Dissertations / Theses on the topic 'Solar Photon'

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.
Includes bibliographical references (p. 150-161).
Photovoltaics is a technology that directly converts photon energy into electrical energy. Depending on the photon source, photovoltaic systems can be categorized into two groups: solar photovoltaics (PV) and thermophotovoltaics (TPV). In solar photovoltaic systems, the photon source is the sun, whereas in thermophotovoltaic systems the photons are from artificially designed thermal emitters that operate at a lower temperature. The differences in the photon sources lead to different research emphases on the two photovoltaic systems in this work. This thesis investigates ways to control photon emission and absorption for solar energy and TPV applications. Several topics are discussed, including photon transport in multilayer structures, measurement of near-field thermal radiation, optical absorption in silicon nanowire structures, surface-plasmon enhanced near-bandgap optical absorption in silicon, and selective absorber surface for solar thermal applications. For thermophotovoltaic systems, the work is focused on thermal emission and photon transport. The study of photon transport in multilayer structures is presented. Results based on wave-optics and ray tracing methods are compared. The analysis shows that for structures contain a large number of layers, the coherence length of the emitting source is no longer a valid criterion to indicate whether ray tracing method is valid. Instead, wave inference effects always play a role. The effects of photon localization are also discussed. Surface-mode enhanced near-field thermal radiation is explored in this work as an effective way to tailor the thermal emission for TPV systems. Calculations based on fluctuation-dissipation theorem and Maxwell's equations are presented to study radiative heat transfer between two closely-spaced glass plates. The theoretical analysis shows that the radiative heat transfer between closely-spaced glass plates is enhanced by surface phonon polaritions and the flux can exceed the far-field upper-limit imposed by Planck's law of blackbody radiation.
(cont.) An experimental system was built to test near-field radiative heat transfer between two parallel glass plates, and the experimental results show good agreement with the theoretical predictions. For solar photovoltaics, the emphasis in this work is on improving optical absorption in silicon-based cells. Two nanostructures, silicon nanowire arrays and silicon embedded with small silver particles, have been analyzed as potential candidates for solar energy harvesting. The study on silicon nanowire structures reveals that nanowires have desirable antireflection characteristics. Several parameters, such as the length and diameter of the nanowires as well as the spacing between the wires, have been studied to provide the basis for the optimization of nanowire based solar cells. The study shows that nanowire structures have low reflectance over a broad spectrum and can absorb shortwavelength photons efficiently. However, the analysis also indicates that silicon nanowire is not efficient in absorbing long-wavelength photons. Longer wires in comparison to the thickness of dense films are generally required to compensate low absorption of the near-bandgap photons. The analysis of surface-plasmon assisted photon absorption is presented to address the problem of inadequate absorption of near-bandgap photons in silicon. Instead of increasing the optical path of photons for more absorption, surface plasmons are explored to enhance the local electromagnetic field and thus the optical absorption. An extended Mie scattering formulation is used to calculate the optical absorption around spherical silver particles embedded in silicon. It is found that local field enhancement by surface plasmon can lead to 50 times more absorption near the bandgap of silicon. An analytical model is developed to study the concentration effects of the surface plasmon field. It is shown that the net absorption gain reaches maximum when the spherical shell surrounding the particle has an outer diameter of 1.26 times of the particle diameter. The absorption loss in the metallic sphere, however, is a main obstacle to overcome.
(cont.) Finally, a different approach of solar energy utilization is discussed in this work. Selective absorber surfaces are studied for solar thermal energy harvesting. The surfaces consist of subwavelength periodic metallic structures. Finite-Difference-Time-Domain (FDTD) analysis is conducted on the metallic structures. The effects of lattice spacing and structure thickness are presented. The numerical simulation indicates that the metallic structures have good spectral selectivity: high absorptance in visible range and low emittance in infrared. Fabrication of the selective absorber surface is attempted. Preliminary experimental results are given in this work. As a proof of concept, nickel is plated in porous anodic aluminum. The resultant structure shows good spectral selectivity which is not found in bulk nickel or aluminum.
by Lu Hu.
Ph.D.

Photon energy upconversion (UC), a process that can convert two or more photons with low energy to a single photon of higher energy, has the potential for overcoming the thermodynamic efficiency limits of sunlight-powered devices and processes. An attractive route to lowering the incident power density for UC lies in harnessing energy transfer through triplet-triplet annihilation (TTA). To maximize energy migration in multicomponent TTA-assisted UC systems, triplet exciton diffusivity of the chromophores within an inert medium is of paramount importance, especially in a solid-state matrix for practical device integration. In this thesis, low-threshold sensitized UC systems were fabricated and demonstrated by a photo-induced interfacial polymerization within a coaxial-flow microfluidic channel and in combination with nanostructured optical semiconductors. Dual-phase structured uniform UC capsules allow for the highly efficient bimolecular interactions required for TTA-based upconversion, as well as mechanical strength for integrity and stability. Through controlled interfacial photopolymerization, diffusive energy transfer-driven photoluminescence in a bi-molecular UC system was explored with concomitant tuning of the capsule properties. We believe that this core-shell structure has significance not only for enabling promising applications in photovoltaic devices and photochromic displays, but also for providing a useful platform for photocatalytic and photosensor units. Furthermore, for improving photon upconverted emission, a photonic crystal was integrated as an optical structure consisting of monodisperse inorganic colloidal nanoparticles and polymer resin. The constructively enhanced reflected light allows for the reuse of solar photons over a broad spectrum, resulting in an increase in the power conversion efficiency of a dye-sensitized solar cell as much as 15-20 %.

In this dissertation, we study the optical property of 2D graded photonic super-crystals (GPSCs) for photon management. We focused primarily on manipulation and control of light by using the newly discovered GPSCs which present great opportunity for electromagnetic wave control in photonic devices. The GPSC has been used to explore the superior capability of improving the light extraction efficiency of OLEDs. The enhancement of extraction efficiency has been explained in term of destructive interference of surface plasmon resonance and out-coupling of surface plasmon through phase matching provided by GPSC and verified by e-field intensity distributions. A large light extraction efficiency up to 75% into glass substrate has been predicted through simulation. We also study the light trapping enhancement in GPSCs. Broadband, wide incident angle, and polarization independent light trapping enhancement is achieved in silicon solar cells patterned with the GPSCs. In addition, novel 2D GPSCs were fabricated using holographic lithography through the interference lithography by two sets of multiple beams arranged in a cone geometry using a spatial light modulator (SLM). Finally, we also report a fabrication of GPSCs with a super-cell size of 12a×12a by using e-beam lithography. Diffraction pattern from GPSCs reveals unique diffraction properties. In an application aspect, light emitting diode arrays can be replaced by a single light emitting diode shinning onto the diffraction pattern for a uniform fluorescence.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 134-148).
Photons from the sun and terrestrial sources have great potential to satisfy the energy demand of humans. This thesis studies two types of energy conversion technologies, photovoltaic solar cells based on crystalline silicon thin films and thermal-radiative cells using terrestrial heat sources, focusing on managing photons but also concurrently considering electron transport and entropy generation. Photovoltaic technology has been widely adopted to convert solar energy into electricity. Crystalline silicon material occupies ~90% of the photovoltaic market. However, the silicon material in a photovoltaic module with ~180-pm-thick silicon material contributes more than 30% of the overall cost, giving rise to an obstacle to compete with fossil fuel energy. One promising solution to break this barrier is the technology of thin-film crystalline silicon solar cells if the weak absorption of silicon can be overcome. To maintain its high energy conversion efficiency, nanostructure is designed considering both light trapping and electron collection. This design guided the fabrication of 10-pm-thick crystalline silicon photovoltaic cells with efficiencies as high as 15.7%. To reach efficiency >20% in industry, multiple strategies have been investigated to further improve the performance including the least-common-multiple rule for the double gratings structure, external optical cavity, high quality silicon in bulk material and interfaces, and optimal contact spacing and doping. For the energy conversion of terrestrial heat source, a direct bandgap solar cell can work in the reverse bias mode to convert energy into electricity companied by emission of photons as entropy carriers. Photon spectral entropy and fluxes are used to develop strategies for improving the heat to electricity conversion efficiency. Near-field radiative transfer, especially using phonon polariton material to couple out emitted photons from electron-hole recombination, is proposed to enhance energy conversion efficiency as well as the power density. We predict that the InSb thermoradiative cell can achieve the efficiency and power density up to 20.4 % and 327 Wm-2, respectively, between a hot source at 500K and a cold sink at 300K, if the sub-bandgap and non-radiative losses could be avoided.
by Wei-Chun Hsu.
Ph. D.

Achieving optimal band-gap combinations of multi-junction solar cells at production level is the most difficult challenge in concentrator photovoltaics. To improve the state-of-the-art InGaP/InGaAs/Ge triple-junction cells, it requires that the band gaps of the top and middle junction to be lower or an additional 1 eV junction. This involves lattice-mismatch growth or introducing dilute nitrides materials, which makes it difficult to scale up to production at low cost. Strain-balanced multiple quantum wells (MQWs) in the middle junction has been very well-studied as a means to adjust the absorption edges of the middle junction in multi-junction solar cells. To fully optimise the efficiency of solar cells with MQW GaAs subcell, an InGaP top cell with MQWs also has to be introduced to achieve current-matching. The aim of this thesis is to address the issues of production multi-junction cell with MQWs. We studied the material properties of MQW InGaP subcells and demonstrated its strong photon coupling effects in multi-junction devices. Several characterisation techniques were developed to acquire deeper understanding of the material qualities and sheet resistance of MQW solar cells.

Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is a photophysical process that facilitates the conversion of two low-energy photons into a single high-energy photon. A low-energy photon is absorbed by a sensitizer molecule that produces a triplet excited state which is transferred to an emitter molecule. When two emitter triplet states encounter each other, TTA can take place to produce a singlet excited state which decays by emission of a high-energy (upconverted) photon. While traditional single-threshold dye-sensitized solar cells (DSSCs) have a maximum efficiency limit of ca. 30%, it has been predicted theoretically that implementation of UC-STTA in DSSCs could increase that efficiency to more than 40%. A possible way to implement UC-STTA into DSSCs, would be to replace the standard sensi- tized nanostructured TiO2 photoanodes by upconverting ones loaded with emitter molecules. Following TTA, the excited emitter molecule would be quenched by injection of a high-energy electron into the conduction band of the TiO2. To explore the practical aspects of this strategy for a highly efficient DSSC, in this thesis UC-STTA is studied in model systems based on nanostructured ZrO2 films. These ZrO2 films are a good proxy for the TiO2 films used in DSSCs, and allow for relatively easy optimization and study of UC-STTA by allowing measurements of the upconverted photons without the complications of electron injection into the film. Herein it is experimentally proven that UC-STTA is viable on nanostructured metal oxide films under non-coherent irradiation with intensities comparable to sunlight. Two different system architectures are studied, differing in the position of the molecular components involved in the UC-STTA mechanism. Both architectures have the emitter molecules adsorbed onto the ZrO2 surface, but the sensitizers are positioned either in solution around the nanostructure, or co-adsorbed with the emitters onto the ZrO2 surface. A set of challenges in the study and optimization of the UC-STTA process is identified for each type of system. Proposals are also given for how to further improve the understanding and UC-STTA optimization of these systems toward application in DSSCs to overcome the present solar energy conversion efficiency limit.

