Cu(In_1−xGa_x)Se₂ (CIGS) is a promising and ideal material for bottom cell in tandem solar cells, which can break the double junction solar cell’s Shockley–Queisser theoretical efficiency to above 40%. However, the high-efficiency CIGS solar cells deposited by the 3-stage process is normally double grading, leading to an incomplete absorption in the bottom cells. In this study, single bandgap grading and quasi-flat bandgap CIGS solar cells are proposed and fabricated for perovskite/CIGS 4-terminal tandem solar cells, which are more favorable for long-wavelength absorption and higher short-circuit current density J_sc. Various characterizations have been performed to investigate the crystallization, crystal defects, composition depth profile, and carrier dynamics of the CIGS thin films. Our study reveals that the performance of CIGS solar cells with high Ga content is worse than expected. Using bandgap engineering, we can obtain CIGS solar cell with an efficiency above 16.5% regardless of the GGI [Ga/(Ga + In)] varying from 0.27 to 0.40. However, CIGS solar cells with less Ga content and low bandgap exhibit superior long-wavelength spectral response, making them more suitable for bottom cell applications in tandem solar cells. In combination with an 18.9% semitransparent inorganic perovskite solar cell, a 25.6% perovskite/CIGS tandem device in 4-T configuration is demonstrated.