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37th European Photovoltaic Solar Energy Conference and Exhibition; 452-458, Feedback on the material quality of silicon ingots is highly beneficial in the photovoltaic production chain. It is crucial for crystal growers to improve the quality and optimize the crystallization process. Moreover, for solar cell manufacturers, knowing the quality beforehand helps to sort out the bad quality material thereby reducing the costs and enhancing the total yield. Therefore, rating material quality already on the brick level is highly valuable for the effective optimization of the value chain in both directions. In this paper, we propose a method to classify the silicon bricks based on their electrical quality. Due to our comprehensive data set and feature detection, the model is capable to predict the quality of even edge and corner bricks of the ingot. We introduce a novel feature extraction method to quantify quality-related features from spatially-resolved microwave-detected photoconductivity (MDP) brick measurements. Further, a machine-learning-based prediction model is developed to predict the open-circuit voltage (Voc) of solar cells from these features. A comparative analysis for brick quality estimation for inner and outer bricks of high-performance multi (HPM) and cast-mono (CM) silicon bricks is provided. The best mean absolute error in prediction achieved for HPM and CM materials is 3.1 mV and 4.8 mV, respectively. more...

36th European Photovoltaic Solar Energy Conference and Exhibition; 361-365, A novel non-destructive and contactless approach for measuring the base resistivity of emitter-diffused, partially-processed wafers is introduced. The method is based on temperature-dependent resistivity analysis and referred as temperature-dependent resistivity slope model (TRSM). It is developed for p-type boron-doped silicon wafers used in industrial applications with base resistivities ranging from 1 to 5 Ωcm. A sensitivity analysis is carried out on TRSM to determine the limits of this simple approach and the results show that TRSM can determine the base resistivity with an accuracy of 90%. The main limiting factor is the reproducibility from the measuring tool with a mean error of 6% on the TRSM results. Finally, the base resistivity of emitter-diffused, partially-processed wafers (in this work referred as precursor wafers) obtained from TRSM is compared to two reference approaches and showed mean error percentages of less than 10%. more...

35th European Photovoltaic Solar Energy Conference and Exhibition; 731-735, In order to connect the information from incoming inspection and IV-characterization or even crystallization information, wafers need to be identified and tracked. Identifying each wafer helps to provide a better analysis of the influence or failures of production processing steps on each individual wafer rather than a general statistical analysis approach of broader number of wafers. Nowadays, several wafer identification methods are available to simplify the tracking of wafers. In this work, we present an approach of exploiting the already integrated cameras within the inline characterisation equipment for automatic detection and recognition of wafer IDs. To do so, Optical Character Recognition (OCR) is applied through adapting Tesseract open source software with optimised preprocessed images. Our approach presents an identifier of wafers which don’t have BSCs or DMCs but alphanumerical wafer IDs. Before using our algorithm, (alpha)numerical wafer labeling is required. For numerical wafer IDs in best cases, an average ID recognition rate of 99% was achieved. For alphanumerical wafer IDs, automatic software correction is highly relevant to solve the confusion of software when switching from numbers to letters and vice versa. A comparison between wafer ID reading techniques revealed that the best average wafer ID reading accuracy of 98% was obtained using brick slice code reader. The data matrix code reader revealed an average wafer ID accuracy of 93%. Finally, our OCR-based ID reader software had an average wafer ID reading accuracy of 85% for alphanumeric IDs. more...

35th European Photovoltaic Solar Energy Conference and Exhibition; 222-228, While for monocrystalline silicon ingot slicing diamond wire sawing processes are widely used, the general approach for slicing multicrystalline bricks has been based on slurry wire processes. Driven by a strong decrease in wafer prices recently, wafer manufacturers of multicrystalline materials switched rather quickly to the diamond-cut process, although several severe technological challenges were to be faced. Unfortunately, sawing is more difficult and an adaptation of solar cell production is essential for texturing processes considering diamond-cut wafer surface morphology. Also characterization methodologies must be adapted to these new material properties. Especially saw mark inspection and thickness topology measurements are key aspects of developments. General imaging methods also need to be evaluated for defect detection and wafer rating purpose. In this paper, an overview of inline characterization methods and their applicability on diamond-cut wafers will be given. Five out of eight methods need hardware or software adjustments to work on diamond-cut wafers. Special attention is paid to the photoluminescence analysis which allows a forecast of solar cell parameters if the relevant defect features are correctly extracted from the images. Due to the increased optical artifacts, this is not a trivial task. more...

