C. Schetter

Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Baden-Württemberg, Germany

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Publications (26)11.32 Total impact

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    ABSTRACT: The present work gives an overview of the activities in multi-step front side metallization for crystalline silicon solar cells featuring copper as conducting layer at Fraunhofer ISE. Different approaches are being followed, the two most important being the deposition of nickel and copper onto printed and fired silver seed layers, or directly onto silicon. The first mentioned technique has been found to exhibit an important potential to reduce the silver consumption per cell. Below 40 mg of printing paste per wafer could be deposited by screen printing, as little as 8 mg by aerosol jet printing and metal inkjet printing. Progress with the challenge of adhesion after plating is presented, although further improvement is still needed. The highest peel force that could be achieved was >1.5 N/mm busbar width. For the second concept, detailed investigations of the contact interface have been done, to tackle the challenge of adhesion. An adaptation of the surface geometry is found to raise adhesion, but only 0.6 N/mm BB have been achieved, which is still insufficient. A silicidation process seems to be necessary to obtain sufficient adhesion.
    27th European PV Solar Energy Conference and Exhibition, Frankfurt, Germany; 01/2012
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    ABSTRACT: Our work deals with the creation of copper-containing stack systems for the front side metallization of silicon solar cells. In this contribution, we give an overview of different approaches from our labs. We have developed processes to apply nickel diffusion barriers onto seed layers and directly onto silicon with both electrolytic and electroless processes. These are reinforced by a light-induced copper plating process. On aerosol-printed seed layers, cell efficiencies equal to those of reference cells with advanced silver metallization have been achieved with a nickel/copper/tin stack system (16.8% on 5×5cm<sup>2</sup> industrial Cz-material, 20.3% on FZ high-efficiency substrates, 2×2cm<sup>2</sup>). As the long term stability of the resulting cells is a critical factor, there is need for a method to characterize this aspect. We developed a thermally accelerated ageing procedure, mirroring the total copper diffusion during a typical cell life cycle. Solar cells with advanced metal stack systems have shown no significant decrease in performance during this thermal stress test.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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    ABSTRACT: In this investigation we compare intrinsic hydrogen diluted amorphous a-Si:H(i) layers deposited by inductively coupled plasma (ICP) to the standard parallel plate (PP) plasma, driven by 13.5 MHz power source. We analyze and compare the growth rate, optical energy gap, homogeneity, passivation quality, and most importantly silicon heterojunction solar cell performance. The ICP a-Si:H(i) layer shows superior properties regarding the growth rate, however, we obtain a slightly better passivation quality with the PP a-Si:H(i) layer, with V<sub>oc</sub> values up to 723 mV. Looking at the overall solar cell performance we were not able to see any difference between ICP and PP silicon heterojunction solar cell. The best solar cell (with an ICP a-Si:H(i) layer) has an efficiency of 18.7%.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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    ABSTRACT: This study deals with some specific characteristics that the light-induced plating (LIP) process, which is used for the metallisation of solar cells, exhibits compared to classical electroplating processes. We contribute to the general understanding of LIP (multiple electrodes, influence of light) with some basic experiments and propose a simplified equivalent circuit scheme. We also address the challenge of process control through potential and current without external connection of the working electrode (front side grid). In this article, we show a possibility to determine the absolute potential of the front side grid for relevant process parameters. Furthermore, we present a method that allows the measurement of the mean current density at the front side grid during the process, which has a great influence on the plating result. KeywordsCrystalline solar cells-Light-induced plating-Absolute potential measurement-Current density
    Journal of Applied Electrochemistry 01/2010; 40(4):757-765. · 1.84 Impact Factor
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    ABSTRACT: In this paper, we present a method to quickly evaluate the long-term effects of copper-containing metal stack systems for silicon solar cell front–side metallization. Copper diffusion, which is detrimental for the solar cell performance, is accelerated by exposing the cell to thermal stress. In this paper, we suggest to quantify the degree of copper diffusion into the cell by the very fast Suns-VOC technique, measuring the pseudo fill factor (pFF). Using three or more different temperatures, and assuming a certain loss in pFF corresponds to a certain depth of diffusion, the effective activation energy for copper diffusion for a given system can be extracted from an Arrhenius plot of the measured data. An extrapolation into temperature regions typical for solar cell modules under outdoor conditions allows an estimation of the fill factor loss for any operation time and temperature. Compared to time- and cost-intensive methods such as transmission electron microscopy or secondary-ion mass spectrometry, this kind of investigation requires only sparse equipment and can typically be done in 1 week per stack system.
