Blue Shift In Band Gap Research Articles

Inverted rear-heterojunction GaInP solar cells using Te memory effect

Tellurium allows attaining heavy n-type doping levels in GaAs, which is suited to achieve very low contact resistivities in solar cells. Besides, it modifies the energy bandgap of MOVPE-grown GaInP by reducing the group-III sublattice ordering and presents a strong memory effect which induces residual n-type doping in subsequent layers, potentially detrimental to the performance of the solar cell. In this work, we present an inverted rear-heterojunction GaInP solar cell that employs a thick Te-doped GaInP layer as absorber, with a doping profile obtained exclusively by controlling the memory effect of Te coming from the preceding growth of a heavily doped GaAs contact layer. In this way, GaInP is partially disordered with the use of no additional surfactant, leading to an increase in the solar cell bandgap of around 35 meV as compared to traditional samples doped with silicon. In the proof-of-concept experimental devices developed so far, the use of a rear-heterojunction configuration and the bandgap increase results in a global open-circuit voltage enhancement of 109 mV. The photocurrent decreases by 1.32 mA/cm2, mostly due to the bandgap blue-shift, with about 0.35 mA/cm2 attributable to lower carrier collection efficiencies. These preliminary results are discussed by analyzing the I-V curve parameters and quantum efficiencies of a Te-doped rear-heterojunction, a Si-doped rear-heterojunction and a Si-doped front-junction solar cell. An additional advantage is that the emitter sheet resistance is reduced from 551 to 147 Ω/□, which offers potential for higher efficiencies through lower front grid shadowing factors, as demonstrated with the concentrator measurements presented.

Effect of Mg doping on structural, morphological, optical and thermal properties of ZnO nanoparticles

Doping of group II elements in ZnO is an efficient way to enhance the optical properties of ZnO nanoparticles. For this purpose, Zn1-xMgxO nanoparticles (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1) are synthesized using co-precipitation technique. Effect of ‘Mg’ doping on structural, morphological, optical and thermal properties of ZnO nanoparticles is investigated. XRD results verify that synthesized nanoparticles are poly-crystalline with typical hexagonal wurtzite structure and possess no other impurity or dopant phases. Crystallite size is increased with the increase in ‘Mg’ content. SEM analysis reveals that polyhedral grains are aggregated with the increase in doping concentration of Mg. For the highest dopant concentration, surface morphology is entirely changed with the formation of nanowires. Optical band gap energy of Mg-doped ZnO nanoparticles is greater than that of pristine ZnO nanoparticles. The blue shift in band gap is observed with Mg content x ≤ 0.04, followed by red shift for higher Mg content. The decrease in phase transition and increase in decomposition temperature for Mg-doped ZnO nanoparticles suggest their thermal stability. Tailoring of band gap makes ZnO nanoparticles a promising material for photocatalysis, optoelectronic and display devices.

Variation in chemical bath pH and the corresponding precursor concentration for optimizing the optical, structural and morphological properties of ZnO thin films

In the present study, ZnO thin films were deposited by chemical bath deposition carried out by selective correlation of varying (i) pH values at fixed concentration and (ii) concentration of the precursors at fixed pH. The selective correlations were done by using the characterization tools like X-ray diffraction, scanning electron microscopy, transmittance, refractive index, dielectric constant, Fourier-transform infrared spectroscopy and IV measurements. Transmittance was found to increase from 57 to 87% on varying the pH from basic side (10.8) to acidic side (pH 6.8) with a blue shift in band gap. The nature and morphology of the deposited films were found to be dependent on pH as well as concentration. Acidic pH 5.0 was found to be most suitable for deposition of highly transparent film with low absorption coefficient, refractive index and dielectric constant. On the other hand, nearly complete coverage of the substrate and high purity was observed in the ZnO thin films which was deposited by taking equal 100 mM concentration of Zn(NO3) and HMTA precursors at a fixed pH 5.0 as desired, sheet resistance was also found to increase on the acidic pH side which is useful in case of buffer layer solar cell application. These studies lay a foundation stone for understanding the optical and morphological parameters by selectively correlating the pH and concentration variation at the same time.

