Thin-film solar cells based on compound semiconductors consist of a multilayer structure with various interfaces and contain a multitude of elements and impurities, etc. A rapid progress of these photovoltaic technologies can only be achieved by an insight-driven optimization/development. Hence it is crucial to characterize and understand the relationship between the chemical and electronic properties of these components. This paper reviews some examples of our recent work characterizing compound semiconductor thin films using laboratory- and synchrotron-based electron and soft X-ray spectroscopic characterization methods. It is demonstrated how these different analytical techniques are extraordinarily powerful to reveal the material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of the related electronic and chemical properties. As examples, the paper will discuss the electronic surface structure of chalcopyrite thin-film solar cell absorbers, the chemical structure of the CdS/chalcopyrite interface, present the band alignment at the CdS/kesterite interface, and report on how post-deposition treatments cause chemical interaction/interdiffusion processes in CdTe/CdS thin-film solar cell structures. (C) 2012 Elsevier B.V. All rights reserved.

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.

In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the chemical and electronic structure of these components. Because of the high complexity of the Cu_2ZnSn(S,Se)_4 compound semiconductor absorber material alone, this is particularly true for kesterite-based devices. Hence, this paper reviews our recent progress in the characterization of Cu_2ZnSnS_4 (CZTS) thin films. It is demonstrated that a combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the article will be on secondary impurity phases, electronic structure, native oxidation, and the CZTS surface composition.

To optimize materials and devices for solar photoelectrochemical hydrogen production, a detailed understanding of the chemical and electronic properties, in particular at the reactive surfaces and interfaces, is needed. In this review article we will show how electron and soft X-ray spectroscopies can provide such information. We will present exemplary studies using X-ray photoelectron spectroscopy, soft X-ray emission spectroscopy, UV photoelectron spectroscopy, and inverse photoemission. While the first two techniques mainly give insight into the chemical properties at and near the surface, the latter two methods allow us to derive the electronic levels relevant for photoelectrochemical water splitting at the surface of the investigated material. Ultimately, the ideal experiment would be performed in situ, in which the device is studied under working conditions, i.e., in a liquid environment and under illumination. We will give a short outlook on how this can be achieved experimentally under the strict requirements of the measurement environment. (C) 2012 Elsevier B.V. All rights reserved.

ABSTRACTTransition‐metal doped chalcopyrite thin films have been proposed as a suitable absorber material for intermediate band solar cells. In this work, CuGa1−xFexS2 thin films were grown by vacuum co‐evaporation at a substrate temperature of 400 °C with various amounts of incorporated Fe. The optical response of thin films grown on soda‐lime glass was evaluated by transmittance/reflectance measurements. Photovoltaic devices were fabricated from CuGa1−xFexS2 thin films concurrently deposited on Mo‐coated glass substrates using the standard chalcopyrite glass/Mo/absorber/CdS/ZnO device structure. The device characteristics of these solar cells were evaluated by current–voltage and quantum efficiency measurements. For Fe‐containing CuGaS2 films, distinct sub‐gap absorption bands at 1·2 eV and 1·9 eV are detected, which increase in prominence with increasing Fe content. On the other hand, the solar cell parameters were found to deteriorate with increasing iron content, indicating an increase in non‐radiative recombination when high levels of iron are incorporated. However, for the lowest iron content (x = 0·003), an increase in the sub‐gap photoresponse at about 1·9 eV is observed, which is attributed to a combination of sufficient intermediate band absorption and carrier collection at this dilution level. Copyright © 2011 John Wiley & Sons, Ltd.