2008/2009
Remarkable interest has recently arisen about the search for Weakly Inter- acting Sub-eV Particles (WISPs), such as axions, Axion Like Particles (ALPs), Minicharged and chameleon particles, all of which are not included in the Stan- dard Model. Precision experiments searching for WISPs probe energy scales as high as 10^6 TeV and are complementary to accelerator experiments, where the energy scale is a few TeV. The axion, in particular, is the oldest studied and has the strongest theoretical motivation, having its origin in Quantum Chromodynamics. It was introduced for the first time in 1973 by Peccei and Quinn to solve the strong CP problem, while later on the cosmological implications of its postulated existence also became clear: it is a good candidate for the cold dark matter, and it is necessary to fully explain the evolution of galaxies. Among the different interactions of axions, the most promising for its detection, from an experimental point of view, is the coupling to two photons (Primakoff effect). Using this coupling, several bounds on the axion mass and energy scale have been set by astrophysical observations, by laboratory experiments and by the direct observation of celestial bodies, such as the Sun. Most of these considerations, as was recently recognized, not only constrain the mass and coupling of the axion, but are more generally applicable to all ALPs. The current best limits on the coupling, over a wide range of ALP masses, come from the the CAST (Cern Axion Solar Telescope) experiment at Cern, which looks for ALPs produced in the solar core. The experiment is based on the Primakoff effect in a high magnetic field, where solar ALPs can be reconverted in photons. The CAST magnet, a 10 T, 10 m long LHC superconducting dipole, is placed on a mobile platform in order to follow the Sun twice a day, during sunrise and sunset, and has two straight bores instrumented with X-ray detectors at each end. The re- generated photon flux is, in fact, expected to be peaked at a few keV. On the other hand, there are suggestions that the problem of the anomalous temperature profile of the solar corona could be solved by a mechanism which could enhance the low energy tail of the regenerated photon spectrum. A low energy photon counter has, for this reason, been designed and built to cover one of the CAST ports, at least temporarily. Low energy, low background photon counters such as the one just mentioned, are also crucial for most experiments searching for WISPs. The low energy photon counting system initially developed to be coupled to CAST will be applicable, with proper upgrades, to other WISPs search experiments. It consists of a Galilean telescope to match the CAST magnet bore cross section to an optical fiber leading photons to the sensors, passing first through an optical switch. This last device allows one to share input photons between two different detectors, and to acquire light and background data simultaneously. The sensors at the end of this chain are a photomultiplier tube and an avalanche photodiode operated in Geiger mode. Each detector was preliminary characterized on a test bench, then it was coupled to the optical system. The final integrated setup was subsequently mounted on one of the CAST magnet bores. A set of measurements, including live sun tracking, was carried out at Cern during 2007-2008. The background ob- tained there was the same measured in the test bench measurements, around 0.4 Hz, but it is clear that to progress from these preliminary measurements a lower background sensor is needed. Different types of detectors were considered and the final choice fell on a Geiger mode avalanche photodiode (G-APD) cooled at liquid nitrogen temperature. The aim is to drastically reduce the dark count rate, al- though an increase in the afterpulsing phenomenon is expected. Since the detector is designed to be operated in a scenario where a very low rate of signal photons is predicted, the afterpulsing effect can be accepted and corrected by an increase in the detector dead time. First results show that a reduction in background of a factor better than 10^4 is obtained, with no loss in quantum e ciency. In addition, an optical system based on a semitransparent mirror (transparent to X-rays and re ective for 1-2 eV photons) has been built. This setup, covering the low energy spectrum of solar ALPs, will be installed permanently on the CAST beamline. Current work is centered on further tests on the liquid nitrogen cooled G-APD concept involving different types of sensors and different layouts of the front-end read-out electronics, with a particular attention to the quenching cir- cuit, whether active or passive. Once these detector studies are completed, the final low background sensor will be installed on the CAST experiment. It is important to note that the use of a single photon counter for low energy photons having a good enough background (<1 Hz at least) is not limited to the CAST case, but is of great importance for most WISPs experimental searches, with special regard for photon regeneration experi- ments, and, in general, for the field of precision experiments in particle physics.
Negli ultimi tempi è riemerso un notevole interesse nel campo della ricerca di particelle leggere debolmenti interagenti (Weakly Interacting Sub-eV Particles - WISPs), come ad esempio assioni, particelle con comportamenti simili agli assioni (Axion Like Particles - ALPs), particelle con carica frazionaria e particelle camaleonte; tutti tipi di particelle non inclusi nel Modello Standard. Vista la loro natura debolmente interagente, la scala di energia coinvolta è dell'ordine dei 10^6 TeV, queste particelle non sono visibili nelle collisioni realizzabili negli attuali acceleratori e possono invece essere studiate in esperimenti di precisione, che, sotto questo punto di vista, diventano complementari agli esperimenti su acceleratori. L'assione in particolare è la prima particella, da un punto di vista cronologico, ad essere stata ipotizzata, ed inoltre la sua esistenza è supportata da forti basi teoriche: la sua origine va infatti ricercata all'interno della Cromodinamica Quantistica (QCD). L'assione fu introdotto per la prima volta nel 1973 da Peccei e Quinn come soluzione del problema di violazione di CP nelle interazioni forti, mentre le sue implicazioni cosmologiche risultarono chiare solo in seguito. L'assione infatti può essere considerato un buon candidato per la materia oscura fredda e la sua introduzione è necessaria per spiegare l'evoluzione delle galassie. Tra le diverse interazione degli assioni con la materia e la radiazione, la più interessante da un punto di vista sperimentale è l'accoppiamento con due fotoni (effetto Primakoff). Usando questo tipo di accoppiamento numerosi limiti, sia sulla massa dell'assione che sulle scale di energia coinvolte, possono essere ottenuti da osservazioni astrofisiche e da esperimenti di laboratorio così come dalla diretta osservazione di oggetti celesti tipo il Sole. Queste considerazioni possono essere applicate non solo all'assione ma più in generale a tutte le ALPs. Attualmente i limiti migliori sulla costante di accoppiamento, su un largo spettro di masse di ALPs, si sono ottenuti dall'esperimento CAST (Cern Axion Solar Tele- scope) al Cern, che guarda agli ALPs prodotti nel Sole. L'esperimento è basato sull'effetto Primakoff in un campo magnetico elevato, dove gli ALPs solari sono riconvertiti in fotoni. Il magnete dell'esperimento CAST è costituito da un prototipo per un dipolo superconduttore di LHC, lungo 10 m e con un campo magnetico totale di 10 T. Il magnete è posto su di un affusto mobile per poter seguire il sole durante le fasi di alba e tramonto. Alle due estremità del magnete sono disposti quattro rivelatori sensibili nel campo degli X molli. Il picco del usso di fotoni rigenerato è infatti atteso a pochi keV. Tuttavia, ci sono suggerimenti che il prob- lema ancora aperto del profilo di temperatura della corona solare può essere risolto tramite un meccanismo che contemporaneamente incrementerebbe le code a bassa energia dell'atteso usso di fotoni rigenerati. A questo scopo un contatore di fotoni sensibile nell'intervallo del visibile è stato progettato ed assemblato per coprire una delle quattro porte del magnete di CAST, almeno temporaneamente. I contatori di fotoni studiati hanno un largo campo di applicazione e possono essere usati in altri tipi di esperimenti per la ricerca di WISPs. Il sistema inizialmente sviluppato per CAST consiste in un telescopio Galileiano per accoppiare una fibra ottica all'apertura del magnete di CAST, la fibra ottica è quindi collegata ad un interruttore ottico che permette di utilizzare due rivelatori contemporaneamente. La fibra in ingresso è infatti collegata alternativamente a due fibre in uscita, in questo modo ciascun rivelatore acquisisce per metà del tempo segnale e per metà del tempo fondo, lasciando inalterato il tempo totale di integrazione. I sensori utilizzati fino ad ora al termine della catena ottica sono un tubo fotomoltiplicatore e un avalanche photodiode operato in modalità Geiger. Ciascun rivelatore è stato preliminarmente caratterizzato su un banco di prova e quindi collegato al sistema ottico. Il sistema finale è stato quindi installato su CAST. Una serie di misure, che includono reali prese dati, sono state condotte al Cern durante il 2007-2008. La misura del fondo ottenuta a CAST è stata la stessa misurata durante i test di prova a Trieste, circa 0.4 Hz, ma risulta chiaro che il vero sviluppo futuro è basato su un sensore a fondo molto più basso. A questo scopo sono stati considerati diversi tipi di sensore e la scelta finale è ricaduta su di un avalanche photodiode operato in modalità Geiger e raffreddato all'azoto liquido. Lo scopo è quello di ridurre drasticamente i conteggi di fondo, sebbene a queste temperature sia atteso un incremento del rateo di afterpulses. Tuttavia il rivelatore è pensato per essere utilizzato in un applicazione a basso rateo e quindi il fenomeno degli afterpulses può essere ridotto agendo direttamente sul tempo morto del rivelatore, cioè aumentandolo. I primi test condotti sul rivelatore mostrano un decremento del fondo pari ad un fattore meglio di 10^4, senza rilevabili variazioni in efficienza. In aggiunta a questo sistema, per ottenere un'installazione permanente sul fascio di CAST, è stato realizzato uno specchio semitrasparente, che lascia pressocchè inalterato il fascio di raggi X e invece de ette il fascio di fotoni con energia nel visibile. Il lavoro attuale è incentrato sullo sviluppo del rivelatore a basso fondo raffreddato all'azoto liquido, includendo anche lo studio di diversi tipi di sensore e diversi tipi di elettronica di lettura, con particolare attenzione all'elettronica di quenching del circuito con le varianti attiva e passiva. Una volta terminati gli studi sui diversi tipi di rivelatori, l'apparato finale sarà installato su CAST. E' comunque importante notare che l'uso di un rivelatore a singolo fotone sensibile tra 1-2 eV con un fondo sufficientemente basso (<1 Hz almeno) non è limitato all'uso su CAST ma in tutti gli altri esperimenti per la ricerca di WISPs, con particolare riguardo agli esperimenti di rigenerazione risonante, e in generale, nel campo di applicazione degli esperimenti di precisione alla fisica delle particelle.
1982

