Antimony sulfide (Sb2S3) and antimony selenide (Sb2Se3) compounds have attracted considerable attention for applications in different optoelectronic devices due to their notable optical and electrical properties, and due to the strong anisotropy of these properties along different crystallographic directions. However, the efficient use of these promising compounds still requires significant efforts in characterization of their fundamental properties. In the present study, Raman scattering and spectroscopic ellipsometry were used to investigate the vibrational and optical properties of Sb2Se3 and Sb2S3 bulk polycrystals grown by the modified Bridgman method. The first technique proved the presence of the desired Sb2S3 and Sb2Se3 phases in the analyzed ingots and confirmed the absence of any preferential crystallographic orientation at the measured surface of the samples. Spectroscopic ellipsometry was performed using a multi-oscillator Tauc-Lorentz dispersion model, and yielded a complex dielectric function of chalcogenides over the range 1.0-4.6 eV with a three phase model (ambient, surface and bulk materials). Finally, spectral data on the refractive index, the extinction coefficient, the absorption coefficient and the reflectivity at normal incidence, R, were obtained, which serve as a reference for the optical modeling of optoelectronic devices based on polycrystalline Sb2S3 and Sb2Se3 compounds.

The present study addresses to the synthesis and determination of the dielectric function of Cu2Zn(GexSi1-x)Se-4 solid solutions with x = 0.4 and 0.8 over the range 1-4.5 eV by spectroscopic ellipsometry analysis, with the aim to achieve a suitable band gap tuning. The dielectric function of the samples is determined using the Adachi model. From the analysis the lowest E-0 transition and high energy E-1A and E-1B transitions are identified. It is found that the band gap varies nonlinearly on composition in the Cu2Zn(GexSi1-x)Se-4 alloys and band gap values as large as 1.87 eV are obtained. These results are essential for the design of efficient tailored photovoltaic solar cells and show the high potential of the kesterite compounds for the development of low-cost sustainable future solar cells.

Semi-transparent solar cells are the next step for photovoltaics into our daily life. Over the last years, kesterite-type material has attracted a special attention to be used as an absorber in thin-film solar cells because of its low toxicity and earth abundant constituents. Here, Cu2ZnGeSe4 (CZGSe) thin films are grown by co-evaporation and subsequent annealing at a maximum temperature of 480 °C or 525 °C onto Mo/V2O5/FTO/glass stacks. The goal of this work is to investigate the influence of the annealing temperature on the composition, morphology, vibrational properties, and transmittance of CZGSe layers, the formation of secondary phases, and distribution of elements within the absorber layer as well as on the optoelectronic properties of the corresponding solar cell devices. Raising the annealing temperature to 525 °C leads to a more uniform distribution of Cu, Zn, Ge and Se throughout the absorber layer, a reduction of the presence of the GeSe2 secondary phase, which is mainly detected at 480 °C, a larger grain size and the formation of a thicker MoSe2 layer at the CZGSe/back contact interface. The strategy of increasing the annealing temperature allows for improved J–V characteristics and higher spectral response resulting in an enhanced device performance of 5.3% compared to 4.2% when using 525 °C and 480 °C, respectively. Both absorber layers present an optical band gap energy of 1.47 eV. Furthermore, higher annealing temperature has beneficial effect to the CZGSe-based devices without losses in total transmitted light because of the higher diffuse transmittance. This work shows first promising semi-transparent CZGSe-based solar cells possibly open up new routes of applications.

