S1 xSex Research Articles

Phase Stability, Band Gap Tuning, and Rashba Splitting in Selenium-Alloyed Bournonite: CuPbSb(S1–xSex)3

Recently, bournonite (CuPbSbS3) has been identified as a potential ferroelectric photovoltaic (PV) material with a 1.3 eV band gap, which falls in an appropriate range for single-junction PV devices. Progress in applying bournonite has yielded several studies reporting successful thin-film processing, the most recent of which demonstrated a 2.65% power conversion efficiency for a bournonite-based PV device. In an effort to explore bournonite band gap engineering for PV and other prospective applications (e.g., thermoelectric, spintronic), we here consider the solid-state synthesis of selenium-alloyed bournonite, CuPbSb(S1–xSex)3, across the full range of x (0.0 ≤ x ≤ 1.0) and report phase purity for 0.0 ≤ x ≤ 0.5. We characterize the crystal structure and band gap of the samples using X-ray diffraction and diffuse reflectance spectroscopy, determining a band gap decrease for single-phase samples from 1.25 eV at x = 0.0 to 1.06 eV at x = 0.5. Formation energies determined using dispersion-corrected hybrid density functional theory (DFT) support experimental findings and are consistent with the stability (instability) of the x = 0.5 (x = 1.0) structure relative to starting materials. The computed band structures show a decreasing trend in the band gap with increasing x, again consistent with experiment, with the states at the conduction (valence) band minima (maxima) being principally derived from Pb (S/Se) states. Spin texture analysis and detailed comparison of spin splitting parameters show that Se alloying modifies the band gap while maintaining Rashba spin splitting character within the electronic structure. Rashba splitting is most pronounced for the conduction band minimum along the Γ–X path in the Brillouin zone. Energy separation between spin states is maintained at ΔE ∼0.2 eV for the various Se contents, with σz as the spin direction. A smaller spin splitting occurs along Γ–Z (decreasing value with increasing x), with σx as the spin direction.

A solution-route processed multicomponent Cu2ZnSn (S1-x Sex)4 nanocrystals: A potential low-cost photocatalyst

An exemplary perspective to comprehend the underlying role of multicomponent nanocrystals such as CZTS/SSe/Se (Cu2ZnSnS4/(SSe)4/Se4) revives the thrust for pioneering the non-vacuum approach for the development of multi-component-based chalcogenides utilizing organic solvents for the formation of nanocrystals, ideal for photo-voltaic/catalysts. This article envisages the employment of Trioctylphosphine (TOP) and Trioctylphosphine Oxide (TOPO) as capping agents cum solvent for CZTS/SSe/Se; the work proceeds with the anionic ratio variations in pristine counterparts of CZTSSe, i.e., to speculate the potential of quaternary chalcogenides where the properties of different materials coalesce and able to improve the characteristics efficiently. Gradually changing the ratio during nanocrystal synthesis via a high-temperature colloidal route imposes precise control over the properties. The synthesized nanocrystals are ingrained in Se-rich and highly crystalline, inhibiting the exigency of selenization and high-temperature annealing. Electrochemical, optical, and structural analysis was intensively studied and the best aliquot Se-ratio was considered to proceed as a photocatalyst. Photocatalysis is an enunciating approach for first aiding wastewater consisting of toxic materials, especially dyes and textile effluents, under sunlight. While, the converging lens deployment uplifts the efficiency and rate of reaction by many folds, it pioneers a new route of energy applications commercially.

(X, T)-phase diagram of TlIn(S1–xSex)2 solid solutions (x = 0, 0.02, 0.06, 0.10, 0.15 and 0.25). Temperature dependences of thermal expansion and optical anisotropy parameters

ABSTRACT Basing on dilatometric and optical anisotropy studies, we have obtained (x, T)-phase diagram for TlIn(S1–xSex)2 solid solutions in the range 0 ≤ × ≤ 0.25 of selenium concentrations. It has been found that increasing selenium concentration leads to decrease in almost all of phase transition (PT) temperatures. At the same time, the temperature Ti of PT into incommensurate phase for the polytype c = c 0 increases at the selenium concentrations higher than 0.1, and we observe a tendency to broadening of the temperature region where the incommensurate phase exists. The incommensurate–ferroelectric PT reveals a notably diffuse character. We have also demonstrated that the TlIn(S1–xSex)2 solid solutions have a polytype structure. It manifests itself in availability of additional PT in the solid solutions with 0 ≤ × ≤ 0.05. Most probably, the latter corresponds to ferroelectric transformation in the polytype with c = 16c 0.

