Search

The thermoelectric properties and thermal conductivity of mixed-phase CH3NH3PbI3-x Cl x thin films have been reported as a function of temperature, ranging from room temperature (RT) to 388 K. Thermoelectric study confirms that CH3NH3PbI3-x Cl x is a p-type material and the charge carrier transport in CH3NH3PbI3-x Cl x is governed by polarons and the thermal scattering process. The Peltier function and power factor are found to decrease initially up to ∼325 K, after which they increase with increasing temperature. The position of (E F - E V) of all samples drops down sharply to zero level around 325 K. The avalanches of thermoelectric properties at ∼325 K indicate the existence of tetragonal-cubic phase transition in CH3NH3PbI3-x Cl x . The calculated thermal conductivity is very low, as desired for thermoelectric materials, due to strong anharmonic interactions. Both the figure of merit (ZT) and device efficiency increase with increasing temperature. However, ZT remains small with temperature. Despite the limitations on the operating temperature range due to phase complexity and small ZT, CH3NH3PbI3-x Cl x exhibits reasonable thermoelectric power and low thermal conductivity. This signifies the possibility of CH3NH3PbI3-x Cl x as a prospective thermoelectric material.

In this paper we explain the temperature dependence of excitonic effective mass and charge carrier conduction mechanism occurs in CH3NH3PbI3−xClx thin films prepared by chemical dip coating (CDC), spray pyrolysis (Spray) and repeated dipping-withdrawing (Dipping). Hall Effect study confirmed that prepared CH3NH3PbI3−xClx samples are p-type semiconductor having carrier concentration of the order of ~ 1016 cm−3. The charge carrier mobility, mean free path and mean free life time were found to decrease with increasing temperature due to polaronic effect. The excitonic effective mass is estimated to (0.090–0.196)me and excitonic binding energy (15–33) meV, well consistent with Wannier-Mott hydrogenic model and the nature of exciton is likely to be Mott-Wannier type. From electrical measurement, it was observed that charge carrier conduction in CH3NH3PbI3−xClx is governed by migration of $${\mathrm{I}}^{-}$$ I - and CH3N $${\mathrm{H}}_{3}^{+}$$ H 3 + vacancies and vacancy-assisted diffusion processes depending on temperature.

In this paper, Ba-doped SnO2 (SnO2:Ba), Mg-doped SnO2 (SnO2:Mg) and Ce-doped SnO2 (SnO2:Ce) nanostructured thin films were prepared on the glass substrate via a simple and low-cost nebulizer spray pyrolysis method. The crystal structure and morphology of all the samples were investigated by X-ray diffraction (XRD) and field-emission-scanning electron microscopy (FE-SEM), respectively. XRD results suggest that all the samples are polycrystalline with the tetragonal rutile structure. FE-SEM analysis exhibits a uniform surface morphology with homogenous distribution of grains. The transmittance measurement suggests that SnO2:Ba sample exhibits high transparency above 90% in the visible region. We find that doping causes an increase in the band gap, this behavior is explained by the Burstein–Moss effect. Two emission bands in the ultraviolet and visible regions are observed in the photoluminescence spectra. Hall effect measurement reveals that all the samples are degenerate and exhibit n-type semiconducting nature with carrier concentration in the order of 1018 cm−3. Ba doping induces the lowest resistivity of 0.047 Ω·cm associated with an increase in carrier concentration of 8.38 × 1018 cm−3 and mobility of 15.87 cm2 V−1 s−1. In contrast, the incorporation of Mg and Ce in SnO2 reduces the mobility and conductivity, which may be associated with the grain boundary scattering.

The mixed halide CH3NH3PbI3−xClx crystalline thin film has been prepared by two-step solution-processed repeated dip coating method at an ambient atmosphere. X-ray diffraction study reveals the presence of tetragonal and cubic phases in deposited film. Raman study confirms the metal halide bond in the inorganic framework and organic CH3 stretching/bending of C–H bond in CH3NH3PbI3−xClx perovskite. Scanning electron microscopy shows that cuboid and polyhedral-like crystal grains of 100 nm to 2 μm may find applications in optoelectronics. The perovskite CH3NH3PbI3−xClx thin film shows high spectral absorption coefficient of the order of 106 m−1. In optical band gap study, we found the coexistence of cubic and tetragonal perovskite phases. The energy band gap is dominated by cubic phase having Eg = 2.50 eV over tetragonal phase with band gap Eg = 1.67 eV. The room-temperature photoluminescence study confirms band edge, shallow and deep-level emissions. The temperature-dependent cathodoluminescence study shows red, green and ultraviolet emissions. The dominating green luminescence evolved for cubic phase at 2.51 eV. The red and ultraviolet emissions are also found for mixed-phase CH3NH3PbI3−xClx thin film, suitable for preparation of light-emitting devices.

