Investigation of Barrier Layer on Flexible Stainless Steel Foils for CIGS Solar Cells

Thin film solar cells will consume a little of material and have potentials in low cost, mass production and flexible substrate applications. The most interesting substrates are stainless steel foils, since they can be coated in a roll-to-roll process at high temperatures of up to 600°C in a Se atmosphere. In the last few years, thin film solar cells draw people’s attention due to their great potential application in the future. However, the efficiency of CIGS solar cells fabricated on stainless steel foils is lower than that on soda-lime glass substrates. There are three possible reasons for this lower efficiency. One is lack of sodium incorporation. From literatures, it is known that the presence of Na during growth leads to electronically and structurally modified CIGS layers. It can increase the carrier concentration and grain size. Therefore, the open circuit voltage and solar efficiency are increased efficiently. Another is metal impurity diffusion during high temperature processing. It worsens the quality of CIGS film and results in a reduction of solar efficiency. The other is surface roughness effects on solar efficiency. The detailed mechanism is still unclear. But, large rough steel surface will result in the reduction of open circuit voltage and fill factor. In this project, we will investigate the processing parameters of fabricating barrier layer for application in flexible stainless steel foils. Three research points are focused. The first one is the process development of barrier layer to prevent metal impurity diffusion effectively. The goal is that to prevent the metal impurity diffusion to CIGS layer is less than 10 cps when the thickness of barrier layer is less than 2 um. The stacked layer of SOG coating/sputtered SiO2 or sputtered SiN3/SOG coating film is investigated. The second one is to provide a sodium diffusion source. CIGS absorbers for high-efficiency cells require the incorporation of typically 0.1 at.% of sodium. The focus point is how to effectively and accurately incorporate sodium into CIGS layer. The coating of Na2S solution is studied. Except providing sodium incorporation, it can also provide sulfur atom and is helpful to enhance the solar efficiency. The last one is the impact of surface roughness on CIGS film properties. We want to know that does a rough substrate result in smaller CIGS grains and more defects as well as increased diffusion of impurities. It is believed that a high efficiency CIGS solar cell on a stainless steel foils can be achieved by optimizing these process parameters.

Project ID:PB10007-2271
External Project ID:NSC100-2221-E182-037