A pilot study for a quantitative surface analysis of multi-element alloy films has been performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee for Amount of Substance (CCQM). The aim of this pilot study is to evaluate a protocol for a key comparison to demonstrate the equivalence of measures by National Metrology Institutes (NMIs) and Designated Institutes (DI) for the mole fractions of multi-element alloy films.A Cu(In, Ga)Se-2 (CIGS) film with non-uniform depth distribution was chosen as a representative multi-element alloy film. The mole fractions of the reference and the test CIGS films were certified by isotope dilution-inductively coupled plasma/mass spectrometry. A total number counting (TNC) method was used as a method to determine the signal intensities of the constituent elements acquired in SIMS, XPS and AES depth profiling. TNC method is comparable with the certification process because the certified mole fractions are the average values of the films. The mole fractions of the CIGS films were measured by Secondary Ion Mass Spectrometry (SIMS), Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), X-Ray Fluorescence (XRF) Analysis and Electron Probe Micro Analysis (EPMA) with Energy Dispersive X-ray Spectrometry (EDX). Fifteen laboratories from eight NMIs, one DI, and six non-NMIs participated in this pilot study.The average mole fractions of the reported data showed relative standard deviations from 5.5 % to 6.8 % and average relative expanded uncertainties in the range from 4.52 % to 4.86 % for the four test CIGS specimens. These values are smaller than those in the key comparison CCQM-K67 for the measurement of mole fractions of Fe-Ni alloy films. As one result it can be stated that SIMS, XPS and AES protocols relying on the quantification of CIGS films using the TNC method are mature to be used in a CCQM key comparison.

In the last years steel foil has become an attractive candidate for use as a flexible substrate material for Cu(Inx,Ga1−x)Se2 thin-film solar cells (CIGS). It is stable at the high temperatures present during CIGS processing and is also commercially available. However, one disadvantage is the diffusion of substrate elements into the CIGS layer, which are detrimental for the performance of solar cells. We found a direct correlation between the iron (Fe) content in the CIGS layer and corresponding solar cell parameters. A deep defect related to Fe impurities could be detected by admittance spectroscopy measurements. The solar cell parameters could be well fitted by simulation with recombination at an acceptor like deep defect in the bulk of the CIGS layer. The simulated density of deep defect states correlates nicely with the Fe concentration in the CIGS layer. From this we conclude that Fe replaces an In (or Ga) site in the CIGS lattice and creates an FeIn2+ (or FeGa2+) deep acceptor state in the bulk of CIGS layers, which is detrimental already at a low concentration in the sub ppm range. The simulations enabled us to estimate the maximum Fe concentration in CIGS layers which is tolerable without disturbing the performance of high-efficiency CIGS solar cells.

The aim of this work is to study the effect of Na on the formation of MoSe2 at the absorber/Mo back contact interface of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells at low process temperatures using polyimide foil as substrate material. As reported previously, the presence of Na has been observed to modify the formation of the back interface, which may in part explain the different electronic properties of the completed device, as was determined by admittance spectroscopy and I–V–T measurements. In order to further study this interface formation, break-off experiments are performed and a lift-off technique is used to enable investigation of the different surfaces via X-ray photoelectron spectroscopy and Raman scattering. Both techniques confirm the dependence of the MoSe2 layer formation at the back interface on the presence of Na. The experiments also reveal the relevance of the composition of the absorber layer to the development of the MoSe2 layer during the Cu(In,Ga)Se2 deposition process. Hence this work describes routines that may be employed to develop an “appropriate” CIGSe/Mo back interface for high-efficiency solar cells.

The diffusion behavior of Zn in solar-grade Cu(In,Ga)Se-2 (CIGSe) is found to be similar to that in epitaxial CuInSe2 (CISe), which indicates that grain boundaries only play a minor role as segregation sites and fast-transport pathways. The diffusivity obeys the Arrhenius equation D-Zn = 3.8 x 10(-3)exp(-1.24 eV/k(B)T) cm(2) s(-1). Surprisingly, the Zn-65 diffusion profiles obtained by the radiotracer technique exhibit anomalous shapes with a second maximum near the CI(G)Se-substrate interface. The observations may be indicative of an interstitial-substitutional diffusion mechanism. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745927]

The German joint research project “Chemical Gradients in Cu(In,Ga)(S,Se)2” (GRACIS) is an initiative to gain a better physical understanding concerning the formation of chemical gradients in the Cu(In,Ga)(S,Se)2 (CIGS) absorber layers and at the CIGS/buffer interface. This article presents the current status of the GRACIS project after three years of execution with results concerning phase formation during chalcogenization and coevaporation processes, influence of Ga grading on structural and electrical parameters as well as on inhomogeneities on the μm scale of CIGS. Special features of the CIGS/Zn(O,S) buffer interface are discussed and compared to CIGS solar cells with CdS buffer. In addition, our experimental results are supported by 3D simulations and calculations using screened-exchanged hybrid density functional theory.

