Raman microprobe characterization of electrodeposited S-rich CuIn „ S , Se ... 2 for photovoltaic applications : Microstructural analysis

This article reports a detailed Raman scattering and microstructural characterization of S-rich CuIn S,Se 2 absorbers produced by electrodeposition of nanocrystalline CuInSe2 precursors and subsequent reactive annealing under sulfurizing conditions. Surface and in-depth resolved Raman microprobe measurements have been correlated with the analysis of the layers by optical and scanning electron microscopy, x-ray diffraction, and in-depth Auger electron spectroscopy. This has allowed corroboration of the high crystalline quality of the sulfurized layers. The sulfurizing conditions used also lead to the formation of a relatively thick MoS2 intermediate layer between the absorber and the Mo back contact. The analysis of the absorbers has also allowed identification of the presence of In-rich secondary phases, which are likely related to the coexistence in the electrodeposited precursors of ordered vacancy compound domains with the main chalcopyrite phase, in spite of the Cu-rich conditions used in the growth. This points out the higher complexity of the electrodeposition and sulfurization processes in relation to those based in vacuum deposition techniques. © 2007 American Institute of Physics. DOI: 10.1063/1.2734103


I. INTRODUCTION
Cu-based chalcopyrite materials ͑Cu-B III -C 2 VI ͒ have become a competitive alternative for the production of photovoltaic cells.These materials have adequate optoelectronic properties, and band gaps well matched to the solar spectrum.Furthermore, their chemical structure allows triggering of their optical properties within a wide range by means of introducing an adequate substitutional atom into the crystallographic structure.Up to now, optimum results in terms of efficiency have been obtained by using a complex alloy Cu͑In,Ga͒͑S,Se͒ 2 as absorber layer, which renders efficiencies above 19%. 1 Nevertheless, a deeper understanding of the properties of this material and the film formation mechanisms is still necessary in order to improve the efficiency and competitiveness of chalcopyrite-based solar cell technologies.
In this article, we present a detailed Raman scattering and microstructural characterization of S-rich CuIn͑S,Se͒ 2 absorbers produced by a two-step process involving the electrodeposition of a nanocrystalline CuInSe 2 precursor and a subsequent reactive annealing under sulfurizing conditions.This technology has a huge market potential, since it re-places several vacuum sputtering deposition processes, which are usually required to produce good-quality absorbers, with a single electrodeposition step. 2 Exploitation of the simplicity, low-cost process capability, and scalability to industrial production characteristic of the electrodeposition processes will allow significant cost reduction to be obtained in relation to the costs of nowadays standard thin-film solar cell technologies.Solar cell efficiencies up to 11% have already been reported for devices fabricated according to this two-step electrodeposition and sulfurization process. 3These efficiencies are close to that achieved in CuInS 2 -based devices synthesized using standard vacuum deposition processes. 4Nevertheless, and despite the work carried out in the last two decades, electrodeposition of chalcopyrite semiconducting compounds is still a relatively unknown process.A more detailed and better knowledge of the morphological and physico-chemical properties of these films is strongly required in order to optimize the film properties, and the absorber manufacturing technology.
Raman scattering constitutes a technique well suited for the microstructural characterization of chalcopyrite-based processes.The zone center phonon representation of the chalcopyrite structure is constituted by 21 optical modes, of which only the two A 2 modes are silent, Chalcopyrite modes are grouped into three frequency bands ͑low, central, and high energy bands͒, which can be interpreted as arising from the ⌫, X, and W points of the Brillouin zone of the zinc-blende analogous pseudobinary compound. 5ost of these modes have a very low scattering cross section and overlap each other, and thus are difficult to resolve even at low temperature using polarization configurations.The A 1 symmetry mode, corresponding to the antiphase vibration of the anions in the lattice, constitutes the most remarkable exception, and it is usually the most intense band in the spectra of chalcopyrite compounds.In addition, it has been demonstrated that the spectral properties of this band in photovoltaic absorbers are closely linked to the crystalline quality of the layer, and to the solar efficiency of the final device. 6,7n order to deep in the crystalline and microstructural characterization of the layers, surface and in-depth resolved Raman microprobe measurements have been correlated with the analysis of the layers by optical and scanning electron microscopy, x-ray diffraction, and in-depth Auger electron spectroscopy.All these measurements have allowed us to identify the main and secondary phases present in the absorbers and the dependence of their main microstructural features on the different process steps.This analysis has revealed the existence of significant differences in relation to the much better known vacuum deposition processes, which point out the higher complexity of the mechanisms involved in the synthesis and recrystallization of the electrodeposited absorbers.

