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36th European Photovoltaic Solar Energy Conference and Exhibition; 326-329, In this paper we investigate the effect of selected processing steps used in Si solar cell fabrication on the recombination activity of grain boundaries (GBs) and other structural defects in high performance multicrystalline silicon (HPMC-Si) wafers. Such defects can in many cases be improved or completely removed using suitable processes, such as impurity gettering and hydrogen passivation, which is accomplished during existing solar cell production. Here we aim to both improve existing production processes with respect to bulk lifetime, as well as introduce new, dedicated processes designed for defect engineering and material quality improvement of HPMC-Si wafers, thus working towards closing the remaining efficiency gap between multi- and monocrystalline Si wafers. Groups of neighbouring wafers from different HPMC Si ingots, have been passed through various processing steps, and subsequently etched down and re-passivated to assess the bulk lifetime of the final solar cells. Photoluminescence (PL) lifetime imaging are then used to generate lifetime maps before and after a selected contact firing process to investigate the effect of GB hydrogenation during this process. The recombination strength of GBs was quantified by numerical simulations of charge carrier diffusion based on line scans of the carrier lifetime. A simplified performance metric was then used to quantify and correlate the recombination of a large number of GBs to the crystallographic properties of the GBs, measured by a Laue scanner on neighbouring wafers.

In this paper experimental results on High Performance multicrystalline silicon ingots (HPmc-Si) grown from reusable silicon nitride crucibles are presented. Five ingots are grown from the same crucible and the material is analyzed for carbon and oxygen, and metallic impurities, by means of Fourier transform infrared (FTIR) spectroscopy and Neutron Activation Analysis (NAA), respectively. Microwave Photoconductance Decay (mW-PCD) and Quasi-Steady-State Photoconductance (QSSPC) is applied on samples to reveal the resulting charge carrier lifetime distribution. Full-area aluminum back surface field (BSF) solar cells are processed from 156 mm × 156 mm wafers and the solar cell efficiency is analyzed. Material properties and solar cell efficiencies are comparable to reference material grown from high purity electronic grade (EG) quartz crucibles. Obtained results are discussed with respect to results published recently.

This presentation was part of the Workshop: Recycling, reuse and resource efficiency: New solutions for a PV circular economy - Results from the projects CABRISS and ECOSOLAR. The workshop was organized within Freiberg Silicon Days 2017.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 309-312, Preliminary results on silicon ingot crystallisation from reusable silicon nitride crucibles are reported. High Performance multicrystalline silicon ingots (HPmc-Si) using granular polysilicon for seeding were grown. The obtained material quality was investigated at ingot, wafer and cell level. Finally, PV modules were manufactured to demonstrate the potential of such material at commercial scale. So far, the silicon nitride crucible was used in five subsequent runs. Material performance and cell efficiencies are comparable to reference material grown from high purity electronic grade quartz crucibles. Two 60 cell modules were assembled and the module power output is comparable to commercial modules of the same category.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1021-1024, The present work investigates the silica slip cast crucible’s role as a contamination source of multicrystalline silicon during directional solidification. The diffusivity of iron in the crucible material and quartz glass was quantified at three different temperatures (1100, 1200 and 1300 °C). Samples of quartz glass and samples of a silica slip cast crucible were heat treated in contact with solid steel. The diffusion profile of iron in quartz glass was obtained with line scans using an Electron Probe Micro Analyser. The diffusion profile of iron in the silica crucible was obtained with Semi-quantitative X-ray mapping and image analysis. The concentration profiles of iron were fit to the solution of the diffusion equation and the temperature independent diffusion coefficients could also be obtained. The diffusivity of iron in both the crucible and quartz glass increased with temperature and the diffusivity was higher in the porous crucible material than in the denser quartz glass. The temperature independent diffusion constants and activation energies were estimated to be 1,12*10-4 cm2/s and 1,52 eV for the crucible, respectively, and 8,1*10-5 cm2/s and 1,8 eV for the quartz glass, respectively. These results indicate that the impurity transport is strongly dependent on the crucible structure. The estimated transport parameters of iron through the crucible material were applied in a numerical model that predicts the contamination of silicon during directional solidification.

The current paper investigates the structure of low-lifetime areas observed in a < 110 >-oriented mono-like silicon ingot grown from monocrystalline seeds. These areas are related to dislocation clusters forming at seed junctions and several generation mechanisms are discussed. Dislocations generated due to physical contact between seeds could only be completely avoided by introducing gaps between the seeds. Large gaps were, however, found to suffer from alternative generation processes not found in small gaps. Dislocations generated in the seeds and in peripheral grains does not necessarily move in to the main crystal and low-lifetime areas are mainly related to dislocations generated above the seeding structure. Dislocations are found to form clusters aligning along < 111 >-directions and are proposed to happen by glide on {111}-planes from the boundary plane between two seed crystals. The extent of low-lifetime areas and corresponding dislocation clusters, for junctions containing no or small gaps, appear to mainly depend on the misorientation between seeds and by attaining sufficiently low misorientation the high bulk lifetime can be retained also at the junctions. Analysis of the misorientations along principal axes indicates that larger misorientations can be tolerated if the misorientation is limited to a single tilt axis.

