450 °C and studied Raman and optical absorption spectra (RS and OAS, respectively) of zeolite single crystals with incorporated Se. At high Se loading density (D), Se8 rings are formed in the AFI channels along with Se chains. The rings are oriented by their 4-fold axis parallel to channels. Se8 is also formed in the LTA large cavities with its 4-fold axis oriented parallel to the 4-fold axis of LTA at any D up to ∼8 Se atoms per cavity (at/c). However, at D > 0.5 at/c, nearly unexplored Se12 rings oriented by their 3-fold axis parallel to the LTA 3-fold axis are formed along with Se8 in neighboring cavities. We discriminate between Se8 and Se12 rings via 1) RS frequencies; 2) RS and OAS band intensities vs. D; 3) RS band intensities vs. excitation wavelength and light polarization configuration. With an increase in D, the number of Se12 rings grows faster than that of Se8. We extract OAS of Se8 and Se12 from the experimental AFI-Se and LTA-Se OAS and show that the 3.0–3.3 eV absorption of Se12 is enhanced compared to that of Se8. This absorption is responsible for the observed resonant Raman effect of Se12 at the 514.5 nm wavelength excitation. Both LTA-confined Se8 and Se12 display rather high thermal stability.

Thin-Film Solar Cells Based on Selenized CuSbS2 Absorber.

Copper antimony sulfide (CuSbS2) has attracted significant interest as an earth-abundant photovoltaic absorber. However, the efficiency of the current CuSbS2 photovoltaic device is too low to meet the requirement of a large-scale application. In this study, selenylation was introduced to optimize the band structure and improve the device performance. Selenized CuSbS2 [CuSbS2(Se)] films were realized using porous CuSbS2 films prepared by spray deposition with a post-treatment in Se vapor. The as-prepared CuSbS2(Se) films exhibited a compact structure. X-ray diffraction and elemental analysis confirmed the effective doping of Se into the lattice by substituting a part of S in CuSbS2. Elemental analysis revealed a gradient distribution for Se from the top surface to the deeper regions, and the substitution rate was very high (>39%). Dark J–V characteristics and AC impedance spectroscopy analysis showed that selenylation significantly reduced the carrier recombination center. As a result, the selenized CuSbS2 device exhibited a significant efficiency improvement from 0.12% to 0.90%, which is much higher than that of the simply annealed device (0.46%), indicating this technique is a promising approach to improve the performance of CuSbS2 solar cells.

Solution-processed CIGS thin film solar cell by controlled selenization process

Cu(In,Ga)Se2 (CIGS) is a prevalent material with superior thermo-chemical stability and excellent optoelectronic properties for solar cell applications. In thin film form, CIGS could be a potentially economicalandbuilding integrated photovoltaicadaptable substitute to Silicon solar cells, solving humanity's extensive energy demands. To be economically sustainable, CIGS thin film processing must be abridged and affordable. We explored a low-cost, simplified wet chemical nano-ink method to make the CIGS thin film absorber layer comprising precursor thin film preparation by spraying CIGS nano-ink and subsequent post-treatment using Selenium vapor under Nitrogen atmosphere at a temperature of 550 °C. The spray-casted nanocrystalline layers were selenized at a low pressure of 1 Torr or 1 atm Nitrogen utilizing elemental pallets of Selenium as a source of Se vapor. The impact of Selenization pressure on the structure, morphology, composition, and electronic conductivity of selenized CIGS thin films absorber is investigated in this study, as well as the device performance associated with the processing parameters.

Effect of Cu-In-Ga Target Composition on Hybrid-Sputtered Cu(In,Ga)Se2Solar Cells

High-efficiency Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGSe) thin-film solar cells are typically fabricated by a multistage coevaporation process. The Cu-rich composition achieved during the second stage is known to favor the grain growth of the polycrystalline CIGSe film and has a beneficial effect on the solar cell performance. In this article, we analyze the effect of copper stoichiometry on the grain size of sputtered CIGSe absorbers and the efficiency of respective solar cells. CIGSe absorber layers were deposited on Mo-coated soda-lime glass substrates by pulsed hybrid reactive magnetron sputtering from Cu-poor and Cu-rich Cu-In-Ga targets and simultaneous Se evaporation. CIGSe films sputtered with the Cu-rich target show larger grain size and lead to better solar cell performance. Furthermore, we also introduce a two-stage process, which further increases the solar cell performance, consisting of an initial CIGSe layer deposited from the Cu-rich target followed by an indium-selenide postdeposition step. This two-stage process effectively eliminates the unwanted Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Se impurity phase and renders the otherwise required and toxic KCN etching unnecessary.

