Sulfur Annealing Research Articles

Cd‐Free High‐Bandgap Cu2ZnSnS4 Solar Cell with 10.7% Certified Efficiency Enabled by Engineering Sn‐Related Defects

AbstractCd‐free high‐bandgap Cu2ZnSnS4 (CZTS) solar cells are expected to offer green and low‐cost solutions to single‐junction and tandem photovoltaic markets. Despite attracting considerable attention in past years, reported certified efficiency has yet to exceed the 10% threshold. Sn‐related defects, especially those originating from Sn2+ oxidation states, lead to severe recombination loss and limit device performance. Here, the formation of these detrimental defects is suppressed by creating a more benign chemical environment during sulfurization annealing. With dominant Sn4+ states in the source material and the precursor, the oxidation state of Sn remains primarily in its native Sn4+ state during annealing and in the final film. Engineering the Sn‐related defects leads to shallower tail states in bulk CZTS and fewer interfacial defects. As a result, both radiative and non‐radiative recombination losses are alleviated, contributing to a certified 10.7% efficiency of the Cd‐free high‐bandgap CZTS solar cell. This strategy may advance various kesterite materials and other technologies with multivalent constituents.

Chemical states of PVD-ZrS2 film underneath scaled high-k film with self-oxidized ZrO2 film as interfacial layer

Zirconium disulfide (ZrS2)—an attractive next-generation channel material because of its high mobility—is stabilized in the air by a zirconium dioxide (ZrO2) film which functions as a high-k film in MISFET. We fabricated high-k/PVD-ZrS2 stacks with a self-oxidized ZrO2 film as an interfacial layer; their chemical properties were analyzed to clarify how each fabrication process affects the ZrS2 under the oxide film. The results clarified that sulfur vapor annealing (SVA) is critical for fabricating high-quality physical vapor deposition (PVD) ZrS2 films and that the change in surface potential of the ZrS2 films due to interface dipoles between the high-k and Zr-compound films is suppressed with scaling of high-k thickness. The SVA with high-k films also prevents degradation of crystallinity and stoichiometry, enhancing the quality of the ZrS2 films without affecting their surface potential. These achievements enable us to control the threshold voltage in ZrS2 MISFETs.

Healing Sulfur Vacancies in Monolayer MoS2 by High-Pressure Sulfur and Selenium Annealing: Implication for High-Performance Transistors

Developing a technology to terminate chalcogen vacancies for transition-metal dichalcogenides is a crucial task for applications, such as transistors, diodes, and sensors, because chalcogen vacancies degrade the electronic and optical properties. This article reports a healing method of S vacancies in MoS2 by high-pressure annealing under a S vapor pressure of 5 atm. The crystal quality after mechanical transfer, S annealing, and vacuum annealing was systematically studied by observing the photoluminescence (PL). The remarkable recovery of the A-exciton emission peak in the PL spectrum indicated repair of the crystal quality in MoS2 by the S annealing. We also demonstrated that the S vacancies could be terminated by Se atoms using a high-pressure annealing method. The crystal quality of MoS2(1–x)Se2x alloys was confirmed by transmission electron microscopy and electron diffraction.

Influence of post-annealing in sulfur atmosphere on thermally evaporated β-In2S3 films

Indium sulfide (In 2 S 3 ) is an n-type semiconductor with wide bandgap energy (2.2–2.7 eV) and is currently used as buffer/window layer in thin film solar cells as an alternative to toxic CdS. In the present study, In 2 S 3 thin films were deposited on soda lime glass substrates using thermal evaporation technique at different substrate temperatures, T s = 200 °C–350 °C. Further, all the as-deposited films were annealed in sulfur ambient at 250 °C for 1 h. The structural, compositional and optical properties of annealed In 2 S 3 films were analyzed using GIXD, EDS and Photon RT spectrophotometer respectively. All the annealed films exhibited polycrystalline nature with improved crystallinity and high optical transmittance in the visible region. Moreover, the as-deposited films were sulfur deficient whereas in annealed layers the S/In ratio was increased due to sulfur annealing. Therefore, annealing of In 2 S 3 films in sulfur atmosphere enhanced the quality of the films. Among all the as-deposited and annealed films, the layers grown at T s = 300 °C followed by annealing at 250 °C have shown better structural and optical properties than the other films.

