Photoluminescence and electrical study of fluctuating potentials in Cu2ZnSnS4-based thin films

In this work, we investigated structural, morphological, electrical, and optical properties from a set of Cu2ZnSnS4 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 Cu2ZnSnS4 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 Cu2ZnSnS4 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.

Growth and characterization of Cu2ZnSn(S,Se)4 thin films for solar cells

Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) with their band gap energies around 1.45 eV and 1.0 eV, respectively, can be used as the absorber layer in thin film solar cells. By using a mixture of both compounds, Cu2ZnSn(S,Se)4 (CZTSSe), a band gap tuning may be possible. The latter material has already shown promising results such as solar cell efficiencies up to 10.1%. In this work, CZTSSe thin films were grown in order to study its structure and to establish the best growth precursors. SEM micrographs reveal an open columnar structure for most samples and EDS composition profiling of the cross sections show different selenium gradients. X-ray diffractograms show different shifts of the kesterite/stannite (1 1 2) peak, which indicate the presence of CZTSSe. From Raman scattering analysis, it was concluded that all samples had traces of CZTS and CZTSSe. The composition of the CZTSSe layer was estimated using X-ray diffraction and Raman scattering and both results were compared. It was concluded that Se diffused more easily in precursors with ternary Cu–Sn–S phases and metallic Zn than in precursors with ZnS and/or CZTS already formed. It was also showed that a combination of X-ray diffraction and Raman scattering can be used to estimate the ratio of S per Se in CZTSSe samples.

Assessment of the potential of tin sulphide thin films prepared by sulphurization of metallic precursors as cell absorbers

In this work, SnxSy thin films have been grown on soda-lime glass substrates by sulphurization of metallic precursors in a nitrogen plus sulphur vapour atmosphere. Different sulphurization temperatures were tested, ranging from 300 °C to 520 °C. The resulting phases were structurally investigated by X-Ray Diffraction and Raman spectroscopy. Composition was studied using Energy Dispersive Spectroscopy being then correlated with the sulphurization temperature. Optical measurements were performed to obtain transmittance and reflectance spectra, from which the energy band gaps, were estimated. The values obtained were 1.17 eV for the indirect transition and for the direct transition the values varied from 1.26 eV to 1.57 eV. Electrical characterization using Hot Point Probe showed that all samples were p-type semiconductors. Solar cells were built using the structure: SLG/Mo/SnxSy/CdS/ZnO:Ga and the best result for solar cell efficiency was 0.17%.

Cu2ZnSnS4 solar cells prepared with sulphurized dc-sputtered stacked metallic precursors

In the present work we report the details of the preparation and characterization results of Cu2ZnSnS4 (CZTS) based solar cells. The CZTS absorber was obtained by sulphurization of dc magnetron sputtered Zn/Sn/Cu precursor layers. The morphology, composition and structure of the absorber layer were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. The majority carrier type was identified via a hot point probe analysis. The hole density, space charge region width and band gap energy were estimated from the external quantum efficiency measurements. A MoS2 layer that formed during the sulphurization process was also identified and analyzed in this work. The solar cells had the following structure: soda lime glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al grid. The best solar cell showed an opencircuit voltage of 345 mV, a short-circuit current density of 4.42 mA/cm2, a fill factor of 44.29% and an efficiency of 0.68% under illumination in simulated standard test conditions: AM 1.5 and 100 mW/cm2.

Study of polycrystalline Cu2ZnSnS4 films by Raman scattering

Cu2ZnSnS4 (CZTS) is a p-type semiconductor that has been seen as a possible low-cost replacement for Cu(In,Ga)Se2 in thin film solar cells. So far compound has presented difficulties in its growth, mainly, because of the formation of secondary phases like ZnS, CuxSnSx+1, SnxSy, Cu2−xS and MoS2. 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, CuxSnSx+1, SnxSy and cubic ZnS based on the sulphurization of stacked metallic precursors. The results of the films’ characterization by XRD, electron backscatter 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 secondary phases.

Growth of the [110] oriented TiO2 nanorods on ITO substrates by sputtering technique for dye-sensitized solar cells

TiO2 films have been deposited on ITO substrates by dc reactive magnetron sputtering technique. It has been found that the sputtering pressure is a very important parameter for the structure of the deposited TiO2 films. When the pressure is lower than 1 Pa, the deposited has a dense structure and shows a preferred orientation along the [101] direction. However, the nanorod structure has been obtained as the sputtering pressure is higher than 1 Pa. These nanorods structure TiO2 film shows a preferred orientation along the [110] direction. The x-ray diffraction and the Raman scattering measurements show both the dense and the nanostructure TiO2 films have only an anatase phase, no other phase has been obtained. The results of the SEM show that these TiO2 nanorods are perpendicular to the ITO substrate. The TEM measurement shows that the nanorods have a very rough surface. The dye-sensitized solar cells (DSSCs) have been assembled using these TiO2 nanorod films prepared at different sputtering pressures as photoelectrode. And the effect of the sputtering pressure on the properties of the photoelectric conversion of the DSSCs has been studied.