Kesterite-type Cu2ZnSnS4 Research Articles

Anharmonic Effects in Ordered Kesterite-Type Cu2ZnSnS4

We performed an in-depth investigation and analysis of the effect of temperature on the Raman-active A-modes of bulk kesterite-type Cu2ZnSnS4 within the 300–460 K temperature range. We acquired the individual contributions to each Raman mode, namely, the thermal expansion and anharmonic interactions terms responsible for the Raman shift and broadening with temperature. Our results indicate that the Raman shift with temperature is dominated by the thermal expansion term, whereas the broadening is mainly governed by three-phonon damping processes in this material. Considering relevant results from the literature, it appears that dimensionality is a key factor in regulating the dominant phonon decay mechanism.

Design and implementation of an ultrathin dielectric/metal/dielectric transparent electrode for Cu2ZnSnS4 thin-film photovoltaics

A proof-of-concept for implementing ultrathin dielectric/metal/dielectric layers of AZO/Ag/AZO (AZO: Al-doped ZnO) as a transparent electrode in the earth-abundant, kesterite-type Cu2ZnSnS4 (CZTS) photovoltaic device, is presented. In a first step, a set of optical simulations for designing the layer sequence and the optimum thickness of AZO/Ag/AZO were performed, resulting in a high CZTS absorption within a device configuration of glass/Mo/MoS2/CZTS/CdS/ZnO/AZO/Ag/AZO. The simulation suggests that a high CZTS absorption in a device can be achieved using an Ag layer with a thickness of 7 nm embedded between a first and second AZO layer, having thicknesses of 50 nm and 40 nm, respectively. The AZO/Ag/AZO films were sputtered on a glass substrate with layer thicknesses based on the optical simulation, showing transmittance of >80% (including the substrate), sheet resistance of 10.5 Ω/sq and figure-of-merit of 29.6 × 10−3 Ω−1. These ultrathin transparent electrode properties were obtained without any substrate heating. The implementation of AZO/Ag/AZO as the transparent electrode in a CZTS photovoltaic device yielded a device efficiency of 8.2% with a short-circuit current density of 24.2 mA/cm2, an open-circuit voltage of 576 mV and a fill factor of 58%. These results suggest an opportunity to reduce the nominal thickness of the front electrode material in CZTS photovoltaics.

High-pressure behavior of disordered kesterite-type Cu2ZnSnS4

We have investigated the high-pressure structural and vibrational behavior of the disordered kesterite-type Cu2ZnSnS4 compound at ambient temperature. Our experimental and theoretical investigations have revealed a clear structural transition to a GeSb-type phase close to 15 GPa, a tetragonally distorted variant of the NaCl-type phase. The latter transformation is accompanied by a cationic coordination increase from fourfold to sixfold with respect to the sulfur anions. In addition, a change in the compressibility rate was detected at about 8 GPa within the pressure stability range of the disordered kesterite-type phase. Upon decompression, a disordered zinc blende/sphalerite structure is recovered. We discuss our findings in close conjunction with our recent high-pressure work on the ordered Cu2ZnSnS4 modification.

Pressure-induced structural and electronic transitions in kesterite-type Cu2ZnSnS4

We have performed structural investigations of ordered kesterite-type Cu2ZnSnS4 up to 30 GPa. Our current X-ray diffraction results clearly excluded the presence of a kesterite → disordered kesterite transition reported previously between 7 and 9 GPa. Nevertheless, specific anomalies connected with the Cu-S bond length of the starting kesterite-type phase are evidenced close to 6 GPa, indicating subtle structural effects at play in this system. Moreover, we have indexed the high-pressure modification of Cu2ZnSnS4 adopted above 16 GPa to a disordered GeSb-type structure, a tetragonally distorted rocksalt-type modification. Full decompression leads to the adoption of a disordered sphalerite/zincblende-type structure. Our complementary density functional theory calculations reproduce accurately the experimental observations and indicate the possibility of a metallic high-pressure GeSb-type phase, unlike the starting semiconducting kesterite-type Cu2ZnSnS4 structure.

Influence of Cu2S, SnS and Cu2ZnSnSe4 on optical properties of Cu2ZnSnS4

We have studied influence of Cu2ZnSnSe4 (CZTSe) and secondary phases of Cu2S and SnS on optical properties of kesterite-type Cu2ZnSnS4 (CZTS) by using the effective medium theory. We found that CZTSe and Cu2S cause band gap reduction and enhance light absorption of CZTS at all photon energies of the sunlight whereas SnS changes optical properties of CZTS only at large photon energies beyond the solar spectrum. Optical spectra of CZTS(Se) has been studied by first principles calculations within hybrid functional that was used as input for the effective medium theory.

