STABILITY OF ALUMINUM THIN FILMS ON FLEXIBLE SUBSTRATE UNDER THERMAL AND ISOTHERMAL CONDITIONS

摘要

Aluminum is an opaque conductive material that is widely used as a back electrode layer in photovoltaic solar cells. The excellent conductivity and ease of deposition on different substrates make aluminum thin films very suitable for such applications. However, during the manufacturing and usage, aluminum thin films could be subjected to thermal and/or isothermal stresses because it is usually exposed to open environment. Therefore, studying the performance of these films under such conditions is important. Furthermore, the thermal expansion coefficient mismatch between the aluminum film and the substrate creates cyclic stresses on the film when the package is subjected to fluctuating temperature. The repetitive stress may cause film fatigue which create micro-cracks and consequently damage the film. In this study, two sets of experiments were conducted on four different film thicknesses, 50, 100, 200 and 500 nm of aluminum sputter-deposited on 5 mil heat-stabilized PolyEthylene Terephthalate (PET) substrate. The first set of experiments consisted of four samples for each thickness. They were thermally aged at 100 °C for 14 days. The second set of experiments also consisted of four samples for each thickness which were thermally cycled for 840 cycles. In each cycle, the temperature was varied from 0 to 100 °C at a ramp rate of 5 °C/ min and the dwell time of 5 minutes. The initial results showed that the aluminum thin film is very stable against thermal aging in term of electrical resistance. No degradation took place during the thermal cycling as well. SEM images showed no difference in the surface morphology for both tests. EDAX results showed stability in material composition.