Varying Band Gap Research Articles

Numerical analysis of the effect of MoS2 interface layers on copper-zinc-tin-sulfur thin film solar cells

MoS2 interface layers are often present in high-temperature sulfurized Cu2ZnSnS4 (CZTS) solar cells, but their effects remain poorly characterized. In this study, the effect of MoS2 on CZTS solar cells was analyzed in simulation. Meanwhile, the quantum confinement effects of MoS2, that is, the varied band gap of MoS2 with the thickness of MoS2 have been considered. When the thickness of MoS2 was varied, the performances of CZTS solar cells were improved by the p-type and n-type MoS2 with suitable thickness due to the reduction of band gap, decreasing height of barrier of p-CZTS/MoS2. The barrier was always observed at the MoS2/Mo interface. When the holes recombination velocity of MoS2/Mo interface was changed, the photovoltaic properties of CZTS solar cells were improved by the suitable recombination velocity of MoS2/Mo interface. The simulated results about the effect of MoS2 thickness were consistent with the reported experiment results.

The effect of the refractive index profile on the optical response of plasmonic nanostructures inside semiconductors

The inclusion of plasmonic nanostructures inside semiconductors has been explored over the last decades with the prominent application of increasing the efficiency in solar cells. The plasmonic properties were often only phenomenologically explained by comparison of experiments with simulations. In this work we investigated the plasmonic properties of a silver particle inside virtual semiconductors with varying band gaps. Because the particle plasmon peaks strictly followed the virtual semiconductor band gap, it was found that only the maxima in the real part of the refractive index were responsible for the presence of particle plasmon peaks. This model, in which the plasmonic response is to a large extent determined by the semiconductor refractive index profile, explains previous investigations of plasmonic nanostructures inside semiconductors in a simple way and will enable a detailed design and explanation of future nanostructured plasmonic systems for solar cells and other devices.

Study of Molarity Concentration Variation Over Optical, Structural and Gas Sensing Response for ZnO Thin Film Based NOx Gas Sensor

Zinc oxide (ZnO) thin films as sensing layer with different molarity concentration are obtained by the sol–gel method using spin coating technique. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O) was taken as starting material. For desired molarity weighed Zinc Acetated dihydrate is dissolved in particular amount of Ethanol. The solution is put on hot plate with Magnetic stirrer at a temperature of 60 °C while stirring 400 rpm to 600 rpm procedure continued for 45 min to 60 min. After that the solution was kept at room temperature for 24 h for ageing effect. By using spin coating method the prepared solution was coated on silicon substrate and then annealed for preparing highly crystalline ZnO thin films. The optical surface morphological and structural studies were done for the synthesized ZnO thin films. The important parameter considered for the gas sensing behavior are operating temperature and sensor response The reported study provides a significant development towards ZnO thin film thickness variations with a very high sensitivity, rapid response and recovery times. The UV–Vis transmittance spectra of ZnO thin films has been studied over quartz substrate under similar deposition parameters. The single coated samples were prepared with different molarities of 1 M, 0.7 M, 0.5 M, 0.33 M, 0.1 M, 0.05 M. Energy band gap of each sample is calculated by using Tauc plot and studied for the varying band gap & the gas sensitivity for the samples.

THE EFFECT OF SEVERAL PARAMETERS ON THE PERFORMANCE of CuInS2-BASED SOLAR CELLS USING THE SCAPS-1D SOFTWARE

In this work, we have used one dimensional solar cells simulator SCAPS-1D (Solar Cell Capacitance Simulator) to design solar cells based on CuInS2 as the absorber material and study their device performances. Solar cells having a typical structure Al/ZnO:Al/CdS/CuInS2/Mo have been modeled. We focus on studying effects of varying band gap and thickness of CuInS2 absorber layer on the performance of the CuInS2 based solar cells. And also, various replacements for conventional cadmium sulphide (CdS) buffer layer, such as ZnSe and In2S3 based buffer layers have been studied to find out the optimum choice. The photovoltaic parameters (short-circuit current density (Jsc), open-circuit voltage (Vco), fill factor (FF) and effciency (η)) have been calculated from the current density-voltage curves. In this study, a simulated effciency of 21.93 % has been obtained with Vco of 0.94 V, Jsc of 27.64 mA/cm2 and FF of 84.25 % for the CuInS2 solar cell with an absorber layer band gap of 1.40 eV, absorber thickness of 2µm and In2S3 buffer layer. The analysis made from this numerical simulation has revealed the good structure Al/ZnO:Al/In2S3/CuInS2/Mo solar cell.

