Post-sulfurization Process Research Articles

Extending the π-conjugation system of covalent organic frameworks for more efficient photocatalytic H2O2production

Three highly crystalline and porous COFs with promising photoredox properties have been prepared.Viaa post-sulfurization process, the imine-linked COFs were converted into thiazole-linked COFs, generating a highly conjugated and ultrastable photocatalyst for H2O2 production.

Enhanced thermoelectric properties of 2H–MoS2 thin film by tuning post sulfurization temperature

Two-dimensional transition metal dichalcogenide semiconductors (TMDCs) like MoS2 are becoming more popular as thermoelectric materials because they are abundant, nontoxic, and have good performance. In the study, the MoS2 thin films have prepared by the sputtering and post-sulfurization process at various temperatures 450 °C, 550 °C, 650 °C, and 750 °C. The XRD data exhibits the formation of the 2H phase of MoS2 thin film with (002), (004), and (006) planes. The Raman spectroscopy has confirmed the 2H–MoS2 thin films with 2LA (M), A1g, E2g, and Eg vibrational modes. The SEM images have shown the thin MoS2 flakes. The Seebeck and electrical conductivity data indicated an enhancement in Seebeck coefficient and electrical conductivity from 20 to 31 μV/°C and 26–53 S/m, respectively, as the post sulfurization temperature increased from 450 °C to 750 °C. The enhancement of the Seebeck coefficient and electrical conductivity have been linked to the perfection of the 2H phase of MoS2 film. The improved crystal structure has increased carrier mobility, leading to a high power factor of the 5.09 μWm−1C−2.

Neo-heterojunction based on the kesterite copper chalcogenide semiconductor with magnetic iron-cation partial-substitution: In-situ room-temperature pulsed laser deposition and optoelectronic properties

For improving optoelectronic properties of copper chalcogenide semiconductors such as Cu2ZnSnS4 (CZTS), we explore to optimize CZTS itself and construct heterostructures on the perspective of semiconductor device design. Specifically, in order to suppress widely existed CuZn and ZnCu antisite defects of pure CZTS, we synthesize 25%-Fe-doped CZTS (CZFTS) thin films through partial substitution of magnetic iron for zinc; we further integrate electron acceptor Bi2S3 to CZFTS and constitute CZFTS/Bi2S3 heterostructure. Our newly-developed in-situ room-temperature pulsed laser deposition (PLD) technique is adopted to prepare the bilayer sulfide films without post-sulfurization process, effectively optimizing CZFTS-Bi2S3 interfaces. Chronoamperometric measurements demonstrate that CZFTS/Bi2S3 heterojunction has at least one order-of-magnitude improvement in photoelectric response speed within visible light (Vis) region compared to single CZFTS film, and responding time is decreased to a few milliseconds. Hereby, the synergistically combining partial cation substitution and in-situ PLD technique is promising to develop the Vis-active CZTS-based thin-film photodetectors in an effective and efficient way as well as to economically produce such analogous thin-film photoelectric devices based on polynary chalcogenide.

Influencing mechanism of post-sulfurization with sulfur flakes on phase evolution and Schottky diode characteristic of Cu2ZnSnS4 thin films sputter deposited from a single target

While single target sputter deposition appears very attractive to prepare kesterite Cu2ZnSnS4 (CZTS) thin films for photovoltaic applications, the growth of secondary phases arising as a consequence of stoichiometric deviation stemming from variation in sputter yield of elements must be prevented. Here, we demonstrate growth of phase pure kesterite CZTS thin films by RF magnetron sputtering of a single target by intuitively manipulating the target composition and post-sulfurization process carried out in a quasi-open environment using sulfur flakes. The influencing mechanism of post-sulfurization process was elucidated from systematic variation in the dwell time, temperature and the sulfur amount. A high temperature or a shorter dwell time yielded a small-grained microstructure associated with the presence of secondary phases. Phase pure CZTS films with better microstructural features were obtained for sulfurization at 500 °C for 60 min with 1.0 g of sulfur flakes. This film exhibited an optical bandgap of ∼1.58 eV indicating its photovoltaic potential. A device in the Mo/CZTS/Ag configuration showed typical features of a Schottky junction. The obtained current–voltage characteristic was analyzed to estimate saturation current, ideality factor and series resistance in correlation with the properties of the CZTS film.

Large-Area MoS2 via Colloidal Nanosheet Ink for Integrated Memtransistor.

