P. Fath

Universität Konstanz, Konstanz, Baden-Wuerttemberg, Germany

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Publications (108)129.49 Total impact

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    ABSTRACT: A record independently confirmed production cell efficiency of 19.3% is presented for a large area P-type CZ silicon solar cell, based on the UNSW laser doped selective emitter technology. In this work, the innovative and patented laser doping technology is simply added to a standard Centrotherm turnkey line, operating with a modified process and the addition of the laser doping and light induced plating steps. Impressively, this record efficiency is achieved by using standard commercial grade p-type CZ grown silicon wafers on standard production equipment and exceeds the previous independently confirmed record for any technology of 19.2% using a standard aluminium back surface field with full rear coverage. The avoidance of laser induced defects is discussed in this work, to overcome previous limitations of the laser doping technology using conventional Q-switched lasers or the laser chemical processing method. It is demonstrated that the use of appropriate lasers can avoid thermal cycling whilst still allowing for the sufficient mixing of dopants, and allow laser doping to be performed through a standard SiN layer with contacts formed through a self-aligning metallisation scheme.
    IEEE Journal of Photovoltaics 07/2011; 1(1):000047-000047. · 3.00 Impact Factor
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    ABSTRACT: This paper reports on the status and perspective of crystalline silicon (c-Si) solar cell production from the viewpoint of a turnkey production line and technology supplier. It exemplifies selected results of centrotherm's strategic research and roadmapping process. The paper addresses the question of cost, timeframe and risk in transferring new technologies from lab level to mass production targeting at the ultimate ambition in PV: Grid parity.
    Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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    ABSTRACT: This paper reports on the development of a masked process for the production of buried contact solar cells on multi-crystalline silicon. The process results in high efficiencies, and only includes steps that would be feasible in an industrial environment. We report here on different mask candidates and on the importance of hydrogenation with the new process. Using the developed process, we produced 111 large area (12 × 12 cm2) cells and achieved an average cell efficiency of 16·2%. The best cell had an efficiency of 16·9%, a Voc of 616 mV, a Jsc of 35·0 mA/cm2 and a fill factor of 78·3%. Copyright © 2008 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 03/2008; 16(6):467 - 477. · 7.71 Impact Factor
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    01/2007;
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    ABSTRACT: Existing models of the silver thick-film contact to an emitter are summarised and recent developments focusing on contact formation and current transport mechanisms are presented. As a glass layer exists at the silicon-thick-film interface the current transport mechanisms are not obvious. The main hypotheses are: current transport via spike-like direct silver–silicon interconnections or via tunnelling through the chemically modified glass layer. Recent investigations showed that silver crystallites grow into the emitter from the glass frit containing dissolved silver [G. Schubert, B. Fischer, P. Fath, in: Proceedings of photovoltaics PV in Europe Conference, Rome, 2002, pp. 343–346 [1]]. These silver islands are covered by a thin glass layer [C. Ballif, D.M. Huljic, A. Hessler-Wyser, G. Willeke, in: Proceedings of the 29th IEEE PVSC, Glasgow, 2002, pp. 360–363 [2]]. Further investigations are necessary to study the crystallite-growth mechanism as well as the current-transport mechanism from the crystallites to the finger.
    Solar Energy Materials and Solar Cells 11/2006; · 5.03 Impact Factor
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    ABSTRACT: In this publication we present a realisation of a large-area metallisation wrap through (MWT) back-contact-solar cell with electroless-plated contacts. The MWT cell is a very promising back-contact solar-cell concept since the additional effort required to process MWT instead of conventional cells is limited to the formation of a small number of holes. In addition, the concept can easily be applied to very large area cells.We introduce a 9.8cm×9.8cm MWT solar cell, which reaches an efficiency of 16.1%. Up to this point cell performance is limited by local shunting, which results from non-optimal nickel sintering.
