Investigation of defects in crystalline silicon solar cells by confocal Raman spectroscopy

Defects in solar cells can be caused during processing or through a benign event like a falling leaf when operating in an outdoor system. Shading caused by such a leaf can result in the cell operating in the reverse direction and ultimately in hotspot formation, which in turn can cause the entire cell to breakdown and essentially become a power dissipator rather than a producer. More often than not, this reverse biasing of the cell will enhance the effect of any inherent defect. In this study, poly-Si cells were reverse biased to enhance the effect of their inherent defect. These defects were then analysed using non-destructive confocal Raman spectroscopy, since this technique allows us to observe small defects in cells/material using the intensity of the transverse optic bands. The intensity of defect-induced Raman band has a direct relationship with the observed morphological defects of the reverse biased cell. The quality of the active layer was also investigated; this includes the chemical composition and the stress level which can be found through the single spectrum bandwidth. The efficiency of solar material depends on the absorption capability of the solar material, while the optical and the electrical properties to a large extent determine the absorption capability of solar cell. However, its structure, defect and stress level can offset the total optical and electronic properties. The present study reveals defect in micro-level and the stress induced in the affected region of the solar cell. Confocal Raman is suitable for characterising stresses in relation to microstructure, defect level as well as the manufacturer-induced defect in the substrate.