The structural evolution of Cu(In,Ga)Se2 thin film and device performance prepared through a three-stage process

Cu(In,Ga)Se2 (CIGS) absorber layer is grown on Mo-coated soda-lime glass (SLG) substrates using co-evaporation deposition technique. The growth characteristics of the CIGS films deposited through a three-stage process are examined by interrupting the deposition along the reaction pathway. In the three-stage process, the absorber layer undergoes several phase transformations with Cu content. The γ-(In,Ga)2Se3 layer is formed first and is then converted to α-Cu(In,Ga)Se2 via β-Cu(In,Ga)3Se5. When α-Cu(In,Ga)Se2 stoichiometry is reached, Cu2−xSe segregation at the surface and at grain boundaries begins to occur. The Cu2−xSe improved the densification and grain growth of the absorber layer. Then, as the absorber layer reverts to substoichiometric composition, the Cu2−xSe phase disappears and the depleted server Cu near the surface instead. This paper reports several types of defects found in absorber layers that act as non-radiative recombination centers, such as impurity phases (Cu2−xSe and Cu(In,Ga)3Se5), deep point defects (InCu), grain boundaries, and voids. The highest efficiency at 10.97% was achieved when the bulk [Cu]/([In]+[Ga]) ratio was 0.98 at the third stage of the process. This result is attributed to the low-concentration deep-level defects that act as recombination centers and to the denser structure with larger grain size.