xSx Solid Solutions Research Articles

ZnO1−xSx Solid Solution as Potential Buffer Layer Materials for Cu2ZnSnS4‐Based Thin Film Solar Cells: Structural and Interfacial Properties

AbstractThe current low conversion efficiency of Cu2ZnSnS4 (CZTS)‐based thin film solar cells is mainly blamed on the high carrier recombination via interface states at the absorber‐buffer. In this work, ZnO1−xSx solid solution is exploited as the potential buffer layer materials to solve this issue by using density functional theory calculations. With the varying of solid solubility, the lattice constant of ZnO1−xSx solid solution follows the first‐order Vegard's Law, while its bandgap follows the second‐order Vegard's Law. Based on the systematical analysis of crystal structure and electronic properties of ZnO1−xSx solid solution, ZnO0.375S0.625 is screened as the optimal buffer layer material for CZTS‐based thin film solar cells, owing to the suitable bandgap (2.33 eV) and the smallest lattice mismatch (<7%). The CZTS/ZnO0.375S0.625 interface has less harmful interface state, small conduction band offset (−0.02 eV) and band bending (−0.23 eV). Moreover, its band alignment belongs to the staggered Type‐II heterojunction, and its built‐in electric field is in the same direction as the carrier migration, which is very favorable for the efficient separation and fast transfer of photogenerated carriers. Therefore, these interface features and effects of CZTS/ZnO0.375S0.625 heterojunction can significantly boost the photovoltaic performance of CZTS‐based thin film solar cells.

Anion substitution and influence of sulfur on the crystal structures, phase transitions, and electronic properties of mixed TiO2/TiS2 compounds

Recent studies of TiO2/TiS2 nanostructures with various morphologies have been reported, usually showing improved properties with applications from electronics and catalysis to solar cells and medicine. However, there is a limited number of studies on the crystal structures of TiO2/TiS2 compounds with corresponding properties. In this research, relevant crystal structures of TiO1–x S x (x = 0, 0.25, 0.5, 0.75 and 1) solid solutions were investigated using an ab initio method. For each composition, crystal structures adopting anatase, rutile and CdI2 structure type were calculated on LDA-PZ and GGA-PBE levels of theory. Novel phase transitions and predicted structures are presented, and apart from several interesting metastable structures, a very interesting pressure-induced phase transition is found in the TiOS compound. Furthermore, electronic properties were studied through the dependence of semiconducting properties on dopant concentration. The first description of the electronic properties of the mixed TiO1–x S x compounds in crystal form has been presented, followed by a detailed study of the structure–property relationship, which will possibly have numerous industrial and technological applications.

High-pressure study of a nanostructured SnSe1−xSx (x = 0.5) solid solution by in-situ X-ray diffraction and ab-initio calculations

A nanostructured solid solution of SnSe1−xSx with x = 0.5 synthesized by mechanical alloying and its structural stability was studied by calorimetric measurements under high pressure conditions. The structural behavior under pressure was investigated using angle-dispersive X-ray diffraction in a synchrotron source and first-principles density functional theory (DFT) calculations. The variations of the cell structure phases were described by a third-order Birch–Murnaghan equation of state.

Stability of SnSe1−xSx solid solutions revealed by first-principles cluster expansion

The configurational thermodynamics of a pseudo-binary alloy SnSe1−xSx in the Pnma phase is studied using first-principles cluster-expansion method in combination with canonical Monte Carlo simulations. We find that, despite the alloy having a tendency toward a phase decomposition into SnSe and SnS at 0 K, the two constituent binaries readily mix with each other to form random SnSe1−xSx solid solutions even at a temperature below room temperature. The obtained isostructural phase diagram of SnSe1−xSx reveals that the alloy is thermodynamically stable as a single-phase random solid solution over a whole composition range above 200 K. These findings provide a fundamental understanding on the alloying behavior of SnSe1−xSx and bring clarity to the debated clustering tendency in this alloy system.

Effects of S-doping on the electronic transition, band gap, and optical absorption of GaSe1−xSx single crystals

The intrinsic evolutions of electronic transition and the band gap of GaSe1−xSx solid solution single crystals (x = 0, 0.133, and 0.439) grown for nonlinear optical applications have been systemically investigated by using spectroscopic ellipsometry and first-principle calculations. Five interband electronic transitions E1, E2, E3, E4, and E5 have been obtained by fitting the second derivatives of the complex dielectric functions and the physical origins were explained with the aid of theoretical calculations. It is found that the interband electronic transition energy E2, E3, and E4 show a blueshift trend from 3.457 eV, 3.736 eV, and 4.810 eV at x = 0 to 3.786 eV, 4.628 eV, and 5.086 eV at x = 0.439, respectively. This is because the larger Se atoms are replaced by smaller S atoms in GaSe1−xSx. The experimental band gap of GaSe1−xSx is increased from 1.908 eV at x = 0 to 2.081 eV at x = 0.439. Moreover, in order to verify the influences of S-doping on the band gap of GaSe1-xSx, we performed the first-principle calculations based on the density-functional theory. The theoretical results also confirm that the band gap energy increases from 2.085 eV at x = 0 to 2.15 eV at x = 0.439, which is in good agreement with the experiment results.

