LAUSR

The diversity of organic cations greatly enriches the species of 2D perovskites; traditional 2D Ruddlesden‐Popper (RP) and Dion‐Jacobson (DJ) perovskites are synthesized by two different organic amines. Here, according to the difference in pKa values between conjugated acids of monoprotonated and biprotonated 4‐(2‐Aminoethyl)pyridine (4AEPy) ions, the 2D perovskites of RP (4AEPy)2PbI4 and DJ (4AEPy)PbI4 from same organic amine is reported, which can realize reversible transformation under the treatment of HI and NH3, respectively. The interaction of N‐H···N hydrogen bond between adjacent organic molecules in (4AEPy)2PbI4 leads to the bending conformation of ethylamine groups, which results in a 2.4 Å reduction in layer spacing compared to typical phenylethylamine lead iodine ((PEA)2PbI4) 2D perovskite. Besides, the ethylamine groups of organic layers in (4AEPy)PbI4 are deeply inserted into octahedral cavities and directly participate in the construction of the conduction band minimum, which leads to a small exciton binding energy of 27.3 meV to generate free charges. The stronger coupling between the organic and inorganic layers and the minor exciton binding energy can promote the DJ phase to possess a more stable structure and better optoelectronic properties. Thus the (4AEPy)PbI4 device displays better light response and X‐ray detection capability with a high sensitivity of 5627 µC Gyair‐1 cm‐2 and the lowest detectable dose rate of 20 nGyair s‐1.