L. Stolt

Uppsala University, Uppsala, Uppsala, Sweden

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Publications (94)176.03 Total impact

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    ABSTRACT: This contribution presents the development of atomic layer deposited (ALD) In2O3 films for utilization as transparent conductive oxide (TCO) layers in Cu(In,Ga)Se2 (CIGSe) solar cells. The effects of ALD process parameters on the morphology and growth of In2O3 are studied and related to the electrical and optical properties of the films. Maintaining similar resistivity values compared to commonly used ZnO:Al (AZO) TCOs (ρ = (5–7) × 10−4 Ωcm), a superior mobility of μ ≈ 110 cm2/Vs could be achieved (more than five times higher than a ZnO:Al reference), which results in a significantly reduced parasitic optical absorption in the infrared region. Application of the optimized In2O3 layers in CIGSe solar cells with varying buffer layers (CdS and Zn1–xSnxOy (ZTO)) leads to a distinct improvement in short circuit current density Jsc in both cases. While for solar cells containing the ZTO/In2O3 window structure, a drop in open-circuit voltage Voc and a deterioration under illumination is observed, the TCO exchange (from AZO to In2O3) on CdS buffer layers results in an increase in Voc without detectable light bias degradation. The efficiency η of the best corresponding solar cells could be improved by about 1% absolute.
    No preview · Article · Jan 2016 · Physica Status Solidi (A) Applications and Materials
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    ABSTRACT: This study highlights the potential of atomic layer deposited In2O3 as a highly transparent and conductive oxide (TCO) layer in Cu(In,Ga)Se2 (CIGSe) solar cells. It is shown that the efficiency of solar cells which use Zn-Sn-O (ZTO) as an alternative buffer layer can be increased by employing In2O3 as a TCO because of a reduction of the parasitic absorption in the window layer structure, resulting in 1.7 mA/cm2 gain in short circuit current density (Jsc). In contrast, a degradation of device properties is observed if the In2O3 TCO is combined with the conventional CdS buffer layer. The estimated improvement for large-scale modules is discussed. Copyright © 2015 John Wiley & Sons, Ltd.
    No preview · Article · Jul 2015 · Progress in Photovoltaics Research and Applications
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    ABSTRACT: Light-induced metastabilities of Cu(In,Ga)Se2 (CIGS)-based thin film solar cells have been extensively researched for many years. It is commonly observed that junction capacitance and p-doping level in CIGS absorbers increase considerably after light soaking (LS). In this work, we focus on the LS behaviors of cells with different minority carrier lifetimes (τn). Experiments show that high efficiency cells with long τn lose open circuit voltage (Voc) and fill factor (FF) upon LS, whereas low efficiency cells with short τn lose less or even gain Voc and FF. The sodium content measured with glow discharge optical emission spectroscopy (GD-OES) increases in the region close to the CdS/CIGS interface with LS and may contribute to the observed LS behaviors. The change in electrical parameters is explained with simulations, which relate the Voc and FF changes to a reduced recombination rate in the space charge region due to the light-induced doping increase. The simulations also suggest that cells with higher n-doping in the CdS are less sensitive to changes in interface recombination rate and doping of CIGS, which agrees with the hypothesis that the CdS buffer deposition is important for the LS behavior.
    No preview · Article · May 2015 · Thin Solid Films

