In this study, α-Al2O3 and γ-Al2O3 nanoparticles were synthesized via the sol–gel method and employed as mesoporous layer in perovskite solar cells with the structure of FTO/Compact TiO2/Mesoporous Al2O3/CH3NH3PbI3/CuInS2/Au. The aim of this study was to investigate the effect of the crystalline phase of aluminum oxide on the structural, optical, and photovoltaic properties of the CH3NH3PbI3 perovskite layer. The structural characteristics of the samples were evaluated using XRD and SEM. The results revealed that the γ-Al2O3 scaffold improved the crystallization of the perovskite layer and increased the perovskite crystallite size from 44.29 to 45.82 nm. Moreover, the average grain size of the perovskite layer increased from 410 nm for the α-Al2O3-based sample to 470 nm for the γ-Al2O3-based sample, indicating enhanced crystal growth and reduced grain boundaries in the perovskite layer. Optical investigations demonstrated that the perovskite layer grown on the γ-Al2O3 scaffold exhibited higher light absorption and improved optical quality. The direct band gaps for the α-Al2O3 and γ-Al2O3-based samples were determined to be 1.566 and 1.533 eV, respectively, while the indirect band gaps were 1.521 and 1.488 eV, respectively. The 0.045 eV difference between the direct and indirect band gaps was attributed to the Rashba effect. Furthermore, the reduction of the Urbach energy from 0.186 eV for the α-Al2O3-based perovskite to 0.170 eV for the γ-Al2O3-based sample indicated decreased structural disorder and electronic defects. The improved structural and optical properties of the perovskite layer on the γ-Al2O3 scaffold enhanced the photovoltaic performance of the corresponding solar cell, increasing the power conversion efficiency from 6.01% for the α-Al2O3-based device to 6.81% for the γ-Al2O3-based device. These results demonstrate that γ-Al2O3 as a mesoporous scaffold can effectively improve the performance of perovskite solar cells by enhancing crystal growth, reducing defects, and increasing light absorption in the perovskite layer.