Microstructural Evolution of Mechanically Deformed Polycrystalline Silicon for Kerfless Photovoltaics

Abstract

Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR‐EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub‐grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post‐deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR‐EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR‐EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities.

Degradation of Edge‐Defined Film‐Fed Silicon Glass–Glass Modules on Florida Rooftop After 22 Years

“…Advancements in processing and cell technologies since the mid-2000s have paved the way for high-efficiency thin silicon devices to enter the market; [1] therefore, a revisiting of module reliability based on kerfless wafers is warranted. Detailed reviews of kerfless wafering technologies can be found in other studies, [1][2][3][4][5][6][7][8] and examples include implant and cleave, [4,9] exfoliation and spalling, [10][11][12][13][14][15] blade cut, [16] laser processes, [17] dendritic web growth, [18,19] string ribbon growth, [20][21][22] direct wafering, [23] the floating silicon method, [24,25] and epitaxial growth. [1,26,27] Epitaxial growth, for example, has shown 4.5 ms carrier lifetime and high (> 20%) efficiencies [26] and shows 4.6-7.0 ms carrier lifetime after phosphorus gettering (before, 1.9-4.3 ms), [27] though there is opportunity for further improvements.…”

Measurements of hardness, young s modulus, and fracture toughness of solar grade silicon obtained from metallurgical grade silicon

“…An important process step in this route is the directional solidification of the MG-Si [1][2][3][4][5] to induce segregation of metallic impurities to the silicon ingot top. Although mechanical properties of silicon play a secondary role in the efficiency of photovoltaic cells [6], its lack of ductility [7] and toughness [8] affects the production of silicon wafers and solar panels and makes the mechanical conformation of silicon practically impossible. Toughness measurements of SoG-Si produced by directional solidification of MG-Si are not easily found in the literature.…”