S2 Films Research Articles

Evidence of Cationic Antiphase Disorder in Epitaxial Cu(In,Ga)S2 Grown on GaP/Si(001)

A transmission electron microscopy study of epitaxial Cu(In,Ga)S2 (CIGS) films coevaporated on GaP/Si(001), in either Cu‐rich or Cu‐poor conditions, is presented. The spatial distribution and the orientation of the different phases by means of electron diffraction are unveiled. From atomically resolved imaging of the CIGS film's atomic structure, it is concluded that different chalcopyrite domains, sharing cation antiphase symmetries of the cation sublattice, coexist in the films. At least three types of cation antiphase boundaries (CAPBs), which do or do not lead to a violation of the octet rule, depending on the propagation direction, are conceptualized. Even though it is observed that epitaxial CIGS is highly prone to cation antiphase disorder (CAPD), and it is found that the growth of CIGS in Cu‐rich conditions leads to a lower density of CAPBs, as compared to Cu‐poor growth conditions. This opens the question of the influence of CAPBs on CIGS electronic properties.

Engineering Biodegradable Sodium Alginate Films with Dragon Fruit Peel Powder and MgO Nanoparticles

This study investigated the effect of the addition of dragon fruit peel powder (DFP) and MgO nanoparticles on sodium alginate (SA) films. The control film (S1 film), DFP-enriched (S2 film), MgO nanoparticles-enhanced (S3 film), and a film combining both MgO nanoparticles and DFP (S4 film) were evaluated. The tensile strength increased by 252.43, 290.27, and 645.13% for the S2, S3 and S4 films, respectively. These enhancements were accompanied by a marked improvement in the UV-blocking property, thermal stability, antioxidant activity,2,2′-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical scavenging activity, opacity and thickness. The water vapor permeability and transparency were decreased to 3.45 × 10−10 g−1 Pa−1 S−1 and 1.82% for the S4 film. An environmental scanning electron microscope (ESEM) study revealed a notable increase in surface roughness with the incorporation of DFP. X-ray diffraction (XRD) analysis indicated that the MgO nanoparticles possessed a crystallographic structure that was compatible with the SA film matrix, with the S4 films exhibiting the highest crystallinity index (28.92%). The overall migration study showed that all SA films complied with the overall migration limit of 60 mg/kg of simulant for fatty food simulants. The biodegradability test revealed complete degradation of the S1, S3, and S4 films before 20 days while the S2 film exhibited a substantial mass loss of 84.99% within the same 20-day timeframe.

Unveiling the role of copper content in the crystal structure and phase stability of epitaxial Cu(In,Ga)S2 films on GaP/Si(001)

Unveiling the role of copper content in the crystal structure and phase stability of epitaxial Cu(In,Ga)S2 films on GaP/Si(001)

Direct Solution Processed Ag(Bix,Sb1−x)S2 Films with Tunable Bandgap for Planar Heterojunction Solar Cells

Silver bismuth antimony sulfide (Ag(Bix,Sb1−x)S2, 0 ≤ x ≤ 1) is considered as a promising light‐harvesting material for solar cell applications because of its excellent stability, abundant element storage, and tunable bandgap. However, the method for depositing the Ag(Bix,Sb1−x)S2 film hasn't been developed, which hinders the investigation of the composition‐dependent photoelectric properties, such as bandgap, band position tunability, and photovoltaic characteristics. In order to apply Ag(Bix,Sb1−x)S2 in photovoltaics, the authors report a convenient solution method to prepare Ag(Bix,Sb1−x)S2 films with continuously tunable constituents through varying the ratio of Bi and Sb in the solution. The bandgaps of Ag(Bix,Sb1−x)S2 films can be turned from 1.70 to 1.22 eV through changing the Bi/Sb ratio. Moreover, for the first time, the authors confirm that Ag(Bix,Sb1−x)S2 can be applied in solar cell applications. Upon device optimizing, the device based on Ag(Bix,Sb1−x)S2 (x = 0.4) shows a champion power conversion efficiency of 2.70%. This research provides an effective method for fabricating Ag(Bix,Sb1−x)S2 films with regulating the bandgap and confirms the photovoltaic application of Ag(Bix,Sb1−x)S2.

