Publications (97)32.73 Total impact
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Article: In situ analysis of Ga accumulation during Cu(In,Ga)S2 synthesis by combined evaluation of synchrotron X-ray fluorescence and diffraction
xxxxx. 01/2013; -
Article: CdS/Cu(In,Ga)S2 based solar cells with efficiencies reaching 12.9% prepared by a rapid thermal process
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ABSTRACT: In this letter, we report externally confirmed total area efficiencies reaching up to 12.9% for CdS/Cu(In,Ga)S2 based solar cells. These are the highest externally confirmed efficiencies for such cells. The absorbers were prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor. Structural, compositional, and electrical properties of one of these champion cells are presented. The correlation between the Ga distribution profile and solar cell properties is discussed.Progress in Photovoltaics Research and Applications 01/2013; 21:88-93. · 5.79 Impact Factor -
Article: 12.6% efficient CdS/Cu(In,Ga)S2-based solar cell with an open circuit voltage of 879mV prepared by a rapid thermal process
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ABSTRACT: A Cu(In,Ga)S2-based solar cell with a confirmed efficiency of 12.6% together with an open circuit voltage of 879 mV, prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor, is reported. The performance of the new cell is superior to those obtained previously with multi-source evaporated absorbers. We show that by carefully adjusting the temperature profile, good absorber properties could be transferred from a long process to a rapid thermal process. The improved efficiency is due to an appropriate degree of gallium diffusion toward the surface, which could be achieved despite the short sulfurization time. Absorber and solar cell characteristics are presented.Solar Energy Materials and Solar Cells 01/2011; 95(3):864-869. · 4.54 Impact Factor -
Article: In situ monitoring the growth of thin-film ZnS/Zn(S,O) bilayer on Cu-chalcopyrite for high performance thin film solar cells
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ABSTRACT: This paper highlights the crucial role that the control of the chemical bath deposition (CBD) process plays for buffer production of Cu-chalcopyrite solar-cell devices. ZnS/Zn(S,O) bilayer was deposited on CuInS2 (CIS) and Cu(In,Ga)(SSe)2 (CIGSSe) and monitored using turbidity measurements of the solution. The results were correlated to the X-ray photoemission spectra of the samples obtained by interruption of the process at sequential stages. Two different feature regimes were distinguished: In the first stage, a heterogeneous reaction takes place on the absorber resulting in the formation of pure ZnS. The second stage of the process is homogeneous, and the in-situ turbidity measurement shows a loss in the transmission of light through the CBD solution. The measured ZnL3M45M45 Auger-peaks, during this second stage of the process, show a shift of the kinetic energy from pure ZnS to a solid-solution ZnS/ZnO (“Zn(S,O)”) with decreasing amount of sulfur. These results are supported by the observations from Energy-filtered transmission electron microscopy. This paper also demonstrates that monitoring of the CBD process combined with the basic understanding using surface and interface analysis have contributed to improve the reproducibility and to enhance the photovoltaic performance of Cu-chalcopyrite thin-film solar modules.Thin Solid Films 01/2009; · 1.89 Impact Factor -
Article: Combined analysis of spatially resolved electronic structure and composition on a cross section of a thin film Cu In1 xGax S2 solar cell
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ABSTRACT: In this work we present a combination of two spatially resolved experimental methods to analyse correlations be tween distributions of element concentrations and elec tronic structure of cross sections of thin film semiconductor devices. We applied these methods to a cross section of a thin film Cu In1 xGax S2 CdS ZnO solar cell. The electronic structure was analysed by measuring the work function of the cross section by Kelvin probe force microscopy. To determine the gallium concentration of the cross section of the Cu In1 xGax S2 layer we performed energy dispersive X ray analysis EDX . We were able to match the spatial coordinates of these measurements. By this we could observe a correlation between the gallium concentration of the Cu In1 xGax S2 layer and its work function. The EDX meas urements show that the Cu In1 xGax S2 layer features a two layer structure one layer with a low gallium concentration x amp; 8776; 0 and one at the back of the solar cell with a high gal lium concentration x amp; 8776; 1 . Furthermore we discuss the influ ence of the two layer structure of the Cu In1 xGax S2 absorber layer on the photovoltaic properties of the solar cell. The spectral quantum efficiency was measured on the same sam ple before the depth resolved data were taken. We observe an improvement of short circuit current and overall efficiency of the Cu In1 xGax S2 CdS ZnO solar cell compared to a gallium free CuInS2 CdS ZnO solar cell.physica status solidi (a) 01/2009; 206:1017-1020. · 1.21 Impact Factor -
Chapter: Performance and Transport Properties of Phthalocyanine:Fullerene Organic Solar Cells
