Gaute Stokkan

Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of...

Spatially resolved modeling of the combined effect of dislocations and grain boundaries on minority carrier lifetime in multicrystalline silicon

A model for the combined effect of dislocations and grain boundaries on minority carrier lifetime has been developed. Lifetime varies with dislocation density, grain boundary misorientation, and the coincidence site lattice (CSL) nature of the boundaries. Minority carrier lifetime was measured with high spatial resolution (50 μm) using the carrier density imaging (CDI) technique on a silicon nitride passivated multicrystalline sample. Dislocation density was measured on the same sample by image recognition of optical microscope pictures of a Secco etched surface. Grain boundaries were then mapped and characterized by electron backscatter diffraction (EBSD). Lifetime was simulated based on the dislocation and grain boundary measurements. Parameters were chosen to match closely the simulated and measured maps. Very good two-dimensional (2D) correlation was obtained by assigning roughly equal importance to recombination at dislocations and grain boundaries. The value for the capture cross section, which give...

Impact of type of crystal defects in multicrystalline Si on electrical properties and interaction with impurities

We investigated impact of type of crystal defects in multicrystalline Si (mc-Si) on electrical properties and their change after gettering process of impurities. A bundle of dislocations gives negative impact on the gettering process, while Σ3 grain boundaries does not affect at all. In addition, we categorized random grain boundaries in mc-Si by the contact angle between adjacent dendrite crystals to form the grain boundary. Change in the contrast of photoluminescence intensity around the grain boundary was found to systematically vary by the contact angle, which showed good correlation with calculated interface energy of the grain boundary. Grain boundaries with low interface energy are concluded to be preferable to weaken recombination activity by the gettering process and improvement of solar cell performance based on mc-Si.

High-Performance and Traditional Multicrystalline Silicon: Comparing Gettering Responses and Lifetime-Limiting Defects

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed.

The microstructure of dislocation clusters in industrial directionally solidified multicrystalline silicon

The microstructure of commonly occurring dislocation patterns in industrial directionally solidified multicrystalline silicon has been systematically studied by light microscopy, electron backscatter diffraction, and transmission electron microscopy. The work has been focused on dislocation clusters on wafers near the top of cast blocks. In near {111} grain surface, dislocation arrays parallel to {110} plane traces are lying in parallel rows of {111} planes inclined to the surface, in mainly 〈112〉30∘ orientation. The dislocation configuration suggests that the microstructure may result from a recovery process. The dislocations formed during crystal growth and cooling have undergone transformations at high temperature in order to achieve low energy configurations for minimization of dislocation and crystal energy.

On the role of stacking faults on dislocation generation and dislocation cluster formation in multicrystalline silicon

The microstructure of highly dislocated stacking fault regions (dislocation density >106 cm−2) in industrial cast multicrystalline silicon has been investigated by light microscopy, scanning electron microscopy, and transmission electron microscopy. Our observations indicate that stacking faults form strong barriers to lattice dislocation movement and to the formation of sub grain boundaries. Stepped and curved stacking fault edges appear to generate dislocations. The observations suggest that stacking faults play an important role in the plasticity as well as in the formation of the microstructure of dislocations in multicrystalline silicon.

High Temperature Annealing of Dislocations in Multicrystalline Silicon for Solar Cells

Dislocation clusters have been shown to constitute the main efficiency reducing factor for multicrystalline silicon solar cells (Sopori et al. 2005). Multicrystalline silicon is made under less ideal conditions compared to monocrystalline silicon, in the sense that thermal fields and the lack of seeding create material with increased density of crystal defects, but also since the direct contact between crystal/melt and crucible/coating provides a rapid channel for impurity contamination. These two factors make multicrystalline silicon inferior compared to monocrystalline silicon in terms of solar cell efficiency (Green et al. 2009). It has been shown by Kveder et al. (Kveder et al. 2001) that the interaction between dislocation levels and impurity levels in the band gap may provide very efficient recombination channels, thus enhancing the efficiency reduction both of the dislocations and the impurities. Furthermore it has been shown that gettering of impurities is far less efficient in regions of high dislocation density (Bentzen et al. 2006).

Modeling of Lifetime Distribution in a Multicrystalline Silicon Ingot

Lifetime distribution of a multicrystalline silicon ingot of 250 mm diameter and 100 mm height, grown by unidirectional solidification has been modeled. The model computes the combined effect of interstitial iron and dislocation distribution on minority carrier lifetime of the ingot based on Shockley Read Hall (SRH) recombination model for iron point defects and Donolato’s model for recombination on dislocations. The iron distribution model was based on the solid state diffusion of iron from the crucible and coating to the ingot during its solidification and cooling, taking into account segregation of iron to the melt and back diffusion after the end of solidification. Dislocation density distribution is determined from experimental data obtained by PVScan analysis from a vertical cross section slice. Calculated lifetime is fitted to the measured one by fitting parameters relating the recombination strength and the local concentration of iron

TEM Characterization of near Sub-Grain Boundary Dislocations in Directionally Solidified Multicrystalline Silicon

A crystal is known to achieve lower energy if lattice dislocations are re-arranged in arrays forming a sub-grain boundary through a recovery process. Interaction of boundary dislocations with glide dislocations is also expected to bring about local equilibrium. In this work, dislocations localised in the vicinity of a sub-grain boundary (mis-orientation ) are studied in detail by transmission electron microscopy in order to determine their source. Contrary to the processes described above, it appears that the sub-grain boundary is the source of these dislocations, which are emitted from some locally stressed parts of the boundary. Several slip systems have been activated along the boundary resulting in high density of dislocations. It appears, further, that dislocation propagation from one or more sources is disrupted by interaction with other dislocations or other defects. The dislocations from various sources will be piled up against the obstacles of the other, resulting in the localization of the dislocations close to the sub-grain boundary

Recombination activity of grain boundaries in high-performance multicrystalline Si during solar cell processing

In this work, we applied internal quantum efficiency mapping to study the recombination activity of grain boundaries in High Performance Multicrystalline Silicon under different processing conditio ...