Polycrystalline silicon solar cell inspection using confocal Raman microscopy

Energy from renewable sources, solar energy for example, is expected to play the important role in the near future. To convert the energy of sunlight directly into electricity, silicon solar cells (crystalline or polycrystalline) are widely used, because silicon technology is dominated in the market due to quickly reducing cost. Poly-Si cells are cheaper than single crystal, but they are less efficient. Polycrystalline cells are made from large blocks of molten and solidified silicon. Microscope optical image of such type of cell is displayed in Fig.1 below.

Blocks of different types of Silicon can be observed on this optical image. The presence of non-crystalline silicon leads to reduced conversion efficiency, and due to this, testing crystallinity of silicon in solar cells is important for the manufacture process. Raman spectroscopy can be used for analyses of crystallinity. This application note demonstrates it on the example of a polycrystalline silicon solar cell displayed by Fig.2.

In crystalline silicon the bond lengths are uniform, and as a result there is only one sharp peak at 520 cm^-1. In amorphous silicon the bond lengths are varied leading to broad diffuse spectral features around 480 cm^-1 (Fig.2). The crystalline fraction is proportional to the intensity ratio of I₅₂₀/I₄₈₀. It is possible to see from Fig.2, that different blocks on the polycrystalline silicon cell have different amount of amorphous phase.