    Solar Energy Materials and Solar Cells 11/2006; 90(18):3232-3237. · 5.03 Impact Factor
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    ABSTRACT: We present low cost, screen printed industrial solar cells with a rear Al-emitter for n-type mono- and mc-Si large area thin wafers showing efficiencies of 16.4% and 14.4% respectively. The gap of 2% absolute between the cell efficiencies for mono- and mc-Si is not material related but is largely due to the non-textured surface of the mc-Si substrate. Applying a surface texture by acidic wet chemical etching could lead to efficiencies above 15% on the mc-Si material. The process is based on the standard industrial p-type firing through solar cell concept with slight modifications in individual processing steps similar to the Phostop cell presented by Ebara Solar. There is still need for further optimisation before industrialisation for instance in the fields of wafer bowing reduction and the implementation of AgAl pads for wafer interconnection. We propose two different ways of pad printing-one directly to the Al-emitter, the other on top of the Al rear contact
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: Multi-crystalline silicon has dominated the photovoltaic market in recent years and with advances in isotexturing and the production of increasingly thinner and larger wafers it is set to play a significant role in the future. As with other cell types, laboratory efficiencies remain higher than those achieved in production. Previous large area efficiency records on multi-crystalline silicon have included a 17.6% efficient cell produced at the University of Konstanz and a 17.7% efficient cell produced by Kyocera. The 17.6% cell was made using the buried contact technique. Again using this technique and multi-crystalline silicon, we have made an 18.1% efficient cell, independently confirmed by the calibration laboratories at Fraunhofer ISE. The area of the cell is 137.7 cm<sup>2</sup>; V<sub>oc</sub> is 636 mV and J<sub>sc</sub> is 36.9 mA/cm<sup>2</sup>. To the best of our knowledge, this is a new world record
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: In this paper we present n-type Si solar cells on large area mc-Si wafers with a boron diffused emitter at the front side. The focus of our studies is mainly related to the front surface of the solar cell. We have optimised BBr<sub>3</sub>-diffusion and in-situ oxidation with respect to the homogeneity of the sheet resistance and substrate degradation. After diffusion even a slight improvement of the minority charge carrier lifetime was measured, which can be related to B-gettering. The emitter is contacted by AgAl-paste and passivated by thermal SiO<sub>2</sub>. The development and optimisation of all processes led to solar cells with efficiencies of 14.7% on mc-Si and 17.1% on Cz-Si substrates. In addition to this we present an innovative interconnection of modules using our developed cell (patent pending). We show an alternate serial interconnection of p- and n-type solar cells resulting in easier module processing
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: The standard industrial multi-crystalline silicon (mc-Si) solar cell is monofacial and includes screen printed aluminum back surface field (BSF). A simple approach to increase performance and reduce costs per W<sub>peak</sub> is to collect the albedo on the rear side. In this work a bifacial, screen printed mc-Si solar cell with boron BSF is demonstrated. Rear to front efficiency ratios of up to 0.83 have been reached on 100times100mm<sup>2</sup> mc-Si wafers with a thickness of about 200mum. The best solar cell processed so far with a boron BSF had an efficiency under front side illumination of eta=16.1% and a back to front efficiency ratio of 0.77. The possible gain in performance in later operation was estimated using PC1D simulation and depends on the albedo that is the amount of light that penetrates into the solar cell from the rear side. The simulation was confirmed by outside module tests, leading to an average gain of 19.5% over one day
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: We present bifacial solar cells processed with a sequence suitable for industrial production. This method uses the LPCVD silicon nitride deposition based on DCS (dichlorosilane) or BTBAS (bis-(tertiary butyl amino)-silane). The bifacial solar cell process on wafers of 200 mum thickness has the following steps: (1) boron doped BSF of R<sub>sheet </sub>=60 ohm/sq; (2) POCl<sub>3</sub> emitter (on front side) of R<sub>sheet</sub>= 50-55 ohm/sq; (3) thermal oxidation of the wafer surfaces; (4) the deposition of DCS or BTBAS based LPCVD silicon nitride on either sides of the wafer (5); finger grid printing on both sides and firing; (6) edge isolation. The solar cells produced with the DCS based silicon nitride process exhibit fill factor (FF) values of 76% on p-type and 75% on n-type solar cells with a rear to front efficiency ratio eta<sub>rear</sub>/eta<sub>front</sub> of 67% for the p-type solar cells and 43% for the n-type solar cells. The solar cells with the BTBAS silicon nitride show FF values close to 72% and eta<sub>rear</sub>/eta<sub>front</sub> 68% for p-type solar cells
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: Record cell efficiencies were achieved with newly developed processes. An integral approach from wafer to module was used. Starting with high-purity feedstock, high-quality large-area mc-Si wafers were made and characterized. Initial lifetimes of up to 160 µs were observed. Three cell concepts were developed further to obtain high efficiency cells: 1) based on screen-printing and complete inline processing; 2) based on V-grooved texturing and roller printing; 3) based on a hybrid buried contact concept with screen-printed Al BSF. With advanced inline processing an independently confirmed efficiency of 17.0% was achieved on 156 cm 2 mc-Si, which is a record for complete inline processing. A full size module made of cells processed with this process has a peak power of 94.3Wp, which corresponds to an encapsulated cell efficiency of 16.8%. Roller printing on V-grooved cells resulted in an independently confirmed efficiency of 17.2%. With the hybrid buried contact concept an efficiency, again independently confirmed, of 18.1% was reached on a cell area of 137 cm 2 ; a record for this size. This record cell was used to determine a roadmap towards 20% mc-Si cell efficiency. For that, it is mainly needed to improve the rear side passivation and rear side internal reflection. This can be done with a di-electric layer for passivation and local contacts. Furthermore, the shading losses should be reduced with for example the Angled Buried Contact concept.