Ce travail a porté sur la synthèse et caractérisations structurales, optiques et électriques des films d’oxyde d'étain (SnOx) dopés avec des éléments de terres rares (RE: Néodyme, Praséodyme ou Ytterbium). L’objectif est de démontrer la conversion de photons UV voire Visible en photons rouges via ces films RE :SnOx, tout en conservant leurs propriétés d’oxydes transparents conducteurs. Les films ont été produits par des méthodes chimiques (sol-gel, précipitation) ou physiques (pulvérisation cathodique). Grâce à des analyses fines, nous avons pu corréler les propriétés structurales et de composition des couches RE :SnOx avec leurs propriétés d’émission de photons. Nous avons pu établir les conditions optimales de conversion photonique dans des systèmes à une seule ou double terre rare. Les mécanismes régissant le transfert dans ces films ont été avancés. Enfin, nous avons appliqué ces couches minces RE :SnOx optimisés sur des cellules solaires en silicium et en CIGS et nous avons montré une amélioration des paramètres photovoltaïques du dispositif ainsi qu’un net gain dans la réponse spectrale de la cellule dans l’UV
Spectral conversion using lanthanide doped materials with excellent performances is a great challenging topic and of particular interest for photovoltaic. This work aims at functionalizing transparent conductive oxide materials with rare earth elements for photons conversion purpose without affecting transparency and transport properties of the TCO. The spectral conversion targeted in this thesis is of type “down”, in other words, we aim at converting high energy UV photons into low energy visible or NIR photons useful to solar cells. For this purpose we investigated the doping process of SnO2 as a host material with different rare earths such as Nd, Tb, Pr, and Yb. To understand the insertion process and the optical activation of the rare earth, RE-doped SnO2 nanoparticles (powders) have been synthesised by two chemical methods: co-precipitation and sol-gel. The results have shown an efficient insertion of the RE into the SnO2 structure with excellent emission properties. In view of application of RE-doped SnOx thin films to solar cells, studies concerning NIR emitting RE have been conducted (Nd, Yb, and co-doping with Yb and Nd) using sputtering. Several deposition parameters and post deposition treatments have been done in order to find the best chemical environment favourable to the RE emission. We have precisely identified the region of the UV light converted into NIR photons and proposed several energy transfer mechanisms occurring between the host SnOx and the REs. In case of co-doping, a second spectral conversion process has been identified; visible photons can be efficiently converted into NIR photons through energy transfer from Nd3+ to Yb3+ ions. Finally, application of these conversion layers to solar cells such as CIGS and Si based have shown an improvement of the cells characteristics, among others the Field factor, the cell efficiency and the increase of the spectral response of the cell in the UV region, thanks to the conversion of the UV photons into NIR photons. The good electrical properties of the RE-doped SnOx layers have been highlighted as well. We believe that these conversion layers will provide a step ahead towards better solar cells performances

This dissertation describes the investigation of the synthesis and characterization of new squaraine-based photonic and electronic materials. In the first part of this thesis, squaraine dyes with large conjugation systems, including extended squaraines consisting of bis(donor)substituted vinylene-heterocycles and bis(indolinylenemethyl)squaraine-based oligomers linking through different π-bridges were designed, synthesized and characterized to exhibit strong two-photon absorption (2PA) for femotosecond and nanosecond optical-power limiting applications in the near-infrared (NIR). One of the dendronized squaraine forms smooth and high optical quality films with large NIR transparency window. In the second part, a series of squaraine- and phthalocyanine-based metal complexes were studied. Those dyes did not show large triplet quantum yield but significantly improved photovoltaic performance compared to the metal-free compounds. In the last part, an effective approach on optimizing bis(indolinylenemethyl)-based squaraine sensitizers with various surface anchor groups and π-linkers, achieved high power conversion efficiencies (PCEs) of 6.7% in liquid dye-sensitized solar cells (DSSCs) and 2.7% in solid-state DSSCs, which stand out all the previous reported squaraine-based sensitizers.

Ce document présente notre travail sur la modélisation en formalisme des fonctions de Green (abrégé formalisme de Green) du transport quantique et de l’intéraction éléctron-photon dans une cellule photovoltaïque composée d’une boîte quantique connectée à deux nanofils semi-infinis, La simulation numérique a été réalisée sur le cluster de calculs MERLIN (IM2NP). Nous présentons le formalisme de Green en général puis appliqué à cellule. Le fonctionnement général de la cellule est déduit de son diagramme de bande qui comporte des contacts sélectifs. Ensuite, nous présentons les résultats obtenus dans l’approximation de bande plate, qui simplifie le contact aux nanofils. Ceux-ci mettent en lumière des effets intriqués du couplage tunnel (couplage entre la boîte et les nanofils) et du couplage optique (couplage avec la lumière). Nous présentons ensuite un calcul analytique effectué dans le régime de fort couplage tunnel et qui explique le fonctionnement contre-intuitif du couplage tunnel dans ce régime. Nous observons également une transition dans le processus de production du courant entre le régime de fort couplage tunnel et le régime de fort couplage optique. Ensuite, nous sortons de l’approximation de bande plate et découvrons que les effets contre-intuitifs sont toujours valides, même si le modèle analytique lui ne l’est plus. Nous présentons le nouvel effet induit par la nouvelle forme du couplage aux réservoirs hors de l’approximation de bande plate: la courbe courant-tension présente une conductance de shunt négative. Cela n’a jamais été observé dans une cellule photovoltaïque auparavant. Enfin, nous présentons une réalisation possible de notre cellule
This document present our work on the modeling of quantum transport coupled to electron-photon interaction in a solar cell composed of one quantum dot connected to two semi-infinite quantum wires. The proposed cell based on a dot in a wire, is a concept imagined in order to investigate quantum effects inside 1D structures in contact with 0D ones. The numerical simulation powered on the Merlin cluster (IM2NP) relies on Green’s function formalism. The philosophy of Green’s function formalism is introduced and then applied to the photovoltaic cell. An overview of the functioning of the cell is given. Results on the cell are presented in the wide band limit (approximation that simplifies the contact to wires). We observe an interlinked impact of the tunneling coupling (dot-wires coupling) and the optical coupling (to light) on the current. In the strong tunneling regime, an increase of the tunneling coupling decreases the current and similarly in the strong optical coupling regime, an increase of the optical coupling decreases the current. We investigate the counter-intuitive impact of the tunneling coupling in the strong tunneling regime through analytical calculations, considering only the first loop of the numerical code instead of the whole self-consistent process. We observe a transition in the current creation process while switching from the strong tunneling regime to the strong optical coupling regime. Results on the cell beyond the wide band limit approximation are presented in which the system exhibits another atypical response to illumination: I-V curve exhibits a negative shunt conductance! Finally, a realization proposal for the concept cell is described

Malaysia is one of the countries that have started to elevate the usage of renewable - specifically solar photovoltaic (PV) - in delivering its energy needs. This thesis is divided into two main sections. The first section evaluates the solar PV installations in the residential sector in Malaysia for the past 20 years; in terms of policies, research and development activities and implementations. Recently, the Feed-in Tariff (FiT) scheme was introduced in Malaysia and based on the financial analysis, any installation of solar PV could generate a lucrative monthly income to the household owner under the FiT scheme. However, a preliminary survey indicated that Malaysians are not interested in investing in a solar PV system, mainly due to the high cost of implementation. The next section focuses on the design of solar concentrators - particularly the family of Dielectric Totally Internally Reflecting Concentrators (DTIRCs) - with the aim of achieving a low cost solar PV system. Part of the PhD project is the optimisation of the concentrator design in the Solar Photonic Optoelectronic Transformer (SPOT), the main component of SolarBrane, a static building integrated PV (BlPV) system developed by SolarEmpower Ltd. An optimised design has been proposed using a DTIRC based on the Maximum Concentration Method (MCM). It has been demonstrated via simulations that the optimised design could potentially increase the output of the SolarBrane, at the cost of having a slightly larger structure. A novel type of DTIRC family. known as the Asymmetrical DTIRC (ADTIRC). has been developed to provide additional gain at the "extrusion" plane of the concentrator, and further reducing the size of the PV cell needed. It is concluded tbat this new design generates a much higher gain compared to the concentrator in the SolarBranel. The results from the indoor experiments indicate that the ADTIRC-PV structure could increase the electrical output by 4.2x when compared with the non-concentrating solar PV cell.

Plusieurs processus photo-induits d'énergie et de transfert d'énergie ont été étudiés en solution et dans le film par spectroscopie d'absorption transitoire et de fluorescence pour deux types de cellules solaires. Combinés avec d'autres expériences et par une analyse globale, ces phénomènes ultrarapides avec leur durée de vie ont été observés et les scénarios photo-induits ont été déterminés. La compréhension approfondie des matériaux moléculaires pourrait aider les chimistes à concevoir des cellules solaires efficaces. La première étude sur l'influence des conceptions chimiques sur la formation et la séparation des charges implique différentes fractions donneuses et différents solvants et les résultats ont été expliqués par la théorie de Marcus-Jortner combinée avec le calcul quantique. La deuxième étude porte sur les complexes Fe (II) comme photosensibilisateurs pour les cellules solaires sensibilisées aux colorants. On a étudié une série de complexes de Fe (II) homo et hétérotéptiques avec des ligands de carbène et de terpyridine en solution et dans le film. La durée de vie de l'état de transfert de la charge métal-ligand du triplet d'enregistrement du complexe Fe (II) est obtenue en solution. La compréhension du film est en cours
Various photo-induced energy and energy transfer processes were investigated in solution and in the film by transient absorption and fluorescence spectroscopies for two types of solar cells. Combined with other experiments and through a global analysis, those ultrafast phenomena with their lifetimes were observed and the photo-induced scenarios were determined. The insight understanding of molecular materials could help chemists to design efficient solar cells.The first study about the influence of chemical designs on charge formation and separation involves different donor moieties and different solvents and the results were explained by Marcus-Jortner theory combined with quantum calculationThe second investigation is about Fe(II) complexes as photosensitizers for dye-sensitized solar cells. A series of homo- and heteroleptic Fe(II) complexes with carbene and terpyridine ligands have been studied in solution and in the film. The record triplet metal-to-ligand charge transfer state lifetime of Fe(II) complex is achieved in solution. The further understanding in the film is in progress

Les travaux présentés dans ce mémoire de thèse portent sur la synthèse et l'étude des propriétés spectroscopiques et électrochimiques de systèmes donneur-accepteur conçus pour des applications variées telles que l'électrofluorochromisme, l'absorption à deux photons et le photovoltaïque. La s-tétrazine a été choisie comme accepteur pour sa forte affinité électronique, ses propriétés émissives remarquables et sa capacité à s'organiser via des interactions intermoléculaires de type --stacking. La triphénylamine a été sélectionnée comme donneur pour son faible potentiel d'ionisation, ses propriétés spectroscopiques (fortes absorption et émission) et la modulation facile de ses propriétés par changement de substituants. Sept dérivés de triphénylamine ont été synthétisés ainsi que dix-huit nouveaux composés multichromophoriques à base de tétrazine et de triphénylamine présentant cinq liens différents et des substituants variés. Ils ont été caractérisés par électrochimie et spectroscopie (stationnaire et résolue en temps). L'étude de la modulation de leurs propriétés photophysiques par le changement de l'état rédox a ensuite été réalisée. Dix composés présentant un lien permettant la conjugaison entre la triphénylamine et la tétrazine ont été synthétisés et caractérisés par électrochimie et spectroscopie. Compte-tenu de leurs propriétés, six d'entre eux ont été testés en absorption à deux photons et deux ont étés retenus pour être utilisés comme donneurs dans une cellule photovoltaïque organique. Par ailleurs, deux réactions ont été étudiées en détail pour expliquer la formation des produits obtenus, inattendus à un premier abord.