Enhancement of Solar Cell Performance of Electrodeposited Ti/n-Cu2O/p-Cu2O/Au Homojunction Solar Cells by Interface and Surface Modification

Cuprous oxide (Cu2O) homojunction thin films on Ti substrates were fabricated by an electrochemical deposition in which a p-Cu2O layer was deposited on an n-Cu2O layer by carefully controlled bath conditions. It was found that the open-circuit voltage of the homojunction solar cell was significantly influenced by the pH of the lactate bath. The variation of the pH was used to achieve the best possible crystal orientation for homojunctions. The crystallinity and morphology of the products were characterized by X-ray diffraction (XRD), high-energy x-ray diffraction (HEXRD), and scanning electron microscopy (SEM). The current density voltage (J-V) analysis showed that the sulfur treatment and annealing enhanced the photocurrent by ten-fold compared to the untreated and unannealed homojunction solar cell. X-ray photoelectron spectroscopy (XPS) studies confirmed that the sulfur treatment eliminated the surface CuO and formed a thin layer of CuS, which was very useful to make the front Ohmic contact. Transient measurements confirmed that the p-type Cu2O layer, which was subjected to sulfur treatment, significantly reduced the recombination, thus enhancing the efficiency of the solar cell. The best sulfur treated annealed Ti/n-Cu2O/p-Cu2O/Au solar cell produced an energy conversion efficiency of 2.64% with an open-circuit voltage of 490 mV and a short circuit current density of 12.8 mA cm−2 under AM 1.5 illumination.

Towards phase pure kesterite Cu2ZnSnS4 absorber layers growth via single step free sulfurization electrodeposition under a fix applied potential on Mo substrate

Abstract This work presents a new synthesis and characterization of Kesterite Cu2ZnSnS4 films by one step electrodeposition with free sulfurization annealing treatment at different applied potentials. This study highlights the effect of applied potential and annealing treatment on the properties of CZTS deposited films. X-ray Diffraction and Raman spectroscopy were employed to assess the structure and composition of the films elaborated at -1V, −1.1V, and −1.2V vs Saturated Calomel Electrode (SCE). The morphological and optical properties were studied using Scanning Electron microscopy and photoluminescence spectroscopy (PL), respectively. The structural properties are improved by annealing treatment, while −1.1V vs SCE was found to be the optimum applied potential to prepare the Kesterite CZTS thin film with a bandgap around 1.5 eV.

Highly efficient photocatalytic methylene blue degradation over Sn(O,S)/TiO2 photocatalyst fabricated via powder atomic layer deposition of SnO and subsequent sulfurization

In this work, tin oxysulfide coated TiO2 nanoparticles for photocatalytic pollutant degradation were fabricated via a two-step vacuum process which involves powder atomic layer deposition (PALD) of SnO on TiO2 nanoparticles followed by sulfurization annealing. The physical and chemical properties of the Sn(O,S)/TiO2 nanoparticles were characterized and compared with TiO2 and SnO/TiO2. From the methylene blue (MB) degradation test, the photocatalytic activities of Sn(O,S)/TiO2 were examined. The Sn(O,S)/TiO2 showed ~ 6.7 and 16-fold improved MB degradation rate constants of 0.002 and 0.114 min−1 compared to the pristine TiO2 under visible light and Xe lamp irradiations, respectively. This may be attributed to the synergetic effects of enhancing the electron-hole charge separation and light absorption characteristics.

Detecting the Repair of Sulfur Vacancies in CVD-Grown MoS2 Domains via Hydrogen Etching

Inherent sulfur vacancies in molybdenum disulfide (MoS2) films grown via chemical vapor deposition have restricted the practical application of MoS2 in optoelectronic devices. The repair of sulfur vacancies is important for improving the quality of MoS2 and enhancing the performance of devices based on MoS2 films. Here we report a route that uses hydrogen (H2) etching to detect the repair effect of sulfur vacancies. The domains with and without sulfur annealing were etched via H2 under the same conditions, and the changes of surface microstructures and properties were compared. It was found that the tolerance of MoS2 to H2 etching was enhanced after sulfur annealing. Results of the photoluminescence spectrum further proved that H2 etching could effectively detect the repair effect of sulfur vacancies. As a practical application, the optimum condition of sulfur annealing was obtained through H2 etching. Our work promotes the practical application of MoS2 in the field of new-generation electronics and optoelectronics.

