Synthesis and characterization of CH3NH3SnI3, Mn doped CH3NH3SnI3 and oxygen-deficient TiO2 as visible-light active semiconductors

Abstract

CH3NH3SnI3 is an environmentally-friendly (lead-free) perovskite whose synthesis depends on SnI2. Some disadvantages of SnI2 include high cost and tin(IV) contamination. Thus, SnSO4 and ‎C16H30O4Sn are presented as cheaper alternatives and different synthesis conditions were studied: 110 oC/10 min, 150 oC/10 min and 150 oC/20 min. Based on X-ray diffraction results, CH3NH3SnI3 was successfully synthesized, being 110 oC/10 min sufficient for phase formation. However, annealing at 150 oC improved crystallinity irrespective of precursor while prolonged annealing time (20 min) was detrimental to the C16H30O4Sn-based perovskite. Film morphology differed by type of precursor. The bandgaps (1.37-1.59 eV) and photoluminescence emissions (831 nm) of the samples characterize them as visible light active semiconductors. Because CH3NH3SnI3 suffers from Sn2+ oxidation to Sn4+, some studies have proposed doping with metal cations. So far, Mn has not been tested. Thus, Mn (2% and 10% mol) doped CH3NH3SnI3 samples were synthesized and characterized. Mn did not disrupt the perovskite structure but slightly increased bandgap. Chlorine from the Mn precursor is not eliminated by annealing, thus could affect material properties. Regarding stability, Mn partially stabilized CH3NH3SnI3, indicating the need for an optimum level of doping. Mn doping does not seem to improve the photovoltaic properties of CH3NH3SnI3, but it should be noted that the solar cells were not optimized. Several questions about CH3NH3SnI3 cells are still open, including alternative TiO2 compatible with the same. Here, oxygen-deficient TiO2 powders were studied. Electron paramagnetic spectroscopy and X-ray photoelectron spectroscopy confirmed oxygen vacancy. Photocatalytic and photocurrent tests showed activity under visible light and an optimum level of oxygen vacancies for lower recombination and high charge separation, important properties for photovoltaic applications.