When producing slices from Cu(In,Ga)(S,Se)(2) thin films for solar cells by use of a focused ion beam (FIB), agglomerates form on the Cu(In,Ga)(S,Se)(2) surfaces, which deteriorate substantially the imaging and analysis in scanning electron microscopy. Similar problems are also experienced when depth-profiling Cu(In,Ga)(S,Se)(2) thin films by means of glow-discharge or secondary ion mass spectrometry. The present work shows that the agglomerates are composed of (mainly) Cu, and that their formation may be impeded considerably by either cooling of the sample or by use of reactive gases during the ion-beam sputtering. The introduction of XeF(2) during FIB slicing resulted in excellent images, in which the microstructures of most layers in the Cu(In,Ga)(S,Se)(2) thin film stack are visible, including the microstructure of the 20 nm thin MoSe(2) layer. Acquisition of high-quality two-dimensional and also three-dimensional electron backscatter diffraction data was possible. The present work gives a basis for enhanced SEM imaging and analysis not only in the case of Cu(In,Ga)(S,Se)(2) thin films but also when dealing with further material systems exhibiting similar formations of agglomerates.

The charm of Cu(In1−xGax)Se2 (CIGS) thin-film solar cells is their potential for flexible, low-weight modules. Steel foil is an attractive candidate for use as a flexible substrate material as it is stable at the high temperatures involved during CIGS processing and is also commercially available. For monolithic series connection of the single cells to a module, an insulating layer is needed on the conducting steel substrate. We tested enamel layers on low-carbon steel as the insulating layer, as source a of sodium doping, and also as a diffusion barrier layer to suppress the diffusion of detrimental elements from the substrate into the CIGS layer, which is more pronounced on mild steel compared to stainless steel. Using enamelled steel, we could achieve a maximum cell efficiency of 17.6% (certified) and a module efficiency of 15.0% on a 10×10cm2 substrate (aperture area=48cm²) with a high temperature multi-stage inline CIGS process. The open-circuit voltage and short-circuit current density values of cells and modules prepared in the multi-stage CIGS process used here were often higher on enamelled steel than on the standard soda lime glass substrates.

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.

The goal of this work is to investigate the influence of the Na incorporation method into CuIn1-xGaxSe2 (x=Ga/(In+Ga)) (CIGSe)-based solar cells on polyimide (PI) foil. In particular we want to compare the effect of a NaF precursor layer with that of NaF co- and post-deposition. Secondary ion and neutral mass spectroscopies (SIMS/SNMS) are used to study the distribution of the elements through the CIGSe layers. Cross-sectional scanning electron microscopy (SEM) shows the dependence of the absorber microstructure on the method of how Na is supplied with and without Ga present. Adding Ga the device microstructure is generally characterized by smaller CIGSe grains next to the Mo back contact, which indicates the very low process temperature used. The use of a NaF precursor, our standard method for the supply of Na, modifies the growth kinetic of the absorber layer and emphasizes the importance of growth parameters such as the Cu flux and max Cu content during the deposition process. Optimization of the deposition process that uses a NaF precursor, so far led to a max efficiency of 15.9 % (ta = 0.95 cm2). In the case of NaF post-deposition the use of a low process temperature is argued to reduce the Na diffusion throughout the absorber layer, which may reduce the quality of the CIGSe/Mo back interface. This may explain the lower FF observed generally for this process in comparison to that where Na is supplied by a precursor layer.

Cu(In,Ga)Se2 films are used as absorber layers in chalcopyrite thin film solar cells. As the gallium concentration in the absorber can be used to control the band gap, there have been many efforts to vary the gallium concentration in depth to gain an optimum balance of light absorption, carrier collection, and recombination at different depths of the absorber film, leading to improved quantum efficiency. In this study, we investigate the effect of the maximum substrate temperature during film growth on the depth dependent gallium concentration. For the in-depth gallium concentration analyses, we use two techniques, covering complementary depth ranges. Angle dependent soft x-ray emission spectroscopy provides access to information depths between 20 and 470 nm, which covers the depth range of the space charge region, where most of the photoexcited carriers are generated. Therefore, this depth range is of particular interest. To complement this investigation we use secondary neutral mass spectrometry, which destructively probes the whole thickness of the absorber (≈2 μm). The two methods show increasingly pronounced gallium and indium gradients with decreasing maximum substrate temperature. The probing of the complementary depth ranges of the absorbers gives a consistent picture of the in-depth gallium distribution, which provides a solid basis for a comprehensive discussion about the effect of a reduced substrate temperature on the formation of gallium gradients in Cu(In,Ga)Se2 and the device performance of the corresponding reference solar cells.

