Reducing the stress concentration of back contact solar cells: Optimizing the process of drilling

Drilling is important for back contact solar cells, which form a good ohmic contact between a front surface and a rear surface. However, drilling causes increases of breakages and cracks in a back contact solar cell when the silicon wafer is thinner and larger than that of conventional cells. In the present paper, the node value method of finite element post-processing, including a stress concentration factor, a Von Mises stress and a sub-model, is used to achieve a quantitative analysis involving the stress concentration of a silicon wafer with multiple holes under thin film residual stress. The results show that drilling which changes the stress field distribution in a silicon wafer generates a stress concentration around the holes. This stress concentration can be reduced by increasing the thickness of a silicon wafer, decreasing the drilling ratio and replacing the morphologies of trapezoidal holes with those of cylindrical holes. The results of the simulation are in accordance with the experimental results adopted by Raman spectra and three-point bending tests. Our results provide a beneficial reference for decreasing stress concentration in back contact solar cells. (C) 2015 Elsevier B.V. All rights reserved.