High-sensitivity and versatile plasmonic biosensor based on grain boundaries in polycrystalline 1L WS2 films

Structural defects play an important role in exploitation of two-dimensional layered materials (2DLMs) for advanced biosensors with the increasingly high sensitivity and low detection limit. Grain boundaries (GBs), as an important type of structural defect in polycrystalline 2DLM films, potentially provide sufficient active defect sites for the immobilization of bioreceptor units via chemical functionalization. In this work, we report the selective functionalization of high-density GBs with complementary DNA receptors, via gold nanoparticle (AuNP) linkers, in wafer-scale polycrystalline monolayer (1L) W(Mo)S2 films as versatile plasmonic biosensing platforms. The large surface area and GB-rich nature of the polycrystalline 1L WS2 film enabled the immobilization of bioreceptors in high surface density with spatial uniformity, while the AuNPs perform not only as bioreceptor linkers, but also promote detection sensitivity through surface plasmon resonance enhancement effect. Therefore, the presented biosensor demonstrated highly sensitive and selective sub-femto-molar detection of representative RNA sequences from the novel coronavirus (RdRp, ORF1ad and E). This work demonstrates the immense potential of AuNP-decorated GB-rich 2DLMs in the design of ultra-sensitive biosensing platforms for the detection of biological targets beyond RNA, bringing new opportunities for novel healthcare technologies.

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

SnxSy MSELD stack thin films: Processing, characteristics and devices for photonic applications

SnxSy varied stoichiometric stack thin films (S1, S2, S3) were obtained using multisource sequential elemental layer deposition (MSELD) technique on annealing. The composition of the SnxSy thin films were varied by supplementing sulphur on predeposited Sn : S (1 : 1) constituents using MSELD technique. Present investigation exhibit formation dynamics of MSEL deposited SnxSy films by condensed state reaction on varying composition and annealing temperatures (473 K, 523 K and 623 K) under constant vacuum. The deposited films exhibit composition variation ranging from 0.5 (S/Sn) to 1.5 (S/Sn) and results into the formation of varied alloy SnxSy phases. These varied alloy phase formation achieved due to diffusion of sulphur atoms on annealing of the varied composition MSELD stack. These variations in phases and composition on annealing allow bandgap tuning from 1.1 eV to 2.1 eV with changed absorption coefficient over the Vis-NIR (Visible - Near infrared) region. The stack films (S1, S2, S3) exhibit p-type electrical resistivity with promising electrical parameters viz. lower resistivity (62–309 Ω cm), mobility (3.1–17 cm2/V s) and carrier concentration (~1014–1015 cm−3) on annealing at 523 K for their use in photovoltaic (PV) technology. To achieve photonic and electronic devices, the schottky (FTO/SnxSy(S2)/Ag) diodes of varied composition and heterojunction (FTO/SnxSy(S2)/n-CdS/Ag) were tested and exhibit nearly ideal behaviour (ideality factor 1.1) for S2 films. In summary, the results demonstrates peculiar characteristics of the SnxSy material system as nano dimension electrical switches, tunable narrow and wide direct bandgaps with higher absorption capabilities in Vis - NIR region for their use in PV technology.

Fabrication of Cu(In,Ga)SxSe2−x films by one-step sputtering with elevated substrate temperature and their characterization

One-step sputtering with elevated substrate temperature was used to fabricate Cu(In,Ga)Se2 (CIGSe) and Cu(In,Ga)S2 (CIGS) films. We investigated the influence of this approach on the structure and electrical properties of CIGSe and CIGS films. Compact films with large grain size were achieved at relatively low substrate temperatures below 400 °C without post-treatments. The films obtained from CIGSe targets showed larger grain and higher crystallinity compared to that prepared from CIGS targets. It was found that the substrate temperature over 400 °C brought some undesirable effects on the film quality, such as voids and secondary phases. The loss of Ga and In during the fabrication process was detected, resulting in the formation of Cu-rich films with low carrier density. HBr solution etching reduced the Cu/(In + Ga) ratio of the sputtered films, leading to the improvement of the performance of the CIGSe solar cells along with the enhancement of carrier density.

Effect of oxygen partial pressure on the structural and optical properties of ion beam sputtered TiO2 thin films

We report the effect of oxygen partial pressure on the structural, electronic and nonlinear optical properties of ion beam sputtered TiO2 thin films deposited on glass substrate at 40% of oxygen (S1) and 20% of oxygen (S2) partial pressure. XRD data shows the crystalline nature of S1 film while the film S2 was amorphous in nature. The energy band gap of the thin films calculated from their UV-Vis spectra was found to be 3.63 eV (S1) and 3.56 eV (S2). The decrease in the band gap with decrease in oxygen partial pressure may be attributed to the amorphous nature of the film. The nonlinear refractive indices for both the films were obtained from the closed aperture Z-scan experiment performed using a cw He-Ne laser source operating at 632.8 nm and were found to be 17.6×10-9 m2/W and -5.64×10-9 m2/W for S1and S2 films, respectively. The reversal in the sign of the nonlinear refractive index may also be ascribed to the crystallinity of the grown films.