Elaboration of wide bandgap CIGS on silicon by electrodeposition of stacked metal precursors and sulfur annealing for tandem solar cell applications

A method was developed for the electrodeposition of Cu-In-Ga precursor layers to elaborate Cu(In,Ga)(S,Se)2 (CIGS) thin films on silicon substrates for future application as silicon/wide-gap CIGS tandem solar cells. An underlayer of Ag was first deposited on silicon substrates to ensure a good adhesion of the electrodeposited stack and to serve as cathode during the deposition process. Cu, In and Ga layers were then sequentially electrodeposited. Ag-Cu-In-Ga precursor layers were finally subjected to elemental sulfur annealing at 600 °C. Formation of compact and adherent AgCIGS is observed. X ray diffraction and photoluminescence analyses confirm the formation of wide-gap CIGS of about 1.6 eV, with a spontaneous gallium grading over the depth of the sample leading to the formation of a bi-layer structure with a gallium rich layer at the interface with silicon.

Metal-organic Framework-driven Porous Cobalt Disulfide Nanoparticles Fabricated by Gaseous Sulfurization as Bifunctional Electrocatalysts for Overall Water Splitting

Both high activity and mass production potential are important for bifunctional electrocatalysts for overall water splitting. Catalytic activity enhancement was demonstrated through the formation of CoS2 nanoparticles with mono-phase and extremely porous structures. To fabricate porous structures at the nanometer scale, Co-based metal-organic frameworks (MOFs), namely a cobalt Prussian blue analogue (Co-PBA, Co3[Co(CN)6]2), was used as a porous template for the CoS2. Then, controlled sulfurization annealing converted the Co-PBA to mono-phase CoS2 nanoparticles with ~ 4 nm pores, resulting in a large surface area of 915.6 m2 g−1. The electrocatalysts had high activity for overall water splitting, and the overpotentials of the oxygen evolution reaction and hydrogen evolution reaction under the operating conditions were 298 mV and −196 mV, respectively, at 10 mA cm−2.

Synthesis and characterization of Cu2CoSnS4 thin films prepared via radio-frequency (RF) magnetron sputtering

Cu2CoSnS4 (CCTS) thin films, for the first time, were successfully fabricated by RF magnetron sputtering with two different metal precursor stacking sequences followed by sulfurization treatment under kinds of temperatures. Structure, morphology, composition, chemical valence states and optical properties of the as-grown films were discussed by X-ray diffraction (XRD), Raman spectroscopy (Raman), Scanning electron microscopy (SEM), Energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible-near-infrared spectrophotometer (UV–vis–NIR spectrophotometer) respectively. XRD and Raman analysis indicated that the as-obtained Cu2CoSnS4 thin films possess a stannite structure and the crystallinity was improved accompanying the incremental sulfurization temperature under both two depositing orders. The growth mechanism of CCTS films during vulcanization process was also discussed. XRD, Raman spectroscopy and SEM analysis showed that the precursors deposited in Cu/Co/Sn sequence had the best crystallinity after 600 °C sulfurization annealing. Under this preparation condition, XPS analysis showed that the chemical states of the four main elements in the film were Cu (I), Co (II), Sn (IV) and sulfide ions,and it exhibited the direct band gap of 1.58 eV by UV–vis–NIR absorption spectra, which is close to the ideal values for photovoltaic utilization.

Fabrication of Pyrite FeS2 Films from Electrochemically Deposited FeOOH by Sulfur Annealing

Fabrication of Pyrite FeS2 Films from Electrochemically Deposited FeOOH by Sulfur Annealing

Effect of vacuum and sulphur annealing on the structural properties of spray deposited Cu2SnS3 thin films