Intermetallic compounds with CaAl2Si2-type structure have been studied extensively due to their exciting set of physical properties. Among various alumo-germanides, MgAl2Ge2 is the new representative of CaAl2Si2-type structures. Our previous study explores the structural aspects, mechanical behaviors and electronic features of intermetallic MgAl2Ge2. The present work discloses the results of optoelectronic, thermodynamic and vibrational properties of MgAl2Ge2 via density functional theory-based investigations. The band structure calculations suggest that MgAl2Ge2 possesses slight electronic anisotropy and the compound is metallic. The Fermi surface topology reveals that both electron- and hole-like sheets are present in MgAl2Ge2. The electron charge density map indicates toward the dominance of covalent bonding in MgAl2Ge2. The optical parameters are found to be independent of the state of the polarization of incident electric field. The large value of the reflectivity in the visible-to-ultraviolet region up to ~ 15 eV suggests that MgAl2Ge2 might be a good candidate as coating material to avoid solar heating. The thermodynamic properties have been calculated using the quasi-harmonic Debye approximation. We have found indications of lattice instability at the Brillouin zone boundary in the trigonal $$P\overline{3}m1$$ P 3 ¯ m 1 phase from the phonon dispersion curves. However, the compound might be stable at elevated temperature and as a function of pressure. All the theoretical findings herein have been compared with the reported results (where available). Various implications of our results have been discussed in detail. Graphic abstract

Nano-fiber structure of ZnO and Ni doped ZnO (Ni:ZnO) transparent thin films have been deposited on glass substrate at 350 °C at an ambient atmosphere via spray pyrolysis technique. The structural, surface morphological and opto-electrical properties of ZnO and Ni doped ZnO thin films have been investigated. The XRD patterns show that the films are of polycrystalline in nature having preferential orientation (0 0 2) plane for ZnO changes to (1 0 1) by Ni doping in ZnO matrix. Optical study exhibits red shifting in band gap energy with Ni doping due to sp-d hybridization and display high absorption coefficient of the order of 107 m−1. The photoluminescence (PL) spectra indicate blue emissions in all samples. Electrical measurement confirms the resistivity of the film decreases remarkably with Ni doping and electrical transport is mainly thermally activated. From Hall Effect study, it is confirmed that all the samples are n-type having carrier concentration of the order of 1018 cm−3. Both mobility and carrier concentrations of the films became higher than ZnO sample with the increase of Ni concentration., Materials science; Thin films; Semiconductor; Band gap; Activation energy

Transparent and conducting CdO and Cr doped CdO (CdO:Cr) crystalline films were prepared on glass substrate at 350 °C by cost effective spray pyrolysis technique. Structural analysis indicates CdO:Cr films are polycrystalline cubic structure comprises with spherical or semi-spherical nano-scale particles. The direct band gap energy of CdO was found to change with Cr doping. The photoluminescence (PL) spectra of CdO show extended band edge emissions accompanied by red emission originated from different impurity states. The optical dispersion parameters calculated using Wemple–DiDomenico single-oscillator model were found to vary with Cr concentration in CdO. The dispersion energy of oscillator found to decrease with increasing Cr concentration. Simultaneously, the variation of third-order nonlinear susceptibility with photon energy was found to decrease with increasing Cr concentrations. Hall study confirms that CdO and CdO:Cr films are n-type semiconductor having carrier concentration of the order of ∼1019 cm−3. The obtained physical properties of CdO film were improved by Cr doping which make them suitable for optoelectronic applications.

Al/ZnO: Al heterojunction was fabricated by depositing ZnO: Al film on Al substrate by spray pyrolysis technique at 220 °C substrate temperature. XRD, SEM and EDAX techniques were used to study the properties of thin films. Heterojunction properties were studied by I–V and C–V measurements. The fabricated Al/ZnO: Al junctions were rectifying in character. The room temperature ideality factors of Al/ZnO: Al junctions are found to vary from 2.56 to 5.45. The reverse saturation currents are 5.21 × 10−9, 1.35 × 10−6, 1.99 × 10−6, 9.99 × 10−7 and 1.02 × 10−7 A for Al/ZnO: Al junctions. Junction forward current depends on doping concentrations and temperature, whereas reverse saturation current remains independent for Al concentration. The built-in-potential calculated from capacitance for Al/ZnO: Al junctions are 2.74, 2.60, 2.0, 2.50 and 2.43 V corresponding to 1, 2, 3, 4 and 5 mol% of Al. X-ray diffraction study confirmed that the films are polycrystalline, orientated in (0 0 2) plane. Scanning electron microscopy study confirmed circular ring patterns with inside ribbon type structure for Al doped ZnO films.

