Molybdenum foil with solar energy
Although aluminum as a substrate for solar cell devices should be ideal due to both the low cost and light nature of this substrate, conventional techniques that effectively anneal the CIGS absorber layer also heat the substrate to high temperatures, resulting in damage to the Al substrate. When Al substrates are exposed to heat and/or selenium-containing compounds for long periods of time, several factors can cause degradation of Al substrates. First, during prolonged heating, individual layers within the molybdenum-plated Al substrate can melt and form an intermetallic back contact of the device that reduces the electronic function intended for the molybdenum layer. Secondly, the morphology of the molybdenum layer interface is changed during heating, which has a negative effect on the subsequent growth of CIGS particles through the change of nucleation mode caused by the molybdenum layer surface. Third, during prolonged heating, Al can migrate into the CIGS absorber layer and disrupt the function of the semiconductor. Fourth, the impurities usually present in the Al foil (such as Si, Fe, Mn, Ti, Zn, and V) can propagate with the moving Al when heated for a long time, diffusing into the solar cell, disrupting the battery's electronic and optoelectronic functions. Fifth, when Se is exposed to Al for a relatively long time and at a relatively high temperature, it can form unstable aluminum selenide. In humid air, aluminum selenide can react with water vapor to form aluminum oxide and hydrogen selenide. Hydrogen selenide is a highly toxic gas, and its free formation can cause safety hazards. For all these reasons, high temperature deposition, annealing, and selenization are not possible for substrates made of aluminum or aluminum alloys.
Because of the high temperatures, long deposition times, and annealing steps, CIGS solar cells cannot be effectively fabricated on aluminum substrates (such as bendable foils made of Al and/or Al based alloys), but instead must be fabricated on stronger (and more expensive) materials, such as stainless steel, titanium or molybdenum strips, glass substrates, or heavier substrates made of metal or metal oxide coated glass. Thus, even though aluminum foil-based CIGS solar cells are lighter, bendable, and less expensive than stainless steel, titanium or molybdenum strips, glass substrates, or metal or metal oxide-coated glass substrates, current practice does not permit the use of aluminum foil for substrates.
