Formation Of Intermediate Band Research Articles

Chromium nano-segregation and intermediate band formation in heavily-doped ZnS:Cr films

X-ray absorption spectroscopy, X-ray diffraction, and UV–Vis spectroscopy were employed to investigate heavily-doped ZnS:Cr films prepared by RF magnetron co-sputtering. EDS indicates ca. 5 at. % Cr incorporation as well as slight S-deficiency. XAS in the Zn K- and Cr K-edges revealed Zn-by-Cr substitution and segregation of Cr nanoclusters which lead to significant lattice distortion. XRD indicates a 44 % increase in the strain, an increase in the lattice parameter, and a decrease in the crystallite size upon Cr incorporation. The presence of Cr nanoclusters contributed to changes in the band gap, a large Urbach tail parameter of 565 meV, and the formation of an intermediate band arising from the Cr 3d spin-up states. The results suggest ZnS:Cr films in the heavily doping regime could lead to applications in photovoltaics and spintronics.

A Novel Approach for Designing a Sub-Bandgap in CuGa(S,Te)2 Thin Films Assisted with Numerical Simulation of Solar Cell Devices for Photovoltaic Application.

As a well-explored chalcopyrite material, copper gallium sulfide CGS has been considered a potential material for solar cell absorber layers. However, its photovoltaic attributes still require to be improved. In this research, a novel chalcopyrite material, copper gallium sulfide telluride CGST, has been deposited and verified as a thin film absorber layer to fabricate high-efficiency solar cells by experimental testing and numerical simulations. The results display the intermediate band formation in CGST with incorporation of Fe ions. Electrical studies showed enhancement in mobility from 1.181 to 1.473 cm2 V-1 s-1 and conductivity from 2.182 to 5.952 S cm-1 for pure and 0.08 Fe-substituted thin films. The I-V curves display the photoresponse and ohmic nature of the deposited thin films, and the maximum photoresponsivity (0.109 A W-1) was observed for 0.08 Fe-substituted films. Theoretical simulation of the prepared solar cells was carried out using SCAPS-1D software, and the obtained efficiency displayed an increasing trend from 6.14 to 11.07% as the Fe concentration increased from 0.0 to 0.08. This variation in efficiency is attributed to the decrease in bandgap (2.51-1.94 eV) and the formation of an intermediate band in CGST with Fe substitution, which is evidenced in UV-vis spectroscopy. The above revealed results open the way to 0.08 Fe-substituted CGST as a promising candidate as a thin film absorber layer in solar photovoltaic technology.

Structural, elemental, and electrical conduction properties of corundum-type Mg2Co2Nb2O9

ABSTRACT is a new member of the corundum-type family. Its crystal structure is trigonal with space group . and ions share the A-site with fifty-fifty occupancy. In , exhibits a 2 + oxidation state, whereas shows a reduction in the charge state from 5 + to 4 + due to the presence of vacancy-trapped electrons in dimmer. The conductivity spectra analysis in view of various transport models states that overlapping large polaron tunneling is the predominant transport process (polaron radius 10.65 Å) in the low-temperature range (303-473 K), while for the high-temperature range (573-673 K), the conduction is intrinsic, and the carriers are primarily small polarons. The proximity between optical bandgap (∼1.60 eV) and conduction activation energy (1.64 eV) is due to the formation of intermediate energy bands near the Fermi level and their participation in high-temperature intrinsic conduction.

Tailoring of structural, optical and electrical properties of anatase TiO2 via doping of cobalt and nitrogen ions