  • No preview · Conference Paper · Jun 2014
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    ABSTRACT: In this paper we present results of the efficiency optimization study performed in our production line for CIGS solar modules. For three different Ga/(Ga+In) ratios two different grading settings were tested. Results indicate high impact of these parameters on efficiency, in the range of 0.5% (abs.). Increasing the Ga content in the film, results in an increase of module voltage. This is almost completely compensated by the decrease of Isc. The resulting change in efficiency is FF driven. The FF increases slightly with higher overall Ga content, but the grading plays a more important role. For the grading setting 1, which was steeper than the setting 2, the FF and consequently efficiency were significantly improved. Optimum Ga content and grading has been defined accordingly.
    No preview · Conference Paper · Jun 2013
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    ABSTRACT: We report a new certified world-record efficiency for thin-film Cu(In,Ga)Se2-based photovoltaic sub-modules of 17.4% (aperture area). The record efficiency of the 16 cm2, monolithically integrated, sub-module has been independently confirmed by Fraunhofer ISE. The record device is the result of extensive co-optimization of all processing steps. During the optimization process, strong focus has been put on the scalability of processes to cost-effective mass production, as reflected, for example, in Cu(In,Ga)Se2 deposition time and substrate temperature. Device manufacturing as well as results of electrical and material characterization is discussed. Copyright © 2012 John Wiley & Sons, Ltd.
    No preview · Article · Nov 2012 · Progress in Photovoltaics Research and Applications
  • E. Wallin · T. Jarmar · U. Malm · M. Edoff · L. Stolt
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    ABSTRACT: The influence of the average Se-to-metal overpressure during three-stage co-evaporation of Cu(InxGa1−x)Se2 solar absorber layers has been investigated. Solar cell devices were fabricated using a baseline process consisting of chemical bath deposited CdS, magnetron sputtered intrinsic and Al-doped ZnO, and e-beam evaporated Ni/Al/Ni current collection grid. For the higher Se-to-metal rate ratios studied, an increased short-circuit current in combination with a decreased fill factor is observed, while the open-circuit voltage stays fairly constant. Based on quantum efficiency measurements, fitting of IV data to a one-diode model, and simulations, we suggest the observed effects to be due to a decreased effective doping in combination with a decreased bulk recombination with higher average Se-to-metal overpressures. This could e.g. be explained by a decreasing number of Se-vacancies or VSe–VCu divacancies with higher Se rate. For the range of lower average Se-to-metal rate ratios studied, device performance drops due to a decreased open-circuit voltage and, for the lowest Se rate investigated, fill factor. In addition to electrical characterization, the effects on absorber microstructure are discussed based on results obtained from X-ray diffraction and scanning electron microscopy.
    No preview · Article · Aug 2011 · Thin Solid Films
  • Source
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    ABSTRACT: IntroductionMaterial PropertiesDeposition Methods Junction and Device FormationDevice OperationManufacturing IssuesThe Cu(InGa)Se2 OutlookReferences
    Full-text · Chapter · Feb 2011
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    ABSTRACT: The effect of deposition pressure and controlled oxygen dosing on the diffusion barrier performance of thin film Ta to Cu penetration was investigated. In-situ resistivity, Auger compositional profiling, scanning electron microscopy and cross-sectional transmission electron microscopy were used to determine the electrical, chemical and structural changes that occur in Cu/Ta bilayers on Si upon heating. A 20 nm Ta barrier allowed the penetration of Cu at temperatures ranging from 320 to 630°C depending on processing conditions. Barrier failure temperature is dependent upon the deposition pressure and oxygen contamination at the Ta/Cu interface. This indicates the importance of understanding how deposition conditions affect diffusion barrier performance.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
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    ABSTRACT: Thin films of YBCO were prepared using a mass-spectrometer controlled coevapora-tion/MBE process with atomic oxygen. Under the conditions of low pressure, necessary for mass-spectrometer rate control, it is shown that an atomic oxygen beam source can be utilized in order to grow high quality thin films of YBCO as well as multi-layer structures involving YBCO and Y2O3. Values of Tc = 88.5 K and Jc = 6–106 A/cm2 at 77 K were determined for a strip with a width of 10 μm of YBCO deposited on a LaAlO3 substrate. Film structure is analyzed by XRD and rocking curve measurements. Magnetic characterization of films and multi-layer structures are reported.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
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    ABSTRACT: Thin film solar cells with the structure soda lime glass/Mo/Cu(In,Ga)Se2/Zn(O,S)/ZnO/ZnO:Al are studied for varying thickness and sulfur content of the Zn(O,S) buffer layer. These Zn(O,S) layers were deposited by atomic layer deposition (ALD) at 120 °C. Devices with no or small concentrations of sulfur in the buffer layer show low open-circuit voltages. This is explained by the cliff, or negative conduction-band offset (CBO), of −0.2 eV measured by photoelectron spectroscopy (PES) and optical methods for the Cu(In,Ga)Se2 (CIGS)/ZnO interface. Devices with ZnS buffer layers exhibit very low photocurrent. This is expected from the large positive CBO (spike) of 1.2 eV measured for the CIGS/ZnS interface. For devices with Zn(O,S) buffer layers, two different deposition recipes were found to yield devices with efficiencies equal to or above reference devices in which standard CdS buffer layers were used; ultrathin Zn(O,S) layers with S/Zn ratios of 0.8–0.9, and Zn(O,S) layers of around 30 nm with average S/Zn ratios of 0.3. The sulfur concentration increases towards the CIGS interface as revealed by transmission electron microscopy and in vacuo PES measurements. The occurrence of this sulfur gradient in ALD‐Zn(O,S) is explained by longer incubation time for ZnO growth compared to ZnS growth. For the Zn(O,S) film with high sulfur content, the CBO is large which causes blocking of the photocurrent unless the film is ultrathin. For the Zn(O,S) film with lower sulfur content, a CBO of 0.2 eV is obtained which is close to ideal, according to simulations. Efficiencies of up to 16.4% are obtained for devices with this buffer layer.
