Effect of sodium diffusion on the structural and electrical properties of Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (CZTS) is a kesterite semiconductor consisting of abundantly available elements. It has a band gap of 1.5 eV and a large absorption coefficient. Hence, thin films made of this material can be used as absorber layers of a solar cell. CZTS films were deposited on soda lime and Na free borosilicate glass substrates through Ultrasonic Spray Pyrolysis. The diffusion of sodium from soda lime glass was found to have a profound effect on characteristics like grain size, crystal texture and conductivity of CZTS thin films. Copper ion concentration also varied during the deposition and it was observed that the carrier concentration was enhanced when there was a deficiency of copper in the films. The effect of sodium diffusion and copper deficiency in enhancing the structural and electrical properties of CZTS films are presented in this paper.     

SiO2 loading combined with high temperature calcination of kesterite Cu2ZnSnS4 nanocrystals towards enhanced photocatalytic H2 evolution

In this paper, highly efficient Cu₂ZnSnS₄ (CZTS) towards photocatalytic H₂ evolution was successfully fabricated by SiO₂ loading and high temperature calcination. X-ray diffraction (XRD) and Raman shifts revealed CZTS was pure phase with kesterite structure. Transmission electron microscope (TEM) and high resolution TEM showed CZTS could maintain nanoscale size and kesterite structure after being calcined. The SiO₂ loading was found to be an efficient approach to avoid the aggregation of CZTS nanocrystals during high temperature calcination. As the elevated calcination temperature, the crystallinity of CZTS increased without observable size change, which suggested the no change on the specific surface area. All CZTS/SiO₂ exhibited superior photocatalytic H₂ evolution. Especially, the photocatalytic H₂ evolution rates for per unit mass CZTS was significantly enhanced, which was mainly attributed to the good dispersion, the improved crystallinity and the less loss of specific surface area during high temperature calcination.     

Investigations on copper zinc tin sulfide thin films grown through nebulizer assisted spray pyrolysis technique

An attempt is made in this work to synthesize the nontoxic Cu₂ZnSnS₄ (CZTS) thin film on FTO coated glass substrates through aerosol assisted nebulizer spray pyrolysis technique at different annealing temperatures for photovoltaic energy conversion. The deposited thin film is further used to form a heterojunction interface with a cadmium sulfide layer to fabricate a solar cell with Glass/FTO/CdS/CZTS/Ag superstrate structure to reveal its photovoltaic application. Various characterization techniques are utilized to study its inherent properties. X-ray diffraction (XRD) is employed to investigate the structural parameters such as crystallite size, microstrain and dislocation density which shows a preferential peak for (112) plane around 28.5° confirming the formation of kesterite CZTS. Raman measurements establish the peak for CZTS at 336 cm⁻¹ and confirm the absence of parasitic secondary phases for an excitation wavelength of 488 nm. Scanning electron microscope and atomic force microscope used to examine the surface morphology and roughness of the films reveal a good surface morphology with a grain size of 555.9 nm and roughness of 123.7 nm for an annealing temperature of 350°C. The bandgap of the deposited CZTS films is found to be around 1.5 eV. The I-V characteristics of CZTS seem to be better for the 350°C annealed film. The performance of the optimized CZTS absorber layer is tested by forming a solar cell structure. The devised solar cell exhibited an open circuit voltage of 213 mV and a short circuit current density of 490 μA cm⁻² with a conversion efficiency of 0.68% substantiating the usage of the prepared film as an absorber for photovoltaic conversion.     

Template synthesis of porous hierarchical Cu2ZnSnS4 nanostructures for photoelectrochemical water splitting

Environmentally friendly and low-cost Cu₂ZnSnS₄ (CZTS) is a promising light absorber for photoelectrochemical (PEC) hydrogen production from water splitting due to the earth-abundant elements, high absorption coefficient, and narrow bandgap. Herein, the hierarchical CZTS film with porous nanostructures was successfully synthesized by a template method. The hierarchical CZTS film was composed of flower-like particles, which were assembled with thin CZTS nanosheets. Macropores were generated owing to the aggregation of flower-like spheres, and mesopores were formed from the stacking of CZTS nanosheets. Compared to the dense CZTS film, the porous hierarchical CZTS film showed a much higher PEC property for water splitting. The improved performance could be attributed to three merits of the porous hierarchical morphology: enhanced light absorption, improved charge separation and transfer, and enlarged electrochemically active surface area. This study provides a useful idea to design efficient semiconductor photoelectrodes for water splitting with delicately controlled morphology.     

