L. Stolt

Uppsala University, Uppsala, Uppsala, Sweden

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Publications (86)157.91 Total impact

  • [Show abstract] [Hide abstract]
    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.
    Progress in Photovoltaics Research and Applications 11/2012; 20(7). · 7.71 Impact Factor
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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.
    Thin Solid Films 08/2011; 519(21):7237–7240. · 1.87 Impact Factor
  • M P Ali, P A Tove, L Stolt
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    ABSTRACT: Measurements of electron and hole barriers of junctions between evaporated Fe films and Si both of p- and n-type are reported. From a study of the forward I-V characteristics, we obtain a value for the hole barrier of Bh ≈ 0.50 ± 0.03 eV using the p-type diodes and a value of Be ≈ 0.62 ± 0.03 eV using the n-type diodes. The sum of the barrier heights comes close to the bandgap, as expected.
    Physica Scripta 12/2006; 24(2):408. · 1.03 Impact Factor
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    ABSTRACT: The diffusion model and the combined thermionic-emission-diffusion model for metal-semiconductor junctions have been used in combination with a modified Gummel-De Mari algorithm to obtain one-dimensional, numerical two-carrier solutions, for silicon Schottky diode structures. Solutions for some diode functions vs. voltage, current density, generation-recombination current injection ratio, minority carrier injection ratio, stored charge, and differential capacitance were obtained. For some applied voltages some diode functions vs. positions were calculated: the band diagram including the quasi-Fermi levels, electron and hole carrier density, the total hole current density, the hole drift current density, the electron drift current density and the net charge density. The parameters that have been varied are the barrier height, the carrier life-times, the doping concentration and the length of the diode structure. The results have been compared to and correlate with previous investigations by Vaitkus and Green & Shewchun. A comparison between the diffusion and the combined diffusion-thermionic emission model have been done. Standard methods for determining structure parameters as the 1n I vs. V plot, the Norde plot and the 1/C2 vs. V plot have been applied to the numerical results. From this plot the ideality factor, series resistance, barrier height, doping concentration have been determined and the results have been compared to the originally given parameters. The various models result in significant differences (a) on barrier height dependence of injected minority carrier current density, and (b) on the differential resistance at zero bias voltage. The results have also been compared with fabricated silicide and silicon Schottky diodes.
    Physica Scripta 12/2006; 24(2):456. · 1.03 Impact Factor
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    ABSTRACT: A comparison of contact resistivity for platinum silicide contacts and sputtered chromium contacts to heavily doped n and p silicon has been done, by a method which is suitable for contacts on a highly conductive surface layer on a less conductive base material. Good statistics was obtained by using 50 separate structures on each wafer. The results showed less spreading and lower specific contact resistance for PtSi contacts on both n+ and p+-Si, than for Cr contacts which on p+ showed a non-ohmic behaviour.
    Physica Scripta 12/2006; 24(2):405. · 1.03 Impact Factor
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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.
    Journal of Applied Physics 08/2006; 100(4):044506-044506-9. · 2.21 Impact Factor
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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
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
  • O. Lundberg, M. Edoff, L. Stolt
    Thin Solid Films 06/2005; · 1.87 Impact Factor
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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.
    Progress in Photovoltaics Research and Applications 04/2005; 13(3):179 - 193. · 7.71 Impact Factor
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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.
    Thin Solid Films 01/2005; · 1.87 Impact Factor
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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.
    Applied Physics Letters 09/2004; 85(13):2634-2636. · 3.79 Impact Factor
