Low Temperature Growth of CIGS Thin Films for Flexible Solar Cells

Low substrate temperatures have to be used for polymer substrates. Therefore, using soda- lime glass (SLG) substrates with and without an alkali barrier (Al 2 O 3 ), a three-step CIGS coevaporation process for a substrate temperature of 450 °C has been developed and compared to film deposition with constant evaporation rates. The three-step process was found to enhance grain nucleation. An efficiency of 14.0% has been achieved with this process for solar cells on SLG. Since polymers in general do not contain Na, a way of Na addition to the absorber is needed. It is shown that NaF coevaporation can be used to control the Na content in CIGS. Also incorporation of Na in CIGS by diffusion from a NaCl layer through a polyimide is demonstrated. With such SLG/NaCl/polyimide structures flexible solar cells can be obtained using a lift-off process. A cell efficiency of 11.6% (0.99 cm 2 area) has been achieved.