Lock-in thermography with depth resolution on silicon solar cells

Lock-in thermography (LIT) is the standard method for imaging and evaluating leakage currents in solar cells. For usually applied lock-in frequencies in the order of 10 Hz, silicon solar cells are considered to be thermally thin. Hence, depth-dependent investigations, as they are performed in non-destructive testing and failure analysis of ICs, were not performed until now by LIT. In this contribution two special LIT investigation and evaluation methods are introduced, which have the potential to judge whether some recombination occurs at the top, in the middle, or at the bottom of a Si solar cell. Such investigations can be useful to evaluate e.g. metal-induced recombination or the influence of crystal defects in multicrystalline solar material on the emitter or backside recombination. The methods are tested at a cell containing a diamond scratch in the emitter and backside recombination at the Ag back contact.