II. EXPERIMENT
S-rich CuIn͑S,Se͒ 2 absorbers have been produced onto Mo-coated glass using a two-step process.First, a nanocrystalline CuInSe 2 precursor layer was obtained by means of one-step electrodeposition in an acidic bath containing Cu 2+ , In 3+ , and H 2 SeO 3 electroactive species.The electrodeposition conditions were chosen in order to produce films with overall stoichiometry given by In/ Cu= 0.9 and Se/ ͑Cu+ In͒ = 1.2.After electrodeposition, the films were annealed in a sulfur-containing atmosphere using a rapid thermal process ͑RTP͒.After the sulfurization reaction, the absorbers were treated in a NaCN bath in order to etch the surface Cu-rich phases related to segregation of Cu excess in the surface of the films.
Nanocrystalline precursor layers and polycrystalline sulfurized absorbers ͑before and after NaCN etching͒ were characterized by means of Raman spectroscopy ͑RS͒ in combination with optical microscopy ͑OM͒, scanning electron microscopy ͑SEM͒, x-ray diffraction ͑XRD͒, and Auger electron spectroscopy ͑AES͒.Raman microprobe measurements were performed using a T64000 Jobin-Yvon spectrometer coupled to an Olympus metallographic microscope.Excitation was provided with the 514.5 nm emission line of an Ar + laser and measurements were performed in backscattering configuration.Measurements on as-grown electrodeposited samples were made using the ϫ50 ͑NA 0.55͒ objective of the optical microscope, which gives a laser spot on the surface of the sample with an estimated size of about 1.2 m, with a penetration depth of about 100 nm.For the annealed samples, submicronic spatial resolution was obtained using the ϫ100 ͑NA 0.95͒ objective of the microscope.This has allowed performance of preliminary in-depth measurements by directly focusing the light spot at different depths in the cross section of the samples.In all cases, power on the sample was 0.5 mW, which corresponds to power densities of 50 kW/ cm 2 ͑ϫ50 objective͒ and 150 kW/ cm 2 ͑ϫ100 ob-jective͒, respectively.For these power conditions, no observable thermal effects on the Raman spectra were found.This configuration also allows one to obtain OM images by coupling the microscope output to a 640ϫ 480 charge-coupled device ͑CCD͒.
A Hitachi S-4100 scanning electron microscope was used in order to investigate the nanostructure and morphology of the layers.In-depth chemical compositional profiles of the sulfurized layers were obtained by means of AES, using a Phi 670 scanning Auger nanoprobe.The electron gun parameters selected for the measurements were electron beam energy 10 keV, and filament current 10 nA.The scanned area for the AES measurements was about 10 ϫ 10 m 2 , for a focused beam diameter below 100 nm.For the in-depth measurements, ion sputtering was carried out with an Ar + beam with energy below 5 keV in order to avoid inducing significant damage.For the sulfurized and NaCN etched samples, in order to obtain quantitative Auger results AES measurements have been performed in standards of CuInS 2 and MoS 2 .These data have allowed calibration of the sensitivity factors used in the quantitative calculations. 8The accuracy obtained by using standard samples is about 5% in absolute elemental concentration.
Furthermore, in order to deepen the in-depth resolved analysis of the layers, combined in-depth Raman/AES measurements were also performed by acquiring sequentially a series of Raman spectra after sputtering the sample in the AES system between every Raman measurement.Since the penetration depth of the green light in CuIn͑S,Se͒ 2 is only around 100 nm, this procedure allows one to obtain complete information on the bulk of the films.A detailed description of this combined technique can be found elsewhere. 9inally, 2 / XRD measurements were done in a Phillips MRS diffractometer and ⍀-scan XRD measurements were done in a Siemens D-500 diffractometer, using the K␣ emission line of copper.

