Thermodynamic pathway for the formation of SnSe and SnSe2 polycrystalline thin films by selenization of metal precursors

In this work, tin selenide thin films (SnSex) 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 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 the hexagonal-SnSe2 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-SnSe2 phase and those at 108 cm−1, 130 cm−1 and 150 cm−1 to the orthorhombic-SnSe phase. All samples presented 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-SnSe2 were close to 0.9 eV and 1.7 eV for indirect forbidden and direct transitions, respectively. The samples with the dominant orthorhombic-SnSe phase presented estimated band gap energies of 0.95 eV and 1.15 eV for indirect allowed and direct allowed transitions, respectively.

Effects of sulphurization time on Cu2ZnSnS4 absorbers and thin films solar cells obtained from metallic precursors

We report the results of a study of the sulphurization time effects on Cu2ZnSnS4 absorbers and thin film solar cells prepared from dc-sputtered tackedmetallic precursors. Three different time intervals, 10 min, 30min and 60 min, at maximum sulphurization temperature were considered. The effects of this parameter' change were studied both on the absorber layer properties and on the final solar cell performance. The composition, structure, morphology and thicknesses of the CZTS layers were analyzed. The electrical characterization of the absorber layer was carried out by measuring the transversal electrical resistance of the samples as a function of temperature. This study shows an increase of the conductivity activation energy from 10 meV to 54meV for increasing sulphurization time from 10min to 60min. The solar cells were built with the following structure: SLG/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni:Al grid. Several ac response equivalent circuit models were tested to fit impedance measurements. The best results were used to extract the device series and shunt resistances and capacitances. Absorber layer's electronic properties were also determined using the Mott–Schottky method. The results show a decrease of the average acceptor doping density and built-in voltage, from 2.0 1017 cm−3 to 6.5 1015 cm−3 and from 0.71 V to 0.51 V, respectively, with increasing sulphurization time. These results also show an increase of the depletion region width from approximately 90 nm–250 nm.

Hopping conduction and persistent photoconductivity in Cu2ZnSnS4 thin films

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu2ZnSnS4 were investigated. It was found that at high temperatures the electrical conductivity was dominated by band conduction and nearest-neighbour hopping. However, at lower temperatures, both Mott variable-range hopping (VRH) and Efros–Shklovskii VRH were observed. The analysis of electrical transport showed high doping levels and a large compensation ratio, demonstrating large degree of disorder in Cu2ZnSnS4. Photoconductivity studies showed the presence of a persistent photoconductivity effect with decay time increasing with temperature, due to the presence of random local potential fluctuations in the Cu2ZnSnS4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu2ZnSnS4.

Growth and Raman scattering characterization of Cu2ZnSnS4 thin films

In the present work we report the results of the growth, morphological and structural characterization of Cu2ZnSnS4 (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 the kesterite structure.

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.

Precursors’ order effect on the properties of sulfurized Cu2ZnSnS4 thin films

A dc magnetron sputtering-based method to grow high-quality Cu2ZnSnS4 (CZTS) thin films, to be used as an 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 Cu2−xS 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 the 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 300–550 nm and 230–250 nm before and after KCN treatment, respectively. All XRD spectra show preferential growth orientation along (1 1 2) at 28.45◦. Raman spectroscopy shows main peaks at 338 cm−1 and 287 cm−1 which are attributed to Cu2ZnSnS4. These measurements also confirm the effectiveness of KCN treatment in removing Cu2−xS phases. From the analysis of the diffuse reflectance measurements the band gap energy for both precursors’ sequences is estimated to be close to 1.43 eV. The KCN-treated films show a better defined absorption edge; however, the band gap values are not significantly affected. Hot point probe measurements confirmed that CZTS had p-type semiconductor behaviour and C–V analysis was used to estimate the majority carrier density giving a value of 3.3 × 1018 cm−3.