Solar cell absorber films of Cu(In,Ga)S2 have been fabricated by multi-stage co-evaporation resulting in compositional ratios [Cu]/([In]+[Ga])=0.93–0.99 and [Ga]/([In]+[Ga])=0.15. Intentional doping is provided by sodium supplied from NaF precursor layers of different thicknesses. Phases, structure and morphology of the resulting films are investigated by X-ray diffraction (XRD) and scanning electron microscopy. The XRD patterns show CuIn5S8 thiospinel formation predominantly at the surface in order to accommodate decreasing Cu content. Correlated with the CuIn5S8 formation, a Ga-enrichment of the chalcopyrite phase is seen at the surface. Since no CuS layer is present on the as-deposited films, functioning solar cells with CdS buffer and ZnO window layers were fabricated without KCN etch. The open-circuit voltage of solar cells correlates with the copper content and with the amount of sodium supplied. The highest efficiency cell (open-circuit voltage 738mV, short-circuit current 19.3mA/cm2, fill factor 65%, efficiency 9.3%) is based on the absorber with the least Cu deficiency, [Cu]/([In]+[Ga])=0.99. The activation energy of the diode saturation current density of such a cell is extracted from temperature- and illumination-dependent current-voltage measurements. A value of 1.04eV, less than the band gap, suggests the heterojunction interface as the dominant recombination zone, just as in cells based on Cu-rich grown Cu(In,Ga)S2.

Air-exposed Cu2ZnSnS4 (“CZTS”) thin-film solar cell absorbers have been investigated by surface-sensitive x-ray photoelectron and x-ray-excited Auger electron spectroscopy, as well as by bulk-sensitive energy dispersive x-ray spectroscopy. We find a native surface oxidation of (mainly) tin, but also (to a lesser extent) of zinc and sulfur as well as evidence for a Cu-poor region at the surface of the absorber, best described by a Cu-free Zn-Sn-S surface layer.

The chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers has been investigated by direct and inverse photoemission. Particular emphasis was placed on the impact of KCN etching, which significantly alters the surface composition and is best explained by a preferred etching of Cu and, to a lesser degree, Sn. As a consequence the surface band gap increased from (1.53 ± 0.15) eV, which agrees with optically derived bulk band gap values, to (1.91 ± 0.15) eV.

The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface.

The incorporation of metal impurities M (M=Ti, Fe, or Sn) into CuGaS2 films is investigated experimentally as a function of impurity concentration. Films are synthesized by thermal co-evaporation of the elements onto glass/Mo substrates heated to 400°C–570°C. The compositions of the resulting films are measured by energy-dispersive X-ray spectroscopy and the structures of the present phases are studied by X-ray diffraction. The formation of Cu–M–S ternary phases is observed in a wide range of conditions. Films of Cu–Ga–Ti–S, synthesized at 500°C, show the presence of a cubic modification of CuGaS2 and Cu4TiS4. Alloying of CuGaS2 and tetragonal Cu2SnS3 is observed for substrate temperatures of 450°C. A miscibility gap opens at 500°C and above with separate Sn-rich and Ga-rich phases. Similarly, alloys of CuFeS2 and CuGaS2 are only found in Cu–Ga–Fe–S films synthesized at lower substrate temperature (400°C), whereas at 500°C a miscibility gap opens leading to separate Fe-rich and Ga-rich phases.

Recrystallization processes during the sulfurization of CuInS2 (CIS) thin films have been studied in-situ using energy dispersive X-ray diffraction (EDXRD) with synchrotron radiation. In order to observe the recrystallization isolated from other reactions occurring during film growth, Cu-poor, small grained CIS layers covered with CuS on top were heated in a vacuum chamber equipped with windows for synchrotron radiation in order to analyze the grain growth mechanism within the CIS layer. In-situ monitoring of the grain size based on diffraction line profile analysis of the CIS-112 reflection was utilized to interrupt the recrystallization process at different points. Ex-situ studies by electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDX) performed on samples of intermediate recrystallization states reveal that during the heat treatment Cu and In interdiffuse inside the layer indicating the importance of the mobility of these two elements during CuInS2 grain growth.

The electronic structure of Cu2ZnSnS4 has been investigated by S L-2,L-3 x-ray emission and absorption spectroscopy. An empirical assignment of the most prominent spectral features of the valence and conduction band, respectively, can be made based on a comparison with spectra of corresponding binary compounds and with calculated densities of states. We find that the top of the valence band is dominated by hybridized antibonding Cu 3d-S 3p states, while the bottom of the conduction band is dominated by respective Sn 5s-S 3p states. We also determine the electronic band gap near the Cu2ZnSnS4 surface to be 1.5 (+/- 0.3) eV.

