Rapid thickness reading of CH3NH3PbI3 nanowire thin films from color maps

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

Core-Shell GaAs Nanowire p-n Junctions from Growth to Photovoltaic and Optoelectronic Applications

Carlo Colombo

Semiconductor nanowires (NWs) are filamentary crystals with the diameter ranging from few tens up to few hundreds of nanometers. In the last 15 years, they have been intensively studied for the prospectives that their unique quasi-one dimensional shape offers to both fundamental and applied physics. More recently particular attention has been dedicated to the advantages that their geometry exhibits in the separation and extraction of photo-generated carriers with respect to standard thin film or bulk technologies. The aim of this thesis is the realization and characterization of single GaAs NW photovoltaic and optoelectronic devices. The work starts with the synthesis of radial NWp-n junctions. The core growth has been realized using the Vapor-Liquid-Solid (V LS) mechanism by Molecular Beam Epitaxy (MBE). Differently from the commonly used V LS method, which requires Au particles as a catalyst, we use a self-catalyzed process which avoids potential contaminations. The shell is then overgrown around, switching the growth parameters toward standard thin film conditions. The doping mechanism of such a structure has been deeply investigated, with the purpose of discriminating in which way the dopants get incorporated in the crystal structure. A reproducible technique has been developed to electrically contact the NWs and test their photovoltaic properties. They have been measured under different illumination conditions. Conversion efficiency of about 4.5% have been measured under diffuse illumination at 1.5AM and more information about the uniformity of the junction have been deduced by scanning photo-current measurements. Due to the size comparable with the wavelength of the visible light, NWs exhibit characteristic absorption peaks as a function of the photon energy, often referred as Mie resonances. In order to engineer these resonances we have deposited metal nanoparticles on their facets and studied how the plasmonic excitation of the latter could couple with the NW leaky modes. The results have been analyzed with Finite Difference Frequency Domain (FDFD) simulations. Since GaAs is a direct band-gap material, the same devices have also been tested in a reverse way as light emitting diodes and plasmon generators. Finally some preliminary studies about the effect of axial wurtzite/zincblende polytypism on the electronic properties have been performed.

EPFL2012

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

Controlled growth of CH3NH3PbI3 nanowires in arrays of open nanofluidic channels

Eric Joseph Bonvin, László Forró, Endre Horvath, Bálint Náfrádi, Andrzej Sienkiewicz, Massimo Spina

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.