A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors

Thin films of Cu2SnS3 and Cu3SnS4 were grown by sulfurization of dc magnetron sputtered Sn–Cu metallic precursors in a S2 atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu2SnS3 phase changes. For a temperature of 350 ◦C the films were composed of tetragonal (I -42m) Cu2SnS3. The films sulfurized at a maximum temperature of 400 ◦C presented a cubic (F-43m) Cu2SnS3 phase. On increasing the temperature up to 520 ◦C, the Sn content of the layer decreased and orthorhombic (Pmn21) Cu3SnS4 was formed. The phase identification and structural analysis were performed using x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Raman scattering analysis was also performed and a comparison with XRD and EBSD data allowed the assignment of peaks at 336 and 351 cm−1 for tetragonal Cu2SnS3, 303 and 355 cm−1 for cubic Cu2SnS3, and 318, 348 and 295 cm−1 for the Cu3SnS4 phase. Compositional analysis was done using energy dispersive spectroscopy and induced coupled plasma analysis. Scanning electron microscopy was used to study the morphology of the layers. Transmittance and reflectance measurements permitted the estimation of absorbance and band gap. These ternary compounds present a high absorbance value close to 104 cm−1. The estimated band gap energy was 1.35 eV for tetragonal (I -42m) Cu2SnS3, 0.96 eV for cubic (F-43m) Cu2SnS3 and 1.60 eV for orthorhombic (Pmn21) Cu3SnS4. A hot point probe was used for the determination of semiconductor conductivity type. The results show that all the samples are p-type semiconductors. A four-point probe was used to obtain the resistivity of these samples. The resistivities for tetragonal Cu2SnS3, cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4 are 4.59 × 10−2 cm, 1.26 × 10−2 cm, 7.40 × 10−4 cm, respectively.

CuxSnSx+1 (x = 2, 3) thin films grown by sulfurization of metallic precursors deposited by dc magnetron sputtering

We report the results of the growth of Cu-Sn-S ternary chalcogenide compounds by sulfurization of dc magnetron sputtered metallic precursors. Tetragonal Cu2SnS3 forms for a maximum sulfurization temperature of 350 ºC. Cubic Cu2SnS3 is obtained at sulfurization temperatures above 400 ºC. These results are supported by XRD analysis and Raman spectroscopy measurements. The latter analysis shows peaks at 336 cm-1, 351 cm-1 for tetragonal Cu2SnS3, and 303 cm-1, 355 cm-1 for cubic Cu2SnS3. Optical analysis shows that this phase change lowers the band gap from 1.35 eV to 0.98 eV. At higher sulfurization temperatures increased loss of Sn is expected in the sulphide form. As a consequence, higher Cu content ternary compounds like Cu3SnS4 grow. In these conditions, XRD and Raman analysis only detected orthorhombic (Pmn21) phase (petrukite). This compound has Raman peaks at 318 cm-1, 348 cm-1 and 295 cm-1. For a sulfurization temperature of 450 ºC the samples present a multi-phase structure mainly composed by cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4. For higher temperatures, the samples are single phase and constituted by orthorhombic (Pmn21) Cu3SnS4. Transmittance and reflectance measurements were used to estimate a band gap of 1.60 eV. For comparison we also include the results for Cu2ZnSnS4 obtained using similar growth conditions.

Study of optical and structural properties of Cu2ZnSnS4 thin films

Cu2ZnSnS4 is a promising semiconductor to be used as absorber in thin film solar cells. In this work, we investigated optical and structural properties of Cu2ZnSnS4 thin films grown by sulphurization of metallic precursors deposited on soda lime glass substrates. The crystalline phases were studied by X-ray diffraction measurements showing the presence of only the Cu2ZnSnS4 phase. The studied films were copper poor and zinc rich as shown by inductively coupled plasma mass spectroscopy. Scanning electron microscopy revealed a good crystallinity and compactness. An absorption coefficient varying between 3 and 4×104cm−1 was measured in the energy range between 1.75 and 3.5 eV. The band gap energy was estimated in 1.51 eV. Photoluminescence spectroscopy showed an asymmetric broad band emission. The dependence of this emission on the excitation power and temperature was investigated and compared to the predictions of the donor-acceptor-type transitions and radiative recombinations in the model of potential fluctuations. Experimental evidence was found to ascribe the observed emission to radiative transitions involving tail states created by potential fluctuations.