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ABSTRACT: Copper phthalocyanine (CuPc)-fullerene (C60) photovoltaic cells are produced by organic vapour phase deposition reaching efficiencies of 3%. The electronic transport properties of the devices are investigated as a function of the CuPc:C60 absorber blend layer composition and its preparation temperature. The analysis of the transport properties of the devices employs the one-diode model. It is shown that the dominant recombination process takes place at the donor-acceptor interfaces of the CuPc and C60 absorber domains. The activation energy of recombination is related to the effective band gap of the blend layer.12/2008: pages 195-198; -
Article: Sulphurisation of gallium-containing thin-film precursors analysed in-situ
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ABSTRACT: It has been demonstrated that rapid thermal sulphurisation of sputtered Cu/In precursor layers is suitable for industrial production of thin film photovoltaic modules. The process is relatively straightforward and the underlying fundamental aspects, such as phase formation sequence and reaction rates, have been studied intensively. Using lab-scale preparation technology, incorporation of gallium is known to improve transport properties of the absorber and to enable the fabrication of wide-gap cells. In this work we have used energy dispersive in-situ X-ray diffraction to study the sulphurisation of sputtered Cu:Ga/In precursors. It is the basis for the future development of an industrially feasible production of Cu(In,Ga)S2 films. Precursor stacking sequence and sulphur partial pressure in relation to precursor temperature have been varied. In many cases, in particular when establishing sulphur partial pressure already at low precursor temperature, we observe a severe reduction of reaction rates after going from pure Cu to Cu:Ga in the precursor. In consequence, single phase films cannot be prepared within the feasible ranges of time and temperature. Adhesion failure and at least intermediate formation of CuIn5S8 are other problems frequently encountered. In spite of these problems, promising pathways to single phase Cu(In,Ga)S2 films prepared from sputtered Cu:Ga/In precursors have now been identified.Thin Solid Films 01/2007; 515:5934-5937. · 1.89 Impact Factor -
Article: TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations
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ABSTRACT: Simple soft-solution method has been developed to synthesize films and powders of TiO2 and mixed TiO2–SiO2 at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO2, we used aqueous acidic medium containing TiOSO4 and H2O2, which results in a peroxo-titanium precursor while colloidal SiO2 has been added to the precursor for the formation of TiO2–SiO2. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO2 (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO2–SiO2 mixed films and powders, TiO2 (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO2 and mixed TiO2–SiO2 films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO2 films. These compact films are highly photoactive when TiO2 is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO2 colloids are added into the solution.Solar Energy Materials and Solar Cells. 01/2006; -
Conference Proceeding: Towards thin film module production using a simplified CIS process
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ABSTRACT: A pilot production for large area thin film solar modules based on CuInS<sub>2</sub> (CIS) is operational in Berlin. The targeted product is a substrate type glass-glass module of 65 W<sub>p</sub> power output. The production line consists of an inline work flow including five deposition steps. It has a projected output of up to 5 MW<sub>p</sub>. The technology is optimized for lowest complexity: sputtering techniques are used for all thickness-defining process steps preventing homogeneity issues on large areas. Rapid thermal processing is applied for compound formation avoiding an elongated tempering line. A minimized number of utilized materials and wide process windows characterize the preparation process. Analyzing the direct manufacturing costs it is shown that a level of 1.50 €/W<sub>p</sub> is feasible in industrial production already at a capacity of 5 MW<sub>p</sub>/a.Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005 -
Conference Proceeding: Improved CuGaSe2-based solar cell performance by In-S surface treatments
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ABSTRACT: CuGaSe<sub>2</sub> (CGSe) thin films for photovoltaic applications have been subjected to surface treatments based on In-S by means of chemical vapor deposition. Structural and electronic characterization of as-grown films and processed devices show the effective incorporation of In and S in the near-surface region of CGSe thin films and a positive impact on the solar cell performance.Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005 -
Article: Wide band gap p-type windows by CBD and SILAR methods
