Characteristics of electroformed low thermal expansion Fe-42 wt. %Ni alloy substrates for thin film silicon solar cells

Abstract

Differences in the thermal expansion of the semi-conductor layer and the substrate of thin film solar cells can lead to thermal deformation of the cell during thermal processing. To control this deformation, the substrate needs to have a thermal expansion behavior similar to that of the semi-conductive layer of the cell. In order to develop such a metal substrate for thin film silicon solar cells, a Fe-42 wt. %Ni alloy with low thermal expansion characteristics was fabricated by electroforming. The thermal expansion behaviors of as-deposited and heat-treated Fe-42 wt. %Ni alloys were measured using a Thermal Mechanical Analyzer. A rapid change in thermal expansion was observed between 350 and 400 °C in the as-deposited Fe-42 wt. %Ni alloy. However, when the alloys were heat treated at temperatures higher than 500 °C, their thermal expansion behaviors were stable and their thermal expansion coefficients were 4–5 × 10−6/ °C. Based on such data, Finite Element Analysis was applied to calculate the thermal deformation of the cell that occurs during cooling from a virtual coating process temperature to room temperature, using the Algor program. The analyses showed that the cell on the heat-treated Fe-42 wt. %Ni alloy substrate had less residual stress and a lower amount of deformation compared to that on a commercial stainless steel substrate.