A. Electrodeposited as-grown layers
Electrodeposited CuInSe 2 films are characterized by nonuniform chemical, structural, and morphological properties.Cross-section SEM images ͓Fig. 1 ͑left͔͒ reveal that the samples are constituted by a 1.6 m thick CuInSe 2 layer, presenting a vertical columnar structure.The layer is also characterized by the presence of vertical pores ͓indicated with arrows in Fig. 1 ͑left͔͒ that extend from the surface down to the midregion in the layers.At the back region, there is an improvement in the layer compactness.The CuInSe 2 / Mo interface appears to be free of voids, thus ensuring good electrical contact and film adherence.Further-more, planar view images show the presence of surface inhomogeneities which are also visible in the OM images ͑as will be shown later͒.According to the results of the microstructural analysis presented below, these inhomogeneities have been related to surface segregation of CuSe and Se crystals.Average grain size inferred from SEM images is in the range of 50 nm.
Surface Raman spectra of the precursor layers are characterized by the presence of fingerprint bands of chalcopyrite ordered CuInSe 2 , as shown in Fig. 2. The most characteristic chalcopyrite band appears at 175 cm −1 , and corresponds to the A 1 mode within the central energy band ͑173-216 cm −1 ͒.This central energy band groups four additional modes ͒, which are hindered in the form of a wide spectral contribution.The main peak at 175 cm −1 is blueshifted in relation to the chalcopyrite CuInSe 2 A 1 mode ͑173 cm −1 ͒. 7 This suggests the presence of a compressive residual stress in the layer.Likewise, the high-energy chalcopyrite band ͑211− 233 cm −1 ͒ comprises four modes ͑B 1

͑3͒
B 2 ͑3͒ 2E ͑5,6͒ ͒, which are especially difficult to resolve due to the nanocrystalline character of the layer, and due to the presence of additional bands arising from the presence of secondary phases.Within the low-frequency chalcopyrite phonon band ͑60− 80 cm −1 ͒, in which four modes B 1 ͑1͒ B 2 ͑1͒ 2E ͑1,2͒ lie, only the lowest frequency E ͑l͒ mode reported at 60͑TO͒ /61͑LO͒ cm −1 can be clearly distinguished. 10ote that the full width at half maximum ͑FWHM͒ of the A 1 mode is more than five times larger than that of a CuInSe 2 single crystal, thus indicating the poor crystalline quality of the layer.In addition, the presence of a lowfrequency shoulder in the 154 cm −1 region is related to the existence of complex point defects in the chalcopyrite structure ͑such as 2V Cu + V Se ͒, which give rise to the formation of ordered vacancies compounds ͑OVCs͒.These compounds have been reported in the literature to retain a chalcopyriterelated structure, while they are able to accommodate different types of complex defects.Their Raman spectrum is characterized by a strong redshift of the A 1 mode with respect to the vacancy-free chalcopyrite crystal, which is caused by the missing bonds in the OVC structure.The energy of the A 1 mode depends on the final stoichiometry of the OVC, 11 being 154 cm −1 for CuIn 3 Se 5 . 12n the other hand, it is evident that the film stoichiometry of the investigated precursors requires a mechanism to accommodate the Cu and Se excess in the film.This mechanism consists of the formation of elemental Se and Cu-Se binary phases, as can be inferred from the analysis of the Raman spectra.The spectra present two peaks at 240 and 261 cm −1 .The peak at 240 cm −1 is attributed to the presence of trigonal elementary selenium ͑Se͒, 13 while the peak at 261 cm −1 is characteristic of Cu-Se compounds.The absence of intense low-frequency bands at 17 and 42 cm −1 characteristic of the covellite-type structure ͑CuSe͒, 14 and the analysis of the XRD spectra, points to a different binary, probably Cu 2 Se, though the coexistence of the covellite CuSe phase cannot be discarded.
Figure 3 ͑left͒ shows an OM image of the surface of the as-grown sample.As already indicated, this image shows the presence of surface inhomogeneities that appear as regions with dark contrast.The micro-RS analysis of these surface inhomogeneities shows a strong increase in the intensity of the vibrational modes related to both Se and Cu-Se phases, together with the appearance of an intense contribution at 42 cm −1 , as can be seen in Fig. 3 ͑right͒.This gives clear evidence of the presence in these inclusions of the covellite CuSe phase, revealing the formation at the surface of the layers of Se and CuSe crystalline inclusions.
