Attempts to remove environmentally harmful materials in mass production industries are always a major issue and draw attention if the substitution guarantees a chance to lower fabrication cost and to improve device performance, as in a wide bandgap Zn_1-xMg_xO (ZMO) to replace the CdS buffer in Cu(In_1-x,Ga_x)Se₂ (CIGSe) thin-film solar cell structure. ZMO is one of the candidates for the buffer material in CIGSe thin-film solar cells with a wide and controllable bandgap depending on the Mg content, which can be helpful in attaining a suitable conduction band offset. Hence, compared to the fixed and limited bandgap of a CdS buffer, a ZMO buffer may provide advantages in V_oc and J_sc based on its controllable and wide bandgap, even with a relatively wider bandgap CIGSe thin-film solar cell. In addition, to solve problems with the defect sites at the ZMO/CIGSe junction interface, a few-nanometer ZnS layer is employed for heterojunction interface passivation, forming a ZMO/ZnS buffer structure by atomic layer deposition (ALD). Finally, a Cd-free all-dry-processed CIGSe solar cell with a wider bandgap (1.25 eV) and ALD-grown buffer structure exhibited the best power conversion efficiency of 19.1%, which exhibited a higher performance than the CdS counterpart.