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D. Biro,
B. Thaidigsmann,
F. Clement,
A. Wolf,
E. Lohmüller,
S. Mack,
T. Fellmeth,
A. Drews,
A. Spribille, E. A. Wotke,
F. Lottspeich,
M. Hofmann,
U. Jäger,
R. Preu
Proceedings of the 37th IEEE Photovoltaic Specialists Conference, Seattle, Washington, USA; 01/2011
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ABSTRACT: Thermal silicon oxides are known to very effectively passivate silicon surfaces. Choosing a water vapor ambient instead of a dry oxygen atmosphere increases the oxidation rate by about one order of magnitude and considerably reduces process time and costs. State of the art pyrox systems produce steam by pyrolysis of hydrogen and oxygen gas. A new approach is the purification of vaporized deionized (DI) water. In this work we present a direct comparison of both steam generation systems, which are connected to the same quartz tube of an industrial high quality tube furnace. The higher steam saturation of the direct steam process enhances the growth rate by about 20% compared to a pyrolytic steam based process. On low-resistivity p-type substrates, excellent surface recombination velocities of around 25 cm/s are found for both systems after a forming gas anneal. Detailed characterization shows similar physical properties of the oxide layers grown by either steam from pyrolysis or purified steam. Moreover, thermal oxide rear surface passivated silicon solar cells show similar high efficiencies exceeding 18.0% irrespective of the applied steam generation technology.
Solar Energy Materials and Solar Cells 01/2011; 95:2570-5. · 4.54 Impact Factor
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ABSTRACT: Most high-efficiency concepts for silicon solar cells utilise passivated surfaces and/or novel metallisation approaches. For these devices an anneal step, preferential in forming gas ambient, is beneficial or even crucial. Annealing is known to activate or improve surface passivation, facilitate metal-silicide formation, improve adhesion of metal contacts and cure potential crystal damage. Challenges for industrial solutions of anneal processes are a precise control of the process atmosphere and temperature, as well as a high throughput and easy integration into production lines. We present a high capacity inline annealing system that addresses these tasks with a throughput of over 1000 wafers per hour. The gas locks, located at the entrance and exit of the furnace, allow for an effective separation of laboratory and forming gas process ambient, which results in a residual oxygen concentration of a few ppm. Inline annealed silicon solar cells with a thermal oxide passivated rear surface show the same conversion efficiency as reference cells, which are annealed in a single wafer reactor. The specific cost of inline annealing in forming gas is calculated to be below 1.1 €ct/W
Proceedings of the 26th European PV Solar Energy Conference and Exhibition, Hamburg, Germany; 01/2011
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ABSTRACT: We use the recently introduced Silicon Nitride Thermal Oxidation (SiNTO) process for the industrial fabrication of silicon solar cells that feature a thermal oxide passivated rear surface and local rear contacts. The SiNTO process represents an innovative approach for the fabrication of a passivated emitter and rear cell (PERC), since the front end part from the conventional process sequence is maintained. We apply mostly industrial production equipment using Czochralski silicon wafers that are partly processed in an industrial production line. Conventional screen printing is used for the formation of the front contacts. A stable conversion efficiency of 18.6% (independently confirmed) is achieved for a PERC device fabricated from conventional boron doped Cz-Silicon by means of the SiNTO process. The average efficiency of a batch of 24 SiNTO cells is 18.4%, measured after fabrication (not stabilized). A test module fabricated from 16 SiNTO solar cells features a fill factor of 76.2% and an open circuit voltage of 10.16 V, corresponding to an average of 635 mV per cell.
Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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ABSTRACT: Higher solar cell efficiencies enable a reduction of the cost per watt ratio, if production effort is maintained at an acceptable level. A proven high-efficiency concept is the passivated emitter and rear cell (PERC). However, the transfer of this solar cell structure from demonstrator level to industrial application is challenging. We present a simple approach for the industrial fabrication of PERC solar cells which utilizes the simultaneous passivation of the front emitter and the rear surface by a thin layer of thermally grown oxide. This Thermal Oxide Passivated All Sides (TOPAS) structure represents an industrially feasible implementation of the PERC concept. Instead of using masking or sacrificial layers to obtain a structure with a textured, diffused front surface and a plain non-diffused rear surface, side selective wet chemical etching is chosen in this work, since it features a higher cost reduction potential. The current cell design features a selective emitter structure, introduced by laser-doping in combination with conventional screen-printed front contacts. With the presented approach we achieve an initial efficiency of 18.9 % on large area (149 cm<sup>2</sup>) 180 μm thick, Czochralski grown, boron doped p-type wafers. The stabilized device reaches a high open circuit voltage of V<sub>oc</sub> = 641 mV. The comparison of the internal quantum efficiency of the TOPAS device and a full Al-back surface field (BSF) reference reveals a strong advantage in the blue and red response for the TOPAS concept.
Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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A. Wolf, E. A. Wotke,
S. Mack,
J -F. Nekarda,
D. Biro,
R. Preu,
K. Schlegel,
T. Weber,
J. Lossen,
T. Böscke,
A. Grohe,
P. Engelhardt,
J. W. Müller,
G. Schubert,
H. Plagwitz,
Y. Gassenbauer
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ABSTRACT: We apply the recently introduced Silicon Nitride Thermal Oxidation (SiNTO) process for the industrial fabrication of silicon solar cells that feature a thermal oxide-passivated rear surface. The SiNTO process utilises a SiNX anti-reflection layer for masking the front side of the solar cell during the thermal oxidation process. This masking layer limits the growth of the thermal oxide to the uncoated rear surface. Laser fired contact (LFC) technology is applied to form the local rear contacts. An efficiency of 18.6% (annealed) and 18.4 % (stable, independently confirmed) is achieved for a PERC device fabricated from boron-doped Czochralski-silicon by means of the SiNTO process. The average efficiency of a batch of 34 SiNTO cells is 18.2%, measured after fabrication (not stabilised). Parallel processed Al-BSF references reach average efficiencies of 17.7%. Thus, the SiNTO approach enables an efficiency increase of 0.5% absolute compared to conventional Al-BSF technology. When introducing soldering pads, the efficiency gain for SiNTO compared to Al-BSF cells even increases to 0.8% absolute. Finally, we use a comprehensive analytical model to estimate the optimum bulk resistivity for locally contacted devices fabricated from conventional Czochralski silicon material. These calculations account for the bulk recombination caused by the formation of boron-oxygen complexes under carrier injection.
Proceedings of the 25th European Photovoltaic Solar Energy Conference and Exhibition, Valencia, Spain; 01/2010
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D. Biro,
S. Mack,
A. Wolf,
A. Lemke,
U. Belledin,
D. Erath,
B. Holzinger, E.A. Wotke,
M. Hofmann,
L. Gautero,
S. Nold,
J. Rentsch,
R. Preu
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ABSTRACT: In this paper various options to integrate thermal oxidation into industrial cell production are presented, maintaining large parts of the standard cell fabrication process. Both the use of thin (15 nm) and thick (200 nm) wet thermally grown oxides are successfully implemented into pilot production at the Fraunhofer production research platform PV-TEC. Solar cells are fabricated with both type of processes. On large area (149 mm<sup>2</sup>) Cz-Si substrates 18% efficiency have been achieved. Furthermore a cost calculation including process and equipment improvements is carried out for the thermal oxidation process and it is shown that the cost for such a process can be well below 10 ¿ct per wafer for thick and below 5 ¿ct per wafer for a thin oxide, thus meeting industrial requirements for cost effective production.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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ABSTRACT: We present a novel method for the industrial fabrication of a silicon solar cell that features an oxide-passivated rear surface. The SiNTO process (silicon nitride thermal oxidation) utilizes a SiN<sub>X</sub> layer for masking the front side of the solar cell during the thermal oxidation process. This masking layer prevents the oxidation of the textured and phosphorus-doped emitter surface and limits the growth of the thermal oxide to the uncoated rear surface. After oxidation the SiN<sub>X</sub> layer remains at the front side of the cell and serves as an anti-reflection coating (ARC). In this work we investigate the impact of the thermal oxidation process on the SiN<sub>X</sub> film and the underlying emitter and analyze the passivation quality of the thermal oxide. The oxidation process results in a sufficiently passivated rear surface with a surface recombination velocity of ~40 cm/s, measured after Al-metallization and post-metallization anneal. Measurements of the emitter sheet resistance and secondary ion mass spectrometry (SIMS) profiling reveal that the SiN<sub>X</sub>-coated emitter reorganizes slightly during the oxidation process whereas an uncoated reference is strongly affected. The emitter dark saturation current density is affected as well. Oxide-passivated solar cells are fabricated from Czochralski (Cz) silicon using the SiNTO approach. A 136 cm<sup>2</sup> large cell fabricated using industrial processing equipment reaches an efficiency of 17.8% (stable), which demonstrates the feasibility of the SiNTO process.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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ABSTRACT: In this paper, we investigate the impact of a solar cell fabrication process on the properties of thermal oxide-passivated Si surfaces. Therefore, symmetric thermally oxidized silicon wafers are exposed to the cell process and analyzed by means of quasi steady-state photoconductance and capacitance-voltage-measurements. The oxide thickness reduces during processing. Alkaline texturing, diffusion and firing processes are shown to decrease the effective carrier lifetime. Moreover, the total charge density decreases and the interface trap density at midgap increases along the manufacturing process. The latter complies with the observed reduction of the effective carrier lifetime. Nevertheless this process induced degradation of the oxide passivation is fully reversible with aluminum deposition and subsequent annealing in forming gas. After a post-metallization anneal, on saw damage etched surfaces of 1
Proceedings of the 24th European Photovoltaic Solar Energy Conference, Hamburg, Germany; 01/2009
