Lock-in thermography investigation of shunts in screen-printed and PERL solar cells

We compare the application of highly sensitive infrared (IR) lock-in thermography for the investigation of local shunts in multicrystalline screen-printed solar cells and in high-efficiency PERL cells made from CZ-Si. In both cell types local shunts are found, showing a nonlinear (diode-like) I-V characteristic. If a reverse bias as large as 13 V is applied, a number of additional hot spots appear in all cells. In the multicrystalline cells, some of these hot spots may be connected with crystal defects, but most of the shunts dominating under forward bias are due to technological imperfections. One of the PERL cells was completely free of shunts acting under forward bias. Only this cell showed an I-V characteristic without any "second diode" contribution. This is a proof that the "second diode" current in these and probably also in most other solar Si cells is essentially due to local shunts.