Optimum Band Gap Research Articles

Study on the band gap optimization and defect state of two-dimensional honeycomb phononic crystals

Abstract

Hydrothermal synthesis of tetragonal and wurtzite Cu2MnSnS4 nanostructures for multiple applications: Influence of different sulfur reactants on growth and properties

The present article reports the hydrothermal synthesis of quaternary Cu2MnSnS4 (CMTS) nanostructures under subcritical reaction conditions. The effect of different chalcogen (sulfur) precursor sources (l-cysteine, elemental sulfur and sodium sulfide) on the growth and properties of the nanostructures is studied. The sulfur precursors showed prominent influence on the structural and morphological properties of the nanostructures. Structural properties indicated the growth of single phase crystalline materials. The crystal phase was controlled and determined by the reactivity of the sulfur sources. Metastable wurtzite phase was achieved with the mildly reactive precursor while tetragonal phase was obtained in the case of highly reactive precursors. Morphological studies showed formation of nanoflakes and anisotropic two-dimensional irregular nanostructures. Surface area analysis indicated that the nanoflakes are mesoporous in nature and exhibited a high specific surface area of 32.24 m2/g. The electrical studies demonstrated p-type conductivity of the material. Optical properties revealed optimum energy band gap value (~1.7 eV) of the nanostructures, suitable for the solar energy conversion applications. The magnetic properties indicated ferromagnetic behavior of the CMTS nanoflakes. The nanostructures were investigated as catalysts for photocatalytic decolaration of malachite green (MG) dye under UV–Visible light illumination. The nanoflakes showed good activity against the dye and photodegraded 95% of the dye in an illumination duration of 140 min. The photocatalysts exhibited good stability during the photocatalytic processes.

SYNTHESIS OF GRAPHENE/Mn0.2Cd0.8S HIERARCHICAL NANOSPHERES FOR PHOTOCATALYTIC DEGRADATION OF ORGANIC POLLUTANTS

The hierarchically structured hybrids of graphene and Mn0.2Cd0.8S solid-solution, resulting in G/Mn0.2Cd0.8S HN, are successfully synthesized by using microwave method with the assistance of L-Histidine. Moreover, X-ray diffraction (XRD), UV-vis absorption spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are employed to analyze the structural and optical properties. Furthermore, the decoloration of methyl orange (MO) under visible light irradiation indicates that the G/Mn0.2Cd0.8S HN is a promising photocatalyst for the degradation of organic pollutants. Herein, a possible photocatalytic mechanism of G/Mn0.2Cd0.8S is proposed, showing the synergistic influence of hierarchical nanospheres morphology, optimal bandgap and conductive graphene scaffold on photocatalytic activity. Overall, the photodegradation performance is improved due to the effective separation of electron/hole pairs and efficient transportation of photogenerated electrons from Mn0.2Cd0.8S to graphene. The current work demonstrates an environment-friendly route to fabricate multi-component chalcogenides for high-performance photocatalytic applications.

Synthesis, Structural and Optical Properties of Erbium-Doped α-Fe2O3 Nanoparticles

In this paper, we report the synthesis and optical studies of α-Fe2O3 and erbium (Er3+) ion-doped α-Fe2O3 nanoparticles (NPs). The sol–gel auto-combustion method was employed to synthesize NPs with varying concentrations of Er3+ ions. The synthesized NPs are thoroughly characterized and discussed. Powder x-ray diffraction analysis confirms the hexagonal (rhombohedral) crystal structure, with no additional phase formation. High-resolution transmission electron microscopy reveals the agglomeration of NPs in the synthesized systems. The average particle size decreases with Er3+ ion doping in α-Fe2O3 NPs. The elemental composition of doped and undoped systems is confirmed by energy dispersive x-ray spectroscopy (EDAX). The optical band gap was calculated from diffuse reflectance spectroscopy using the Kubelka–Munk relation. Band gap decreases with an increase in Er3+ ion doping up to a specific doping concentration. Er3+ ion-doped α-Fe2O3 NPs show enhanced luminescence as compared to pristine α-Fe2O3 NPs. An optimum band gap and enhanced luminescence of these doped systems make them as good candidates for optoelectronic devices.