Pure (pristine) anatase and mono–doped and co–doped derivatives of TiO2 having nitrogen (N) and cobalt (Co) as dopants with respective fixed doping concentrations of 0.7 mol.% and 1.0 mol.% were synthesized using auto–combustion sol–gel technique. The doping effects at corresponding non–metal and transition metal sites of TiO2 on the basis of the structural, optical and electrical properties have been investigated. X–ray diffraction (XRD) measurement confirms the formation of pure anatase phase of TiO2 for all samples having I41/amd space group of tetragonal structure which has been also verified by the Raman spectroscopy measurement. Various crystallographic parameters have been calculated by performing Rietveld refinement of XRD data including average crystallite size that has been observed in the range of 10–15 nm. Pure anatase phase indicates the incorporation of Co2+ into TiO2 lattice which assists the substitution of N in place of oxygen in co–doped TiO2. The band gap tuning towards the visible region from 3.2 to 2.1 eV has been achieved with mono–doping and co–doping of the N and Co in TiO2 lattice. This can be described in terms of the formation of localized levels of N–2p and Co–3d states in mono–doping and an isolated intermediate band formation in co–doping case. Electrical properties have been investigated in details and explained as the synergetic effects of structural and inherent ionic characters of various dopants. The observed band gaps of all doped samples lie within the visible region which makes them pertinent as the solar energy harnessing materials for photocatalytic and photovoltaic applications.

Fe-doped CuGaS2 (CuGa1-xFexS2) - Detailed analysis of the intermediate band optical response of chalcopyrite thin films based on first principle calculations and experimental studies

We have theoretically and experimentally studied the tailoring of intermediate bands in the CuGa1-xFexS2 chalcopyrite system. An ab initio density functional theory calculations within GGA + U approach have been performed to model pristine and Fe-doped CuGaS2 to understand their structural and electronic properties and thereby understand the origin of intermediate states produced with Fe doping. Thin films of the pristine and Fe-doped CuGaS2 of various compositions are deposited using chemical spray pyrolysis for the experimental studies. The crystal structure, morphology, and topography of the deposited thin films are examined using Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM), respectively. The binding energy and chemical composition of CuGa0.95Fe0.05S2 thin films are determined using X-ray Photoelectron Spectroscopy (XPS). The direct and intermediate band optical responses have been probed through UV–Vis–NIR Spectroscopy. Pristine CuGaS2 exhibits a direct bandgap of 2.43 eV, with formation of intermediate bands in Fe-doped CuGaS2 at 1.82 eV, 1.75 eV, 1.54 eV and 1.49 eV with varying Fe atomic percentage (X = 0.025, 0.05, 0.075, and 0.1 at. %). The optimum Fe concentration is determined to be X = 0.05 (CuGa0.95Fe0.05S2) as it exhibits the highest photocurrent (1.46 mA), electrical conductivity, and intermediate band optical response. The IB formation is further supported by the calculated density of states of the Fe doped CuGaS2 structure.

Role of intermediate band and carrier mobility in Sn/Fe doped CuAlS2 thin film for solar cell: An ab-initio study

Formation of intermediate band in environmentally stable and wide band gap semiconductor, CuAlS2, have been studied towards potential, low cost and efficient thin film solar cell material. We investigate the effect of sustainable low-cost dopant (Sn and Fe) in CuAlS2 film by means of density functional theory within the modified Becke-Johnson approximation for the calculation of electronic and optical properties. Polar film of CuAlS2 in (001) direction shows lower band gap than the bulk CuAlS2 due to the excess Cu and Al atoms. Doping of Sn and Fe in CuAlS2 form partially filled, isolated intermediate band. Absorption coefficients for Sn and Fe doped films show higher absorption under visible energy region than the pristine bulk and film. Fe and Sn doped films show efficiencies of 34.67 and 35.64%, respectively within spectroscopic limited efficiency. By means of present study, Sn and Fe doping formed intermediate band, improved absorption in visible light and power conversion efficiency. We suggest Sn doped CuAlS2 as the promising material due to higher effective mass ratio of charge carriers, hence lower rate of recombination, indicates improved performance of CuAlS2 based thin film solar cell.

