Precipitation of CH3NH3PbCl3 in CH3NH3PbI3 and Its Impact on Modulated Charge Separation

A precipitation of CH3NH3PbCl3 (MAPbCl(3)) in CH3NH3PbI3 (MAPbI(3)) has been observed for deposition from solutions containing CH3NH3I (MAI), PbCl2 and small amounts of ammoniumvaleric acid iodide (HOOC(CH2)(4)NH3I or AVAI) in N,N-dimethylformamide. The influence of the precipitation of MAPbCl(3) on the electronic properties of the free perovskite layer and of the nanoporous TiO2/perovskite nanocomposite has been investigated by modulated surface photovoltage spectroscopy. The lowest characteristic energy of exponentially distributed defect states below the band gap was found for the highest amount of AVAI in the solution (2.6% for the given experiments). Stabilization of MACl by AVA and the role of coadsorption of ions on TiO2 surfaces for charge separation and layer formation are discussed on the basis of interaction and adsorption energies obtained by molecular modeling.