The formation of these secondary phases is in fact expected under the selected electrodeposition conditions. 2 However, the appearance of OVC phases in the film clearly shows the complexity of the electrodeposition process.In the present case, the Cu and Se excess conditions favor the nucleation of Se, Cu 2 Se, and CuSe nanocrystallites.Once nucleated, their growth likely induces a local deficiency of Se and Cu atoms, which results in the formation of OVC domains within the chalcopyrite CuInSe 2 crystalline grains.The introduction of V Cu and V Se defects in the lattice allows the electric valence of the Cu ͑1+͒ , In ͑3+͒ , and Se ͑2−͒ atoms in the lattice to be preserved, and ensures the chemical stability of the compound.On the other hand, we note the existence of a film aging process that is characterized by an increase of the Se and CuSe inclusions located at the film surface.Room temperature recrystallization of these secondary phases is responsible for this film aging.Nanometric CuSe/Se grains at the film surface likely act as nucleation grains.Crystal growth could be assisted by the redissolution of energetically metastable nanometric bulk crystallites, and by the grain boundary diffusion toward the film surface.Further investigations are required to fully understand this behavior, which gives another clear indication of the complexity of the electrodeposition process.
The spectra obtained do not allow ruling out the existence of a Cu-Au ordered CuInSe 2 phase ͑which is characterized by an A 1 mode at 183 cm −1 ͒, though it can be stated that, in any case, it is not a dominant phase in the film.Despite the fact that the formation of this metastable phase is known to be favored by low-temperature processing conditions and stoichiometric deviations during film formation, 11,15 the electrodeposition process do not seem to promote the formation of Cu-Au ordering, while clearly induces the formation of OVCs, even though the overall film stoichiometry is copper rich.
XRD analysis of electrodeposited samples supports the conclusions previously presented ͑see the spectra in Fig. 4͒.Detailed analysis of the spectra show the poor quality of the chalcopyrite phase, as indicated by the broadening of the reflection peaks, and by the absence of the characteristic tetragonal splitting ͑220͒/͑204͒.In addition to chalcopyrite reflections, the spectra show reflections corresponding to the Mo substrate ͑with low intensity under the selected grazing incidence conditions͒ and to Se, Cu 2 Se, and CuSe secondary phases in the films.Grazing incidence XRD spectra at different values reveal that Se and CuSe secondary phases tend to accumulate at the surface of the film, as pointed out by a greater relative contribution of the corresponding reflections.This agrees with the formation of Se and Cu-Se surface aggregates observed from the micro-RS analysis of surface of the samples.

B. Sulfur annealed films
Sulfurization of the precursor layers leads to important modifications in the morphology, composition, and chemical structure of the layers.This can be clearly seen in the crosssection SEM image in Fig. 1 ͑right͒, which corresponds to a sulfurized and NaCN etched sample.SEM observations show the formation of large crystals ͑1-2 m in size͒, which constitute a major part of the layer.Moreover, the sulfurization process also affects significantly the Mo back contact, giving rise to the formation of an intermediate layer with a thickness of about 1 m.As it will be shown later on, the main phase constituting this layer corresponds to MoS 2 .In principle, the presence of a MoS 2 interface layer between the absorber and the back Mo contact is necessary from the point of view of the solar cell performance, since it avoids the formation of a rectifying junction in the back contact of the cell, allowing for an ohmic one.However, the relatively high thickness of this sulfurized region constitutes a potential source of series resistance losses that could limit the device performance.In this sense, it is important to take into account that the sulfurization step leads to a significantly thinning ͑in more than 50% of the original thickness͒ of the Mo back contact layer.
The large grain size of the sulfurized films allows easy determination of the chemical composition of the sample by means of micro-RS.Figure 5 shows the OM image of the surface of the sulfurized layer before NaCN etching, together with the Raman spectra obtained at different points.Dark spots in the OM images present Raman spectra characteristic of a Cu͑S,Se͒ alloy.In this alloy, the A 1g mode of the covellite phase of CuS and CuSe binaries gives rise to three different peaks in the spectra, corresponding to the S-S, S-Se, and Se-Se vibrations from the anions located at the 4e sites in the covellite structure. 14Taking into account the experimental dependence of the intensity of the peaks, these spectra indicate a S-rich composition of the Cu͑S,Se͒ alloy, with more than 70% of S atoms at 4e sites ͑which correspond to 2/3 of the total anion sites͒.On the other hand, spectra obtained from lower contrast areas in the OM image correspond to a pure chalcopyrite phase.The stoichiometry of this phase can also be inferred by analyzing the spectral characteristics of the A 1 mode.In this case, the A 1 mode of the CuIn͑S,Se͒ 2 quaternary presents a bimodal behavior, with two bands involving pure S-S and Se-Se vibrations. 16As shown in Fig. 5, the Raman spectra are characterized by a dominant peak corresponding to the S-S vibrational mode, which indicates that S atoms have almost totally replaced the Se atoms in the crystalline lattice of the chalcopyrite phase, the percentage of remaining Se atoms being of the order of x Յ 5%.On the other hand, it is interesting that the narrower Raman bands in the spectra of these samples also indicate a considerable improvement on their crystalline quality after the sulfurization process, in agreement with the SEM observations.