Lattice dynamics of Pnma Sn(S1–xSex) solid solutions: energetics, phonon spectra and thermal transport

Alloying is widely used as a means to fine-tune the properties of thermoelectric materials by reducing the lattice thermal conductivity. However, the effects of compositional variation on the lattice dynamics of alloy systems are not well understood, due in part to the difficulty of building realistic first-principles models of structurally-complex solid solutions. This work builds on our previous study of Snn(S1–xSex)m solid solutions (Gunn et al 2019 Chem. Mater. 31 3672) to explore the lattice dynamics of the Pnma Sn(S1–xSex) system, which has been widely studied for potential thermoelectric applications. We find that the vibrational internal energy and entropy have a large quantitative impact on the mixing free energy and are likely to be particularly important in alloy systems with competing phases. The thermodynamically-averaged phonon dispersions and density of states curves show that alloying preserves the structure of the low-frequency bands of modes associated with the Sn sublattice but broadens the high-frequency chalcogen bands into a near-continuous spectrum at the 50/50 mixed composition. This results in a general reduction in the phonon mode group velocities and an increase in the number of energy-conserving scattering channels for heat-carrying low-frequency modes, which is consistent with the decrease in thermal conductivity observed in experimental measurements. Finally, we discuss some of the limitations of our first-principles modelling approach and propose methods to address these in future studies.

Controlled Synthesis of Sb2(S1–xSex)3 (0 ≤ x ≤ 1) Solid Solution and the Effect of Composition Variation on Electrocatalytic Energy Conversion and Storage

Solid solutions provide better control over optical and electronic properties of a material as a function of the composition. However, to achieve precise control over stoichiometry, carefully designed precursors with comparable reactivity are required. We have used the molecular precursor route to prepare Sb2(S1–xSex)3 (0 ≤ x ≤ 1) solid solution over an entire range. The p-XRD analysis confirms the formation of a solid solution, and EDX analysis shows a uniform distribution of all elements in the synthesized samples. Morphology examination by SEM and TEM analysis indicates crystal splitting in all the samples. Raman analysis confirms the absence of phase segregation, and UV–vis–NIR analysis shows a gradual change in the band gap as a function of composition. The electrocatalytic behavior of the samples for energy generation and energy storage was also examined. For hydrogen evolution reaction, the selenium-enriched sample with a small amount of sulfur showed the lowest overpotential of 210 mV at a current...

Thermodynamics, Electronic Structure, and Vibrational Properties of Sn n (S1-x Se x ) m Solid Solutions for Energy Applications.

The tin sulfides and selenides have a range of applications spanning photovoltaics and thermoelectrics to photocatalysts and photodetectors. However, significant challenges remain to widespread use, including electrical and chemical incompatibilities between SnS and device contact materials and the environmental toxicity of selenium. Solid solutions of isostructural sulfide and selenide phases could provide scope for optimizing physical properties against sustainability requirements, but this has not been comprehensively explored. This work presents a detailed modeling study of the Pnma and rocksalt Sn(S1–xSex), Sn(S1–xSex)2, and Sn2(S1-xSex)3 solid solutions. All four show an energetically favorable and homogenous mixing at all compositions, but rocksalt Sn(S1–xSex) and Sn2(S1–xSex)3 are predicted to be metastable and accessible only under certain synthesis conditions. Alloying leads to a predictable variation of the bandgap, density of states, and optical properties with composition, allowing SnS2 to be “tuned down” to the ideal Shockley–Queisser bandgap of 1.34 eV. The impact of forming the solid solutions on the lattice dynamics is also investigated, providing insight into the enhanced performance of Sn(S1–xSex) solid solutions for thermoelectric applications. These results demonstrate that alloying affords facile and precise control over the electronic, optical, and vibrational properties, allowing material performance for optoelectronic applications to be optimized alongside a variety of practical considerations.