Cadmium Chloride and Cadmium Iodide Thiosemicarbazone Complexes as Single Source Precursors for CdS Nanoparticles

Cadmium sulfide (CdS) nanoparticles have been synthesized from CdX2 (X = Cl–, I–) complexes of 4-chlorobenzaldehyde and benzophenone thiosemicarbazone. The complexes have been studied by thermogravimetric analysis (TGA), elemental analysis, FT-IR, and NMR spectroscopy. The solvothermal decomposition of the complexes has afforded oleylamine-capped CdS nanoparticles with a wurtzite hexagonal phase. The transmission electron microscopy (TEM) studies show different morphologies which are influenced by reaction temperature and the nature of the ligands on the precursor complexes. Particles in the form of irregular cubes, elongated cubes and nanodendrites have been observed. The UV-Vis spectroscopy measurements show temperature and nature of complex-dependant optical properties. Blue-shifted band gap energies have been observed at lower reaction temperatures.

Influence of Li ion implantation on LO phonon broadening and bandgap opening in ZnO thin films

Doping of nonmagnetic impurities in technologically important ZnO has opened a new window for achieving room temperature ferromagnetism, p-type carrier conduction, and enhancement of ferroelectric properties. Here, we report on the confined optical phonon and bandgap engineering in highly oriented Li implanted ZnO thin films. Using resonance Raman scattering condition, the confined longitudinal optical phonon lineshapes in uniaxial hexagonal wurtzite crystal are analyzed in detail using the phonon confinement model. We have demonstrated that phonon confinement model can yield a meaningful result for the interpretation of resonance Raman lineshapes if one considers the contribution of both the E1 (LO) and A1 (LO) modes, particularly while dealing with oriented ZnO thin films. Furthermore, with the increase in Li dose, the bandgap of ZnO is found to show a blue shift, and such blue shift in bandgap is explained using first principles calculation.

Biogenic ZnO nanoparticles: a study of blueshift of optical band gap and photocatalytic degradation of reactive yellow 186 dye under direct sunlight

Abstract Synthesis of nanoparticles (NPs) using plant extracts has been suggested as an environmentally friendly alternative to chemical synthesis of semiconductor NPs. In the present study, ZnO NPs were synthesized by a simple and cost-effective method using Coriandrum sativum leaf extract and zinc acetate as precursors. The as-synthesized ZnO NPs were characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) analysis. The results confirmed the formation of ZnO NPs with a wurtzite structure, spherical shape and average particle size of 24 nm. The photocatalytic degradation of reactive yellow 186 (RY 186) dye was carried out under direct sunlight irradiation and its degradation efficiency and apparent rate constant (K’app) of reaction were calculated to be 93.38%, and 0.0019 min−1, respectively. The optical band gap value of the as-synthesized ZnO NPs was found to be 3.4 eV, which indicates the presence of blueshift. Owing to the presence of blueshift and a wide band gap of synthesized biogenic ZnO NPs, the overall absorption of sunlight irradiation will be enhanced, which leads to higher degradation efficiency of the dye. The current study thus highlights the optical band gap properties of biogenic ZnO NPs and their significance as a heterogeneous catalyst for the purification of polluted water.

Tuning the Optical Absorption Property of GeSe Thin Films by Annealing Treatment

As an emerging promising photovoltaic absorber material, GeSe has attracted significant interest recently due to its simple binary composition, attractive optical and electrical properties as well as earth‐abundant and low‐toxic constituents. However, no systematic study on the absorption property tuning of GeSe has been reported. Here, first it is shown that the crystallization temperature of amorphous GeSe is about 330 °C through differential thermal analysis and temperature‐dependent X‐ray diffraction. Next, GeSe films with tunable absorption property in the range of 1.79–1.14 eV are fabricated by annealing amorphous GeSe films at different temperatures. Finally, through the combined analysis of bandgaps measured by transmission spectroscopy and nanosheet thickness characterized by cross‐sectional high‐resolution transmission electron microscopy, the blue shift of bandgap with decreasing thickness of nanosheets has been attributed to the quantum confinement effect. Such continuously tunable absorption property of GeSe films makes it more useful for further optoelectronics.

