Kesterite Phase Research Articles

Structural and Thermodynamic Properties of Cu2ZnSnS4 Material: Theoretical Prediction

Abstract The present work aims to investigate the structural and thermodynamic properties of homogenous tetragonal Cu2ZnSnS4 (CZTS) absorber material in Kesterite phase using first-principle calculations based on density functional theory (DFT). This approach requires only knowledge of the atomic species and crystal structure to predict several physical properties of materials. The Quantum Espresso code within the Ultra Soft pseudopotentials (USPP) and the local density approximation (LDA) were used in the calculations. Equilibrium unit cell volumes, bulk modulus, as well as the pressure derivative of the bulk modulus are predicted. In addition, several thermodynamic properties, especially: the Debye’s vibrational energy, the vibrational free energy, the vibrational entropy, and the constant volume heat capacity at different temperatures T of Cu2ZnSnS4 material are calculated and discussed. Our study shows that the vibrational energy, the entropy and the constant volume heat capacity increase with increasing temperature, while the vibrational free energy decreases monotonously with the increase of temperature.

Influence of Hubbard U correction on the structural, electronic and optical properties of Kesterite Cu2XSnS4 (X= Zn, Fe)

The new materials like quaternary chalcogenides semiconductors is an approach towards environment-friendly photovoltaic materials due to their promising potential as thin film solar cell absorbers. This work investigates the density functional theory (DFT) and density functional theory plus Hubbard U (DFT + U) approach on the kesterite phase of sulfide-based chalcogenides, Cu2XSnS4, CXTS (X = Zn and Fe) materials. The inclusion of the potential correlation term, U, plays a vital role in aiding the understanding of the complex many-electron problem, which LDA and GGA from DFT might not adequately describe. It was found that, by applying Hubbard U terms on p and d orbital states, the value of electronic band gaps can be significantly fixed close to the experimental value (∼1.3 eV–1.5 eV). The parametrized dependence of the band gap was well explained. The investigation of optical properties associated with the thin film applications on imaginary parts of the dielectric function shows that both structures have greater absorption at the energy range of 0–5 eV. Moreover, the refractive index and optical absorption show good results for both kesterite CXTS in the most effective wavelength of light to absorb sunlight (visible spectrum), which could exhibit a better finding for a suitable candidate for cost-effective new thin film solar cell application.

Controlling Selenization Equilibrium Enables High-Quality Kesterite Absorbers for Efficient Solar Cells

Kesterite Cu2ZnSn(S, Se)4 is considered one of the most competitive photovoltaic materials due to its earth-abundant and nontoxic constituent elements, environmental friendliness, and high stability. However, the preparation of high-quality Kesterite absorbers for photovoltaics is still challenging for the uncontrollability and complexity of selenization reactions between metal element precursors and selenium. In this study, we propose a solid-liquid/solid-gas (solid precursor and liquid/vapor Se) synergistic reaction strategy to precisely control the selenization process. By pre-depositing excess liquid selenium, we provide the high chemical potential of selenium to facilitate the direct and rapid formation of the Kesterite phase. The further optimization of selenium condensation and subsequent volatilization enables the efficient removal of organic compounds and thus improves charge transport in the absorber film. As a result, we achieve high-performance Kesterite solar cells with total-area efficiency of 13.6% (certified at 13.44%) and 1.09 cm2-area efficiency of 12.0% (certified at 12.1%).

Coupling solvent-mediated modulations with the morphological, optical, and antimicrobial attributes of solvothermally synthesized multifunctional Cu2ZnSnS4 nanoparticles

In this study, two distinct reaction media- water (CZTS1) and ethylene glycol (EG) (CZTS2) were used to synthesize nanocrystalline Cu2ZnSnS4 (CZTS) samples using the facile solvothermal method. X-ray diffraction (XRD) analysis indicated the presence of a tetragonal kesterite phase in both samples with CZTS2 exhibiting broader peaks and smaller crystallite size (6.04 nm) compared to CZTS1 (8.78 nm). Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray (EDX) analysis revealed the superior surface morphology and better stoichiometric composition of CZTS2 nanoparticles. In terms of optical properties, the direct bandgap values of CZTS1 and CZTS2 show variation from 1.45 eV to 1.50 eV, as determined from the Tauc plot. Furthermore, the antimicrobial assay of samples was identified by their zone of inhibition of a bacterial (Bacillus subtilis) and a fungal (Tilletia indica) strain demonstrating CZTS1 exhibited more robust antimicrobial activity compared to CZTS2 suggesting its potential in agriculture, food safety, and healthcare.

