, where ε is the dielectric constant and µ = (m * e × m * h )/(m * e + m * h ) is the reduced effective mass and e is the charge of an electron. The calculated exciton binding energy (Table ) of all selected PEA-based 2D monolayer hybrid halide perovskites are much larger with respect to that of MAPbI 3 (∼15 meV), which undoubtedly inhibit the photoninduced electron-hole separations. Compared to 2D monolayer BA 2 MX 4 , the selected 2D monolayer PEA 2 MX 4 possess smaller exciton binding energy and larger exciton radius. The small exciton binding energy of 2D monolayer PEA 2 MX 4 should stem from the relatively high dielectric constant of PEA + spacer cation, as evaluated by the dielectric constant model.