Studies on Cu2ZnSnS4 (CZTS) absorber layer using different stacking orders in precursor thin films

Cu₂ZnSnS₄ (CZTS) thin films were deposited by sputtering on glass substrates using stacked precursors. The stacked precursor thin films were prepared from Cu, SnS₂ and ZnS targets at room temperature with different stacking orders of Cu/SnS₂/ZnS/glass (A), ZnS/Cu/SnS₂/glass (B) and SnS₂/ZnS/Cu/glass (C). The stacked precursor thin films were sulfurized using a tubular rapid thermal annealing system in a mixed N₂ (95%)+H₂S (5%) atmosphere at 550 °C for 10 min. The effects of the stacking order in the precursor thin films on the structural, morphological, chemical, electrical and optical properties of the CZTS thin films were investigated. X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy studies showed that the annealed CZTS thin film using a stacking order A had a single kesterite crystal structure without secondary phases, whereas stacking orders B and C have a kesterite phase with secondary phases, such as Cu_2−xS, SnS₂ and SnS. The annealed CZTS thin film using stacking order A showed a very dense morphology without voids. On the other hand, the annealed CZTS thin films using stacking orders B and C contained the volcano shape voids (B) and Sn-based secondary phases (C) on the surface of the annealed thin films. The direct band gap energies of the CZTS thin films were approximately 1.45 eV (A), 1.35 eV (B) and 1.1 eV (C).     

Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E---B evaporated precursors

By sulfurization of E---B evaporated precursors, CZTS(Cu₂ZnSnS₄) films could be prepared successfully. This semiconductor does not consist of any rare-metal such as In. The X-ray diffraction pattern of CZTS thin films showed that these films had a stannite structure. This study estimated the optical band gap energy as 1.45 eV. The optical absorption coefficient was in the order of 10⁴cm⁻¹. The resistivity was in the the order of 10⁴ Ω cm and the conduction type was p-type. Fabricated solar cells, Al/ZnO/CdS/CZTS/Mo/Soda Lime Glass, showed an open-circuit voltage up to 400 mV.     

CZTS based thin film solar cells: a status review

Abstract

Today’s thin film photovoltaic technologies comprising CuInS₂ (CIS), CuInGaSe₂ (CIGS) and CdTe rely on elements that are costly and rare in the earth’s crust (e.g. In, Ga, Te) and are toxic (e.g. Cd). Hence, in future cost reduction and increased production, using abundantly available non-toxic elements, seem to be the main issues. Cu₂ZnSnS₄ (CZTS), having the kesterite structure, is one of the most promising absorber layer candidates for low cost thin film solar cells, because of its suitable direct band gap between 1·4 and 1·5 eV and large absorption coefficient, over 10⁴ cm^?1. Also it is composed of earth abundant and non-toxic elements, promising price reductions in future. Recently, research in this area has gained momentum due to the desirability of producing Ga, In and Cd free absorber layers and the potential to obtain new insights. Hence, a review of recent literature is urgently warranted. The CZTS progress and present status of CZTS thin film solar cells has been reviewed, with the hope of identifying new paths for productive research.     

The effect of hydrogen on the mechanical properties of FeTi for hydrogen storage applications

In this paper, the density functional theory (DFT) within the generalized gradient approximation (GGA) was used. The single crystal elastic constants for the intermetallic FeTi and its hydrides FeTiH and FeTiH₂ are successfully obtained from the stress–strain relationship calculations and the strain energy-strain curves calculations, respectively. The shear modulus, Young's modulus, Poisson's ratio and shear anisotropic factors are also calculated. The bulk moduli derived from the elastic constants calculations of the cubic FeTi, orthorhombic P222₁ FeTiH and Cmmm FeTiH₂ are calculated. For cubic FeTi compound, the bulk modulus is in a good agreement with both theoretical results and experimental data available in the literature. More importantly, it is found that, the insertion of hydrogen into the FeTi crystal structure causes an increase in the bulk modulus. From the analysis of shear-to-bulk modulus ratio, it is found that FeTi compound and its hydrides are ductile and that this ductibility, changes with changing the concentration of hydrogen.     

