One of the major perovskites used as a light absorber in perovskite solar cells (PSCs) is methylammonium lead iodide (MAPI). MAPI perovskite shows many optimal optoelectronic properties making it a high-performance solar cell material. Nonetheless, PSCs face some limitations related to stability and degradation against moisture, and toxicity due their lead content. The goal of this work is to study the partial substitution of lead iodide (PbI₂) with the inorganic compound copper iodide (CuI) to enhance the solar cell stability thanks to the hydrophobic properties of the latter. XRD showed a tetragonal crystal structure growth for the MAI[(PbI₂)_1−x(CuI)_x] perovskite films. Even for 20 mol%, CuI was well incorporated into the perovskite lattice structure producing a slight change in the lattice parameters. SEM analysis showed a clear improvement of the film’s morphology with the CuI substitution (less pinholes, better uniformity). The optical absorption edges and calculated optical bandgap, around 1.55 eV, remain unchanged with CuI partial substitution. With the increase in CuI/PbI₂ ratio photovoltaic properties of the MAI[(PbI₂)_1−x(CuI)_x] devices improved, higher V_OC and J_SC are observed. Finally, the stability was studied during 150 days in air and an enhancement of PSCs properties was observed for CuI substituted PbI₂ PSCs.