Abstract: Understanding and controlling the ultrafast charge carrier and exciton dynamics at the interface of semiconductor nanocrystals (NCs) offer an excellent opportunity to improve the charge collection and the overall performance of many optoelectronic and energy-based devices. In this dissertation, we study how interfacial engineering of these materials can have a direct influence on controlling the charge transfer and the nonradiative losses in different donor-acceptor systems. The first introductory chapter provides an overview of all the fundamental photophysical processes controlling the interfacial phenomena. Then, the second chapter highlights all the chemicals and synthesis methods employed during this thesis. The subsequent two chapters discuss the detailed experimental studies and observations related to different materials and interfaces. First, it describes how we can dramatically tune the intersystem crossing (ISC) rate, the triplet state lifetime, turn on/off the electron injection at the CdTe-Prophyrin interface via tuning either the quantum dot size or the porphyrin molecular structure. Also, how the intermolecular distances, electronic coupling, and subsequently, the photoinduced charge transfer can be controlled by the interfacial electrostatic interactions at CdTe-Fullerene interfaces. Second, due to the promise that of perovskite NCs holds for improving many solar cell and optoelectronic applications, chapter 3 highlights the tremendous effect that the shape of perovskite nanocrystals has on the rate and the mechanism of charge transfer at the MAPbBr3- TCNE interface. Besides, it demonstrates how the confinement effect brought by changing the dimensionality influence the charge transfer dynamics at the MAPbBr3-BQ interface. Finally, it explains how the effective passivation of the surface defects and the subsequent suppression of the formation of surface nonradiative recombination centers in CsPbCl3 NCs controls the photoluminescence quantum yield and the photodetector performance.

As the global population approaches 11 billion people, demands for sustainable food, energy, and water (FEW) are approaching unprecedented levels. Current technology places sustainable FEW production methods in direct competition with one another for global surface area, such as land area for agriculture versus photovoltaic farms. This is because current installation methods for solar modules create deep shading that suppresses plant growth. The field of agrivoltaics (AgPV) addresses this issue directly by optimizing coproduction strategies for FEW and developing systems where competition is reduced; however, previous work has seen reductions in agricultural output. AgPV, where module architecture is also modified and agricultural output is minimally impacted, requires novel multi-level experimental design and characterization. In my proposed thesis, I will address the following two aspects of the project: (1) a farm-level experimental analysis of existing PV and (2) a device-level analysis of new and emerging PV material candidates. To establish the plausibility of this work, I designed and implemented an agrivoltaic system with two treatments that was successfully farmed this year. In my thesis, I will demonstrate a fully characterized utility scale AgPV array through several steps. First, I will validate the prior simulation work on the constructed AgPV array. Then I will experimentally correlate crop growth underneath the AgPV experiment. Next, the effects of the shadowing from the array on crop growth will be quantified. I will optimize the tracking algorithm for the array to maximize crop growth during the summer and power production during other seasons. Finally, I will investigate a platform for evaluation of novel PV materials and devices tailored for AgPV systems using Photoluminescence Excitation Spectroscopy (PLE) where I redesigned, constructed, and validated a new experimental design.

Small clusters of [6,6] phenyl-C61-butyric acid methyl ester (PCBM) molecules are analyzed with respect to their equilibrium geometries and associated electronic as well as energetic properties. Plane wave density functional theory (PWDFT) computations, assisted by molecular dynamics (MD) simulations, are performed on systems of the form PCBMn (n = 1-5). The bonding operative in these units is described as a cooperation between HO bonding, involving the C5H9O2 groups of the PCBM molecule, and fullerene-fullerene attraction. The maximally stable structures identified tend to include a dimer motif that combines both interaction modes. The great importance of van-der-Waals effects in stabilizing the studied clusters is demonstrated by comparing the PCBM3 series with and without inclusion of a van-der-Waals term in the PWDFT procedure. The two approaches yield reverse orders of stability. A decreasing tendency in the Kohn-Sham HOMO-LUMO gaps of PCBMn with the cluster size may be used to monitor PCBM aggregation in the active layer of organic photovoltaic devices by optical spectroscopy.

