Evaluation of Local Electrical Parameters of Solar Cells by Dynamic (Lock-In) Thermography

The local current flow through biased solar cells can be monitored by measuring the local heating at the cell surface. Based on the discussion of heat diffusion phenomena and on practical aspects, optimum strategies for dynamic local current density measurements are derived. The practical realization of dynamic contact thermography is reviewed. Local inhomogeneities of the injected current density can be measured by this technique with a spatial resolution of down to 100 μm and a sensitivity of about 200 μA/cm2 in forward direction of a silicon cell. Moreover, local I—V characteristics can be measured thermally in a non-destructive way. The applicability of this technique is demonstrated by investigating shunts in multicrystalline silicon solar cells. Dynamic thermography is shown to be a useful supplement to LBIC and EBIC investigations, since it reveals the locations limiting the open circuit voltage and the fill factor of solar cells.