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ABSTRACT: Chemical deposition methods, namely, chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) have been used to deposit wide band gap p-type CuI and CuSCN thin films at room temperature (25 °C) in aqueous medium. Growth of these films requires the use of Cu (I) cations as a copper ions source. This is achieved by complexing Cu (II) ions using Na2S2O3. The anion sources are either KI as iodine or KSCN as thiocyanide ions for CuI and CuSCN films, respectively. The preparative parameters are optimized with the aim to use these p-type materials as windows for solar cells. Different substrates are used, namely: glass, fluorine doped tin oxide coated glass and CuInS2 (CIS). X-ray diffraction, scanning electron microscopy, atomic force microscopy and optical absorption spectroscopy are used for structural, surface morphological and optical studies, and the results are discussed.Thin Solid Films 01/2004; · 1.89 Impact Factor -
Conference Proceeding: Preparation of CuGaSe/sub 2/ solar cells and their optimization
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ABSTRACT: First Page of the ArticlePhotovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003 -
Conference Proceeding: Charge carrier transport in polycrystalline CuGaSe2 thin films
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ABSTRACT: The mechanism of charge carrier transport in stoichiometric polycrystalline CuGaSe<sub>2</sub> has been studied in the temperature range of 80-350 K using Hall effect and conductivity measurements. The layers were grown on soda-lime glass substrates by physical vapour deposition. At 300 K we found mobility values in the range of 10-20 cm<sup>2</sup>/Vs. A thermally activated behavior of the mobilities in these films was observed in the temperature range of 150-300 K. Intergrain potential barriers limiting the charge carrier transport were determined to vary between 60 and 130 meV. Using the grain boundary barrier trapping model developed for polycrystalline Si the density of charged states at the grain boundaries was calculated to be about 1.2e12 cm<sup>-2</sup>. This value does not depend on the net doping concentration of the samples significantly, suggesting that the position of the relevant defect states at the grain boundaries is above the Fermi level. Above 300 K the carrier transport is limited not only by potential barriers but also the in-grain mobility.Photovoltaic Specialists Conference, 2002. Conference Record of the Twenty-Ninth IEEE; 06/2002 -
Conference Proceeding: Cd-free CIGSSe solar cells and minimodules
World Renewable Energy Congress VII, Cologne, Germany; 06/2002 -
Conference Proceeding: n-ZnSe/p-GaAs heterojunction solar cells
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ABSTRACT: For PV applications, epitaxial layers of n-ZnSe were grown by metal-organic vapour phase epitaxy (MOVPE) at 340°C on GaAs(001) substrates. n-type net carrier concentration in the range between 10<sup>17</sup>-10<sup>19</sup> cm<sup>-3</sup> was demonstrated. By increasing the doping concentration, the double crystal X-ray diffraction full width at half maximum of 1.2 μm thick ZnSe-layers increased from 400" to 600". A first n-ZnSe/p-GaAs solar cell achieved an open circuit voltage of 706 mV, a fill factor of 65% and a short circuit current density of 10 mA cm<sup>-2</sup> (total area, ELH-lamp, 100 mW cm<sup>-2</sup>, no AR-coating). J<sub>SC</sub> was increased by 4 mA cm<sup>-2</sup> by depositing sputtered n<sup>+</sup>-ZnO on the n-ZnSe layer. In order to improve the PV performance further, n-ZnSe will be grown on GaAs buffer layers, the doping profile will be optimised systematically and MgF<sub>2</sub> will be deposited on the ZnO layerPhotovoltaic Specialists Conference, 1997., Conference Record of the Twenty-Sixth IEEE; 11/1997 -
Article: Role of side groups in pyridine and bipyridine ruthenium dye complexes for modulated surface photovoltage in nanoporous TiO2
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ABSTRACT: The influence of molecular side groups on the electron backtransfer into ruthenium-dye molecules adsorbed on ultra-thin nanoporous TiO2 layers has been investigated by modulated surface photovoltage spectroscopy in argon atmosphere. All ruthenium-dye molecules contained one bipyridine (bpy) ligand with two carboxylic groups for anchoring at the TiO2 surface. Ruthenium (II) complexes with a second bpy ligand with branching and non-branching side groups containing C and H only and ruthenium (II) complexes with a second pyridine (py) ligand with side groups containing different numbers of –C–O–C– sequences were synthesized. The electron backtransfer was practically not affected by branching or non-branching side groups in the bpy–bpy complexes. In contrast, electron backtransfer was much less for bpy–py complexes in comparison to bpy–bpy complexes and increased strongly with increasing number of –C–O–C– sequences in the side groups.Solar Energy Materials and Solar Cells 94(4):686-690. · 4.54 Impact Factor -
Article: CVD of CuGaSe2 for thin film solar cells with various transport agents
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ABSTRACT: Chemical vapor deposition (CVD) in an open tube system was employed to deposit single-phase CuGaSe2 thin films on plain and Mo-coated glass substrates. The use of HCl and ternary CuGaSe2 source material resulted in non-stoichiometric volatilization of the source material. The use of binary source materials – Cu2Se and Ga2Se3 – in combination with I2 and HCl as the respective transport agents yielded single-phase CuGaSe2 thin films while the source materials were volatilized stoichiometrically. Mo/CuGaSe2/CdS/ZnO devices were fabricated from these samples exhibiting an open-circuit voltages up to Voc=853 mV.Solar Energy Materials and Solar Cells 67:105-112. · 4.54 Impact Factor -
