J. Álvarez-García

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. ...

Comprehensive comparison of various techniques for the analysis of elemental distributions in thin films

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis.The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

Growth process monitoring and crystalline quality assessment of CuInS(Se)2 based solar cells by Raman spectroscopy

Abstract This work reviews the use of Raman scattering for the crystalline quality assessment of CuInS(Se) 2 thin film chalcopyrite materials for photovoltaic applications. The Raman spectra of these semiconductors show a dominant A 1 mode with spectral features sensitive to the microcrystalline quality of the layers. The spectra are also characterised by additional modes, which have been interpreted and modelled as related to the existence of polymorphic domains with Cu–Au ordering. This interpretation is supported by the detailed microstructural analysis of various specimens, including epi- and polycrystalline layers obtained under different stoichiometric regimes leading to significant differences in final solar cell efficiency. The relative intensity of these additional modes in the spectra is proposed for the monitoring and quality assessment of the absorber layers for photovoltaic devices. Besides, identification of the main vibrational modes related to secondary phases characteristic of the different thin film growth processes allows the use of the technique for process monitoring. Moreover, thanks to its non-destructive character, its implementation can be in principle designed at both in situ and ex situ levels.

In-depth resolved Raman scattering analysis of secondary phases in Cu-poor CuInSe2 based thin films

Raman scattering analysis of Cu-poor CuInSe2 layers shows the coexistence of the ordered vacancy compound (OVC), CuAu–CuInSe2 and chalcopyrite (CH) CuInSe2 phases as function of the Cu/In content ratio x. In-depth resolved measurements from layers with x≤0.57 show a strong inhibition in the relative intensity of the CH-CuInSe2 mode at the back region. Micro-Raman spectra directly measured at different regions from the layers with 0.66≤x≤0.71 also suggest a higher content of the OVC phase at this back region. These data suggest an enhancement in the formation of OVC at this region in the layers.

Raman scattering structural evaluation of CuInS2 thin films

Abstract This work reports the spectral characterisation of CuInS 2 (CIS) layers for solar cell devices by Raman scattering. The Raman spectrum from CIS is characterised by a dominant A 1 mode at approximately 290 cm −1 , being the position and width of the mode sensitive to the CIS structural quality. Besides, layers with poor structural quality show the appearance in the spectra of an additional line, not allowed for chalcopyrite crystals, at approximately 305 cm −1 . The relative intensity of this line has been proposed as a further marker for film quality. To deepen in the knowledge of the origin of this mode, Raman scattering measurements are performed in both polycrystalline and epitaxial layers under different conditions, including resonant Raman and different polarisation configurations. The origin of the mode is discussed in terms of (i) the presence of phases with different symmetry as Cu-Au ordering, and (ii) the possible existence of defect-related local vibrational modes.

Combined in-depth scanning Auger microscopy and Raman scattering characterisation of CuInS2 polycrystalline films

Abstract In this work, the combination of in-depth scanning Auger microscopy with Raman microprobe spectroscopy is applied for the detailed microstructural characterisation of CuInS 2 (CIS) thin films. CIS films are used for the fabrication of high efficiency solar cell devices. These films are obtained by sequential sputtering of Cu and In layers on a Mo-coated glass substrate, followed by a sulphurisation step at 500°C in a rapid thermal processing furnace. In order to study this process, samples obtained at intermediate steps are investigated. The obtained data show the formation of the CIS phase already at the first stages of the sulphurisation process, although with a highly disordered structure. Moreover, segregation of CuS towards the surface is observed before sulphurisation is completed. This fact is accompanied by a significant increase of the structural quality of the CIS film, which allows for the fabrication of high efficiency solar cell devices. The performed analysis corroborates the strong complementarity between the used techniques for the detailed microstructural analysis of complex multilayer systems.

Analysis of S-rich CuIn(S,Se)2 layers for photovoltaic applications: Influence of the sulfurization temperature on the crystalline properties of electrodeposited and sulfurized CuInSe2 precursors

This paper reports the microstructural analysis of S-rich CuIn(S,Se)2 layers produced by electrodeposition of CuInSe2 precursors and annealing under sulfurizing conditions as a function of the temperature of sulfurization. The characterization of the layers by Raman scattering, scanning electron microscopy, Auger electron spectroscopy, and XRD techniques has allowed observation of the strong dependence of the crystalline quality of these layers on the sulfurization temperature: Higher sulfurization temperatures lead to films with improved crystallinity, larger average grain size, and lower density of structural defects. However, it also favors the formation of a thicker MoS2 interphase layer between the CuInS2 absorber layer and the Mo back contact. Decreasing the temperature of sulfurization leads to a significant decrease in the thickness of this intermediate layer and is also accompanied by significant changes in the composition of the interface region between the absorber and the MoS2 layer, which bec...

Process monitoring of chalcopyrite photovoltaic technologies by Raman spectroscopy: an application to low cost electrodeposition based processes

This work describes the use and capabilities of Raman scattering for process monitoring and quality control applications in thin film chalcopyrite photovoltaic technologies. Main vibrational modes in the Raman spectra from the chalcopyrite layers are very sensitive to features related to their crystalline quality, chemical composition and the presence of secondary phases that are relevant for the optoelectronic properties of the absorbers and the efficiency of the solar cells and modules. Measurements performed at different process steps allow for the monitoring of the synthesis process of the chalcopyrite layers, giving information directly related to their processing conditions. These techniques have been successfully applied for the monitoring at on-line and in situ (real time) levels of the electrodeposition processes involved in the fabrication of low cost electrochemical based chalcopyrite solar cells. The results obtained corroborate the strong potential of Raman scattering for these applications, and open interesting perspectives on the development of new real time process control strategies.

Quality assessment of chalcopyrite thin films using Raman spectroscopy

The relationship of the linewidth of the Raman A1-mode of CuInS2-based thin films is investigated. Thereby it is assumed that the linewidth is predictive for the crystal quality. Comparison with the parameters of the solar cells formed out of these thin films reveal a direct correlation between the linewidth and solar cell data, such as open circuit voltage and fill factor. A correlation of the linewidth with the crystal sizes as determined by SE images is also found. For small linewidths, a saturation of the solar cell data is found. This behaviour indicates other origins of performance limitation to be present in CuInS2-based devices.

Electrochemical synthesis of CuIn(S,Se)2 alloys with graded composition for high efficiency solar cells

This work reports the electrochemical synthesis and characterization of CuIn(S,Se)2 layers with graded composition from a S-rich surface region toward a Se-rich back region. The process includes the single step electrodeposition of a CuInSe2 precursor followed by a sulfurization step. Cross-sectional Raman microprobe measurements have allowed to characterize the complex graded structure of the layers. These data have been correlated with in-depth Auger electron spectroscopy and x-ray diffraction. Preliminary simulations of solar cells with different kinds of absorbers corroborate the potential of these complex graded layers for the development of low cost high efficiency devices.