Fabrication et caractérisation de cellules solaires organiques nanostructurées par la méthode de nanoimpression thermique

La morphologie des couches actives des cellules solaires organiques joue un rôle important sur l’efficacité de conversion de l’énergie solaire en énergie électrique de ces dispositifs. Les hétérojonctions planaires et les hétérojonctions en volume sont les plus communément utilisées. Cependant, la morphologie idéale pour l’efficacité se situerait à mis chemin entre celles-ci. Il s’agit de l’hétérojonction nanostructurée qui augmenterait la surface entre les couches actives de matériaux tout en favorisant le transport des porteurs de charge. L’objectif de ce projet de maîtrise est d’étudier l’impact de l’implantation de nanostructures dans les cellules solaires organiques sur leurs performances photovoltaïques. Pour ce faire, on utilise la méthode de nanoimpression thermique sur le matériau donneur, le P3HT, afin que celui-ci forme une interface nanostructurée avec le matériau accepteur, le PCBM. Pour effectuer les nanoimpressions, des moules en alumine nanoporeuse ont été fabriqués à l’aide du procédé d’anodisation en deux temps développé par Masuda et al. Ces moules ont subi un traitement afin de faciliter leur séparation du P3HT. Les agents antiadhésifs PDMS et FTDS ont été utilisés à cette fin. Les résultats obtenus témoignent de la complexité d’exécution du procédé de nanoimpression. Il a été démontré que la pression appliquée durant le procédé, la tension superficielle des éléments en contact et les dimensions des nanopores des moules sont des paramètres critiques pour le succès des nanoimpressions. Ceux-ci ont donc dû être optimisés de manière à réussir cette opération. Ainsi, des cellules à interface nanostructurée à 25% avec des nanobâtonnets de 35 nm de hauteur ont pu être fabriquées. Les cellules nanostructurées ont démontré une efficacité 2,3 ± 0,6 fois supérieure aux cellules sans nanostructures, dites planaires. D’autre part, un solvant a été proposé pour diminuer l’interdiffusion entre les couches de P3HT et de PCBM pouvant altérer les nanostructures. Ce phénomène bien connu survient lors du dépot de la couche de PCBM avec le dichlorométhane, un solvant orthogonal avec ces matériaux. Des mesures au TOF-SIMS ont démontré que le limonène permet de diminuer l’interdiffusion entre les couches de P3HT et de PCBM, ce qui en fait un meilleur solvant orthogonal que le dichlorométhane.

Design, Fabrication, and Mechanical Property Analysis of 3D Nanoarchitected Materials

Recent developments in micro- and nanoscale 3D fabrication techniques have enabled the creation of materials with a controllable nanoarchitecture that can have structural features spanning 5 orders of magnitude from tens of nanometers to millimeters. These fabrication methods in conjunction with nanomaterial processing techniques permit a nearly unbounded design space through which new combinations of nanomaterials and architecture can be realized. In the course of this work, we designed, fabricated, and mechanically analyzed a wide range of nanoarchitected materials in the form of nanolattices made from polymer, composite, and hollow ceramic beams. Using a combination of two-photon lithography and atomic layer deposition, we fabricated samples with periodic and hierarchical architectures spanning densities over 4 orders of magnitude from ρ=0.3-300kg/m³ and with features as small as 5nm. Uniaxial compression and cyclic loading tests performed on different nanolattice topologies revealed a range of novel mechanical properties: the constituent nanoceramics used here have size-enhanced strengths that approach the theoretical limit of materials strength; hollow aluminum oxide (Al₂O₃) nanolattices exhibited ductile-like deformation and recovered nearly completely after compression to 50% strain when their wall thicknesses were reduced below 20nm due to the activation of shell buckling; hierarchical nanolattices exhibited enhanced recoverability and a near linear scaling of strength and stiffness with relative density, with E∝ρ^1.04 and σy∝ρ^1.17 for hollow Al₂O₃ samples; periodic rigid and non-rigid nanolattice topologies were tested and showed a nearly uniform scaling of strength and stiffness with relative density, marking a significant deviation from traditional theories on “bending” and “stretching” dominated cellular solids; and the mechanical behavior across all topologies was highly tunable and was observed to strongly correlate with the slenderness λ and the wall thickness-to-radius ratio t/a of the beams. These results demonstrate the potential of nanoarchitected materials to create new highly tunable mechanical metamaterials with previously unattainable properties.

