Role of the distance between crack tip and grain boundary in mechanics of crack propagation in bicrystal silicon

Abstract

Fracture in polycrystalline silicon solar cell is affected by the distance (DCG) between the crack tip and grain boundary (GB). The mechanics of crack propagation processes can be understood by studying the bicrystal silicon as a primer. In the present work, the role of distance between the crack tip and GB on the mechanics of crack propagation has been studied using molecular dynamics (MD) simulations. Four hypothetical bicrystals of different DCG have been considered for the present study. A side edge crack has been made and an uniaxial extension has been applied perpendicular to the initial crack plane.

The analysis of fracture processes like crack propagation initiation, crack/GB interaction, crack arrest and re-initiation requires the determination of stress intensity factor (SIF), crack tip opening displacement (CTOD) and energy release rate (ERR) through near-tip mechanics. SIFs calculated using the near-tip stress field are quite useful for this analysis. However, SIF cannot be calculated when crack tip is close to the GB (DCG $\le$ 50 Å). To overcome this limitation, ERR, evaluated using CTOD, has been used for all the analysis when crack tip is close to the GB. The CTOD determination is not an established method at atomistic scale. Hence, in the present work, ERR determined through CTOD will be verified for atomistic scale through atomistic and continuum scale $J$ integral. $J$ integral at atomistic scale is determined using volume integral method and verified by its path independence.

From the present analysis, it has been found that when the crack tip is close to GB (DCG $\le$ 50 Å), the crack propagation initiation occurs at lower ERR and remote strain. With the increase in DCG, the crack propagation initiation happens at a higher ERR and strain. The effect of DCG becomes less relevant after a certain value, beyond which the crack propagation initiation resembles that of Single Crystal (SC). Further, the crack GB interaction and crack propagation in these bicrystals happened differently due to the different DCGs.