An insight on the growth mechanism of Cu2ZnSnS4 via hydrothermal route

In the current study, we successfully prepared Cu2ZnSnS4 (CZTS) microparticles using the hydrothermal method at 180 °C, where thiourea was used as a sulphur precursor. To investigate the growth mechanism of CZTS, time-dependent X-ray diffraction (XRD)pattern was studied. The morphology and shape of the synthesized microparticles were examined by field emission scanning electron microscopy (FE-SEM). UV-DRS(UV- Diffusion Reflectance Spectrometer) studies were used to study the optical properties. The morphology played a vital role in the formation of CZTS. This is clearly observed in SEM images where there is a transformation from agglomeration to a hierarchical flower-like structure. The synthesized powder using hydrothermal processes demonstrated the Cu2ZnSnS4 is formed in kesterite phase. The band gap of as-produced CZTS microparticles determined by UV-DRS spectra measurements was 1.57 eV, which is close to the ideal value for photo absorption applications. We analyze the changes in surface properties of particles using X-ray photoelectron spectroscopy (XPS). Understanding the mechanism in preparation of such hierarchical flower like CZTS enables the application of such structure in a wide sector, including photocatalysis and photovoltaics.

A comparative study of Cu2ZnSn(SxSe1−x)4 nanoparticles for solar cells: From chemical synthesis to devices

Thin-film solar cells using quaternary compounds Cu2ZnSn(SxSe1−x)4 (0 ≤ x ≤ 1; CZTSe, CZTSSe, or CZTS) as absorber materials have attracted a lot of attention from the photovoltaic research community. Traditionally, vacuum-based techniques have been used for the synthesis of Cu2ZnSn(SxSe1−x)4 absorbers. In this work, an easy, less toxic, non-vacuum, single-reaction chemical route is used for the synthesis of Cu2ZnSn(SxSe1−x)4 (x = 0.0,0.4,1.0) nanoparticles. Using structural characterization techniques, the presence of kesterite phase in the as-synthesised nanoparticles is confirmed. The surface micrographs revealed increase in nanoparticle grain size with the selenium doping and nearly stoichiometric composition with compact morphology of nanoparticles. Further, the optical characterization of as-synthesised nanoparticles exhibited bandgaps in the range 1.52 – 1.02 eV which are optimal for thin film solar cell applications. The fabricated solar cells using synthesised compounds exhibited promising photo response capabilities with a highest efficiency of 0.752% for Cu2ZnSn(S0.4Se0.6)4 absorber-based device under AM1.5 illumination without any buffer layer. Results presented in this work indicate the suitability of the kesterite compounds synthesized through the chemical route as absorber materials for cheap solar cells.

Dependence of CZTS thin film properties and photovoltaic performance on heating rate and sulfurization time

In this study, effect of heating rate and sulfurization time on the properties of CZTS thin films and their photovoltaic performances was investigated. CZTS layers were prepared by a two-stage technique comprising deposition of precursor films by sputtering on Mo-foil substrates (Mo-foil/ZnS/CuSn/Cu) and a sulfurization process utilizing Rapid Thermal Annealing (RTA) approach. Heating rates and sulfurization times at 550 °C were changed to determine the best conditions. CZTS films were characterized by several techniques. All samples showed Cu-poor and Zn-rich chemical composition regardless of the heating rate and annealing time of the sulfurization process. XRD diffraction patterns showed that only kesterite CZTS phase was formed in all samples. Although Raman spectroscopy measurements confirmed formation of kesterite CZTS phase in all samples, forming of Cu2SnS3 (CTS) secondary phase was detected in all films except for the sample prepared utilizing a relatively low heating rate of 1 °C/s and 60 s sulfurization time (CZTS-I-60). The largest crystallite size and the less micro strain and dislocation density were also found in this sample. All CZTS thin films displayed dense and compact polycrystalline surface microstructure. Optical characterizations showed that the optical band gaps of the films were in the range of 1.48–1.58 eV. PL emission spectra of the films exhibited transition from conduction band to acceptor/defect levels instead of band-to-band transition. Solar cells were fabricated and the highest Voc and conversion efficiency (4.39%) were achieved employing the film obtained with ramp rate of 1 °C/s and the sulfurization time of 60 s, because of better structural properties and purer crystal structure determined for this sample.

