Evidence for Cu2–xSe platelets at grain boundaries and within grains in Cu(In,Ga)Se2 thin films

Cu In,Ga Se2 CIGS based solar cells reach high power conversion efficiencies of above 22 . In this work, a three stage co evaporation method was used for their fabrication. During the growthstages, the stoichiometry of the absorbers changes from Cu poor [Cu] [In ] [Ga] lt; 1 to Cu rich [Cu] [In] [Ga] gt; 1 and finally becomes Cu poor again when the growth process is completed. It is known that, according to the Cu In Ga Se phase diagram, a Cu rich growth leads to the presence of Cu2 xSe x 0 0.25 , which is assumed to assist in recrystallization, grain growth, and defect annihilation in the CIGS layer. So far, Cu2 xSe precipitates with spatial extensions on the order of 10 100 nm have been detected only in Cu rich CIGS layers. In the present work, we report Cu 2 xSe platelets with widths of only a few atomic planes at grain boundaries and as inclusions within grains in a polycrystalline, Cu poor CIGS layer, as evidenced by high resolution scanningtransmission electron microscopy STEM . The chemistry of the Cu Se secondary phase was analyzed by electron energy loss spectroscopy, and STEM image simulation confirmed the identification of the detected phase. These results represent additional experimental evidence for the proposed topotactical growth model for Cu Se assisted CIGS thin film formation under Cu rich condition