Using plane-wave pseudopotential density functional theory (DFT), the first-principle calculations are performed to investigate the structural aspects, mechanical behaviors and electronic features of the newly synthesized CaAl2Si2-prototype intermetallic compound, MgAl2Ge2 for the first time and the results are compared with those calculated for its silicide and carbide counterparts MgAl2Si2 and MgAl2C2. The calculated lattice constants agree fairly well with their corresponding experimental values. The estimated elastic tensors satisfy the mechanical stability conditions for MgAl2Ge2 along with MgAl2Si2 and MgAl2C2. The level of elastic anisotropy increases following the sequence of X-elements Ge → Si → C. MgAl2Ge2 and MgAl2Si2 are expected to be ductile and damage tolerant, while MgAl2C2 is a brittle one. MgAl2Ge2 and MgAl2Si2 should exhibit better thermal shock resistance and low thermal conductivity and accordingly these can be used as thermal barrier coating (TBC) materials. The Debye temperature of MgAl2Ge2 is lowest among three intermetallic compounds. MgAl2Ge2 and MgAl2Si2 should exhibit metallic conductivity; while the dual characters of weak-metals and semiconductors are expected for MgAl2C2. The values of theoretical Vickers hardness for MgAl2Ge2, MgAl2Si2, and MgAl2C2 are 3.3, 2.7, and 7.7 GPa, respectively, indicating that these three intermetallics are soft and easily machinable.

MAX phases have diverse physical properties and as such are an important research field linked to numerous applications. The recent addition of boride member to the MAX family has further extended the physical properties and diversity of these compounds. In this study, the structural, electronic, mechanical, thermal and optical properties were investigated using the density functional theory (DFT) for Nb2SB. The effects of C and N substitution at the B-site of Nb2SB are also investigated. To do so the conventional carbide and nitride counterparts of Nb2SB have also been calculated. In most cases, the first boride MAX phase Nb2SB is mechanically stronger than its carbide and nitride counterparts. Its elastic constants C11 and C44, elastic moduli G and E, Vickers hardness HV, Debye temperature θD, lattice thermal conductivity κph, minimum thermal conductivity κmin, and melting temperature Tm are higher than those of Nb2SC and Nb2SN. Nb2SB is brittle in nature, whereas Nb2SC and Nb2SN are ductile. Non-central forces are dominant in Nb2SB while the central forces are dominant in Nb2SC and Nb2SN. Nb2SB is more covalent and more resistant to shear deformation than its carbide and nitride counterparts. Elastically and optically, Nb2SB is less anisotropic than its carbide and nitride counterparts. Data availability Data will be made available upon reasonable request.

Heterojunction diodes of n-ZnO/p-Si (100) and n-ZnO:Al/p-Si (100) were fabricated by spray pyrolysis technique. X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), and field emission scanning electron microscopy (FESEM) were used to characterize the as-prepared samples. The XRD pattern indicates the hexagonal wurzite structure of zinc oxide (ZnO) and Al-doped ZnO (AZO) thin films grown on Si (100) substrate. The compositional analysis by EDX indicates the presence of Al in the AZO structure. The FESEM image indicates the smooth and compact surface of the heterostructures. The current–voltage characteristics of the heterojunction confirm the rectifying diode behavior at different temperatures and illumination intensities. For low forward bias voltage, the ideality factors were determined to be 1.24 and 1.38 for un-doped and Al-doped heterostructures at room temperature (RT), respectively, which indicates the good diode characteristics. The capacitance–voltage response of the heterojunctions was studied for different oscillation frequencies. From the 1/C 2–V plot, the junction built-in potentials were found 0.30 V and 0.40 V for un-doped and Al-doped junctions at RT, respectively. The differences in built-in potential for different heterojunctions indicate the different interface state densities of the junctions. From the RT photoluminescence (PL) spectrum of the n-ZnO/p-Si (100) heterostructure, an intense main peak at near band edge (NBE) 378 nm (3.28 eV) and weak deep-level emissions (DLE) centered at 436 nm (2.84 eV) and 412 nm (3.00 eV) were observed. The NBE emission is attributed to the radiative recombination of the free and bound excitons and the DLE results from the radiative recombination through deep level defects.

This work illustrates thickness dependent physical properties of copper indium diselenide CuInSe 2 (CIS) thin films fabricated by a novel two stage electron-beam evaporation technique. Structural analysis exhibits CuInSe 2 phase chalcopyrite structure having preferred orientation in the (112) plane. Absorption coefficient of CIS films have been found very high and associated with band-to-band transition. The optical band gap displays a slight variation from 1.80 eV to 1.90 eV depending on film thickness. Photoluminescence (PL) study reveals the presence of acceptor-type defects in the films. Some other important optical constants, such as refractive index, real and imaginary parts of dielectric constant, optical conductivity and energy loss function are also calculated and found dependent on film thickness. Electrical conductivity measurement confirms CIS films are semiconducting in nature and the activation energy values indicate electrical transport is mainly thermally activated. Hall Effect study confirms CIS films are p-type having carrier concentration of the order of ∼10 26 m -3 .