INFLUENCE OF HEATING RATE ON THE STRUCTURAL AND OPTICAL PROPERTIES OF SILVER AND GERMANIUM CO-DOPED CZTS THIN FILM

The effect of heating rate on the structural and optical properties of Ag+Ge co-doped CZTS thin film were investigated and compared with the undoped CZTS sample. The undoped and Ag+Ge co-doped CZTS samples obtained by two-stage technique consisting of the sequential deposition of the precursor stacks by sputtering systemand sulfurization of these layers at elevated temperature in the RTP system by employing heating rate of 1°C/s, 2°C/s and 3°C/s. Ag and Ge co-doped precursor stack as well as undoped stack demonstrated Cu-poor, Zn-rich composition. In addition, the dopant ratio of the Ag+Ge co-doped stack was close to the targeted content considering to EDS measurement. Regardless of the employed heating rate or the doping process, all of the samples crystallized in a kesterite structure. However, it was confirmed by XRD measurements that high heating rates caused phase separation in kesterite phase formation. On the other hand, The Raman peaks assigned to Cu-vacancy and CuZn antisite defects formation inhibited with incorporating Ag and Ge into the CZTS structure. Ag and Ge co-doped CZTS sample produced with a heating ramp rate of 1°C/s showed better structural and optical results among them.

Antimicrobial particles based on Cu2ZnSnS4 monograins

In this research, Cu2ZnSnS4 (CZTS) particles were successfully fabricated via the molten salt approach from the copper, zinc and tin sulphides as raw precursors. SEM analysis revealed that CZTS particles are tetragonal-shaped with sharp edges, smooth flat plane morphology, and crystal size varying from 10.8 to 28.7 µm. The phase and crystalline structure of synthesized powders were investigated using XRD analysis, which confirms the presence of a tetragonal crystal structure kesterite phase. The chemical composition of CZTS particles was evaluated by EDX spectroscopy, which identified the nearly stoichiometric composition with an averaged formula of Cu1.88Zn1.04SnS3.97. The TG/DTA-MS and ICP-OES analysis showed the possible decomposition pathways and predicted their degradation rate in aqueous solutions. The CZTS particles possessed highly effective concentration and time-dependent antimicrobial properties against medically relevant bacteria and yeast strains. The CZTS particles (1 g L−1) exhibited over 95.7 ± 1.9% killing efficiency towards M. luteus. In contrast, higher dosages (3.5 and 5 g L−1) led to its complete inactivation and reduced the P. aeruginosa cell viability to 43.2 ± 3.2% and 4.1 ± 1.1%, respectively. Moreover, the CZTS particles (0.5 g L−1) are responsible for causing 54.8 ± 1.8% of C. krusei and 89.7 ± 2.1% of C. parapsilosis yeasts death within the 24 h of exposure, which expanded to almost 100% when yeasts were treated with two times higher CZTS concentration (1.0 g L−1). The mechanism of action has been proposed and evidenced by monitoring the 2′,7′-dichlorofluorescein (DCF) fluorescence, which revealed that the overproduction of reactive oxygen species (ROS) is responsible for microorganism death.