    Journal of The Electrochemical Society 01/2010; 157(10):H942-6. · 2.59 Impact Factor
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    ABSTRACT: Fraunhofer ISE's concept for an advanced metallization of silicon solar cells is based on a two-step process: the deposition of a seed layer to form a mechanical and electrical contact and the subsequent thickening of this seed layer by a plating step, preferably by light-induced plating (LIP). The concept of a multi-layer metallization is used for most of the relevant high-efficiency cell types in industry. The main advantage of this concept is that each layer can be optimized individually, i.e. the seed layer to achieve an optimal electrical and mechanical contact and the plated layer in terms of high lateral conductivity and good solderability. Solar cells results with seed layers fabricated by aerosol printing, chemical Ni plating on cells with a laser-structured dielectric layer and laser-enhanced Ni plating are presented.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 06/2008
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    ABSTRACT: We report the successful transfer of passivating, intrinsic a-Si x C 1-x :H layers from a laboratory batch-type to an industrial in-line plasma-enhanced chemical vapour deposition (PECVD) reactor. In both cases silane (SiH 4 ) and methane (CH 4 ) are used as precursor gases and the plasma energy is provided by a high frequency (13.65 MHz) as well as by a microwave (2.45 GHz) generator. Intensive process parameters such as temperature (350–400°C) and pressure (0.3–0.4 mbar) could be directly transferred from the lab to the industrial system whereas power and gas composition had to be adjusted carefully to the different dimensions and geometry of the in-line reactor. By means of a statistical design of the experiments a parameter range for passivating a-Si x C 1-x layers could be found resulting in surface recombination velocities as low as S ≪ 10 cm/s. These values could be achieved without applying any wetchemical step to the silicon samples as the cleaning of the surface was performed in-situ in the plasma chamber. The deposition rate is 100 nm/min and therefore an order of magnitude higher than in our laboratory-type system. Fourier transform infrared spectroscopy (FT-IR) measurements performed on the in-line deposited a-Si x C 1-x layers reveal an elevated carbon content compared to their counterparts originating from our static laboratory PECVD reactor.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 06/2008
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    ABSTRACT: This paper presents a method to improve the efficiency of large area screen-printed silicon solar cells by about 0.3% to 0.4% absolute. By light-induced plating (LIP) the series resistance of screen-printed solar cells is reduced. The line conductivity of the front side metal conductors is improved without increasing the shaded area. This plating technique has been optimized for years at Fraunhofer ISE and is a fast method to homogeneously galvanize metal contacts on n-doped material. High efficiencies of 18.4% on 20times20 mm<sup>2</sup> screen-printed FZ silicon solar cells and 16.6% on 156times156 mm<sup>2</sup> industrial processed mc-Si solar cells have been obtained. It is believed that the significant increase of efficiency combined with the reduction of the amount of screen-printing paste required, overcompensates the cost for this additional step at the end of the process sequence
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: All work published so far on C-SiTF on ceramics was based on either non-conductive ceramics, or on "model" ceramics never able to cope with the cost requirements. For a wafer equivalent approach, where c-Si films on substrate can be directly processed to solar cells with nowadays technology, the respective substrates however need to be low-cost and electrically conductive. We therefore decided to use tape-casted RBSiC (Reaction Bonded) ceramics as a cost-effective substrate which can be used in a production scenario without any further change. The porosity present in this substrate material causes special problems during layer and solar cell preparation. We realized multicrystalline Si thin-films with high quality on these substrates. Here we report our experience and results on the way to prepare the films and in further solar cell processing. We firstly present results concerning SiC diffusion barrier performance. The concentration of transition metals in our cell bulk was measured by Glow Discharge Mass Spectrometry (GDMS). The data show a significant concentration decrease of 3 to 4 orders of magnitude for Fe, V and Co from substrate to cell bulk. The second focus of this work is the porosity of the ceramic substrate. We were able to observe on cross sections that the pores are filled with Si and the intermediate layer (IL) covers their inner surface. Furthermore the applicability of several industrial cell process steps was tested. Especially plasma processes for texturization were realized and led to promising results. Solar cell results of cells with an area of 1 cm will be presented
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
  • S. Janz, S. Reber, F. Lutz, C. schetter
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    ABSTRACT: Crystalline silicon thin-film solar cells (CSITF; high temperature approach) on low-cost substrates will only get their chance on the market when they can be introduced into an industrial solar cell process. Therefore, a conductive diffusion barrier intermediate layer (IL) is necessary. The most promising candidate is, in our opinion, SiC. Not just because doping with boron or phosphorous can change the electrical behavior effectively. It also matches well in thermal expansion coefficients with RBSiC (reaction-bonded) ceramics which is a very promising low-cost substrate. The passivation performance and the possibility to use the thin layer as a dopant source for an in situ back surface field are additional benefits. In this paper, we will show that the electrical conductivity of SiC can become sufficiently high with temperature treatments. We will furthermore present FTIR and SIMS measurements which would give information about the changing of bonding conditions and the distribution of the dopant boron with different deposition temperatures and annealing.