A Rapid One-Pot Synthesis of CuO Rice-Like Nanostructure and Its Structural, Optical and Electrochemical Performance

We report the production of high stable rice-like CuO nanostructures synthesized by a facile and one-step hydrothermal method using urea as a fuel agent. The thermogravimetric nature, phase purity, morphology, and structure of rice-like CuO nanostructures have been characterized by thermogravimetric–differential thermal analysis, powder x-ray diffraction (PXRD), field-emission scanning electron microscopy (FESEM), and energy dispersive x-ray spectroscopy. PXRD revealed the formation of monoclinic CuO with an average crystallite size of around 20 nm. FESEM showed a rice-like morphology with an average size of 40–50 and 100–120 nm along the shorter axis and longer axis, respectively. UV-DRS analysis confirmed a considerable blue-shift in the optical band gap (E− = 1.45 eV) owing to a quantum confinement effect. X-ray photoelectron spectroscopy and electron paramagnetic resonance spectra confirmed the presence of copper in Cu2+ state. Furthermore, we can use these electrodes for their electrochemical supercapacitive properties, like cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectra. The nano-rice CuO showed an enhanced specific capacitance of 305 F/g at a current density of 1 A/g.

Fabrication of ZnO nanostructures using Al doped ZnO (AZO) templates for application in photoelectrochemical water splitting

ZnO nanostructured thin films have been fabricated by continuous spray pyrolysis (CoSP) technique using different seed layers and implemented as photoanodes for photoelectrochemical (PEC) water splitting. Effects of Al loading (0.02–0.2 wt%) in the ZnO seed and the voltage applied during the spray on morphological, structural, optical and photoelectrochemical properties of ZnO nanostructures have been studied. In the ZnO films grown on Al doped ZnO(AZO) seed with lower Al wt%, a red shift in the band gap has been observed along with increased (0 0 2) orientation in the XRD pattern. A blue shift in band gap is observed with AZO seed having higher Al wt% which can be attributed to the Burstein-Moss effect. The decrease in the band gap with stronger (0 0 2) orientation becomes more prominent on applying the voltage. Linear sweep voltammetry measurements carried out on the ZnO films grown on AZO seed and with applied voltage show a significant increase in photocurrent under illumination as compared to that for unmodified ZnO thin films. The significantly improved photoresponse of ZnO nanostructured thin films using a simple and cost effective fabrication method can find applications for water splitting under light leading to hydrogen generation and other related products.

Electrical properties of grain size tuned CdSe nanocrystal films for practical applications

CdSe nanocrystal (NC) thin films of different NC sizes were synthesized using vacuum deposition technique. The films showed blue shift of band gap and the NC sizes calculated size using Brus equation were 9.40, 9.68 and 9.82nm. DC conductivity and photoconductivity of as-deposited and annealed films increased and activation energy for conduction decreased with increase in the NC size. Also, both carrier concentration and mobility of the films increased with increase in the size of the NCs which was attributed to the increased electronic coupling between the NCs. The slow response part of photocurrent response pulse analyzed using power law: Iph ∞ tα gave larger values for α for larger NC size indicating stronger electronic coupling between grains in films of larger grain sizes. Charge transport in the CdSe NC films was akin to that of solid films of CdSe quantum dots, of sizes given by Brus equation, connected together by an outer shell of CdSe material where charge carriers had free movement. Additionally, measurement of the characteristics of the hetero-junction formed by the CdSe NC film with a similarly deposited Cu2Se film demonstrated usefulness of the CdSe NC films synthesized using the present technique for practical applications.

Infrared photoconductivity and photovoltaic response from nanoscale domains of PbS alloyed with thorium and oxygen

Thin films of lead sulfide alloyed with thorium and oxygen were deposited on GaAs substrates and processed to produce a photo-diode structure. Structural, optical and electrical characterizations indicate the presence of small nanoscale domains (NDs) that are characterized by dense packaging, high quality interfaces and a blue-shift of the energy bandgap toward the short wavelength infrared range of the spectrum. Photocurrent spectroscopy revealed a considerable photoconductivity that is correlated with excitation of carriers in the NDs of lead sulfide alloyed with thorium and oxygen. Furthermore, the appearance of a photovoltaic effect under near infrared illumination indicates a quasi-type II band alignment at the interface of the GaAs and the film of NDs.

