Herein we demonstrate the favorable synergistic effect of an NH4Cl additive and DMSO solvent on 2D perovskites that are synthesized at room temperature. Interestingly, we observe the spontaneous formation of 3D phases within either NH4Cl or DMSO treated 2D perovskites, which plays a pivotal role in facilitating charge transport. It is revealed that NH4Cl increases charge carrier lifetime and passivate trap states within the 3D phase while DMSO promotes 2D/3D inter-phase charge transfer. These two competitive processes reach a delicate balance in DMSO and NH4Cl co-treated devices, which deliver a maximum PCE up to 13.41% with excellent air-stability and eliminated hysteresis. This is among the highest values reported for 2D RP perovskite (n = 4) based planar solar cells, particularly via all low-temperature solution fabrication.