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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. [ABSTRACT FROM AUTHOR]

The surface Cu-depletion of chalcopyrite thin films and its influence on the interface properties of related solar cells have been subject of a controversial debate for many years. Although the nature of this Cu-depletion and its extension in depth are crucial for the device physics, there are only a few contradictory experimental results that address this topic. To clarify this issue, we performed depth-dependent compositional analysis by angle dependent soft x-ray emission spectroscopy (AXES) on Cu(In,Ga)Se2 thin films with different integral Cu-contents. By considering depth profiles from literature and by taking the accuracy of AXES into account, our numerical AXES simulations predict a pronounced angle dependence for our samples. However, our experimental data show only a minor angle dependence, which leads to the conclusion that the Cu-depleted surface layer must be restricted to a very thin surface layer, which is not accessible by AXES. This conclusion is consistent with the result from our previous investigation by hard x-ray photoelectron spectroscopy, where we found a Cu-depleted surface layer in the subnanometer regime. Consequently the present study gives further experimental evidence for the surface reconstruction model proposed by first-principles calculations. Supported by secondary neutral mass spectroscopy, we show that the minor angle dependence in our AXES data can be attributed to a Ga-gradient in the chalcopyrite material. [ABSTRACT FROM AUTHOR]

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin-film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. After identifying the deposited Zn compound, as ZnS/Zn(S,O) bilayer buffer in former investigations [M. Bär et al., J. Appl. Phys. 99, 123503 (2006)], this time the focus lies on potential diffusion/intermixing processes at the buffer/absorber interface possibly, clarifying the effect of the heat treatment, which drastically enhances the device performance of respective final solar cells. The interface formation was investigated by x-ray photoelectron and x-ray excited Auger electron spectroscopy. In addition, photoelectron spectroscopy (PES) measurements were also conducted using tunable monochromatized synchrotron radiation in order to gain depth-resolved information. The buffer side of the buffer/absorber heterointerface was investigated by means of the characterization of Zn(S,O)/ZnS/CIS structures where the ZnS/Zn(S,O) bilayer buffer was deposited successively by different deposition times. In order to make the (in terms of PES information depth) deeply buried absorber side of the buffer/absorber heterointerface accessible for characterization, in these cases the buffer layer was etched away by dilute HClaq. We found indications that while (out-leached) Cu from the absorber layer forms together with the educts in the chemical bath a [Zn(1-Z),Cu2Z]S-like interlayer between buffer and absorber, Zn is incorporated in the uppermost region of the absorber. Both effects are strongly enhanced by postannealing the Zn(S,O)/ZnS/CIS samples. However, it was determined that the major fraction of the Cu and Zn can be found quite close to the heterointerface in the buffer and absorber layer, respectively. Due to this limited (in the range of one monolayer) spatial extent, these “diffusion” mechanisms were rather interpreted as a chemical bath deposition induced and heat-treatment promoted Cu-Zn ion exchange at the buffer/absorber interface. Possible impacts of this intermixing on the performance of the final solar cell devices will also be discussed. [ABSTRACT FROM AUTHOR]

Zn(O,OH) layers deposited by the ion layer gas reaction (ILGAR) technique have the potential to replace the conventionally used CdS buffer layer in Cu(In(1-X)GaX)(SYSe(1-Y))2-based thin-film solar cells. To avoid stability issues, the fraction of metastable Zn(OH)2 should be reduced in the final buffer layer. However, hydroxide-poor or -free ZnO “buffers” result in noncompetitive devices. We have therefore investigated the impact of different oxide/hydroxide ratios on the electronic band alignment at the absorber/buffer heterointerface. The surface composition as well as the position of the valence-band maximum (VBM) of respective ILGAR-Zn(O,OH) samples was determined by photoelectron spectroscopy. The position of the conduction-band minimum (CBM) was estimated using optical band gaps determined from optical reflection/transmission measurements. From the comparison of these VBM and CBM values with the respective values of the absorber surface, predictions are made in terms of valence- and conduction-band offsets at the crucial absorber/buffer interface. The results are compared with previous findings, and the drawn conclusions are correlated with the performance of respective solar cell devices. [ABSTRACT FROM AUTHOR]