2D transition metal dichalcogenides (TMDs) exhibit intriguing properties for applications in optoelectronics and electronics, among which memtransistors received extensive attention as multifunctional devices. For practical applications of 2D TMDs, large-area fabrication of the materials via reliable processes, which is in trade-off with their quality, has been a long-standing issue. Here, a simple and effective way is proposed to fabricate large-area and high-quality molybdenum disulfide thin films using MoS2 colloidal ink through a spray coating, followed by a postsulfurization process. High-quality MoS2 thin films exhibit excellent optical and electrical properties that can be utilized in field-effect transistors (FETs) and memtransistor arrays. The MoS2 FETs show an average on/off ratio of 5 × 106 and a high electron mobility of 10.34 cm2 V-1 s-1 , which can be understood by the healing of sulfur vacancies, recrystallization, and the removal of the carbon contamination of the MoS2 . These MoS2 -based memtransistors present stable operations with a high switching ratio tuned by back gate and light illumination, which is promising for multiple-levels memory and complex neuromorphic computing. This study demonstrates a new strategy to fabricate 2D TMDs with large-area and high quality for integrated optoelectronic and memory device applications.

Visualization of band offsets at few-layer MoS2/Ge heterojunction

The visualization of band alignment for designing heterostructures between transition metal dichalcogenides and germanium plays a vital role in a deeper understanding of carrier dynamics at the heterointerface. Here, to study the band alignment across the MoS2/Ge heterojunction, we have deposited a wafer-scale highly crystalline few atomic layers MoS2 film via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The Raman and XRD spectra of as-fabricated MoS2/Ge heterojunction expose the presence of highly crystalline few atomic layer MoS2 on top of Ge substrate. Interestingly, we found a type-II band alignment at the MoS2/Ge heterointerface having valence band, and conduction band offset values of 0.88 and 0.21 eV, which can provide very efficient recombination through spatially confining charge carriers. The calculation of band offset parameters offers a promising way for device engineering across the MoS2/Ge heterojunction interface. Moreover, to demonstrate the practicability of the fabricated heterostructure, we explored the suitability of our device for broadband photodetection applications.

Photoconversion Optimization of Pulsed-Laser-Deposited p-CZTS/n-Si-Nanowires Heterojunction-Based Photovoltaic Devices.

We report on the achievement of novel photovoltaic devices based on the pulsed laser deposition (PLD) of p-type Cu2ZnSnS4 (CZTS) layers onto n-type silicon nanowires (SiNWs). To optimize the photoconversion efficiency of these p-CZTS/n-SiNWs heterojunction devices, both the thickness of the CZTS films and the length of the SiNWs were independently varied in the (0.3–1.0 µm) and (1–6 µm) ranges, respectively. The kësterite CZTS films were directly deposited onto the SiNWs/Si substrates by means of a one-step PLD approach at a substrate temperature of 300 °C and without resorting to any post-sulfurization process. The systematic assessment of the PV performance of the ITO/p-CZTS/n-SiNWs/Al solar cells, as a function of both SiNWs’ length and CZTS film thickness, has led to the identification of the optimal device characteristics. Indeed, an unprecedented power conversion efficiency (PCE) as high as ~5.5%, a VOC of 400 mV, a JSC of 26.3 mA/cm2 and a FF of 51.8% were delivered by the devices formed by SiNWs having a length of 2.2 µm along with a CZTS film thickness of 540 nm. This PCE value is higher than the current record efficiency (of 5.2%) reported for pulsed-laser-deposited-CZTS (PLD-CZTS)-based solar cells with the classical SLG/Mo/CZTS/CdS/ZnO/ITO/Ag/MgF2 device architecture. The relative ease of depositing high-quality CZTS films by means of PLD (without resorting to any post deposition treatment) along with the gain from an extended CZTS/Si interface offered by the silicon nanowires make the approach developed here very promising for further integration of CZTS with the mature silicon nanostructuring technologies to develop novel optoelectronic devices.

A high-performance hydrogen sensor based on a reverse-biased MoS2/GaN heterojunction

We report a MoS2/GaN heterojunction-based gas sensor by depositing MoS2 over a GaN substrate via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The microscopic and spectroscopic measurements expose the presence of highly crystalline and homogenous few atomic layer MoS2 on top of molecular beam epitaxially grown GaN film. Upon hydrogen exposure, the molecular adsorption tuned the barrier height at the MoS2/GaN interface under the reverse biased condition, thus resulting in high sensitivity. Our results reveal that temperature strongly affects the sensitivity of the device and it increases from 21% to 157% for 1% hydrogen with an increase in temperature (25–150 °C). For a deeper understanding of carrier dynamics at the heterointerface, we visualized the band alignment across the MoS2/GaN heterojunction having valence band and conduction band offset values of 1.75 and 0.28 eV. The sensing mechanism was demonstrated based on an energy band diagram at the MoS2/GaN interface in the presence and absence of hydrogen exposure. The proposed methodology can be readily applied to other combinations of heterostructures for sensing different gas analytes.