Influence of calcination on the structural properties of earth abundant Cu2ZnSnS4

In the present work, we report on the synthesis of nano-crystalline kesterite copper zinc tin sulfide (CZTS) in powder form and the post-annealing process at high temperatures (calcination), and study their physical properties. We have successfully synthesized CZTS by direct fusion, and the resulting material was crushed to obtain a fine powder. Then, the resulting powder was calcined at high temperatures: 800–1000 ∘C. The calcined samples have been characterized with a number of different structural: X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and optical techniques: photoluminescence (PL), Raman and diffuse reflectance spectroscopy (DRS). From the XRD analysis we grant that the material presents mainly the kesterite structure and it is polycrystalline, with a preferential orientation along the [112] direction. From XRD, SEM, EDX and Raman spectroscopy, we conclude that the best crystallinity and stoichiometry was obtained for a calcination temperature of 900 ∘C. From te PL and DRS results we deduce that the sample with smaller emission and an energy gap close to 1.5 is that calcined at 900∘. In summary, we can confirm from the present study made with a number structural and optical techniques, that the CZTS has grown in the kesterite structure and the sample calcined at 900∘C has the best crystallinity and the optimum band gap to be used as an absorber in solar cell devices.

Role of the substrates in the ribbon orientation of Sb2Se3 films grown by Low-Temperature Pulsed Electron Deposition

Antimony selenide (Sb2Se3) is a p-type semiconductor, considered as an excellent photovoltaic absorber for its high absorption coefficient in the visible region and a nearly optimal band-gap for single-junction solar cells. The simple binary composition, together with non-toxic and earth-abundant constituents make this material very promising toward industrial production of low-cost thin-film solar cells. Theoretical calculations and experimental data have confirmed that Sb2Se3 exhibits strong anisotropic optoelectronic properties, due to the presence of infinite covalent (Sb4Se6)n ribbons along the (001) direction, whereas weak van der Waals interactions occur between ribbons. In such anisotropic materials, electronic transport is strongly affected by crystal structure: when ribbons are aligned perpendicularly to the solar cell surface, a better collection of photogenerated current occurs, leading to larger photovoltaic conversion efficiencies.In order to understand the alignment mechanisms of ribbons, Sb2Se3 thin films were grown by Low-Temperature Pulsed Electron Deposition (LT-PED) technique at different temperatures (from 200 °C to 400 °C) on a variety of substrates, such as glass, molybdenum, CdS, Fluorine-doped Tin Oxide (FTO) and nanostructured ZnO. The structural and morphological characterization of the Sb2Se3 films, performed by out-of-plane and in-plane X-Ray Diffraction and micro-Raman spectroscopy, demonstrated that ribbon alignment along the surface normal is strongly dependent on the used substrate. The comparison between solar cells grown in substrate configuration on Mo and FTO, clearly demonstrates the influence of ribbon orientation on short-circuit current.

Facile assembled N, S-codoped corn straw biochar loaded Bi2WO6 with the enhanced electron-rich feature for the efficient photocatalytic removal of ciprofloxacin and Cr(VI)

To the best of our knowledge, in few studies, biochar (BC)-based materials have been used as the photocatalyst for water purification, and their application is limited to a great extent due to catalyst agglomeration and inefficient electron migration. In this study, a novel Bi2WO6 loaded N, S co-doping corn straw biochar (Bi2WO6/NSBC) was successfully synthesized with a simple solvothermal method for the removal of ciprofloxacin (CIP) and Cr(VI) under visible light irradiation. The Bi2WO6/NSBC was featured with efficient and rapid catalytic removal toward CIP (5 mg/L) and Cr(VI) (10 mg/L), with efficiencies of ∼90.33% and ∼99.86% within 75 min, respectively. More attractively, this composite can be applied in a wide pH range (3.0–9.0) and with weak effects by coexisting ions (Cl−, CO32−, SO42−, and Ca2+). The facile synthesized porous graphitized structure demonstrates an outstanding performance of superior conductivity and promoted photoelectron transport. Meanwhile, it is found that N, S co-doping of the BC induces highly interconnected fibrous structures, high catalytic property, and favorable specific surface areas, which is considered to avoid agglomeration of Bi2WO6. The increased photocatalytic activity results from the synergistic effects of Bi2WO6 and NSBC by the optimized band gap and enhanced visible light response, due to higher migration and utilization efficiency of photoinduced carriers in photocatalytic reactions. In this approach, a cheap catalyst is provided, and at the same time, a synergistic effect of N, S co-doping is formed to rapidly remove contaminants in wastewater treatment.

