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C7.04 - The Impact of Crystallization Processes on the Structural and Optical Properties of Hybrid Perovskites from Molecular to Mesoscopic Level 
April 9, 2015   11:30am - 11:45am

In the last two years methylammonium lead halide perovskite based solar cells have been characterized by a fast development, achieving power conversion efficiencies approaching 20% 1, 2. Impressively, most of the latest advancements have been the result of a higher control on the material processing and on the crystallization steps. On the other hand, the poor understanding of the relationship between structure and optoelectronic properties is still a limiting factor for a further improvement in the efficiency. Here, we demonstrate how the crystallization procedure as well as the nature of the precursors, i.e. the presence of chlorine, can affect the macroscopic structural and optical properties of CH3NH3PbI3 and the chlorine-doped CH3NH3PbI3 perovskite films. We investigate the optical and micro-structural properties of hybrid perovskite polycrystalline films deposited either on flat glass substrate or infiltrated into a mesoporous scaffold combining Raman spectroscopy 3 with X-Ray Diffraction measurements, High Resolution Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopy 4. The specific organic-inorganic interactions are revealed by monitoring the Raman signal related to the Pb-I stretching modes and the CH3NH3+ rocking modes, as important markers of local distortion of the inorganic cage3. The experimental findings point to a clear trend, that is, an ordered arrangement of the organic cation in CH3NH3PbI3 crystallites grown on �flat� substrates with respect to a fully randomly orientation in the mesoporous scaffold. Interestingly, we observe that the displacement of the organic cations and their interaction with the inorganic cage result in a strain felt by the lattice that manifest as a band gap shrinking moving from meso to flat CH3NH3PbI3 film. On the other hand the presence of chlorine in the precursor to obtain the Chlorine-doped CH3NH3PbI3 also affects the crystal formation both at a molecular scale and at a mesoscopic level during crystal growth. In particular, the chlorine assists the crystallization favouring a preferential ordered arrangement of the organic cation in the unit cell even in the mesoporous scaffold, as revealed by XRD. We demonstrate that, although we do not find any chloride signature in the crystal unit cell, chlorine ions play an important role at a larger scale in driving the formation of crystals with a preferential order. We show that, when the crystallization is carried out on a flat substrate, most chlorine ions are segregated from the film in the forms of large perovskite crystals. When instead the perovskite film is crystallized in the presence of an oxide scaffold, these are retained at the mesoporous interface. References [1] Zhou, H. et al. Science. 345, 542�546 (2014). [2] Im, G. H. et al. Nat. Nanotechnol (2014). doi:10.1038/nnano.2014.181 [3] Grancini, G. et al. J. Phys. Chem. Lett. (2014). doi:10.1021/jz501877h [4] Quarti, C. et al. J. Phys. Chem. Lett. 5, 279?284 (2014)

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