Fabrication and characterization of perovskite CH3NH3PbI3 films via two-step sol-gel process: Impact of soaking time of PbI2

Perovskite MAPbI 3 (MA = methylammonium, CH 3 NH 3 ) films and solar cells have thoroughly been studied in recently years. However, the formation mechanism of MAPbI 3 in the two-step sol-gel process is not reported and not well understood yet. Thus in this work, the cubic perovskite MAPbI 3 films with preferred 〈110〉 orientation were fabricated on the glass substrates via the two-step sol-gel process. Impact of soaking time (t soaking ) of PbI 2 in MAI in isopropyl alcohol (IPA) solution was thoroughly studied on the microstructure and the optical properties of the MAPbI 3 films. The formation mechanism of MAPbI 3 in the two-step sol-gel process was in particular proposed. The blue and red shifts of the absorption edge of the films with increase of t soaking are attributed to the improved crystallization and to the weakened surface effect of the grains, respectively. An intermediate metal-organic frameworks (MOF) structure of PbI 2 :MAI:IPA is formed during the soaking due to Pb weakly coordinating with IPA organic molecules. The unstable MOF structure is collapsed during the following annealing, and PbI 2 and ions I − from the hydrolyzed MAI form the stable PbI 6 4− cubic frame. Ions (MA) + diffuse to the interstice positions in the cubic frame structure under the electrostatic force, thus producing the ultimate MAPbI 3 .