Root Cause Analysis of Solar Cell Cracks at Shingle Joints
In this work, we report a cracking mechanism occurring on shingle solar cells in PV modules subjected to thermal cycling. Experimental investigations of six different ECAs show that the positions of cracks are precisely limited to the applied ECA in the joint and the occurrence confined to the rear side of the solar cells. Structural mechanic simulations based on the Finite Element Method (FEM) obtain maximum stresses of up 400 MPa to on the rear surface of the solar cell orientated towards the back sheet. On the opposing surface, orientated towards the glass, the stresses are on a far lower level at around 230 MPa. We measured the characteristic fracture stresses and found values of 96 MPa on the rear and 265 MPa on the front side of the separated shingle solar cells. The lower values on the rear originate from microfractures at the solar cell edges caused by the laser scribe and mechanical cleave process. We discuss two mechanisms taking place at the shingle joint during cooling from lamination temperature at 160 ͦC to -40 ͦC. First a relative shift of the solar cells caused by a mismatch in the thermal expansion in y-direction. Second a mismatch in thermal expansion in z-direction. We apply an anisotropic thermal expansion behavior in our simulations to separate both mechanisms and reveal the driving mechanism behind crack formation. We find that the cracks are caused by thermal contraction of the encapsulant in z-direction. Transversal contraction of the encapsulant causes additional strain in z-direction.