33rd European Photovoltaic Solar Energy Conference and Exhibition; 870-874, Multiple tools and methods for the optical and electrical quality control of high efficiency silicon solar cells during their industrial production are available on the market, even more are discussed in the literature. We summarize the possibilities of these tools along the value chain from wafer to cell for the case of passivated emitter and rear cells (PERC) and discuss some showcases. We recommend a broad inline quality control before solar cell production in order to discard wafers with insufficient quality. Especially PL-Imaging can reveal material defects and process variations and thus acts as a versatile tool for the whole production chain. An economic evaluation shows that a CAPEX spending of 250,000 EUR for metrology equipment is already justified if cell efficiency is increased by only 0.03 %abs from a reference efficiency 20.61 % to 20.64 %. Combining technological and economical aspects, this paper gives an overview over recent inline metrology tools and upcoming challenges. more...

Purpose – The purpose of this paper is to show how knowledge intensive information technology (IT) applications within an organisation can be identified and analysed to achieve two corporate goals: First, an optimisation of the corporate IT landscape that avoids inefficiencies or redundancies. Second the implementation of a knowledge management (KM) system that is aligned with the corporate IT infrastructure. Design/methodology/approach – Methodically, the approach can be described as a practical two-step procedure. In the first step the knowledge intensive IT systems are identified through a questionnaire that is performed in the IT department of the organisation. Based on the expertise of the IT management adequate information concerning benefits and utilization of the applications and the description of technical conditions can be determined. On the basis of the work of the first step, selected user groups (key-user, admin-user, heads of departments, etc.) are surveyed on a broader base through semi-structured interviews. The focus here is to determine the application within its processes and to identify the importance in the fulfilment of the daily tasks as well as the capabilities in knowledge management. Therefore the survey covers the main questions regarding the classification of KM and provides a solid foundation for optimisations regarding the IT infrastructure. The two-step approach also provides the flexibility to identify future processes concerning an appropriate KM system and to identify practical adaptions of the existing IT landscape. Originality/value – The suggestion of a newly developed method to identify and assess knowledge intensive IT systems – what includes hard- and software – within an organisation. The results of the method can be used to develop recommendations to improve the conceivably of already existing KM or to originate an organisational KM as well as to enhance the existing IT landscape. This includes in particular the consideration of the processes in which knowledge is generated, stored, used and shared. Practical implications – The identification, utilization and harmonization of KM intensive systems can be a substantial advantage during the implementation or enhancement process of KM for two reasons: First, the important and implicitly for KM purposes used systems are identified and evaluated before the inception of the organisational KM. Second, the knowledge management orientation of the approach allows reducing both, the complexity and the variety of IT applications within an organisation. more...

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1038-1043, In this paper, seven Czochralski (Cz) and one Float Zone p-type silicon materials are processed as PERC solar cells in the Fraunhofer ISE PVTEC pilot line. In order to compare materials with different bulk resistivities, the rear contact distance is varied in order to keep the spreading resistance in the same range. Asprocessed and regenerated, energy conversion efficiencies between 20.5% and 21.0% are achieved. Within processing and measurement accuracy, all materials performed on the same level when comparing the solar cell conversion efficiencies. Implied open-circuit voltage samples revealed to be more sensitive to the material quality than the solar cells and give some more details. Due to the small differences in the results, we will not provide a ranking of the material tested in this paper. However, all the materials tested here are capable of achieving at least an efficiency of 20.7%, when applied in a PERC solar cell process. more...

31st European Photovoltaic Solar Energy Conference and Exhibition; 477-484, In this work, we thoroughly investigate the impact of parameter and process variations in an industrially feasible multicrystalline silicon (mc-Si) passivated emitter and rear cell (PERC) process. In an experiment, where in total more than 800 mc-Si PERC cells are fabricated, we distribute 51 neighboring highperformance mc-Si wafers and characterize them in depth to garner only the process-induced variations within a larger experiment. We use numerical three-dimensional (3-D) device simulations to model the width of the resulting distribution of solar cell efficiencies of this group. The main responsible parameters leading to a broadening of the distribution are extracted and ranked by their impact, highlighting the parameters that need to be tuned and controlled most accurately. In our case, from the variations in cell efficiencies amounting to 0.9 %abs., we identify the rear side passivation and a wrap-around during the emitter etch back process to be in charge of almost 60 % of the total fluctuations. With the presented approach and its findings, a better understanding of the underlying dependencies can be developed and subsequently applied to improve the ramp-up and the development of mc PERC cell production. In addition, the relevant parameters and their range obtained and denoted in this work are a fundamental input for modelling a mc-Si PERC solar cell, drawing a realistic illustration of parameters achieved in today’s research facilities and during ramp-up of PERC production. more...