Influence of Ag-doping on the performance of Cu2ZnSnS4 solar cells

We report an improvement in Cu2ZnSnS4 (CZTS) solar cell efficiency with Ag-doping. The highest efficiency obtained for the fabricated CZTS solar cells was 5.84% and that for Ag-doped (ACZTS) cell efficiency was 6.66% without anti-reflection coating. The precursor film was prepared by co-sputtering of copper, tin sulfide (SnS), and zinc sulfide (ZnS) targets on a molybdenum (Mo) coated glass substrate. For the ACZTS, a thin layer of Ag (∼5 nm) was thermally evaporated on top of the CZTS precursor before annealing in an H2S/N2 environment for crystallization. X-ray diffraction (XRD) patterns of both films showed Kesterite phase. Scanning electron microscopy (SEM) imaging showed that Ag-doping increased grain size and lowered porosity. Energy-dispersive X-ray spectroscopy (EDS) indicated that the ACZTS film was able to prevent some Sn loss compared to bare CZTS. Current Density – Voltage (J-V) measurements showed an improved efficiency for ACZTS was mostly due to increase in short-circuit current density (Jsc), while open circuit voltage (Voc) improved slightly. The solar cell device with Ag-doping also exhibited lower series resistance and higher shunt resistance (Rsh). The lower series resistance is attributed to the suppression of MoS2 formation for ACZTS film, which was confirmed by Raman spectra. The optical band gaps for both CZTS and ACZTS films were measured to be 1.525 eV. Capacitance-Voltage (C-V) measurement showed that built-in voltage (Vbi) for CZTS and ACZTS were 0.75 V and 0.70 V, respectively. The Voc of the ACZTS solar cell being closer to the built-in voltage indicated reduction in defects after Ag-doping.

Impact of the ZnS layer position in a stacked precursor film on the properties of CZTS films grown on flexible molybdenum substrates

In the present study, CZTS thin films were prepared by annealing and reaction of Cu–Sn–ZnS precursor layers. First, sputter deposition was carried out on flexible molybdenum (Mo) foil to form Mo-foil/CuSn/ZnS/Cu, Mo-foil/CuSn/Cu/ZnS and Mo-foil/ZnS/CuSn/Cu stacked precursor structures. Annealing process was performed in sulfur atmosphere using Rapid Thermal Processing (RTP) method to obtain kesterite CZTS phase in the reacted layers. All prepared precursors and CZTS thin films displayed Cu-poor and Zn-rich chemical composition, as targeted. XRD patterns of CZTS samples showed that the kesterite phase was obtained in all samples regardless of the stacking order of the precursor films. However, the full width at half maximum (FWHM) values of the (112) preferential peaks extracted from the XRD patterns, and the corresponding structural parameters (crystallite size and microstrain), indicated that the Mo-foil/ZnS/CuSn/Cu precursor structure yielded more promising crystalline quality. The occurrence of kesterite phase in all samples and existence of low amount of CTS phase were verified by Raman spectroscopy measurements. The CZTS sample prepared employing the Mo-foil/ZnS/CuSn/Cu precursor film structure presented more prominent, homogenous and compact surface microstructure as observed in SEM images. Optical band gap values were found to be in the range of 1.44–1.50 eV. The room temperature photoluminescence (PL) measurements showed that the transitions from the conduction band to intrinsic defect levels dominated the spectra instead of the band-to-band transitions. Electrical characterization of the films showed that Mo-foil/CuSn/ZnS/Cu and Mo-foil/ZnS/CuSn/Cu precursor films yielded lower electrical resistivity and higher carrier concentration due to better crystalline quality. Overall, it was seen that the CZTS thin films produced using the Mo-foil/ZnS/CuSn/Cu stacked precursor layers displayed better properties in terms of crystalline quality, surface microstructure, and optical and electrical properties, which are favorable for photovoltaic applications.