    Thin Solid Films 01/2006; · 1.87 Impact Factor
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    ABSTRACT: Emitter formation by in-line deposition of a PECVD PSG and subsequent diffusion in an in-line belt furnace is suggested as an alternative to quartz tube diffusion. The PSG deposition process reached excellent layer homogeneities, both across a single wafer and across the whole carrier, resulting in very uniform sheet resistance distributions. Relative standard deviations of 2,5 % have been achieved. Solar cells with the developed emitter reached efficiencies of up to 17,5 % and 16,8 % on textured and untextured Cz-Si respectively.
    Proceedings of the 21st European Photovoltaic Solar Energy Conference; 01/2006
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    ABSTRACT: We present first results on the application of PECVD silicon carbide as intermediate layer for crystalline silicon thin-film solar cells. Silicon carbide layers were deposited by PECVD and characterized by Auger spectrometry and SEM. The subsequent sample processing included high-temperature anneal, deposition of a silicon seeding layer by CVD, recrystallization of the seeding layer by zone-melting, epitaxial growth of the base layer and finally a solar cell process where conventional and one-side contact scheme were realized. All process steps were successfully accomplished but characterization of the samples revealed that the silicon carbide intermediate layers were partly damaged or perforated. Efficiencies up to 7.1% were reached using a conventional contact scheme.
    Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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    ABSTRACT: Thinner wafers and the reduction of breakage losses make it attractive for solar cell manufacturers to use in-line production systems. Closing the gap between diffusion and in-line silicon nitride deposition systems a plasma etching system has been designed suitable for a throughput of 1000 wafer/h with an automated transport system. Different plasma sources appropriate for etching large areas were tested and etch rates of around 13 /spl mu/m/min for saw damage removal could be reached with sufficient homogeneity. For phosphorous glass (PSG) removal selectivities between PSG and silicon above 10 could be achieved with a CF/sub 4//C/sub 2/H/sub 4/ etch gas mixture with sufficient PSG etch rates of around 80 nm/min. Solar cells processed with the plasma PSG removal step show slightly decreased efficiencies attributed to the plasma induced damage to the emitter layer and the silicon bulk. Further process optimization is needed to reduce this damage.
    Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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    ABSTRACT: At Fraunhofer ISE the fabrication of high-efficiency solar cells was extended from a laboratory scale to a small pilot-line production. Primarily, the fabricated cells are used in small high-efficiency modules integrated in prototypes of solar-powered portable electronic devices such as cellular phones, handheld computers etc. Compared to other applications of high-efficiency cells such as solar cars and planes, the illumination densities found in these mainly indoor applications are significantly below 1 sun. Thus, special care was taken to keep the cell efficiency level high even at very low illumination levels. For this reason, particularly the cell border was analyzed and optimized carefully. The excellent cell characteristics achieved at low illumination densities increase the benefit of a solar power supply for such devices by an order of magnitude if compared to standard solar cells.
    Photovoltaic Specialists Conference, 2002. Conference Record of the Twenty-Ninth IEEE; 06/2002
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    ABSTRACT: An interdigitated front grid structure for both the emitter and base was simulated and realized. This contact design is suitable for thin-film solar cells on insulating substrates or insulating intermediate layers. Confirmed efficiencies of up to 18.2% were achieved on a 46 mu m thick epitaxial silicon layer which was grown on a SIMOX wafer with an implanted compact SiO2 intermediate layer. Samples with and without a highly doped back surface field were prepared to study the influence of the back-side recombination velocity. Leff values of 250 and 52 mu m, corresponding to Sback values of 800 and 105 cm/s, respectively, were measured, thus, underlining the importance of a low back-side recombination velocity. The optical confinement properties of the SiO2 intermediate layers were calculated depending on the angle of the incident rays. An angle from the plane normal which is larger than 23 degrees is necessary in order to achieve the condition of total internal reflection. Future work will focus on recrystallized Si layers on foreign substrates. Since the surface of the silicon layer is fairly rough after the recrystallisation process, another set of masks was designed which is more tolerant to aligning accuracy. This is mainly relevant for the area where the base contacts are located between the locally diffused emitter. The technology for CVD Si-layer deposition, zone melting recrystallization ZMR, as well as for a simplified solar cell process is under investigation.