Strain-optoelectronic coupling properties of externally deformed nanoribbons with embedded quantum well

Strain-engineering provides a critical mechanism for tuning the band structure of quantum wells (QWs). However, due to the inherent rigidity of the semiconductor materials, the strain induced by lattice-mismatch is restricted to be invariable, uniform, thus cannot be subject to direct mechanical loading processes. Therefore, the flexible nanoribbons (NRs) in a variety of configurations offer entirely new research perspectives in modulation of the optoelectronic characteristic of QWs by curvature-induced inhomogeneous strain. In this paper, we propose a strategy to fabricate the QW layer embedded in uniaxially wavy NRs. To thoroughly figure out the internal strain-optoelectronic coupling mechanism, we establish a simple and accurate calculation model for externally deformed QW NRs using the eight-band k·p perturbation method for the first time. Luttinger-Kohn-Pikus-Bir Hamiltonian (LKPBH) for strained semiconductor is adopted and solved by finite-difference method (FDM). Theoretical calculations reveal inclined band edges, which result in the quantum-confined stark effect (QCSE) in the bended QW NRs. Continuously and periodically varied band gap of the wavy QW is revealed with a blue-shift from peak to valley of the sample NR, which agrees well with the μ-photoluminescence measurements. Further adjustments of the external configuration are explored to study the impact of the larger deformed structure on the curvature and band gap of the wavy QW within the confinement of the fracture limit.

1T-Phase Tungsten Chalcogenides (WS2, WSe2, WTe2) Decorated with TiO2 Nanoplatelets with Enhanced Electron Transfer Activity for Biosensing Applications

Layered transition metal dichalcogenides (TMDs) have received a great deal of attention due to fact that they have varied band gap, depending on their metal/chalcogen composition and on the crystal structure. Furthermore, these materials demonstrate great potential application in a myriad of electrochemical technologies. Heterogeneous electron transfer (HET) abilities of TMD materials toward redox-active molecules occupy a key role in their suitability for electrochemical devices. Herein, we introduce a promising biosensing strategy based on improved heterogeneous electron transfer rate of WS2, WSe2, and WTe2 nanosheets exfoliated using tert-butyllithium (t-BuLi) and n-butyllithium (n-BuLi) intercalators decorated with vertically aligned TiO2 nanoplatelets. By comparison of all the nanohybrids, decoration of TiO2 on t-BuLi WS2 (TiO2@t-BuLi WS2) results in the fastest HET rate of 5.39 × 10–3 cm s–1 toward ferri/ferrocyanide redox couple. In addition, the implications of decorating tungsten dichalcogenides ...

Ultrafast exciton many-body interactions and hot-phonon bottleneck in colloidal cesium lead halide perovskite nanocrystals