Article: Nanoscale investigations of the electronic surface properties of Cu(In,Ga)Se2 thin films by scanning tunneling spectroscopy
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ABSTRACT: In this work we investigate the electronic surface properties of polycrystalline Cu(In,Ga)Se2 thin films by locally resolved scanning tunneling spectroscopy (STS). From current imaging tunneling spectroscopy (CITS) maps of an area of we observe distinct granular inhomogeneities, where current–voltage (I(U)) spectra differ from grain to grain and vary between metallic and semiconducting characteristics. Due to the high density of defect states at the Cu(In,Ga)Se2 surface, the metallic I(U) characteristics is not surprising. In the case of the semiconducting I(U) characteristics, we suggest a preferential oxidation of particular grains, which passivates defect levels at the surface. This is supported by the presence of gallium and indium oxides detected by global X-ray photoelectron spectroscopy. Furthermore, we recorded I(U) spectra from different grains under supra band gap laser illumination, which always show semiconducting characteristics. This behavior can be explained by a saturated occupation of defect states by photoexcited charge carriers. By evaluating differential conductance (dI/dU) spectra under illumination from various grains, we estimate the average surface band gap to and compare the valence band onset with results from macroscopic ultraviolet photoelectron spectroscopy. The high lateral resolution of our CITS data allows also to study electronic properties at grain boundaries, which are discussed with regard to a recent STS study on a non-oxidized sample.Graphical AbstractGranular inhomogeneities of a Cu(In,Ga)Se2 thin film observed using current imaging tunneling spectroscopy on an area of 2×2 μm2at various tunneling voltages U. In this work we discuss lateral inhomogeneities, effects at grain boundaries and I(U) tunneling spectra under supra band gap laser illumination.Solar Energy Materials and Solar Cells 95(6):1537-1543. · 4.54 Impact Factor -
Article: Photoluminescence and sub band gap absorption of CuGaSe2 thin films
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ABSTRACT: A detailed study on the optical properties of polycrystalline thin film CuGaSe2 as a function of chemical composition has been carried out using photoluminescence and photothermal deflection spectroscopy. The CuGaSe2 thin films were grown by a novel open-tube chemical vapor deposition process. This method is suitable for the preparation of films within a wide range of composition. Luminescence due to shallow defect levels can be directly compared with the photoluminescence spectra of MOCVD-grown epitaxial CuGaSe2 thin films. In addition, we have observed a broad excitonic luminescence peak 31±2 meV below the band gap correlated with a donor–acceptor pair-like luminescence transition at approximately 1:29±0:01 eV. By a detailed analysis of the binding nature of the exciton, we were able to identify an acceptor-like defect band at 243±20 meV, presumably caused by Ga-vacancies in slightly Cu-rich films. The composition dependence of the luminescence intensities is correlated with composition-dependent sub band gap absorption measurements performed by photothermal deflection spectroscopy. These data show shallow defect structure-dependent Urbach-tail energies (varying from 25±2 to 35±2 meV) as well as variations of defect absorption far below the band edge. By comparing the photoluminescence data with the absorption data, we could attribute this defect absorption to the same acceptor-like defect band.Thin Solid Films · 1.89 Impact Factor -
Article: CuGaSe2 thin films prepared by a novel CCSVT technique for photovoltaic application
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ABSTRACT: A novel chemical close-spaced vapor transport (CCSVT) technique has been developed for the growth of the CuGaSe2 (CGSe) thin films on areas as large as 10×10 cm2. Cu precursors deposited on clean and Mo-coated soda lime glass substrates are thermally and chemically treated under gaseous GaClx/H2Se atmosphere in the CCSVT cell. The Ga2Se3 employed as source material is stoichiometrically volatilised at 550 °C by a controlled amount of HCl/H2 agent. Single phase CGSe thin films are prepared with a growth rate of 230–240 nm/min by using a single stage process. A two-stage process is applied for the fine tuning of the CGSe composition and electronic properties appropriate for the subsequent solar cell preparation. Film characterisation including X-ray diffraction measurements, scanning electron microscopy observations, X-ray fluorescence analysis and elastic recoil detection analysis has been carried out. An 8.7% active area efficient ZnO/CdS/CuCaSe2 solar cell under AM1.5 conditions has been achieved.Thin Solid Films.
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2013
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Helmholtz-Zentrum Berlin
Berlin, Land Berlin, Germany
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