(Invited) Hydrogen Evolution from Water Using Modified Chalcogenide Photocathodes

Copper chalcopyrite semiconductors of Cu(In,Ga)(S,Se)2 include a wide range of compounds that are of interest for photoelectrochemical (PEC) water splitting due to their band gaps in the range of 1.0 eV to 2.5 eV by varying the alloy ratio. The conduction band of the compounds is also well positioned above the reduction potential of water, enabling them to be used as photocathodes for H2 generation. Two of important factors required for photocathodes of PEC water splitting are photocurrent (photocurrent density) and onset potential (photovoltage), i.e., photocathodes having positive onset potentials coupled with high photocurrents are promised in order to induce water splitting by using a tandem system without applied bias energy. For chalcopyrite materials applied to the photocathode part, this can be achieved through band engineering by altering the composition of the alloy. In this study, wide-gap Cu(In,Ga)S2 (CIGS) films with various amounts of Ga were fabricated using spray pyrolysis followed by sulfurization. Photoelectrochemical water splitting properties of photocathodes based on these films were evaluated after modification with a CdS layer and Pt nanoparticles. Both photocurrent densities and onset potentials of the photocathodes were gradually improved by increase in the amount of Ga in the CIGS films up to a Ga/(In+Ga) ratio of 0.25 using 0.1 M Na2SO4 (pH 9) as an electrolyte under illumination of simulated sunlight (AM 1.5G). Further inclusion of Ga in the CIGS film was detrimental for both photocurrent density and onset potential. The maximum photocurrent density of 6.8 mA cm-2 (at 0 V vs. RHE) and the highest photocurrent onset potential of 0.84 V vs. RHE were obtained by using such photocathode. Achievements of a relatively wide interface band gap of the CIGS/CdS heterointerface and formation of relatively large grains in the CIGS with Ga/(In+Ga) ratio of 0.25 were likely to be responsible for such superior water reduction properties. The preliminary result of band engineering via replacement of Cu with Ag in the Cu(In,Ga)(S,Se)2 system in relation with its application as photocathode also will be presented.

Ecofriendly and Nonvacuum Electrostatic Spray-Assisted Vapor Deposition of Cu(In,Ga)(S,Se)2 Thin Film Solar Cells

Chalcopyrite Cu(In,Ga)(S,Se)2 (CIGSSe) thin films have been deposited by a novel, nonvacuum, and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) method. The generation of a fine aerosol of precursor solution, and their controlled deposition onto a molybdenum substrate, results in adherent, dense, and uniform Cu(In,Ga)S2 (CIGS) films. This is an essential tool to keep the interfacial area of thin film solar cells to a minimum value for efficient charge separation as it helps to achieve the desired surface smoothness uniformity for subsequent cadmium sulfide and window layer deposition. This nonvacuum aerosol based approach for making the CIGSSe film uses environmentally benign precursor solution, and it is cheaper for producing solar cells than that of the vacuum-based thin film solar technology. An optimized CIGSSe thin film solar cell with a device configuration of molybdenum-coated soda-lime glass substrate/CIGSSe/CdS/i-ZnO/AZO shows the photovoltaic (j-V) characteristics of Voc=0.518 V, jsc=28.79 mA cm(-2), fill factor=64.02%, and a promising power conversion efficiency of η=9.55% under simulated AM 1.5 100 mW cm(-2) illuminations, without the use of an antireflection layer. This demonstrates the potential of ESAVD deposition as a promising alternative approach for making thin film CIGSSe solar cells at a lower cost.

Properties of brush electrodeposited CuIn1− x Ga x S2 films

CuIn1−xGaxS2 films were deposited for the first time by the brushplating technique. The films were deposited at different electrolyte temperatures in the range of 30°C–80°C and at a constant deposition current density of 5.0 mA cm−2. The films exhibited singlephase copperindiumgalliumsulfide, and the grain size increased with increase of indium concentration. The optical band gap of the films varied from 1.60 to 2.275 eV as the gallium concentration increased. Surface morphology studies indicated that the grain size varied from 100 nm to 600 nm with increasing indium concentration. Room temperature resistivity decreased from 14.74 to 4.52 Ω cm with increasing indium concentration. Solar cells made with n-CdS yielded an open circuit voltage of 0.71 V and short circuit current density of 12.0 mA cm−2.

