Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se-2 (CIGS) material have demonstrated excellent power conversion efficiency. However, developing a low-cost fabrication process with less safety risk that compatible with large-scale production still stands as a major challenge. Using cheap and low toxic element Se vapor to replace the expensive and highly toxic H2Se which is widely used in sputtering-based industrial manufacture is an attractive solution. Received considerable attention, yet the devices fabricated from metallic precursor and element Se vapor still suffer from an inferior performance. In this work, we disclosed the main culprit behind this limitation-the ordered vacancy compounds (OVC) phase at the CIGS/Mo rear interface, which is induced by the drastic out-diffusion of Cu during the selenization. The detrimental OVC phase at the rear interface acts as a blocking barrier, and also results in enhanced rear interface recombination, and weakens the band bending in the hetero-junction, as manifested by the temperature-dependent current-voltage (J-V) measurements, KPFM measurements, and SCAPS-1D simulation. We found that the out-diffusion of Cu and the associated formation of OVC phase at the rear interface can be efficiently suppressed by turning the element Cu and In in as-sputtered precursor into Cu-9(In,Ga)(4) and Cu9Ga4 alloys using a co-sputtered metallic precursor with a thermal alloying treatment. With the rear interface modification, we significantly improved the device performance, with pronounced increase of Voc and fill factor. Our results indicate even higher efficiency can be achieved by further improving the doping density of the rear interface and optimizing the band gap.