Cu2ZnSn(SxSe1−x)4 thin film solar cell with high sulfur content (x approximately 0.4) and low Voc deficit prepared using a postsulfurization process

High-efficiency (11.1%) Cu2ZnSn(SxSe1−x)4 (CZTSSe) solar cells have been obtained only with low-sulfur absorbers because the incorporation of high sulfur content is typically accompanied by a large open-circuit voltage (Voc) deficit. In this research, a sulfur content of 40% (x = 0.4) was obtained by postsulfurization. In comparison with a low-sulfur CZTSSe with x = 0.13, an efficiency of 9.8%, a band gap of 1.05eV, and a Voc of 446mV, the proposed cell had an efficiency of 11.1%, a band gap of 1.2eV and a Voc of 578mV; the post-sulfurization caused a very small increase in the Voc deficit (approximately 18mV). Approximately 250-nm-thick S-rich CZTSSe layer was found near the surface and was close to the depletion width (approximately 238.5nm) of the p-n junction, indicating a double-layered CZTSSe included a high-sulfur top layer was responsible for the high Voc. Admittance spectroscopy showed the activation energy of the bulk defect was 138meV; this revealed some deep-level defects were associated with the low short-circuit current at long wavelengths. The activation energy of the interfacial defects was 1.08eV, indicating the Voc deficits of future devices may be decreased by suitable surface treatment of high-sulfur-content CZTSSe devices.

Controlled post-sulfurization process for higher efficiency nontoxic solution-deposited CuIn0.7Ga0.3Se2 absorber thin films with graded bandgaps

Abstract A practical way of adjusting post-sulfurization process with variable durations is proposed to improve the cell-conversion efficiency of non-toxic solution driven CuIn 0.7 Ga 0.3 Se 2 absorber thin films. The degree of bandgap grading was controllable by changing the sulfurization time from 2 min to 30 min due to the dissimilar distribution of sulfur across the absorber layer. Deeper intensified distributions of Ga and S were responsible for enhancements in the open-circuit voltage and cell-conversion efficiency. A competitive efficiency of ∼8.81% for the 10 min-sulfurized CIGSSe cell was achieved with other promising parameters, i.e., short-circuit current density of 33.17 mA/cm 2 , open-circuit voltage of 0.496 V, and fill factor of 53.5%, resulting from the improved carrier collection and reduced recombination by the bandgap grading. This improved efficiency corresponds to a ∼15.6% enhancement compared to the unsulfurized sample.

A Novel Strategy for Preparation of Cu<sub>2</sub>(M<sup>II</sup>)(M<sup>IV</sup>)S<sub>4</sub> Thin Films for Solar Cell: Sulfurization of Mixed Metal Oxides Precursor Synthesized by Combustion Method

In this study, a novel strategy for preparation of Cu2(MII)(MIV)S4 thin films was creatively proposed. Three light absorption materials Cu2MnSnS4, Cu2ZnSnS4 and Cu2ZnGeS4 were successfully synthesized based on combustion method. It was found that the combustion synthesized powder is mainly consist of several metal oxides. The band gap energy of Cu2MnSnS4, Cu2ZnSnS4 and Cu2ZnGeS4 are determined to be 1.3 eV, 1.49 eV and 2.15 eV, respectively, showing a promising application on thin film solar cells. According to our experiment results, the combustion method was confirmed to be an efficient tool to synthesize various Cu-based absorber materials by using a post-sulfurization process, which provides a new view for fabrication of high-efficient low-cost photovoltaics.

Reconstructing the energy band electronic structure of pulsed laser deposited CZTS thin films intended for solar cell absorber applications

We report here on the use of pulsed KrF-laser deposition (PLD) technique for the growth of high-quality Cu2ZnSnS4 (CZTS) thin films onto Si, and glass substrates without resorting to any post sulfurization process. The PLD-CZTS films were deposited at room temperature (RT) and then subjected to post annealing at different temperatures ranging from 200 to 500°C in Argon atmosphere. The X-ray diffraction and Raman spectroscopy confirmed that the PLD films crystallize in the characteristic kesterite CZTS structure regardless of their annealing temperature (Ta), but their crystallinity is much improved for Ta≥400°C. The PLD-CZTS films were found to exhibit a relatively dense morphology with a surface roughness (RMS) that increases with Ta (from ∼14nm at RT to 70nm at Ta=500°C with a value around 40nm for Ta=300–400°C). The optical bandgap of the PLD-CZTS films, was derived from UV–vis transmission spectra analysis, and found to decrease from 1.73eV for non-annealed films to ∼1.58eV for those annealed at Ta=300°C. These band gap values are very close to the optimum value needed for an ideal solar cell absorber. In order to achieve a complete reconstruction of the one-dimensional energy band structure of these PLD-CZTS absorbers, we have combined both XPS and UPS spectroscopies to determine their chemical bondings, the position of their valence band maximum (relative to Fermi level), and their work function values. This enabled us to sketch out, as accurately as possible, the band alignment of the heterojunction interface formed between CZTS and both CdS and ZnS buffer layer materials.