Uno de los retos actuales de la ciencia y la tecnología es el desarrollo y utilización de fuentes de energías limpias, sostenibles y seguras, con el fin de sustituir el uso de combustibles fósiles. La energía solar, única alternativa viable, puede convertirse y almacenarse en forma de enlaces moleculares, mimetizando el proceso de fotosíntesis de las plantas, para la obtención de combustibles u otros productos de valor añadido. Para ello se requieren materiales semiconductores que puedan absorber y transformar la energía solar en energía química de manera eficiente. En la presente tesis doctoral se abordó el estudio de materiales semiconductores empleados en la obtención fotoelectrocatalítica de combustibles solares. Dicha investigación se realizó desde diferentes enfoques, que inlcuyen: la modificación de fotoelectrodos con recubrimientos catalíticos obtenidos a partir de marcos metal-orgánicos; la implementación de un nuevo método para la comprensión de los mecanismos de operación de fotoelectrodos; la integración de dispositivos electrocatalíticos y fotovoltaicos; y la evaluación y establecimiento de nuevos sistemas con potencial aplicación en procesos foto-electrocatalíticos.
The development and use of clean, sustainable and safe energy sources, in order to substitute the use of fossil fuels, is a current challenge of science and technology. Solar energy, the only viable alternative, can be converted and stored in the form of molecular bonds, mimicking the photosynthesis process in green plants, to obtain fuels or other added-value products. This process requires semiconductor materials that can efficiently harvest and transform solar into chemical energy. In the present doctoral thesis, the study of semiconductor materials for photo-electrocatalytic applications was addressed from different approaches. That includes: the modification of photoelectrodes with catalytic coatings, obtained from a metal-organic framework; the implementation of a new method for the understanding of the photoelectrodes operating mechanisms; the integration of electrocatalytic and photovoltaic devices from Earth-abundant materials; and, finally, the investigation of new systems with potential application in photo-electrocatalytic processes. (Signatura
Programa de Doctorat en Ciències

Les pérovskites hybrides halogénées (ABX3) sont utilisées depuis cinq ans comme couches absorbantes pour de nouvelles cellules solaires à bas coût combinant les avantages des matériaux organiques (molécule A) et inorganiques (métal B et halogène X). Très récemment, des cellules solaires à boîtes quantiques à bases de pérovskites purement inorganiques ont également montré des efficacités prometteuses, ce qui en fait une alternative potentiellement stable et efficace à leurs cousins hybrides.Le but de cette thèse de doctorat est d'étudier et de mieux comprendre les instabilités structurelles et thermodynamiques de ces pérovskites halogénées, avec un focus sur la pérovskite purement inorganique CsPbI3.Dans un premier temps les propriétés vibrationnelles et électroniques des différentes phases de CsPbI3 sont étudiées grâce à différentes techniques ab-initio, dont la plupart sont basées sur la théorie de la fonctionnelle de la densité (DFT) et son approche en réponse linéaire (DFPT). Alors que la phase γ noire, cruciale pour les applications photovoltaïques, se comporte de manière harmonique autour de l'équilibre, pour les trois autres phases nos calculs de phonons froids révèlent une instabilité de double puits au centre de la zone de Brillouin. Nos calculs montrent également que le terme d'entropie d'ordre-désordre lié à ce double puits est crucial pour empêcher la formation de la phase pérovskitoïde jaune. Nous analysons ensuite en détail les changements structurels et l’effet Rashba dynamique le long de trajectoires de dynamique moléculaire à la lumière de ces résultats.La seconde partie de la thèse porte sur la stabilité thermodynamique de la pérovskite hybride MAPbI3. Notre étude expérimentale par ellipsométrie apporte une meilleure compréhension de la décomposition chimique de MAPbI3 en ses deux précurseurs, l’iodure de méthylamonium et l'iodure de plomb, que nous avons prédite grâce à des calculs de diagrammes de stabilité DFT et que nous confirmons par diffraction des rayons X. Enfin, nous démontrons que la pérovskite hybride MAPbI3 se comporte davantage comme les composés inorganiques (grande constante diélectrique, faible énergie de liaison des excitons) que comme les matériaux organiques (faible constante diélectrique, forte énergie de liaison d'exciton)
Hybrid halide perovskites (ABX3) have emerged over the past five years as absorber layers for novel high-efficiency low-cost solar cells combining the advantages of organic (molecule A) and inorganic (metal B, halogen X) materials. Very recently, fully inorganic perovskite quantum dots also shown promising efficiencies, making them a potentially stable and efficient alternative to their hybrid cousins.The aim of this PhD thesis is to study and better understand both the structural and thermodynamic instabilities of these halide perovskites, with a specific focus on purely inorganic CsPbI3 structures.We first use various ab-initio techniques, the majority of which are based on Density Functional Theory (DFT) and its linear-response approach (DFPT), to investigate the vibrational and electronic properties of the different phases of CsPbI3. While the black γ-phase, crucial for photovoltaic applications, is shown to behave harmonically around equilibrium, for the other three phases frozen phonon calculations reveal a Brillouin zone center double-well instability. We also show that avoiding the order-disorder entropy term arising from these double-well instabilities is key in order to prevent the formation of the yellow perovskitoid phase, and evidence a Rashba effect when using the symmetry breaking structures obtained through frozen phonon calculations. We then analyze the structural changes and the dynamical Rashba splitting along molecular dynamics trajectories in the light of our findings.In a second phase, we investigate the thermodynamical stability of hybrid perovskite MAPbI3. Our experimental ellipsometry-based study brings better understanding of the chemical decomposition of MAPbI3 into its two precursors, methylammonium and lead iodides, which we predicted using DFT stability diagram calculations and which we confirm by X-Ray diffraction. Last, we prove that hybrid perovskite structure MAPbI3 behaves more like inorganic compounds (high dielectric constant, low exciton binding energy) than like organic materials (low dielectric constant, high exciton binding energy)

Negli ultimi decenni, l’uso dei materiali organici per la realizzazione di dispositive elettronici si è guadagnato l’attenzione di molti gruppi di ricercatori. Questo è dovuto principalmente alla possibilità di usare, con questi materiali, tecniche di fabbricazione a basso costo da fase liquida, adatte anche per lo sviluppo di dispositivi su supporto flessibile, e di poter modificare le proprietà dei materiali stessi secondo le esigenze di applicazione. Particolarmente in optoelettronica, l’uso di questi materiali per la realizzazione di sorgenti luminose (OLED, diodo organico emettitore di luce, o OTFL, laser organici a film sottile), foto-diodi e celle solari è stato già dimostrato. In questo contesto, la combinazione di differenti dispositivi organici potrebbe spianare la strada a nuove applicazioni nel campo della comunicazione dati, sensoristica, digitalizzazione di immagini ed energia solare rinnovabile. I dispositivi foto-voltaici a eterogiunzione-bulk di polimeri coniugati depositati da soluzione liquida rappresentano una promessa nel campo della conversione di energia solare e della comunicazione dati, grazie ad una efficienza di conversione fino al ~5%, e una risposta temporale ad una sorgente ottica a ~200KHz.
In the last few decades, the use of organic materials for the realization of electronic devices has gained the attention of many research groups. This is mainly due to the possibility to use low-cost techniques for fabrication as solution-processing, suitable also to flexible substrates, and to tailor the material properties for specific applications. In the field of optoelectronics, the use of such materials for the realization of light sources (OLED, Organic Light-Emitting Diode, or OTFL, Organic Thin-Film Lasers), photo-diodes and solar cells has already been demonstrated. In this context, the combination of different organic devices for integrated optical systems, can pave the way to new applications in the field of data communication, sensing application, imaging and solar energy. Conjugated polymer bulk-heterojunction photo-voltaic device made from blend solution could be a good promise for solar energy conversion and data communication purpose, with its solar conversion efficiencies up to ~5% and a time-resolved response of ~200KHz to an optical source.

Fundamental studies for the improvement of photo-microbial fuel cells (pMFCs) within this work comprised investigations into ceramic electrodes, toxicity of metal-organic frameworks (MOFs) and hot-pressing of air-cathode materials. A novel type of macroporous electrode was fabricated from the conductive ceramic Ti2AlC. Reticulated electrode shapes were achieved by employing the replica ceramic processing method on polyurethane foam templates. Cyclic voltammetry of these ceramics indicated that the application of potentials larger than 0.5 V with regard to a Ag/AgCl reference electrode results in the surface passivation of the electrode. Ti2AlC remained conductive and sensitive to redox processes even after electrochemical maximisation of the surface passivation, which was shown electrochemically and with four terminal sensing. Application of macroporous Ti2AlC ceramic electrodes in pMFCs with green algae and cyanobacteria resulted in higher power densities than achieved with conventional pMFC electrode materials, despite the larger surface area of the Ti2AlC ceramic. The effect of electrode surface roughness and hydrophobicity on pMFC power generation and on cell adhesion was examined using atomic force and confocal microscopy, contact angle measurements and long-term pMFC experiments. The high surface roughness and fractured structure of Ti2AlC ceramic was beneficial for cell adhesion and resulted in higher pMFC power densities than achieved with materials such as reticulated vitrified carbon foam, fluorine doped tin oxide coated glass or indium tin oxide coated plastic. Toxicity of the MOF MIL101 and its amine-modified version MIL-101(Cr)-NH2 on green algae and cyanobacteria was assessed on the basis of both growth in liquid culture and by exclusion zones of agar colonies around MOF pellets. MOF MIL101 was found harmless in concentrations up to 480 mg L-1 and MIL-101(Cr)-NH2 did not exhibit toxic effects at a concentration of 167 mg L-1. Air-cathodes were produced from a range of carbon materials and ion-exchange membranes. Hot-pressing of Zorflex Activated Carbon Cloth FM10 with the proton-selective Nafion® 115 membrane provided the best bonding quality and pMFC performance.

