Synthesis of CH₃NH₃PbI₃ microcubes via Solvothermal Method

Full text: Methylammonium lead iodide (CH₃NH₃PbI³) has been attracted great attention from scientific community due to its excellent photovoltaic performance. This compound with a perovskite-like crystal structure can be used as light-harvesting layer in solar cell architectures with efficiency close to silicon solar cells. These materials can be obtained via facile and low cost processing methods. In this work, we have shown the formation of CH₃NH₃PbI₃ micro cuboids via microwave assisted solvothermal method using PbI₂ and methylamine as precursors and HI in isopropyl alcohol solution. We have systematically changed the temperature and reaction time during the synthesis process. Scanning electron microscopy reveals well-defined cuboid shape of perovskites. The cube-side length is around 10 μm. We have also observed the presence of hollow spaces and/or hopper-type morphology, i. e., the interior spaces of the microcubes are not completely filled. The highly homogeneous formation of cuboid-shaped polyhedral may be attributed to rearrangement of smaller particles in the solution. A complete understanding of this mechanism remains in debate. X-Ray diffraction confirmed the CH₃NH₃PbI₃ tetragonal phase with I4/mcm space group symmetry for the microcubes. The width at half height of Bragg reflections reveals the same crystallite size regardless the change in reaction time. Dynamic light scattering evidenced the presence of three populations of scattering particles. We have determined the average hydrodynamic radius of the assemblies which reveal 100, 200 and 700 nm. These results suggest that the particles belonging to smaller populations may be the basic units acting as building blocks for the microcube formation. UV-Vis absorption measurement indicated an optical band gap energy around 1.5 eV for all samples. The self-assembly microcube formation will be discussed. (author)