Universiẗat, Diss., 2005--Hamburg. / Erscheinungsjahr an der Haupttitelstelle: 2004.

Zugl.: Konstanz, Univ., Diss., 2005

Techn. Hochsch., Diss., 2006--Aachen.

Zugl.: Darmstadt, Univ., Diss., 2009

Universiẗat, Diss., 2000--Konstanz.

University, Diss., 2003--Stuttgart.

Techn. Universiẗat, Diss., 2004--Darmstadt.

Higher conversion efficiencies while reducing costs at the same time is the ultimate goal driving the development of solar cells. Multi-crystalline silicon has attracted considerable attention because of its high stability against light soaking. In case of solar grade multi-crystalline silicon the rigorous control of metal impurities is desirable for solar cell fabrication. It is the aim of this thesis to develop a new manufacturing process optimized for solar-grade multi-crystalline silicon solar cells. In this work the goal is to form solar cell emitters in silicon substrates by plasma immersion ion implantation of phosphine and posterior millisecond-range flash lamp annealing. These techniques were chosen as a new approach in order to decrease the production cost by reducing the amount of energy needed during fabrication. Therefore, this approach is called “Low Thermal Budget” process. After ion implantation the silicon surface is strongly disordered or amorphous up to the depth of the projected ion range. Therefore, subsequent annealing is required to remove the implantation damage and activate the doping element. Flash lamp annealing in the millisecond-range is demonstrated here as a very promising technique for the emitter formation at an overall low thermal budget. During flash lamp annealing, only the wafer surface is heated homogeneously to high temperatures at a time scales of ms. Thereby, implantation damages are annealed and phosphorous is electrically activated. The variation of pulse time allows to modify the degree of annealing of the bulk region to some extent as well. This can have an influence on the gettering behavior of metallic impurities. Ion implantation doping got in distinct consideration for doping of single-crystalline solar cells very recently. The efficient doping of multi-crystalline silicon remains the main challenge to reduce costs. The influence of different annealing techniques on the optical and electrical properties of multi-crystalline silicon solar cells was investigated. The Raman spectroscopy showed that the silicon surface is amorphous after ion implantation. It could be demonstrated that flash lamp annealing at 1000 °C for 3 ms even without preheating is sufficient to recrystallize implanted silicon. The sheet resistance of flash lamp annealed samples is in the range of about 60 Ω/□. Without surface passivation the minority carrier diffusion length in the flash lamp annealed samples is in the range of 85 µm. This is up to one order of magnitude higher than that observed for rapid thermal or furnace annealed samples. The highest carrier concentration and efficiency as well as the lowest resistivity were obtained after annealing at 1200 °C for 20 ms for both, single- and multi-crystalline silicon wafers. Photoluminescence results point towards phosphorous cluster formation at high annealing temperatures which affects metal impurity gettering within the emitter. Additionally, in silicon based solar cells, hydrogen plays a fundamental role due to its excellent passivation properties. The optical and electrical properties of the fabricated emitters were studied with particular interest in their dependence on the hydrogen content present in the samples. The influence of different flash lamp annealing parameters and a comparison with traditional thermal treatments such as rapid thermal and furnace annealing are presented. The samples treated by flash lamp annealing at 1200 °C for 20 ms in forming gas show sheet resistance values in the order of 60 Ω/□, and minority carrier diffusion lengths in the range of ~200 µm without the use of a capping layer for surface passivation. These results are significantly better than those obtained from rapid thermal or furnace annealed samples. The simultaneous implantation of hydrogen during the doping process, combined with optimal flash lamp annealing parameters, gave promising results for the application of this technology in replacing the conventional phosphoroxychlorid deposition and diffusion.

Die vorliegende Arbeit befasst sich mit den Untersuchungen zum Einfluss der Züchtungsbedingungen auf die Eigenschaften von multikristallinen (mc) Silizium-Kristallen. Im Mittelpunkt stehen Züchtungsexperimente mit einer gezielten Variation der Züchtungsaufbauten und Züchtungsgeschwindigkeiten. Die gezüchteten Kristalle wurden umfassend charakterisiert im Hinblick auf die Kohlenstoffkonzentration, die Kornstruktur, die Vesetzungsdichte, Verteilung der Ausscheidungen und Ladungsträgerlebensdauer. Zusätzlich wurde die Versetzungsanordnung in Abhängigkeit von der Wachstumsrate bzw. Abkühlrate systematisch untersucht. Als Ergebnis wurde gezeigt, dass die Züchtungsbedingungen die Kohlenstoffkonzentration, die Versetzungsdichte, die Bildung von den Ausscheidungen sowie die Ladungsträgerlebensdauer beeinflussen können, jedoch nicht die Korngröße. Es wurde ein direkter Zusammenhang zwischen Ausscheidungsgebieten und erhöhte Versetzungsdichte beobachtet. Im Rahmen der Arbeit wurde festgestellt, dass die endgültige Versetzungsstruktur sich als Resultat von Gleit- und Erholungsprozessen darstellt.

