Controlled growth of CH3NH3PbI3 nanowires in arrays of open nanofluidic channels

Spatial positioning of nanocrystal building blocks on a solid surface is a prerequisite for assembling individual nanoparticles into functional devices. Here, we report on the graphoepitaxial liquid-solid growth of nanowires of the photovoltaic compound CH3NH3PbI3 in open nanofluidic channels. The guided growth, visualized in real-time with a simple optical microscope, undergoes through a metastable solvatomorph formation in polar aprotic solvents. The presently discovered crystallization leads to the fabrication of mm2-sized surfaces composed of perovskite nanowires having controlled sizes, cross-sectional shapes, aspect ratios and orientation which have not been achieved thus far by other deposition methods. The automation of this general strategy paves the way towards fabrication of wafer-scale perovskite nanowire thin films well-suited for various optoelectronic devices, e.g. solar cells, lasers, light-emitting diodes and photodetectors.

Hybrid Halide Perovskite Nanostructures

Massimo Spina

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 Youngâs 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 âslip coatingâ. 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.

EPFL2015

Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping

Alberto Casadei, Anna Fontcuberta i Morral, Fauzia Jabeen, Federico Matteini

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10(18) cm(-3) for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.

American Chemical Society2016

Rapid thickness reading of CH3NH3PbI3 nanowire thin films from color maps

László Forró, Claudio Grimaldi, Endre Horvath, Bálint Náfrádi, Massimo Spina

Hybrid halide perovskite photovoltaic materials show a remarkable light conversion efficiency in various optoelectronic devices. In the fabrication of these solar cells, light emitting diodes, laser and photodetector prototypes the thickness of the perovskite is an important parameter since the light is absorbed within a thin layer of a few hundred nanometers. Nevertheless, making perovskite coatings with various solution-based and evaporation methods showing highly reproducible thickness and area coverage is still an issue. Therefore, rapid and reliable quality-control of the film morphology is needed. This report shows a simple, rapid, and calibration-free method for reading the thickness directly from the color map of nanowire perovskite films seen in standard optical microscope with visible light. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Wiley-VCH Verlag Berlin2016

Hybrid Halide Perovskite Nanostructures

Massimo Spina

EPFL2015