Cu 2 ZnSnS 4 (CZTS) is a p-type semiconductor that has been seen as a possible low-cost replacement for Cu(In,Ga)Se 2 in thin film solar cells. So far compound has presented difficulties in its growth, mainly, because of the formation of unwanted phases like ZnS, Cu x SnS x+1 , Sn x S y , Cu 2−x S and MoS 2 . X-ray diffraction analysis (XRD), which is mostly used for phase identification cannot resolve some of these phases from the kesterite/stannite CZTS and thus the use of a complementary technique is needed. Raman scattering analysis can help distinguishing these phases not only laterally but also in depth. Knowing the absorption coefficient and using different excitation wavelengths in Raman scattering analysis, one is capable of profiling the different phases present in multi-phase CZTS thin films.This work describes in a concise form the methods used to grow chalcogenide compounds, such as, CZTS, Cu x SnS x+1 , Sn x S y and cubic ZnS based on the sulphurization of stacked metallic precursors. The results of the films' characterization by XRD, electron backscattered diffraction and scanning electron microscopy/energy dispersive spectroscopy techniques are presented for the CZTS phase. The limitation of XRD to identify some of the possible phases that can remain after the sulphurization process are investigated. The results of the Raman analysis of the phases formed in this growth method and the advantage of using this technique in identifying them are presented. Using different excitation wavelengths it is also analysed the CZTS film in depth showing that this technique can be used as non destructive methods to detect unwanted phases.

Abstract:In the present work we report the results of the growth, morphological and structural characterization of Cu 2 ZnSnS 4 (CZTS) thin films prepared by sulfurization of DC magnetron sputtered Cu/Zn/Sn precursor layers. The adjustment of the thicknesses and the properties of the precursors were used to control the final composition of the films. Its properties were studied by SEM/EDS, XRD and Raman scattering. The influence of the sulfurization temperature on the morphology, composition and structure of the films has been studied. With the presented method we have been able to prepare CZTS thin films with a kesterite structure.

Abstract:Thin film Cu 2 SnS 3 and Cu 3 SnS 4 were grown by sulfurization of dc-magnetron sputtered Sn-Cu metallic precursors in a S 2 atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu 2 SnS 3 phase changes. For a temperature of 350 ºC the films were constituted by tetragonal (I-42m) Cu 2 SnS 3 . The films sulfurized at a maximum temperature of 400 ºC presented a cubic (F-43m) Cu 2 SnS 3 phase. Increasing the temperature up to 520 ºC, the Sn content of the layer lowered and orthorhombic (Pmn21) Cu 3 SnS 4 was formed.The phase identification and structural analysis was performed using X-ray Diffraction (XRD) and Electron Back-Scattered Diffraction (EBSD) analysis. Raman scattering analysis was also performed and the comparison with XRD and EBSD data allowed the assignment of peaks at

Thin film solar cells based in Cu(In,Ga)Se2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%.

In this work, we investigated structural, morphological, electrical, and optical properties from a set of Cu 2 ZnSnS 4 thin films grown by sulfurization of metallic precursors deposited on soda lime glass substrates coated with or without molybdenum. X-ray diffraction and Raman spectroscopy measurements revealed the formation of single-phase Cu 2 ZnSnS 4 thin films. A good crystallinity and grain compactness of the film was found by scanning electron microscopy. The grown films are poor in copper and rich in zinc, which is a composition close to that of the Cu 2 ZnSnS 4 solar cells with best reported efficiency. Electrical conductivity and Hall effect measurements showed a high doping level and a strong compensation. The temperature dependence of the free hole concentration showed that the films are nondegenerate. Photoluminescence spectroscopy showed an asymmetric broadband emission. The experimental behavior with increasing excitation power or temperature cannot be explained by donor-acceptor pair transitions. A model of radiative recombination of an electron with a hole bound to an acceptor level, broadened by potential fluctuations of the valence-band edge, was proposed. An ionization energy for the acceptor level in the range 29-40 meV was estimated, and a value of 172 ± 2 meV was obtained for the potential fluctuation in the valence-band edge.

In this work, tin selenide thin films (SnSe x ) were grown on soda lime glass substrates by selenization of dc magnetron sputtered Sn metallic precursors. Selenization was performed at maximum temperatures in the range of 300 • C to 570 • C. The thickness and the composition of the films were analysed using step profilometry and energy dispersive spectroscopy, respectively. The films were structurally and optically investigated by X-Ray diffraction, Raman spectroscopy and optical transmittance and reflectance measurements. X-Ray diffraction patterns suggest that for temperatures between 300 • C and 470 • C the films are composed of hexagonal-SnSe 2 phase. By increasing the temperature, the films selenized at maximum temperatures of 530 • C and 570 • C show orthorhombic-SnSe as the dominant phase with a preferential crystal orientation along the (400) crystallographic plane. Raman scattering analysis allowed the assignment of peaks at 119 cm −1 and 185 cm −1 to the hexagonal-SnSe 2 and 108 cm −1 , 130 cm −1 and 150 cm −1 to the orthorhombic-SnSe phase. All samples present traces of condensed amorphous Se with a characteristic Raman peak located at 255 cm −1 . From optical measurements, the estimated band gap energies for hexagonal-SnSe 2 were close to 0.9 eV and 1.7 eV for indirect forbidden and direct transitions, respectively. The samples with the dominant orthorhombicSnSe phase presented estimated band gap energies of 0.95 eV and 1.15 eV for indirect allowed and direct allowed transitions, respectively.

We report the results of the growth of Cu-Sn-S ternary chalcogenide compounds by sulfurization of dc magnetron sputtered metallic precursors.

Abstract:A dc magnetron sputtering based method to grow high quality Cu 2 ZnSnS 4 (CZTS) thin films, to be used as absorber layer in solar cells, is being developed. This method combines dc sputtering of metallic precursors with sulfurization in S vapour and with post growth KCN treatment for removal of possible undesired Cu 2-x S phases. In this work we report the results of a study of the effects of changing the precursors' deposition order on the final CZTS films' morphological and structural properties. The effect of KCN treatment on the optical properties was also analysed through diffuse reflectance measurements. Morphological, compositional and structural analyses of the various stages of the growth have been performed using stylus profilometry, SEM/EDS analysis, XRD and Raman Spectroscopy. Diffuse reflectance studies have been done in order to estimate band gap energy of the CZTS films.We tested two different deposition orders for the copper precursor, namely:Mo/Zn/Cu/Sn and Mo/Zn/Sn/Cu.The stylus profilometry analysis shows high average surface roughness in the ranges 300 nm to 550 nm and 230 nm to 250 nm before and after KCN treatment, respectively. All XRD spectra show preferential growth orientation along (112) .