Sulfurization and post-selenization of oxides precursors for high quality CuIn(S,Se)2 thin films

Oxide nanoparticles-based process is one of the successful approaches to prepare CuIn_1−xGa_xSe₂ and CuInSe₂, which has achieved high power conversion efficiencies. In order to transform the oxides into selenide, the oxide precursors were annealed with solid Se which was used as a source of Se vapor rather than highly toxic and explosive H₂Se and H₂ gas. However, the In₂O₃ phase frequently remains in the final films after selenization because of the high stability of In₂O₃ and the poor activity of Se during selenization. So, in order to eliminate the impurity phase of In₂O₃ and improve the morphology of the final thin films, the oxide precursors were sequentially sulfurized and selenized. The CuIn(S, Se)₂ (CISSe) thin films which have pure phase, improved crystallinity, larger grain size and optimized band gap were obtained in this work.     

Phase identification and XPS studies of Cu(In,Ga)Se2 thin films

In this paper we present results concerning the effect of preparation conditions on the surface chemistry and crystalline phase of Cu(In,Ga)Se₂ (CIGS) thin films grown by a chemical reaction of the precursor species in two and three stage processes. The CIGS samples were studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis. It was found that the bulk of the samples grown in a three stage process contains mainly the CIGS phase; however, secondary phases like In₂Se₃, Cu₂Se and In₂O were additionally identified at the surface of CIGS samples grown in two stages.     

Composition-Dependent Thermoelectric Properties of n-Type Bi2Te2.7Se0.3 Doped with In4Se3

We present the effects of In₄Se₃ addition on thermoelectric properties of n-type Bi₂Te_2.7Se_0.3. In this study, polycrystalline (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x pellets were prepared by mechanical alloying followed by spark plasma sintering (SPS). The thermoelectric properties such as Seebeck coefficient and electrical and thermal conductivities were measured in the temperature range of 300 K to 500 K. Addition of In₄Se₃ into Bi₂Te_2.7Se_0.3 resulted in segregation of In₄Se₃ phase within Bi₂Te_2.7Se_0.3 matrix. The Seebeck coefficient of the (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x samples exhibited lower values compared with that of pure Bi₂Te_2.7Se_0.3 phase. This reduction of Seebeck coefficient in n-type (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (In₄Se₃) _x and (Bi₂Te_2.7Se_0.3)_1?x as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of In₄Se₃ leads to an enhanced thermoelectric figure of merit (ZT) at a temperature range over 450 K: a maximum ZT of 1.0 is achieved for the n-type (In₄Se₃)_0.03-(Bi₂Te_2.7Se_0.3)_0.97 sample at 500 K.     

Specific features of the low-temperature conductivity and photoconductivity of CuInSe2-ZnIn2Se4 alloys

n-type CuInSe₂-ZnIn₂Se₄ alloy single crystals are grown by the horizontal variant of the Bridgman method. The slight temperature dependence of the conductivity, high electron concentration, and the low photoconductivity of single crystals containing a low (5–10 mol %) fraction of ZnIn₂Se₄ are indicative of the nearly degenerate state of the crystals. It is established that, in the CuInSe₂-ZnIn₂Se₄ single crystals containing 15 and 20 mol % of ZnIn₂Se₄, the hopping mechanism of conductivity is dominant at temperatures of T ～ 27–110 K. At T ≥ 110 K, hopping conductivity gives way to activated conductivity. It is found that the specific feature of the low-temperature (27–77 K) photoconductivity spectrum of single crystals with ～15 and 20 mol % of ZnIn₂Se₄ is a single narrow peak at a wavelength of λ_max = 1190–1160 nm.     

Phase composition of CuInSe2 in different annealing process

CuInSe₂ was synthesized by a low cost, non-vacuum hydrothermal solution method using copper chloride, indium chloride and Se powder as raw materials. The reaction schemes of CuInSe₂ in different annealing processes were investigated by in-situ X-ray diffraction measurements. Its phase composition, crystal structure and morphology properties of CuInSe₂ were studied. The results show that CuInSe₂ has chalcopyrite crystal structure and it remains stable below the temperature of 773 K, but it decomposes to CuSe and InSe at temperature above 773 K in vacuum annealing. While in oxygen annealing, CuInSe₂ is oxidized to CuO, In₂O₃ and SeO₂ at the temperature of 523 K. Therefore, the temperature of selenization or annealing must be lower than 773 K in order to reduce the amount of CuSe and InSe in large production scale. The results were demonstrated by energy dispersive spectrometer as well. The process of its reaction mechanism was discussed based on the experimental data.     