The effect of isovalent doping on the electronic band structure of group IV semiconductors

The band gap engineering of group IV semiconductors has not been well explored theoretically and experimentally, except for SiGe. Recently, GeSn has attracted much attention due to the possibility of obtaining a direct band gap in this alloy, thereby making it suitable for light emitters. Other group IV alloys may also potentially exhibit material properties useful for device applications, expanding the space for band gap engineering in group IV. In this work the electronic band structure of all group IV semiconductor alloys is investigated. Twelve possible A:B alloys, where A is a semiconducting host (A = C, Si, and Ge) and B is an isovalent dopant (B = C, Si, Ge, Sn, and Pb), were studied in the dilute regime (0.8%) of the isovalent dopant in the entire Brillouin zone (BZ), and the chemical trends in the evolution of their electronic band structure were carefully analyzed. Density functional theory with state-of-the-art methods such as meta-GGA functionals and a spectral weight approach to band unfolding from large supercells was used to obtain dopant-related changes in the band structure, in particular the direct band gap at the Γ point and indirect band gaps at the L(X) points of the BZ. Analysis of contributions from geometry distortion and electronic interaction was also performed. Moreover, the obtained results are discussed in the context of obtaining a direct fundamental gap in Ge:B (B = C, Sn, and Pb) alloys, and intermediate band formation in C:B (B = Sn and Pb) and Ge:C. An increase in localization effects is also observed: a strong hole localization for alloys diluted with a dopant of a larger covalent radius and a strong electron localization for alloys with a dopant of smaller radius. Finally, it is shown that alloying Si and Ge with other elements from group IV is a promising way to enhance the functionality of group IV semiconductors.

Electronic structure of yttrium-doped zinc ferrite – Insights from experiment and theory

Abstract In this work we have studied the effects of yttrium-doping to electronic structure of zinc ferrite. Our present study encompasses combination of experimental and computational procedures, with the aim of understanding how important and typical properties of title nanomaterial could be modified with frequently employed rare-earth element. Co-precipitation approach was used for the synthesis of zinc ferrite and its yttrium-doped derivatives, followed by characterization and experimental measurements of activation energies. Structures of zinc ferrite and yttrium-doped zinc ferrite were then subjected to detailed computational investigations in the framework of density functional theory (DFT). It has been shown that yttrium doping at the sites of zinc atoms produces significantly higher increase of unit cell parameter. DFT calculations encompassed geometrical optimizations, thanks to which information about structural properties have been acquired. Details about band structure have been obtained by DFT + U calculations and the calculated band gaps were related with the experimentally obtained information on activation energies. It has been shown by calculations that band gap of pristine ZFO is 0.922 eV, and that it can be manipulated by replacing iron ions with yttrium. The band gap values decrease to 0.899 eV when two yttrium ions are introduced to sites of iron ions, while the third Y increases band gap to the value of 1.027 eV. On the other side, replacing of zinc ions with yttrium leads to the formation of n-type semiconductor. Detailed insights into the effects of yttrium were also discussed through investigation of density of states. The results indicate different effects of yttrium doping, depending on whether iron or zinc atoms are replaced. It was particularly interesting to identify the formation of intermediate bands in cases when mixed spinel phase zinc ferrite was doped with yttrium.

Intermediate band formation by InGaAs/GaAs multistacked quantum dots without strain compensation

Quantum dots (QDs) are widely used for enhancing the performance of optical devices. Intermediate-band solar cells that use multistacked QDs can surpass the conversion efficiency of conventional silicon cells. We performed the optical characterization of In0.4Ga0.6As/GaAs multistacked QDs without strain compensation. We used the theoretical and experimental techniques prescribed for intermediate-band solar cells, i.e., photoluminescence (PL) spectroscopy and two-color excitations spectroscopy. The interdot spacings were not uniform and were found to be 15 nm and 7 nm. The results verify the formation of intermediate bands by the multistacked QDs. Using the theoretical studies and experimental results, we performed an in-depth study on the mechanism underlying the formation of intermediate bands by the multistacked QDs and the effect of different interdot spacings.

