Adopting the non-equilibrium Green's function theory and the density functional theory (NEGF-DFT), we herein investigated the impact of two widely applied spacer cations BA + (C 4 H 9 NH + 3 ) and PEA + (C 6 H 5 C 2 H 4 NH + 3 ) on the optoelectronic and quantum transport properties of 2D monolayer hybrid halide perovskites. Compared to BA + , the PEA + spacer cations have strong π-π stacking interactions between each other, together with strong hydrogen bonding interactions and van der Waals interactions between the PEA + with MX 4 - 6 backbone, which effectively narrows the electronic band gap via shortening the M-X bond length (and consequently promoting the coupling of antibonding orbitals). Three lead-free monolayer PEA-based hybrid halide perovskites are predicted to possess moderate electronic bandgaps for sunlight harvesting (1.3∼1.9 eV) and reasonably good optical absorption in the visible-light range (their maximum absorption coefficient all exceeds 105 cm -1 ). Among them, 2D monolayer PEA 2 SnI 4 has the most promising electronic bandgap of 1.35 eV, which makes it the most attractive candidate for single-junction solar cells. Due to the relatively high dielectric constant of PEA + , it has a weak exciton binding effect in the selected 2D monolayer PEA 2 MX 4 , which contributes to the easy photo-generated electron/hole separations. Further calculations reveal that the presence of the conjugated benzene ring in PEA + dramatically weakens the potential barrier of 2D monolayer PEA 2 SnI 4 between the inorganic backbone and organic spacer. The potential barrier of 2D monolayer PEA 2 SnI 4 -based devices (1.07 eV) is observed to be nearly 50% lower than that of 2D monolayer BA 2 SnI 4based devices (1.56 eV). Consequently, the bias-voltage-induced current in the 2D monolayer PEA 2 SnI 4 -based device is observed to be one order of magnitude higher than that of the