Impurities influence on multicrystalline photovoltaic Silicon (original) (raw)
Related papers
A TEM study of SiC particles and filaments precipitated in multicrystalline Si for solar cells
Solar Energy Materials and Solar Cells, 2008
The microstructure of SiC particles and SiC filament-type precipitates found in block-cast multicrystalline Si was studied in detail by transmission electron microscopy (TEM). TEM investigations showed that the SiC particles are single crystalline and the SiC filaments are microcrystalline. Both types of precipitates consist of cubic SiC. However, a high density of planar defects was found in the filaments. Very wavy and rough interface between SiC filaments and silicon (Si) was revealed by high-resolution TEM. In addition, SiC filaments do not show a special orientation relationship with respect to the Si matrix. The growth mechanisms of SiC precipitates are discussed. Finally, the influence of SiC inclusions in terms of device performance is considered.
physica status solidi (a)
All silicon (Si) ingot fabrication processes share challenges to control grain structure, defect and impurity contamination during the solidification step to improve the material properties. The final grain structure and inherent structural defects issued from the solidification step are responsible for the photovoltaic (PV) properties for a large part, all the more as they are often associated with impurity distribution. Impurities play a major role as they not only can modify the development of the grain structure formation but interact as well with structural defects creating regions of deleterious minority carrier lifetime recombination. Samples containing different levels of impurities and solidified with different processes were selected and analyzed as-grown or observed by X-ray imaging during re-solidification from as-grown seeds. The growth features and relative crystallographic orientation of neighbor grains were characterized. Moreover, minority carrier lifetime measurements were performed and correlated to the growth features. The complementarity of the different techniques allows improving the understanding of phenomena at stake during the formation of grains and twins, the effect of impurities and their impact on photovoltaic properties. The results show the significant influence of light and metallic impurities such as copper on the grain structure and on the electrical properties.
One-dimensional model of the equiaxed grain formation in multi-crystalline silicon
Journal of Crystal Growth, 2011
During solidification of low purity silicon for photovoltaic (PV) cells, solute rejection at the growth interface leads to an increase of the carbon concentration in the liquid phase and then to the precipitation of silicon carbide (SiC). When the precipitate radius becomes higher than the silicon critical nucleus radius, SiC can act as a refining agent for the Si and Si equiaxed grains appear in the liquid. The grain structure of the ingot changes from columnar to small grains, also known as grits. We developed a one-dimensional analytical model of this series of phenomena, including C segregation, SiC nucleation and growth, Si nucleation on the SiC precipitates and subsequent growth of the Si equiaxed grains. The equations are implemented under Matlab software in order to predict the columnar to equiaxed transition (CET) during the directional solidification of PV Si. We carried out calculations of the position and thickness of the equiaxed areas and of the number and size of Si grits as a function of the main process parameters: thermal gradient and growth velocity. Recommendations in order to adapt the growth process parameters to the initial carbon content are given. It is expected that coupling this model to global 3D numerical simulation codes could help improving the yield of ingot solidification.
Journal of Crystal Growth, 2010
SiC and Si 3 N 4 precipitates in multi-crystalline (mc) silicon for photovoltaic application have detrimental effects on wafer sawing process and solar cell performance. In this study the influence of the growth rate on the incorporation of carbon and nitrogen and the SiC/Si 3 N 4 precipitate formation during directional solidification of mc-silicon was investigated. Cylindrical silicon ingots with 6 cm diameter and 4-5 cm length were grown in a laboratory scale vertical gradient freeze (VGF) furnace at different growth rates R (R = 0.2, 1.0 and 2.2 cm/h) and characterized by infrared-transmission (IR-TM), FTIR-spectroscopy and lateral photovoltage scanning (LPS). The results show that the growth rate R is influencing the shape of the phase boundary, the distribution of carbon and nitrogen in the silicon melt and crystal, and the formation of SiC and Si 3 N 4 precipitates. It will be shown that an improved crystallization process with increased convective transport in the melt leads to precipitate-free crystals even at high growth rates.