Defect-tolerant perovskite nanocrystals of the general formula Cs-Pb-${X}_{3}$ (where $X=\mathrm{Cl}$, Br, and I) have shown exceptional potential in fundamental physics as well as in novel optoelectronic applications as the next generation of solar cells. Although exciton many-body interactions such as biexciton Stark shift, state filling, and Auger recombination are studied extensively, other important correlated effects, such as band gap renormalization (BGR) and hot-phonon bottleneck, are not explored in these nanocrystals. Here we experimentally demonstrate the carrier density dependence of the BGR and an effective hot-phonon bottleneck in $\mathrm{CsPb}{(\mathrm{C}{\mathrm{l}}_{0.20}\mathrm{B}{\mathrm{r}}_{0.80})}_{3}$ mixed-halide nanocrystals. The results are compared with two other halide compositions, namely, ${\mathrm{CsPbBr}}_{3}$ and $\mathrm{CsPb}{(\mathrm{B}{\mathrm{r}}_{0.55}{\mathrm{I}}_{0.45})}_{3}$ nanocrystals with varying band gaps. The optical response of the nanocrystals changes dramatically across the spectral range of many hundreds of meV at high carrier density due to large BGR. We have calculated the BGR constant $\ensuremath{\approx}(6.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ cm for $\mathrm{CsPb}{(\mathrm{C}{\mathrm{l}}_{0.20}\mathrm{B}{\mathrm{r}}_{0.80})}_{3}$ nanocrystals that provides the amount of band gap shift as a function of carrier density. In these nanocrystals, an efficient hot-phonon bottleneck is observed at a carrier density of $3.1\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ that slows down the thermalization by 1 order of magnitude. Our findings reveal that the complex kinetic profile of the exciton dynamics can be analyzed by the global target analysis using the sequential model with increasing lifetimes.

Pressure-Tuneable Visible-Range Band Gap in the Ionic Spinel Tin Nitride.

The application of pressure allows systematic tuning of the charge density of a material cleanly, that is, without changes to the chemical composition via dopants, and exploratory high‐pressure experiments can inform the design of bulk syntheses of materials that benefit from their properties under compression. The electronic and structural response of semiconducting tin nitride Sn3N4 under compression is now reported. A continuous opening of the optical band gap was observed from 1.3 eV to 3.0 eV over a range of 100 GPa, a 540 nm blue‐shift spanning the entire visible spectrum. The pressure‐mediated band gap opening is general to this material across numerous high‐density polymorphs, implicating the predominant ionic bonding in the material as the cause. The rate of decompression to ambient conditions permits access to recoverable metastable states with varying band gaps energies, opening the possibility of pressure‐tuneable electronic properties for future applications.

Pressure‐Tuneable Visible‐Range Band Gap in the Ionic Spinel Tin Nitride

AbstractThe application of pressure allows systematic tuning of the charge density of a material cleanly, that is, without changes to the chemical composition via dopants, and exploratory high‐pressure experiments can inform the design of bulk syntheses of materials that benefit from their properties under compression. The electronic and structural response of semiconducting tin nitride Sn3N4 under compression is now reported. A continuous opening of the optical band gap was observed from 1.3 eV to 3.0 eV over a range of 100 GPa, a 540 nm blue‐shift spanning the entire visible spectrum. The pressure‐mediated band gap opening is general to this material across numerous high‐density polymorphs, implicating the predominant ionic bonding in the material as the cause. The rate of decompression to ambient conditions permits access to recoverable metastable states with varying band gaps energies, opening the possibility of pressure‐tuneable electronic properties for future applications.

Composition‐Tuned Wide Bandgap Perovskites: From Grain Engineering to Stability and Performance Improvement

AbstractWide bandgap (WB) organic–inorganic hybrid perovskites (OIHPs) with a bandgap ranging between 1.7 and 2.0 eV have shown great potential to improve the efficiency of single‐junction silicon or thin‐film solar cells by forming a tandem structure with one of these cells or with a narrow bandgap perovskite cell. However, WB‐OIHPs suffer from a large open‐circuit voltage (Voc) deficit in photovoltaic devices, which is associated with the phase segregation of the materials under light illumination. In this work the photoinstability is demonstrated and Voc loss can be addressed by combining grain crystallization and grain boundary passivation, achieved simultaneously through tuning of perovskite precursor composition. Using FA0.17Cs0.83PbI3–xBrx (x = 0.8, 1.2 1.5, and 1.8), with a varied bandgap from 1.72 to 1.93 eV, as the model system it is illustrated how precursor additive Pb(SCN)2 should be matched with a proper ratio of FAX (I and Br) to realize large grains with defect‐healed grain boundaries. The optimized WB‐OIHPs show good photostability at both room‐temperature and elevated temperature. Moreover, the corresponding solar cells exhibit excellent photovoltaic performances with the champion Voc/stabilized power output efficiency reaching 1.244 V/18.60%, 1.284 V/16.51%, 1.296 V/15.01%, and 1.312 V/14.35% for WB‐OIHPs with x = 0.8, 1.2, 1.5, and 1.8, respectively.