Continuously enhanced photoactivity of hierarchical β-Bi2O3/Bi2S3 heterostructure derived from novel BiO2CH3 octagonal nanoplates

BiO2CH3 nanoplate, a new organometallic crystal, with octagonal morphology is successfully prepared using methanol solvothermal method and further employed as template to fabricate porous β-Bi2O3 octagonal nanoplates with a thickness of less than 50nm. Then a series of hierarchical octagonal haystack-like β-Bi2O3/Bi2S3 hybrids with large heterointerface and small bandgaps (1.36–1.44eV) are constructed. It is found that, different from many regular photocatalysts that suffer from shrink of photocatalytic efficiency, the photoactivities of all β-Bi2O3/Bi2S3 heterostructures are continuously enhanced with growing test cycles, owing to the formation of Bi2O3−xSx solid solution, in which the built-in electric filed induced by lattice distortion can brings about higher e−/h+ separation efficiency. The gradually increasing photocatalytic activity can reach to an up limit in about 5 photocatalytic cycles and maintain at this level. β-Bi2O3/Bi2S3 heterostructure loaded with 20mol% of Bi2S3 is demonstrated to be the best photocatalyst and capable of fully decomposing Rh B in 30min. Our work provides deep insights into the nature of the inversed photocatalytic behavior and facilitates the advanced design of photocatalysts with unabated photoactivities.

Crystal growth of ZnSe1 −x S x solid solutions at the lowest possible vapor pressure

We have developed a procedure for the crystal growth of ZnSe1 − xSx solid solutions from the vapor phase at pmin, which makes it possible to reproducibly grow partially faceted, large-block crystals at temperatures below the wurtzite-sphalerite phase transition (1020°C) at a rate of at least 0.04 mm/h at a temperature of 945°C.

Characteristics of lattice thermal conductivity and carrier mobility of undoped PbSe-PbS solid solutions

Solid solution has been considered as an effective approach to advanced thermoelectrics. However, the benefit from the decrease in lattice thermal conductivity induced by the impurity scattering will be compensated by the reduction in carrier mobility, therefore suppressing the maximum figure of merit (zT). We demonstrate the characteristics of the fundamental properties, including the lattice thermal conductivity and carrier mobility, of undoped PbSe1−xSx solid solutions (0 < x < 1) at room temperature, which provide an efficient way to evaluate the maximum zT of the materials with optimum doping. The lattice thermal conductivity is found to decrease significantly after adding a small amount of PbSe into PbS matrix up to the composition of PbSe0.7S0.3 when the carrier mobility remains virtually unchanged, indicating the beneficial effect of impurity scattering to improve the thermoelectric performance.

Temperature dependent electroreflectance study of CdTe solar cells

Cadmium telluride is a promising material for large scale photovoltaic applications. In this paper we study CdS/CdTe heterojunction solar cells with electroreflectance spectroscopy. Both CdS and CdTe layers in solar cells were grown sequentially without intermediate processing by the close-space sublimation method. Electroreflectance measurements were performed in the temperature range of T=100–300K. Two solar cells were investigated with conversion efficiencies of 4.1% and 9.6%. The main focus in this work was to study the temperature dependent behavior of the broadening parameter and the bandgap energy of CdTe thin film in solar cells. Room temperature bandgap values of CdTe were Eg=1.499eV and Eg=1.481eV for higher and lower efficiency solar cells, respectively. Measured bandgap energies are lower than for single crystal CdTe. The formation of CdTe1−xSx solid solution layer on the surface of CdTe is proposed as a possible cause of lower bandgap energies.

Growth of ZnSe1 − x S x solid solution crystals from vapor phase

Partially faceted ZnSe1 − xSx solid solution crystals have been grown from a vapor phase in a closed stationary horizontal system. The growth has been performed in cylindrical quartz ampoules—∼10 mm in diameter and evacuated to a residual pressure of less than ∼10−3 Pa-at a temperature of 1050°C and a temperature drop between the source and growth zones of 10–16°C. The crystal structure, optical absorption, and composition inhomogeneity along the ingot length have been investigated by X-ray diffractometry and optical spectroscopy in the wavelength range of 300–3000 nm. The solid-solution composition range corresponding to cubic polymorphs is determined. The concentration dependences of the lattice parameter and band gap are presented.