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    ABSTRACT: Thin films of Cu(In,Ga)Se2 are grown by a co-evaporation process in which the In, Ga, and Se fluxes, as well as the substrate temperature, are constant and the only variable is the Cu flux. This Cu flux varies in three steps in such a way that the growing film evolves from Cu-poor to Cu-rich and then back to Cu-poor. The film growth is monitored by the ‘end point detection’ method, and film thicknesses of the order of 2 μm are deposited in less than 20 min. Quality devices (efficiencies above 15%) are produced in our baseline processes for all of the other synthesis steps. The Cu(In,Ga)Se2 layers are studied from a (112) versus (220) (204) orientation and recrystallization point of view. Including the results from a previous study on the influence of the substrate temperature to the present X-ray diffraction and scanning as well as tunneling electron microscopy data, a five-stage growth model for the films is described. The specific features of these films are that they are weakly (220) (204) oriented and exhibit crevices in their top fractions. The growth model hypothesizes about the origins of these crevices and on how to avoid them. Copyright © 2003 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 07/2003; 11(5):319 - 331. · 7.71 Impact Factor
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    ABSTRACT: We have analyzed the potential for light trapping in Cu(In,Ga)Se/sub 2/ cells. Key quantities such as back contact reflectance and reflectance at the absorber-window interface are discussed and calculated from measured optical properties. Two model cases, perfectly specular interfaces and Lambertian scattering from the absorber front and back surfaces, are compared in terms of integrated AM 1.5 absorption in the absorber as a function of absorber thickness d/sub a/ for Ag, TiN, Mo back reflectors. Relative to the Mo reflector in the specular model, the potential gain of an Ag reflector at d/sub a/ = 0.5 /spl mu/m is estimated to 2.0 and 3.5 mA/cm/sup 2/ in the specular and scattering cases, respectively. Improved long wavelength quantum efficiency is experimentally demonstrated with a TiN back reflector. The resulting gain in J/sub sc/ is 0.8 mA/cm/sup 2/ at 0.45 /spl mu/m absorber thickness, in accordance with our model results.
    Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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    ABSTRACT: Cd-free solar cells based on Cu(In,Ga)Se/sub 2/, with efficiencies of up to 16.0%, are achieved by replacing the (CBD)CdS layer with a Zn(O,S)/ZnO bilayer deposited by ALD. Problems with reproducibility of the device results are observed and are found to be correlated with thickness variations of the Zn(O,S) layer, probably induced by differences of the CIGS surface. An ultra-thin Zn(O,S) layer, possibly not completely covering the CIGS surface, is observed by XPS analysis for high efficiency devices. Degradation of fill factor is observed on some, but not all, Zn(O,S) devices after about 2 months of storage, but these devices recover after light-soaking at elevated temperature. In order to improve the reproducibility, a new Zn(O,S) process is developed that includes longer ALD pulses. Using this new process, a promising average result of 12.1% (without AR coating) for 10 consecutive ALD runs is obtained with a maximum spread of /spl plusmn/1% unit.
    Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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    ABSTRACT: Thin-film solar cells with Cu(In,Ga)Se2 (CIGS) absorber layers ranging from 1.8 to 0.15 μm in thickness were fabricated by co-evaporation, with both homogeneous and Ga/(Ga + In) graded composition. The absorption of the CIGS layers was determined and compared with corresponding QE measurements in order to obtain the optical related losses. The material characterization included XRD as well as cross-sectional SEM analysis. Devices with CIGS layers of all thicknesses were fabricated, and down to 0.8–1 μm they showed a maintained high performance (η ∼ 15%). When the CIGS layer was further reduced in thickness the loss in performance increased. The main loss was observed for the short-circuit current, although the loss was not only due to a reduced absorbance. The open-circuit voltage was essentially not affected by the reduction of the CIGS thickness, while the fill factor showed a slight decrease. The fill factor loss was eliminated by introducing a Ga/(Ga+In) graded CIGS, which also resulted in an increased open-circuit voltage of 20–30 mV for all CIGS thicknesses. Device results of 16.1% efficiency at 1.8 μm CIGS thickness, 15.0% at 1.0 μm and 12.1% at 0.6 μm (total area without anti-reflective coating) were achieved. Copyright © 2002 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 02/2003; 11(2):77 - 88. · 7.71 Impact Factor
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    ABSTRACT: In this contribution we give an overview of the mechanisms behind degradation of Cu(In,Ga)Se2-based modules. Based on the results from a detailed analysis of power losses in modules, prior to and after extended damp heat exposure, we discuss to what extent modules can be designed to achieve enhanced long-term performance. For conventional modules, we show that the stability can be improved by optimizing the interconnect and the front contact. Furthermore, we argue that gridded modules are better from a long-term performance point of view. A novel interconnect structure, specifically designed for long-term durability, is briefly discussed.