Synthesis of CuInSe2 monodisperse nanoparticles and the nanorings shape evolution via a green solution reaction route

High-quality CuInSe₂ nanoparticles were successfully synthesized through a green, simple, low-cost route using environmentally friendly N-oleoylmorpholine as a new solvent of Se powder and oleylamine as capping ligand. The as-synthesized nanoparticles were characterized by X-ray diffraction, TEM, HRTEM, and energy dispersive X-ray spectroscopy. The experimental results revealed that the as-prepared CuInSe₂ nanoparticles with chalcopyrite tetragonal structure have an average size about 8 nm. The facile and green synthesis strategy was also used to synthesize hexagonal shaped CuInSe₂ nanorings by altering the synthesis parameters. Possible shape evolution and crystals growth mechanisms have been suggested for the formation of spherical shaped CuInSe₂ nanoparticles and hexagonal shaped CuInSe₂ nanorings, respectively.     

Metastable Hexagonal In2O3 Nanofibers Templated from InOOH Nanofibers under Ambient Pressure

Single‐crystal, metastable, hexagonal In₂O₃ (H‐In₂O₃) nanofibers with an average diameter of 80 nm and length of up to several micrometers were synthesized on a large scale, for the first time under ambient pressure, by annealing InOOH nanofibers at 490 °C. The InOOH nanofibers were prepared by a controlled hydrolysis solvothermal reaction, using InCl₃·4H₂O as the starting material and ether as the solvent, in the temperature range of 190–240 °C. The solvent has significant effects on the formation of the metastable phase and the morphology of the In₂O₃ nanocrystals during the synthesis of the precursor InOOH. Room‐temperature optical absorption spectra of the hexagonal In₂O₃ nanofibers showed strong absorption peak located at 325 nm (3.83 eV) with a slight blue‐shift compared with that of bulk In₂O₃ (3.75 eV). The H‐In₂O₃ nanofibers photoluminesce at room temperature with emission peaks at 378 nm, 398 nm, and 420 nm. The successful production of metastable hexagonal In₂O₃ nanofibers in large scale under mild conditions could be of interest both for applications and fundamental studies.     

The structural evolution of Cu(In,Ga)Se2 thin film and device performance prepared through a three-stage process

Cu(In,Ga)Se₂ (CIGS) absorber layer is grown on Mo-coated soda-lime glass (SLG) substrates using co-evaporation deposition technique. The growth characteristics of the CIGS films deposited through a three-stage process are examined by interrupting the deposition along the reaction pathway. In the three-stage process, the absorber layer undergoes several phase transformations with Cu content. The γ-(In,Ga)₂Se₃ layer is formed first and is then converted to α-Cu(In,Ga)Se₂ via β-Cu(In,Ga)₃Se₅. When α-Cu(In,Ga)Se₂ stoichiometry is reached, Cu_2−xSe segregation at the surface and at grain boundaries begins to occur. The Cu_2−xSe improved the densification and grain growth of the absorber layer. Then, as the absorber layer reverts to substoichiometric composition, the Cu_2−xSe phase disappears and the depleted server Cu near the surface instead. This paper reports several types of defects found in absorber layers that act as non-radiative recombination centers, such as impurity phases (Cu_2−xSe and Cu(In,Ga)₃Se₅), deep point defects (In_Cu), grain boundaries, and voids. The highest efficiency at 10.97% was achieved when the bulk [Cu]/([In]+[Ga]) ratio was 0.98 at the third stage of the process. This result is attributed to the low-concentration deep-level defects that act as recombination centers and to the denser structure with larger grain size.     