Nanocrystalline ZnS thin layers are prepared by ion layer gas reaction, a low cost deposition method for compound semiconductor thin films. They are investigated by optical spectroscopy, x-ray fluorescence and energy dispersive x-ray analysis. It is seen that the absorption edges are shifting towards lower photon energies with increasing layer thickness. Also, the amount of unconverted precursor salt is decreased. A simulation procedure for fitting the determined absorption edges of these layers is introduced. This leads to particle size distribution functions showing an increasing broadening with increasing layer thickness. Moreover, the distribution maximum is moving to larger crystallite radii with larger film thickness. Both results indicate a growth in crystallite size during the layer deposition. Additionally, the increase in strength of the phonon-photon-coupling confirms the conclusions of transmission electron micrographs. Later investigations show that the noncrystalline precursor matrix which surrounds the ZnS crystallites is vanishing with increasing layer thickness. Therefore, the nanocrystallites are more frequently in contact with each other. This again favors the propagation of phonons. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)(S,Se)2- (“CIGSSe”) based solar cells with a ZnO layer deposited by the ion layer gas reaction (ILGAR) method yield superior efficiencies (15.0%) than the references with a chemical bath-deposited CdS buffer (14.1%). However, this high performance is only reached if the absorber is pretreated in a Cd2+- and aqueous ammonia-containing bath prior to the ILGAR-ZnO deposition. The photovoltaic as well as the dark device parameters are strongly influenced by this treatment. Scanning and transmission electron microscopy (TEM) as well as x-ray diffraction measurements reveal a different morphology and structure of ILGAR-ZnO layers on top of Cd2+/NH3-treated and on as-deposited absorbers, indicating a considerably modified absorber surface. By energy dispersive x-ray analysis in the TEM, Cd could only be identified at the ILGAR-ZnO/Cd2+/NH3-treated-CIGSSe interface of the respective cross sections, if the absorber was treated in a bath with an atypically high Cd2+-concentration. In this case a Cd-containing thin layer between ZnO and CIGSSe was observed in TEM images. [ABSTRACT FROM AUTHOR]

A simple model is introduced which determines the optical band-gap energy Eg for penternary Cu(In(1-X)GaX)(SYSe(1-Y))2 (CIGSSe) alloys from its Ga/(Ga+In) ratio as well as from its S/(S+Se) ratio. In order to verify the model the depth dependent composition of a CIGSSe sample was revealed by elastic recoil detection analysis. Applying the model, the concentration profiles were transferred in an Eg profile. Finally, these values were compared with optical band-gap energies, which were obtained directly by independent characterization methods. [ABSTRACT FROM AUTHOR]

The chemical and electronic structure of the interface between liquid water and a CuIn(S,Se)[sub 2] thin film surface was studied with synchrotron-based, high energy-resolution soft x-ray emission spectroscopy (XES). By probing the local environment around the sulfur atoms, an x-ray-induced sulfate formation at the CuIn(S,Se)[sub 2] surface can be monitored, correlated with a substantial enhancement of sodium impurity atoms from the CuIn(S,Se)[sub 2] film and its glass substrate. The results demonstrate that, with XES, an experimental probe is available to in situ study chemical reactions at liquid–solid interfaces or at surfaces in a high-pressure gas environment in a chemically sensitive and atom-specific way. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nanocrystalline CdS thin layers can be prepared using the ion layer gas reaction (ILGAR), a low-cost deposition technique for compound semiconductor thin films. We investigated the layers by optical spectroscopy and transmission electron microscopy, and found that lower CdC1[sub 2] precursor concentrations and thicker films favor larger nanoparticles and a lower band gap energy, whereby the concentration effect is more pronounced. The nucleation mechanism of the CdC1[sub 2] precursor seems to determine the particle size and the probability of its growth. A minimum CdS particle diameter of 4.7 nm has been extrapolated, which corresponds to an optical band gap energy of 2.83 eV, as a result of quantum confinement. ILGAR enables deposition of nanoparticles of controlled size. [ABSTRACT FROM AUTHOR]

The formation of buffer-free charge carrier selective Mg–Ag cathodes for donor-copper phthalocyanine (CuPc):acceptor-C60 bulk heterojunction organic solar cells is investigated. An optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag3Mg. In addition, Mg diffusion into CuPc:C60 layer is observed. As a result, an interaction between Mg and Cu2+ with a concurrent change in oxidation state of both metals takes place. However, no formation of magnesium phthalocyanine is observed. [ABSTRACT FROM AUTHOR]

