Three-step vapor Se/N₂/vapor Se reaction of electrodeposited Cu/In/Ga precursor for preparing CuInGaSe₂ thin films

The three-step H₂Se/Ar/H₂Se reaction of CuInGa precursors for preparing high-quality CuInGaSe₂ thin films is extensively used to control their composition and adhesion. In this work, a non-toxic, low-cost, solid Se source is used instead of toxic H₂Se gas to selenize electrodeposited Cu/In/Ga precursors by the three-step vapor Se/N₂/vapor Se reaction. Experimental results reveal that the Se content of the film in the first selenization step importantly affects the Ga distribution. Incomplete selenization (0.74<Se/Metal<0.96) in the first selenization step favors subsequent Ga diffusion. The second annealing step in inert gas atmosphere generates significant grain growth with Ga re-diffusion and does not change the composition in comparison with the first step treatment. The third selenization step in Se vapor atmosphere does not cause a significant additional change in the Ga distribution or microstructure of the film, but it causes Ga depletion at the top surface. Finally, CIGS solar cells on stainless steel substrate were fabricated using the three-step reaction and devices with efficiency=8.23% and V_OC=434 mV were produced by a preliminary process parameter optimization.