Utiliser l'hydrogène en tant que vecteur énergétique pour stocker l'énergie solaire et/ou remplacer le pétrole comme carburant est très attrayant, d'autant qu'il peut être produit de façon propre par photo-électrolyse de l'eau. Dans ce procédé, des paires électron-trou, générées par éclairement dans des semi-conducteurs immergés dans une solution aqueuse, réalisent les réactions d’oxydo-réduction de l’eau (production d'oxygène à la photo-anode et production d'hydrogène à la photo-cathode). Les oxydes de métaux de transition, en particulier l'hématite (α-Fe2O3) qui présente un gap quasi-idéal pour cette application, sont les matériaux de photo-anode les plus prometteurs. Des films minces d'hématite ont été déposés sur des monocristaux par épitaxie par jets moléculaires assistée par plasma d’oxygène. Ces échantillons modèles ainsi que l’utilisation de techniques de pointe, notamment utilisant le rayonnement synchrotron, rendent possible l’identification des paramètres pertinents influençant les propriétés de photo-électrolyse. Je me suis d'abord intéressé à l'impact de la structure cristallographique, de la stœchiométrie et de la morphologie de surface. Ensuite, les effets d'un dopage avec du titane ont été analysés, montrant l'existence d'un taux de dopage optimal et l'augmentation de la longueur de diffusion des porteurs de charges induisant un fort gain en photo-courant. J'ai également étudié la structure électronique et la dynamique des recombinaisons en surface d'hétérojonctions TiO2 - hématite dopée Ti, révélant une interface diffuse. Enfin, le champ électrique interne créé par un film mince ferroélectrique de BaTiO3/Nb:SrTiO3 a été considéré pour améliorer les propriétés des photo-anodes. Un premier pas vers la compréhension du lien entre polarisation ferroélectrique et photo-courant a été fait, mettant en évidence un champ électrique interne favorable pour séparer les charges
Using hydrogen as an energy carrier for solar energy storage and/or fuel alternative to oil is very appealing, especially as it can be cleanly produced by solar water splitting. In this process, electron-hole pairs, generated in illuminated semiconductors dipped in an aqueous solution, realize the water oxidoreduction reactions (oxygen production at the photoanode and hydrogen production at the photocathode). Transition metal oxides, in particular hematite (α-Fe2O3) which features a quasi ideal band-gap for this application, are the most promising photoanodes materials. Hematite thin films were deposited on single crystals by oxygen plasma assisted molecular beam epitaxy. These model samples along with the use of high-end techniques, in particular using synchrotron radiation, make possible the identification of the relevant parameters affecting the photoelectrochemical properties. I firstly focused on the impact of the crystallographic structure, the stoichiometry and the surface morphology. Then the effects of doping with titanium were investigated, demonstrating the existence of an optimal doping level and an increase of the charges diffusion length inducing a high photocurrent gain. In addition, I studied the electronic structure and the surface recombinations dynamics of TiO2 - Ti-doped hematite heterojunctions, revealing a diffuse interface. Lastly, the internal electric field created by a ferroelectric thin film of BaTiO3/Nb:SrTiO3 was considered in order to enhance the performances of photoanodes. A first step toward the comprehension of the link between ferroelectric polarization and photocurrent was achieved through the evidence of an internal electric field favourable for the separation of charges

- Einführung in das Projekt Solar Impulse (Entstehung, Herausforderung, Errungenschaften) - Beschreibung der eingesetzten Software Tools zur Entwicklung des Flugzeuges - Fallbeispiele für den Einsatz von Mathcad

Mesoporous semiconductors such as TiO2 nanoparticles, as well as transparent conducting oxides (TCOs) such as indium tin oxide films are typically employed for setting up the photo-electrode module in variety of photoelectrochemical cells including Dye-Sensitized Solar Cells (DSSCs). In order to exhibit a high performance efficiency, the photo-electrodes in such applications are required to be able to harvest the light and transport the generated electrons effectively. Accordingly mesoporous layers with high values of surface area and well-established pore structure along with highly transparent and conductive TCOs are deposited on suitable substrates through the physical or chemical vapor deposition methods. The processing facilities and materials required to fabricate such high-quality devices with high values of efficiency are complicated and expensive, whereas devices of lower quality do not fulfill the demands. This issue is of particular importance regarding the energy production and developing the solar cell technologies, as it is considered by the concept of “cost per watt”. Thus, a great deal of effort is being carried out globally to enhance the efficiency of affordably-produced solar cells such as low-cost DSSCs. Utilizing the wet chemical techniques such as sol-gel method which provide a considerably more affordable route to synthesize nanoparticles and deposit thin films without the need of applying high temperature or vacuum condition is a widely-used approach to decrease the processing expenses. However, to achieve an acceptable cost-per-watt ratio requires enhancing the obtained efficiency value as well, and therefore, modifying the processing procedures to improve the required features of the products are highly encouraged. This thesis focuses on two individual activities: synthesis of TiO2 nanoparticles, and also thin film deposition of a promising TCO called aluminum-doped zinc oxide (AZO); both obtained through the sol-gel route that is modified to contribute to nanostructures with suitable features for application in photoelectrochemical devices such as DSSC. In the first part, mesoporous anatase nanoparticles were synthesized through the surfactant-mediated sol–gel route. Through changing the refluxing time and water-to-surfactant molar ratio, as-prepared nanocrystals of high density and large and narrowly-distributed pore sizes were obtained, displaying surface area values up to 240 m2·g-1, much higher than the reported values for commercial TiO2-based catalysts. In the second part, sol–gel dip–coating of ZnO thin films doped with 2 at.% of aluminium ions was carried out. By altering the hydrolysis reaction and changing the thermal treatment procedure, thin films of highly c-axis preferred orientation were obtained with optical transmittance of around 80% and resistivity values down to 6 – 15 mΩ·cm, corresponding to sheet resistance of around Rsh ~ 500 Ω/sq. The obtained conductivity values, even though one order magnitude lower than those reported for the AZO thin film prepared via expensive techniques, are in the suitable range to improve the cost per watt ratio in applications such as inkjet printing of low-cost printed electronics and more affordable DSSC devices.

Mesoporous semiconductors such as TiO2 nanoparticles, as well as transparent conducting oxides (TCOs) such as indium tin oxide films are typically employed for setting up the photo-electrode module in variety of photoelectrochemical cells including Dye-Sensitized Solar Cells (DSSCs). In order to exhibit a high performance efficiency, the photo-electrodes in such applications are required to be able to harvest the light and transport the generated electrons effectively. Accordingly mesoporous layers with high values of surface area and well-established pore structure along with highly transparent and conductive TCOs are deposited on suitable substrates through the physical or chemical vapor deposition methods. The processing facilities and materials required to fabricate such high-quality devices with high values of efficiency are complicated and expensive, whereas devices of lower quality do not fulfill the demands. This issue is of particular importance regarding the energy production and developing the solar cell technologies, as it is considered by the concept of “cost per watt”. Thus, a great deal of effort is being carried out globally to enhance the efficiency of affordably-produced solar cells such as low-cost DSSCs. Utilizing the wet chemical techniques such as sol-gel method which provide a considerably more affordable route to synthesize nanoparticles and deposit thin films without the need of applying high temperature or vacuum condition is a widely-used approach to decrease the processing expenses. However, to achieve an acceptable cost-per-watt ratio requires enhancing the obtained efficiency value as well, and therefore, modifying the processing procedures to improve the required features of the products are highly encouraged. This thesis focuses on two individual activities: synthesis of TiO2 nanoparticles, and also thin film deposition of a promising TCO called aluminum-doped zinc oxide (AZO); both obtained through the sol-gel route that is modified to contribute to nanostructures with suitable features for application in photoelectrochemical devices such as DSSC. In the first part, mesoporous anatase nanoparticles were synthesized through the surfactant-mediated sol–gel route. Through changing the refluxing time and water-to-surfactant molar ratio, as-prepared nanocrystals of high density and large and narrowly-distributed pore sizes were obtained, displaying surface area values up to 240 m2·g-1, much higher than the reported values for commercial TiO2-based catalysts. In the second part, sol–gel dip–coating of ZnO thin films doped with 2 at.% of aluminium ions was carried out. By altering the hydrolysis reaction and changing the thermal treatment procedure, thin films of highly c-axis preferred orientation were obtained with optical transmittance of around 80% and resistivity values down to 6 – 15 mΩ·cm, corresponding to sheet resistance of around Rsh ~ 500 Ω/sq. The obtained conductivity values, even though one order magnitude lower than those reported for the AZO thin film prepared via expensive techniques, are in the suitable range to improve the cost per watt ratio in applications such as inkjet printing of low-cost printed electronics and more affordable DSSC devices.

En esta tesis doctoral se han estudiado procesos fundamentales de reacciones de transferencia de carga en películas mesoporosas de dióxido de titanio sensitivizado con puntos cuánticos, en películas finas de mezclas polímero:puntos cuánticos y en dispositivos completos de mezclas del polímero PCPDTBT y puntos cuánticos de CdSe en operaciones de trabajo reales. Los resultados obtenidos permiten abordar la fabricación de dispositivos fotovoltaicos con un conocimiento de los procesos de recombinación que limitan la eficiencia de las celdas más amplio. Y por tanto, se demuestra la posibilidad de fabricar celdas solares basadas en puntos cuánticos con eficiencias iguales o superiores a los dispositivos fotovoltaicos orgánicos.
The fundamental processes of the charge transfer reactions between titania dioxide mesoporous films and quantum dots, in blend films of the semiconductor polymer P3HT and CdSe quantum dots and in complete devices fabricated with the polymer PDPCTBT and CdSe quantum dots in working conditions have been studied in this doctoral thesis. The obtained results allow the fabrication of photovoltaic devices with a deeper and wider knowledge of the recombination processes that limit the device efficiency. Therefore, it is demonstrated the possibility of fabrication of quantum dot based solar cells with efficiencies similar or higher than the organic photovoltaic devices.

