Sergio Castellanos

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.

Dislocation Density Reduction During Impurity Gettering in Multicrystalline Silicon

Isothermal annealing above 1250 °C has been reported to reduce the dislocation density in multicrystalline silicon (mc-Si), presumably by pairwise dislocation annihilation. However, this high-temperature process may also cause significant impurity contamination, canceling out the positive effect of dislocation density reduction on cell performance. Here, efforts are made to annihilate dislocations in mc-Si in temperatures as low as 820 °C, with the assistance of an additional driving force to stimulate dislocation motion. A reduction of more than 60% in dislocation density is observed for mc-Si containing intermediate concentrations of certain metallic species after P gettering at 820 °C. While the precise mechanism remains in discussion, available evidence suggests that the net unidirectional flux of impurities in the presence of a gettering layer may cause dislocation motion, leading to dislocation density reduction. Analysis of minority carrier lifetime as a function of dislocation density suggests that lifetime improvements after P diffusion in these samples can be attributed to the combined effects of dislocation density reduction and impurity concentration reduction. These findings suggest there may be mechanisms to reduce dislocation densities at standard solar cell processing temperatures.

Variation of dislocation etch-pit geometry: An indicator of bulk microstructure and recombination activity in multicrystalline silicon

Dislocation clusters in multicrystalline silicon limit solar cell performance by decreasing minoritycarrier diffusion length. Studies have shown that the recombination strength of dislocation clust ...

Exceptional gettering response of epitaxially grown kerfless silicon

The bulk minority-carrier lifetime in p- and n-type kerfless epitaxial (epi) crystalline silicon wafers is shown to increase >500× during phosphorus gettering. We employ kinetic defect simulations and microstructural characterization techniques to elucidate the root cause of this exceptional gettering response. Simulations and deep-level transient spectroscopy (DLTS) indicate that a high concentration of point defects (likely Pt) is “locked in” during fast (60 °C/min) cooling during epi wafer growth. The fine dispersion of moderately fast-diffusing recombination-active point defects limits as-grown lifetime but can also be removed during gettering, confirmed by DLTS measurements. Synchrotron-based X-ray fluorescence microscopy indicates metal agglomerates at structural defects, yet the structural defect density is sufficiently low to enable high lifetimes. Consequently, after phosphorus diffusion gettering, epi silicon exhibits a higher lifetime than materials with similar bulk impurity contents but highe...

The Impact of Dislocation Structure on Impurity Decoration of Dislocation Clusters in Multicrystalline Silicon

Light microscopy, electron backscatter diffraction and transmission electron microscopy is employed to investigate dislocation structure and impurity precipitation in commonly occurring dislocation clusters as observed on defect-etched directionally solidified multicrystalline silicon wafers. The investigation shows that poligonised structures consist of parallel mostly similar, straight, well-ordered dislocations, with minimal contact-interaction and no evidence of precipitate decoration. On the other hand, disordered structures consist of various dislocation types, with interactions being common. Decoration of dislocations by second phase particles is observed in some cases. Enhanced recombination activity of dislocations may therefore be a result of dislocation interaction forming tangles, microscopic kinks and jogs, which can serve as heterogeneous nucleation sites that enhance metallic decoration.

Iron precipitation upon gettering in phosphorus-implanted Czochralski silicon and its impact on solar cell performance

Phosphorus implantation can provide a direct route to a high-performing emitter, with no surface dead layer and improved blue response, and potentially higher open-circuit voltage. Here, iron precipitation during gettering is investigated in phosphorus-implanted, low-oxygen monocrystalline silicon and its impact on device performance evaluated. Previously, it has been shown that higher levels of initial iron contamination lead to lower final interstitial iron concentration after gettering with ion-implanted emitters, resulting in longer final bulk diffusion lengths in the more-highly contaminated materials. In this contribution, we show that despite longer bulk diffusion lengths, the open circuit-voltage of devices made from the highly iron-contaminated material can be strongly reduced. Using synchrotron-based Xray fluorescence we reveal the presence of micron-sized iron precipitates in the near surface region. While not measured over wafer-sized areas, the density of these precipitates correlates with the annealing profile. Slow-cooling from the activation anneal and proceeding directly to a 620-750°C gettering anneal results in large precipitates that are indicated as the underlying cause for the disastrous open-circuit voltage. On the other hand, quickly cooling to room temperature and then re-inserting the wafers for gettering results in very small precipitates that do not appear to have significant detrimental affects on open-circuit voltage. It is thus critical to consider the precipitation behavior of iron during gettering of ion-implanted emitters - even in monocrystalline silicon - and during low-temperature annealing in general.

Quantitative residual stress imaging of multicrystalline, quasi-mono, and thin kerfless silicon wafers by infrared birefringence and sectioning

One of the parameters with highest impact on photovoltaic module cost is manufacturing yield during solar cell production and handling. This work presents a method to quantify residual stress in silicon wafers, through a combination of infrared birefringence imaging (IBI) and sectioning. We spatially resolve and decouple in-plane residual stress components in silicon wafers produced by four different growth methods.

Inferring dislocation recombination strength in multicrystalline silicon via etch pit geometry analysis

Dislocations limit solar cell performance by decreasing minority carrier diffusion length, leading to inefficient charge collection at the device contacts. However, studies have shown that the recombination strength of dislocation clusters within millimeters away from each other can vary by orders of magnitude. In this contribution, we present correlations between dislocation microstructure and recombination activity levels which span close to two orders of magnitude. We discuss a general trend observed: higher dislocation recombination activity appears to be correlated with a higher degree of impurity decoration, and a higher degree of disorder in the spatial distribution of etch pits. We present an approach to quantify the degree of disorder of dislocation clusters. Based on our observations, we hypothesize that the recombination activity of different dislocation clusters can be predicted by visual inspection of the etch pit distribution and geometry.

Infrared birefringence imaging of residual stress and bulk defects in multicrystalline silicon

We explore the potential of infrared birefringence imaging (IBI) to reveal a complete picture of macro- and microscopic internal stresses and their origins in multicrystalline silicon (mc-Si). We present a method to decouple macroscopic thermally induced residual stresses and microscopic bulk defect-related stresses, and validate this method in mc-Si wafers via microstructural analysis. We then describe the unique IR birefringence signatures, including stress magnitudes and directions, of common microdefects in mc-Si solar cell materials: β-SiC and β-Si3N4 microdefects, twin bands, non-twin grain boundaries, and dislocation bands. We relate observed stresses to other topics of interest in solar cell manufacturing, including wafer mechanical strength and minority carrier lifetime.

Stress and temperature coupling effects on dislocation density reduction in multicrystalline silicon

In multicrystalline silicon (mc-Si), the presence of dislocation-rich areas limits solar cell conversion efficiencies [1–2]. Previous studies have demonstrated that dislocation densities higher than 106 cm−2 can dramatically decrease the minority carrier lifetime [3]. High dislocation densities, and their decoration with impurities, can limit minority carrier lifetime even after phosphorous diffusion or hydrogen passivation [4–5]. We previously proposed a method to remove dislocations from mc-Si by high-temperature annealing, demonstrating dislocation density reductions of 95% approximately [6]. We demonstrated that the dependence of dislocation density reduction on annealing temperature is much more pronounced that the dependence on annealing time [7]. In this contribution, we propose stress as an additional mechanism to enhance dislocation density reduction. We discuss the relationship between temperature, stresses and dislocation density in string ribbon.