    Solar Energy Materials and Solar Cells 01/1997; 48(1-4):335-42. · 5.03 Impact Factor
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    ABSTRACT: In a study we applied different solar cell processes of varying complexity to several different directionally solidified mc-Si materials. The materials differed with respect to the crystallization velocity, planarity of the crystallization interface, and feedstock quality. In general, higher solar cell efficiencies were observed with increasing process complexity. Emitter passivated solar cells with integrated gettering yielded efficiencies up to 17.4% for materials with planar crystallization interface while materials with strongly bent crystallization interface seem to be more vulnerable to prolonged processing times and higher temperatures. The limiting factor for solar cell efficiency seems to be the grown-in quality of the silicon like homogeneity and defect density rather than the solar cell process itself
    Photovoltaic Specialists Conference, 1997., Conference Record of the Twenty-Sixth IEEE; 01/1997
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    ABSTRACT: Mini solar cell arrays allow all relevant solar cell parameters, including V<sub>0C</sub>, I<sub>SC</sub> and efficiency to be locally measured. From individual mini solar cells, larger ensembles can be constructed by means of parallel connection. The data of individual and connected cells can be used to test models for multicrystalline Si solar cells
    Photovoltaic Specialists Conference, 1996., Conference Record of the Twenty Fifth IEEE; 06/1996
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    ABSTRACT: Using optically heated furnaces, silicon layers of 30-50 μm thickness were recrystallized on 2 μm thick SiO<sub>2</sub> intermediate layers which were deposited on Si substrate wafers. In one case, the SiO<sub>2</sub>-layers were prepared with 1% of the area being opened as contact and seeding holes, and the recrystallization was done with a large area heater (LAH). In the second case, a zone melting recrystallization (ZMR) process was applied on a compact SiO<sub>2</sub>-layer without any seeding. Since the active layer is not electrically contacted to the substrate, a newly developed interdigitated front grid was used to contact both the base and the emitter from the front side. The differential spectral response of the solar cells prepared on a SiO<sub>2</sub>-intermediate layer with seeding holes depends on the bias light for long wavelengths, where the contribution of the base is dominant. Rocking curve analysis was used to characterize the crystallographic structure of the recrystallized layers. It was found that the seeding holes mostly caused a tiled structure of squares in the silicon layer which, like the substrate, was completely (100)-oriented
    Photovoltaic Specialists Conference, 1996., Conference Record of the Twenty Fifth IEEE; 06/1996
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    ABSTRACT: Silicon films 40-90 μm thick were epitaxially deposited on fast-grown mc-SSP pre-ribbons. Special attention was focused on the design of the authors' self-constructed CVD system. It is principally convertable into a conveyer-belt system for a high continuous throughput, which is needed to be economically competitive. With high deposition rates of up to 10 μm/min, the epi-layers revealed diffusion lengths of 250 μm on 〈100〉-Cz substrates, 150 μm on SILSO and 11-30 μm on SSP pre-ribbons (all substrates highly doped). Solar cells were manufactured using the authors' standard cell process. No passivation or gettering steps were performed and no texturing was applied. Solar cell efficiencies of 12.8% on Cz, 11.1% on SILSO wafers and 6.1% on SSP were achieved
    Photovoltaic Specialists Conference, 1996., Conference Record of the Twenty Fifth IEEE; 06/1996
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    ABSTRACT: The liquid-phase epitaxy etchback regrowth (LPE-ER) technique was used to grow AlGaAs/GaAs heteroface and AlGaAs/Al<sub>y</sub>Ga<sub>1-y </sub>As(0.1<y<0.3) heteroface solar cell structures. The influence of substrate position (vertical or horizontal) during the LPE-ER process was investigated. A model for the growth of the LPE-ER structure in vertical substrate position is proposed. The LPE-ER grown structures were used to fabricate one-sun and concentrator solar cells. Efficiencies of up to 22.4% (AM1.5G) were achieved for solar cells of 4 cm<sup>2</sup> area. For concentrator application an efficiency of 24.8% (AM1.5, direct) was measured at 105 suns on a 13 mm<sup>2</sup> solar cell. A 243 cm<sup>2</sup> aperture area concentrator module consisting of 12 Fresnel lenses and 12 LPE-ER grown GaAs solar cells showed an efficiency of 20.3% under outdoor conditions
    Photovoltaic Energy Conversion, 1994., Conference Record of the Twenty Fourth. IEEE Photovoltaic Specialists Conference - 1994, 1994 IEEE First World Conference on; 01/1995