The choice of a transparent conductive oxide (TCO) as the top contact of a CIGS PV cell is critical to the long term performance of the cell. While most PV applications are forced to match the warranties of roofing contractors...25-30 years..., the life of the cells over that time period can be called into question by the choice of the TCO.
Aluminum Zinc Oxide (AZO) is a common choice of TCO because of cost and relatively good optical transmission performance in the solar spectrum. However, if even the slightest amount of moisture penetrates the cell, the AZO coating reacts with the water and ceases to operate as a TCO, rendering the cell useless.
I have not been privy to all environmental life testing of CIGS solar cells, but my belief is that it is virtually impossible to create a flexible seal on a thin film CIGS cell that will withstand 25-30 years of storms on a rooftop in my neighborhood. And if you can't withstand the storms and moisture for 25-30 years, you should not use AZO. Your cell will not survive.
The main concern about another TCO, indium tin oxide (ITO) is the cost. ITO can be priced at several times that of AZO. However, ITO does consistently defeat AZO in almost every performance category including chemical resistance to moisture. ITO is not affected by moisture and it can survive in a CIGS cell for 25-30 years on a rooftop.
While the sputtering target or evaporative material that is used to deposit the ITO is significantly more costly than AZO, consider that the amount of material placed on each cell is quite small. Therefore the cost penalty per cell is quite small too.
It is my belief that the cell lifetime benefits of ITO greatly outweigh the cost penalty of the material and I strongly recommend to all CIGS manufacturers that they use ITO as their TCO.