Browsing by Author "Sousa, M. G."
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- Annealing of RF-magnetron sputtered SnS2 precursors as a new route for single phase SnS thin filmsPublication . Sousa, M. G.; Cunha, A. F.; Fernandes, P. A.Tin sulphide thin films have been grown on soda-lime glass substrates through the anneali ng of RF-mag netron sputtered SnS2 precursors . Three different approaches to the annealing were compared and the resulti ng films thoroughly studied. One series of precursors was annealed in a tubular furnace directly exposed to a flux of sulphur vapour plus formi ng gas, N 2 + 5%H2, and at a constant pressure of 500 mbar. The other two series of identical precursors were annealed in the same furnace but inside a graphite box with and without elemental sulphur evaporation again i n the presence of N 2 + 5%H2 and at the same pres sure as for the sulphur flux experiments. Different maximum annealing temperatures for each set of sam ples , in the range of 300-570 °c,were tested to study their effects on the properties of the final films.The resulti ng phases were structurally investigated by X-Ray Diffraction (XRD) and Raman spectroscopy. Annealing of SnS2 precursors in sulphur flux produced films where SnS2 was dominant for temperatures up to 480 °C. Increasing the temperature to 530 °c and 570 °c led to films where the dominant phase became Sn2S3• Annealing of SnS2 precursors in a graphite box with sulphur vapour at temperatures in the range between 300 °c and 480 °c the films are multi-phase , containing Sn2 S3, SnS2 and SnS. For high annealing temperatures of 530 °c and 570 °c the films have SnS as the dominant phase.Annealing of SnS2 precursors in a graphite box without sulphur vapour at 300 °c and 360 °c the films are essentially amor phous, at 420 °c SnS2 is the dominant phase. For temperatures of 480 °c and 530 °c SnS is the dominant phase but also same residual SnS2 and Sn2S3 phases are observed. For annealing at 570 °c, according to the XRD results the films appear to be single phase SnS. The composition was studied usi ng energy dis persive spectroscopy being then correlated with the annealing temperature. Scanning electron micros copy studies revealed that the SnS films exhibit small grain structure and the film surface is rough. Optical measurements were performed, from which the band gap energies were estimated .These studies show that the direct absorption transitions of SnS are at 1.68 eV and 1.41 eV for annealing in graphite box with and without elemental sulphur evaporation, respectively . For the indirect transition the val ues var ied from 1.49 eV to 1.37 eV.The results of this work show that the third approach is better suited to pro duce single phase SnS films. However, a finer tunning of the duration of the high temperature plateau of the anneali ng profile is required in order to eliminate the -Sn top layer.
- Comparison of fluctuating potentials and donor-acceptor pair transitions in a Cu-poor Cu2ZnSnS4 based solar cellPublication . Teixeira, J. P.; Sousa, R. A.; Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.; Salomé, P. M. P.; González, J. C.; Leitão, J. P.The structure of the electronic energy levels of a single phase Cu2ZnSnS4 film, as confirmed by Raman Scattering and x-ray diffraction, is investigated through a dependence on the excitation power of the photoluminescence (PL). The behavior of the observed asymmetric band, with a peak energy at ∼1.22 eV, is compared with two theoretical models: (i) fluctuating potentials and (ii) donor-acceptor pair transitions. It is shown that the radiative recombination channels in the Cu-poor film are strongly influenced by tail states in the bandgap as a consequence of a heavy doping and compensation levels. The contribution of the PL for the evaluation of secondary phases is also highlighted.