Preparation and layer-by-layer solution deposition of Cu(In,Ga)O2 nanoparticles with conversion to Cu(In,Ga)S2 films.

We present a method of Cu(In,Ga)S2 (CIGS) thin film formation via conversion of layer-by-layer (LbL) assembled Cu-In-Ga oxide (CIGO) nanoparticles and polyelectrolytes. CIGO nanoparticles were created via a novel flame-spray pyrolysis method using metal nitrate precursors, subsequently coated with polyallylamine (PAH), and dispersed in aqueous solution. Multilayer films were assembled by alternately dipping quartz, Si, and/or Mo substrates into a solution of either polydopamine (PDA) or polystyrenesulfonate (PSS) and then in the CIGO-PAH dispersion to fabricate films as thick as 1–2 microns. PSS/CIGO-PAH films were found to be inadequate due to weak adhesion to the Si and Mo substrates, excessive particle diffusion during sulfurization, and mechanical softness ill-suited to further processing. PDA/CIGO-PAH films, in contrast, were more mechanically robust and more tolerant of high temperature processing. After LbL deposition, films were oxidized to remove polymer and sulfurized at high temperature under flowing hydrogen sulfide to convert CIGO to CIGS. Complete film conversion from the oxide to the sulfide is confirmed by X-ray diffraction characterization.

Morphology and structure evolution of Cu(In,Ga)S2 films deposited by reactive magnetron co-sputtering with electron cyclotron resonance plasma assistance

Cu(In,Ga)S2 (CIGS) films were deposited on Mo coated soda lime glass substrates using an electron cyclotron resonance plasma enhanced one-step reactive magnetron co-sputtering process (ECR-RMS). The crystalline quality and the morphology of the Cu(In,Ga)S2 films were investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy, and X-ray fluorescence. We also compared these CIGS films with films previously prepared without ECR assistance and find that the crystallinity of the CIGS films is correlated with the roughness evolution during deposition. Atomic force microscopy was used to measure the surface topography and to derive one-dimensional power spectral densities (1DPSD). All 1DPSD spectra of CIGS films exhibit no characteristic peak which is typical for the scaling of a self-affine surface. The growth exponent β, characterizing the roughness Rq evolution during the film growth as Rq ∼ dβ, changes with film thickness. The root-mean-square roughness at low temperatures increases only slightly with a growth exponent β = 0.013 in the initial growth stage, while Rq increases with a much higher exponent β = 0.584 when the film thickness is larger than about 270 nm. Additionally, we found that the H2S content of the sputtering atmosphere and the Cu- to-(In + Ga) ratio has a strong influence of the morphology of the CIGS films in this one-step ECR-RMS process.

Vapor-phase growth and characterization of Mo1−xWxS2(0 ≤ x ≤ 1) atomic layers on 2-inch sapphire substrates

Atomically thin Mo(1-x)W(x)S2 (0 ≤ x ≤ 1) ternary compounds have been grown on 2-inch c-plane sapphire substrates with high uniformity by sulfurizing thin Mo(1-x)W(x) layers that were deposited at room temperature using a co-sputtering technique. Atomic force microscopy (AFM), Raman scattering, and optical absorbance spectroscopy (OAS) studies reveal that the Mo(1-x)W(x)S2 films consist of crystallites of two-to-four monolayers in thickness. X-ray photoelectron spectroscopy (XPS) shows that the core levels of Mo3d and W4f shift to lower binding energies while that of S2p shifts to higher ones with the increase in W compositions, which can be related to the larger electron affinity of W (0.8163 eV) than that of Mo (0.7473 eV). OAS has also shown that the direct bandgap of Mo(1-x)W(x)S2 is tuned from 1.85 to 1.99 eV by increasing x from 0 to 1. Both E(1/2)(g) and A(1g) phonon modes of the Mo(1-x)W(x)S2 films exhibit a two-mode behavior. The bandgap tuning and the two-mode phonon behaviors are typically the same as those recently observed in monolayer Mo(1-x)W(x)S2 obtained by mechanical exfoliation, thus shedding light on the bottom-up growth of large-scale two-dimensional Mo(1-x)W(x)S2 ternary alloys.