Wide band-gap kesterite-based solar cells are very attractive to be used for tandem devices as well as for semi-transparent photovoltaic cells. Here, Cu2ZnGe(S,Se)(4) (CZGSSe) thin films have been grown by sulfurization of co-evaporated Cu2ZnGeSe4. The influence of a NaF precursor layer and of a Se capping film on CZGSSe absorbers and solar cells has been investigated. It has been found that the distribution of [S]/([S]+[Se]) through the CZGSSe absorber layer is strongly dependent on the Na content. Na promotes the diffusion of S towards the bulk of the absorber layer. Thicker NaF layers >6 nm lead to a higher S content in the bulk of the absorber layer, but to a decreased accumulation of sulphur at the surface, as detected by GIXRD, GD-OES, and Raman spectroscopy measurements. A relationship between J(sc), FF and Na-content supplied was found; higher Na content resulted in improved solar cell efficiencies. It has also been possible to modify the [S]/([S]+[Se])-gradient throughout the CZGSSe film by the absence of the Se capping layer, achieving devices with 2.7% performance and E-g = 2.0 eV. This work reveals two ways to control the [S]/([S]+[Se]) depth-profile to produce wide band gap CZGSSe absorber layers for efficient solar cells. (C) 2021 The Author(s). Published by Elsevier B.V.

Cu2ZnSnS4 (CZTS) thin films deposited using direct current magnetron sputtering and sulfurized at argon atmosphere pressures of 950, 460, and 50 mbar are studied in a view of solar cell absorber fabrication. The main novelty in the work is the influence of the radio frequency (RF) electromagnetic field treatment on the Cu2−xS secondary phase. The treatment reduces the amount of the Cu2−xS secondary phase, which is confirmed by Raman spectroscopy. The RF effect is long-term, at least one year later. So, this treatment is a promising technique to achieve higher purities of CZTS absorber layers for solar cells.

The influence of the Na content and incorporation procedure into Cu2ZnGe(S,Se)4 (CZGSSe) thin films and solar cells is investigated. The effects of the presence/absence of Se during the deposition of a 15 nm thick NaF layer before and/or after the Cu2ZnGeSe4 (CZGSe) co-evaporation, are compared. Both the Na content, and Na-supply method significantly influence the incorporation of S into the CZGSe lattice and its distribution in the absorber. A [S]/([S] + [Se])-gradient throughout the CZGSSe layers is observed for all the samples and correlated with effects induced by the Na incorporation procedure. For instance, the evaporation of Se together with NaF leads to an increased S concentration in the surface-region of the CZGSSe layer and a modified surface morphology. CZGSSe-based solar cells with band gap energies of about 2 eV are obtained, regardless of the NaF addition method used, while the absence of the NaF layer reduces the S incorporation and the Eg. However, the evaporation of Se together with NaF results in higher Eg and open circuit voltage VOC, probably related to a higher S accumulation near the surface, demonstrating the importance of the [S]/([S] + [Se]) distribution. CZGSSe-based photovoltaic devices with efficiency of 3.2 % and Eg of 2 eV are obtained.

The pseudo dielectric function of Cu2ZnSn(SxSe1-x)(4) [x = 0.35, 0.62, 0.81] bulk polycrystals is determined over the range 1.1-4.6 eV at room temperature from the analysis of spectroscopic ellipsometry data using the Adachi model. From the analysis, the lowest E-0 transition and high energy E-1A and E-1B transitions are clearly identified, and used to follow the evolution of the pseudo dielectric function as a function of the composition. It is shown that the fundamental E-0 and high energy E-1A transitions can be tuned by increasing the sulfur content over a range of 0.3 eV. These results show the potential of the kesterite compounds for the design of efficient tailored photovoltaic solar cells. (C) 2020 Elsevier B.V. All rights reserved.

A Cu2ZnSnS4 (CZTS) thin film deposited on Mo contact film using direct current magnetron sputtering and sulfurized is studied. The morphological and structural investigations are focused on the interface between the CZTS film and the back Mo layer. The film is shown to be polycrystalline with an average grain size of 0.8 mu m and of a high conductivity of the grain boundaries. It is also characterized by a suitable elemental composition with a noncritical deviation from the stoichiometry across the film depth. This results in the optical bandgap of 1.48 eV, which is optimal for solar cell absorbers. Raman spectra show low FWHMs of two A-symmetry dominant bands for CZTS thin film, which confirms a high quality of the crystal structure over a large area. At the same time, ZnS secondary phase is found on the film surface, while MoS2 is detected in the depth using a resonant excitation. The Raman mapping shows a non-uniform distribution of MoS2 along the interface between the CZTS film and the back Mo layer.

