The characteristics of sub-grains in the mono-like silicon crystals grown with directional solidification method

Mono-like wafer or quasi-mono wafer is becoming a more and more important solar cell material along with the conventional mono and multi wafers. The large area of one single crystal makes it possible that the alkaline based texturing can be applied to the mono-like wafers. Therefore the light reflection on the solar cell surface can be significantly reduced. Also, the mono-like wafers generally contain less crystal defects comparing to conventional multi-wafers due to large single crystals. However, it was found in our research that sub-grains can sometimes be found within the large single crystals. The sub-grains can be observed on the top surface of an ingot after growth. With alkali texturing on the wafer, it is more obvious that patches of sub-grains exist within a single crystals. Those sub-grains tilt slightly around <;100>;, which is confirmed by crystal orientation test. It is also interesting to observe that dislocations can be found usually in the sub-grains area. This can be verified with several inspection methods, including PL, EL and acid etching methods. Such crystal defects also impact the solar cell performance as demonstrated with standard solar cell processes. And the lifetime measurement indicated that the gap between the seeds is responsible for the formation of sub-grains.