Comparative study of nano-sized Al2O3 powder synthesized by sol-gel (citric and stearic acid) and aldo-keto gel method

Inorganic Al2O3 nano-powders were synthesized by citric acid sol-gel, stearic acid sol-gel and aldo-keto gel method. The prepared samples were characterized by X-ray powder diffraction (XRD) for structural studies and the average particle size was calculated by Debye-Scherrer formula. The optical investigation was done through UV–vis and FL spectra analysis. The florescence properties were studied by fluorescence spectrophotometer (F-7000), indicating the emission of radiation of 397, 399, 401 nm by stearic acid sol-gel, aldo-keto gel and citric acid sol-gel method respectively, when nano powder absorbed UV radiation at 224 nm. However, it is found that blue shift in band gap was observed in stearic acid sol-gel, aldo-keto gel and citric acid sol-gel method respectively.

Optical properties of zinc titanate perovskite prepared by reactive RF sputtering

Abstract In this paper we report results from optical transmittance spectroscopy complemented with data on structure from XRD measurements to determine optical properties of a series of ZnTiO3 perovskite thin films deposited on glass by reactive magnetron co-sputtering. The members of the series differ by the titanium content that was revealed as an origin of the changes not only in structure but also in dispersive optical properties. Low porosity has been discovered and calculated using the Bruggeman effective medium approximation. An apparent blue-shift of the optical band gap energies with increasing titanium content was observed. The observed band gap engineering is a good prospective for eg optoelectronic and photocatalytic applications of ZnTiO3.

High near-infrared transmittance, high intense orange luminescence in vanadium doped indium oxide (V: In2O3) thin films deposited by electron beam evaporation

The V (5wt.%) doped In2O3 (V: In2O3) thin films with the thickness of 50–150nm were deposited on to glass substrate by electron beam evaporation technique and the deposited films were annealed at 200°C in air for 2h. The annealed films were characterized by structural, electrical, optical and photoluminescence properties. X-ray diffraction (XRD) pattern shows that the film is an amorphous nature. The negative sign of the Hall coefficient indicates the n-type conductivity. Experimental results show that the resistivity and transmittance of the films are strongly influenced by film thickness and post deposition annealing. A minimum resistivity of 6.22×10−3Ωcm, a carrier concentration of 5.23×1019cm−3, a Hall mobility of 19.21cm2V−1s−1 and a figure of merit of 1.04×10−4Ω−1 are observed for the film with thickness of 150nm. After annealing the films, the average transmittance is enhanced up to 84% in the near infrared region. The blue-shift of optical band gap is found to increase from 1.62 to 2.25eV with increasing the thickness, whereas the refractive index decreases from 2.92 to 2.64. Structural disorder was observed from the Urbach’s tail. A prominent and high intense orange emission is achieved at room temperature, which may be attributed to oxygen deficiencies or intrinsic defects.

Two-Dimensional GeSe as an Isostructural and Isoelectronic Analogue of Phosphorene: Sonication-Assisted Synthesis, Chemical Stability, and Optical Properties

Monochalcogenides of germanium (or tin) are considered as isoelectronic and isostructural analogues of black phosphorus. Here, we demonstrate the synthesis of atomically thin GeSe by direct sonication-assisted liquid phase exfoliation (LPE) of bulk microcrystalline powders in organic solvents. The thickness of the GeSe sheets is dependent on the exfoliation conditions, and highly crystalline few-layer GeSe sheets of 4–10 layer stacks with lateral sizes over 200 nm were obtained. In ambient atmosphere, the LPE sheets deposited on the substrate demonstrate strong resistance against degradation, while decomposition into elemental Ge and Se nanostructures occurs at a moderate rate for ethanol dispersions. Density functional theory calculation together with optical characterizations confirm the blue-shifted bandgap for the GeSe sheets as a result of strong quantum confinement effect. In addition, we show that the few-layer GeSe sheets with favorable optical bandgap allow for efficient solar light harvesting fo...

Room temperature ferromagnetism and red shift of band gap in (Mn2+, Mg2+) co-doped ZnO nanoparticles

In this study we report, for the first time, room temperature ferromagnetism in (Zn1−xMgxMn0.01)O nanoparticles having Mg concentration in the range 0 < x < 0.10 prepared by sol–gel method. It is observed that the room temperature ferromagnetic ordering varies with Mg concentration. Compared with Mn-doped ZnO (Zn0.99Mn0.01O) nanoparticles, the (Mn, Mg) co-doped ZnO nanoparticles exhibit decreased room temperature ferromagnetism. This study also reports the balance of the two competing effects from the blue shift of band gap as a result of Mg doping and sp-d ferromagnetic exchange interaction leading to red-shift of band-gap.

