Synchrotron-based investigation of transition-metal getterability in n-type multicrystalline silicon

Abstract

Solar cells based on n-type multicrystalline silicon (mc-Si) wafers are a promising path to reduce the cost per kWh of photovoltaics; however, the full potential of the material and how to optimally process it are still unknown. Process optimization requires knowledge of the response of the metal-silicide precipitate distribution to processing, which has yet to be directly measured and quantified. To supply this missing piece, we use synchrotron-based micro-X-ray fluorescence (μ-XRF) to quantitatively map >250 metal-rich particles in n-type mc-Si wafers before and after phosphorus diffusion gettering (PDG). We find that 820 °C PDG is sufficient to remove precipitates of fast-diffusing impurities and that 920 °C PDG can eliminate precipitated Fe to below the detection limit of μ-XRF. Thus, the evolution of precipitated metal impurities during PDG is observed to be similar for n- and p-type mc-Si, an observation consistent with calculations of the driving forces for precipitate dissolution and segregation gettering. Measurements show that minority-carrier lifetime increases with increasing precipitate dissolution from 820 °C to 880 °C PDG, and that the lifetime after PDG at 920 °C is between the lifetimes achieved after 820 °C and 880 °C PDG.

Cu gettering by phosphorus-doped emitters in p-type silicon: Effect on light-induced degradation

“…24, we find an average Cu concentration of ∼7 × 10 13 cm 3 in the reference samples with no emitter, which approximately corresponds to the Cu contamination levels reported by several authors in as-cut solar-grade wafers. 1,12,25 In Group A, the residual Cu concentration after gettering is estimated to be ∼3.8 ×10 13 cm 3 (i.e. a gettering efficiency < 50%).…”

“…The emitter was then formed by depositing a phosphosilicate glass (PSG) via a phosphorus oxychloride (POCl 3 ) process and then diffusing the dopant at 830 • C for 20 minutes followed by a 5 min anneal in oxidizing ambient. Next, the PSG was removed in a HF:DIW solution (1:50) and the sheet resistance of the phosphorus layer was measured with a four-point probe to be ∼80 Ω sq 1 . Electrochemical Capacitance-Voltage (ECV) measurements performed on similarly processed wafers also indicated a peak electrically active phosphorus concentration of ∼2 × 10 20 cm 3 and a junction depth of ∼0.4 µm.…”

“…Besides being present in significant concentrations in the as-grown feedstock material, [1][2][3][4] additional sources of Cu contamination are wafer sawing 5 and the use of cost-effective Cu alloys for contacts and interconnects. The presence of parasitic Cu contamination is known to progressively deteriorate the bulk minority carrier lifetime during exposure to illumination.…”

The Relationship between Chemical Flexibility and Nanoscale Charge Collection in Hybrid Halide Perovskites

“…The high sensitivity of nano‐XRF has enabled detection and detailed study of trace constituents or impurities in a wide variety of materials, ranging from biological to impurities in organic and inorganic solar cells . Na is typically the lightest detectable element in practice using X‐ray fluorescence.…”

Electronic Quality Improvement of Highly Defective Quasi‐Mono Silicon Material by Phosphorus Diffusion Gettering

“…[28,29] Iron precipitates are known to cause charge carrier recombination, [30,31] and are known to be difficult to getter via PDG. [32][33][34] They can also dissolve during thermal processing following the PDG, which is typical in, e.g., silicon solar cell processing, [35] which increases their harmful impact. [36] In order to facilitate the hard core removal, a modified PDG process was implemented.…”