Red phosphorus in its two-dimensional limit: novel clathrates with varying band gaps and superior chemical stabilities.

First-principles calculations within density functional theory reveal the preferred structures of red phosphorus in the two-dimensional (2D) limit to be porous with intriguing structural, electronic, and chemical properties. These few-atomic-layer structures are stabilized as novel 2D clathrates with tunable pore sizes and varying semiconducting band gaps, labelled as V-Hex, P-Monoclinic, P-Hex, and V-Tetr in descending energetic stabilities. The cohesive energies of the 2D clathrates are all substantially higher than that of white phosphorus. More strikingly, the V-Hex structure is energetically as stable as black phosphorene, but possesses distinctly superior chemical stability when exposed to O2 due to the presence of a much higher activation barrier against chemisorption. We also exploit the salient properties of these 2D clathrates for their important application potentials, including serving as effective elemental photocatalysts for visible-light-driven water splitting, and as a new class of sieves for molecular separation and DNA sequencing.

Functionally graded nanocrystalline silicon powders by mechanical alloying

Functionally graded nanocrystalline silicon powder, having varied bandgap. has been prepared using mechanical alloying technique. The solar grade (99.999% pure) nanocrystalline silicon (nc-Si) powder was ball milled in argon atmosphere in an attrition mill for 4 h to 20 h (4 h, 12 h and 20 h) to reduce crystallite size in it. The milled powder samples were then degassed, to remove entrapped gases, in a vacuum of 10−2 torr at 200 °C for 1 h. Transmission electron microscopy and X-ray diffraction studies show the crystallite size of all the powders are in the nanometer range and decrease in crystallite size was seen with increasing milling time. Raman spectroscopy shows nanocrystalline phases in the milled powder particles with 100% crystalline volume fraction. UV–Vis-IR spectroscopy was used to determine bandgap of the powder samples using Tauc formula. The bandgap and crystallite size in silicon powder is found to be the function of milling duration, without addition of any alloying element. The nc-Si powder with different bandgaps may find potential applications in the field of functionally graded solar cells.

Effects of alkaline-earth dopants on structural, optical and magnetic properties of Bi2Fe4O9 powders

The pure Bi2Fe4O9 and Bi2 (1−x) A2xFe4O9 (A = Ca, Ba, x = 0.03) powders are synthesized via a modified solid-state reaction method to study the effects of alkaline-earth metal ions doping on crystal structural, optical and magnetic properties. Both X-ray diffraction and Raman spectroscopy data reveal that all the powders are Mullite-type Bi2Fe4O9 orthorhombic single phase without any impurities. Much greater structural distortion in Bi1.94Ba0.06Fe4O9 than that of Bi1.94Ca0.06Fe4O9 is observed. The chemical compositions of Ba2+ and Ca2+ doped powders have been investigated with energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy results indicate that oxygen vacancies could be found in all doped powders. The ratio of Fe2+ in the total Fe ions is almost unchanged by Ca doping and increases a little with Ba substitution. Compared with that of pure Bi2Fe4O9, the band gap values decrease slightly in Bi1.94Ca0.06Fe4O9 but drop dramatically in Bi1.94Ba0.06Fe4O9. A clear and obvious ferromagnetic behavior is found in Bi1.94Ba0.06Fe4O9 at 10 K. However, Bi1.94Ca0.06Fe4O9 shows a weak ferromagnetism with enhanced magnetization and Bi2Fe4O9 exhibits antiferromagnetism with a linear M–H relationship. The varied bandgap and magnetization resulting from the alkaline-earth metal ionic species are discussed in terms of structural distortion due to the ionic radius size effect.