Cu2SnS3 thin films have been deposited by using non-vacuum spray pyrolysis technique on soda lime glass substrates from single aqueous solution containing all the constituents. The effect of post deposition annealing of the Cu2SnS3 thin films on the composition, structural, optical and electrical properties has been studied. The XRD study reveals that vacuum annealing of the thin film gives rise to the evolution of CuxSy secondary phases by the loss of elemental tin, while the annealing held at sulphur atmosphere increases the crystallinity of the film without giving rise to any secondary phases. The Cu2SnS3 phase with preferred orientation along the (112) crystal direction grew to greater extent upon sulphur annealing associated with an improvement in the optical band gap. From the Raman analysis it was observed that as-deposited films are dominated by the presence of tetragonal crystal symmetry of the Cu2SnS3 phase. Upon vacuum annealing the peak appearing at 474 cm−1 grows to greater extent which corresponds to CuS phase, and is in good agreement with the XRD spectra. Eventually its sulphur annealing gives the pure phase of Cu2SnS3 with dominating tetragonal crystal structure.

Effect of sulfur annealing on the morphological, structural, optical and electrical properties of iron pyrite thin films formed from FeS2 nano-powder

Iron pyrite (FeS2) thin films were fabricated by spin coating the solution of FeS2 nanocrystals of ∼40 nm in size on glass substrates, followed by annealing in a sulfur environment at different temperatures. The effect of sulfurization temperature on the morphology, structural, optical and electrical properties was investigated. With increase of the sulfurization temperature, the grain size and crystallinity of the films was improved, although some cracks and voids were observed on the surface of thin films. The band gap of the FeS2 films was decreased at higher sulfurization temperature. The electrical properties were also changed, including the increasing in resistivity and the decrease in Hall mobility, with increase of sulfurization temperature. The change in the optical and electrical properties of the FeS2 thin films was explained based on the changes of phase, morphology, surface, and grain boundary property.

Hydrogen evolution enhancement of ultra-low loading, size-selected molybdenum sulfide nanoclusters by sulfur enrichment

Size-selected molybdenum sulfide (MoSx) nanoclusters obtained by magnetron sputtering and gas condensation on glassy carbon substrates are typically sulfur-deficient (x = 1.6 ± 0.1), which limits their crystallinity and electrocatalytic properties. Here we demonstrate that a sulfur-enriching method, comprising sulfur evaporation and cluster annealing under vacuum conditions, significantly enhances their activity towards the hydrogen evolution reaction (HER). The S-richness (x = 4.9 ± 0.1) and extended crystalline order obtained in the sulfur-treated MoSx nanoclusters lead to consistent 200 mV shifts to lower HER onset potentials, along with two-fold and more-than 30-fold increases in turnover frequency and exchange current density values respectively. The high mass activities (∼111 mA mg−1 @ 400 mV) obtained at ultra-low loadings (∼100 ng cm-2, 5% surface coverage) are comparable to the best reported MoS2 catalysts in the literature.

Microporous novolac-derived carbon beads/sulfur hybrid cathode for lithium-sulfur batteries

Abstract Novolac-derived nanoporous carbon beads were used as conductive matrix for lithium-sulfur battery cathodes. We employed a facile self-emulsifying synthesis to obtain sub-micrometer novolac-derived carbon beads with nanopores. After pyrolysis, the carbon beads showed already a specific surface area of 640 m2 g−1 which was increased to 2080 m2 g−1 after physical activation. The non-activated and the activated carbon beads represent nanoporous carbon with a medium and a high surface area, respectively. This allows us to assess the influence of the porosity on the electrochemical performance of lithium-sulfur battery cathodes. The carbon/sulfur hybrids were obtained from two different approaches of sulfur infiltration: melt-infusion of sulfur (annealing) and in situ formation of sulfur from sodium thiosulfate. The best performance (∼880 mAh gsulfur−1 at low charge rate; 5th cycle) and high performance stability (>600 mAh gsulfur−1 after 100 cycles) were found for the activated carbon beads when using melt infusion of sulfur.

Basal-Plane Ligand Functionalization on Semiconducting 2H-MoS2 Monolayers.