- Cu2ZnSnS4 absorber layers obtained through sulphurization of metallic precursors: Graphite box versus sulphur fluxPublication . Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.; Teixeira, J. P.; Leitão, J. P.; Salomé, P. M. P.In this work we employed a hybrid method, combining RF-magnetron sputtering with evaporation, for the deposition of tailor made metallic precursors, with varying number of Zn/Sn/Cu (ZTC) periods and compared two approaches to sulphurization. Two series of samples with 1×, 2× and 4× ZTC periods have been prepared. One series of precursors was sulphurized in a tubular furnace directly exposed to a sulphur vapour and N2+5% H2 flux at a pressure of 5.0×10+4 Pa. A second series of identical precursors was sulphurized in the same furnace but inside a graphite box where sulphur pellets have been evaporated again in the presence of N2+5% H2 and at the same pressure as for the sulphur flux experiments. The morphological and chemical analyses revealed a small grain structure but good average composition for all three films sulphurized in the graphite box. As for the three films sulphurized in sulphur flux grain growth was seen with the increase of the number of ZTC periods whilst, in terms of composition, they were slightly Zn poor. The films' crystal structure showed that Cu2ZnSnS4 is the dominant phase. However, in the case of the sulphur flux films SnS2 was also detected. Photoluminescence spectroscopy studies showed an asymmetric broad band emission whichoccurs in the range of 1–1.5 eV. Clearly the radiative recombination efficiency is higher in the series of samples sulphurized in sulphur flux. We have found that sulphurization in sulphur flux leads to better film morphology than when the process is carried out in a graphite box in similar thermodynamic conditions. Solar cells have been prepared and characterized showing a correlation between improved film morphology and cell performance. The best cells achieved an efficiency of 2.4%.
- Effect of rapid thermal processing conditions on the properties of Cu2ZnSnS4 thin films and solar cell performancePublication . Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.; Teixeira, J.P.; Sousa, R.A.; Leitão, J.PIn the present work, we have studied the effect of several sulphurization conditions on the properties of Cu2ZnSnS4 thin films obtained through rapid thermal processing (RTP) of multi-period precursors with 8 periods of Zn/SnS2/CuS. In this study we varied the heating rate, the maximum sulphurization temperature, the time at maximum temperature and the amount of evaporated sulphur. The samples were characterized through scanning electron microscopy, energy dispersive spectroscopy, Raman scattering spectroscopy, X-ray diffraction, photoluminescence and I–V measurements. We have observed that at heating rates above 0.5 1C/s the samples delaminated severely. As a result further tests were carried out at 0.2 1C/s heating rate. The morphological studies revealed that the samples sulphurized at higher temperatures, shorter times and higher amount of evaporated sulphur exhibited larger grain sizes. The structural analysis based on Raman scattering and XRD did not lead to a clear distinction between the samples. Photoluminescence spectroscopy studies showed an asymmetric broad band characteristic of CZTS, which occurs in the range of 1.0–1.4 eV and a second band, on the high energy side of the previous one, peaking at around 1.41 eV. The intensity of this latter band varies from sample to sample revealing substantial differences in their optical properties. This band appears to originate either from the surface of the absorber or from the CdS layer and has a clear correlation with cell efficiency. The higher the intensity of this band the lower the cell efficiency, presumably due to the increase in recombination resulting from CZTS surface decomposition and eventually from the CdS with modified optoelectronic properties. The cell results hint toward a detrimental effect of long sulphuriza-tion times and a positive effect of higher sulphur vapour pressure and higher sulphurization temperature. Solar cell efficiencies improved with increased grain size in the absorber layer. The highest cell efficiency obtained in this study was 3.1%.
- Effect of selenization conditions on the growth and properties of Cu2ZnSn(S,Se)4 thin filmsPublication . Ranjbar, S.; Menon, M. R. Rajesh; Sousa, M. G.; Fernandes, P. A.; Cunha, A. F. daCu2ZnSn(S,Se)4 (CZTSSe) is a potentially cost effective candidate for future thin film solar cells (TFSCs). We report the growth of CZTSSe using a hybrid process involving the sequential evaporation of Zn and sputtering of the sulfide precursors of Cu and Sn, followed by a selenization step. Two approaches for selenization were followed, one using a tubular furnace (TF) and the other using a Rapid Thermal Processor (RTP). SEM and EDS were employed to investigate the morphology and composition of the films. Structural analyses were done using XRD and Raman spectroscopy. Structural analyses revealed the formation of CZTSSe. The effects of annealing conditions on the morphological and structural properties of the films were investigated.