Graphene/BiVO4/TiO2 nanocomposite: tuning band gap energies for superior photocatalytic activity under visible light

A facile, ultrasonic wave-assisted one pot hydrothermal method has been developed to fabricate reduced graphene oxide/bismuth vanadate/titanium oxide (RGO/BiVO4/TiO2) ternary nanocomposites. By utilizing graphene oxide (GO) as multifunctional structure, RGO/BiVO4/TiO2 (GBT) with diverse percentage composition possessing varying band gap energies is obtained. XRD and Raman spectroscopy evince formation of tetragonal and monoclinic phases of BiVO4. The band gap energies of the components of the composite were determined by applying modified Kubelka–Munk function on UV–Vis DRS data. Tuning of band gap energy of the BiVO4 and TiO2 were simultaneously achieved by modifying the concentrations of GO and TiO2 during synthesis. The GBT exhibited enhanced photocatalytic degradation of methylene blue (MB) under visible light irradiation. The relative photocatalytic activity rates of the composites in GBT series are in agreement with the photoluminescence data. The mechanism behind the activity suggests GO acting as an electron trapper and TiO2 behaving as an efficient mediating co-catalyst. The band gap energy tuning led to reduction in time needed for complete MB degradation from 40 min with RGO/BiVO4 to 10 min with the ternary composite GBT. It is expected that the work would encourage new vistas to engineer different combinations of graphene based ternary composites which might lead to potential applications guided by band gap tuning.

Adsorption of alkali and alkaline earth metal atoms and dimers on monolayer germanium carbide

First-principles plane wave calculations have been performed to study the adsorption of alkali and alkaline earth metals on monolayer germanium carbide (GeC). We found that the favourable adsorption sites on GeC sheet for single alkali and alkaline earth adatoms are generally different from graphene or germanene. Among them, Mg, Na and their dimers have weakly bounded to GeC due to their closed valence electron shells, so they may have high mobility on GeC. Two different levels of adatom coverage ( and ) have been investigated and we concluded that different electronic structures and magnetic moments for both coverages owing to alkali and alkaline earth atoms have long range electrostatic interactions. Lithium atom prefers to adsorbed on hollow site similar to other group-IV monolayers and the adsorption results in metallisation of GeC instead of semiconducting behaviour. Na and K adsorption can induce 1 total magnetic moment on GeC structures and they have shown semiconductor property which may have potential use in spintronic devices. We also showed that alkali or alkaline earth metal atoms can form dimer on GeC sheet. Calculated adsorption energies suggest that clustering of alkali and alkaline earth atoms is energetically favourable. All dimer adsorbed GeC systems have nonmagnetic semiconductor property with varying band gaps from 0.391 to 1.311 eV which are very suitable values for various device applications.

Development and studies of Cd1−xMgxTe thin films with varying band gaps to understand the Mg incorporation and the related material properties

In this paper we report a systematic work involving the development of Cd1−xMgxTe thin films by co-evaporation of CdTe and Mg. The evaporation rate of both materials were adjusted to obtain ternary films of varying stoichiometry and hence the band gap. We have deposited films with band gap ranging from 1.47 to 2.41eV. The films were characterized for structural, morphological, optical, opto-electronic, and spectroscopic properties. The film stoichiometry was studied across the thickness using SIMS data. SEM images showed that the grain size has a dependence on Mg content in the film, which inhibits the grain growth. The structural parameters showed a systematic dependence on Mg content in the film, however, there was no noticeable change in the XRD reflections with respect that of pure CdTe for lower concentrations of Mg. XPS analysis shed light on the incorporation of Mg further supporting the band gap variations observed with the UV–Vis spectroscopic studies. The photoresponse of the film was affected by Mg incorporation. Prototype devices of the type Cd1−XMgxTe/CdS were fabricated and the results are discussed.

Precursor Triggering Synthesis of Self-Coupled Sulfide Polymorphs with Enhanced Photoelectrochemical Properties.