Structural and morphological properties of PLD Sb2Se3 thin films for use in solar cells

Sb2Se3 has been promising absorber layer for photovoltaics application because of its high absorption coefficient, optimal bandgap, non-toxicity with earth-abundant components. However, the control of Sb2Se3 absorber orientation is not fully achieved, although the importance of this issue had been realized in early studies. Whereas (hkl, l ≠ 0) crystal orientations of Sb2Se3 are favorable for photovoltaic absorber. In this work, pulsed-laser deposition (PLD) has been used to systematically investigate the correlation between the growth parameters and crystallographic orientation of Sb2Se3 thin films. Unlike literature reports, it has been shown that not only growth temperature, but also background pressure and/or selenium pressure should be considered to grow Sb2Se3 thin films with preferential crystallographic orientations. Independent of the substrate temperature between room temperature and 400 °C, Sb2Se3 thin films tend to grow along (hkl, l = 0) crystal orientations under vacuum. These peaks were also dominant in post-annealed samples under both Ar and N2 atmosphere at 400 °C. On the other hand, Sb2Se3 thin films with (hkl, l ≠ 0) crystal orientations were successfully deposited by introducing the Ar partial pressure and changing the Sb:Se ratio in the target. It has been demonstrated that not only growth temperature, but also background pressure and Se content in the target (or Se vapor pressure) have strong influence on the formation of Sb2Se3 thin film with desired crystallographic orientations. We anticipate that this work could accelerate further optimization of material properties for high efficiency photovoltaic cells.

Synthesis of CZTS without sulfurization process and its performance evaluation on n-Si substrate as ITO-free photovoltaic cell

Kesterite Cu2ZnSnS4 (CZTS) nanostructure is synthesized without the sulfurization process using a solvothermal technique. The induced effects in the optical properties of the fabricated film of this nanostructure are studied. As XRD analysis revealed the as-prepared film is polycrystalline in nature which indexed in the tetragonal Kesterite structure of the CZTS phase. Optical properties related to solar cell parameters, such as absorption coefficient, penetration depth, Urbach energy, and energy band gap are studied. The absorption coefficient of 1.5 × 105 cm−1 is observed at 1.5 eV, which matches the optimum bandgap of the solar spectra. The penetration depth as a function of photon energy for CZTS is shown. Al/Si/CZTS/Al heterojunction photovoltaic cell is fabricated. The as-prepared fabricated cell shows a low short circuit current (Isc) of 1.5 mA and a low fill factor (FF) of 27.2%. Upon the annealing at 350 °C in the N2 atmosphere without any sulfurization, the cell performance was much enhanced, where the ISC improved up to 8.9 mA and the FF up to 33.5%. In addition, dramatic improvement in the power conversion efficiency (η) was observed from 0.91% up to 6.72% for the as-prepared and annealed cell, respectively.

Cu2Zn(Si,Ge)Se4 quaternary semiconductors as potential photovoltaic materials

The geometric and electronic structures of Cu2ZnSixGe1-xSe4 alloys are studied using density functional theory calculations. The variation of lattice constants is less than 1.2%. Negative formation enthalpies are obtained, indicating these alloys are completely miscible. The conduction band minimum gradually shifts upward with the Si concentration, resulting in a tunable bandgap from 1.27 to 2.26 eV. The bandgap of the Cu2ZnSi0.25Ge0.75Se4 alloy is very close to the optimal value. Compared with other two well-studied alloys with the optimal bandgap (Cu2ZnSi0.5Sn0.5Se4 and Cu2ZnSnS4), the Cu2ZnSi0.25Ge0.75Se4 has a red-shift in the optical absorption edge, highest absorption coefficient and smallest transmittance.

