Optical properties of iron-doped cadmium stannate thin films

Thin film technology has been on the rise recently. The thin film technology is rapidly overtaking the bulk technology in almost all squares of the scientific world. Optical properties of thin films doped with metals is an interesting field, since doping a material with a metal is expected to reduce optical transmittance due to introduction of free charge carriers. What are the advantages of doping a material with a metal on the optical properties?