Energy Procedia, 2011
This study is focussed on the growth of multicrystalline silicon ingots with large grains by controlling the silicon melt cooling rate to initiate dendritic nucleation in the initial stage of the solidification. Two ingots were grown with different undercooling rates and compared with a reference ingot grown by standard cooling conditions. All ingots were grown in a lab scale directional solidification system. The wafers cut from all three ingots have been characterized for resistivity, minority carrier lifetime and dislocation density measurements by four point probe, quasi steady state photo conductance and PV Scan, respectively. The wafers were converted into solar cells and their electrical parameters have been measured. The cells fabricated from ingot 2 show slightly higher efficiencies in comparison with ingot 1 and the reference one. The present cooling rate was not enough to initiate the dendrite nucleation in the beginning of the solidification. Hence, there is no significant difference was observed in the crystal quality of the grown ingots 1 and 2.
Growth of Silicon Carbide Filaments in Multicrystalline Silicon for Solar Cells
Solid State Phenomena, 2009
This work introduces two different approaches to explain the growth of silicon carbide (SiC) filaments, found in the bulk material and in grain boundaries of solar cells made from multicrystalline (mc) silicon. These filaments are responsible for ohmic shunts. The first model proposes that the SiC filaments grow at the solid-liquid interface of the mc-Si ingot, whereas the second model proposes a growth due to solid state diffusion of carbon atoms in the solid fraction of the ingot during the block-casting process. The melt interface model can explain quantitatively the observed morphologies, diameters and mean distances of SiC filaments. The modeling of the temperature-and time-dependent carbon diffusion to a grain boundary in the cooling ingot shows that solid state diffusion based on literature data is not sufficient to transport the required amount of approximately 3.4 × 10 17 carbon atoms per cm 2 to form typical SiC filaments found in grain boundaries of mc-Si for solar cells. However, possible mechanisms are discussed to explain an enhanced diffusion of carbon to the grain boundaries.
Electronic activity of SiC precipitates in multicrystalline solar silicon
physica status solidi (a), 2007
In the upper part of block-cast multicrystalline silicon one often finds silicon carbide and silicon nitride precipitates and inclusions. These contaminants can cause severe ohmic shunts in solar cells and thus decrease the efficiency of the solar cells very strongly. It is well known that the silicon carbide precipitates cause the ohmic shunts. However, the electrical properties of the silicon carbide was unknown so far. To study the electrical properties of these silicon carbide particles we isolated them from the silicon bulk material and performed different electrical measurements on them. The measurements show that the silicon carbide precipitates are highly conductive. An investigation of the heterojunction silicon -silicon carbide was also performed and a simulation of this heterojunction leads to a new model of the ohmic shunt mechanism. It is concluded that the shunt current flows inside of the filaments.
The crucible/silicon interface in directional solidification of photovoltaic silicon
Acta Materialia, 2017
Photovoltaic silicon ingots are currently grown in silica crucibles coated with a porous silicon nitride layer which acts as an interface releasing agent between the silicon and the crucible. The interactions between Si and the Si 3 N 4 coating determine the infiltration and sticking phenomena occurring at the interface and also affect the pollution of Si by the components of the coating. In this investigation the interfacial interactions and microstructure are studied in crystallization experiments performed in crucibles involving high silicon masses (tens of kg) and long contact time between the silicon and the coated silica (tens of hours). It is shown that for long times, a dramatic change in the nature of the coating/Si interface takes place, with the formation of a self-crucible which prevents the direct contact between the silicon and the coating. The stability of the self-crucible is modeled taking into account the capillary and hydrostatic pressures. The influence of the self-crucible on different practical aspects of the photovoltaic silicon crystallization process is discussed.
Journal of the European Ceramic Society, 2017
Photovoltaic silicon is currently grown in silica crucibles coated with an oxidized silicon nitride powder, which acts as an interface releasing agent between the silicon and the crucible. A series of experiments was performed to study the reactions between coating components under high vacuum, varying the temperature, the holding time and the oxygen content in the coating. The results are discussed with the help of a simple analytical model taking into account the diffusive transport of reaction species from the inside of the porous coating to its surface and then their evaporation into the vapour phase.