Multi-stage co-evaporation process for active Ga gradient control in CIGS solar cells

The control of the in-depth compositional gradient in Cu(In,Ga)Se 2 (CIGS) absorber layers is known to be crucial to achieve very high solar to electric energy conversion efficiencies in CIGS-based solar cells. In this work we varied the Ga-gradient extrinsically, i.e. by changing the ratio of In and Ga deposition rates throughout the deposition of CIGS. We show that the double Ga gradient intrinsically achieved by the 3-stage co-evaporation process can be further optimized. The challenge thereby is to lower the optical band gap while maintaining a high open circuit voltage and fill-factor.