Evolution of the Cu2ZnSnS4 phase based on the sulfurization-crystallisation duration of the CuS/SnS/ZnS stack formed by thermal evaporation

ABSTRACT Binary sulfides were deposited by sequential thermal evaporation with the stacking order glass/CuS/SnS/ZnS and subsequently subjected to a sulfurization-crystallization process, considering two thermal treatment time intervals, 5 and 20 min. The objective of implementing different annealing durations was to identify the best conditions to form CZTS films in the pure kesterite phase. After being subjected to the annealing, the films show structural characteristics of the kesterite phase. However, XRD data showed that prolonged annealing causes degradation of the kesterite phase, leading to the formation of traces of CuS and Cu5Sn2S7. The films annealed for shorter duration, in this case 5 min, present a denser and more uniform surface morphology, better degree of preferential orientation, small Urbach energy of 0.302 eV, and higher photosensitivity. The band gap of the films was 1.46 eV and 1.53 eV for annealing durations 5 and 20 min, respectively.

Non-toxic solution processed Cu2ZnSn(SSe)4 thin films for photovoltaic Applications: A review

Due to its earth-abundant and environmentally safe constituents, as well as suitable optical properties like a high absorption coefficient > 104 cm−1 and a direct bandgap of 1.5 eV Kesterite Cu2ZnSnS4 (CZTS) has emerged as a celebrated successor to CuInGaS2 (CIGS) as the absorber layer in thin film solar cells. The binary chalcogenides are first synthesized, and then they are dispersed in hydrazine, yielding the solution process, which has so far the highest conversion efficiency of 12.6% for any CZTSSe fabrication process. In light of hydrazine’s explosive, heptotoxic and carcinogenic properties, further study has been done on the simple synthesis of CZTS films from non-toxic solutions. Numerous non-hydrazine solutions, which are significantly safer to use, have been researched. The formation of single kesterite phase with better control over secondary phases has been the key to its success in photovoltaic applications. In all solution-based approaches for semiconductor thin films, the film growth proceeds from the release of cations and anions from the precursor salts, and their subsequent reactions.

First principles investigations for structural, electronic, optical and elastic properties of Ag2BeSn(S/Se)4 chalcogenide in kesterite and stannite phases with theoretical SLME calculations

In the present work, density functional theory based computational analysis is carried out for structural, electronic, optical, and mechanical properties of novel chalcogenide Ag2BeSn(S/Se)4 materials. We have employed highly accurate Heyd-Scuseria-Ernzerhof (HSE-06) hybrid functional with generalised gradient approximation (GGA) functional within density functional theory (DFT). The Ag2BeSn(S/Se)4 in their thermodynamically stable phases kesterite (Kes) and stannite (Sta) are optimized and using the optimized lattice constants the bandgap values are calculated. Out of all four structures studied here, the estimated bandgap values are found to be 1.5 eV and 1.26 eV, respectively, for Ag2BeSnS4 (Sta) and Ag2BeSnSe4 (Kes) materials. Partial density of states (PDOS) for all four structures have also been analysed to get more insights into their electronic behaviour. Various optical properties such as the dielectric function, refractive function, absorption spectrum, extinction coefficient, reflectivity and optical conductivity have been calculated and studied in detail for all structures. Calculated elastic constants and moduli to validate the mechanical stability of the materials are studied here. Also, we have calculated spectroscopic limited maximum efficiency (SLME) for all four materials studied here and found that Ag2BeSnS4 (Sta), and Ag2BeSnSe4 (Kes) have single layer efficiencies of 23.7%. 31.3%, respectively. We expect our computational results to encourage photovoltaic researchers globally to experimentally realise these materials and their solar cells.

Elastic Constants of Tetragonal Cu2ZnSnS4 Semiconductor: AB-Initio Calculation

Abstract In this work, an ab-initio calculation is used to investigate the elastic constants and some other mechanical and thermal parameters of tetragonal Cu2ZnSnS4 (CZTS) quaternary semiconducting bulk material in Kesterite (KS) and Stannite (ST) phases. The Quantum Espresso code within the Ultra Soft pseudo potentials (USPP) and the local density approximation (LDA) approach were used in the calculation. Firstly,, studies are started with the prediction of the elastic stiffness constants Cij and the normal and shear anisotropy factors. Then some other mechanical moduli, especially the isotropic bulk modulus B, the shear modulus G, the Young modulus E, the Poisson’s ratio ν, and the Pugh’s criteria (G/B) are delivered. The analysis of the mechanical stability criteria at equilibrium shows that our elastic stiffness constants Cij of CZTS material obey all the stability conditions. Additionally, some other parameters of the CZTS semiconductor, especially: the Vickers hardness HV , the sound velocity, the Debye temperature θD and the melting temperature Tm were also calculated. The obtained values of the elastic constants Cij and other mechanical and thermal parameters agree well with experimental and other theoretical results of the literature. The Debye temperature θD of the KS phase was found at around 332.7 K, and that of the stannite phase was found equal to 329.1 K, respectively.