Mechanism of charge transfer in n-CdS/p-CdTe heterostructures with a thick layer of the CdTe1 − x S x solid solution

The current-voltage and capacitance-voltage characteristics of the n-CdS/p-CdTe heterosystem are investigated. An analysis of the results obtained has demonstrated that a high-resistance i layer (the CdTe1 − xSx solid solution), which is inhomogeneous not only in the electrical conductivity but also in the composition, is formed at the n-CdS/p-CdTe heterointerface. The thicknesses of the solid-solution layers are estimated from the capacitance-voltage characteristics, and regions with different types of conduction are found to exist in the intermediate layer. It is shown that the ambipolar diffusion and drift in the high-resistance CdTe1 − xSx solid-solution layers are directed toward each other, thus resulting in the appearance of a sublinear portion in the current-voltage characteristics described by the dependence V = exp(Iaw). The observation of a sublinear portion in the current-voltage characteristics measured in both the forward and reverse electric current directions over a wide temperature range 77–323 K suggests that the diffusion-drift regime can occur in different parts of the i layer, depending on the electric current density and the ambient temperature. The changes observed in the electric current and capacitance, as well as in the current-voltage and capacitance-voltage characteristics, after ultrasonic irradiation indicate that, in the CdTe1 − xSx solid solutions, there exist metastable states, which most probably decay under ultrasonic irradiation and then again form solid solutions with more stable states.

Optical properties of nonlinear solid solution GaSe1-x S x (0 &lt; x ≤ 0.4) crystals

Transmission spectra and phase matching conditions for second harmonic generation in GaSe1–x S x (0 < x ≤ 0.4) solid solution crystals are experimentally investigated. An algorithm is suggested and dispersion equations are derived that allow the phase matching angles describing best the available experimental data to be estimated as a function of the mixing ratio x.

Formation of interfacial CdTe1−x S x solid solutions in CdTe/CdS heterojunctions

Experimental evidence is presented that interfacial CdTe1−x S x solid solutions graded in composition can be formed in CdTe/CdS heterojunctions.

Mechanism of charge transfer in n-CdS/p-CdTe heterojunctions

The capacitance-voltage and current-voltage characteristics of the n-CdS/p-CdTe heterosystem are investigated. Analysis of these characteristics demonstrates that the CdTe1−xSx solid solution formed at the n-CdS/p-CdTe heterointerface is inhomogeneous in both the conductivity and composition. The thickness of solid solutions is estimated from the capacitance-voltage characteristics. It is shown that, for the n-CdS/p-CdTe heterosystem, the current-voltage characteristic in the current density range 10−8-10−5 A cm−2 is governed by the thermal electron emission, whereas the current in the heterostructure at current densities in the range 10−4-10−2 A cm−2 is limited by recombination of charge carriers in the electroneutral region of the CdTe1−xSx solid solution. The lifetime and the diffusion length of minority charge carriers in the CdTe1−xSx solid solution and the surface recombination rate at the interface between the CdS layer and the CdTe1−xSx solid solution are determined. It is demonstrated that the n-CdS/p-CdTe heterostructure operates as a p-i-n structure in which CdTe is a p layer, CdTe1−xSx is an i layer, and CdS is an n layer.

A p-i-n Model of CdTe/CdS Heterostructures

CdTe/CdS heterostructures are analyzed in terms of a p-i-n model in which the i layer consists of a CdTe1 − x Sx solid solution resulting from Te and S diffusion into CdS and CdTe, respectively. The lattice parameters of the solid solution are determined. The i layer is shown to be too thick for tunneling.

Single Crystal Structural Investigations on NiyMo6Se8−xSx Solid Solution: A New Location of Nickel Counterions

The structures of nickel sulfoselenide Chevrel phases have been determined. All the richest nickel phases crystallize in rhombohedral symmetry. A single-crystal structural study makes it possible to identify the special nickel cation locations inside the host network for both pure sulfide and selenide Chevrel phases. We also show that these nickel atoms can form some kind of cluster in the cavities of the host or Ni–Mo bondings.

Exciton Thermoabsorption of GaSe1−xSx Solid Solutions

AbstractResults are reported obtained at liquid nitrogen temperature by thermally modulating the absorption of GaSe1−xSx solid solutions. In order to clarify the relations among different polytypes, their optical properties, and growth methods, a complete series of measurements is performed, varying the compositional parameter x from 0 to 1, for single crystals grown both from the vapour and from the melt. The results are consistent with the existence of a unique polytype (ε) for x ≈ 0 in crystals grown by both methods. Discontinuities in the exciton ground state energy E0 versus x curves are related to interpolytype transitions, namely γ → β andε → β, for melt‐ and vapour‐grown crystals, respectively.