Recently, the 2D perovskite layer is employed as a capping/passivating layer in the perovskite solar cells (PSCs). The 2D perovskite layer is prepared by inserting a large-sized hydrophobic cation spacer into the perovskite crystal lattice. The large-sized cation in the 2D perovskite lattice can successfully suppress the moisture intrusion and hence improve the stability of the PSCs. However, a deep understanding of the interfacial mechanisms at the 2D/3D heterojunction and the relative contributions of the mobile ions and trapped charge carriers is still lacking. In this work, deep levels transient spectroscopy (DLTS) and reverse DLTS (RDLTS) have been performed to characterize the n-i-p structured 3D and 2D/3D PSCs. DLTS and RDLTS have been used to distinguish between the spectral contribution made by mobile ionic species, electron/hole traps, and to investigate the presence of ordinary deep electron and hole traps in the bandgap of perovskite. Besides, the PSCs have been characterized by photoinduced voltage transient spectroscopy (PIVTS) to study the decay of the open-circuit voltage (V-OC) under illumination. For both 3D and 2D/3D PSCs, the contribution of mobile ions was found to be dominant; however, in the case of 2D/3D samples, the intensity of the mobile ions signal was several times lower. The lower intensity can be correlated with a lower amplitude of slow tails in V-OC decay curves in 2D/3D solar cells as compared to 3D solar cells. The PIVTS study also endorses the 2D/3D structures as more robust than the 3D structures.