Nanorod like crystalline CH3NH3PbI3-xClx thin film was prepared by spray pyrolysis at an ambient atmosphere. X-ray diffraction (XRD) and Raman studies reveal the tetragonal perovskite phase of spray deposited film. The optical band gap of CH3NH3PbI3-xClx is estimated to ~1.64 eV which is consistent with vacuum based deposited perovskites. The CH3NH3PbI3-xClx film exhibits a high absorption coefficient of the order of 106 m−1. Photoluminescence (PL) study suggests that band-to-band radiative recombination occurs in CH3NH3PbI3-xClx film. Cathodoluminescence study confirms the emission of 1.67 eV at 80 K and 1.80 eV at 300 K. In this study, the second harmonic generation at photon energy 3.6 eV is reported for the first time at room temperature (RT). It is noted that the band edge peak position at 1.67 eV is being diffused and shifted to 1.80 eV at RT while the second harmonic peak position at 3.6 eV remains unchanged.

Optical and photo-electrical properties of ZnO–CdO films with the ratio of Zn/Cd=1:0, 3:1, 1:1, 1:3 and 0:1 has been studied. XRD study confirms the combination of hexagonal ZnO and cubic CdO phase present in the polycrystalline sample. Atomic force microscopy results indicate that the crystal grains are agglomerated and surface roughness enhanced due to higher Cd concentration in ZnO. From optical studies, it is found that the transmittance and the band gap decreased as Cd content increased. Photoluminescence studies on ZnO–CdO films showed intense near-band edge emissions at room temperature and is attributed to recombination of excitons localized within band tail states likely caused by non-uniform Cd distribution in ZnO–CdO matrix. The dispersion of refractive index was analyzed by the Wemple–DiDomenico single-oscillator model. The third-order nonlinear polarizability is found high with higher concentration of cadmium at higher photon energies. Some other optical parameters such as dissipation factor, optical conductivity, interband transition strength, surface and volume energy loss have been calculated depending on dielectric constant evaluated from optical data. Finally, photoconductive gain and carrier lifetime have been calculated and found dependent on Zn/Cd ratio.

First-principles calculations are employed to investigate and compare the structural, elastic and optical properties of tantalum hemicarbide Ta2C and tantalum monocarbide TaC. Calculated lattice constants of these carbides are in good agreement with available theoretical and experimental results. The mechanical stability of these carbides is affirmed by the estimated values of elastic constants. The estimated values of Vickers hardness indicate that TaC is harder than Ta2C. The values of Pugh’s ratio and Poisson’s ratio suggest the brittle nature of both Ta2C and TaC. The optical properties of Ta2C along two polarization directions [001] and [100] are studied in details for the first time. Our study shows that the optical properties of both Ta2C and TaC compounds compliment the electronic structure calculations. Optical constants of Ta2C show significant dependence of the state of polarization of the incident electric field, and predict this material as optically anisotropic.

A series of ZnO-CdO thin films of different molar ratios of Zn and Cd have been deposited on glass substrate at substrate temperature ~ 360 ℃ by the spray pyrolysis technique at an ambient atmosphere. X-ray diffraction (XRD) studies confirmed the polycrystalline nature of the film and modulated crystal structures of wurtzite (ZnO) and cubic (CdO) are formed. The evaluated lattice parameters, and crystallite size are consistent with literature. Dislocation density and strain increased in the film as the grain sizes of ZnO and CdO are decreased. The band gap energy varies from 3.20 to 2.21 eV depending on the Zn/Cd ratios in the film. An incident photon intensity dependent I-V study confirmed that the films are highly photosensitive. Current increased with the increase of the intensity of the light beam. The optical conductivity and the optical constants, such as extinction coefficient, refractive index and complex dielectric constants are evaluated from transmittance and reflectance spectra of the films and these parameters are found to be sensitive to photon energy and displayed intermediate optical properties between ZnO and CdO, making it preferable for applications as the buffer and window layers in solar cells.

Transparent and highly photoconductive nano-composite ZnO–CdO films were fabricated on glass substrate at 360 °C substrate temperature. X-ray diffraction (XRD) studies indicate that the films are nano-crystalline and contain a combination of hexagonal ZnO and cubic CdO structures. According to scanning electron microscopy (SEM) studies, the surface of the films is found to be rough over large areas favoring photo absorption. From the optical and electrical measurements, it is found that ZnO–CdO composite combines the properties of optical transparency in the visible region with a high electrical conductivity. Based on the characteristics of photoluminescence (PL) and photoconductivity, it seems that Zn/Cd = 3:1 nano-crystalline film is preferable for application as buffer layers in solar cells.