Contrast enhancement in the detection of defects in transparent layered structures: The use of optothermal interference technique in solar cell investigation

This article shows the enhanced sensitivity of the optothermal interference technique in the detection of local differences (nonhomogeneity in thickness and optothermal parameters), compared to the conventional optical interference, when investigating layered transparent structures. The measured signal is sensitive to the reflectance variation at the distinct interfaces, function of temperature, as well as to the optical phase lag between the reflected beams. Measurements made on solar cells show contrast of the order of 100% in the optothermal interference, while the conventional optical interference presents a contrast of only 15%. A model based on the reflectance variation at each interface describes the signal behavior as a function of modulation frequency. Theoretical calculation based on this model evidences the influence of the optothermal parameters in the signal contrast. (C) 2000 American Institute of Physics. [S0021-8979(00)04821-0].