Integration of single layer graphene into CZTS thin film solar cells

In this study, CZTS samples were produced on Mo and graphene/Mo coated glass substrates using quaternary target. The CZTS thin films deposited by RF magnetron sputtering were annealed using rapid thermal processing (RTP) method in sulphur atmosphere at 500, 525, and 550 °C so as to obtain glass/Mo/CZTS and glass/Mo/graphene/CZTS (g-CZTS) structures. The obtained CZTS and g-CZTS thin films were then characterized by several methods such as EDX, XRD, Raman spectroscopy, SEM etc. The EDX data demonstrated that all CZTS thin films had Cu poor composition regardless of the sulfurization temperature and increasing the temperature led to Sn loss from the films. Diffraction peaks of kesterite CZTS phase were observed in all the samples; additionally, SnS and CuS secondary phases were also observed in CZTS samples annealed at 500 °C. The crystallite size of the CZTS thin films were found to be increasing with both increasing annealing temperature and use of graphene film as an inter-layer. The creation of kesterite phase with a very small CTS phase in all the samples were verified by the Raman spectroscopy measurement. The SEM images of the samples indicated that using graphene improves the crystalline quality of the CZTS films and contributes to forming more compact, homogenous and larger crystal structure. The determined optical band gap values varied from 1.41 to 1.44 eV depending on the Sn-content of the samples. The produced solar cells selected from the more promising absorber layers showed that implementing graphene in CZTS cell structure enhanced the conversion efficiency from 2.40% to 3.52% due to improvement of crystalline quality of the absorber layer.

Solution-processed CZTS thin films and its simulation study for solar cell applications with ZnTe as the buffer layer.

Using zinc tellurium (ZnTe) as the buffer layer in the Cu2ZnSnS4 (CZTS)-based solar cells showed an improvement in overall efficiency. ZnTe is investigated as an alternative to replace the conventional toxic Cd-contained buffer layers. It may also reduce the overall cost of these cells as both layers (ZnTe and CZTS) have eco-friendly and earth-abundant constituents. The sol-gel spin coating method is used for the deposition of CZTS thin films on the corning glass substrates. The X-ray diffraction studies showed the peaks corresponding to (112), (200), (220), and (312) planes which confirmed the formation of the essential kesterite phase. The optical band gap of the deposited films was found at around 1.45eV by the UV-visible-NIR spectrophotometer. The optimum thickness of the absorber layer (CZTS) and buffer layer (ZnTe) was investigated based on the performance of the ZnO:Al/ZnO/ZnTe/CZTS/Mo cell structure by using the AMPS-1D simulation tool. In contrast, the tool was molded by the experimentally investigated data for the constituent materials of the cell structure. The solar cells' efficiency was increased by 23.47% at 2500nm and 50nm thickness of the CZTS and ZnTe layers, respectively. In addition, it was analyzed and found that the current density value showed an improvement with operating temperature as it is one of the requirements in the high solar radiation areas where the temperature even rises more than 50°C in the summer.

Synthesis and Characterization of Novel Sprayed Ag-Doped Quaternary Cu2MgSnS4 Thin Film for Antibacterial Application.