Effects of Bi2Se3 Nanoparticle Inclusions on the Microstructure and Thermoelectric Properties of Bi2Te3-Based Nanocomposites

A series of thermoelectric nanocomposite samples were prepared by integrating Bi₂Se₃ nanoparticles into a bulk Bi₂Te₃ matrix. Primarily, spherical Bi₂Se₃ nanoparticles with diameter of ～30 nm were synthesized by combining bismuth acetate with elemental Te in oleic acid solution. Bi₂Te₃-based nanocomposite samples were prepared by consolidating the appropriate quantity of Bi₂Se₃ nanoparticles with the starting elements (Bi and Te) using typical solid-state synthetic reactions. The microstructure and composition of the Bi₂Te₃-based nanocomposites, as well as the effects of the Bi₂Se₃ nanoparticles on their thermoelectric properties, are investigated. Transmission electron microscopy observation of the Bi₂Te₃-based nanocomposites reveals two types of interface between the constituent materials, i.e., coherent and incoherent, depending on the Bi₂Se₃ concentration. The Bi₂Se₃ nanoparticles in the Bi₂Te₃ matrix act as scattering centers for a wider range of phonon frequencies, thereby reducing the thermal conductivity. As a result, the maximum ZT value of 0.75 is obtained for the Bi₂Te₃ nanocomposite with 10 wt.% Bi₂Se₃ nanoparticles at room temperature. It is clear that the reduction in the thermal conductivity plays a central role in the enhancement of the ZT value.     

Optical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films

In₂Se₃ films are produced by ion-beam evaporation at substrate temperatures of 313 and 623 K. As the target, In₂Se₃ single crystals grown by the vertical Bridgman method are used. The composition and structure of the crystals and films are determined by the X-ray spectral analysis and X-ray diffraction techniques, respectively. It is established that the crystals and films crystallize with the formation of a hexagonal structure. The band gap and refractive index of the In₂Se₃ films are determined from the transmittance and reflectance spectra. It is found that, as the substrate temperature is increased, the band gap increases.     

Anisotropic Properties of Quasi-1D In4Se3: Mechanical Exfoliation,Electronic Transport,and Polarization-Dependent Photoresponse

Theoretical and experimental investigations of various exfoliated samples taken from layered In₄Se₃ crystals are performed. In spite of the ionic character of interlayer interactions in In₄Se₃ and hence much higher calculated cleavage energies compared to graphite, it is possible to produce few-nanometer-thick flakes of In₄Se₃ by mechanical exfoliation of its bulk crystals. The In₄Se₃ flakes exfoliated on Si/SiO₂ have anisotropic electronic properties and exhibit field-effect electron mobilities of about 50 cm² V⁻¹ s⁻¹ at room temperature, which are comparable with other popular transition metal chalcogenide (TMC) electronic materials, such as MoS₂ and TiS₃. In₄Se₃ devices exhibit a visible range photoresponse on a timescale of less than 30 ms. The photoresponse depends on the polarization of the excitation light consistent with symmetry-dependent band structure calculations for the most expected ac cleavage plane. These results demonstrate that mechanical exfoliation of layered ionic In₄Se₃ crystals is possible, while the fast anisotropic photoresponse makes In₄Se₃ a competitive electronic material, in the TMC family, for emerging optoelectronic device applications.     

Effect of terbium doping on structural,optical and gas sensing properties of In2O3 nanoparticles

In this work, the effect of terbium (Tb³⁺) as dopant on the structural, optical, electrical and gas sensing properties of In₂O₃ (indium oxide) nanoparticles has been discussed. In₂O₃ and Tb³⁺-doped In₂O₃ nanoparticles were synthesized by a facile and cost effective co-precipitation method. XRD analysis revealed the formation of bixbyite-type cubic phase for In₂O₃ and Tb³⁺-doped In₂O₃ nanoparticles which was further supported by Raman studies. It was observed that the crystallite size of In₂O₃ nanoparticles decreased, while structural disorder increased with increase in Tb³⁺ concentration. SEM micrographs showed that particles were spherical in shape and EDS corroborated the presence of Tb³⁺ in doped In₂O₃ nanoparticles. A broadening and shifting of Raman peaks with increase in Tb³⁺ content was also observed. For gas sensing characteristics, the nanoparticles were applied as thick film onto the alumina substrate and tested at different operating temperatures for various volatile organic compounds (such as methanol, ethanol, acetone) and ammonia. The results indicated that the sensor based on 5%Tb³⁺-doped In₂O₃ nanoparticles presented much higher sensor response to 50ppm ethanol at 300 °C temperature than the pure In₂O₃ sensor. The enhancement of the response may be attributed to high surface basicity, small size and large lattice distortion of doped In₂O₃ sensor.     

