Recently emerging low-dimensional (2D) organic-inorganic hybrid halide perovskites, constructed by inserting large ammonium cation into conventional three-dimensional perovskites to form a layered structure of alternating stacked insulating organic spacer and inorganic slabs, have the promising potential to improve the long-term stability due to their intrinsic structural features. The hydrophobic organic spacer acts as a natural protective barrier against moisture, thus maintaining long-term stability under ambient operation conditions. Furthermore, the strong van der Waals interactions between organic spacers lead to an increased formation energy, and consequently higher natural lattice stability. Further reducing of the dimensionality was found to improve the perovskites' stability, with a much wider electronic bandgap away from the visible light absorption which usually results in a poor power conversion efficiency. On the other hand, the giant dielectric difference between the organic spacer (potential barrier) and inorganic MX 6 backbone layer (potential well) in the 2D hybrid halide perovskites structures creates a quantum well (QW), which results in the strong exciton effect, severe charge recombination, as well as the retardation of out-of-plane charge carrier transport (leading to inferior carrier mobility). All these effects seriously hamper the photovoltaic performance. Furthermore, the impact of functional groups and the interactions between the spacer cations and inorganic backbone on the electronic structure and carrier transport is still not clear. How to design the composition and structure of 2D monolayer hybrid halide perovskites to narrow the electronic bandgap and promote the charge carrier transport without sacrificing long-term stability, therefore, remains a critical issue of broad interest.