Phase engineering of CBD grown tin sulfide films by post-sulfurization and solar cell application

We report on post-annealing effects on structures, compositions and surface morphology of chemical bath deposition (CBD) grown tin sulfide thin films. For post-annealing of as-grown tin sulfide films, we carried out three types of sulfurization at different temperatures. As grown tin sulfide films followed by sulfurization at different conditions showed modified surface morphologies and crystalline phases; SnS, Sn2S3 and SnS2 according to the sulfurization conditions. Experimental results showed that these earth-abundant thin films could be engineered to have different structural, electrical and optical properties by simple post-sulfurization process, possessing significant potentials for wide fields of applications. Moreover, we made solar cell using SnS as photo absorber, which showed ∼0.1% power conversion efficiency.

Influence of sodium incorporation on kesterite Cu2ZnSnS4 solar cells fabricated on stainless steel substrates

Kesterite Cu2ZnSnS4 (CZTS) thin film solar cells are fabricated based on sputtering and post sulfurization process on stainless steel substrates. NaF layers with different thickness are used as external sodium source to address the lack of Na issue in Na-free steel substrates and 10nm of NaF layer is proved to be the optimal thickness. Sodium incorporation into CZTS induces the metal element redistribution, facilitates CZTS film recrystallization and promotes the phase evolution. Appropriate amount of sodium is evidenced to yield an enhancement in power conversion efficiency from 3.07% to 4.10% whilst further increase in sodium tends to deteriorate the device efficiency. Device performance deterioration is correlated to the rather small grain size accompanied with a large amount of grain boundaries and unfavorable SnS2 secondary phase for CZTS samples without sodium, while pinholes and lower carrier density in absorber for CZTS samples with over amount of sodium.

Layered MoS2 grown on c ‐sapphire by pulsed laser deposition

Layered growth of molybdenum disulphide (MoS2) was successfully achieved by pulsed laser deposition (PLD) method on c ‐plane sapphire substrate. Growth of monolayer to a few monolayer MoS2, dependent on the pulsed number of excimer laser in PLD is demonstrated, indicating the promising controllability of layer growth. Among the samples with various pulse number deposition, the frequency difference (A1g–E12g) in Raman analysis of the 70 pulse sample is estimated as 20.11 cm–1, suggesting a monolayer MoS2 was obtained. Two‐dimensional (2D) layer growth of MoS2 is confirmed by the streaky reflection high energy electron diffraction (RHEED) patterns during growth and the cross‐sectional view of transmission electron microscopy (TEM). The in‐plane relationship, (0006) sapphire//(0002)MoS2and sapphire//MoS2 is determined. The results imply that PLD is suitable for layered MoS2 growth. Additionally, the oxide states of Mo 3d core level spectra of PLD grown MoS2, analysed by X‐ray photoelectron spectroscopy (XPS), can be effectively reduced by adopting a post sulfurization process. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Fabrication of Cu2ZnSnS4 thin films using a ceramic quaternary target

Cu2ZnSnS4 (CZTS) films were fabricated from a ceramic quaternary target, through middle frequency magnetron sputtering. A post sulfurization process was applied for the sputtered films and sulfurization temperatures ranging from 460 °C to 580 °C were utilized. It has been found that obvious Zn loss exists during the sputtering, making the sputtered films become Zn poor although the target is Zn rich. The sputtered films were not totally amorphous, but composed of small CZTS grains at the size of several nanometers. After a post sulfurization process the crystallinity were greatly improved and films composed of densely packed CZTS grains at the size of several hundred nanometers can be obtained at sulfurization temperatures higher than 520 °C. Yet minor SnS coexists with CZTS since the films were Zn poor and Sn rich. And finally the CZTS films sulfurized at 580 °C depict an optical band gap of around 1.55 eV.

Effects of Cu/In ratio of electrodeposited precursor on post-sulfurization process in fabricating quaternary CuIn(S,Se) 2 thin films

This paper reports the analysis of S diffusion into electrodeposited CuInSe 2 (CISe) precursors during post-sulfurization treatment at 500 °C in an Ar/H 2S ambient. The characterizations of the sulfurized films by X-ray diffraction, grazing-incidence X-ray diffraction, Auger electron spectroscopy and micro-Raman spectroscopy allow the observation of the strong dependence of S incorporation into these films on the Cu/In ratio of the precursor. AES profiles reveal higher S content along the depth of Cu-rich film than Cu-poor film after sulfurization. Raman Scattering shows that copper sulfoselenides Cu–(Se,S) are only detected in Cu-rich samples. The re-crystallization of films during sulfurization was analyzed and it is presumed that quasi-liquid Cu–Se phases, which are related to Cu/In ratio of precursor, promote continuous incorporation of S into these films.