Tailoring sub-bandgap of CuGaS2 thin film via chromium doping by facile chemical spray pyrolysis technique

Tailoring sub-bandgaps using intermediate states or bands with wide bandgap semiconductors is a promising and novel technique for the application in high efficiency solar cells. Pure and chromium (Cr) doped chalcopyrite CuGaS2 (CGS) thin films were prepared by facile chemical spray pyrolysis technique and annealed in vacuum, nitrogen and argon atmospheres. Structural characterization confirmed that the prepared films are in tetragonal chalcopyrite structure with polycrystalline nature. No secondary phases were present in both pure and Cr doped CGS thin films. Presence of Cr ions was confirmed by X-ray photoelectron spectroscopy and energy dispersive analysis of X-rays analyses. The optical direct and sub band gap of pristine and Cr doped CGS thin films were measured from UV absorption data. It was revealed that the pure CGS film has a band gap of 2.40 eV sub-band gap values were observed at 2.25 and 2.15 eV for 1 and 2 wt% of Cr doping, respectively. These gaps can be ascribed to the formation of intermediate bands due to hybridization of Cr d-states into the host electronic structure. Meanwhile, photoconductivity study demonstrated the photo-electric activity of the intermediate bands in the Cr doped thin films.

Intermediate Bands in Zero-Dimensional Antimony Halide Perovskites

Using density functional theory, the structural and electronic-structure properties of a recently discovered, zero-dimensional antimony halide perovskite are studied. It is found that the herein considered material EtPySbBr6 exhibits very promising electronic-structure properties: a direct band gap close to the peak of the solar spectrum and effective masses allowing for efficient carrier transport of electrons in particular. These results are rationalized by analysis of the electronic structure, which reveals the formation of intermediate bands due to orbital-hybridization effects of the Sb s-states. This study shows that the formation of intermediate bands can lead to highly favorable electronic-structure properties of zero-dimensional perovskites and discusses the possibility of fabricating lead-free halide perovskites with promising optoelectronic properties by targeted substitution of ions and emergence of intermediate bands. These insights are important when understanding and further enhancing the capabilities of antimony and other promising lead-free compounds.

Enhancement of low energy photon absorption in Bi2S3 based Ti replacement: Prospect for high performance solar cells

Full-potential (linearized) augmented plane wave plus local orbitals (FP-LAPW+lo) scheme as implemented in WIEN2K code is used to study the effect of intermediate band (IB) formation in chalcogenide-type semiconductor at single level of Ti substitution. By using Tran-Blaha’s modified Becke and Johnson with generalized gradient approximation/ local density approximation (mBJ-GGA/LDA) energy functionals; structural, electronic, and optical properties, including the dielectric function, refractive index, extinction coefficient, absorption spectrum, and optical reflectivity for the pure Bi2S3 and two slabs model Bi4Ti4S12 and Bi12Ti4S24 are calculated and analysed. Our calculated electronic properties show how the effects of interaction between the d electrons of Ti and the p electrons of the parent atoms do influence the formation of IB when Ti is substituted Bi at a 25% level. In addition, the parent structure has shown a direct band gap and high absorption coefficient α(ω), and other calculated optical properties indicate Bi2S3 to be a promising material for photovoltaic application. Moreover, at 25% substitution the enhancement in the absorption due to the intermediate band appears mainly in the most intense part of the sun spectrum.

Efficient inactivation of Pseudomonas aeruginosa by Cu/Co-α-NiMoO4 in visible light

The Cu/Co-α-NiMoO4 nanorods were fabricated by microwave hydrothermal process and characterized by various techniques. According to XRD/HRTEM/SAED/EDS analysis, the dopant ions were existed in the lattice of α-NiMoO4.The Cu/Co-α-NiMoO4 samples have capacity to inactivate the multidrug resistant Pseudomonas aeruginosa in visible light within 180 min. Also, they revealed the excellent stability with reusability. The scavengers test indicates that H2O2/OH/O2−/h+ play a crucial work for pathogen inactivation. Also, NBT assay suggested the enhancement of intracellular O2− production in Cu (33 folds) and Co doped (29 folds) α-NiMoO4. The FESEM analysis revealed dead bacteria cells by attack of active species on cell wall. Also, DNA fragmentation (gel electrophoresis) and protein degradation (Bradford assay) confirmed the inactivation of bacteria by active species. The evidence of superior biofilm penetration ability (69%) by doped photocatalyst was exhibited by well plate based crystal violet assay and TEM/EDS mapping. The band gap reduction, surface plasmon resonance effect, oxygen vacancy, electron-hole trapping by Cu2+/Co2+/Co3+, formation of intermediate energy band, and efficient separation of electron-hole pair were contributed for enhancement of the pathogen inactivation in visible light by doping of Cu and Co in α-NiMoO4. Also, Cu doped photocatalyst has slightly more pathogen inactivation efficacy than Co doped photocatalyst. The toxicity of photocatalyst was evaluated by using RAW 264.7 cells via MTT assay and the result revealed the toxicity. In conclusion, the Cu/Co-α-NiMoO4 photocatalyst can be applied for efficient inactivation of multidrug resistant pathogen in biological waste water treatment.