Fluorescent solar collectors are cheap plates made generally of glass or plastic that is doped with fluorescent molecules. Fluorescence can be used to trap incident solar radiation in order to concentrate light and also to shift the radiation to wavelengths converted at higher efficiencies. This project investigates both these aspects. Solar concentration using fluorescent solar collectors is examined for different combinations of solar cells and mirrors coupled to different surfaces of fluorescent solar collectors. The effects of the fabrication method i.e. spin coating or moulding is also studied. Modelling of the fluorescence re-absorption is the key to obtain the efficiency of such systems and these models will be shown to be useful in the optimisation of certain configurations of fluorescent collectors that behave like ideal models. On comparison with experimental results these models can also be used to quantify and identify losses in configurations that suffer additional losses. Devices tested in the lab are estimated to have the potential to achieve power conversion efficiencies close to 4% while results of optimisation results indicate possible further improvements. Angular resolved measurements of the fluorescence exiting the fluorescent solar collector edge is also used to study the photon transport of light within these devices. The combination of solar concentration and wavelength shifting (to wavelengths with superior solar cell performance) has been exploited for application to cadmium telluride solar cells for the first time. The theory of operation of these devices is used to model the current output measured experimentally. Novel photonic-based structures that incorporate fluorescent molecules within one-dimensional photonic crystals has also been fabricated and characterised. It will be shown that these devices have the potential to reduce fundamental loss mechanisms found in conventional fluorescent solar collectors by suppressing and reducing emission within loss cones and tuning emission reaching the solar cells to certain wavelengths. The theory of operation of these devices will used to describe the propagation of light within the structure and explain the experimentally measured emission characteristics. The maximum theoretical efficiencies of photonic-based fluorescent solar collectors have also been derived and adapted from previous work to show the potential of such devices.

Le mie attività di ricerca hanno indagato i meccanismi di ricombinazione eccitonici in nanocristalli semiconduttori colloidali (NC), promuovendo lo sviluppo di nuovi paradigmi per la manipolazione delle proprietà ottiche e di scintillazione. Grazie all'ampia gamma di tecniche spettroscopiche e alle preziose collaborazioni intraprese, le mie conclusioni sono state pubblicate in prestigiose riviste scientifiche, contribuendo al progresso della comunità di scienziati dei nanomateriali. La mia ricerca ha trattato principalmente due temi di attuale rilevanza tecnologica: i) l'origine della fotoluminescenza in NC di calcogenuri ternari I-III-VI2 Cd-free come CuInS2 e AgInS2 ii) l'impiego di nanostrutture di perovskiti in schemi di rivelazione e/o conversione in energia di radiazione ionizzante. Nello specifico, l’uso di tecniche spettroscopiche complementari in regime di temperatura controllata ha validato la presenza di sottolivelli intrinseci, con diversa parità, nella banda di valenza di NC stechiometrici di CuInS2 responsabili delle proprietà ottiche di questa classe di NC. I miei risultati, supportati da simulazioni di propagazione Monte Carlo, hanno portato alla fabbricazione di un concentratore solare luminescente - con efficienza record - basato su NC di CuInS2 con taglia ottimale. Lo studio è stato quindi esteso a NC di AgInS2, un materiale meno studiato ad oggi, ma molto promettente per applicazioni di bioimaging grazie all’assenza di elementi tossici. Quindi, ho investigato la rivelazione di radiazione ionizzante attraverso nanostrutture ad alto numero atomico come le perovskiti a base di alogenuri di piombo (LHP), ed in particolare il CsPbBr3. Attraverso lo studio dettagliato delle proprietà di foto- e radio-luminescenza, ho evidenziato gli effetti dell’interazione tra eccitone di bordo banda e stati di difetto shallow/deep in nanostrutture di CsPbBr3 con diversa dimensionalità. Questo studio fondamentale ha offerto una piattaforma per sviluppare nuove strategie sintetiche atte a passivare i siti di intrappolamento sulle superfici degli NC che hanno portato ad un incremento di efficienza di scintillazione del 500%. La stabilità dei NC di CsPbBr3 è stata infine verificata in termini di resistenza alla radiazione gammma fino a dosi estreme di 1 MGy. Inoltre, per estendere la loro applicazione alla radiation detection con dispositivi a guida d’onda, ho studiato la sensibilizzazione di un colorante organico accoppiato a NC di CsPbBr3, realizzando il primo esempio di scintillatore plastico con ampio Stokes-shift e luminescenza veloce basato su LHP. Per superare i limiti imposti dalla presenza di Pb nelle LHP, ho infine esplorato le proprietà ottiche e di scintillazione di nuove classi emergenti di perovskiti doppie green. Le informazioni raccolte incoraggiano il proseguimento di questa linea di ricerca indicando la passivazione superficiale come strategia più promettente per raggiungere prestazioni analoghe alle controparti a base di Pb.
During my PhD I investigated excitonic recombination mechanisms in colloidal semiconductor nanocrystals (NCs), promoting the development of new paradigms for the manipulation of optical and scintillation properties. Thanks to the wide range of spectroscopic techniques and the valuable collaborations undertaken, my conclusions have been published in prestigious scientific journals, contributing to the advancement of the community of nanomaterials scientists. My research mainly dealt with two topics of current technological importance: i) the origin of photoluminescence in NC of Cd-free ternary I-III-VI2 chalcogenides such as CuInS2 and AgInS2 ii) the use of perovskite nanostructures in detection schemes and/or energy conversion of ionizing radiation. Specifically, the use of complementary spectroscopic techniques in a controlled temperature regime has validated the presence of intrinsic sublevels, with different parity, in the valence band of the stoichiometric CuInS2 NCs responsible for the optical properties of this class of NC. My results, supported by Monte Carlo ray-tracing simulations, led to the fabrication of a luminescent solar concentrator - with record efficiency - based on CuInS2 NCs with optimal size. The study was then extended to AgInS2 NCs, a less investigates material so far, but very promising for bioimaging applications thanks to the absence of toxic elements. Then, I investigated the detection of ionizing radiation through high atomic number nanostructures such as lead halide perovskites (LHP), and in particular CsPbBr3. Through the detailed study of photo- and radio-luminescence properties, I highlighted the effects of the interaction between band edge exciton and shallow/deep defect states in CsPbBr3 nanostructures with different dimensionality. This fundamental study offered a platform to develop novel synthetic strategies to passivate trap sites on NC surfaces that led to a 500% enhancement of scintillation yield. The stability of CsPbBr3 NCs was finally verified in terms of radiation hardness, up to extreme gamma doses of 1 MGy. Furthermore, to extend their application to radiation detection with waveguiding devices, I studied the sensitization of an organic dye coupled to CsPbBr3 NCs, creating the first example of a plastic scintillator with wide Stokes-shift and fast luminescence based on LHP. To overcome the limitations imposed by the presence of Pb in LHPs, I finally explored the optical and scintillation properties of new emerging classes of green double perovskites. The information gathered encourages the continuation of this line of research, indicating surface passivation as the most promising strategy for achieving performance similar to the Pb-based counterparts.

Hydrogen energy is an ideal energy resource owing to its clean and efficient utilization. As an energy carrier without natural abundance, the limited reserve makes the high consumption a big challenge. In the meantime, fossil fuels, e.g., coal, oil and gas, have been important carbon carriers in the long-term carbon cycle, but their upgrading is restricted to conventional thermocatalysis. Solar energy with the advantages of large abundance, widespread distribution, and high flux appeals extensive attention, but unfortunately is underutilized at the moment. Photocatalysis initiated with semiconductors is a promising pathway towards the conversion and storage of solar energy into chemical stocks, and has been studied for several decades. However, due to the low photoresponse capacity and solar energy conversion efficiency of the existing photocatalysts, the prospect of their industrialization is still unclear. Photothermal catalysis integrating photocatalysis and thermocatalysis into one unit has been proposed in the past several years. Although its quantum efficiency and reaction turnover frequency were significantly improved, the reaction mechanisms have not yet been well illustrated. This PhD study is to develop photo assisted catalysis to obtain high performances for energy preparation and fossil fuels upgrading, and to have a deep insight into their reaction mechanisms. First, in-plane heterostructured graphene/carbon nitride photocatalyst was prepared via a hydrogeninitiated chemical epitaxial growth strategy. With the insert of nano-graphene into the porous carbon nitride, the quantum efficiency of the water splitting reaction for hydrogen generation was significantly enhanced (Chapter 3). Considering the unsatisfied incident light to electron efficiency, the study unveiled the potential difference as the internal electrical field affecting the separation, transfer and output of photoinduced charge carriers. Meanwhile, the quantum efficiency and utilization of solar light were both improved via the optimization of potential differences in photocatalytic systems (Chapter 4). In addition, the active sites (Chapter 5) and relationships between photocatalysis and thermocatalysis (Chapter 6) in photothermal catalytic systems were both in-depth studied. With the available reaction mechanism and optimization of reaction conditions, the photothermal catalytic performances in the upgrading of fossil fuels are increased to a industrialization level. This PhD project contributes to the improvements of quantum efficiency via catalyst modification, reaction optimization and mechanism investigation and then expects to provide both technological and scientific knowledge for the full storage and conversion of solar energy into fuels.

