Herein, flexible, Cd-free, and all-dry process Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells on stainless steel (SUS) substrates are developed with manufacturing CIGSSe absorbers fabricated in real production line with large sample size of 0.94 × 1.23 m². Air-annealing process under a temperature of 130 °C, 1 atmospheric pressure, and air humidity of about 50% (accelerated oxidizing process) is conducted on absorbers for their oxidized surface. It is demonstrated for the first time that oxidized surface of CIGSSe absorbers after air-annealing process gives rise to considerable increases in photovoltaic performances of CIGSSe solar cells. This occurs because of formation of native In_x(O,S)_y buffer near CIGSSe surface formed after air-annealing process, which yields self-forming heterojunction, acting as charge separation and hole-blocking barrier with increased valence band offset of native In_x(O,S)_y/CIGSSe to 1.4 eV, thereby avoiding sputtering damage on CIGSSe surface. Therefore, carrier recombination rates at the interface and depletion regions of the solar cell are reduced, implying improvement of interface and near-surface qualities. Ultimately, conversion efficiency of 16.7% for flexible, Cd-free, and all-dry process CIGSSe solar cell on SUS substrate is attained after the air-annealing process (130 °C and 6 h) with potential toward Lab-to-Fab transition.