A computer vision system for fast AR film thickness measurement of polysilicon solar cells

For increasing marketing competence, silicon solar cell manufacturers have adopted optical inspection techniques in production lines to perform product classification and statistical process analysis. The product classification is based on overall photoelectric conversion efficiency of the solar cell itself. Two factors directly influence the overall photoelectric conversion efficiency of the solar cell, i.e., composed materials and antireflection film coating on substrate. Since film thickness variation of the antireflection layer will induce color change on the surface of solar cell, a cost-effective computer vision measurement system is proposed to perform fast AR film thickness measurement of polysilicon solar cells. The proposed system first uses a color CCD to capture the red-green-blue color image of inspected polysilicon solar cell, and transforms it to hue-saturation-lightness (HSL) image format. And then the area and boundary of different hue-value images are calculated and sorted with the image thresholding and label operation. With the corresponding measurement procedure on specified hue-value regions of using a high accuracy optical film thickness measurement instrument, the regression equation between the hue value and antireflection film thickness is obtained, and then implemented into the proposed system to perform large area scanning antireflection film thickness measurement of polysilicon solar cells. Compared to the optical ellipsometry, the measurement speed of the proposed system is fast. It take only 0.1 second to finish the antireflection film thickness measurement of an image of 768×768 pixels (15 cm×15 cm), and the measurement accuracy of the proposed system can reach 3 nm.