This Ph.D work reports the studies of photovoltaic devices produced by solution processable methods. Two material systems are of interest: one is based on organic semiconductors, and another on organic/inorganic hybrid composites. Specifically, organic photovoltaic (OPV) devices are made using photoactive materials consisted of a -conjugated polymer [Poly(3-hexylthiophene), or P3HT] and fullerene derivative [phenyl-C60-butric acid methyl ester, or PCBM] in a bulk heterojunction (BHJ) structure of donor/acceptor network. On the other hand, hybrid photovoltaic (HPV) devices are made from blend of quantum dots and -conjugated polymers. The QD material presented here are of the lead sulfide (PbS), and lead selenide (PbSe), whereas the polymers are either P3HT or Poly(3-dodecyl thienylene vinylene) (PTV)with controlled regio-regularity. For OPV devices, two different device geometries are investigated, namely, the conventional or normal structure where indium tin oxide (ITO) is used an anode, and a metal cathode is fabricated by thermal vapor deposition (TVD). In this geometry, thin layer (about 30~35nm) of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is deposited from aqueous solution onto ITO as hole transport layer (HTL). The second geometry, called the inverted structure, uses ITO as the cathode of the device. A thin layer of cesium carbonate (Cs2CO3) (about 1~2nm) is applied over the ITO and functions as electron transport layer (ETL), thereby decreasing the work function of ITO and allowing it to function as the cathode. In this case, PEDOT:PSS is mixed with 5vol.% of dimethylsulfoxide to increase conductivity for serving as anode. Two solution processable methods, spin-coating and spray processes were investigated, and a detailed study of nanomorphology influence under different annealing conditions, different solvents and thickness are reported. The main contribution of this Ph.D. work was the development and implementation of a layer-by-layer (LBL) all-spray solution-processable technique to fabricate large-scale OPV arrays, with more than 30% transmission in the visible to near IR range. Comparing with traditional laboratory OPV fabrication based on spin-coating and using metal as cathode contact, which greatly limits transparency of solar cells and posts difficulty for large scale manufacturing, this LBL spray process solves these two problems simultaneously. This technology eliminates the need for high-vacuum, high temperature, low rate and high-cost manufacturing associated with current silicon and in-organic thin film photovoltaic products. Furthermore, this technology could be used on any type of substrate including cloth and plastic. Single cell OPV with active area of 4mm2 was used as preliminary test device to obtain fabrication parameters for multi-cell OPV arrays. Three different sizes of OPV arrays were fabricated and tested under various illumination conditions. Starting from a 4" x 4" array with 50 cells in series connection 4" x 4" substrate consisting of 50 cells with total active area of 30cm2, a scaled up 1' x 1' array was fabricated as a proof of concept, and whose results are reported. Scaled down arrays, called micro arrays, are also presented in this work. OPV micro array has the potential application in DC power supplies for electrostatic Microelectromechanical systems (MEMS) devices. The first generation micro array consists of 20 small (1mm2) solar cells connected in series for a total device area of approximately 2.2cm2. The 2nd generation micro array with 60 cells shares the same size substrates and single cell active area as the first generation. However, the 2nd generation micro array cell has a new design with reduced series resistance and improved cell occupancy by 3 fold. Infrared quantum dots (QD) such as PbS and PbSe have potential in photovoltaic applications. These solution processable quantum dots with tunable electronic properties offer very attractive approach for expanding spectral sensitivity of -conjugated polymers to infrared region of solar spectrum. However, these QDs often have defects originated from either incomplete surface passivation or imperfections in the quantum Dots. The electronic levels of defects often are within the bandgap of the semiconductor. These in-gap states are of great importance since they affect the final destiny of excitons. Continuous wave photoinduced absorption spectroscopy has proven to be a convenient and successful technique to study long-lived photoexcitations of in-gap states. Part of this Ph.D work was the investigation of a peculiar gap state found in films of PbS QDs. This gap state bears confinement dependence, with a lifetime about 2μs. A detailed analysis of the Stokes shift, temperature dependence of PL, absorption and photoinduced absorption reveals the unconventional GS is a new state of a trapped exciton in a QD film. This gap state is directly relevant to exciton dissociation and carrier extraction in this class of semiconductor quantum dots. As synthesized PbSe and PbS quantum dots usually have bulky ligands such as oleic acids or TOPO (trioctylphosphine oxide). This capping layer is necessary to prevent nanocrystals from coalescence, however, the bulky ligands hinder charge extraction from and charge transport through the nanocrystals, as well as exciton dissociation at the nanocrystal/polymer interface. Common ways to manipulate ligands include ligand wash and ligand exchange in solution, and ligand removal on films. Through this Ph.D. work, a novel method using electric field to manipulate quantum dots ligands for interface of quantum dots and polymer, which possibly could facilitate charge extraction from the quantum dots and charge transfer between quantum dots and polymers, without the need of harmful chemicals. Over four orders improvement of photoconductivity at zero bias and more than six orders improvement at 5V reverse bias in a sandwich structure quantum dots photovoltaic device, and more than 5x improve in film smoothness. After thorough fundamental study on QD optoelectronic properties, hybrid photovoltaic (HPV) device was fabricated using a blend solution of PbS QDs and P3HT. Two different solution processes are used to form the QD/polymer active layer, one is the traditional spin coating method, and another is the spray technique developed in this Ph.D. Work. Different film morphology was observed with these two methods. Although the film is slightly rougher in the case with sprayed QD/polymer active layer, the phase segregation is more distinct and with smaller domain, which is beneficial for charge transport.

This thesis presents the design and synthesis of nanowires (NW) with targeted and tunable optical properties. Moreover, we show how single and assembled NW devices can enable new photovoltaic (PV) and photonic platforms. Beginning with an investigation of axially modulated p-i-n junction NWs, we established several fundamental parameters dictating solar cell performance at the nanoscale and demonstratred the first series integration of multiple solar cells on a single NW. Thereafter, implementation of the first silicon NW photovoltaic device with radially modulated p-n junctions showed that power conversion efficiencies of 3-4% are attainable from a nanoscale architecture, exceeding efficiencies for many organic and hybrid organic-inorganic solar cells. Despite these achievements, the poor electrical characteristics and insufficient control over absorption properties characterizing the aforementioned devices would limit the promise of silicon NWs for next generation solar cells. We overcome these limitations with a class of polymorphic core/multi-shell silicon NWs with highly-crystalline hexagonally-faceted shells and embedded coaxial p/i/n junctions. NW PV devices 200-300 nm in diameter exhibit open-circuit voltages of 0.5 V and fill-factors of 73% under one-sun solar illumination. Single-NW wavelength-dependent photocurrent measurements agree quantitatively with FDTD simulations. Synthetic manipulation of NW size and morphology drives tuning of optical resonances such that optimized structures can yield current densities double those for films of comparable thickness. Further optimized NW devices achieve current densities of 17 mA/cm2 and power conversion efficiencies of 6%. We also present steps toward rational assembly of larger-scale NW PV arrays. Parallel integration of NWs preserves PV metrics while assembly of vertically-stacked NWs yields current densities of \(25 mA/cm^2\) and projected efficiencies of ~15% for \(1 \mu m\) thick assemblies. Finally, we present the first ever NW material possessing 3 degrees of structural freedom, thus expanding the NW "structome." Such NWs were achieved through the first demonstration of facet selective growth of silicon and germanium in the gas phase. Photonic devices based on this new material present intriguing optical properties, including selective attenuation, enhancement, and wavelength tunability of resonant cavity modes.
Chemistry and Chemical Biology

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2012.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 153-159).
The photovoltaic technology has been attracting widespread attention because of its effective energy harvest by directly converting solar energy into electricity. Thin-film silicon solar cells are believed to be a promising candidate for further scaled-up production and cost reduction while maintaining the advantages of bulk silicon. The efficiency of thin-film Si solar cells critically depends on optical absorption in the silicon layer since silicon has low absorption coefficient in the red and near-infrared (IR) wavelength ranges due to its indirect bandgap nature. This thesis aims at understanding, designing, and fabricating novel photonic structures for efficiency enhancement in thin-film Si solar cells. We have explored a previously reported a photonic crystal (PC) based structure to improve light absorption in thin-film Si solar cells. The PC structure combines a dielectric grating layer and a distributed Bragg reflector (DBR) for effcient light scattering and reflection, increasing light path length in the thin-film cell. We have understood the operation principles for this design by using photonic band theories and electromagnetic wave simulations. we discover that this DBR with gratings exhibit unusual light trapping in a way different from metal reflectors and photonic crystals. The light trapping effects for the DBR with and without reflector are numerically investigated. The self-assembled anodic aluminum oxide (AAO) technique is introduced to non- lithographically fabricate the grating structure. We adjust the AAO structural parameters by using different anodization voltages, times and electrolytes. Two-step anodization is employed to obtain nearly hexagonal AAO pattern. The interpore periods of the fabricated AAO are calculated by fast Fourier transform (FFT) analysis. We have also demonstrated the fabrication of ordered patterns made of other materials like amorphous Si (a-Si) and silver by using the AAO membrane as a deposition mask. Numerical simulations predict that the fabricated AAO pattern exhibits light trapping performance comparable to the perfectly periodic grating layer. We have implemented the light trapping concepts combining the self-assembled AAO layer and the DBR in the backside of crystalline Si wafers. Photoconductivity measurements suggest that the light absorption is improved in the near-IR spectral range near the band edge of Si. Furthermore, different types of thin-film Si solar cells, including a-Si, mi- crocrystalline Si ([mu]-Si) and micromorph Si solar cells, are investigated. For demonstration, the designed structure is integrated into a 1:5 [mu]m thick [mu]c-Si solar cell. We use numerical simulations to obtain the optimal structure parameters for the grating and the DBR, and then we fabricate the optimized structures using the AAO membrane as a template. The prototype devices integrating our proposed backside structure yield a 21% improvement in efficiency. This is further verified by quantum efficiency measurements, which clearly indicate stronger light absorption in the red and near-IR spectral ranges. Lastly, we have explored the fundamental light trapping limits for thin-film Si solar cells in the wave optics regime. We develop a deterministic method to optimize periodic textures for light trapping. Deep and high-index-contrast textures exhibit strong anisotropic scattering that is outside the regime of validity of the Lambertian models commonly used to describe texture-induced absorption enhancement for normal incidence. In the weak ab- sorption regime, our optimized surface texture in two dimensions (2D) enhances absorption by a factor of 2.7[pi]n, considerably larger than the classical [pi]n Lambertian result and exceeding by almost 50% a recent generalization of Lambertian model for periodic structures in finite spectral range. Since the [pi]n Lambertian limit still applies for isotropic incident light, our optimization methodology can be thought of optimizing the angle/enhancement tradeoff for periodic textures. Based on a modified Shockley-Queisser theory, we conclude that it is possible to achieve more than 20% efficiency in a 1:5 [mu]m thick crystalline Si cell if advanced light trapping schemes can be realized.
by Xing Sheng.
Ph. D.

