In a nutshell, this study outlines the efficacy of mixed dimensional (2D/3D) hybrid perovskites by developing a new class of triple-cation quasi-2D perovskites having (S0.97S′0.03)2[Cs0.05(FA0.97MA0.03)0.95]n-1Pbn(I0.07Br0.03)3n+1 general composition, in which a mixture of ý5-ammonium valeric acid ýiodideý (S) and tetra-n-octylammonium ýbromide (S′) was employed ýas a spacer.ý The effect of the 2D and 3D structures molar ratios (i.e., C=2D/2D+3D) in the range of 0-100 % on photovoltaic performance of the deposited photoanodes was systemically studied. Drawing a comparison between such compounds and an analogous triple-cation 3D counterpart (i.e., Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3) as a reference (i.e., C=0) showed that the C=10 film containing 10 % 2D and 90 % 3D perovskites had optimal properties for solar cell applications. The pronounced (0 k 0) peak series at low angles and the blue shift of the 3D perovskites peaks observed in X-ray diffraction set forth that quasi-2D perovskites were formed in the layers. The C=10 film having the bandgap of around 1.62 eV exhibited the optimal photon ýabsorption, light harvesting, and crystallinity compared to all the deposited layers, ensuring ýadequate photocurrent. CuInS2 nanoparticles were employed as a novel hole-transporting material to assemble solar cells. The C=10 cell also indicated the highest power conversion efficiency (i.e., 12.33 %), recombination resistance (i.e., 1650.23 Ω), and electron lifetime (20.73 μs) as well as the lowest series resistance of 10.23 Ω amongst all the 2D/3D films, demonstrated by electrochemical impedance spectroscopy (EIS). Although the efficiency of this cell was not yet sufficient to compete with that of the C=0 (i.e., 12.87 %), it could retain 80 % of its own performance over 2500 h in ambient conditions without encapsulation, endowing more stability than its 3D counterpart which sustained 64 % of the initial efficiency. Therefore, developing triple-cation 2D/3D perovskites is a facile, straightforward strategy for fabricating stable, efficient solar cells.