31st European Photovoltaic Solar Energy Conference and Exhibition; 571-574, A quasi-single-crystalline silicon ingot has been produced in a pilot scale furnace. The directional growth is controlled using a seed layer of equally oriented monocrystalline plates. This seed layer has been specially designed with different junction gaps. Junctions between seed plates are known to be nucleation sites for dislocation growth. Angular mismatch between the seeds has been measured using a Laue X-ray scanning method. Both increased spacing between the seed plates as well as angular mismatch lead to increased dislocation growth. While the large mono-like areas respond well to phosphorus gettering the improvements are minimal in the dislocation rich areas directly above the seed junctions.

The purpose of this work is to verify the possibility to process highly doped Si supporting substrates using a 2-step process: (i) sintering of a low-cost Si powder based ingot using hot pressing and a ELKEM Silgrain material as a feedstock; and (ii) wafering of such ingots using multi-wire sawing technique similar to that, which is used for cast multi-Si or Cz-Si grown ingots. Moreover, the possibility to dope Si powder ingot at sintering temperatures below melting point of Si, using a mixture of Si and boron powders to fabricate highly conductive Si wafers is verified as well. The slurry technique has been chosen for multi-wire sawing of sintered Si powder based ingots. Surface properties of Si powder based substrates as well as their chemical composition have been studied by optical microscopy imaging and energy-dispersive X-ray spectroscopy (EDX). Although the overall concentrations of oxygen, carbon and possibly also metal impurities, which are initially present in a low-cost Si feedstock, are too high to achieve acceptable semiconducting properties, it is concluded, that sintered Si powder based wafers have high enough conductivity (resistivity similar to 0.001 Omega cm) to serve as supporting substrates for low cost Si wafer equivalent structures.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1418-1421, Grain boundary properties and dislocation density were investigated for an ingot produced in a pilot scale directional solidification furnace. The material is characteristic of traditional multicrystalline silicon, different from the current trend of producing small grained material, which has significantly lower dislocation density. Large, twinned grains were present from the bottom part of the ingot, and grain size increased as growth proceeded. While the total density of grain boundaries decreased, the fraction of all types of grain boundaries remained stable. The ingot developed extensive dislocation networks towards the top, covering large areas in the range of several cm2. Together with these regions also large regions with very low dislocation density exists. The characteristics of grain boundary types and their evolution during growth in this, former generation of multicrystalline silicon is a useful tool for evaluating product and process parameters of high performance multicrystalline silicon, although no direct comparison to such material has been performed in this work.

28th European Photovoltaic Solar Energy Conference and Exhibition; 427-430, Highly conductive sintered silicon is a promising material which can be used as substrates for high efficiency devices deposited on or bonded to the surface of such supporting substrates. This work presents results concerning sintering of such material by hot pressing (HP) and Spark Plasma Sintering (SPS) methods. The samples were characterized by Light and electron microscopy, Electron Backscatter Diffraction, Energy Dispersive Diffraction and Glow Discharge Mass Spectrometry. The texture was substantially different between the two materials with random orientation and grain boundaries for HP and considerable Coincidence Site Lattice (CSL) and twinning for the SPS material. Considerable particulate contamination of W, Fe, Cr and Al was detected by EDS. Dislocation density was high, but could not be reliably characterized by etching method. Such defect and contamination levels are not necessarily detrimental for hybrid structures, although presence of fast diffusing elements may pose challenges due to transfer to the high quality layer during depositing or bonding.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1319-1324, Minority charge carrier lifetimes are measured in multicrystalline silicon using a series of different techniques; photoluminescence-imaging, microwave photoconductance decay, carrier density imaging and quasi steady state photoconductance, both with front and back located coils.The different measurements tools are compared and evaluated with respect to trends through the height of an ingot and spatial characteristics within wafers. Mechanical polishing results in lower surface recombination than chemical polishing of as-sawn wafers, and offer more details using the lifetime mapping tools. The different techniques are found to have different advantages with respect to spatial resolution, tolerance for variations in surface properties and calibration of the measurements. All techniques describe the general trend in the ingot, with some variation in the absolute values.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1018-1022, Calcium is a sparsely studied impurity in silicon for solar cell applications, even though adding calciumcontaining compounds to metallurgical grade silicon (MG-Si) melt with subsequently repeated directional solidifications [1] or with subsequent leaching [2],[3] are two known purifying methods. Calcium is present as a potential impurity in multicrystalline silicon as a result of this process. Therefore, the behavior of Calcium as added contaminant in directional solidification of multicrystalline Silicon (mc-Si) has been studied. Ca was added to the feedstock prior to the casting of a 12 kg pilot scale mc-Si ingot to obtain 100 ppm Ca in the melt [4]. In this study, a sample covering the whole height of the ingot was studied on a larger scale and the most striking feature of the block is the line of Ca-rich regions containing S3iN4 particles in the middle of the block. Dislocated areas extend from the Ca-rich regions, and upwards into the ingot.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1307-1311, Dislocations are viewed as the most detrimental defect in terms of efficiency reduction for multicrystalline and quasi-monocrystalline silicon, and measuring the dislocation density quantitatively is crucial for understanding the generation processes and optimizing the production environment. A scanning instrument was built which records the reflection pattern from a polished and etched surface and analyzes the obtained images. It is possible to distinguish the pattern resulting from scattering by twins/grain boundaries and obtain measurements which only includes signal from dislocation etch pits. The resulting intensity plot can be calibrated against manual counting of etch pits, and a simple linear calibration factor is sufficient to obtain quantitative measurements of dislocation density over entire wafer areas.