In this work, the effects of silver doping with different Ag/(Ag + Cu) ratios (i.e., 2%, 5% and 10% at.% in the spray solution) on the structural, morphological, optical, electrical and antibacterial properties of Cu2MgSnS4 (CMTS) thin film grown by spray pyrolysis have been studied. The X-ray diffraction (XRD) and selected area electron diffraction (SAED) results have shown that the kesterite phase of CMTS thin films has a maximum crystallite size of about 19.60 nm for 5% Ag/(Ag + Cu). Scanning electron microscopy (SEM) images have shown spherical grain shapes. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) microscopy observations confirmed the intrinsic reticular planes of CMTS thin film with (112) as a preferred orientation and interplanar spacing value of 3.1 Å. The optical properties showed high absorbance and an absorption coefficient of about 104 cm−1 in the visible region with an optical band gap energy of 1.51 eV. Impedance analysis spectroscopy demonstrated good electrical properties of the CMTS film obtained using 5% Ag/(Ag + Cu). The antibacterial activity of the undoped and Ag-doped particles of CMTS obtained using 5% Ag/(Ag + Cu) against different strains of pathogenic bacteria was tested using the agar well diffusion method. These results showed a significant antibacterial activity of the Ag-doped CMTS particle, which was much higher than the undoped CMTS particles. These experimental findings may open new practices for the Ag-doped CMTS compound, especially the one obtained using 5% Ag/(Ag + Cu), in antibacterial application.

A library of polytypic copper-based quaternary sulfide nanocrystals enables efficient solar-to-hydrogen conversion

Designing polytypic homojunction is an efficient way to regulate photogenerated electrons and holes, thereafter bringing desired physical and chemical properties and being attractive photocatalysts for solar-to-hydrogen conversion. However, the high-yield and controllable synthesis of well-defined polytypes especially for multinary chalcogenide - the fundamental factor favoring highly efficient solar-to-hydrogen conversion - has yet to be achieved. Here, we report a general colloidal method to construct a library of polytypic copper-based quaternary sulfide nanocrystals, including Cu2ZnSnS4, Cu2CdSnS4, Cu2CoSnS4, Cu2MnSnS4, Cu2FeSnS4, Cu3InSnS5 and Cu3GaSnS5, which can be synthesized by selective epitaxial growth of kesterite phase on wurtzite structure. Besides, this colloidal method allows the precise controlling of the homojunction number corresponding to the photocatalytic performance. The single-homojunction and double-homojunction polytypic Cu2ZnSnS4 nanocrystal photocatalysts show 2.8-fold and 3.9-fold improvement in photocatalytic hydrogen evolution rates relative to the kesterite nanocrystals, respectively. This homojunction existed in the polytypic structure opens another way to engineer photocatalysts.

Cu2ZnSnS4 QDs anchored 2-D few-layer graphene bridge enhanced photo induced charge carrier transport behavior for high efficient kesterite photovoltaic cell

Quaternary chalcogenide Cu2ZnSnS4 (CZTS) is a promising absorber for photovoltaic cell application due to its excellent properties like natural abundancy, non-toxic material incorporation, high absorption coefficient ∼104 cm−1 and tuneable band gap energy of 1.4–1.6 eV. However, still there is a huge gap of efficiency between the market oriented Si based photovoltaic device and kesterite device as it is hindered by poor charge carrier mobility. To improve the carrier mobility of semiconducting CZTS material, incorporation of highly conducting material like graphene would be helpful. The incorporation of graphene into CZTS enhances the separation and transfer of charge carriers in an efficient way inhibiting recombination. In this present work, the photovoltaic performance of solution processed single phase kesterite CZTS QDs anchored on electro-chemically prepared few-layer graphene sheet (FLGS) as active layer has been reported. The QD decoration has been carried out by a simple, scalable, environment friendly solution based approach without surface modifications and post-sulphurization treatment. The prepared hybrid structure demonstrates significant increase in photocurrent and optical absorbance indicates the fast charge carrier transport behaviour. The power conversion efficiency (PCE) of fabricated hybrid CZTS QDs-FLGS photovoltaic cell with device configuration of Mo/CZTS QDs-FLGS/CdS/ZnO/Al approached ∼7.64% (for 2.5 wt% FLGS loading) with short circuit current density (Jsc) of 19.07 mAcm−2, superior to individual CZTS QDs (∼6.11% PCE and 15.6% Jsc).