Black silicon (b‐Si) has many attractive properties and is currently being adopted in various fields of semiconductor technology. It is shown here that b‐Si during thermal activation may serve as an external gettering layer to remove metallic impurities and associated defects from bulk Si. The gettering efficiency by b‐Si is estimated according to the results of studying the resistivity, defect density, interstitial iron concentration, and effective minority carrier lifetime on gettered test and ungettered reference Si samples. It is shown that in the thermal oxidation process, the b‐Si layer serves as an effective drain for excess iron atoms and other fast‐diffusing impurities, thereby significantly reducing the density of oxidation‐induced stacking faults in Si. In addition, the resistivity of the substrates decreases, and the bulk carrier lifetime increases significantly. Finally, gettering by b‐Si is introduced into the solar cells fabrication process and its effect on solar cell current–voltage characteristics is investigated. It has been shown that all electronic parameters of the solar cells are improved. The conversion efficiency of ungettered solar cells is 16.8%, and for gettered solar cells, depending on the oxidation temperature, it increases by 1.36–1.96%.
Spatial distribution of slip dislocations in saddleshaped Czochralski silicon wafers was characterized using an X-ray topography and optical microscope. It is shown that slip dislocations are preferentially generated and randomly distributed on the concave region versus the convex region of the wafers. Asymmetrical distribution of dislocations increases with increase in treatment temperature. The obtained results are explained by the combined action of internal compressive stresses, generated because of lattice mismatch between SiO x precipitates and Si matrix, and additional thermal bending stresses on the concave region of the thermally deformed wafers.
Black Si (b-Si) is a needle-like surface of crystalline Si. The optical radiation incident
on it is almost completely absorbed due to multi-reflections from the side surfaces of the needles, which makes b-Si attractive for solar cell applications as antireflection surfaces. Currently, b-Si solar cells are fabricated based on the sequences used for traditional (pyramidal textured) solar cells, namely first, an antireflection surface is formed, and then high-temperature treatments such as oxidation and diffusion are carried out. However, in this case, due to large surface area, the diffusion of dopants within b-Si is much more efficient than in a planar, without texture, wafer. It leads to high doping concentrations and non-uniformity in the dopant. In addition, during the diffusion process, a surface layer of glass is grown, and subsequently removed by wet etching, which leads to noticeable changes in the morphology and structure of b-Si. As a result, after thermal and chemical treatments, the optical characteristics of the b-Si are retrogressed.
A new fabrication sequence of b-Si solar cells has been proposed and tested, according to which, the b-Si with preferable height of needles is formed after the main chemical
and thermal treatments of the technological process. It has been shown that all performance parameters (open-circuit voltage, short current, fill factor and conversion efficiency) of the new b-Si solar cells are higher than those of the conventional solar cells. Joint use of b-Si and pyramidal textured surfaces can get a more tangible effect.
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