26th European Photovoltaic Solar Energy Conference and Exhibition; 1895-1899, Investigations with optical microscope and SEM have been carried out in order to study the microstructure of silicon wafers from the bottom parts of a silicon block, where the main focus was to study dislocation formation at ∑=27a boundaries. A ∑=27a boundary was found to dissociate into relatively large and stable triangular grains consisting of ∑=3 and ∑=9 boundaries, where one of the ∑=9 boundaries was an asymmetric CSL-boundary. The asymmetric ∑=9 boundary lowered its energy by dissociating further into ∑=3 boundaries. Dislocations were produced in the junctions between the symmetric and asymmetric ∑=9 boundaries. Some of these will be geometrically necessary due to translations at the facet junctions. Further dislocation formation is expected to be caused by stress concentrations which develop at these boundary defects during crystallization and cooling of the silicon ingot. Also, different etching behaviour at boundaries is discussed.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1529-1532, The growth rate of the interface of a 12 kg silicon ingot was measured by dipping quartz rods into the melt during directional solidification in three different locations at intervals of 20 minutes through the whole solidification experiments. Wafers from the resulting ingots were analysed and a circular area of decreased minority carrier lifetime was detected in the wafers. Iron was shown to cause the decrease in lifetime and different mechanisms for the iron- and lifetime patterns are discussed. Gettering experiments were conducted and most of the iron was removed, but the minority carrier lifetime was not completely restored. The latter is explained by clustering of the impurities above the solubility limit.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2612-2617, Within the framework of a Norwegian national project, KMB-Defect Engineering, a report describing the currently available techniques for characterization of silicon solar cell has been written. The report is divided into four chapters. The first chapter defines and describes the physical parameters. The second chapter is a summary of the cost and availability of the different characterization techniques. The third chapter describes all currently available techniques for silicon solar cell. The fourth chapter reviews the most common sample preparation methods. In this work, we present a summary of this report and describe briefly most of the characterization tools currently available for silicon solar cell. The techniques are divided according to the characterization targets which are i) electrical parameters, ii) dimension and topographic parameters, iii) crystallographic parameters and structural defect, iv) mechanical parameters, v) optical parameters and vi) impurity concentration and defect parameters. We believe that this review is relevant for both the research- and industry communities, and can be a useful tool for further development of measurement techniques for the photovoltaic industry.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1034-1037, Ingots containing varying amounts of aluminum were compared by solar cell processing. The impact on cell performance was found to be related to the amount of Al. However, also contra intuitive results were obtained, in which a larger impact of the Al contamination on cell performance was observed for less Al and vice versa. Formation of harmful Al- O complexes, known to be responsible for Al-related minority carrier recombination, can qualitatively explain the differences between the cells, based on variations in the total oxygen content in the ingots. Possibly, competition between Al and B in the formation of oxygen complexes can also explain the improved cell performances in the presence of B.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1257-1259, The life time of minority charge carriers in conventional as-cast boron doped multicrystalline silicon (mc-Si) blocks has been studied as a function of thermal treatment at various temperatures in the range 100-400°C . Life time measurements have been performed by QSSPC after each thermal treatment. The ingot exhibited lifetime degradation from more than 200μs immediately after treatment, to a stable value of about 40μs on the order of months after thermal treatment. Formation of metal-boron pairs of a fast diffusing transition metal may be identified as the source of the lifetime degradation. A reference ingot doped with aluminium instead of boron crystallized under the same conditions did not show the behaviour. The time scale of the effect taking place after thermal treatment indicates the dissolution and subsequent formation of Cr-B pairs to be the source of the effect. A sample has been repeatedly thermally treated at a range of temperatures to regain the high lifetime.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1253-1256, Two observations frequently found in industrially scale as well as pilot scale directionally solidified multicrystalline silicon ingots have been explained by a mechanism where dislocations close to the solid liquid interface are attracted by the interface and where slip on the {11 0} system occur and creates single ended Frank-Read sources. This mechanism explains why: Samples cut parallel to the growth direction generally shows a lower dislocation density than wafers cut perpendicular to the growth direction, and that grains filled with dislocations in the samples cut parallel to the growth direction often is V-shaped. It can also explain the dislocation pattern where dislocations are lining up perpendicular to twin boundaries in twinned areas.