    Tres Cantos (Madrid) Spain. 01/2006; 8.
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    ABSTRACT: We present further results of a surface passivation study of p+-Si emitters by both intrinsic and boron-doped amorphous SiCx films, deposited in two different standard PECVD reactors. For comparison, thermally grown SiO2 and PECVD-SiN x layers with refractive indices of n=2.0 and n=2.4 were examined on the same test structures. While thermal SiO2 exhibits passivating properties comparable to those on n+-Si emitters, PECVD-SiNx is found to even deteriorate the surface passivation, especially after firing (without metal contacts). On the other hand, PECVD-SiCx yields, to our knowledge, the best p +-Si passivation so far obtained by an industrially relevant low temperature process. It is expressed by an implied Voc of 635 mV for a symmetrically 60 Omega/sq BBr3-diffused n-type CZ-wafer with a base resistivity of 4.6 Omegacm
    01/2006;
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    ABSTRACT: We present the characterisation of directionally solidified n-type Si ingots. Three ingots with a range of bulk resistivities and different n-type doping elements (Sb, P and As) were studied. We show from Hall measurements that the mc-Si material has excellent electrical transport properties. The mobilities are close to the theoretical limit, which is given mainly by scattering at acoustical phonons. Mobilities so close to the theoretical value have, to our knowledge, not been demonstrated for comparable p-type mc-Si wafers. Additional measurements on high quality p-type mc-Si material support this statement. This means that other scattering mechanisms reduce the mobility in p-type mc-Si material, but are not present in n-type silicon. Lifetime measurements were conducted by μW-PCD using an iodine-ethanol surface passivation. This passivation was used preferably to SiN<sub>x</sub>, as in some experiments the hydrogen from the PECVD SiN seemed to passivate the bulk at deposition temperatures. Average values in excess of 120 μs over large areas were measured. In order to exploit the good material properties of n-type mc-Si, solar cell concepts must be developed and the processes optimised. B-diffusion is the most problematic step as it is considered to be both destructive to material quality and energy consuming. In this paper, we show that a BBr<sub>3</sub>-emitter diffusion is possible at moderate temperatures without degrading the carrier lifetime of the mc-Si material. An additional contribution from Libal et al. on solar cell processing is included in this conference.
    Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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    ABSTRACT: A key issue for future solar cell processes is a diffusion sequence which enables fast and clean production, for example by spin-on and spray-on dopants. These diffusion processes have in common that the emitter diffusion is applied on one wafer side, whereas the POCl<sub>3</sub> emitter process leads to double-sided doping. For higher throughput the POCl<sub>3</sub> process could be adapted by using one slot for two wafers. This so called back-to-back process leads to one-sided emitters and increases industrial throughput but could result in lower cell efficiency due to reduced gettering during phosphorus diffusion. In this study we investigate the difference in solar cell performance for cells processed in a back-to-back and standard POCl<sub>3</sub> process. Furthermore, X-ray fluorescence and SIMS measurements were performed on Cz material to study the effect of emitter over-compensation by aluminium. These experiments should clarify if the phosphorus of double-sided emitter cells is partly overcompensated or diffuses into the Al layer of the rear contact.
    Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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    ABSTRACT: A simplified laboratory process with one photolithographic step for front junction solar cells on n-type multicrystalline (mc) silicon has been developed. The emitter diffusion is done in an open tube furnace with BBr<sub>3</sub> and back-surface-field diffusion using POCl<sub>3</sub>, loading the wafers front-to-front and back-to-back respectively and thus avoiding additional etching steps. The front surface has been passivated by a 10 nm thermal oxide grown in a tube furnace. With this simple process, efficiencies of 11.0% on n-type mc-Si and 11.5% on n-type Cz-Si have been realized without antireflection coating and without surface texture. Applying a double layer antireflection coating (DARC) on these cells, efficiencies of 16.4% on Cz-Si and 14.7% on mc-Si have been achieved.
    Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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    ABSTRACT: The influence of gettering or defect passivation steps on recombination activity in the vertically grown, multicrystalline ribbon materials edge-defined film-fed growth and string ribbon silicon has been investigated with the help of photoconductance decay. In contrast to well-known results of integral measurements, spatially resolved lifetime mappings have been obtained by applying microwave detection technique.This aspect of spatial resolution has been found to be indispensable for investigating the impact of different processing steps on material quality in an accurate way. Apart from strong variations in as-grown lifetimes that have been found throughout vertically grown silicon wafers, this is due to areas of comparable starting lifetimes which have been revealed to react very differently to applied processing steps. After processing, some of them reach minority charge carrier lifetimes of more than 300μs whereas others just show values of a few microseconds. As a consequence, the results of integral measurements strongly depend on the nature of areas incorporated in the specific sample. An impression of the corresponding uncertainties inherent to integral measurements has been obtained by statistical evaluation of spatially resolved lifetime data.
    Solar Energy Materials and Solar Cells 02/2005; 85(4):559-572. · 5.03 Impact Factor
  • Holger Knauss, M. McCann, Peter Fath
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    ABSTRACT: In this paper we present a realisation of large area metallization wrap through (MWT) back contact solar cell with electroless plated contacts. The MWT cell is a very promising back contact solar cell concept, since if electroless metallization is used the additional effort required to process MWT compared to conventional buried contact cells is limited to the formation of a small number of holes. In addition, the concept can easily be applied to very large area cells and is suited to thin wafers. Two different metallization processes are investigated. The best MWT cells of each process reached an efficiency of 16.6% and 17.0% on an area of 140 cm2. The efficiency difference originates from a difference in bulk lifetime, resulting from the generation of meta-stable defects during the Ni sintering step required for one of the processes.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2005;
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    ABSTRACT: This paper presents the characterisation of casted n-type mc-Si material using microwave detected photocurrent decay (µW-PCD) measurements of the minority charge carriers and other characterisation methods. The as-received material shows a very high average lifetime of the minority charge carriers (>100 µs), which confirms recent results on n-type mc-Si reported in the literature (1). In particular, this paper focuses on material improvement by various gettering-techniques and by hydrogen-passivation. Application of these processes leads to effective charge carrier lifetimes of up to 220 µs averaged over the whole wafer. After testing different surface passivation methods for lifetime measurements, we found the PECVD SiNx used at our laboratory to be bulk-passivating even without firing, therefore we chose iodine/ethanol-solution (I/E) for surface passivation in order to measure the real properties of the bulk. Boron-diffusion is considered to be the most destructive step during n-type cell-processing. We show that BBr3-emitter diffusion is possible without degrading the carrier lifetime of the mc-Si material (and that the degradation occurring when using a spin-on-dopant with belt-furnace diffusion can be partially repaired by a subsequent phosphorous-diffusion.)
    01/2004;
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    ABSTRACT: This paper describes the further development of an industrial processing sequence for large area multicrystalline silicon solar cells applying the buried contact technology for solar cell metallisation. Passivation of crystal defects was investigated by remote plasma hydrogenation at different stages of the processing sequence. The reduction of emitter recombination was examined by the optimisation of the emitter diffusion as well as the growth of a thermal oxide. A record high efficiency for a large area multicrystalline silicon solar cell of 17.6% (V/sub oc/=632.5 mV, J/sub sc/=35.85 mA/cm/sup 2/, FF=77.7%, cell area 144 cm/sup 2/, independently confirmed at FhG-ISE, Germany) was achieved. For this cell, a loss analysis was done to determine the potential for further improvements in solar cell efficiency.
    Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003