    No preview · Article · Aug 2006 · Journal of Applied Physics
  • Marika Edoff · Lina Malmberg · Ulf Malm · Lars Stolt
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    ABSTRACT: CdS films were deposited with chemical bath deposition (CBD) onto Cu(In,Ga)Se<sub>2</sub>/Mo/soda-lime glass structures. The basic ingredients for the CBD were ammonia (NH<sub>4</sub>OH), cadmium acetate (CdAc), and thiourea CS(NH<sub>2</sub>)<sub>2</sub>. Two recipes with very low thiourea concentration were compared to the baseline recipe. From quantum efficiency measurements we observe an increased carrier collection for longer wavelengths for the modified buffer recipes as compared to baseline. The higher carrier collection is explained by an increase in the depletion region width. A lower bandgap is observed for the modified buffer. The solar cells with modified buffer layers have lower voltages and fill factors than the solar cells with our baseline buffer, leading to a lower efficiency for the modified cell structures in spite of slightly higher current densities. A tentative explanation is given assuming an inverted CIGS layer at the CIGS/buffer interface, possibly with Cd as a donor element
    No preview · Conference Paper · Jun 2006
  • O. Lundberg · M. Edoff · L. Stolt
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    ABSTRACT: In this paper the effect of an in depth variation of the Ga/(In+Ga) ratio in GIGS based thin film solar cells is presented. The conclusions made are based on a review of earlier publications, theoretical considerations and results from a large set of new devices. For standard devices with normally thick CIGS films (1.5-2 mu m) deposited at a relatively long deposition time (60 min) an improved efficiency of around 0.4% units for the devices with an increased Ga/(In+Ga) ratio towards the back contact is observed. This improvement is due to a field assisted carrier collection resulting in an improved QE response at long wavelengths. When the CIGS thickness is reduced the importance of the increased Ga/(In+Ga) ratio towards the back contact is enhanced and at a GIGS thickness of 0.5 mu m a gain of 2.5% units is obtained. The gain is due to an improved V proportional to and FF. The main reason for the improvement is passivation of the back contact, which becomes increasingly detrimental for the device performance as the CIGS thickness is reduced. Also for pure CIS a significant improvement of the device performance is obtained by introducing an increased Ga concentration towards the back contact. This improvement is, however, more related to the introduction of Ga itself than the gradient of the Ga-concentration. From many simulations the largest gain is predicted for an increased Ga/(In+Ga) ratio towards the GIGS surface. However, neither in the literature nor from our own experiments we can find evidence for an improved device performance due to an increased Ga-concentration towards the GIGS surface. (c) 2004 Elsevier B.V. All rights reserved.
    No preview · Article · Jun 2005 · Thin Solid Films
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    ABSTRACT: CuIn1−xGaxSe2 (CIGS) and CuInSe2 (CIS) thin-film solar cells, with ZnO buffer layers deposited by Atomic Layer Deposition (ALD), are examined with respect to dominant recombination path. They are compared with reference cells with CdS buffer layers. The principal method of examination is temperature-dependent J–V characterization (J(V)T), and the analysis of the J(V)T data has been modified in order to more reliably discern the dominant recombination path.Compared to the CIS cells with the traditional CdS buffer layer, the CIS cells with ALD–ZnO buffer layer exhibit the same dominant recombination path, i.e., recombination in the bulk of the absorber. For the CIGS cells (with [Ga]/([Ga]+[In])=0.3), however, the analysis of the cells with ALD–ZnO buffer points to dominant interface recombination, while the CdS buffer cells are dominated by bulk recombination.For CIGS, the difference between the recombination in ALD–ZnO and CdS cells is consistent with the negative conduction band offset found by photoelectron spectroscopy in these ALD–ZnO cells in a previous study. This offset leads to increased interface recombination.For CIS/ALD–ZnO, it was previously found that there is no negative conduction band offset since the conduction band minimum of the absorber is lower. Consistently there is no difference in dominant recombination path between ALD–ZnO buffer cells and traditional CdS buffer cells.
    Preview · Article · Jun 2005 · Thin Solid Films
  • J. Sterner · J. Malmström · L. Stolt
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    ABSTRACT: The formation of the interface between In2S3 grown by atomic layer deposition (ALD) and co-evaporated Cu(In,Ga)Se2 (CIGS) has been studied by X-ray and UV photoelectron spectroscopy. The valence band offset at 160°C ALD substrate temperature was determined as −1·2±0·2 eV for CIGS deposited on soda-lime glass substrates and −1·4±0·2 eV when a Na barrier substrate was used. Wavelength dependent complex refractive index of In2S3 grown directly on glass was determined from inversion of reflectance and transmittance spectra. From these data, an indirect optical bandgap of 2·08±0·05 eV was deduced, independent of film thickness, of substrate temperature and of Na content. CIGS solar cells with ALD In2S3 buffer layers were fabricated. Highest device efficiency of 12·1% was obtained at a substrate temperature of 120°C. Using the bandgap obtained for In2S3 on glass and a 1·15±0·05 eV bandgap determined for the bulk of the CIGS absorber, the conduction band offset at the buffer interface was estimated as −0·25±0·2 eV (−0·45±0·2 eV) for Na-containing (Na-free) CIGS. Copyright © 2005 John Wiley & Sons, Ltd.
    No preview · Article · Apr 2005 · Progress in Photovoltaics Research and Applications
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    ABSTRACT: A reactively sputtered ZrN reflector layer on top of the conventional Mo back contact yields enhanced absorber/back contact reflectance in Cu(In,Ga)Se2 thin film solar cells. Improved long wavelength quantum efficiency is demonstrated with a ZrN reflector at a Cu(In,Ga)Se2 thickness of 0.5 μm. The optical gain with respect to a standard Mo back contact is initially offset by increased back contact recombination and contact resistance, but these electronic losses can be suppressed by Ga grading of the absorber or by inclusion of a contact layer of MoSe2. This allows for a significantly improved power conversion efficiency of devices with sub-micron Cu(In,Ga)Se2 thickness.
    No preview · Article · Sep 2004 · Applied Physics Letters

