D. Brémaud

École Polytechnique Fédérale de Lausanne, Lausanne, VD, Switzerland

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Publications (14)23.05 Total impact

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    ABSTRACT: Development of a Cu(In,Ga)Se2 thin film solar cell on a polyimide film with a conversion efficiency of 17.1%, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process. Copyright © 2011 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 01/2011; 19(5):560 - 564. · 7.71 Impact Factor
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    ABSTRACT: For the development of high efficiency Cu(In,Ga)Se<sub>2</sub> (CIGS) flexible solar cells on polymer films with efficient and robust deposition process the so-called three-stage evaporation processes were modified. CIGS layers were grown by co-evaporation of constituent elements at low substrate temperature compatible with the stability of the polyimide film. The CIGS layers grown with our standard process are compositionally graded, especially the Ga has a strong grading profile across the layer thickness. Different variations of the standard evaporation process were investigated, especially in view of changing the energy band gap profiles in the absorber layer for achieving further improvements in high efficiency solar cells. Further on, the incorporation of sodium during the CIGS deposition was optimized. Structural and chemical composition properties of CIGS layers were characterized with SEM, XRF, SIMS and the photovoltaic properties were characterized with I-V and quantum efficiency measurements. A flexible CIGS solar cell on polyimide film with a record efficiency of 17.6% has been developed.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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    ABSTRACT: Solar cells based on polycrystalline Cu(In,Ga)Se<sub>2</sub> (CIGS) absorber layers have shown high potential for low cost photovoltaic energy conversion. Our group has achieved 18.1% efficient cells on glass substrates and 14.1% on flexible polyimide foils without antireflection (AR) coating. These results were achieved by applying a three-stage evaporation process with common growth rates of about 35 nm/min. Aim of this study was to enhance the growth rates of layers deposited at low substrate temperature (450°C) suitable for polyimide foils and to investigate structural properties of CIGS layers and photovoltaic properties of solar cells. It was found that very high deposition rates of 500 nm/min during the 2nd and 3rd stage of the evaporation process are feasible, maintaining high absorber quality and performance of solar cells.
    Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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    ABSTRACT: CuIn1 − xGaxSe2 (CIGS) solar cells show a good spectral response in a wide range of the solar spectrum and the bandgap of CIGS can be adjusted from 1.0 eV to 1.7 eV by increasing the gallium-to-indium ratio of the absorber. While the bandgaps of Ga-rich CIGS or CGS devices make them suitable for top or intermediate cells, the In rich CIGS or CIS devices are well suited to be used as bottom cells in tandem solar cells. The photocurrent can be adapted to the desired value for current matching in tandem cells by changing the composition of CIGS which influences the absorption characteristics. Therefore, CIGS layers with different [Ga]/[In + Ga] ratios were grown on Mo and ZnO:Al coated glass substrates. The grain size, composition of the layers, and morphology strongly depend on the Ga content. Layers with Ga rich composition exhibit smaller grain size and poor photovoltaic performance. The current densities of CIGS solar cells on ZnO:Al/glass varied from 29 mA cm− 2 to 13 mA cm− 2 depending on the Ga content, and 13.5% efficient cells were achieved using a low temperature process (450 °C). However, Ga-rich solar cells exhibit lower transmission than dye sensitized solar cells (DSC). Prospects of tandem solar cells combining a DSC with CIGS are presented.
    Thin Solid Films 02/2009; · 1.87 Impact Factor
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    ABSTRACT: Cu(In,Ga)Se2 (CIGS) solar cells on aluminum foils offer the advantage to be flexible, lightweight and, because of the low cost substrate, can be used for several applications, especially in buildings, where aluminum is already commonly used. There are reports of a-Si solar cells on Al foil, but to our knowledge development of CIGS solar cells on Al foils has not been reported. We have developed CIGS solar cells on coated Al-foil samples. When using Al as substrate, CIGS layers of suitable structural and opto-electronic properties should be grown at low (< 450 °C) deposition temperatures, because of the difference in the thermo-physical properties of layers and substrates. We have grown CIGS layers by evaporation of elemental Cu, In, Ga, and Se at different substrate temperatures and investigated the properties of these CIGS layers by different methods (SEM, SIMS, and EDX). The photovoltaic properties of small area solar cells were characterized with I–V and quantum efficiency measurements. An efficiency of 6.6% has been achieved. We have also observed that some Al from the foil dissolves during chemical bath deposition (CBD) of CdS. The presence of Al in the bath seems, in some cases, to be beneficial for the electrical properties of the CIGS solar cells. Thinner and more homogenous CdS layers are obtained. Elastic Recoil Detection Analysis (ERDA) and SIMS measurements have shown incorporation of Al in the CdS.