The cross-section SEM images of these samples show the presence of inclusions in the grain boundaries of the chalcopyrite grains, which are attributed to the Cu͑S,Se͒ crystals ͑indicated with arrows in Fig. 6͒.As can be appreciated, these inclusions are not localized only at the surface of the samples, as it is usually found in the sulfurization of metallic precursor layers, but tend to penetrate down the absorber layer through the grain boundary.In some regions ͑not shown in Fig. 6͒ Cu͑S,Se͒ crystallites can also be observed close to the interface with the MoS 2 region, although the majority of these aggregates forms at the surface region.This morphology suggests that the sulfurization reaction is not able to promote the segregation of the Cu excess to the surface of the film, but just leads to a chemical and morphological modification of the binary phases already formed during the precursor formation.
The RS analysis of the samples after NaCN etching demonstrates that this process effectively removes these Cu͑S,Se͒ aggregates.Despite the contrasts observed in the OM images from the surface of the etched samples ͑see Fig. 7͒, dark areas are associated with surface roughness, as cor-roborated by micro-RS.This roughness is related to the etching of the Cu͑S,Se͒ inclusions penetrating into the surface region of the absorber layer.
Figure 8 ͑left͒ shows the series of Raman spectra measured with the laser spot focused at different positions in the cross section of the etched sample, from the surface region ͑spectrum 1 ͒ to the interface with the back Mo contact layer ͑spectrum 4 ͒.These measurements have allowed clear experimental evidence to be obtained regarding the formation with the sulfurization process of a MoS 2 region below the absorber layer.As shown in the figure, the spectra measured with the laser spot in this region have two dominant peaks at 377 and 404 cm −1 , which agree with the main vibrational modes reported for MoS 2 . 15In addition, it is possible to observe some weak spectral contributions related to the presence of Cu͑S,Se͒ inclusions.This can be seen in Fig. 8 ͑right͒, which corresponds to the spectrum measured with the laser spot centered at the MoS 2 region ͓spectrum 3 in Fig. 8 ͑left͔͒.These contributions are located at 270 and 372 cm −1 , the last one appearing as a shoulder at the low-frequency side of the 377 cm −1 MoS 2 mode, and correspond to the Se-Se and S-S bands of the A 1g vibration of the covellite structure.The correlation observed between the intensity of these weak modes and that of the intense MoS 2 ones suggests that traces of the Cu͑S,Se͒ phase are located at the intermediate MoS 2 region.
To deepen the in-depth microstructural analysis of these samples, combined Raman/AES measurements have also been performed in the NaCN etched absorber layers.Figure 9 shows the AES compositional depth profiles, as obtained from the spectra corresponding to the main constituents identified in these samples ͑S, Cu, In, and Mo͒, as well as the Raman spectra obtained after different sputtering steps.
The AES profiles presented in Fig. 9 show that after the chemical etching, the composition of the absorber layer is close to stoichiometric CuInS 2 but slightly Cu poor, despite the fact that the precursor was grown under Cu excess conditions.Furthermore, the Raman spectra from the bulk of the absorber layer show the presence of a broad contribution in the 320-360 cm −1 spectral range, which can be related to CuIn 5 S 8 domains. 17The presence of this In-rich secondary phase is in agreement with the slightly Cu-poor composition of the layer.The spectra measured at different sputter times also confirm the existence of an abrupt transition between the absorber layer ͑up to 40 min of sputter time͒ and the MoS 2 region ͑sputter times between 48 and 90 min͒, these last spectra characterized by the main MoS 2 vibrational modes.According to the AES profiles, the main chemical constituents of this region are Mo and S, which agrees with the identification of MoS 2 as the main phase in this region.Another feature of interest is the penetration of the Cu profile into this region, the estimated Cu content being in the range 5-10%.This supports the assignment of the weak Cu͑S,Se͒ modes in the spectra directly measured on the cross section of the samples with the presence of traces of this secondary phase in the MoS 2 intermediate region.
