Characterization of Cu(In,Ga)Se2 Solar Cells Grown on Na-Free Glass with an NaF Layer on a Mo Film

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Cu(In,Ga)Se2 (CIGS) solar cells fabricated on soda-lime glass (SLG) exhibited high efficiency due to the supply of Na from the SLG substrate. As a simple doping method, Na can be supplied into CIGS films using a Na compound deposited on Mo electrodes. In this study, the authors compared the properties of CIGS thin films grown on a NaF layer deposited on Mo-coated Na-free glass with those of CIGS films on Mo-coated SLG by a standard through high-resolution transmission electron microscopy and low-temperature photoluminescence analyzes. After NaF deposition on the Mo film, an amorphous interlayer was detected at the CIGS/MoSe2 interface, and the MoSe2 layer that formed on the Mo surface was thin. The photoluminescence study showed that NaF doping did not effectively prevent the formation of deep donors in the CIGS film, whereas Na supplied by SLG effectively prevents their formation. We concluded that the poor performance of CIGS solar cells incorporating a NaF precursor is due to the amorphous layer at the CIGS/Mo interface and different luminescence characteristic with standard CIGS films, resulting in a low minority carrier collection.