  • No preview · Article · Jan 2004
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    ABSTRACT: The purpose of this investigation was to model the thermodynamic behavior of a Cu(In,Ga)Se2 sample during the growth process and especially during the conversion from copper-rich to copper-poor material in a two-step process. Starting from a very simple model of a directly heated substrate the model was refined until it qualitatively and quantitatively explained the features observed in the real experiment. The results can be used to determine more accurate criteria for the endpoint detection in a real-world evaporation system.
    Full-text · Article · Jan 2004
  • M. Igalson · M. Bodegård · L. Stolt
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    ABSTRACT: The reversible persistent changes of the fill factor (FF) induced by the illumination and voltage bias along with changes in the electronic properties of the ZnO/CdS/Cu(In,Ga)Se2 photovoltaic devices have been studied. Admittance spectroscopy and capacitance–voltage characterization reveal a correlation between the FF and the space charge distribution within the absorber. Our experiments provide evidence that a major source of FF loss in efficient devices is caused by excess negative charge close to the interface. We explain the persistent changes in the net acceptor concentration in the interface region by the relaxation effects due to compensating donors—the same mechanism, which leads to metastable changes of the doping level in the bulk of the absorber.
    No preview · Article · Oct 2003 · Solar Energy Materials and Solar Cells
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    M. Igalson · M. Bodegård · L. Stolt
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    ABSTRACT: Minority carrier traps in the absorber layer of the ZnO/CdS/Cu(In,Ga)Se2 photovoltaic devices have been investigated by use of deep level transient spectroscopy. In the efficient baseline structures a recombination process involving deep electron trap is revealed in the presence of blue light introducing holes from the buffer into absorber. A large capture cross-section for minority carriers and high concentration exceeding net acceptor concentration suggest that this trap plays a significant role as a recombination center in these devices. Its specific features indicate that it might also be a center involved in the metastable phenomena characteristic for these devices. Another deep electron trap, observed in the cells of inferior performance, has been investigated by double pulse DLTS. We conclude, that its concentration and values of capture cross-sections for holes and electrons are too low to account for the low efficiency of these structures.
    Preview · Article · Sep 2003 · Journal of Physics and Chemistry of Solids

Publication Stats

2k Citations
176.03 Total Impact Points

Institutions

  • 1995-2011
    • Uppsala University
      • Department of Engineering Sciences
      Uppsala, Uppsala, Sweden
  • 1996
    • Alimetrics Ltd, Espoo, Finland
      Helsinki, Uusimaa, Finland
  • 1993-1994
    • KTH Royal Institute of Technology
      Tukholma, Stockholm, Sweden
  • 1990-1991
    • IBM
      Armonk, New York, United States