Hybrid Halide Perovskite Nanostructures

Hybrid organometallic halide perovskites have been intensively investigated in the past few years, especially the CH3NH3PbI3, as highly efficient light harvesters for various optoelectronic application both for sensing and emitting light. However, despite the intense research in improving the chemical composition, the material morphology and its implementation in prototypal devices, still little is known about the physical mechanisms behind these outstanding performances. The central theme of my PhD dissertation is the CH3NH3PbI3 nanowires. Before starting to study its physical properties at nanoscale I investigated some transport coefficients of bulk crystals. The intrinsic d.c. electric resistivity under white light illumination gave insight into the electronic states that govern the charge carrier transport in dark and under illumination. Furthermore, the measurement of the thermal conductivity and thermoelectric power of CH3NH3MI3 (M= Pb, Sn) revealed the possibility to enhance the figure of merit ZT towards unity by a suitable doping. Characterization of CH3NH3PbI3 single crystals by nanoindentation permitted to extract its Young’s modulus (E, 20.0 GPa) and hardness (H, 1.0 GPa). Upon exposure to water vapor and aging both mechanical quantities decreased considerably. Chemical treatment of the degraded crystal in methylammonium iodine solution partially recovered E and H, showing to be a promising procedure to fight against the loss of advantageous characteristics of the material. The nanowires were prepared by a method, named “slip coating”. Their height can be easily determined by optical microscopy, thanks to the different colors resulting from interference patterns of light. Photoelectric characterization of the nanowires and comparison with perovskite-based film of nanoparticles, revealed the superior charge collection efficiency of single domain structure vs polycrystalline films. Due to this feature CH3NH3PbI3 nanowires were combined with carbon nanomaterials (i.e. graphene and single walled carbon nanotubes (SWCNTs)) to realize highly performant photodetectors. Moreover, the possibility to tune the charge carrier density of these carbon nanomaterials by the application of a gate voltage (phenomenon not observed in the perovskite nanowires alone) gave insights into the quality of the interface that forms between these materials in terms of chemical reactions, defect formations and charge transfer. The transfer of photo-generated charges from the hybrid organic-inorganic perovskite to the carbon material and the consequent photo-induced gating effect allowed to achieve responsivities comparable with state of the art photodetectors. Finally, we realized a photodiode by mechanically putting together crystals of MASnI3 and MAPbI3. The quality of the junction that forms between the two organometallic halide perovskites is remarkable and the resulting photodetectors showed external quantum efficiencies above 100%. These results suggest that multilayer perovskite structures could be an additional way to further improve the performance of optoelectronic devices based on hybrid organometallic halide perovskites.