A precision method for laser focusing on laser beam induced current experiments

Using the laser beam induced current (LBIC) technique for the study of solar cells and photovoltaic devices, it is possible to obtain images representing the different degrees of quantum efficiency observed on the surface of these elements. Since the resolution of these images depends on the size of the spot used, the laser beam is directed through a focusing lens, which allows us to obtain significantly small sizes. This article puts forward a method for precise focusing of the laser beam used in the LBIC study of photovoltaic devices by analyzing line scans over sharp structures. The technique is applied to characteristic heterogeneities of solar cells such as the fingers used to collect the induced current, the grain boundaries characteristic of polycrystalline solar cells, and the small surface areas with a quantum efficiency different from that of its adjacent areas. The validity of the proposed method has been evaluated by carrying out virtual experiments where the focusing technique has been applied to heterogeneities reproducing the ones we mentioned above. A criterion for calculating the Gaussian diameter of the laser spot using LBIC signal analysis is also discussed in this work from a theoretical point of view. (C) 2002 American Institute of Physics.