Elimination of severe shunting problems due to air-side electrode formation on evaporated poly-Si thin-film solar cells on glass

Recent progress in the metallization of poly-silicon thin-film solar cells on glass, created by solid phase crystallization (SPC) of evaporated amorphous silicon (EVA), revealed that this type of solar cell suffers from severe shunting problems when the air-side (i.e., rear) metal contact is deposited. Pinholes of sizes between 5 and 20 microns exist in this material at moderate density (0.1–1 mm⁻²), and at first glance these pinholes appear to be the reason for the shunting. However, experiments with pinhole filling prior to the metallization process and attempts to find a correlation between the magnitude of the shunt and the density of the pinholes both lead to the conclusion that (i) there has to be a different shunting path and (ii) the pinholes visible in the optical microscope are unproblematic for the metallization of the rear surface. The shunting is shown to be of a spatially distributed nature and to happen at a rather high shunt density of at least several shunts per mm². Contacting only a small fraction of the rear Si surface via a point contacting scheme, whereby the Al/poly-Si contact area must not exceed ∼ 5%, is shown to work well on EVA solar cells.