Step up in performance

Antimony selenosulfides are promising photovoltaic materials but obtaining high-quality absorber layers is challenging. Researchers now show that layers deposited using a hydrothermal method have optimal bandgap, good morphology and favourable growth orientation, enabling solar cells with 10% efficiency.

First-Principles Study on the Photoelectric Properties of CsGeI3 under Hydrostatic Pressure

CsGeI3 has been widely studied as an important photoelectric material. Based on the density functional theory (DFT), we use first-principles to study the photoelectric properties of CsGeI3 by applying successive hydrostatic pressure. It has been found that CsGeI3 has an optimal optical band gap value of 1.37 eV when the applied pressure is −0.5 GPa, so this paper focuses on the comparative study of the photoelectric properties when the pressure is −0.5 GPa and 0 GPa. The results showed that CsGeI3 has a higher dielectric value, conductivity, and absorption coefficient and blue shift in absorption spectrum when the pressure is −0.5 GPa. By calculating and comparing the effective masses of electrons and holes and the exciton binding energy, it was found that their values are relatively small, which indicates that CsGeI3 is an efficient light absorbing material. CsGeI3 was found to be stable under both pressure conditions through multiple calculations of the Born Huang stability criterion, tolerance factor T, and phonon spectrum with or without virtual frequency. We also calculated the elastic modulus of both pressure conditions and found that they are both soft, ductile, and anisotropic. Finally, the thermal properties of CsGeI3 under two kinds of pressure were studied. It was found that the Debye temperature and heat capacity of CsGeI3 increased with the increase of thermodynamic temperature, and the Debye temperature increased rapidly after pressure, while the heat capacity slowly increased and finally stabilized. Through the calculation of enthalpy, entropy, and Gibbs free energy of CsGeI3, it was found that the Gibbs free energy decreases faster with the increase of temperature without applied pressure, which indicates that CsGeI3 has a higher stability without pressure. Through the comparative analysis of the photoelectric properties of CsGeI3 under pressure, it was found that CsGeI3 after applied pressure is a good photoelectric material and suitable for perovskite solar cells (PSCs) material.

Effect of thickness, bandgap, and carrier concentration on the basic parameters of Cu2O nanostructures photovoltaics: numerical simulation study

ABSTRACT There is no clear data about the optimum thicknesses, band gaps, and charge densities of Cu2O thin films to fabricate solar cells. Therefore, here, Solar Cell Capacitance Simulator (SCAPS) program was employed to simulate the Cu2O nanostructures solar cells. Effect of window layer thickness, absorber layer thickness, bandgap, and carrier concentration on basic parameters of Cu2O solar cells were studied. Results revealed that window layer thickness in range from 0.3 to 0.4 µm is optimum to produce a higher performance of about 6.5%. Bandgap should be greater than 2.1 eV and donor carrier concentration under 1×1016 cm-3 are required to improve solar cell efficiency of about 8%. Built-in potential, width of the depletion layer, collection length of charge carrier, lifetime of minority carrier, and recombination rate are the main factors directing performance of devices. Consequently, employing our results to fabricate Cu2O solar cells is a step forward to improve efficiencies.