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%.

Detection of ZnS phases in CZTS thin-films by EXAFS

Copper zinc tin sulfide (CZTS) is a promising Earthabundant thin-film solar cell material; it has an appropriate band gap of ~1.45 eV and a high absorption coefficient. The most efficient CZTS cells tend to be slightly Zn-rich and Cu-poor. However, growing Zn-rich CZTS films can sometimes result in phase decomposition of CZTS into ZnS and Cu2SnS3, which is generally deleterious to solar cell performance. Cubic ZnS is difficult to detect by XRD, due to a similar diffraction pattern. We hypothesize that synchrotron-based extended X-ray absorption fine structure (EXAFS), which is sensitive to local chemical environment, may be able to determine the quantity of ZnS phase in CZTS films by detecting differences in the second-nearest neighbor shell of the Zn atoms. Films of varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor) were examined using the EXAFS technique. Differences in the spectra as a function of Cu/Zn ratio are detected. Linear combination analysis suggests increasing ZnS signal as the CZTS films become more Zn-rich. We demonstrate that the sensitive technique of EXAFS could be used to quantify the amount of ZnS present and provide a guide to crystal growth of highly phase pure films.

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.

Properties of tantalum oxynitride thin films produced by magnetron sputtering: the influence of processing parameters

The main purpose of this work is to present and to interpret the change of structure and physical properties of tantalum oxynitride (TaNxOy) thin films, produced by dc reactive magnetron sputtering, by varying the processing parameters. A set of TaNxOy films was prepared by varying the reactive gases flow rate, using a N2/O2 gas mixture with a concentration ratio of 17:3. The different films, obtained by this process, exhibited significant differences. The obtained composition and the interpretation of X-ray diffraction results, shows that, depending on the partial pressure of the reactive gases, the films are: essentially dark grey metallic, when the atomic ratio (N + O)/Ta < 0.1, evidencing a tetragonal β-Ta structure; grey-brownish, when 0.1 < (N + O)/Ta < 1, exhibiting a face-centred cubic (fcc) TaN-like structure; and transparent oxide-type, when (N + O)/Ta > 1, evidencing the existence of Ta2O5, but with an amorphous structure. These transparent films exhibit refractive indexes, in the visible region, always higher than 2.0. The wear resistance of the films is relatively good. The best behaviour was obtained for the films with (N + O)/Ta ≈ 0.5 and (N + O)/Ta ≈ 1.3.

Mechanical and tribological characterization of TiB2 thin films

Titanium Diboride (TiB2) presents high mechanical and physical properties. Some wear studies were also carried out in order to evaluate its tribological properties. One of the most popular wear tests for thin films is the ball-cratering configuration. This work was focused on the study of the tribological properties of TiB2 thin films using micro-abrasion tests and following the BS EN 1071-6: 2007 standard. Due to high hardness usually patented by these films, diamond was selected as abrasive on micro-abrasion tests. Micro-abrasion wear tests were performed under five different durations, using the same normal load, speed rotation and ball. Films were deposited by unbalanced magnetron sputtering Physical Vapour Deposition (PVD) technique using TiB2 targets. TiB2 films were characterized using different methods as Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), Electron Probe Micro-Analyser (EPMA), Ultra Micro Hardness and Scratch-test Analysis, allowing to confirm that TiB2 presents adequate mechanical and physical properties. Ratio between hardness (coating and abrasive particles), wear resistance and wear coefficient were studied, showing that TiB2 films shows excellent properties for tribological applications.

Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells

A persistent photoconductivity effect (PPC) has been investigated in Cu2ZnSnS4 thin films and solar cells as a function of temperature. An anomalous increase of the PPC decay time with temperature was observed in all samples. The PPC decay time activation energy was found to increase when temperature rises above a crossover value, and also to grow with the increase of the sulfurization temperature and pressure. Both the anomalous behavior of the PPC decay time and the existence of two different activation energies are explained in terms of local potential fluctuations in the band edges of CZTS.