A review on growth optimization of spray pyrolyzed Cu2ZnSnS4 chalcogenide absorber thin film

The progress of solar cell technology in the development of clean and economic quaternary compound copper zinc tin sulfide (CZTS)‐based absorber thin films using the spray pyrolysis technique are presented in this review. CZTS (Cu₂ZnSnS₄) is the only potential competitor for the existing solar thin film absorbing materials owing to its environment‐friendly Earth abundant constituents. Even though different nonvacuum thin film technologies have been developed for the large area fabrication of this nontoxic absorber material, spray pyrolysis technique offers more versatility in changing the process parameters which has a direct impact on the cell efficiency. It can be used for depositing a wide variety of materials even with complex composition with good crystallinity, and the method has the advantage of being flexible and straightforward to design and can be quickly adopted for extensive area deposition. A survey on the effects of experimental conditions as well as the nature of precursors on the structural, morphological, electrical, and optical properties on the spray pyrolyzed CZTS thin films is discussed in detail. This analysis certainly could provide a potential to obtain new insights in the fabrication of high‐efficiency CZTS‐based solar cells and to launch it into the commercial market to satisfy the ever‐growing future energy demand.     

Comparative studies on hierarchical flower like Cu2XSnS4[X= Zn,Ni, Mn & Co] quaternary semiconductor for electrocatalytic and photocatalytic applications

CXTS [X = Zn, Co, Mn &Ni] has gained increasing interest owing to its application in photovoltaic devices, photocatalysis and electrocatalysis due to its favorable electrical and optical properties. In this work, CXTS has been prepared by hydro-thermal technique and explored as an electrocatalyst and photocatalyst for hydrogen evolution reaction and dye-degradation. From the XRD analysis, we observe that the prepared compound shows tetragonal structure. SEM images reveal that CXTS exhibits flower like architecture. The optical bandgap has been determined to be 1.53, 1.44, 1.42 and 1.37eV for CZTS, CCTS, CMTS and CNTS respectively. For, electrocatalytic activity, it is found that CNTS is preferred for hydrogen evolution with lower Tafel slope value of 76mV/dec and higher current density among the other materials in CXTS. As a photocatalyst CZTS resulted in better degradation of MB dye for about 60% under visible light. The effect of the position of energy bands, surface activity in hydrogen evolution reaction and the impact of recombination of charge carriers on the photocatalytic degradation has been discussed.     

Surface modification of the p-type Cu2ZnSnS4 photocathode with n-type zinc oxide nanorods for photo-driven salt water splitting

The sulfurization of co-sputtering Cu–Zn–Sn metal precursors was employed to prepare the quaternary copper-zinc-tin-sulphide (Cu₂ZnSnS₄, CZTS) photocathodes on substrates. Influence of [Zn]/[Sn] ratios in CZTS photocathodes on their phases, morphologies, and the efficiencies of photo-driven salt-water splitting was examined. Pristine p-type CZTS photocathodes showed the highest photo-driven performance of 0.61 mA cm^?2 in an electrolyte containing 1 M sodium chloride with the external bias kept at ?1.0 V vs. Ag/AgCl. An n-type zinc oxide (ZnO) nanorod arrays layer was then coated on the CZTS photocathode to improve its photo-driven salt-water splitting performance. The CZTS/ZnO photoelectrode had the best photo-driven performance of 1.87 mA cm^?2 in the 1 M NaCl solution under illumination with the external bias set at ?1.0 V vs. Ag/AgCl. From results of electrochemical impedance spectra measurements for the samples in the electrolyte, the CZTS/ZnO sample had good photo-driven salt-water splitting performance due to its lowest charge transfer resistance and p-n junction formed at the sample. Intensity modulated photocurrent spectroscopy and electrochemical impedance spectra results of samples indicated that the surface states at the CZTS/ZnO interface were the recombination centers with the electrons from the CZTS sample and holes from the ZnO and therefore improved its photo-driven salt-water splitting performance.     