Phase equilibrium of a CuInSe2–CuInS2 pseudobinary system studied by combined first-principles calculations and cluster expansion Monte Carlo simulations

We report the phase diagram of a CuInSe₂–CuInS₂ pseudobinary system calculated by a combination of first-principles calculations based on density functional theory, cluster expansion, and Monte Carlo simulations. All formation energies of CuIn(Se_1−xS_x)₂ (CISS) alloys are positive, indicating that CISS alloy is a miscibility gap system and has a tendency to phase separation. The phase diagram computed with conventional cluster expansion shows a miscibility gap with consolute temperature T_C=170 K. The contribution of lattice vibrations lowers T_C to 130 K. The miscibility gaps for the CuInSe₂–CuInS₂ system are predicted to be asymmetric. The effect of lattice vibrations on the miscibility gap is found to be large, and the size mismatch mechanism can be used to explain the large vibrational effect in the CuInSe₂–CuInS₂ system.     

Nonvolatile Ferroelectric Memory Effect in Ultrathin α‐In2Se3

High‐density memory is integral in solid‐state electronics. 2D ferroelectrics offer a new platform for developing ultrathin electronic devices with nonvolatile functionality. Recent experiments on layered α‐In₂Se₃ confirm its room‐temperature out‐of‐plane ferroelectricity under ambient conditions. Here, a nonvolatile memory effect in a hybrid 2D ferroelectric field‐effect transistor (FeFET) made of ultrathin α‐In₂Se₃ and graphene is demonstrated. The resistance of the graphene channel in the FeFET is effectively controllable and retentive due to the electrostatic doping, which stems from the electric polarization of the ferroelectric α‐In₂Se₃. The electronic logic bit can be represented and stored with different orientations of electric dipoles in the top‐gate ferroelectric. The 2D FeFET can be randomly rewritten over more than 10⁵ cycles without losing the nonvolatility. The approach demonstrates a prototype of rewritable nonvolatile memory with ferroelectricity in van der Waals 2D materials.     

Physical investigations on (In2S3)x(In2O3)y and In2S3−xSex thin films processed through In2S3 annealing in air and selenide atmosphere

In₂S_3−xSe_x and (In₂S₃)_x(In₂O₃)_y thin films have been prepared on glass substrates using appropriate heat treatments of In evaporated thin films. X-ray analysis shows that In thin films which were annealed under sulfur atmosphere at 350 °C were mainly formed by In₂S₃. A heat treatment of this binary in air at 400 °C during one hour leads to (In₂S₃)_x(In₂O₃)_y ternary material which has a tetragonal structure with a preferred orientation of the crystallites along the (109) direction. Similarly, a heat treatment of In₂S₃ in selenium atmosphere at 350 °C during six hours leads to a new In₂S_3−xSe_x ternary material having tetragonal body centered structure with a preferred orientation of the crystallites along the (109) direction. Optical band gap, refractive index and extinction coefficient values of In₂S_3−xSe_x and (In₂S₃)_x(In₂O₃)_y thin films have been reached. Moreover, correlations between optical conductivity, XRD, AFM and Urbach energy of such ternary thin films have been discussed. Finally, the recorded formation disparity between the quaternary (In₂S₃)_x(In₂O₃)_y and ternary In₂S_3−xSe_x compounds has been discussed in terms of the Simha–Somcynsky and Lattice Compatibility theories.     

Photoluminescence Studies of CuInS2-CuInSe2 Alloy Crystals

Photoluminescence spectra are presented for single crystals of CuInS₂, CuInSe₂ and CuInS_2ySe_2-2y alloys. The PL spectrum of stoichiometric CuInSe₂ is dominated by free exciton emission of 9 meV FWHM at 1.03 eV, but structure at 090, 0.94 and 0.97 eV is observed due to transitions involving residual native defect states. For pure CuInS₂ broad deep luminescence bands are obtained that involve several deep native defect states, e.g. donors at 45 and 160 meV below the CBE and acceptors at 85 meV above the VBE. These defects persist in S-rich CuInS₂/CuInSe₂ alloys, but excitonic emission is observed, in addition to the deep luminescence. Surprisingly CuInS_2ySe_2-2y crystals at y ≳ l are totally dominated by free exciton emission. This result shows that an increase of at least 20% over the bandgap of CuInSe₂ and excellent crystal quality can be achieved by partial substitution of Se by S.     