- Enhanced SnS phase purity of films produced by rapid thermal processing of SnS2 precursorsPublication . Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.In this work, we present a procedure to grow single phase SnS thin films consisting on the annealing of RF magnetron sputtered SnS2 precursors . A series of samples was produced by rapid thermal processing of precursors deposited both on bare and Mo coated glass. For those samples the time at maximum temperature and heating rate were varied
- Growth and Characterization of single phase SnS thin films by sulphurization of sputtered sulphide precursorsPublication . Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.Single-phase SnS thin films have been grown on soda-lime glass substrates based on the sulphurization of RF-magnetron sputtered SnS2 precursors. Two different approaches to sulphurization were compared and thoroughly studied. The two series of identical precursors were sulphurized in the same furnace, inside a graphite box with and without elemental sulphur evaporation. Different maximum sulphurization temperatures, in the range 300ºC to 570ºC, were tested. Sulphurization of SnS2 precursors in a graphite box with and without sulpur vapour at high temperature produces SnS films which appear to be single-phase from the structural analysis. The studies show that the direct absorption transitions of SnS are at 1.41 eV and 1.68 eV for sulphurization in graphite box with and without elemental sulphur evaporation, respectively. The indirect absorption transition values varied from 1.49 eV e 1.37 eV.
- Growth and Characterization of SnSe2 by selenization of sputtered metallic precursorsPublication . Fernandes, P. A.; Sousa, M. G.; Salomé, P. M. P.; Teixeira, J. P.; Leitão, J. P.; Cunha, A. F. daIn the present work, we present a process to grow tin diselenide thin films by selenization at a maximum temperature of 470 ºC, of tin metallic precursor layers deposited by dc magnetron sputtering. For this maximum temperature, disklike grain morphologies were observed. Prominent XRD reflections at 2θ= 30.75º, 40.10º and 47.72º and vibration modes located at 119 cm-1 and 185 cm-1 were observed. These results allowed concluding that the dominant phase is SnSe2. The composition analysis, done by energy dispersive spectroscopy (EDS), showed that the films were close to being stoichiometric SnSe2 with a Se to Sn ratio of 1.95. Photoluminescence characterization was performed and revealed a dominant band at 0.874 eV and two other bands at ~0.74 and 1.08 eV with a lower relative intensities. The observed radiative transitions depend critically on the temperature.
- Influence of defects on photoluminescence from a Cu2ZnSnS4 thin filmPublication . Teixeira, J. P.; Sousa, R. A.; Leitão, J. P.; Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.; Salomé, P. M. P.GubZnSnS2 VCZTS0 p re sent s p romi sing p rope rtie s fo r thin film photovoltaics VPV0 technology [;]Z • non toxic character and availability of the constituentsS • high absorption coefficient VR;M2 cm-;0 in the near-infrared and visible spectral regionsS • almost optimal direct bandgap energy V;F3 eV0F
- Radiative transitions in highly doped and compensated chalcopyrites and kesterites: The case of Cu2ZnSnS4Publication . Teixeira, J. P.; Sousa, R. A.; Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.; Salomé, P. M. P.; Leitão, J. P.The theoretical models of radiative recombinations in both CuIn1−xGaxSe2 chalcopyrite and Cu2ZnSnS4 kesterite, and related compounds, were revised. For heavily doped materials, electrons are free or bound to large donor agglomerates which hinders the involvement of single donors in the radiative recombination channels. In this work, we investigated the temperature and excitation power dependencies of the photoluminescence of Cu2ZnSnS4-based solar cells in which the absorber layer was grown through sulphurization of multiperiod structuresofprecursorlayers.Forbothsamplestheluminescenceisdominatedbyanasymmetricbandwithpeak energy at∼1.22 eV, which is influenced by fluctuating potentials in both conduction and valence bands. A value of ∼60 meV was estimated for the root-mean-square depth of the tails in the conduction band. The radiative transitions involve the recombination of electrons captured by localized states in tails of the conduction band with holes localized in neighboring acceptors that follow the fluctuations in the valence band. The same acceptor level with an ionization energy of∼280 meV was identified in both absorber layers. The influence of fluctuating potentials in the electrical performance of the solar cells was discussed