Preparation and characterization of the Si:Co layer for intermediate band solar cell applications

Abstracts The Co-implanted silicon layers with rapid thermal process (RTP) have been prepared for intermediate band materials. The characterization of the Si:Co layer has been analyzed and discussed. The results show that the Co concentrations in effective implanted region of the Si layers exceed the Mott limit, satisfying the requirement for the formation of intermediate band in silicon. The Co-implanted samples are well crystallized by RTP treatment. The effective lifetime of the carriers in the samples increases with the Co dose implanted. This indicates that the non-radiative recombination has been suppressed in the post-annealed samples. Furthermore, the optical absorptance of the samples is much enhanced in the wavelength range of above 1100 nm. It can be concluded that the Si:Co layer can be prepared by Co implantation in Si followed by RTP method, and it is a promising material that can be used for intermediate band solar cells.

Intermediate band formation in a δ-doped like QW superlattices of GaAs/AlxGa1−xAs for solar cell design

It is reported a numerical computation of the local density of states for a δ-doped like QW superlattices of AlxGa1−xAs, as a possible heterostructure that, being integrated into a solar cell device design, can provide an intermediate band of allowed states to assist the absorption of photons with lower energies than that of the energy gap of the solar-cell constituent materials. This work was performed using the nearest neighbors sp3s* tight-binding model including spin. The confining potential caused by the ionized donor impurities in δ-doped impurities seeding that was obtained analytically within the lines of the Thomas-Fermi approximation was reproduced here by the Al concentration x variation. This potential is considered as an external perturbation in the tight-binding methodology and it is included in the diagonal terms of the tight-binding Hamiltonian. Special attention is paid to the width of the intermediate band caused by the change in the considered aluminium concentration x, the inter-well distance between δ-doped like QW wells and the number of them in the superlattice. In general we can conclude that this kind of superlattices can be suitable for intermediate band formation for possible intermediate-band solar cell design.

Mixed Valence Perovskite Cs2 Au2 I6 : A Potential Material for Thin-Film Pb-Free Photovoltaic Cells with Ultrahigh Efficiency.

New light is shed on the previously known perovskite material, Cs2 Au2 I6 , as a potential active material for high-efficiency thin-film Pb-free photovoltaic cells. First-principles calculations demonstrate that Cs2 Au2 I6 has an optimal band gap that is close to the Shockley-Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs2 Au2 I6 a double-perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs2 Au2 I6 has a direct-band-gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state-of-the-art multiscale simulations connecting from the material to the device, strongly suggests that Cs2 Au2 I6 will serve as the active material in highly efficient, nontoxic, and thin-film perovskite solar cells in the very near future.

Formation of intermediate band and low recombination rate in ZnO-BiVO4 heterostructured photocatalyst: Investigation based on experimental and theoretical studies