The crystal structure of kesterite type compounds: A neutron and X-ray diffraction study

The atomic structure of the potential photovoltaic materials Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) is discussed on the basis of a structural analysis of neutron powder diffraction data refined by the Rietveld analysis. Both compounds were found to crystallize in the kesterite type structure, but with a disorder within the Cu-Zn layers at z=1/4 and 3/4. The latter causes Cu_Zn and Zn_Cu anti-site defects, whose concentration depends on the sample growth conditions.The temperature dependent structural phase transition in CZTS was studied by in-situ high temperature diffraction experiments using high energy synchrotron X-rays. The transition from the tetragonal kesterite to the cubic sphalerite type structure is discussed by means of the interplay of increasing and decreasing cation-anion-cation bond angles in the coordination tetrahedra, reflected by the shift of the anion atomic coordinates to more ideal (x and y) or non-ideal (z) values. This striking behavior may be connected with frustrations that occur due to the triangular geometry of the coordination tetrahedra.     

Thermal Transport Properties of Black Phosphorus: A Topical Review

Black phosphorus (BP) is a layered direct bandgap semiconductor. Few-layer BP, an emergent and promising two-dimensional semiconductor, has attracted increasing interest for novel electronic, photonic, and energy conversion applications due to its unique tunable direct bandgap and anisotropic in-plane transport properties. Understanding the thermal transport properties is critical for thermal management of electronics and thermoelectric applications. In this review, we cover the thermal transport properties of single-layer, few-layer, and bulk BP from most recent theoretical and experimental work, with a particular focus on the anisotropic in-plane thermal conductivity.     

Phase transformation of solution‐based p‐type Cu2ZnSnS4 thin film: applicable for solar cell

In this present work, quaternary Cu₂ZnSnS₄ thin films were deposited on commercial glass substrates at room temperature by a novel solution growth dip coating technique. The influence of annealing temperature of the films at 300 °C in a hot air furnace without the presence of any inert gas, on structural, optical, and electrical properties was investigated and discussed. The structural analyses were analyzed by X‐ray diffraction and Raman spectroscopy, whereas optical and electrical properties were analyzed by means of ultra violet infrared (UV‐ViS/IR). The results analyzed showed that there exists a phase formation from orthorhombic to kesterite crystal structure with an increase in optical bandgap and an optical conductivity, with an increase in annealing temperature. The electrical conductivity was observed of the order of 10^?6 ohm cm^?1. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of CZTS thin films solar cells by pulsed laser deposition: Influence of pulse repetition rate

High-quality Cu₂ZnSnS₄ (CZTS) thin films were synthesized by pulsed laser deposition as a function of pulse repetition rate onto the SLG substrates. Influence of pulse repetition rate onto the structural, morphological, compositional and optical properties have been investigated for as-deposited and annealed thin films. X-ray diffraction study shows transformation of amorphous to crystalline phase after tuning pulse repetition rate and annealing of samples. FESEM images of thin films show increase in grain size upon annealing. Films are nearly stoichiometric deposited at 10 Hz repetition rate has been confirmed with the help of EDAX and XPS analysis. The direct band gap energy of the deposited CZTS thin films are in the solar energy range. The performance of solar cell based on CZTS absorber layer has been tested and the efficiency is about 2%.     

Electronic structure,thermomechanical and phonon properties of inverse perovskite oxide (Na3OCl): An ab initio study

Within first principles calculations, the electronic structure, thermodynamic, mechanical stability, magnetism, and phonon properties of the inverse perovskite (Na₃OCl) have been summed up. The Birch-Murnaghan derived lattice constant and bond-lengths are identical, when compared to the experimental data. A direct energy gap of 2.18 eV observed from the band structure reveals the semiconducting nature of the present oxide. Also, the application of strain on electronic properties predicts the decrease in bandgap with respect to compressive strain and vice versa. The constituent nonmagnetic atoms in its crystal propose the total magnetic moment to be zero and the same is supported by susceptibility data. In addition to the negative Cauchy's pressure, the small bulk modulus compared to Young's modulus determined from elastic constants, possibly claims it as a brittle material. Also, the temperature dependent Gruneisen parameter (1.58) and Debye temperature (382.27 K) are determined to reveal the lattice thermal conductivity (κ = 6.48 W/mK) at room temperature.     