Sodium (Na) is an important doping element for Cu(In,Ga)Se2 (CIGS) solar cells. However, when using Na-free flexible substrates like steel foil or polyimide film, it is necessary to ensure an efficient supply of sodium to achieve high cell efficiencies. The common incorporation methods for Na on these Na-free substrates are either to deposit a Na-containing precursor layer (e.g. NaF) onto the molybdenum (Mo) back contact prior to CIGS growth or to coevaporate a Na compound during CIGS growth. Another way is to incorporate sodium after CIGS growth by a post-deposition treatment with NaF. In this work, we tested two alternative Na doping methods which are well suited for a production line due to their easy controllability. One approach is to dope the molybdenum target with Na. With Na-doped Mo layers (Mo:Na) as the back contact, we could achieve efficiencies of 13.1% both on titanium (Ti) and stainless Cr steel foil using a single-stage inline CIGS process. With a low-temperature single-stage CIGS process on polyimide (PI) we reached an efficiency of 11.2% using a Mo:Na back contact. Another doping method involves sol–gel-deposited silicon oxide layers which contain Na (SiOx:Na). We have successfully deposited these sol–gel layers onto stainless steel foil by a roll-to-roll (R2R) method with short annealing times as needed in production. With these SiOx:Na layers we could achieve efficiencies of 13.7% on stainless steel foil and 11.5% on mild steel sheet using a single-stage inline CIGS process.

The sodium supply via thermal evaporation of NaF during different stages of a three-stage Cu(In,Ga)Se2 (CIGS) evaporation process has been investigated. Solar cells were processed on soda lime glass with Si3N4 diffusion barrier and on polyimide foils at low substrate temperature of 475°C compatible with the stability of the polyimide foil. Secondary electron micrographs (SEM) of CIGS layers show inhomogeneous microstructure containing regions of small grains near the back contact when sodium is evaporated during the 1st and the 2nd CIGS growth stage, respectively. The CIGS layer structure is affected only to minor extent if sodium is incorporated in the 3rd stage. In order to correlate the layer inhomogeneities with the composition profiles, the CIGS layers were investigated with depth resolved Raman scattering and sputtered neutral mass spectroscopy (SNMS). Both analyzing techniques reveal a strongly graded composition across the CIGS absorber, with an intermediate Ga-poor region and Ga-rich surface and back regions. The performance of resulting solar cells was characterized by means of current-voltage (J-V) and external quantum efficiency (EQE) measurements. It is found that the photovoltaic performance of the cells depends significantly on the NaF incorporation method. Cells developed with a low temperature growth process yielded high efficiencies of up to 16.4% without antireflection coating when NaF was supplied during the 3rd stage of the CIGS growth process.

Diffusion of Fe in solar-grade Cu(In,Ga)Se2 (CIGSe) layers was investigated over the temperature range from 200 to 600 °C. Either natural or radioactive iron was diffused from the front-surface of CIGSe/Mo/soda lime glass samples. Penetration profiles of stable F56e and radioactive F59e were measured by secondary ion mass spectrometry (SIMS) and ion-beam sputtering in conjunction with activity counting, respectively. The Fe diffusivity can be described by the Arrhenius equation D=1.6×10−4 exp(−0.97 eV/kBT) cm2 s−1. Taking into account Fe solubility data obtained from the SIMS profiles, we provide evidence that D is representative of diffusion along grain boundaries.

The objective of this work is to study the influence of the maximum Cu content during the deposition of Cu(In,Ga)Se2 (CIGSe) by multi-stage co-evaporation on the phases present in the final film, the film structure and the electrical properties of resulting solar cell devices. The variation of the composition is controlled by the Cu content in stage 2 of the deposition process. The different phases are identified by Raman spectroscopy. The in-depth Ga gradient distribution is investigated by in-depth resolved Raman scattering and secondary neutral mass spectroscopy. The morphology of the devices is studied by scanning electron microscopy. Efficiencies of 9.2% are obtained for ordered-vacancy-compound-based cells with a Cu/(In+Ga) ratio=0.35, showing the system’s flexibility. This work supports the current growth model: a small amount of Cu excess during the absorber process is required to obtain a quality microstructure and high performance devices.

