Joining and mechanical behaviour of poly-crystalline silicon solar cells

Abstract

The most advanced production lines for H-pattern poly-crystalline solar cells use wafers of less than 200 micron thickness. The reason for this development is the cost reduction that can be achieved by reducing the amount of silicon required. Unfortunately, the cells produced from thinner wafers suffer from bowing. The extent of bowing and the delicacy of the cells are problems for the building of modules from cells thinner than 200 micron. In this report the origin and extent of bowing is examined for wafers and cells of different thicknesses, by analysis at several stages in the production cycle. Not only the production of the cells is taken into account. A functioning solar panel requires cells that will have a good performance for 20 years. To realise such long lifetimes, the damage induced by the production process should be minimal. In several steps of the production, stress and possible damage is introduced. The presence of surface cracks at several stages was examined by microscopy for this purpose. The presence of microcracks in soldered solar cells was studied using acoustic emission monitoring. Four point bending experiments of cells taken from different stages of the production process were also monitored by acoustic emission. Surface cracks have not been observed and the presence of microcracks after soldering could not be confirmed in this research, although acoustic emission. measurements imply the possibility of their propagation or initiation during soldering. Losses in the production process did not occur during the soldering of the cells, but in production steps preceding soldering. The research shows that the bowing is caused primarily by the addition and more specifically the thermal contraction of the aluminium underside of the solar cells. Bowing increases as the thickness of the cell decreases and the experimental fracture strength of the cells and wafers reduces as the thickness reduces. The weakest area of the solar cells appears to be the transition between aluminium and silver on the rear of the cell. This work reports on a preliminary research study, which will be followed-up by a more detailed graduate study. ECN is currently working on poly-crystalline cells with an open underside. These cells, even thinner ones, should have a higher fracture stress, than those with an aluminium layer.