The effect of Zn content on compositional, structural and vibrational properties of Cu2ZnSn0.5Ge0.5Se4 (CZTGSe, x similar to 0.5) thin films is studied. Kesterite layer is deposited by co-evaporation onto 5 x 5 cm(2) Mo/SLG substrate followed by a thermal treatment at maximum temperature of 480 degrees C, obtaining areas with different composition and morphology which are due to the sample position in the co-evaporation system and to the non-uniform temperature distribution across the substrate. Kesterite layers with higher Zn amounts are characterized by lower Cu and Ge contents; however, a uniform Ge distribution through the absorber layer is detected in all cases. The excess Zn concentration leads to the formation of ZnSe secondary phase on the surface and in the bulk of the absorber as determined by Raman spectroscopy. When higher Ge content and no ZnSe are present in the absorber layer, a compact structure is formed with larger grain size of kesterite. This effect could explain the higher V-oc. of the solar cell. The Zn content does not affect the bandgap energy significantly (E-g near 1.3 eV), although the observed effect of Zn excess in CZTGSe results in a decreased device performance from 6.4 to 4.2%. This investigation reveals the importance of the control of the off-stoichiometric CZTGSe composition during the deposition process to enhance solar cells properties.

Cu2ZnSn1-xGexSe4 (CZTGSe) thin films have been grown onto Mo/SLG and Mo/V2O5/FTO/SLG substrates using thermal co-evaporation followed by a subsequent thermal annealing. A NaF precursor layer was evaporated prior to the deposition of the kesterite absorber layer. In the samples grown on Mo/SLG, it has been found that Na promotes Ge incorporation into the Cu2ZnSnSe4 lattice. The high concentration of incorporated Ge leads to the segregation of Sn-Se secondary phases as well as to an accumulation of Sn next to the Mo layer. The use of 12 and 16 nm NaF thick precursor layers prior to the CZTGSe deposition leads to absorber band gaps of 1.30 and 1.34 eV, and to device performances of 4.7 and 4.0%, respectively. A higher Na content, furthermore, caused the formation of bigger grains, a higher charge carrier concentration and a shorter depletion width. A 12 nm NaF precursor layer was used for the devices grown on FTO-based substrates, producing an optimal back contact that allows achieving efficiencies of 5.6% and transmittance of 30% in the near infrared range. This enhanced performance can be associated with the absence of secondary phases and Ge distribution through the absorber layer. The formation of a MoSe2 layer at the back interface in all the investigated devices seems to play a crucial role to improve the solar cell efficiency.

This paper provides an overview of the physical vapor technologies used to synthesize Cu2ZnSn(S,Se)4 thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films have been fabricated using sequential stacking or co-sputtering of precursors as well as using sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition, pulsed laser deposition of composite targets and monograin growth by the molten salt method were developed as alternative methods for kesterite layers deposition. This review presents the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical description of the main issues limiting this efficiency and of the experimental pathways designed to prevent or limit these issues is provided and discussed as well. A final section is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiency, such as alkali doping and bandgap grading.

Cu2ZnSnS4 (CZTS) thin films deposited using direct current magnetron sputtering and sulfurized at different argon atmosphere pressures of 950, 460 and 50 mbar are studied focusing on the control of disorder degree and secondary phases. The films are in detail characterized employing X-ray diffraction, scanning electron and conductive atomic force microscopies, energy dispersive X-ray, mu-Raman spectroscopies and mappings. Such a comprehensive approach shows how the pressure variation influences morphology, structural and electrical properties of the films in submicron scale. The films sulfurized at the lowest applied pressure of 50 mbar are found to be single-phase CZTS with low disordering degree. Nevertheless, in those sulfurized at 950 and 460 mbar, Cu2-xS and ZnS secondary phases with concomitant spatial inhomogeneities are detected. At the same time, sulfurization at higher pressures enlarges the grains, which however demonstrate wide size distribution. Formation of CZTS crystal structure and secondary phases as well as acceleration of grain growth depending on pressure are interpreted and discussed.

