Enhanced Material Quality in SMART Moni-Si Block Cast Ingots by Introduction of Functional Defects

The use of cast-mono technology to control the development of structural defects during the block casting process enables the crystallization of Silicon for solar cells with good material quality at high throughput and low cost. The obstacles, ingrowth of parasitic grains and development of dislocation clusters, might be overcome by the SMART seeding technique, i.e. the introduction of functional defects. Ingots with the cast-mono and SMART seeding configuration, as well as a new approach including a monocrystalline spacer have been crystallized. Whereas the ingrowth of parasitic grains was reproduced with the standard cast-mono technique, the functional defects suppressed the ingrowth of parasitic grains until an ingot height of approx. 80 mm. By the introduction of an additional monocrystalline spacer, the ingrowth of parasitic grains could be avoided for the whole ingot height of 210 mm. The spreading of dislocation clusters in the monocrystalline area could be reduced significantly using the SMART approach in contrast to the standard cast-mono configuration. This reduction of structural defects enabled high bulk lifetimes above 1 ms on large wafer areas for both p- and n-type material after typical high efficiency solar cell processing steps.