Structural transitions of ordered kesterite-type Cu2ZnSnS4 under pressure

We have investigated the high-pressure structural and vibrational behavior of the ordered kesterite-type Cu2ZnSnS4 compound. Our investigations have revealed two structural transitions: a kesterite-to-disordered kesterite transition was observed between 7 and 9 GPa, which involves a Zn/Cu disorder within the respective cationic sublattice, whereas a rocksalt-type structure was realized at ∼15 GPa. The latter transition is accompanied by a cationic coordination increase from fourfold-to-sixfold with respect to the sulfur anions. The predicted kesterite-to-stannite transition was not detected. Furthermore, our high-pressure Raman studies have shown that the aforementioned Zn/Cu cationic disorder will always be present in Cu2ZnSnS4 under relatively moderate compression.

Study of the mechanochemical process to crystalline Cu2ZnSnS4 powder

Kesterite-type Cu2ZnSnS4 was synthesized from the corresponding binary sulfides by a mechanochemical route in a planetary ball mill. The reaction progress during this milling step was followed within a time range of 10–180min by powder X-ray diffraction. In addition, the crystallization of the milled material was studied in situ by high-temperature X-ray diffraction methods in the temperature range of 300–500°C. Significant disorder (cation distribution) was observed at 500°C, strongly decreasing during cooling down to ambient temperature with a rate of 60K/h.

A facile and green synthesis of copper zinc tin sulfide materials for thin film photovoltaics

Kesterite-type Cu2ZnSnS4 (CZTS) has been attracting a lot of attention in thin-film solar cells due to its relatively low cost, earth abundant and environmentally benign nature compared to its analog Cu(In, Ga)Se2 materials. Until now, solution-based processes are considered as promising methodologies for mass production of CZTS materials for industrial demands. However, most material sources are highly toxic as well as dangerous. In this study, we proposed a facile and green synthesis strategy for the CZTS absorber by taking advantage of an ultrasonic spray technique where we adopted metallic nanopowders and ethanol as our nontoxic precursors as well as solvents, respectively. The phase formation and transition mechanism of the synthesized CZTS film are investigated through varying sulfurization temperatures and analyzed using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The solar cell device can be made at a scale as large as 20×20mm2, and the cell shows a power conversion efficiency of 1.5%.

Effects of substrate temperature on the Cu2ZnSnS4 films deposited by radio-frequency sputtering with single target

Kesterite-type Cu2ZnSnS4 (CZTS) thin films were directly deposited from a single quaternary CZTS target. In this study, we investigated the influence of substrate temperature on structural, compositional, electrical and optical properties of CZTS films. All four samples show the peaks corresponding to the kesterite-type structure. The diffraction peaks of (112) are sharp and the small characteristic peaks of the kesterite structure such as (220)/(204) and (312)/(116) are also clearly observed in X-Ray Diffraction patterns. The dislocation density and strain could be decreased to 0.33×1015cm-3 and 2.37×10-4, respectively. Besides, the films exhibit a high absorption coefficient of the order of 104cm-1, an optical band gap of 1.56eV, a low carrier concentration of 2.7×1018cm-3, a resistivity of 2.0Ω-cm, and a mobility of 1.5cm2/V-s. These results indicate that the quaternary CZTS would be a potential candidate for solar cell applications. Devices built with these CZTS films exhibit efficiencies as high as 3%.

Directly electrospinning growth of single crystal Cu2ZnSnS4 nanowires film for high performance thin film solar cell

Quaternary kesterite-type Cu2ZnSnS4 nanowires can be used as absorbing materials for thin film solar cells. The structural, morphological, compositional, and optical properties of the CZTS nanowires have been studied using X-ray diffraction (XRD), UV–vis spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and optical absorption techniques respectively. The Cu2ZnSnS4 nanowires, fabricated by electrospinning process and sintered at 600 °C in Argon atmosphere, the cells exhibits a power conversion efficiency of 6.18% under AM 1.5 solar irradiation in the thin film solar cells. This study suggests that the optimized Cu2ZnSnS4 composite nanowire is a promising absorbing material for high performance solar cells.

Cellulose acetate assisted synthesis and characterization of kesterite quaternary semiconductor Cu2ZnSnS4 mesoporous fibers by an electrospinning process

Quaternary kesterite-type Cu2ZnSnS4 (CZTS) mesoporous fibers for low-cost thin film solar cells were successfully synthesized by a relatively simple electrospinning process. Semiconductor CZTS mesoporous fibers were obtained from CA/CZTS precursor after annealing treatment at 450 °C for 3 h. CZTS mesoporous fibers were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The optical properties of the CZTS mesoporous fibers were also recorded by UV-vis absorption spectroscopy. The results showed that the synthesized CZTS mesoporous fibers had a single phase, good crystallinity and a stoichiometric composition. Moreover, the prepared mesoporous fibers had a size ranging from 200–500 nm and a band gap of 1.46 eV, which demonstrates that they are suitable for use in a thin film solar cell light absorption material.