Cu(In,Ga)Se 2 (CIGSe) thin film solar cells are the most efficient thin film photovoltaic technology available. Deposited onto the appropriate substrate they are potentially flexible, very light, robust and low cost. Due to their excellent radiation hardness and potentially high specific power, they have also attracted interest for use in space applications. Highest quality CIGSe absorber layers are usually grown at temperatures well above 500°C. So far only metal foils are a suitable choice as flexible substrate material in this temperature range. However, as those are conductive, the use of monolithic integration for solar cell interconnection requires an electrically insulating barrier between substrate and solar cell back contact. A non-conductive alternative to metal is polyimide foil. Commercially available polyimide foils are only tolerant to temperatures of up to around 400°C. It is therefore necessary to identify and understand the influence of main process parameters in order to achieve growth of high quality absorber material at these low temperatures. Former work has already highlighted that the amount of sodium present during film growth is a key parameter regarding optimum growth results. The work that is presented here summarizes previous work and investigates the complex relationship between the growth temperature and the effect of Na on the compositional, structural and electronic properties of CIGSe thin films.

In-depth resolved composition inhomogeneities of polycrystalline Cu(In,Ga)Se-2 (CIGS) complex layers for high efficiency solar cells were investigated with Raman scattering measurements. In-depth resolved analysis of the frequency of the main CIGS Raman mode in the spectra measured after sputtering of the layers at different depths lead to identification of different compositions across the layer thickness. These data are in good agreement at both qualitative and quantitative levels with the in-depth resolved composition analysis of the samples by sputtered neutral mass spectroscopy. In addition, Raman measurements also allow detection of additional phases as ordered vacancy compounds.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Steel foil is an attractive candidate for use as a flexible substrate material for Cu(Inx,Ga1−x)Se2 solar cells (CIGS). It is stable at the high temperatures involved during CIGS processing and is also commercially available. Stainless chromium (Cr) steel is more expensive than Cr-free steel sheets, but the latter are not stable against corrosion. We processed CIGS solar cells on both types of substrates. The main problem arising here is the diffusion of detrimental elements from the substrate into the CIGS absorber layer. The diffusion of iron (Fe) and other substrate elements into the CIGS layer was investigated by Secondary Ion Mass Spectrometry (SIMS). The influence of the impurities on the solar cell parameters was determined by current voltage (JV) and external quantum efficiency (EQE) measurements. A direct correlation between the Fe content in the CIGS layer and the solar cell efficiency was found. The diffusion of Fe could be strongly reduced by a diffusion barrier layer. Thus we could process CIGS solar cells with a conversion efficiency of 12.8% even on Cr-free steel substrate.

Diffusion of Fe in CIGS was investigated on solar-grade CIGS layers using radiotracer sputter-profiling of Fe-59 or in-depth secondary ion mass spectrometry (SIMS) of natural iron isotopes. In both cases natural or radioactively labelled iron was deposited on the front-surface of CIGS/Mo/float-glass samples. Fe penetration profiles were measured after isothermal annealing in a lamp oven at different temperatures. A diffusivity of 4 x 10(-13) cm(2) s(-1) was deduced from Gaussian-type SIMS profiles originating from annealing at 300 degrees C. It was found that pronounced sputter-broadening effects may complicate the interpretation of the diffusion profiles. (C) 2008 Elsevier B.V. All rights reserved.

The development of Cd-free buffer layers by vacuum process for Cu(In,Ga)Se2 (CIGS) solar modules becomes even more interesting regarding environmental aspects and the implementation in industrial production. This work presents the latest results of CIGS modules with indium sulfide (In2S3) as buffer layer deposited by the atomic layer chemical vapour deposition technique. A module efficiency close to 13% was realised on the area of 30×30 cm2 (η=12.9%, VOC=27.8 V, FF=72.6%, ISC=0.457 A, aperture area: 714 cm2 and 42 cells). Diffusion processes at the buffer layer interfaces, dependent on deposition temperature and post annealing, have been investigated by X-ray photoelectron spectroscopy, secondary ion mass spectrometry and sputtered neutral mass spectrometry analysis. Diffusion of Cu and Na into the buffer layer and intermixing of S and Se at the In2S3/CIGS interface have been detected.