29th European Photovoltaic Solar Energy Conference and Exhibition; 1132-1137, For texture quality assessment, a detailed microscopic analysis acts as a valuable addition to reflection measurements. The pyramidal surface structure of alkaline texture is suitable to be characterized by geometric parameters. After metallization, microscopic measurements also contribute to the assessment of printing results. The latter depend on the surface of the solar cell and are thus interconnected with texture surface properties. In the following, two tools are presented which allow a combined quantification of texture and contact finger geometry. Relevant parameters are identified for this purpose and the conformity of these parameters with manual evaluations and established measurement methods is demonstrated. Pyramids are detected with high precision for diverse textures (97±2%) and the contact finger parameters agree with reference methods (relative deviation of ±2%). The combined analysis is found suitable to investigate the relation between texture and contact finger parameters. more...

29th European Photovoltaic Solar Energy Conference and Exhibition; 1171-1176, Thermal donors can form in Czochralski grown silicon during the cooling of the ingot. Base resistivity measurements in the as-cut state as well as the inline control of the emitter sheet resistance by inductive base resistivity measurements are influenced by these oxygen clusters. The influence of the thermal donors on the measurements results as well as on calculation results is quantified within this paper. Additionally, a new PL based method (PL-BR) for the determination of the dopant-related base resistivity despite the presence of thermal donors is presented. With the PL-BR method the dopant-related base resistivity and therefore the emitter sheet resistivity can be calculated with a significant higher accuracy than standard base resistivity measurements. more...

Most high-efficiency solar cells are fabricated from monocrystalline Czochralski silicon (Cz-Si) wafers because the material quality is higher than multicrystalline silicon (mc-Si) wafers. However, the material study presented in this paper reveals strong variations in the material quality of commercially available Cz-Si wafers, leading to a loss in solar cell efficiency of 4% absolute. The reason for this is the presence of defects, which appear as dark rings in photoluminescence (PL) images of the finished solar cells. It is shown that these efficiency-limiting defects originate from oxygen precipitation during emitter diffusion. It is demonstrated that an incoming inspection in the as-cut state is difficult, as strong ring structures in as-cut wafers turn out to originate most often from thermal donors. These are dissolved during high-temperature treatments and are therefore harmless, whereas moderate ring structures in the as-cut state may become severe. That is why critical wafers can be identified and sorted out reliably only after emitter diffusion, by using QSSPC-based lifetime measurements or PL imaging. The two-year statistics gathered from the research line at Fraunhofer ISE on the occurrence of ring defects in Cz-Si wafers indicate that ring defects are highly relevant in terms of material yield. more...

27th European Photovoltaic Solar Energy Conference and Exhibition; 636-641, Photoluminescence images of the side faces of a multi-crystalline silicon brick yield valuable information of the electric quality of the material. However, the signal originates from only the first millimeters of the surface and thus contains no bulk information. If consecutive PL-images of wafers sliced from a whole brick are taken and combined, a tomographic reconstruction of the brick is possible. This approach opens a new insight into the silicon crystallization process and its influence on solar cell parameters. By means of image processing crystal growth, impurities, grain boundaries and defect clusters can be reconstructed three dimensionally helping to identify the origin of certain defects and to further optimize the crystallization process. more...

Acidic texturing of multi crystalline silicon (mc-Si) wafers often leads to rough surfaces with areas of strong etch attacks which we call trench structures. Particularly at sites containing crystal defects such as grain boundaries and dislocations, trench structures are created. These trench structures have often been reported as detrimental to solar cell performance. In this paper, a detailed statistical analysis of the influence of trench structures on solar cell efficiency is carried out. It is shown that their influence on cell efficiency, as well as open circuit voltage, short circuit current and fill factor, is only very minor. The material quality determines the diffusion length of the minority carriers and predominantly impacts the solar cell efficiency. Moreover, it has been established that trench structures do not significantly influence surface roughness and do not lead to shunting losses. However, the statistical model indicates that materials with many crystal defects, and thus lower quality and a smoother (and hence more reflecting) texture lead to slightly higher efficiencies compared to solar cells with a rougher texture. The higher efficiency obtained with smoother textures for this kind of material might be caused by the absence of trench structures. Furthermore, it has been found in this work that for these acidic textures, surface roughness correlates to weighted reflection and hence only one of these parameters has to be determined to analyze an acidic texture. more...