    Solar Energy Materials and Solar Cells 01/2003; · 5.03 Impact Factor
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    ABSTRACT: In this study, we have decreased the deposition time from 60 min down to 3.75 min for three sets of Cu(In,Ga)Se2 (CIGS) layers: baseline CIGS (≈2 μm thick and homogeneous composition), Ga-graded CIGS (≈2 μm thick) and Ga-graded thin CIGS (≈1 μm thick). The CIGS layers were fabricated with co-evaporation and analysed with a scanning electron microscope and X-ray diffraction. The complete devices were analysed with I–V and quantum efficiency. By reducing the deposition time from 60 to 3.75 min, the efficiency was reduced from 14.7% down to 12.3% (for the baseline CIGS). This reduction is explained by increased recombination, which correlates with decreased grain size for CIGS layers with shorter deposition times. In order to decrease the deposition time, with a maintained high efficiency, a 1.8–2 μm thick CIGS layer with a Ga-gradient is shown to be the best alternative down to 7.5 min deposition time, at which a CIGS layer, resulting in a 14.6% efficient solar cell, was fabricated. At 3.75-min deposition time the efficiency was improved when the CIGS thickness was reduced from 2 μm down to 1 μm.
    Thin Solid Films 01/2003; · 1.87 Impact Factor
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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.
    Journal of Physics and Chemistry of Solids 01/2003; 64:2041-2045. · 1.53 Impact Factor
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    ABSTRACT: Thin film Cu(In,Ga)Se2 is grown using constant substrate temperatures and deposition times of less than 20 min for film thicknesses of 2 μm. In the growth process used, referred to as the CUPRO process, only the Cu flux varies, in such a manner that the film evolves from Cu-poor to Cu-rich to Cu-poor. The evolution of the film is monitored by tracking the change in the thermal response of the substrate. Films and devices are produced using a given evolution of the Cu content and their quality and characteristics are correlated to the substrate temperature, between 475 and 550 °C. The films are analysed by scanning electron microscopy and X-ray diffraction (XRD), the devices by current–voltage and quantum efficiency (I(V) and QE(λ)) measurements. Morphologically, the bottom fraction of the layers, grown prior to the film becoming Cu-rich, does not appear to be very temperature dependent, whereas differences are observed in the upper fraction, which result from the Cu-rich and back to Cu-poor stages of the growth. From the device parameters a shift in the bandgap of the Cu(In,Ga)Se2 with substrate temperature is found. The XRD study shows (2 2 0)(2 0 4) orientation of the films grown at the lower temperatures. Total area device efficiencies up to 15.5%, without AR, have been obtained, without large impact of the substrate temperature.
    Thin Solid Films 01/2003; · 1.87 Impact Factor
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    ABSTRACT: The electronic properties of the interface region in the ZnO/CdS/Cu(In,Ga)Se2 devices have been investigated in the various metastable states induced by voltage bias and illumination. Capacitance spectroscopy has been used to gain information about the level spectrum in the interface region of absorber and space-charge distribution within the structures. The results of capacitance spectroscopy are analyzed in conjunction with the current–voltage characteristics. We have differentiated between the metastable effect due to the changes of the space-charge distribution in the absorber and a process involving the persistent changes of the Fermi-level position at the interface. We attribute the first one to the electronic processes due to relaxing donor-type VSe centers. The second one in our opinion involves the process of forming a quasi-equilibrium between the positive and negative charges in the immediate vicinity of the interface. In the admittance and DLTS spectra of interface levels a signal belonging to bulk donors (most probably to InCu defects) has been identified.
    Thin Solid Films 01/2003; 431:153-157. · 1.87 Impact Factor

Publication Stats

585 Citations
157.91 Total Impact Points

Institutions

  • 1983–2006
    • Uppsala University
      • Department of Engineering Sciences
      Uppsala, Uppsala, Sweden
  • 1996
    • Universität Stuttgart
      • Institute of Physics
      Stuttgart, Baden-Württemberg, Germany
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States
  • 1994
    • KTH Royal Institute of Technology
      Tukholma, Stockholm, Sweden
  • 1990–1991
    • IBM
      Armonk, New York, United States
  • 1989
    • Stanford University
      Palo Alto, California, United States
  • 1982
    • Tel Aviv University
      Tell Afif, Tel Aviv, Israel