The Origins of Void formation in Sputtered CdSe

The introduction of selenium to the front of thin film CdTe photovoltaic devices has led to a sharp increase in conversion efficiency. One way to introduce the selenium is to first deposit a layer of CdSe on to the buffer layer before depositing a CdTe layer at the back. The two layers are then inter-diffused during the cadmium chloride activation. Magnetron sputtering is an attractive deposition method to deposit the CdSe, but previous studies have revealed the presence of deleterious voids in the finished sputtered CdSe/CdTe devices. In this paper, we show that the voids are caused by the accumulation of argon into bubbles formed during activation with experimental and theoretical evidence. The bubbles are similar to those previously observed in sputtered CdTe. These bubbles can also develop into blisters causing significant exfoliation of the film surface.