C. Fella

Empa - Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Zurich, Switzerland

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Publications (25)175.25 Total impact

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    ABSTRACT: Cu2ZnSn(S,Se)4 absorbers were processed from metal salt solutions in dimethyl sulfoxide (DMSO) and annealed in reactive Se atmosphere. A simple way to introduce sodium doping by adding NaCl to the initial precursor solution is presented. The additional sodium has no significant effect on the absorber morphology, but it improves the device performance from 5.01% to 6.09%. The sodium doping clearly reduces the recombination rate as manifested in a higher Jsc. and enhanced carrier collection. It is also shown that the absorber layer thickness should be optimized to ∼1.5 µm. By reducing the absorber thickness we found a facile way to extend the well crystallized top crust over the whole absorber and thus improve the solar cell performance.
    Physica Status Solidi (A) Applications and Materials 09/2014; · 1.53 Impact Factor
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    ABSTRACT: Cu2ZnSnS4 (CZTS) layers are deposited by a direct solution coating of metal salts dissolved in dimethyl sulfoxide (DMSO) and thiourea as sulphur source. An intentional Sn-rich precursor is used and the influence of reactive and inert atmosphere on the layer morphology is investigated during high temperature sintering. Larger grain growth is observed when annealing the precursor in inert N2 atmosphere as compared to reactive sulphur atmosphere. An initial Sn-rich precursor composition with Zn/Sn < 1 promotes the grain growth while Sn leaves the sample during crystallization ending up with a final Zn-rich (Zn/Sn > 1) layer composition.
    Physica Status Solidi (A) Applications and Materials 08/2014; · 1.53 Impact Factor
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    ABSTRACT: Large-grained CuInSe2 absorber layers are synthesized using a non-vacuum process based on nanoparticle ink precursors and selenization by rapid thermal processing (RTP). The use of hydroxide-based particles in organic solvents allows for the conversion with elemental selenium without the need to employ explosive and/or toxic H2 or H2Se gasses. Lateral grain sizes up to 4 µm are obtained through a novel RTP route, overcoming the inherently high layer porosity for previous nanoparticle processes. Morphological and elemental characterization at interrupted selenization steps suggests that liquid selenium can play a beneficial role in promoting layer densification and grain growth. Long carrier collection lengths in CuInSe2 enable notable conversion efficiencies, despite the low minority carrier lifetimes of below 1 ns. Record efficiencies up to 8.73% highlight the potential of low-cost, non-vacuum deposition of chalcopyrite absorber layers with safe and simple precursors and processing routes. Copyright © 2014 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 07/2014; · 7.71 Impact Factor
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    ABSTRACT: Deposition of Cu(In,Ga)Se2 (CIGS) thin film solar cells on metallic substrate is an attractive approach for development of low cost solar modules. However, in such devices, special care has to be taken to avoid diffusion of impurities, such as Fe, Ni, and Cr, from the substrate into the active layers. In this work, the influence of Ni and Cr impurities on the electronic properties of CIGS thin film solar cells is investigated in detail. Impurities were introduced into the CIGS layer by diffusion during the CIGS deposition process from a Ni or Cr precursor layer below the Mo electrical back contact. A high temperature and a low temperature CIGS deposition process were applied in order to correlate the changes in the photovoltaic parameters with the amount of impurities diffused into the absorber layer. Solar cells with Ni and Cr impurities show a reduction in the device performance, whereas the effect was most pronounced in Ni containing devices. The presence of deep defect levels in the absorber layer was identified with admittance spectroscopy and can be related to Ni and Cr impurities, which diffused into the CIGS layer according to secondary ion mass spectroscopy depth profiles and inductively coupled plasma mass spectrometry. Copyright © 2014 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 04/2014; · 7.71 Impact Factor
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    ABSTRACT: The reliable and homogenous addition of Na to the Cu(In,Ga)Se2 (CIGS) solar cell absorber represents one major challenge for the industrial implementation of the CIGS technology faces. In this study, an alternative Na source from a Na-doped Mo (MoNa) layer is compared to the conventionally used soda-lime glass (SLG) substrate. In particular, the Na out-diffusion mechanism from different MoNa back contacts into the CIGS layer is investigated for different multilayer designs and magnetron sputtering conditions. From the obtained experimental results a Na diffusion model for the MoNa back contact system is developed, which proposes that Na is mainly collected in the MoNa grain boundary region and then diffuses from these grain boundaries towards the CIGS layer. It is demonstrated that with increasing porosity of the MoNa layer the Na diffusion into the CIGS layer is enhanced. In addition, the low activation energy for Na diffusion found for the MoNa back contact proposes that this Na doping technology is suitable for both low and high substrate temperature CIGS processes. For CIGS solar cells with MoNa back contacts, best cell efficiency of 15% was achieved without anti-reflection coating, which exceeded the performance of the reference sample on SLG with standard Mo back contact.
