Understanding the “Molten Salt” Synthesis of MAPbI3 – Characterization of New Lead(II)‐Ammine Complexes as Intermediates

Abstract

AbstractThe so‐called “molten salt process” is used to enhance the properties of perovskite‐based solar cells using CH3NH3PbI3 (MAPbI3) as an absorber. We have characterized intermediates of the reaction of CH3NH2 with both PbI2 and MAPbI3. The reaction of PbI2 or MAPbI3 in liquid CH3NH2 yields white [Pb(CH3NH2)6]I2 (cubic K2PtCl6 type), representing a typical coordination compound and the first homoleptic ammine Pb(II)‐complex. Reducing the CH3NH2 vapor pressure and/or increasing the temperature gives pale‐yellow platelets of the neutral complex Pb(CH3NH2)4I. The coordination of Pb2+ consists of four neutral CH3NH2 ligands and two iodide anions with quite different distances, so the complex could also be written as [Pb(CH3NH2)4I]+I−. Traces of humidity in the molten salt process result in the formation of Pb(OH)I and a new compound, namely (CH3NH3)5(CH3NH2)2Pb2I9 with isolated [Pb2I9]5− di‐octahedra. Charge compensation takes place by CH3NH3+ cations and novel dimers [CH3NH3⋅⋅⋅NH2CH3]+, where a cation and neutral molecule are connected via a N−H⋅⋅⋅N hydrogen bond.