Current-voltage characteristics of silicon based solar cells in the presence of cracks: MD simulations

The presence of micro-cracks in solar cells hinders the movement of charges leading to charge accumulation around the crack surfaces. Cracks grow during real-time operation and also new cracks will be formed, leading to further charge accumulation. In this study, the influence of cracks on the movement of charges and hence the current-voltage characteristics of silicon based solar cells is investigated through molecular dynamics simulations. Simulations are performed considering a domain of dimensions 260.64 Å × 222.63 Å × 43.44 Å with five different cases: (1) without any initial defects, (2) an edge crack, (3) a center crack, (4) two angled edge cracks and (5) two oblique cracks from a circular crack at the center of the domain, considering a time step of 1 fs. Charges of the atoms at a given time instant are estimated after charge equilibration. The electric current, voltage, and power are estimated based on the charges. As the crack starts propagating, the charge fluctuations of a group of atoms around the crack tip are observed to be in the range of -9 × 103e to -6 × 10-3e, where the highest fluctuation is noticed in case of angled edge cracks. The electric current for the same atoms is found to be fluctuating between -0.02 nA and -0.098 nA, peak fluctuations observed in the case of the edge crack. Similar ranges of charge and current without any initial crack are found to be: 0 × 10-3 e and 3 × 10-3 e, and 0 nA and 0.01 nA, respectively. This confirms that the presence of cracks can hinder the charge flow. Furthermore, the peak voltage estimated between two groups of charges: near and far away from the crack tip is found to be 1.24 × 10-22 V. Whereas, the peak voltage between two groups considered away from the crack tip is equal to 0.1 × 10-22 V, indicating that the potential difference is significant due to charge accumulation around the damaged areas. Moreover, the electric power associated with the group of atoms around the crack tip is observed to be fluctuating rapidly. The peak electrical power of the intact cell is estimated to be 2.14 nW, whereas it is dropped down to 31.93% in the presence of two inclined edge cracks. The electric power associated with a group of charges lying within the included region between two inclined cracks with a hole is observed to converge to zero, in agreement with the experimental observations in the literature.