We present systematic investigations on the relationship between interface formation and enhanced photocatalytic activity of ZnO-BiVO4 nanocomposite based on experimental techniques supported by theoretical calculations. The interaction between ZnO (101) nanosheet and BiVO4 surface at the heterojunction was explored to study the charge transfer and separation mechanism responsible for enhanced photocatalytic response. XPS results and DFT computations mutually validate the reasonable existence of ZnO-BiVO4 interface. The nanocomposite photocatalytic activity, tested for various weight ratios, was found to be highest for ZnO-BiVO4 (1 : 1) under visible-light irradiation. Moreover, the percentage removal of MB was found to be greater than RhB for the same time duration. Steady state and time resolve photoluminescence were employed to understand the carrier lifetime and emissivity. Visible light driven high photoactivity exhibited by ZnO-BiVO4 (1 : 1) was ascribed to the formation of intermediate band and comparatively low recombination rate, which facilitates the separation of electron-hole pairs. Based on the theoretical outcome, we found that valence band maximum was occupied by Bi s orbital and conduction band minimum was occupied by Zn s orbital, which indicates the maximum electron transition from BiVO4 valence band to ZnO conduction band in ZnO-BiVO4 composite. These results demonstrated that heterojunction semiconductors are an effective strategy that can be successfully applied to develop photocatalysts that respond to visible light for organic pollutant degradation.

Multicolor Electroluminescence from Intermediate Band Solar Cell Structures

Intermediate band solar cells are a new generation of photovoltaics that allow for better utilization of the solar spectrum. The key and most challenging requirement for these cells is an efficient optical coupling between the intermediate band and the charge conducting bands. GaNAs based intermediate band solar cells have been used to generate electroluminescence. Two electroluminescence peaks are generated in the structure with electrically blocked intermediate band. The peaks are observed for both forward and reverse bias configuration and are attributed to optical transitions from the conduction to the intermediate band, and from the intermediate band to the valence band. The origin of the electroluminescence is confirmed by the temperature dependence of the electroluminescence peak energies that is consistent with the band anticrossing model of the intermediate band formation in dilute nitride alloys. This is the first direct observation of the optical transitions required for the operation of intermediate band solar cells. The results also demonstrate that properly modified intermediate band solar cell structures could be used as multicolor light emitters.

Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing

Hyperdoped black silicon fabricated with femtosecond laser irradiation has attracted interest for applications in infrared photodetectors and intermediate band photovoltaics due to its sub-bandgap optical absorptance and light-trapping surface. However, hyperdoped black silicon typically has an amorphous and polyphasic polycrystalline surface that can interfere with carrier transport, electrical rectification, and intermediate band formation. Past studies have used thermal annealing to obtain high crystallinity in hyperdoped black silicon, but thermal annealing causes a deactivation of the sub-bandgap optical absorptance. In this study, nanosecond laser annealing is used to obtain high crystallinity and remove pressure-induced phases in hyperdoped black silicon while maintaining high sub-bandgap optical absorptance and a light-trapping surface morphology. Furthermore, it is shown that nanosecond laser annealing reactivates the sub-bandgap optical absorptance of hyperdoped black silicon after deactivation by thermal annealing. Thermal annealing and nanosecond laser annealing can be combined in sequence to fabricate hyperdoped black silicon that simultaneously shows high crystallinity, high above-bandgap and sub-bandgap absorptance, and a rectifying electrical homojunction. Such nanosecond laser annealing could potentially be applied to non-equilibrium material systems beyond hyperdoped black silicon.

Review on the degradation and device physics of quantum dot solar cells

Briefly, we reviewed the latest progress in energy conversion efficiency and degradation rate of the quantum dot (QD) solar cells. QDs are zero dimension nanoparticles with tunable size and accordingly tunable band gap. The maximum performance of the most advanced QD solar cells was reported to be around 10%. Nevertheless, majority of research groups do not investigate the stability of such devices. QDs are cheaper replacements for silicon or other thin film materials with a great potential to significantly increase the photon conversion efficiency via two ways: (i) creating multiple excitons by absorbing a single hot photon, and (ii) formation of intermediate bands (IBs) in the band gap of the background semiconductor that enables the absorption of low energy photons (two-step absorption of sub-band gap photons). Apart from low conversion efficiency, QD solar cells also suffer from instability under real operation and stress conditions. Strain, dislocations and variation in size of the dots (under pressure of the other layers) are the main degradation resources. While some new materials (i.e. perovskites) showed an acceptable high performance, the QD devices are still inefficient with an almost medium rate of 4% (2010) to 10% (2015).