Fabrication and characterization of perovskite CH_3NH_3PbI_3 films via two-step sol-gel process: Impact of soaking time of PbI_2

摘要

Perovskite MAPbI(3) (MA = methylammonium, CH3NH3) 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 PbI2 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 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 PbI2: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 PbI2 and ions I- from the hydrolyzed MAI form the stable PbI64- cubic frame. Ions (MA)(+) diffuse to the interstice positions in the cubic frame structure under the electrostatic force, thus producing the ultimate MAPbI(3).