    Solar Energy Materials and Solar Cells 01/2014; 124:10–16. · 5.03 Impact Factor
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    ABSTRACT: Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20% (refs , ). This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.
    Nature Material 11/2013; · 35.75 Impact Factor
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    ABSTRACT: A solution deposition approach for high-performance aluminum-doped zinc oxide (AZO) thin films (visible transparency > 90% and sheet resistance down to 25 Ω/sq) with process temperatures not exceeding 85 °C is presented. This allows the non-vacuum deposition of AZO on temperature sensitive substrates such as polymer films for flexible and transparent electronics, or inorganic and organic thin film photovoltaics.
    Advanced Materials 10/2013; · 14.83 Impact Factor
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    ABSTRACT: Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.
    Nature Communications 08/2013; 4:2306. · 10.74 Impact Factor
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    ABSTRACT: Phase-pure Cu2ZnSnSe4 (CZTSe) layers are necessary for achieving efficient thin film solar cells. This requires the knowledge of intermediate phases and their existence regions during the evolution of the CZTSe phase within its homogeneity range. Here we investigate the growth mechanism of different phases when solution deposited metal salt precursors are selenized into CZTSe layers. A combination of in situ and ex situ X-ray diffraction, Raman spectroscopy, energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy at successively increasing substrate temperatures is used to track evolving crystal phases. The growth starts with the fast formation of binary Cu–Se phases that are present between 190 °C and 320 °C. Overlapping diffraction patterns of CZTSe/Cu2SnSe3/ZnSe phases evolve from 280 °C onwards and remain until a final temperature of 550 °C. The ternary Cu2SnSe3 phase co-existing with CZTSe between 340 °C and 370 °C is confirmed by Raman spectroscopy and point EDX measurements. No individual zinc or tin binary phases can be detected. We propose the kinetically driven formation mechanism, which starts with the selenization of Cu requiring the lowest activation energy for reaction, and then proceeds via the gradual incorporation of Sn and Zn to yield the final CZTSe phase.
    Journal of Alloys and Compounds 08/2013; 567:102-106. · 2.73 Impact Factor
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    ABSTRACT: The presence of Na in Cu(In,Ga)Se2 (CIGS) is well known to improve the solar cell performance. To incorporate Na into the CIGS absorber, Na-doped Mo (MoNa) back contact layers were grown on stainless steel foils. Three different back contact designs deposited from MoNa sputtering targets with Na concentrations of 3 at.%, 5 at.% and 10 at.% were investigated. A multistage CIGS evaporation process at low (~ 450 °C) substrate temperature was used to deposit the absorbers.Measurements from time-of-flight secondary ion mass spectroscopy depth profiles indicate that Na is preferentially collected at the internal interfaces and out-diffuses from the grain boundaries of the multilayer MoNa back contact into the CIGS absorber. From the [Ga]/([In] + [Ga]) grading profiles, a more pronounced Ga dip was found with increasing Na concentration. Moreover, at high Na concentrations (10 at.% MoNa target), a change in the CIGS texture was observed by X-ray diffraction. Best solar cell efficiency of 14.4% was achieved for the 5 at.% MoNa sample without antireflective coating, which is a significant improvement compared to the 9.8% efficiency measured for the Na-free reference.
    Thin Solid Films 05/2013; 535:214–219. · 1.87 Impact Factor
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    ABSTRACT: The following paper presents a processing route for Cu(In,Ga)Se-2 absorber layers that is based on nanoparticle dispersions which are applied by doctor blade deposition and converted with elemental selenium vapors. In particular, the preparation of the precursor layers is investigated by systematically assessing the influence of the stacking sequence of mono-and multi-metallic layers on sintering, elemental distribution and solar cell efficiency. By applying suitable stacking sequences, precursor layers with both local Cu-rich and over-all Cu-poor stoichiometry could be prepared that allowed improved sintering properties and modifications of the gallium gradient. Despite the still prevailing porosity of the absorber layer, solar cells with efficiencies exceeding 5% could be obtained. (C) 2013 Elsevier B.V. All rights reserved.