InAs/GaSb Type-II superlattice photodiode array inter-pixel region blue-shift by femtosecond (fs) laser anneal

A post-growth blue-shift in the band gap of an undoped InAs/GaSb Type-II superlattice (5.5 μm cutoff wavelength), as a result of 775 nm, 150 fs laser annealing, is presented. A band gap blue-shift of ∼72 meV in the - and p-layer etched inter-pixel region, laser annealed superlattice is achieved. Using an inter-diffusion model, the dominant group-III and group-V diffusion coefficients are found to be and respectively. Confirmation of the unaltered condition of the superlattice in a Ti/Au masked pixel area establishes this blue-shifted superlattice band gap to be the reason behind the improved inter-pixel isolation resulting from the fs laser annealing technique.

Spectrally resolved nonlinearity and temperature dependence of perovskite solar cells

An accurate electrical device characterization of hybrid organic-inorganic halide perovskite solar cells (PSCs) is an important prerequisite for further improvement and industrial transfer of this promising photovoltaic technology. In this work, we study the nonlinearity of current versus irradiance as well as the temperature dependence of PSCs in a spectrally resolved manner by highly accurate differential external quantum efficiency (EQE) measurements. We investigate three different types of PSCs fabricated by different research groups. The nonlinearity of all samples is found to be spectrally invariant, which significantly simplifies spectral mismatch corrections. We demonstrate that misinterpretation of EQE measurements can result in a more than 10% relative error in efficiency measurements, if solar simulators are adjusted to photocurrents determined from differential EQEs. For obtaining an accurate integrated photocurrent from EQEs, we introduce a new, convenient approach that accounts for cell nonlinearities but avoids the time-consuming full analysis of spectrally resolved nonlinearity. Moreover, for the samples investigated here, it is shown that the differential EQE measured at 0.35 suns bias irradiance represents a reasonably good estimate of the actual EQE at 1 sun. Furthermore, we determine spectrally resolved temperature coefficients (TCs) and show how the band gap blue shift varies with perovskite absorber and temperature.

Triangular CdS nanostructure: effect of Mn doping on photoluminescence, electron spin resonance, and magneto-optical properties

In this paper, we report synthesis and study of magneto-optic Faraday effect for dilute magnetic semiconductor nanostructure. The colloidal CdS nanocrystals were prepared via hot injection method and successfully doped with Mn2+ cations. The synthesized nanoparticles were characterized by using UV–Vis spectroscopy, X-ray diffraction, photoluminescence spectroscopy, transmission electron microscopy, and electron spin resonance spectroscopy. Systematic studies on effect of Mn2+ doping on photoluminescence, electron spin resonance, and magneto-optical properties are carried out. UV–Vis spectral analysis confirms blue shift in bandgap of CdS nanoparticles due to quantum confinement effect. The X-ray diffraction study confirms hexagonal wurtzite phase formation of CdS nanoparticles without any impurity phases. TEM analysis confirms uniform particle size, having particle size distribution around 5 nm. As-synthesized undoped CdS shows triangular-shaped nanocrystals with hexagonal structure; however, triangular shape of CdS nanoparticles is not conserved after Mn2+ doping. The photoluminescence characteristic spectra of Mn2+-doped CdS nanocrystals showed emission band at 660 nm and its intensity was found to increase with increasing Mn2+ concentration. Electron spin resonance signal, with six-line hyperfine structure splitting, confirmed doping of Mn2+ ions in CdS lattice. Magneto-optic measurements showed linear variation of Faraday rotation with respect to applied magnetic field, indicating paramagnetic behavior of Mn-doped CdS. The highest Verdet constant 24.81 deg/T cm was observed for 2% Mn-doped CdS nanocrystals, which further decreases with increasing Mn2+ concentration.

Impact of stoichiometry on the linear and nonlinear optical response of SnOx thin films

SnO is a promising p-type oxide semiconductor materials for applications such as transparent electronics and solar cells. However, further improvement of its performance is hindered by its diverse stoichiometry. We investigated the nonlinear and saturable absorption characteristics of pristine SnO and O-rich SnOx films by femtosecond degenerate pump-probe measurements at 515 nm. UV-Vis absorption data indicate bandgap blueshift with increasing oxygen concentration. Pristine SnO film exhibit saturable absorption while nonlinear absorption is observed in O-rich SnOx films. Our results shed light on the utilization of SnO in future device applications.