First Principles Calculations of Defect Formation in In-Free Photovoltaic Semiconductors Cu2ZnSnS4and Cu2ZnSnSe4

To quantitatively evaluate the formation energies of Cu, Zn, Sn, and S vacancies in kesterite-type Cu2ZnSnS4 (CZTS), first-principles pseudopotential calculations using plane-wave basis functions were performed. The formation energies of neutral Cu, Zn, Sn, and S vacancies were calculated as a function of the atomic chemical potentials of constituent elements. We compared the vacancy formation in the In-free photovoltaic semiconductor CZTS with those of Cu2ZnSnSe4 (CZTSe) and CuInSe2 (CIS). The obtained results were as follows. (1) Under the Cu-poor and Zn-rich condition, the formation energy of the Cu vacancy was generally smaller than those of the Zn, Sn and S vacancies in CZTS, as is the case for CZTSe. (2) The formation energies of Cu, Zn, and Sn vacancies in CZTS were larger than those in CZTSe. On the other hand, the formation energy of the S vacancy is smaller than that of the Se vacancy in CZTSe. (3) Under the Cu-poor and Zn-rich condition, the formation energies of the Cu vacancy in CZTS and CZTSe are much larger than that in CIS. These results indicate that in kesterite-type CZTS and CZTSe, the Cu vacancy is easily formed under Cu-poor, Zn-rich, and S(Se)-rich condition, but it is more difficult than that in CIS.

First Principles Calculations of Defect Formation in In-Free Photovoltaic Semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4

To quantitatively evaluate the formation energies of Cu, Zn, Sn, and S vacancies in kesterite-type Cu2ZnSnS4 (CZTS), first-principles pseudopotential calculations using plane-wave basis functions were performed. The formation energies of neutral Cu, Zn, Sn, and S vacancies were calculated as a function of the atomic chemical potentials of constituent elements. We compared the vacancy formation in the In-free photovoltaic semiconductor CZTS with those of Cu2ZnSnSe4 (CZTSe) and CuInSe2 (CIS). The obtained results were as follows. (1) Under the Cu-poor and Zn-rich condition, the formation energy of the Cu vacancy was generally smaller than those of the Zn, Sn and S vacancies in CZTS, as is the case for CZTSe. (2) The formation energies of Cu, Zn, and Sn vacancies in CZTS were larger than those in CZTSe. On the other hand, the formation energy of the S vacancy is smaller than that of the Se vacancy in CZTSe. (3) Under the Cu-poor and Zn-rich condition, the formation energies of the Cu vacancy in CZTS and CZTSe are much larger than that in CIS. These results indicate that in kesterite-type CZTS and CZTSe, the Cu vacancy is easily formed under Cu-poor, Zn-rich, and S(Se)-rich condition, but it is more difficult than that in CIS.

Synthesis and Characterization of Sphere-Like Cu<sub>2</sub>ZnSnS<sub>4</sub> Nanocrystals by Solvothermal Method

A simple solvothermal route has been successfully used to prepare Cu2ZnSnS4 nanocrystals using metal chloride and L-cysteine as precursors at 180 °C for 16 h. L-cysteine was used as the sulfide source and complexing agent. The phase, structure, morphology, and optical properties of the as- synthesized products were characterized by powder X-ray diffraction (XRD), field-emission scan electron microscopy (FESEM), energy dispersive spectrometry (EDS), high-resolution electron transmission microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-Vis) spectrophotometer. The results showed that pure kesterite-type Cu2ZnSnS4 nanocrystals were prepared under this condition and the diameters of the microspheres were about 400-800 nm while the microspheres consisted of nanoflakes with thickness of 20 nm. The band gap of CZTS nanoparticles was about 1.58 eV,which was close to the optimum band gap of thin film solar cells. A possible formation mechanism was also discussed.

A mild solvothermal route to kesterite quaternary Cu2ZnSnS4 nanoparticles

Quaternary kesterite-type Cu2ZnSnS4 (CZTS) nanoparticles for low-cost thin film solar cell were successfully synthesised using a relatively simple and convenient solvothermal route. Nanoparticles with diameters of about 5–10nm were obtained at the temperature of 180°C, analyzed by transmission electron microscopy (TEM). The morphologies of the continuous CZTS films with satisfactory stoichiometry were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA). The crystallinity of CZTS nanoparticles was greatly improved by annealing in H2S (5%)/Ar mixed gases analyzed by X-ray diffraction (XRD). High-resolution X-ray photo-emission spectroscopy (XPS) analysis of the four constituent elements confirmed the purity and composition of CZTS nanoparticles. UV–vis absorption spectra measurement indicated that the band gap of as-synthesised CZTS nanoparticles was about 1.5eV, which was near the optimum value for photovoltaic solar conversion in a single-band-gap device.