Effects of Surface Termination on CuIn1-xGaxSe2 Prepared from Mixed Cu-In-Ga-Se Precursors

Thin film CuIn(Ga)Se₂ samples were prepared by a two-stage method starting from amorphous or nano-crystalline precursor structures. Deposition of individual Cu+Se and In+Se layers as a function of substrate temperature revealed the onset of detectable crystal structures at T_sub = 100°C and T_sub ≥ 200°C, respectively. For the quaternary system the formation of CuInSe₂ was observed at 400°C and evidence was found for liquid Cu_xSe assisted growth. Further focus was on surface termination schemes with the objective to enhance the open-circuit voltage. Ga+Se, In+Se, Ga+Se/In+Se, and In+Se/Ga+Se terminating layers with varying amounts of Ga and In are addressed. Schemes studied to date have resulted in an increase in V_oc at the expense of short-circuit current. The use of Ga containing termination layers resulted in the formation of large voids in the CIGS which could be explained by the volume changes during formation of the quaternary material and a Cu₂Se free surface in these instances.