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

Abstract

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 growth stages, the stoichiometry of the absorbers changes from Cu-poor ([Cu]/([In] + [Ga]) < 1) to Cu-rich ([Cu]/([In] + [Ga]) > 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 Cu2–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 scanning transmission 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 conditions.

Secondary-Phase-Assisted Grain Boundary Migration in CuInSe2

“…During the third stage of the three-stage growth process, the ratio of Cu=In decreases, and the CS secondary phase at the GBs might transform back to the CIS or CIGS phases. Nevertheless, such CS secondary phases are occasionally found at the GBs of bulk CIGS after a full three-stage growth [42], consistent with the present observation.…”

“…FIG [40,41]. Occasionally, small platelets with Cu 2−x Se phases were found at GBs in Cu-poor CIGS [42]. Our results show that such topotactic transitions between CIS and Cu 2−x Se phases commonly exist near the CIS GBs when sufficient Cu diffuses into GBs.…”

Elucidating the Mechanism of an RbF Post Deposition Treatment in CIGS Thin Film Solar Cells

“…A depletion of Se and In as well as an enrichment of Cu, O, and Rb at the grain boundary is observed. Similar results (in studies without Rb) have been reported previously . But also contrary results have been published …”

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

“…1c). Recent high-resolution TEM evidence from Simsek Sanli et al 30 has revealed the presence of interspersed Cu 2 Se domains within CIGS grains, to which they attributed a topotactical grain growth effect during the three-stage process 31 . At this point, it is not possible to resolve with certainty which of the two scenarios occurs in reality.…”