Nucleation and phase formation during reactive magnetron co-sputtering of Cu(In,Ga)S2 films, investigated by in situ EDXRD

Energy dispersive X-ray diffraction (EDXRD) has been used to observe in situ the formation of Cu(In,Ga)S2 (CIGS) films during reactive magnetron co-sputtering. By simultaneous sputtering from a CuGa and an In target in an Ar:H2S atmosphere the CIGS films, which can be used as absorber layers for thin film solar cells, can be grown in a single step without any post-treatments.By carefully analyzing the in situ experiments, several effects have been found which should be considered during designing the recipe for an absorber deposition process in order to achieve CIGS films with the desired stoichiometry and good crystallinity. For instance substrate temperatures below 380°C should be used in order to achieve a sufficient condensation of In atoms which is needed to avoid the formation of intermetallic Cu–Ga phases. Since, on the other hand, high substrate temperatures are necessary in order to grow large crystallites, a process which starts at a low temperature and has temperature ramp up later on is possible because In which is bound in the chalcopyrite phase evaporates only for much higher temperatures.These results demonstrate the opportunities of EDXRD for the real-time analysis of the growth process of thin films which can provide information not obtainable by any ex situ characterization method.

Growth and morphology of thin Cu(In,Ga)S2 films during reactive magnetron co-sputtering

The roughness and the morphology of chalcopyrite Cu(In,Ga)S2 (CIGS) films on different substrates prepared by dc reactive magnetron co-sputtering with thicknesses ranging from 10 to 1000nm were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy (AFM). One-dimensional power spectral density (1DPSD) functions, derived from the AFM profiles, were used to compare the microstructure scaling behavior of the thin films. For CIGS films on Mo coated glass substrates, the variation of the grain size (dg) obtained from the AFM measurements, with film thickness (df) in the columnar grained films was dg(AFM)~df0.33. The growth exponent β, characterizing the roughness evolution of the films, changes with the film thickness. The root-mean-square roughness (Rrms) increases only slightly as Rrms~dfβ (β=0.13±0.09) in the initial growth stage, while Rrms increases with β=0.56±0.03, when the film thickness is larger than 50nm. 1DPSD analysis showed three spatial-frequency regimes of the roughness evolution during room temperature deposition, where the intermediate region disappears when the substrate temperature is increased. The nucleation and growth mechanisms are discussed in terms of surface diffusion, grain growth and shadowing effects. The substrate morphology does not significantly affect CIGS growth, which was proved for TiN and float glass. From the viewpoint of surface roughness and film growth, TiN is comparable to molybdenum and is suited as a potential back contact for chalcopyrite solar cell applications.

Cu(In1−xGax)S2 nanocrystals and films: low-temperature synthesis with size and composition control

We demonstrate a single-step X-ray irradiation process that yields high-quality Cu(In1-xGax)S2 nanocrystals in colloidal solutions, with complete control of size and composition. Thin films produced by drop-casting exhibit high-quality photoresponse, confirming that our process is suitable for microelectronics applications.

Dependence of the Properties of Cu(In,Ga)S2/Mo Films Prepared by Two-Stage Evaporation Method on Degree of Vacuum during Deposition

The influence of background pressure in multisource evaporation is not straightforward to explain, but it is clear that the amount of absorbed gases on the surface of the growing grains change with pressure. This may influence surface energies, and consequently the growth mechanism. We investigated the relation between the orientation of Cu(In,Ga)S2 films and pressure during deposition. We varied the background pressure during the deposition of the precursor film (first stage) by varying the temperature of the sulphur source, by throttling the vacuum pump, or by introducing N2 gas. We found that Cu(In,Ga)S2 films prepared from In–Ga–S precursors show (112) orientation if the pressure during precursor deposition is less than 0.03 Pa when the pressure is mainly attributable to sulphur. When it is mainly due to N2 gas, the pressure at which the orientation changes is 0.14 Pa. The orientation of the final film reflects the orientation of the (In,Ga)2S3 precursor immediately before entering the second stage.

Dependence of the Properties of Cu(In,Ga)S2/Mo Films Prepared by Two-Stage Evaporation Method on Degree of Vacuum during Deposition

The influence of background pressure in multisource evaporation is not straightforward to explain, but it is clear that the amount of absorbed gases on the surface of the growing grains change with pressure. This may influence surface energies, and consequently the growth mechanism. We investigated the relation between the orientation of Cu(In,Ga)S2 films and pressure during deposition. We varied the background pressure during the deposition of the precursor film (first stage) by varying the temperature of the sulphur source, by throttling the vacuum pump, or by introducing N2 gas. We found that Cu(In,Ga)S2 films prepared from In–Ga–S precursors show (112) orientation if the pressure during precursor deposition is less than 0.03 Pa when the pressure is mainly attributable to sulphur. When it is mainly due to N2 gas, the pressure at which the orientation changes is 0.14 Pa. The orientation of the final film reflects the orientation of the (In,Ga)2S3 precursor immediately before entering the second stage.