Durante il dottorato ho investigato il processo fotofisico di "upconversion" assistito da annichilazione tripletto-tripletto (TTA-UC) tramite studi di spettroscopia in sistemi profondamente differenti gli uni dagli altri. In TTA-UC radiazione ad alta energia è emessa dalla ricombinazione radiativa dello stato di singoletto eccitato di una molecola emettitore, popolato precedentemente dall'annichilazione dei tripletti di due emettitori. Un sensibilizzatore immagazzina la luce incidente a bassa energia e trasferisce l'eccitazione agli emettitori tramite trasferimento di energia alla Dexter. Poiché il suo funzionamento si basa su tripletti mestastabili, TTA-UC può essere altamente efficiente anche in condizioni di luce non coerente e a bassa energia. Come tale, è particolarmente adatto per dispositivi che sfruttano l'energia solare poiché è in grado di aumentarne l'efficienza di conversione limitando le perdite per trasmissione. Mi sono concentrata su due problemi importanti che tuttora limitano l'impiego di materiali che attuano TTA-UC (upconverters), ossia la limitata capacità di immagazzinare energia dei comuni sensibilizzatori organici e le scarse prestazioni di TTA-UC in upconverters a stato solido, i quali sono più adatti per applicazioni tecnologiche rispetto a sistemi liquidi. Per risolvere il primo problema ho investigato sensibilizzatori ibridi, composti da nanostrutture a semiconduttore decorate con molecole organiche, con ampio assorbimento. Nanocristalli di CdSe drogati con cationi d'oro e decorati con acido antracenico carbossilico si sono dimostrati essere sensibilizzatori ibridi efficienti ed innovativi. Il drogante introduce nel gap energetico dei nanocristalli livelli localizzati su cui le lacune si localizzano sulla scala dei picosecondi, più velocemente dell'estrazione di lacune sul livello HOMO dei leganti. Con tale strategia ho raggiunto l'efficienza di UC del 12%, record per sistemi ibridi. Ho poi mostrato come le proprietà superficiali e fotofisiche di nanoplatelets di CdSe le rendano ottimali candidati in sensibilizzatori ibridi. Ho mostrato che il ricoprimento delle superfici non è omogeneo, ma procede ad isole e l'interazione di "π- π stacking" porta alla formazione di aggregati sulle superfici delle nanoplatelets, con il risultato di ridurre l'energia dei tripletti dei leganti con profonde ripercussioni sulle prestazioni di TTA-UC e sulla scelta della specie emettitrice. Riguardo al secondo problema, ho studiato due upconverters a stato solido, polimeri vetrosi nanostrutturati che mostrano proprietà macroscopiche simili ma realizzati con tecniche differenti. Essi presentano domini liquidi di dimensione inferiore a 50 nm dove le specie che attuano TTA-UC si accumulano, racchiuse in una matrice rigida polimerica che fornisce protezione da ossigeno e qualità ottica eccellenti e stabilità a lungo termine. Il confinamento molecolare permette di aumentare la densità locale di eccitoni aumentando l'efficienza di UC a basse potenze grazie alle ridotte distanze intermolecolari e all'attivazione del regime di TTA-UC confinato.Ho inoltre studiato un nuovo emettitore derivato da perilene, realizzato con lo scopo di aumentarne l'efficienza di fluorescenza. Grazie a questo emettitore ho raggiunto l'efficienza record di UC di 42%, dovuta proprio alla struttura molecolare dell'emettitore che permette di limitare la formazione di aggregati, garantendo un'eccellente efficienza di generazione di singoletti tramite TTA. Infine, ho presentato una prospettiva riguardo alle prestazioni che possono essere raggiunte combinando le due tematiche trattate, ossia inserendo sensibilizzatori ad ampio assorbimento in polimeri nanostrutturati. Trovando il giusto compromesso tra taglia dei domini liquidi e distribuzione dell'energia di eccitazione si raggiungerebbe la massima efficienza di UC a potenze minori dell'irradianza solare, promuovendo lo sviluppo di upconverters a stato solido per tecnologie a energia solare
In my PhD project, I investigated the photophysical process of photon upconversion assisted by triplet-triplet annihilation (sTTA-UC) through spectroscopy studies in a variety of systems, profoundly different on many levels. In sTTA-UC high energy radiation is emitted from the fluorescent recombination of the excited singlet of an emitter molecule, previously populated via annihilation of the metastable triplet states of two emitters. This is a sensitized process since a sensitizer is necessary to harvest the low energy incident light and to transfer the stored energy to the emitters via Dexter energy transfer. Because its functioning relies on long-lived metastable triplets, this process can be highly efficient also under low power, noncoherent light. As such, sTTA-UC is particularly suited for solar applications as it can increase the conversion efficiency by reducing transmission losses. During my studies, I focused on addressing two crucial issues that still limit the application of upconverters in solar technologies, i.e. the limited storage ability of common organic sensitizers and the poor sTTA-UC performance in solid-state upconverters, which are intrinsically better suited than liquid solutions for technological applications. To solve the first problem, I investigated hybrid sensitizers, composed of semiconductor nanostructures decorated with conjugated organic ligands characterized by broadband absorption. CdSe nanocrystals (NCs) doped with gold cations and decorated with 9-anthracene carboxylic acid demonstrated to be efficient innovative broadband hybrid sensitizers. The doping strategy inserts into the NCs energy gap localized hole-accepting states where the holes localize on the picosecond timescale, outpacing hole transfer to the ligand HOMO. With this strategy, I achieved the UC efficiency of 12%, the record performance obtained so far for hybrid upconverters. I then discussed how the CdSe nanoplatelets surface and photophysical properties make them potential optimal light harvesters. My studies on the nanoplatelets-to-ligands energy transfer dependency on the surface ligand density revealed that the surface coverage is not homogeneous but proceeds in an island-like way promoted by π- π stacking and results in the formation of ligands aggregates on the nanoplatelets surfaces, which causes a redshift of the ligand triplet energy with critical repercussions on the sTTA-UC performance and on the emitter selection. To address the second issue, I investigated two solid-state upconverters, i.e. nanostructured glassy polymers that show similar macroscopic properties but fabricated via different approaches. They both feature liquid droplets of mean size less than 50 nm where the upconverting dyes accumulate, embedded in a rigid polymer matrix that grants excellent oxygen protection and optical quality and long-term stability. The dyes confinement allows to increase the effective local excitons density resulting in an enhanced UC efficiency at low excitation intensities, thanks to the reduced intermolecular distances and the activation of the confined sTTA-UC regime. I also introduced a new perylene derivative as emitter, specifically designed to prevent molecular aggregation to maximize its fluorescence efficiency. By employing this emitter, I achieved the record UC efficiency of 42%, which directly stems from the emitter molecular structure, as it limits the formation of aggregates, while guaranteeing excellent singlet generation efficiency upon TTA. I finally presented a perspective of the performances that can be achieved by combining the two topics considered, i.e. loading broadband sensitizers in nanostructured polymers. I highlighted that if the best trade-off between nanostructure size and energy distribution is met the maximum UC efficiency can be achieved at excitation powers orders of magnitude lower that the solar irradiance, therefore promoting the development of real-world solid-state upconverters.

The development of organic solar cells as a novel form of renewable energy has been driven by their potential for low-cost, large-scale fabrication though the solution-processing of semiconducting polymers and small molecules. Certified power conversion efficiencies have reached 13% as of 2016 thanks to the development of new donor-acceptor molecules, but the efficiency of any given device is still highly sensitive to the morphology that these materials adopt during deposition. It is essential that morphology is characterized thoroughly in order to establish the relationships between molecular structure, morphological properties and device performance; in order to maximise efficiency and make organic solar cells an economically competitive source of renewable energy. In this thesis, several spectroscopic techniques are used to probe the impact of various processing parameters on the molecular order, crystallinity and phase separation of polymer:fullerene blends. For the model blend system P3HT:PCBM, P3HT molecular order can be measured by resonant Raman spectroscopy, and PCBM is found to dissolve in the amorphous domains of the semi-crystalline polymer up to a miscibility limit of 25 %wt, above which it can only be accommodated by increased disorder. In situ annealing demonstrates that when heated above a glass transition temperature of ~50C, disordered blends separate into purer domains of high molecular order that correlate well to improved charge transport and efficiency for thermally annealed devices. Raman spectroscopy is also used to probe the stability of the high-efficiency PTB7:PC70BM blend. Photo-oxidation of PTB7 was found to induce specific vibrational changes that correlate to formation of a hydroxyl group on the benzodithiophene unit. In situ experiments reveal that hydroxylation precedes the loss of chromophores that results in deterioration of device performance, and is accelerated by blending with PC70BM. Understanding the impact of morphology on charge extraction from the active layer requires the selective probing of interfacial properties at the top and bottom of the organic film, which we demonstrate using SERS. For both a polymer:fullerene blend (PTB7:PC70BM) and a polymer:polymer blend (P3HT:F8TBT), spin-coated films exhibit interfacial compositions different from that of the bulk film and favourable to charge extraction from inverted device architectures, but can be modified by pre- or post-annealing treatments. Finally, we investigated the morphology of a novel low band-gap polymer, a tellurium analogue of polythiophene, in order to understand the impact of the heavy atom on chain planarity and polymer crystallinity. The Raman spectrum of P3ATe exhibited a much stronger sensitivity to molecular order, which was highly dependent on the length and linearity of the alkyl side chain, but there was no clear morphological reason why P3ATe reportedly performs poorly compared to P3HT, despite the superior absorption of its smaller band-gap.

碩士
國立臺灣大學
光電工程學研究所
99
In this thesis, first, the nanowire array (NWA) layers with controlled structure profiles fabricated by maskless galvanic wet etching on Si substrates are found to exhibit extremely low specular reflectance (&lt; 0.1 %) in the wavelengths of 200-850 nm. The significantly suppressed reflection is accompanied with other favorable antireflection (AR) properties, including omnidirectionality and polarization-insensitivity. The NWA layers are also effective in suppressing the undesired diffuse reflection. These excellent AR performances benefit from the rough interfaces between air/NWA layers and NWA layers/substrate and the decreased nanowire densities, providing the gradient of effective refractive indices. The Raman intensities of Si NWAs were enhanced by up to 400 times as compared with the signal of the polished Si, confirming that the NWA layers enhance both insertion and extraction efficiencies of light. This study provides an insight into the interaction between light and nanostrucutres, and should contribute to the structural optimization of various optoelectronic devices. Second, wafer-scale nanowire arrays (NWAs) with hierarchical structure, combined the nanowire and interface micro-roughness were fabricated by single process of costless wet etching. The NWA based solar cells with designed hierarchical structure demonstrate excellent light-harvesting characteristics, such as broadband working ranges and omnidirectionality in external quantum efficiency and reflectance measurement. Compared to the polished Si and conventional NWAs, the solar cell with hierarchical structure exhibits significantly superior photovoltaic characteristics, i.e., short-circuit current of 32.7 mA/cm2 and conversion efficiency of 11.25 %. The enhanced photovoltaic performances agree with the theoretical analysis based on a finite-difference time-domain method. A viable scheme for light harvesting using the hierarchical structure employing micro-roughness/nanoscale surface textures on single crystalline Si solar cells has been demonstrated. Third, antireflective Si/oxide core-shell nanowire arrays (NWAs) were fabricated by galvanic etching and subsequent annealing process. The excellent light-harvesting characteristics of the core-shell NWAs, such as broadband working ranges, omnidirectionality, and polarization-insensitivity, ascribed to the smooth index transition from air to the substrates, have been demonstrated. By tuning core-shell volume ratios, we obtained enhanced light trapping regions implemented in either the planar Si underneath NWAs or the core regions of NWAs, greatly benefiting the geometry design of planar and radial p-n junction cell structures, respectively. This photon management scheme indicates the potential use in nanostructured photovoltaic applications. Finally, rough AZO films were employed to enhance the internal scattering and consequent optical absorption of thin film (amorphous/polycrystalline Si) tandem solar cells. Through the optimization work by simulations, the matched current densities from the top and the bottom cells were obtained with the device structure containing 1.5-μm roughened polycrystalline Si layer, which produces the efficiencies comparable to those of the 3.5-μm layer without roughening. The simulation results were supported by the device performances measured experimentally. The significantly enhanced light scattering in the thin rough active region was revealed by the calculation results based on finite-difference time-domain method. The concept and technique presented in this study should benefit the development of next generation of thin film solar cells.

Taking into consideration the importance of a detailed model in the trajectory propagation, three space missions using solar photon sailing has been studied with a different thrust model. Then, an equipment has been designed and built to measure the deformation of a real sample of solar sail on several work conditions. An analysis of the deformations and they distributions has been taken in account to extrapolate a more accurate model for thrust. A comparison between models in function of the sail parameters has been presented to compare the optimal time of travel to reach a circular-to-circular orbital change.