Chalcopyrite CuIn(S1–xSex)2 for Photoelectrocatalytic H2 Evolution: Unraveling the Energetics and Complex Kinetics of Photogenerated Charge Transfer in the Semiconductor Bulk

Cu(In,Ga)(S,Se)2 (CIGS) chalcopyrites and their derivatives are potential candidates for use in photoelectrochemical (PEC) H2 generation because of their well-matched solar absorption properties and agreeable band positions relative to the water reduction redox potential. In this work, we present the photoelectrochemical characterization and H2 evolution performance of the highly tunable anion CIGS derivative series CuIn(S1–xSex)2. The photocathodes were subjected to Mott–Schottky analysis and chopped-light irradiation to determine their flat band potential. With excellent agreement between the two techniques, the flat band potentials were determined to increase successively with increasing x in CuIn(S1–xSex)2, with all band positions possessing appropriate energetics for the thermodynamic requirement of reducing H2O to H2. The photocathodes were then subjected to identical bulk H2 evolution conditions for 10–12 h, maintaining ∼90% faradaic efficiency. The rates of photoelectrocatalysis were found to vary...

Low-temperature Raman studies of sulfur-rich TlIn(S1−xSex)2 single crystals

Raman spectra of TlIn(S1–xSex)2 (0 ≤ x ≤ 0.25) single crystals in the frequency range 16–400 cm−1 were studied at T = 30 K in the Z(XX+XY)Z¯ configuration. The experimental Raman spectra of TlIn(S1-xSex)2 single crystals were analyzed by multi-peak simulation by Lorentzian contours, enabling the peak frequencies, halfwidths and intensities to be determined. Their compositional dependences were studied. Compositional behaviour of Raman features revealed in the spectral regions 40–85 cm−1 and 190–220 cm−1 is discussed in view of different number of isovalent S and Se atoms in structural groups corresponding to the general formula of In4S10–ySey.

A novel solid solution LiGa(S1–xSex)2 for generating coherent ultrafast mid-IR sources

With renewed refractive indices, the potential of a solid solution () in optical frequency conversion—especially in phase matching and group velocity matching—is theoretically investigated, together with the composition ratio limitation. It is found that the solution has excellent features for generating coherent ultrafast mid-IR sources covering 8–11 μm, which can be realized by type II down-conversion in the ba-plane with perfect group velocity matching, or type I in the bc-plane with part group velocity matching. This will have broad applications in LiDAR monitoring and precision spectroscopy, as well as life and environmental sciences.

Alloying as a Route to Monolayer Transition Metal Dichalcogenides with Improved Optoelectronic Performance: Mo(S1–xSex)2 and Mo1–yWyS2

On the basis of first-principles and cluster expansion calculations, we propose an effective approach to realize monolayer transition metal dichalcogenides with sizable band gaps and improved optoelectronic performance. We show that monolayer Mo(S1–xSex)2 and Mo1–yWyS2 with x = 1/3, 2/3 and y = 1/3, 1/2, 2/3 are stable according to phonon calculations and realize 1T′ or 1T″ phases. The transition barriers from the 2H phase are lower than for monolayer MoS2, implying that the 1T′ or 1T″ phases can be achieved experimentally. Furthermore, it turns out that the 1T″ monolayer alloys with x = 1/3, 2/3 and y = 1/3, 2/3 are semiconductors with band gaps larger than 1 eV, due to trimerization. The visible light absorption and carrier mobility are strongly improved as compared to 2H monolayer MoS2, MoSe2, and WS2. Thus, the 1T″ monolayer alloys have the potential to expand the applications of transition metal dichalcogenides, for example, in solar cells.

Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material: The Clear Role of Chalcogen Ratio on Light Absorption and Charge Transport for Cu2–xZn1+xSn(S1–ySey)4

Photovoltaic (PV) devices based on bulk polycrystalline Cu2ZnSn(S1–xSex)4 (CZTSSe) as the absorber material have historically shown the best efficiency with high Se compositions. The selenization process, which is employed in the formation of absorber layer, has been shown to result in maximum device efficiency at a lower than predicted optimal band gap (Eg= ∼1.1 eV as compared to the 1.34 eV predicted by the Shockley–Queisser detailed balance model). It is still not clear if this deviation is due to changes in the chalcogen composition, grain growth in the film, or increased order in the lattice. In contrast, CZTSSe nanocrystals (NCs) offer a unique opportunity to evaluate the effect of chalcogen ratio on light absorption, charge transport, and photovoltaic performance excluding the impact of the uncertain effects of the conventional selenization step and, importantly, offer a potential path to a dramatic reduction in PV manufacturing cost. Despite an abundance of literature reports on this compound, the...

Characterization of Primary Carrier Transport Properties of the Light-Harvesting Chalcopyrite Semiconductors CuIn(S1–xSex)2

We report the carrier transport properties of CuIn(S1–xSex)2 (0 ≤ x ≤ 1), a promising chalcopyrite semiconductor series for solar water splitting. A low concentration Mg dopant is used to decrease the carrier resistivity through facilitating bulk p-type transport at ambient temperature. Temperature-dependent resistivity measurements reveal a four-order magnitude decrease in bulk electrical resistivity (from 103 to 10–1 Ohm cm) for 1% Mg-doped CuIn(S1–xSex)2 as x increases from 0 to 1. Hall effect measurements at room temperature reveal p-type majority carrier concentrations that vary from 1015 to 1018 cm–3 and mobilities of approximately 1–10 cm2 V–1 s–1. These results provide insights into the fundamental carrier transport properties of CuIn(S1–xSex)2 and will be of value in optimizing these materials further for photoelectrochemistry applications.

ChemInform Abstract: Impact of Minor Phases on the Performances of CZTSSe Thin‐Film Solar Cells

AbstractReview: 293 refs.

Impact of Minor Phases on the Performances of CZTSSe Thin-Film Solar Cells

Cu2ZnSn(S1–xSex)4 (CZTSSe) alloys are promising candidates for low-cost and high-efficiency thin-film solar cells. Compared to CdTe and Cu(InxGa1–x)Se2, CZTSSe uses chemicals found more abundantly on earth. This key aspect as well as its optical properties—band gap range of 1.0–1.5 eV and absorption coefficient larger than 104 cm–1—makes CZTSSe a feasible option for photovoltaic thin-film technologies scalable to TW/year. The key limitation to high efficiency in this technology remains a deficit in the open-circuit voltage with respect to the band gap: many groups have linked this drawback of CZTSSe solar cells to the presence of minor phases. This review aims to summarize what is known about CZTSSe minor phases and their impact on device characteristics.

Electronic Structure and Photocatalytic Water-Splitting Properties of Ag2ZnSn(S1–xSex)4

The silver-based quaternary chalcogenides have potential applications in solar cell absorbers and photocatalytic water splitting to produce hydrogen. In the present work, the electronic structures and photocatalytic properties of Ag2ZnSn(S1–xSex)4 (x = 0–1) were investigated by the combination of GGA+U and hybrid functional methods. The results indicate that Ag2ZnSnS4 (x = 0) and Ag2ZnSnSe4 (x = 1) have a dispersive conduction band and small electron effective masses, which are beneficial to the photogenerated carrier separation. The hole effective masses are remarkably direction dependent, and that along the [100] and [010] directions are sensitive to strain, which indicates easy modulation of photocatalytic properties. In addition, the band gaps of the Ag2ZnSn(S1–xSex)4 solid solution can be tuned continuously by controlling the x component, which indicates the easy manipulation of light response range. The transfer abilities of charge carriers can also be improved with increasing Se incorporation. In a...