Heteronanostructures have attracted intensive attention due to their electronic coupling effects between distinct components. Despite tremendous advances of nanostructure fabrication, combining independent polymorphs by forming heterojunction is still challenging but fascinating, such as copper sulfides (Cu2-xS), exhibiting varying band gaps and crystal structures with various stoichiometries. Herein, self-coupled Cu2-xS polymorphs (Cu1.94S-CuS) by a facile one-pot chemical transformation route have been reported for the first time. Unprecedentedly, a manganous precursor plays a crucial role in inducing and directing the formation of such a dumbbell-like architecture, which combines 1D Cu1.94S with 2D CuS. During the transformation, Mn2+ ions mediate the Cu+ ions diffusion and phase conversion process particularly. Furthermore, this self-coupled polymorphic structure exhibits significantly enhanced photoelectrochemical properties compared with the individual Cu1.94S nanocrystals and CuS nanoplates, originating from the unique heterointerfaces constructed by intrinsic band alignment and the enhanced contact between high conductivity hexagonal planes and the working electrode revealed by density functional theory (DFT) calculations. So we anticipate this emerging interfacial charge separation could provide useful hints for applications in optoelectronic devices or photocatalysis.

Observation of scattering parameters for bandgap-tuned graphene oxide under 488 nm illumination

Excellent high frequency and optoelectronic material properties of graphene oxide have been highlighted recently. Our study reveals interactions between high frequency up to 4 GHz and 488 nm light wave in the thermally tunable band-gap of annealed graphene oxide to investigate the effect of 488 nm wave induces additional carriers. Graphene oxide is a strong candidate for studying high frequency and optical interactions due to excellent scattering parametric characteristics being a few nano-meters thick and having outstanding optical characteristics with a variable band-gap. Measurement results of the high frequency device composed of annealed graphene oxide with optical illumination demonstrate that the photon-assisted carriers affect variation of scattering parameters. The signals were changed due to π–π* interaction and excitons affected by varied band gap of graphene oxide and illumination intensity, respectively. The graphene oxide interconnector is a promising candidate for a controllable high frequency device. This initial study not only gives further insight into the interaction of high frequency and 488 nm wave with graphene oxide reduced to different levels, but also reveals a way to control the scattering by optic power.

Crystalline to amorphous phase transformation of Ta2O5 quantum dots driven by residual stress

Simple vacuum thermal evaporation technique was used to grow Ta2O5 quantum dots. The small dots with large voids between them in ultra-thin films were found to gradually grow in size with decreasing voids with increase in film thickness. The development of residual stress was observed with increasing dot size or film thickness. Residual stress effect on band gap of orthorhombic Ta2O5 quantum dots is studied. Both SAED and XRD confirm that the films thicker than 45 nm become amorphous. Significantly varying optical band gap with film thickness was observed by studying optical absorption of the films. The variation of residual stress with film thickness scales linearly very well with the varying band gap with film thickness. Stress free band gap (3.94 eV) and pressure coefficient of band gap (∂Eg/∂P)T = −26.4 meV/GPa) were determined from this study. Transformation from crystalline to amorphous phase in orthorhombic form of Ta2O5 has been determined to be at about 18 GPa. Both crystalline to amorphous transition and pressure coefficient of band gap for orthorhombic Ta2O5 driven by residual stress have been determined for the first time in the literature.

(Invited) Charge Separation and Recombination at Semiconducting Single-Walled Carbon Nanotube Interfaces

The time scales and mechanisms for interfacial charge separation and recombination play crucial roles in determining efficiencies of excitonic photovoltaics. Semiconducting SWCNTs are robust light absorbers that have garnered increasing attention as the electron-donating or electron-accepting components in excitonic solar cells. In particular, near-infrared photons are harvested efficiently by semiconducting single-walled carbon nanotubes (SWCNTs) paired with appropriate electron acceptors, such as fullerenes (e.g. C60). While the reported AM1.5 power conversion efficiencies of such devices are steadily increasing, significant improvements remain to be made if a better fundamental understanding is reached for the kinetics and thermodynamics of exciton dissociation and charge recombination. In this presentation, I will discuss a number of time-resolved spectroscopic studies on donor:acceptor heterojunctions that incorporate semiconducting SWCNTs with widely varying band gaps and frontier orbital energies. Following the evolution of excitons and charges with multiple techniques provides a platform for understanding the time scales and quantum yields for interfacial charge transfer, recombination rates and mechanisms, and the dependence of charge transfer rates and yields on interfacial energetics.