The structural analysis of the absorbers has been completed with XRD 2 / measurements ͑see Fig. 10͒.The XRD spectra also show a drastic improvement in the crystalline quality of the chalcopyrite phase, with it now possible  to resolve the characteristic tetragonal splitting in the peaks ͑200͒/͑004͒, ͑220͒/͑204͒, etc.In addition, weak reflections appear in the spectrum, which cannot be indexed according to either the chalcopyrite or the Mo substrate diffraction patterns.Most of such reflections can be interpreted with the diffraction pattern of the CuIn 5 S 8 spinel, thus supporting the in-depth Raman observations.Moreover, a second group of weak peaks could be indexed as ͑200͒ and ͑400͒ of the CuIn 5 S 9 compound. 18,19No other diffractogram in the JCPDS could explain these reflections.
These results point to the existence of significant differences in the microstructural evolution of the electrodeposited and sulfurized layers in relation to the behavior of films synthesized using vacuum deposition processes.These are related to the absence of a significant contribution of Cu-Au metastable domains in the spectra from the annealed layers, in spite of the In-rich secondary phases present in the absorbers and their final Cu-poor composition.For samples obtained using vacuum deposition processes such as coevaporation, the presence of these secondary phases is accompanied by a degradation of the microstructural quality of the absorbers, concomitant with the existence of CuAu metastable ordered microcrystalline domains. 6,7n the other hand, the final Cu-poor composition of the sulfurized layers is likely related to the presence of OVC Cu-poor domains in the CuInSe 2 precursors, in spite of the Cu and Se excess conditions during their electrochemical growth.These results suggest the existence of a direct relationship between the final microstructure of the sulfurized layers and the binary and ternary phases present in the electrochemically grown nanocrystalline precursor.

IV. CONCLUSIONS
This work describes the detailed Raman scattering and microstructural characterization of S-rich CuIn͑S,Se͒ 2 photovoltaic absorbers produced by sulfurization of electrodeposited CuInSe 2 precursors.
Electrodeposited layers are mainly constituted by nanocrystalline CuInSe 2 grains with chalcopyrite structure.Elemental Se, CuSe, and Cu 2 Se are the most significant secondary phases present in the precursors.Despite the selected Cu and Se excess growth conditions, the Raman spectra from these samples also reveal the presence of OVC domains.The sulfurization process leads to a significant improvement of the crystalline quality of the chalcopyrite phase, as well to an increase of the average crystal size.
The high microcrystalline quality of the synthesized precursors is responsible for the high efficiency reported for solar cells fabricated with these absorbers.This contrasts with the Cu-poor composition of the absorber, which is likely related to the coexistence of Cu-poor OVC domains with the main chalcopyrite phase in the electrodeposited precursors.This likely results in the formation of secondary In-rich phases such as CuIn 5 S 8 and CuIn 5 S 9 , though the OVC phase does not seem to persist after the sulfurization reaction.It was observed also that despite the Cu-poor final composition of the absorbers, no trace of CuAu ordering was found in the resulting CuIn͑S,Se͒ 2 phase.Furthermore, the sulfurization process also leads to the formation of a thick intermediate MoS 2 region.The in-depth detailed analysis shows the presence of a significant Cu content in this region, which has been related to the identification of a secondary Cu͑S,Se͒ phase remaining in the final samples.This, together with the high thickness of the MoS 2 region, is a potential source of lowering of the efficiency of the solar cell devices.
To summarize, the investigated growth process is adequate to produce good-quality chalcopyrite CuIn͑S,Se͒ 2 absorbers.Even though the resulting layers are free of Cu-Au ordered phase, the presence of which has been previously related to the degradation of the optoelectronic properties of photovoltaic absorbers, the coexistence of In-rich secondary phases such as CuIn 5 S 8 , together with the formation of a thick intermediate MoS 2 region with Cu-rich secondary phases, may limit the performance of these samples in photovoltaic devices.

FIG. 5 .
FIG. 5. Micro-Raman spectra obtained with the laser spot focused in two different regions of the surface of a sulfurized precursor.The figure also shows the OM image of the surface of the sample.