Application of real time spectroscopic ellipsometry for analysis of roll-to-roll fabrication of Si:H solar cells on polymer substrates

Real time spectroscopic ellipsometry (RTSE) has been developed to monitor the cassette roll-to-roll deposition of thin film Si:H n-i-p solar cells on flexible polymer substrates coated with a back-reflector (BR). The methodology is first demonstrated here in growth studies from nucleation to the final thickness for a magnetron sputtered ZnO layer on top of opaque Ag in the BR structure. The methodology is then extended to plasma-enhanced chemical vapor deposition (PECVD) of the i and p-layers in succession on the BR/n-layer stack. RTSE data collection is initiated before the plasma is ignited so that the nucleation of the layers can be observed. The film thickness increases with time until a steady state is reached, after which the bulk layer thickness at the monitoring point is constant with time. This occurs when the elapsed deposition time equals the time required for the moving substrate to travel from the leading edge of the deposition zone to the monitoring point. Although a constant substrate speed is selected such that the final film thickness is achieved in the time required to move through the entire deposition zone, this speed does not permit study of film growth that occurs after the substrate passes the monitoring point. To address this deficiency, the substrate speed is reduced only over an initial length of the roll such that the final film thickness of interest is reached at the monitoring point. In this way, RTSE can be used to analyze the entire layer on an initial length of the roll before its full length is coated. The goal of this work is to develop and verify optimum deposition procedures based on optical monitoring of thin film Si:H solar cell structures in roll-to-roll multi-chamber deposition.