Functionalized Carbon Micro/Nanostructures for Biomolecular Detection

Advancements in the micro-and nano-scale fabrication techniques have opened up new avenues for the development of portable, scalable and easier-to-use biosensors. Over the last few years, electrodes made of carbon have been widely used as sensing units in biosensors due to their attractive physiochemical properties. The aim of this research is to investigate different strategies to develop functionalized high surface carbon micro/nano-structures for electrochemical and biosensing devices. High aspect ratio three-dimensional carbon microarrays were fabricated via carbon microelectromechanical systems (C-MEMS) technique, which is based on pyrolyzing pre-patterned organic photoresist polymers. To further increase the surface area of the carbon microstructures, surface porosity was introduced by two strategies, i.e. (i) using F127 as porogen and (ii) oxygen reactive ion etch (RIE) treatment. Electrochemical characterization showed that porous carbon thin film electrodes prepared by using F127 as porogen had an effective surface area (Aeff 185%) compared to the conventional carbon electrode. To achieve enhanced electrochemical sensitivity for C-MEMS based functional devices, graphene was conformally coated onto high aspect ratio three-dimensional (3D) carbon micropillar arrays using electrostatic spray deposition (ESD) technique. The amperometric response of graphene/carbon micropillar electrode arrays exhibited higher electrochemical activity, improved charge transfer and a linear response towards H2O2 detection between 250μM to 5.5mM. Furthermore, carbon structures with dimensions from 50 nano-to micrometer level have been fabricated by pyrolyzing photo-nanoimprint lithography patterned organic resist polymer. Microstructure, elemental composition and resistivity characterization of the carbon nanostructures produced by this process were very similar to conventional photoresist derived carbon. Surface functionalization of the carbon nanostructures was performed using direct amination technique. Considering the need for requisite functional groups to covalently attach bioreceptors on the carbon surface for biomolecule detection, different oxidation techniques were compared to study the types of carbon–oxygen groups formed on the surface and their percentages with respect to different oxidation pretreatment times. Finally, a label-free detection strategy using signaling aptamer/protein binding complex for platelet-derived growth factor oncoprotein detection on functionalized three-dimensional carbon microarrays platform was demonstrated. The sensor showed near linear relationship between the relative fluorescence difference and protein concentration even in the sub-nanomolar range with an excellent detection limit of 5 pmol.

Development, Evaluation and Optimization of Fabrication Processes for Daylight Guiding Systems Based on Micromirror Arrays

In this work, fabrication processes for daylight guiding systems based on micromirror arrays are developed, evaluated and optimized.Two different approaches are used: At first, nanoimprint lithography is used to fabricate large area micromirrors by means of Substrate Conformal Imprint Lithography (SCIL).Secondly,a new lithography technique is developed using a novel bi-layered photomask to fabricate large area micromirror arrays. The experimental results showing a reproducible stable process, high yield, and is consuming less material, time, cost and effort.

The optical properties of metamaterials

This thesis studies the parametric investigation, polarisation dependence and characterization of fishnet structure at near infrared wavelengths. Detailed simulations are performed to understand the behaviour of the structure at near infrared and optical wavelengths. Simulations are performed to obtain negative refractive index of the fishnet structure formed from nanoimprint lithography (NIL) by taking into account the effect of substrate and polymethyl methacrylate (PMMA) beneath it. Two different structures have been designed and fabricated of varying dimensions using NIL and their resonant wavelength measured in the near infrared at 1.45 µm and 1.88 µm. Simulations suggest that a negative refractive index real part with the magnitude -0.24 is found at 1.53 µm and this decrease to a maximum magnitude of -0.57 at 1.9 µm. The PMMA and suppressed pillars are here responsible for the increasing material losses and limiting the value of negative refractive index. An analytical approach has been suggested to characterise fishnet structures at oblique incidence. The expressions for an absorbing medium are rewritten for an alternative definition of refractive index. The expressions are initially validated for a dielectric slab and a metal film. These results provide the possibility that this proposal may yield a general algorithm for obtaining the complex reflection and transmission coefficients for artificial structures. FDTD simulations have been extensively used in this thesis to understand the optical metamaterials and their characterization.

Dispositivi organ-on-chip per l' ingegnerizzazione di tessuto cardiaco destinato allo screening farmacologico

Negli ultimi anni, nell' ambito dell' ingegneria dei tessuti, ha avuto un rapido aumento la generazione di tessuti cardiaci miniaturizzati, per lo studio della fisiologia cardiaca e delle patologie. In questa tesi, viene analizzato un processo di realizzazione di un dispositivo heart-on-a-chip recentemente pubblicato da Jayne et al. Per il processo di fabbricazione dei dispositivi è stata utilizzata una combinazione di Soft Lithography e Direct Laser Writing (DLW). Quest' ultima, in particolare, ha fornito due importanti caratteristiche ai dispositivi deputati alla semina cellulare: una struttura curva lungo l’ asse verticale e strutture 3D di diverse altezze sullo stesso piano. Tramite DLW sono stati realizzati anche precisi punti di adesione per le cellule staminali pluripotenti indotte, che hanno consentito di controllare la geometria dei tessuti ingegnerizzati. In particolare, oltre al processo di fabbricazione, in questo lavoro vengono anche illustrate le procedure necessarie al fine di calibrare i microsensori utilizzati per monitorare i costrutti. La prima fase della calibrazione si occupa di determinare la responsività meccanica dei sensori di spostamento, mentre la seconda valuta quella dei sensori elettrici, deputati alla conversione di spostamenti in variazioni di resistenza elettrica.