Molybdenum disulfide (MoS2) is a two-dimensional material promising for electronic, optical, and catalytic applications. To fully harness its potential, functionalization is essential to controlling its properties. However, MoS2 functionalization has been mostly limited to either 1T-phase MoS2 or the edges of 2H-phase MoS2, and the chemistry of covalent functionalization on the basal plane of 2H-MoS2 is poorly understood. Here, we report a facile approach to covalently functionalize chemical vapor deposition (CVD) grown 2H-MoS2 monolayers (MLs), as well as mechanically exfoliated MoS2, via thiol conjugation at sulfur vacancies on the basal plane. Thorough characterization confirmed the functionalization by thiol molecules on MoS2 MLs, and we experimentally proved that sulfur vacancies in MoS2 MLs play a key role in the functionalization of basal planes. By the controlling of the amount of sulfur vacancies via sulfur annealing, the degree of MoS2 functionalization was effectively tuned. Because thiol conjugation partially repairs or passivates sulfur vacancies, enhanced photoluminescence response and decreased active sites for hydrogen evolution catalysis were observed for functionalized MoS2. Moreover, such functionalization can be utilized for making MoS2-based heterostructures, an example of which was demonstrated using a dithiol molecule to link MoS2 layers and PbSe quantum dots. These results provide new understanding and insights on the surface chemistry of MoS2 and open up more opportunities for MoS2 MLs with well-controlled properties and broader applications.

Hierarchical Ni3.5Co5.5S8 nanosheet-assembled hollow nanocages: Superior electrocatalyst towards oxygen evolution reaction

Developing highly efficient and cost-effective electrocatalysts for oxygen evolution reaction (OER) is crucial for renewable energy storage technologies. Here, we synthesized hierarchical Co-based bimetallic sulfide nanostructures as an efficient electrocatalyst for OER. Ni3.5Co5.5S8 nanostructures were synthesized by solvothermal sulfurization and thermal annealing of pre-synthesized homogenous bimetallic metal–organic frameworks (MOFs). Electron microscopy studies revealed that the Ni3.5Co5.5S8 has hollow nanocage like morphology with thin nanosheets grown on the surface. In addition, Co9S8 hollow nanocages were also synthesized for comparative electrocatalytic evaluation with the Ni3.5Co5.5S8 nanosheet-assembled hollow nanocages (NAHNs). The Ni3.5Co5.5S8 NAHNs has exhibited a low overpotential of 333 mV at the current density of 10 mA cm−2 (1.563 V vs RHE) and Tafel slope of 48.5 mV dec−1 for the oxygen evolution reaction in 1 M KOH. Benefitting from their structural merits, Ni3.5Co5.5S8 NAHNs manifest excellent OER electrocatalytic activity compared to most of the recently reported non-precious catalyst for OER.

Sulfur vacancy-induced reversible doping of transition metal disulfides via hydrazine treatment.

Chemical doping of transition metal dichalcogenides (TMDCs) has drawn significant interest because of its applicability to the modification of electrical and optical properties of TMDCs. This is of fundamental and technological importance for high-efficiency electronic and optoelectronic devices. Here, we present a simple and facile route to reversible and controllable modulation of the electrical and optical properties of WS2 and MoS2via hydrazine doping and sulfur annealing. Hydrazine treatment of WS2 improves the field-effect mobilities, on/off current ratios, and photoresponsivities of the devices. This is due to the surface charge transfer doping of WS2 and the sulfur vacancies formed by its reduction, which result in an n-type doping effect. The changes in the electrical and optical properties are fully recovered when the WS2 is annealed in an atmosphere of sulfur. This method for reversible modulation can be applied to other transition metal disulfides including MoS2, which may enable the fabrication of two-dimensional electronic and optoelectronic devices with tunable properties and improved performance.

Effects of Reaction Conditions on MoS2 Thin Film Formation Synthesized by Chemical Vapor Deposition using Organic Precursor

ABSTRACTMolybdenum disulfide (MoS2) thin films were fabricated by two-step chemical vapor deposition (CVD) using (t-C4H9)2S2 and the effects of temperature, gas flow rate, and atmosphere on the formation were investigated in order to achieve high-speed low-temperature MoS2 film formation. From the results of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) investigations, it was confirmed that c-axis orientation of the pre-deposited Mo film has a significant involvement in the crystal orientation after the reaction low temperature sulfurization annealing and we successfully obtained 3 nm c-axis oriented MoS2 thin film. From the S/Mo ratios in the films, it was revealed that the sulfurization reaction proceeds faster with increase in the sulfurization temperature and the gas flow rate. Moreover, the sulfurization under the H2 atmosphere promotes decomposition reaction of (t-C4H9)2S2, which were confirmed by XPS and density functional theory (DFT) simulation.