Back contact-absorber interface modification by inserting carbon intermediate layer and conversion efficiency improvement in Cu2ZnSn(S,Se)4solar cell

Carbon layers have been employed as intermediate layers between Mo back contact and Cu2ZnSn(S1–xSex)4(CZTSSe) absorber film prepared by sol–gel and post-selenization method. Carbon layers with appropriate thickness can significantly inhibit the formation of MoSe2 and voids at bottom region of the absorber, and therefore reduce the series resistance remarkably. The conversion efficiency can be boosted by the introducing of the carbon layer from 6.20% to 7.24% by enhancement in short current density, fill factor and open voltage in comparison to the reference sample without carbon layer. However, excess thickness of carbon layer will worse device performance due to the deteriorated absorber crystallinity. In addition, the time-resolved photoluminescence analysis shows that inserting the carbon layer with suitable thickness does not introduce recombination and lower minority lifetime. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Cu2ZnSnS4absorber layers deposited by spray pyrolysis for advanced photovoltaic technology

In this work, pneumatic spray pyrolysis was used to deposit Cu2ZnSnS4 kesterite thin film layers onto soda lime glass substrates/Mo coated substrates (substrate configuration). Furthermore, pneumatic spray pyrolysis was also tested as a potential way to grow kesterite thin film layers on ZnO nanorod arrays on SnO2:F coated glass substrates (superstrate cell configuration). In this case semi-transparent samples were obtained. A summary of the process parameters optimization to effectively coat the ZnO nanostructures is presented. In both solar cell configurations, absorbers were selenized at 400–500 °C and compared. For the substrate configuration, we present a device with a power conversion efficiency of 1.86%, Voc = 292 mV, Jsc = 16.2 mA/cm2 and FF = 39.6%. (a) Cross-sectional SEM image of ZnO nanorods covered by Cu2ZnSn(S1–xSex)4 in a superstrate configuration, (b) layout of the conceptual idea.

Cu2ZnSn(S1–xSex)4 thin film solar cells prepared by water‐based solution process

AbstractThe kesterite Cu2ZnSn(S1–xSex)4 (CZTSSe) thin film solar cell has been developed rapidly due to its excellence in structural and optical properties and its abundance in raw materials. Both vacuum‐based and solution‐based methods have been successfully employed to fabricate CZTSSe thin film solar cells. In this Letter, we report an environmentally friendly, water‐based, solution process for fabrication of high‐efficiency CZTSSe thin film solar cells. High quality CZTSSe thin film is obtained by selenization under high temperature and Se vapor. An efficiency of 6.2% is achieved on CZTSSe thin film solar cell fabricated by such water‐based solution process. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystal structures and band‐gap energies of Cu2Sn(S,Se)3 (0≤ x ≤1.0) solid solution

AbstractCu2Sn(S1–xSex)3 (CTSSe) solid solutions with 0 ≤ x ≤ 1.0 were synthesized by mixing the elemental powders and post‐annealing at 600 °C. The crystal structure of Cu2SnS3 (CTS), Cu2SnSe3 (CTSe), and their solid solution were determined by Rietveld refinement of the powder X‐ray diffraction data. CTSSe solid solution had a monoclinic crystal system [space group: Cc (No. 9)]. The refined lattice parameters of CTS were a =6.654(8) Å, b =11.534(2) Å, c =6.659(4) Å, and β =109.40(5)° and those of CTSe were a =6.964(6) Å, b =12.056(6) Å, c =6.972(4) Å, and β =109.49(1)°. The lattice parameters, a, b, and c of CTSSe solid solution monotonically increased with increasing Se content. The band gap of the CTSSe solid solution linearly decreased from 0.87 eV (x=0.0) to 0.67 eV (x=0.60) with increasing Se content. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic and optical features of the mixed crystals Ag0.5Pb1.75Ge(S1–xSex)4

A single crystal with the Ag0.5Pb1.75GeS3Se composition (25 mol% Ag0.5Pb1.75GeSe4) was grown and characterized. Its phase diagram was explored. The section is non-quasi-binary. The role of defects in the conductivity mechanism was explored by electrical, optical and photoconductivity measurements. The band gap value was around 2.0 eV at 100 K. Two photoconductivity maxima with λm1 = 615 nm and λm2 = 775 nm were dominant at 100 K. We have performed X-ray crystallographic data analysis of Ag0.5Pb1.75GeS3Se and the obtained data were used as the starting point for a comprehensive theoretical investigation of the electronic structure using the all-electron full potential linearized augmented plane wave method within density functional theory. The calculations have confirmed that the investigated compound possesses an indirect band gap of about 1.6 eV in good agreement with the experimental value. The angular momentum decomposition of the atoms projected density of states show there exists a strong hybridization between various states, and evaluated the origin of the chemical bonding.