Luminescence imaging is a very rapid technique for the characterisation of silicon samples. Megapixel luminescence images on silicon bricks, unprocessed wafers, partially processed wafers and fully processed cells can be captured with acquisition times of typically a few seconds or less than one second. A number of specific luminescence imaging applications have been developed over the last three years, allowing quantitative spatial information to be gained about a variety of crucial material and device parameters. This paper reviews some of the intriguing possibilities for inline monitoring in PV production at an early stage of production that result from the above mentioned short measurement times. more...

26th European Photovoltaic Solar Energy Conference and Exhibition; 1025-1030, Recent progress with photoluminescence (PL) imaging shows that applying pattern recognition techniques on the images of as-cut multicrystalline silicon (mc-Si) wafers allows a prediction of final solar cell parameters. Based on PL a reliable quality control for electrical parameters can thus be established for mc-Si wafers. High-efficiency solar cells are fabricated from Czochralski silicon (Cz-Si) wafers in most cases because of their high material quality. In this study, however, solar cells from Cz-Si suffered an efficiency loss of more than 4 % absolute due to a defect which appears as dark rings in PL images. In this paper we will show that the origin of this defect lies in oxygen precipitation during emitter diffusion. By using QSSPC-based lifetime measurements or PL-imaging, critical wafers can be identified and sorted out reliably at an early state of production and thus yield and average efficiency of production lines can be increased. more...

Only recently, methods for quality control of multicrystalline silicon wafers have been published, which allow the efficiency of solar cells to be predicted precisely from photoluminescence (PL) images taken in the as-cut state. In this letter it is shown that oxygen precipitates, present in standard Czochralski silicon wafers, can cause efficiency losses of more than 4% (absolute) within an industrial solar cell proc- ess. These efficiency losses correlate with ring-like defect structures of reduced intensity in the PL image. In comparison with QSSPC-based lifetime measurements, we introduce a PL-based method of quality control which allows the critical wafers to be identified and sorted out reliably at an early state of production and thus increases yield and average efficiency of production lines. more...

26th European Photovoltaic Solar Energy Conference and Exhibition; 2063-2068, Automated wafer handling is inevitable during the production of crystalline silicon solar cells. Additional to the risk of wafer breakage, the influence of wafer handling devices on the optical and electrical properties of the solar cell proofs to be of great importance. This work investigates the influence of different wafer grippers on the optical properties of grown random pyramids as well as their impact on the quality of hydrogenated amorphous silicon (a-Si) passivation layers. The homogeneity of the random pyramids is characterized by examination of scanning electron microscopy (SEM) images and reflectance measurements. The influence of the wafer grippers on the electrical properties is investigated by calibrating quasi-steady-state photoconductance (QSSPC) measurements to photoluminescence (PL) images. We find that using grippers which distribute their exerted force on a larger area of the wafer surface provides better optical and electrical properties than using grippers with smaller, locally defined contact areas. more...

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1078-1082, In this paper we present a novel method to describe the quality of multi-crystalline as-cut wafer based on photoluminescence imaging (PL). PL has a high potential to detect efficiency relevant defects already on as-cut wafers. Defects that can be detected are for instance crystal dislocations and contaminations from iron precipitates or the crystallization crucible. We present reliable image processing algorithms to detect and quantify quality features related to specific defects. For an interpretable presentation the quality features are combined in a histogram. We show that the histogram contains a large fraction of the physically relevant information to predict the open circuit voltage of the finished solar cells by an artificial neural network. This proves that the features can be used to establish a meaningful rating of the wafer quality. more...

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1867-1870, A dispensing process was developed, allowing for a single step, contactless front side metallization of silicon solar cells. Compared to screen printing, the dispensed fingers showed superior geometry, with line widths down to 65μm across the whole wafer area and aspect ratios (height:width) of up to 0.5. Silicon solar cells were produced using both multi- and monocrystalline (cz) material. The efficiency gain on industrial solar cells was 0.6% absolute on multicrystalline material and 0.4% absolute on monocrystalline material, comparing the best dispensed cell with the best screen-printed cell. The average efficiency gain of the two groups was 0.3% (mc) and 0.2% (cz). more...

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1825-1827, Electro (EL) and photoluminescence (PL) imaging are rather new methods for fast spatially resolved characterization of the electrical properties of crystalline silicon solar cells. EL imaging is experimentally quite simple. Unfortunately in general a quantitative evaluation of the diode quality is not possible, but is limited to very low quality devices. Conversely, PL imaging is experimentally more challenging but allows also a quantitative evaluation of the diode quality for state-of-the-art devices. Experimentally the two techniques are compared analyzing two differently processed solar cells made of multi crystalline sister wavers. more...