Synthesis and Growth of New Compounds Au3In5Se9 and Au3Ga5Se9

The phase analysis was performed and a character of interaction in AuInSe₂ - In₂Se₃ and AuGaSe₂ -Ga₂Se₃ quasi-binary systems were considered. The technology of synthesis and growth of single crystals of new semiconducting compounds Au₃In₅Se₉ and Au₃Ga₅Se₉ were developed. The materials for ohmic contacts were chosen and the electrical, optical and thermal properties of obtained compounds have been investigated. The forbidden band gap, its temperature coefficient, the type of the optical transitions and heat capacity of Au₃In₅Se₉ and Au₃Ga₅Se₉ compounds were determined. The charge of entropy and enthalpy has been estimated by numerical integration.     

Properties of In2O3 films obtained by thermal oxidation of sprayed In2S3

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using indium chloride and thiourea as precursors at a molar ratio of S:In=2.5. The deposition was carried out at 350 °C on quartz substrates. The film thickness is about 1 µm. The films were then annealed for 2 h at 550, 600, 650 and 700 °C in oxygen flow. This process allows the transformation of nanocrystal In₂O₃ from In₂S₃ and the reaction is complete at 600 °C. X-ray diffraction spectra show that In₂O₃ films are polycrystalline with a cubic phase and preferentially oriented towards (222). The film grain size increases from 19 to 25 nm and RMS values increase from 9 to 30 nm. In₂O₃ films exhibit transparency over 70–85% in the visible and infrared regions due to the thickness and crystalline properties of the films. The optical band gap is found to vary in the range 3.87–3.95 eV for direct transitions. Hall effect measurements at room temperature show that resistivity is decreased from 117 to 27 Ω cm. A carrier concentration of 1×10¹⁶ cm^?3 and mobility of about 117 cm² V^?1 s^?1 are obtained at 700 °C.     

Sodium‐Mediated Epitaxial Growth of 2D Ultrathin Sb2Se3 Flakes for Broadband Photodetection

As an important member of group VA–VIA semiconductors, 2D Sb₂Se₃ has drawn widespread attention thanks to its outstanding optoelectronic properties as compared to the bulk material. However, due to the intrinsic chain‐like crystal structure, the controllable synthesis of ultrathin 2D planar Sb₂Se₃ nanostructures still remains a huge challenge. Herein, for the first time, the crystal structure limitation is overcome and the successful structural evolution of 2D ultrathin Sb₂Se₃ flakes (as thin as 1.3 nm), by introducing a sodium‐mediated chemical vapor deposition (CVD) growth method, is realized. The formation of 2D planar geometry is mainly attributed to the preferential growth of (010) plane with the lowest formation energy. The thickness‐dependent band structure of 2D Sb₂Se₃ flakes shows a wide absorption band from UV to NIR region (300–1000 nm), suggesting its potential application in broadband photodetection. Strikingly, the Sb₂Se₃ flakes–based photodetector demonstrates excellent performance such as broadband response varying from UV to NIR region, high responsivity of 4320 mA W^?1, fast response time (τ_rise ≈ 13.16 ms and τ_decay ≈ 9.61 ms), and strong anisotropic ratio of 2.5@ 532 nm, implying promising potential application in optoelectronics.     

Synthesis of Indium and Indium Oxide Nanoparticles from Indium Cyclopentadienyl Precursor and Their Application for Gas Sensing

Decomposition of the organometallic precursor [In(η⁵‐C₅H₅)] in toluene in the presence of methanol (8 vol.‐%) at room temperature leads to the immediate formation of aggregates of indium nanoparticles of 15 ± 2 nm mean diameter. The aggregates are roughly spherical with a mean size of 400 ± 40 nm. The particles were characterized by means of transmission electron and high‐resolution transmission electron microscopies (TEM and HRTEM), and X‐ray diffraction (XRD) studies indicate that the powder consists of the tetragonal phase of indium. The thermal oxidation in air of these nanoparticles yields well‐crystallized nanoparticles of In₂O₃ with unchanged morphology (aggregates of nanoparticles of 16.6 ± 2 nm mean diameter with aggregate mean size of 400 ± 40 nm) and without any sign of coalescence. XRD pattern shows that the powder consists of the cubic phase of In₂O₃. The electrical conductivity measurements demonstrate that this material is highly sensitive to an oxidizing gas such as nitrogen dioxide and barely sensitive to a reducing gas such as carbon monoxide. Its association with SnO₂‐based sensors allows the selective detection of carbon monoxide (30 ppm) and sub‐ppm amounts of nitrogen dioxide (400 ppb) in a mixture at 21 °C and at a relative humidity of 60 %.