Simulation and optimization of CdS-n/Cu2ZnSnS4 structure for solar cell applications

In this work, the performance of solar cell based on CdS-n/Cu₂ZnSnS₄-p hetero-junction is numerically simulated. The aim of the study is to investigate the influence of thickness, defects density and bandgap energy of absorber layer CZTS and the thickness of the buffer layer CdS of the solar cell on electrical parameters J_sc, V_oc, FF and efficiency η of the cell. The results of our simulation allowed us to optimize the parameters above mentioned in order to get the best efficiency at the optimal band gap which corresponds to the maximum of the solar spectrum with optimal values of the electrical performances of the cell. This results lead to develop CZTS solar cells with high efficiency and low cost and give a help full indication for fabrication process.     

Hydrogen storage performance enhancement and bandgap opening of M-Decorated (M = Li,Na and K) III4–V4 monolayer by fluorine functionalization

The effects of fluorine functionalization on the hydrogen storage capability of alkaline decorated III₄–V₄ monolayers is studied. This structure can store up to two hydrogen molecules per alkaline atom. Here, we demonstrate that functionalizing alkaline decorated monolayer with a high electronegative element of fluorine can significantly enhance both binding energy and the maximum number of the stored hydrogen molecules. In this regard, the hydrogen molecule storage capability is notably improved from two to four and when absolute binding is considered. In addition, F-functionalization of M-decorated III₄–V₄ can demonstrate bandgap opening effect, introduce semiconducting characteristics and forming a new two-dimensional semiconductor structure. The bandgap for Li–Al₄P₄–F is 1.23 eV which is very close to the solar peak. The resulted bandgap in the M–III₄–V₄-F structure is even significantly larger than that of pristine III₄–V₄ monolayer. The binding of the hydrogen molecule to the alkaline atom is also improved from 0.114 to 0.272 eV by the fluorine functionalization.     

Role of precursor solution in controlling the opto-electronic properties of spray pyrolysed Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄, a potential candidate for application as absorber layer in thin film solar cells, were successfully deposited on soda lime glass substrates using spray pyrolysis and the effect of variation of precursor on the structural and opto-electronic properties was investigated. We used stannous as well as stannic chloride as precursors of tin in the spray solution. All the films exhibited kesterite structure with preferential orientation along the (1 1 2) direction. But crystallinity and grain size were better for stannic chloride based films. Also they possessed a direct band gap of 1.5 eV and the absorption coefficient was >10⁴ cm⁻¹. Carrier concentration and mobility could be enhanced and the resistivity reduced by two orders by using stannic chloride in spray solution. Junction trials were performed with CZTS films prepared using stannic chloride precursor as the absorber layer and indium sulfide as the buffer layer. XPS depth profiling of the junction was done. Formation of CZTS could be confirmed and also information about the junction interface could be obtained from the XPS results. We obtained an open-circuit voltage of 380 mV and short-circuit current density of 2.4 mA/cm².     

Janus Ga2SeTe and In2SeTe nanosheets: Excellent photocatalysts for hydrogen production under neutral pH

In the past few years, Janus nanosheets have attracted much interest according to their specific structure and considerable potential to address the energy and environmental issues. Herein, the electronic, optical and photocatalytic properties of two-dimensional Janus Ga₂SeTe and In₂SeTe have been studied using ab-initio computations based on the density functional theory. The obtained results show that these nanomaterials exhibit a semiconductor behavior with direct and moderate bandgaps using hybrid HSE06 functional. Subsequently, the understudied compounds present suitable optical conductivity, absorption, transmission and reflectivity for water splitting under the ultraviolet–visible light irradiation. Interestingly, the band edge positions of Janus Ga₂SeTe and In₂SeTe excellently straddle the redox potentials of water under neutral pH. Additionally, the free energy values for the formation of H₂ from H adsorbed on the Ga₂SeTe and In₂SeTe compounds are respectively 1.304eV and 0.976eV at pH = 7. More excitingly, the present study proposes strain engineering approach to improve the photocatalytic performance of the Janus Ga₂SeTe and In₂SeTe monolayers. Specifically, the investigated semiconductors show more appropriate band edge alignment and better hydrogen evolution reaction activity under biaxial tensile strain, which fulfil the water splitting requirements at neutral pH conditions. Our findings conclude that the Janus Ga₂SeTe and In₂SeTe nanosheets are promising candidates for photocatalytic hydrogen production.