    Thin Solid Films 01/2007; 515(15):5857-5861. · 1.87 Impact Factor
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    ABSTRACT: In order to replace chemical bath deposited (CBD) CdS buffer layers in Cu(In,Ga)Se2 (CIGS) solar cells by an alternative material, InxSy thin-film buffer layers were prepared by ultrasonic spray pyrolysis at various substrate temperatures. X-ray Diffraction measurements confirmed that the films contained primarily the tetragonal In2S3 phase. X-ray Photoelectron Spectroscopy measurements revealed a small concentration of chlorine impurity throughout the InxSy layer. By depositing the indium sulphide layer as buffer layer in the CIGS solar cell configuration, a maximum solar cell efficiency of 8.9% was achieved, whilst the reference cell with CdS/CIGS on a similar absorber exhibited 12.7% efficiency. Additionally, light soaking enhanced the efficiency of InxSy/CIGS cells primarily by improvements in fill factor and open circuit voltage.
    Thin Solid Films 01/2007; · 1.87 Impact Factor
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    ABSTRACT: Multijunction stacked (tandem) solar cells can increase the overall photovoltaic conversion efficiency by optimal utilization of the solar spectrum in individual cells. We demonstrate that a photovoltaic tandem cell comprising a nanocrystalline dye-sensitized solar cell as a top cell for high-energy photons and a copper indium gallium selenide thin-film bottom cell for lower-energy photons produces AM 1.5 solar to electric conversion efficiencies greater than 15%.
    Applied Physics Letters 06/2006; · 3.79 Impact Factor
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    ABSTRACT: Incorporation of a small amount of sodium into Cu(In,Ga)Se2 (CIGS) absorbers for thin-film solar cells is well known to enhance conversion efficiencies. Usually, Na is added in a way such that it is present during CIGS growth and therewith influences the growth kinetics. We have used post-deposition Na in-diffusion into as-grown, Na-free absorbers and observed typical efficiency improvements. This suggests that in general the main positive Na effect originates from changes in the electronic absorber properties rather than from modification of CIGS growth kinetics. At low substrate temperatures, Na impedes CIGS phase formation. This may explain why absorbers grown at substrate temperatures below 450 °C in the presence of Na yield inferior cells compared with post-deposition-treated CIGS. We have developed post-deposition Na incorporation for the processing of flexible CIGS solar cells on polyimide substrates. A conversion efficiency of 14.1% under AM1.5 standard test conditions was independently measured. This represents the highest reported efficiency of any solar cell grown on a polymer substrate to date.
    Journal of Applied Physics 03/2005; 97(8):084903-084903-5. · 2.21 Impact Factor
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    ABSTRACT: Development of Cu(In,Ga)Se<sub>2</sub> (called CIGS) solar cells on polymers is challenging because of the thermo-physical properties of layers and substrates. CIGS layers of suitable structural and opto-electronic properties should be grown at low temperature (< 500°C) as polyimides tend to degrade at higher deposition temperatures. Additionally, a method for controlled incorporation of an optimum amount of Na in CIGS is needed for high-efficiency cells since polyimides do not contain Na. Solar cells were developed on commercially available Upilex foils. CIGS layers were grown by evaporation of elemental Cu, In, Ga and Se at different substrate temperatures. Na from a NaF film was incorporated into CIGS layers with a post-deposition diffusion method that is suitable for in-line production of solar cells. Independent measurements have confirmed 14.1% efficiency under simulated AM1.5 standard test conditions. This is the highest efficiency reported to date for any kind of solar cell grown on polymer films. An average reflectance loss of about 13% was measured for these cells. Application of a commonly used anti-reflection coating would enable more than 15% efficiency flexible CIGS solar cells on polyimide foils.
    Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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    ABSTRACT: Incorporation of sodium into Cu(In,Ga)Se2 (CIGS) absorber layers and low-temperature absorber growth processes are important issues for the processing of flexible CIGS solar cells, particularly when polymer substrates are used. Na present during growth of CIGS is known to influence the growth kinetics and to lead to structurally and electronically modified absorber material. With post-deposition Na incorporation, predominantly electronic properties are altered. We compared the performances of solar cells processed with Na diffusing from the glass substrate during growth with those of cells with post-deposition Na incorporation and thus obtained an indication of the significance of Na-induced modifications of CIGS growth for cell efficiency. When the absorbers were grown at low substrate temperatures (below 500 °C), we found that absorbers with post-deposition-incorporated Na performed better. This is attributed to an impeding influence of Na on CIGS phase formation, which became most apparent when the growth temperature was 370 °C. With higher substrate temperatures (around or above 500 °C), the performance of cells grown in the presence of Na became similar or superior, which might indicate a beneficial influence of Na on CIGS growth once a certain growth temperature is exceeded. With post-deposition Na incorporation, a cell efficiency of 13.8% was achieved at 400 °C substrate temperature.
    Thin Solid Films 01/2005; · 1.87 Impact Factor
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    ABSTRACT: The reaction kinetics of the MoSe2 formation have been investigated by selenizing Mo layers in Se vapor at different temperatures and for different durations. The samples were characterized by means of Rutherford backscattering spectrometry, X-ray diffraction (XRD), electron diffraction, and by bright-field and high-resolution transmission electron microscopy. It was found that in all samples, a homogeneous MoSe2 layer is formed on top of the Mo layer. The temperature dependence shows that the MoSe2 layer thickness increases strongly for temperatures higher than ca. 550 °C. At a substrate temperature of 450 °C, no difference in the MoSe2 thickness was detected for samples selenized for different durations. The diffusion constant of Se in MoSe2 is estimated from the selenization duration dependence of MoSe2 layer thicknesses at 580 °C. Finally, Cu(In,Ga)Se2 (CIGS) solar cells in substrate configuration were developed on indium tin oxide (ITO) transparent back contacts. An intentionally grown MoSe2 intermediate layer on ITO, prior to CIGS deposition, causes a significant efficiency improvement, suggesting that MoSe2 can facilitate a quasi-ohmic contact. Solar cell efficiencies of up to 11.8% are obtained using an ITO/MoSe2 back contact.
    Thin Solid Films 01/2005; 480:433-438. · 1.87 Impact Factor
  • 19th EU Photovoltaic Solar Energy Conference. 01/2004;
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    ABSTRACT: ABSTRACT Three independent European CIS manufacturers of different countries, a big player in glass coating industry, three universities and three research institutes work together in the EU project LARCIS in order to advance CIS manufacturing at high yields on glass substrates. The two main approaches for the realisation of enhanced module efficiencies at reduced costs are inline co evaporation of Cu In,Ga Se2 and electrodeposition of the CuInSe2 CIS precursor. Alternative back contacts such as ZrN and TiN with high optical reflection have been tested and molybdenum layers with enhanced conductivity for homogeneous electrodeposition on 30 x 30 cm2 have been DC sputtered. A chemical bath deposited Cd free ZnS based buffer has been applied on both CIS types resulting in high photocurrents and efficiencies which are at least as high as reference cells with CdS buffers. In parallel, an In2S3 buffer has been evaporated ex situ from powder with a certified maximum efficiency of 14.16 . Solar cells of up to 17.8 efficiency certified, with ARC were fabricated by using co evaporated multi stage CIGS from an inline reactor. CuIn Se,S 2 was electrodeposited homogeneously on substrate areas of 30 x 30 cm2. The mean efficiency of test cells distributed on this area was lt;8 gt;.