Bifacial Cu2ZnSn(S,Se)4 Thin Film Solar Cell Based on Molecular Ink and Rapid Thermal Processing

AbstractThe use of transparent conducting oxides (TCO) as a back contact for Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cell enables light absorption from both front and rear sides and allows for the fabrication of semi‐transparent photovoltaic devices. However, the CZTSSe solar cell based on TCO substrate suffers from high parasitic losses owing to the long post‐annealing time at elevated temperature required for high‐quality absorber. This work aims to overcome this issue by carefully tailoring the annealing conditions to obtain high‐performance solar cell. Rapid thermal processing is used to instantaneously generate high Se vapor pressure at target temperature and shorten the annealing time. The crystalline quality, morphology, and particularly the photovoltaic performance of the resulting CZTSSe thin film are systematically investigated. It is found that the increase of annealing temperature and time improves the crystalline quality but unfortunately deteriorates the device performance owing to increasing parasitic losses. Increasing the amount of Se allows to get highly crystalline absorber within 7 min of annealing, yielding a total area 5.56% efficient CZTSSe solar cell with front illumination resulting from reducing parasitic losses. This work provides a simple approach to preserve the back contact without using a complex interfacial layer.

Inkjet printed CuIn(1-X)GaXSe2 thin film by controlled selenium distribution for improved power conversion efficiency in chalcopyrite solar cells

Selenium (Se) vapor pressure is a key factor during the selenization of CuIn1-XGaX (CIG) film to obtain a high-quality CuIn1-XGaXSe2 (CIGS) absorber layer. To investigate the effect of Se vapor distribution on the grain growth of inkjet printed precursor film, two geometries of graphite box (square and circular) are used. The results revealed that selenization in the round graphite box give rise to uniform surface coverage and suppressed fine-grained layer due to adequate and uniform distribution of Se vapor. In contrast, film selenized in a square graphite box exhibits high strain and low crystallinity with a thick fine-grained layer. Probable Se vapor distribution inside the graphite box based on internal geometrical constraint and its impact on crystal phase and microstructure is discussed. Finally, CIGS devices fabricated using films selenized in a round graphite box demonstrates higher power-conversion efficiency of 5.2%, owing to high light absorption and efficient carrier separation. Based on J-V and EQE results, probable losses and recombination in the devices are examined and discussed.

Wide band gap Cu2ZnGe(S,Se)4 thin films and solar cells: Influence of Na content and incorporation method

The influence of the Na content and incorporation procedure into Cu2ZnGe(S,Se)4 (CZGSSe) thin films and solar cells is investigated. The effects of the presence/absence of Se during the deposition of a 15 nm thick NaF layer before and/or after the Cu2ZnGeSe4 (CZGSe) co-evaporation, are compared. Both the Na content, and Na-supply method significantly influence the incorporation of S into the CZGSe lattice and its distribution in the absorber. A [S]/([S] + [Se])-gradient throughout the CZGSSe layers is observed for all the samples and correlated with effects induced by the Na incorporation procedure. For instance, the evaporation of Se together with NaF leads to an increased S concentration in the surface-region of the CZGSSe layer and a modified surface morphology. CZGSSe-based solar cells with band gap energies of about 2 eV are obtained, regardless of the NaF addition method used, while the absence of the NaF layer reduces the S incorporation and the Eg. However, the evaporation of Se together with NaF results in higher Eg and open circuit voltage VOC, probably related to a higher S accumulation near the surface, demonstrating the importance of the [S]/([S] + [Se]) distribution. CZGSSe-based photovoltaic devices with efficiency of 3.2 % and Eg of 2 eV are obtained.