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The electronic surface structure of Mo-incorporated WO3 ("WO3:Mo") is investigated using direct and inverse photoemission and compared to that of pure (Mo-free) WO3. The films are found to be n-type with an electronic surface band gap of 3.27 (+/- 0.15) eV. The conduction band minimum (valence band maximum) is 0.64 (+/- 0.10) eV above [2.63 (+/- 0.10) eV below] the Fermi level and at most 0.38 (+/- 0.11) eV above the H+/H-2 reduction potential [at least 1.66 (+/- 0.11) eV below the H2O/O-2 oxidation potential]. The findings suggest an explanation why WO3:Mo/WO3 bilayer structures show improved photoelectrochemical performance compared to respective single layer photoanodes.

Solar cells based on kesterite‐type Cu2ZnSnS4 (CZTS) were fabricated on molybdenum coated soda lime glass by evaporation using ZnS, Sn, Cu, and S sources. The coevaporation process was performed at a nominal substrate temperature of 550°C and at a sulfur partial pressure of 2–3 × 10−3 Pa leading to polycrystalline CZTS thin films with promising electronic properties. The CZTS absorber layers were grown copper‐rich, requiring a KCN etch step to remove excess copper sulfide. The compositional ratios as determined by energy‐dispersive X‐ray spectroscopy (EDX) after the KCN etch are Cu/(Zn + Sn): 1.0 and Zn/Sn: 1.0. A solar cell with an efficiency of 4.1% and an open‐circuit voltage of 541 mV was obtained. Copyright © 2010 John Wiley & Sons, Ltd.

We monitor the recrystallization of Cu-poor CuInS2 thin films assisted by pure Cu or pure Ag by means of real-time synchrotron-based polychromatic X-ray diffraction. In both cases a new microstructure is formed accompanied by an increase in grain size. In the case of Cu, the onset temperature of the thin-film recrystallization is higher than 370°C. In the case of Ag, the thin-film recrystallization comes to an end at 270°C. The Ag-assisted recrystallization occurs in the presence of the body-centered cubic β-Ag2S phase. We find that domain growth and diffusion of silver into the film occur simultaneously.

Titanium has been incorporated in CuInS2 thin films and devices by diffusion of Ti from the substrate during the CuInS2 co-evaporation growth process. The CuInS2 crystal structure is unaffected but the grain size is reduced compared to Ti-free reference samples. X-ray photoelectron spectroscopy shows the presence of TiO2 at the front of the absorber layer, which appears at the heterojunction interface of the completed CuInS2/TiO2/CdS/ZnO solar cells. Low temperature photoluminescence spectra show no additional transitions that could be assigned to Ti-based impurity defect levels. Ti-containing cells show conversion efficiencies higher than the Ti-free reference cells due to higher open-circuit voltage (efficiency 11%, VOC=731mV). No reduction in short circuit current is detected, indicating that the titanium does not introduce additional bulk recombination. Temperature-dependent current-voltage measurements indicate a reduced interface recombination for cells containing Ti, which is attributed to the TiO2 interlayer detected by X-ray photoelectron spectroscopy.

Different CU2ZnSnS4 (CTZS) thin-film solar cell absorbers have been investigated by bulk-sensitive energy-dispersive spectroscopy (EDS) and surface-near bulk-sensitive soft x-ray emission spectroscopy (XES). While we find a good agreement between the computed Zn/Sn composition ratio based on the EDS and XES data, the XES determined Cu/(Zn+Sn) composition ratio significantly deviates from that based on EDS measurements for some samples. While the first can be explained by a homogenous Zn/Sn composition throughout the CTZS samples, the latter is interpreted as a variation of the Cu depth profile in the respective thin-film solar cell absorbers.