Single crystalline and polycrystalline CuInS2 samples prepared by different methods are characterized by Raman spectroscopy. The measured spectra are fitted according to the phonon confinement model. Correlation lengths were obtained, which correspond to the size of domains of perfect crystallinity. These correlation lengths are in good agreement with distances between twin defects observed by transmission electron microscopy in polycrystalline CuInS2. Additionally, the strain present in the samples was determined from the Raman spectra. A tensile strain was obtained for the polycrystalline CuInS2 thin films, which agrees well with published values for the same material. [ABSTRACT FROM AUTHOR]

Recently, Cd-free Cu(In,Ga)(S,Se)2-based “CIGSSe” thin film solar cells with a nominal In2S3 buffer layer deposited by the spray ion layer gas reaction technique resulted in photovoltaic performances comparable to that of CdS buffered references. In the past it was argued that diffusion processes across the In2S3/CIGSSe interface play a significant role for the device quality. Investigating the interface formation by using x-ray photoelectron spectroscopy, the authors were able to confirm a strong interfacial diffusion involving Cu and Na from the CIGSSe. [ABSTRACT FROM AUTHOR]

CuGaSe2 (CGSe)-based high-gap thin-film solar cells have to date not reached their potential level of electrical performance. In order to elucidate possible shortcomings of the electronic interface structure, we have studied the initial stage of the CdS/CGSe interface formation by use of a simple Cd2+/NH3 treatment. As in the case of low-gap chalcopyrites, we find a Cd-containing surface layer, in the present case comprised of approximately one monolayer of CdSe. The results indicate that the CdS/CGSe interface is not abrupt, but contains intermediate layers. Furthermore, they shed light on possible surface modification schemes to enhance the overall performance of high-gap CGSe chalcopyrite solar cells. [ABSTRACT FROM AUTHOR]

We report the direct observation of a photoinduced oxidation process at the buried buffer/absorber interface in high-efficiency Zn(O,OH)/Cu(In,Ga)(S,Se)2 thin-film solar cell structures by means of x-ray emission and photoelectron spectroscopy. We propose a reaction mechanism that involves the decomposition of a hydroxide compound in the buffer layer into water and an oxide and present evidence that this process also occurs with visible light excitation and after accelerated lifetime tests of nonencapsulated devices. This suggests a possible photoinduced aging effect in solar cell devices with other hydroxide containing buffer layers or under humid conditions. [ABSTRACT FROM AUTHOR]

The surface modifications induced by treating Cu(In,Ga)(S,Se)[sub 2] films in an aqueous ammonia hydroxide-based solution of Cd[sup 2+] ions—as used in record Cu(In,Ga)(S,Se)[sub 2] solar cells without a CdS buffer layer—have been investigated for different Cd[sup 2+] concentrations. Employing a combination of x-ray photoelectron spectroscopy, Auger electron spectroscopy, and x-ray emission spectroscopy, it is possible to distinguish two different surface modifications. For Cd[sup 2+] concentrations below 4.5 mM in the solution we observe the formation of a CdS monolayer, while higher Cd[sup 2+] concentrations lead to the additional deposition of a cadmium hydroxide film on the CdS/Cu(In,Ga)(S,Se)[sub 2] surface. [ABSTRACT FROM AUTHOR]

Zinc oxide films prepared by the recently developed ion layer gas reaction (ILGAR) technique and by rf magnetron sputtering are compared with respect to their linear optical properties. Spectral ellipsometry as well as reflectance and transmittance measurements in the UV-visible-near infrared range were employed to deduce the complex refractive index. The band gap energy, Urbach energy, and packing density were determined. The results are compared to literature data of single crystals and thin films. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Single crystalline and polycrystalline CuInS2 samples prepared by different methods are characterized by Raman spectroscopy. The measured spectra are fitted according to the phonon confinement model. Correlation lengths were obtained, which correspond to the size of domains of perfect crystallinity. These correlation lengths are in good agreement with distances between twin defects observed by transmission electron microscopy in polycrystalline CuInS2. Additionally, the strain present in the samples was determined from the Raman spectra. A tensile strain was obtained for the polycrystalline CuInS2 thin films, which agrees well with published values for the same material. [ABSTRACT FROM AUTHOR]