The efficiency of multicrystalline (mc) silicon solar cells depends strongly on the fraction of recombination active crystal defects. This work focuses on a systematic analysis of how the area fraction of recombination active crystal defects and thus the solar cell efficiency is af-fected by the grain structure of mc-silicon wafers, i.e. grain size, grain orientation and type of the grain boundaries between adjacent grains. For that purpose a new characterization method was developed which allows the measurement of the grain orientation and grain boundary type of full 156x156 mm² mc-silicon wafers. The results of the grain structure analysis were correlated with the etch pit density, the recombination active area fraction measured by photo-luminescence imaging, and the solar cell efficiency in order to quantify the most important features of the grain structure, which were relevant to obtain high quality mc-silicon wafer material. For the determination of the grain orientation and grain boundary type two metrology sys-tems were combined. The so-called grain detector determines the geometrical data of each grain (size and form) by a reflectivity measurement. Afterwards the wafer with the geomet-rical information of all grains is transferred into the so-called Laue Scanner. This system irra-diates each grain larger 3 mm² with white x-rays and creates a backscatter diffraction pattern (Laue pattern) for each grain. From this Laue pattern the grain orientation and the grain boundary type of neighboured grains is calculated and statistically analysed in combination with the geometrical data of the grain detector. In this work the grain structure of twelve industrially grown mc-silicon bricks, which were produced by different manufacturers, and two laboratory grown bricks were investigated. Seven of these bricks show a fine grain structure. This material named class F is considered to be typical for so-called High Performance Multi (HPM) silicon. The other bricks show a coarse-grained structure. This grain structure was called class G and corresponds to the con-ventional mc-silicon material. The results show that the grain structures of the start of the crystallization process differ sig-nificantly between class F and class G. The class F mc-silicon wafers have a uniform initial grain size (characterized by coefficient of variation CV¬KG < 2.5) and grain orientation (charac-terized by coefficient of variation CVKO < 1.5) distribution with a small mean grain size (< 4 mm²) and a high length fraction of random grain boundaries (> 60 %) in comparison to the class G wafers. Despite the totally different initial grain structure for the class F and class G bricks, the grain structure of the wafers which represent the end of the crystallization process is more or less comparable. It can be concluded that the development of the grain structure along the crystal height of the class F bricks is driven by an energy minimization due to the surface energy and the grain boundary energy, that means that the share of (111) oriented grains having the lowest surface energy and the share of ∑3 grain boundaries having the lowest interface energy increase from the start of crystallization to the end. This phenomenon could not be observed for the class G bricks, which show a decreasing ∑3 length fraction and a decreasing area fraction of {111} oriented grains. This energetically unfavourable grain structure development is not clear so far but it means another kind of energy minimization effect must exist within class G. This could be for instance the formation of dislocations. The grain structure investigations show clearly that especially the initially fine-grained struc-ture of the class F bricks, i.e. at the start of crystallization, influences beneficially the area fraction of recombination active defects and the solar cell efficiency subsequently. This ob-servation can be explained as follows. Reduced dislocation cluster formation: • The small grain sizes in combination with the low length fraction of ∑3 grain bounda-ries capture the dislocations within a grain. Dislocations are not able to move across the grain boundaries which have not the ∑3-type within moderate stress and tempera-ture fields. This prohibits the formation and expansion of large dislocation cluster. • The previously described energetically driven grain selection and the continuously in-creasing grain size from bottom to top leads to an overgrowth of grains. This means that also dislocated grains will disappear which also prohibits the formation of large dislocation cluster. Reduced possibility of dislocation formation: • Compared to the class G bricks the area fraction of {111} oriented grains is reduced. Therefore, the possibility of the formation of dislocations is reduced, because they would be activated first in {111} oriented grains taking the Schmidt factor in account which is lowest for {111} oriented grains. After the dislocation generation within a {111} oriented grain, the dislocation can move forward on 3 of 4 possible {111} slip planes which have an angle of 19.5° with regard to the growth direction. No other ori-entation has more slip planes for the dislocation movement which have an angle smaller 20° with regard to the growth direction. These arguments in combination with the high reproducibility of the characteristic initial class F structure can explain the observed low recombination active area fraction from start to end of crystallization which was smaller 5 % and especially the low variation of 2 % of the electrical active wafer area in between the class F bricks. One can also easily explain the higher recombination active area fraction up to 14 % and the large variation of 10 % between the class G bricks due to the obtained grain structure data. These differences in the recombination active area fractions are reflected in the solar cell efficiency which is 0.4 % higher for the class F bricks compared to the class G bricks. In consideration of the above mentioned reasons it is not beneficial for the industrial ingot production technology to increase the ingot height further, due to the fact that the advanta-geous initial grain structure properties of class F bricks disappear with increasing crystal height.