The effect of gradient conductivity of doped BiFeO3 as filler on the surface insulation performance of epoxy composite

In this paper, we studied the effect of filler conductivity on the surface insulation of composites. We proposed using different valence element doping as a strategy to prepare filler with gradient conductivity. The pristine BiFeO3 (BFO), Bi0.95Ca0.05FeO3, and Bi0.95Sm0.05FeO3 nanoparticles were prepared to fill into epoxy resin. The Rietveld refinement of the x-ray diffraction patterns was performed to investigate the transition of the lattice structure. According to the Kubelka–Munk function, the energy bandgap was calculated to verify the regulation of the BFO conductivity. Subsequently, we measured the flashover voltage of the composites with three types of BFO in ambient and vacuum atmosphere, respectively. We found the vacuum flashover is more sensitive to filler conductivity, and BFO with higher conductivity as filler is more conducive to surface insulation, whether in the air or vacuum. Finally, the optimal BFO bandgap of 2.1819 eV is determined for enhancing surface insulation of composites. Our research supplies a novel method for constructing high insulation composites for more wide applications and provides a scientific basis on flashover mechanism from the perspective of filler characteristics.

Recent Progresses on Metal Halide Perovskite-Based Material as Potential Photocatalyst

Recent years have witnessed an incredibly high interest in perovskite-based materials. Among this class, metal halide perovskites (MHPs) have attracted a lot of attention due to their easy preparation and excellent opto-electronic properties, showing a remarkably fast development in a few decades, particularly in solar light-driven applications. The high extinction coefficients, the optimal band gaps, the high photoluminescence quantum yields and the long electron–hole diffusion lengths make MHPs promising candidates in several technologies. Currently, the researchers have been focusing their attention on MHPs-based solar cells, light-emitting diodes, photodetectors, lasers, X-ray detectors and luminescent solar concentrators. In our review, we firstly present a brief introduction on the recent discoveries and on the remarkable properties of metal halide perovskites, followed by a summary of some of their more traditional and representative applications. In particular, the core of this work was to examine the recent progresses of MHPs-based materials in photocatalytic applications. We summarize some recent developments of hybrid organic–inorganic and all-inorganic MHPs, recently used as photocatalysts for hydrogen evolution, carbon dioxide reduction, organic contaminant degradation and organic synthesis. Finally, the main limitations and the future potential of this new generation of materials have been discussed.

Silicon-Integrated III–V Light Emitters and Absorbers Using Bipolar Diffusion

The ultimate target for silicon photonics is to merge together all electronic and photonic functions on the same CMOS platform. This requires efficient light emitters and absorbers directly integrated on silicon chips, the lack of which presently forms one of the major bottlenecks for further progress. In this work, a possible solution is presented where diffusion-driven charge transport (DDCT), lateral doping of silicon, and III--V nanowire growth are combined to create fully integrated near-surface light emitters and absorbers controlled solely by biasing the underlying silicon wafer. According to the three-dimensional full-device simulations carried out in this paper, DDCT enables high efficiencies for both light emission and photodiode operation in technically feasible silicon-integrated free-standing nanowire structures. Moreover, the results indicate that DDCT is especially well suited for the commonly used 1.55 $\ensuremath{\mu}\mathrm{m}$ wavelength due to the optimal band-gap difference with silicon, which promotes both a high injection efficiency and low voltage losses.

Bandgap tuning and optimization of green-emitting Zn2SnO4-Mg2SnO4:Mn2+ using combinatorial pulsed laser deposition

We investigated a compositional spread Mn2+-doped Zn2SnO4-Mg2SnO4 (Mn-ZMTO) film prepared by combinatorial pulsed laser deposition. The composition of the prepared film gradually changed from Zn2SnO4 (ZTO) to Mg2SnO4 (MTO). Moreover, the lattice constant decreased with an increase in MTO composition. Under ultraviolet irradiation, the film exhibited bright green luminescence attributable to the transition from 4T1 to 6A1 of Mn2+. This is the report of the observation of green luminescence from Mn-ZMTO on the ZTO-rich side. In addition, the most intense luminescence was located at the center of the sample. The emission intensity and the most intense position were changed by irradiation at various excitation wavelengths. This was clarified by the positional dependence of PLE spectra. The bandgap of the prepared film, which corresponded to the PLE peak wavelength, was larger than that of the bulk material and increased with an increase in Mg concentration. Since emission cannot usually be observed from Mn-doped ZTO, we suggest that the luminescence from the prepared film was caused by an increase in the bandgap attributable to the 1-dimentional thin film size effect along thickness direction. Thus, using the thin film technique we revealed that bandgap tuning is important in phosphor material.

