Optical inspection of solar cells using phase-sensitive optical coherence tomography

The inspection and measurement of solar cells has become an important technique in evaluating the efficiency and quality of solar cell devices. Currently, surface defect detection of solar cells can be achieved by several approaches such as machine vision, photoluminescence, and electroluminescence imaging techniques. Nevertheless, it is still difficult to inspect the inner structures of solar cells. In response to this need, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have become common techniques for investigating the nanostructures of solar cells. However, both carry the disadvantages of destructive imaging, high cost, and a small inspection area. Moreover, it is difficult to simultaneously estimate multiple optical properties with the aforementioned techniques. In this study, we propose the use of phase-sensitive optical coherence tomography (PS-OCT) for the inspection of solar cells. We develop a two-reference-arm configuration to reduce the phase noise that intrinsically accompanies the OCT system. Based on the proposed approach to extract the amplitude and phase terms from OCT interference signals, the 3D microstructure of solar cells can be obtained while simultaneously probing the nanostructures on arbitrary planes of the solar cell. The OCT microstructural results show that the structures of different layers can be nondestructively visualized and quantitatively evaluated. From the phase signal, the inverted pyramid structure, which is commonly used for the reduction of interface reflection, can be visualized. Moreover, based on the two-reference-arm configuration, the optical reflection coefficient (ORC) can be estimated in order to evaluate the interface reflection from the surface of solar cell. Results show that PS-OCT can be a valuable tool for providing nondestructive inspection of the micro/nanostructures of solar cells.