The sulfurization of co-evaporated Cu2ZnSnSe4 (CZTSe) thin films is studied. In this work, a relationship between the Na concentration and the further S incorporation in the CZTSe absorber layer is found. Na is added by a NaF precursor layer before CZTSe co-evaporation and/or by diffusion from the soda-lime glass substrate. Higher Na concentrations result in bigger grain sizes, higher S/(S + Se) atomic ratios and photovoltaic devices with higher VOC, as investigated by SEM, GIXRD, Raman and I-V measurements. Results using an alternative substrate as ceramic verify the importance of the control of the Na content, which alters the carrier concentrations and elements distribution with much higher S and Na concentrations at the surface in this particular case. By using this methodology, it is possible to tune the kesterite band gap energy by the creation of S-gradient through the absorber thin film. This may provide a new approach for the control of Cu2ZnSn(S,Se)4 growth and development of high efficiency kesterite solar cells. Efficiencies of 5.5% and 6.4% are obtained using ceramic and SLG substrates respectively (without antireflection coatings and without grids).

The linear optical properties of Cu2ZnSnS4 bulk poly-crystals have been investigated using spectroscopic ellipsometry in the range of 1.2–4.6 eV at room temperature. The characteristic features identified in the optical spectra are explained by using the Adachi analytical model for the interband transitions at the corresponding critical points in the Brillouin zone. The experimental data have been modeled over the entire spectral range taking into account the lowest E0 transition near the fundamental absorption edge and E1A and E1B higher energy interband transitions. In addition, the spectral dependences of the refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity values have been accurately determined and are provided since they are essential data for the design of Cu2ZnSnS4 based optoelectronic devices.

Bismuth sulfide thin films were prepared by chemical bath deposition using thiourea as sulfide ion source in basic medium. First, the effects of both the deposition parameters on film growth as well as the annealing effect under argon and sulfur atmosphere on as-deposited thin films were studied. The parameters were found to be influential using the Doehlert matrix experimental design methodology. Ranges for a maximum surface mass of films (3 mg cm−2) were determined. A well-crystallized major phase of bismuth sulfide with stoichiometric composition was achieved at 190 °C for 3 h. The prepared thin films were characterized using grazing incidence X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis. Second, the bandgap energy value was found to be 1.5 eV. Finally, the thermal properties have been studied for the first time by means of the electropyroelectric (EPE) technique. Indeed, the thermal conductivity varied in the range of 1.20–0.60 W m−1 K−1, while the thermal diffusivity values increased in terms of the annealing effect ranging from 1.8 to 3.5 10−7 m2 s−1.

Compounds of the chalcogenide family Ag–In–VI (VI = S, Se, Te) are interesting materials due to their stoichiometric stability and potential application in nonlinear optics and solar cells. A polycrystalline ingot of AgIn5S8, an ordered vacancy semiconductor, was prepared by direct fusion of the stoichiometric mixture of the elements in an evacuated quartz ampoule. The presence of a single phase with cubic structure was confirmed by X-ray powder diffraction at room temperature. The lattice parameter, [Formula: see text], was calculated, giving 10.821750 Å. Samples in evacuated quartz ampoules were used to perform Differential Thermal Analysis measurements, showing congruent melting at 1110[Formula: see text]C. Transmittance and reflectivity measurements were used to calculate the absorption coefficient [Formula: see text]. From the plot of ([Formula: see text])2 versus [Formula: see text], two direct transitions are observed at 1.25 eV and 1.88 eV. While the higher energy direct transition has been observed by other authors, the direct nature of the lower energy transition was confirmed from the fitting of the plot of the reflectivity versus 1/[Formula: see text] between 0.53 eV[Formula: see text] (1.89 eV) and 0.55 eV[Formula: see text] (1.82 eV), obtaining a value of 1.29 eV. The real refractive index [Formula: see text] and the high-frequency dielectric constant [Formula: see text] were also obtained from the fit of the reflectivity, resulting to be 2.68 and 7.2, respectively.