Formation of CuInAlSe2 film with double graded bandgap using Mo(Al) back contact

Using Al added Mo back electrode to provide Al source to form CuInAlSe2 (CIAS) absorber with self-formed double graded bandgap (or Al concentration) is reported. The double Al grading is self-forming and requires no process tweaking or modification. A 15 at % Al in Mo(Al) film yielded 0.39 Al/(In+Al) ratio in the CIAS film with a bandgap of 1.54 eV at the surface. The benefits of doping Al into Mo film are: lower resistance of the Mo layer, improved Mo to glass adhesion, increased surface electric field or improved minority carrier collection from the graded Al content (graded bandgap), and supply Al to form a CIAS absorber layer.