Fabrication of N-GQDs and AgBiS2 dual-sensitized ZIFs-derived hollow ZnxCo3-xO4 dodecahedron for sensitive photoelectrochemical aptasensing of ampicillin

A sensitive photoelectrochemical (PEC) aptasensing platform based on Nitrogen-doped graphene quantum dots (N-GQDs) and AgBiS2 dual-sensitized Zn/Co bimetallic oxide (ZnxCo3-xO4) dodecahedron as a matrix was constructed for the ultrasensitive detection of ampicillin (AMP). Concretely, the ZnxCo3-xO4 dodecahedron derived from ZnCo-ZIFs retained the unique hollow porous structure, which provided large specific surface and efficacious active sites, and thereby significantly enhancing the light utilization and shortening the photoelectrons transmission path. Both N-GQDs with the up-conversion effect and eco-friendly AgBiS2 with optimal band gap for solar absorption were utilized to dual-sensitize ZnxCo3-xO4 dodecahedron to further strengthen the PEC performance. Subsequently, the aptamer recognition technology was integrated with the modified photoelectrode, adopting AMP-aptamer to catch the target AMP, accompanied the hole oxidation of bind AMP causing the remarkable increase of photocurrent signal, the AMP could be specific and sensitive detect. Under optimized conditions, the photocurrent of developed aptasensor increased linearly displayed large liner response of 0.5 pmol/L – 10 nmol/L with a detection limit of 0.25 pmol/L (S/N = 3). Besides, the novel PEC AMP-sensoring exhibited excellent reproducibility, perfect stability and selectivity, which could open a potential avenue for antibiotic residues detection in environmental media.

25.1% High‐Efficiency Monolithic Perovskite Silicon Tandem Solar Cell with a High Bandgap Perovskite Absorber

Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p–i–n tandem configuration. A methylammonium‐free FA0.75Cs0.25Pb(I0.8Br0.2)3 perovskite with high Cs content is investigated for improved stability. A 10% molarity increase to 1.1 m of the perovskite precursor solution results in ≈75 nm thicker absorber layers and 0.7 mA cm−2 higher short‐circuit current density. With the optimized absorber, tandem devices reach a high fill factor of 80% and up to 25.1% certified efficiency. The unencapsulated tandem device shows an efficiency improvement of 2.3% (absolute) over 5 months, showing the robustness of the absorber against degradation. Moreover, a photoluminescence quantum yield analysis reveals that with adapted charge transport materials and surface passivation, along with improved antireflection measures, the high bandgap perovskite absorber has the potential for 30% tandem efficiency in the near future.

Enhancement of the performance of kesterite thin-film solar cells using dual absorber and ZnMgO buffer layers

A new structure of kesterite thin-film solar cells is simulated using SCAPS software. In addition, the structures of CZTS/ZnMgO, CZTSSe/CZTS/CdS and CZTSSe/CZTS/ZnMgO are suggested and scrutinized. In CZTS/ZnMgO structure, the efficiency, open-circuit voltage and short circuit current at the absorber layer thickness of 3 μm have been improved 35%, 15%, and 15% respectively. Furthermore, the efficiency of the proposed solar cell has been enhanced 60% by adding the CZTSSe absorber layer on the back of the CZTS layer. The optimum buffer layer thickness in the structures of CZTSSe/CZTS/CdS and CZTSSe/CZTS/ZnMgO are 150 nm and 125 nm, respectively. The simulation results show that the basic photovoltaic parameters (VOC, JSC, FF and η) have been decreased by variation of temperature from 300 K to 400 K. The optimal bandgap of the CZTSSe and ZnMgO in the structure of CZTSSe/CZTS/ZnMgO are obtained 0.9eV and 3.48eV, respectively in the ZnMgO optimal thickness of 50 nm. The highest efficiency of the kesterite solar cell with Zn0.81Mg0.19O buffer layer is 17.05%.