    Thin Solid Films 01/2013; 535:138-142. · 1.87 Impact Factor
  • Solar Energy Materials and Solar Cells 01/2013; 119:276-277. · 5.03 Impact Factor
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    ABSTRACT: Abstract This review describes a specific group of non-vacuum methods for depositing kesterite solar cell absorbers, which we refer to as direct solution coating (DSC). These methods work by coating a layer of precursor-containing solution on the surface of a substrate followed by appropriate thermal processing to yield the desired kesterite Cu2ZnSn(S,Se)4 phase. General aspects, challenges and future prospects of the DSC are discussed in detail. Two research highlights are presented in which kesterite layers are processed from solutions of metal salts in simple non-toxic organic solvents to yield up to 7.5% efficient solar cells.
    Solar Energy Materials and Solar Cells 01/2013; 119:181-189. · 5.03 Impact Factor
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    physica status solidi (a) 06/2012; 209(6):1043-1048. · 1.21 Impact Factor
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    ABSTRACT: 910HF Times Cited:0 Cited References Count:1 , The following values have no corresponding Zotero field: Author Address: Buecheler, S Empa, Swiss Fed Labs Mat Sci & Technol, Lab Thin Films & Photovolta, Uberlandstr 129, CH-8600 Dubendorf, Switzerland Empa, Swiss Fed Labs Mat Sci & Technol, Lab Thin Films & Photovolta, Uberlandstr 129, CH-8600 Dubendorf, Switzerland Empa, Swiss Fed Labs Mat Sci & Technol, Lab Thin Films & Photovolta, CH-8600 Dubendorf, Switzerland Univ Ghent, Elect & Informat Syst ELIS, B-9000 Ghent, Belgium
    Thin Solid Films 01/2012; 520(9):3710. · 1.87 Impact Factor
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    ABSTRACT: Compared with rigid glass, manufacturing of Cu(In,Ga)Se2 (CIGS) solar cells on flexible stainless steel (SS) substrates has potential to reduce production cost because of the application of roll-to-roll processing. Up to now, high-efficiency cells on SS could only be achieved when the substrate is coated with a barrier layer (e.g. SiOx or Si3N4) for hindering the diffusion of impurities, especially Fe, into the CIGS layer. In this paper, the effect of these impurities on the electronic transport properties of the device is investigated. Using admittance spectroscopy, the presence of a deep defect level at around 320 meV is observed, which deteriorates the efficiency of the solar cells. Furthermore, it is shown that reducing substrate temperature during CIGS deposition is an effective alternative to a barrier layer for reducing diffusion of detrimental Fe impurities into the absorber layer. By applying a CIGS growth process for deposition at low substrate temperatures, an efficiency of 17.7%, certifie
    Progress in Photovoltaics Research and Applications 01/2012; · 7.71 Impact Factor
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    ABSTRACT: Non-vacuum methods for Cu(In,Ga)Se2 (CIGS) absorber deposition have gained wide interest because of their inherent cost and energy saving potential. Here, a solution-based processing route for CIGS absorber layers is presented that employs binder-free solutions of metal salts in non-toxic, alcohol solvents. Despite the low-boiling-point nature of the employed solvents, a residual carbon-rich layer is observed between the CIGS and metal back contact. Based on comprehensive investigations by scanning electron microscopy, energy-/wavelength dispersive X-ray spectroscopy, X-ray fluorescence, X-ray diffraction, thermogravimetric analysis, differential thermal analysis, and extended X-ray absorption fine structure spectroscopy, a formation reaction mechanism through intermediate metalorganic complexes is proposed. In this route, the CIGS layer is formed in selenium atmosphere via a gradual decomposition of the carbon-rich layer comprising carboxylic chelate complexes of metals. A compositional gradient occurs in the CIGS absorber, whereas a significant amount of metals remains in the carbon-rich layer. The incorporation of Ga into CIGS is affected by the initial salt concentration and the selenization temperature. Fabricated solar cells exhibit active area efficiencies of up to 7.7% on 0.3?cm2 area without anti-reflection coating, which is among the highest reported efficiencies for solar cells from a solution process with non-explosive gases or solvents. Copyright (c) 2012 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 01/2012; 20:526-533. · 7.71 Impact Factor
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    ABSTRACT: Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.
    Nature Material 09/2011; 10(11):857-61. · 35.75 Impact Factor
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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: We have used soft x-ray emission spectroscopy to study the impact of the CdCl2 activation treatment on the chemical structure of the CdTe/ZnO1−xSx interface. We find a pronounced chemical interaction, most prominently the interfacial intermixing of Cd and Zn. Furthermore, the formation of S-Cd bonds at the expense of S-Zn bonds can be observed.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2011;