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27th European Photovoltaic Solar Energy Conference and Exhibition; 1014-1017, A main goal in manufacturing crystalline silicon wafers is to produce wafers with flat and low damaged surfaces. The breakage ratio during the wafer sawing process and solar cell production is still a main problem for every crystalline silicon wafer producer. Hence, it is necessary to know the subsurface damage after the wafering process. Earlier investigations show that surface roughness measurements are not sufficient for subsurface damage conclusions. Therefore, it is important to determine the maximum crack depth and the crack depth distribution of representative wafers. In our contribution we present a new and quick method for investigating the subsurface damage at bevel polished wafer samples and compare it with standard methods.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1045-1048, Carrier lifetime measurements are a very powerful tool in the semi-conductor material characterization as they are very sensitive to variations in the material quality. However, the measurable carrier lifetime always depends on both material and surface properties. A number of techniques have been established to separate these two contributions to the measured lifetime for thin wafers on the one side or thick blocks on the other side. In this work, we present a systematic analysis of the results that can be obtained on samples of arbitrary thickness. In particular we study the measurable transient lifetime on unpassivated and passivated silicon-slices of varying thickness. It turns out that measurement tool specific parameters such as measurement time, sensor penetration length, and length of the initial flash influence the measurable lifetime in a significant way.

26th European Photovoltaic Solar Energy Conference and Exhibition; 1492-1496, The results of a round-robin experiment, which determines the electrical quality of silicon, are presented. Five participants measure the excess charge carrier lifetime (using RF-PCD) and the excess charge carrier decay time (using MW-PCD) on mono- and multicrystalline ingots as well as on adjacent surface passivated wafers. Applying a defined set of measurement parameters a quite good reproducibility is found for each type of measurement. The standard deviation within the measurement methods ranges from 3 % to maximal 19 %. Between the measurement methods large differences are found, which are explained by the properties of each measurement method. For the investigated samples the estimated lifetime determined by RF-PCD on the ingot comes closest to the lifetime measured on the surface passivated wafers.

X-ray optics fabrication at Brookhaven National Laboratory has been facilitated by a new, state of the art magnetron sputtering physical deposition system. With its nine magnetron sputtering cathodes and substrate carrier that moves on a linear rail via a UHV brushless linear servo motor, the system is capable of accurately depositing the many thousands of layers necessary for multilayer Laue lenses. I have engineered a versatile and automated control program from scratch for the base system and many subsystems. Its main features include a custom scripting language, a fully customizable graphical user interface, wireless and remote control, and a terminal-based interface. This control system has already been successfully used in the creation of many types of x-ray optics, including several thousand layer multilayer Laue lenses.Before reaching the point at which a deposition can be run, stencil-like masks for the sputtering cathodes must be created to ensure the proper distribution of sputtered atoms. Quality of multilayer Laue lenses can also be difficult to measure, given the size of the thin film layers. I employ my knowledge of software and algorithms to further ease these previously painstaking processes with custom programs. Additionally, I will give an overview of an x-ray optic simulatormore » package I helped develop during the summer of 2010. In the interest of keeping my software free and open, I have worked mostly with the multiplatform Python and the PyQt application framework, utilizing C and C++ where necessary.« less

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2344-2347, We investigate the variation of the interstitial iron content in CZ silicon wafers, which exhibit increased interstitial iron content, during different annealing steps. In particular, phosphorous diffusion gettering is studied experimentally and by simulations. Applying a slow cooling ramp after phosphorous diffusion, an additional reduction of the interstitial iron content by a factor of ten is found. With increasing phosphorous diffusion time slightly increasing interstitial iron content is observed. The iron solubility integral in the phosphorous doped region increases with increasing phosphorous diffusion temperature and is correlated to the reciprocal sheet resistance.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2486-2489, In recent years photoluminescence (PL) has been introducd as a fast and versatile tool for characterising silicon wafers and solar cells. For PL imaging a laser as excitation source is usually used. In this contribution we replaced the laser with light emitting diode (LED) arrays, which have the advantage of avoiding the need to widen and homogenise the laser beam. By means of LED arrays, PL images can be obtained with the same quality as the laser beam excited photoluminescence. There is experimental evidence that LED excited PL is capable of measuring the lifetime distribution without artifacts that are possibly caused by internal electrical fields and that are encountered in other methods like microwave detected photoconductance decay (MWPCD). A possibly new effect is encountered in a solar cell under open circuit condition where finger-interrupted regions show unexpected high PL signal in comparison to electroluminescence.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2157-2159, We investigated the impact of using low purity single crystalline CZ silicon on the performance of silicon solar cells manufactured in the industrial process lines. Groups of wafers carefully chosen from a highly contaminated ingot as well as an initially pure one (as reference) underwent various high temperature steps as encountered at different stages of the process chain. The analysis was carried out by means of minority carrier lifetime measurements, which also serve to assess the interstitial iron content. The total concentration of the existing impurities at the beginning and end of the process line was evaluated using neutron activation analysis. Our results show that initially contaminated CZ silicon responds very positively to segregation gettering provided by phosphorous diffusion, aluminum alloying and post-diffusion annealing. A skilled tailoring of these steps results in a huge reduction of the interstitial iron content and a very considerable increase of the charge carrier lifetime. Results of the neutron activation analysis pointed to the gettering of other impurities than iron.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2060-2064, Minority carrier trapping, which is often observed in silicon used for photovoltaics, influences the determination of the excess carrier density from photoconductance measurements. In this contribution the Hornbeck- Haynes-model is used to simulate the apparent excess carrier density, which can be determined by MWPCD measurements. The three trap parameters: trap density, emission and trapping time constant are varied and their impact on the apparent excess carrier density is discussed. A kink in the apparent excess carrier density is found to correlate with the trap density. This result is used to determine the trap density from MWPCD measurements. The impact of a constant bias light on the apparent excess carrier density is investigated by simulation and experiment. The influence of trapping on the MWPCD signal can be eliminated by the bias light.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1640-1644, We have performed a systematic variation of the wet chemical oxidation and the subsequent oxide etching steps during the cleaning of Czochralski (CZ) single crystalline silicon wafers prior to surface passivation. The optimization of these preconditioning steps was carried out on saw-damage etched or textured Si wafers subsequently passivated by amorphous silicon nitride (a-SiNx:H) or chemical passivation by an iodine ethanol (I/E) solution. Measuring the carrier lifetime using the spatially resolved microwave detected photoconductance decay, we monitored the impact of the wet-chemical surface conditioning on the surface morphology and wafer base doping type. For damage etched surfaces in alkaline potassium oxide solutions and passivated by iodine ethanol, an optimum surface passivation is obtained by omitting the last water rinse (as used in the standard clean) and adding a hot water treatment as final step in the cleaning procedure. While this results was found for both p-type and n-type wafers, suggesting that the passivation mechanism is based on the saturation of dangling bonds, a strong dependence on the doping type was observed for the passivation by a-SiNx:H. Prior to passivation by a-SiNx:H, the best preconditioning for n-type was achieved by adding a hot water treatment and subsequent etching of the oxide by hydrofluoric acid (HF). For p-type however, no improvement was achieved with respect to the standard cleaning step. This dependency on the base doping type could be attributed to the passivation mechanism of a-SiNx:H based on the field effect. By employing ammonium fluoride as final etching solution instead HF, followed by hot water treatment, compared to the standard treatment, an improvement of the lifetime can be observed for both surface structures, indicating that the surface micro-roughness can be reduced and the contamination can be removed.