The re-emergence of interest in organic semiconducting small molecules and polymers during past several decades can be attributed to their advantage of utility, flexibility, ease of access, and turnability over silicon based inorganic semiconductors. The library of organic semiconductors containing p-type (hole conducting) and n-type (electron conducting) materials have grown in numbers and efficiency. The p-type semiconducting materials hold an advantage over n-type materials owing to their stability and ease of synthesis. The widespread use of fullerenes (C60 and C70) as n-type materials in organic photovoltaics OPVs and their known downsides of poor absorption in visible and NIR region and limited charge carrier transport have triggered the development of non-fullerene based electron accepting (NFEA) materials . By taking advantage of the electron accepting behavior of cyclopenta[hi]aceanthrylene fragment of C70, we have synthesized a new class of cyclopentafused polyaromatic hydrocarbons (CP-PAHs). These new contorted CP-PAHs have been prepared utilizing the modified version of our previously developed palladium catalyzed cyclopentannulation strategy. The target molecules broaden the scope of annulation chemistry to 1,2-bis(5-hexylthiophen-3-yl)ethyne with aryl dibromo derivatives of anthracene, pyrene and perylene to yield 4,4',4'',4'''-(cyclopenta[hi]aceanthrylene-1,2,6,7-tetrayl)tetrakis(2-hexylthiophene), 4,4',4'',4'''-(dicyclopenta[cd,jk]pyrene-1,2,6,7-tetrayl)tetrakis(2-hexylthiophene) and 1,2,7,8-tetrakis(5-hexylthiophen-3-yl)-1,2,7,8-tetrahydrodicyclopenta[cd,lm]perylene. Scholl cyclodehydrogenation of the cyclopentafused thiophene units with suitably substituted hydrocarbons chains provided access to p-extended polyaromatic systems including 2,5,11,14-tetrahexylrubiceno[5,4-b:6,7-b':12,11-b'':13,14-b''']tetrathiophene, 2,5,11,14-tetrahexyldithieno-[4,5:6,7]indeno[1,2,3-cd]dithieno[4,5:6,7]indeno-[1,2,3-jk]pyrene, and 2,9,12,19-tetrahexyldithieno[4,5:6,7]indaceno[1,2,3-cd]dithieno[4,5:6,7]indaceno[1,2,3-lm]perylene. The fully conjugated p-electronic core of these small molecules provide low optical band gaps, decent mobilities and broad absorption. The HOMO and LUMO energies of these CP-PAHs were found to be in the range of -5.48 to -5.05 eV and -3.48 to -3.14 eV, respectively. Besides showing broad band absorption features, some derivative were found to operate as a p-type semiconductor when tested in organic field effect transistors. Anthradithiophene (ADT) is an isoelectronic analogue of pentacene and became a point of interest. A soluble, and functionalizable ADT derivative, 5,11-dibromoanthradithiophene was prepared and then polymerized utilizing Stille, Sonogashira and Yamamoto cross coupling strategies. The newly developed ADT polymers were found to operate in p-type regime when tested in organic field effect transistors. To explore the effects of solvent on growing polymer chains in step-growth polymerizations, we developed a library of Yamamoto and Glaser polymers. The hypothesis tested was that growing polymer chains can recruit further monomer units to create block character in the growing polymer chains. Our investigations reveals that the solvent conditions altering the polarity of the reaction mixture can cause up to 40% preference of blockiness in the growing polymer chains.

Le but de la thèse était de simuler le spectre d'absorption de méta-matériaux pour les applications photovoltaïques. Par méta-matériaux, nous entendons une assemblée d'objets de taille nanométrique situés à distance mésoscopique. L'idée sous-jacente est qu'en modifiant la taille du nano-objet et l'arrangement géométrique, on peut ajuster le seuil d'absorption. Pour calculer ces quantités, j'ai utilisé l'état de l'art du formalisme, c'est-à-dire des méthodes ab initio.La première étape du travail a été dédiée au calcul de l'absorption d'un objet isolé (tranche de silicium, graphène, hBN). Dans le cadre de codes périodiques, on utilise une supercellule avec du vide pour isoler l'objet, et une méthode a été développée précédemment dans le groupe de Spectroscopie Théorique du LSI, pour obtenir des résultats indépendants du vide. Elle est appelée Selected-G, et a été appliquée avec succès aux surfaces de silicium. Pour une tranche isolée, une expression modifiée du potentiel coulombien dans l'espace réciproque, appelé "slab potential", doit être utilisée. Pour valider l'utilisation du potentiel de slab pour le calcul de la matrice diélectrique microscopique, j'ai simulé les spectres de perte d'énergie d'électrons pour des empilements de quelques plans de graphène, et reproduit avec succès les données expérimentales disponibles. Cela a offert la possibilité d'étudier la dispersion du plasmon d'un plan de graphène, et discuter la nature des excitations électroniques dans ce système (transitions interband ou plasmon 2D).La second étape a été consacrée à l'étude du spectre d'absorption d'une assemblée de tranches en interaction. Comme il a été mis en évidence que le formalisme de supercellule agit comme une théorie de matériau moyen avec du vide, avec l'effet erroné d'avoir des spectres dépendant de la taille de la supercellule, j'ai renversé la procédure pour extraire le spectre de la tranche en interaction, affranchi du problème du vide. La faisabilité a été démontrée sur les tranches de hBN, dont le caractère semi-conducteur à large bande interdite évite les instabilités numériques.Cela a permis de comprendre la raison pour laquelle l'absorption de la tranche en interaction de silicium apparaît à plus basse énergie que celle du matériau massif: cela vient de la présence des états de surface dans la bande interdite de la structure de bandes du massif. Néanmoins, la différence avec la tranche isolée doit être encore étudiée.La troisième partie a été dédiée à l'étude de matériaux utilisés, ou candidats, aux applications photovoltaïques comme InP et InSe. J'ai étudié dans un premier temps les structures de bandes des massifs. Pour corriger la sous-estimation de la bande interdite calculée dans l'approximation de la densité locale (LDA), j'ai calculé les corrections GW, et utilisé la fonctionnelle d'échange et corrélation de Heyd-Scuseria-Ernzerhof (HSE). Le spectre d'absorption de InP massif a été calculé en résolvant l'équation de Bethe-Salpeter, qui permet de tenir compte des effets excitoniques. Comme ce calcul est très lourd numériquement, j'ai également comparé avec le calcul beaucoup plus léger de TDDFT avec le kernel à longue portée pour introduire les effets excitoniques. Pour le massif de InSe, j'ai calculé les corrections HSE pour les valeurs propres et obtenus un bon accord avec la bande interdite expérimentale. Les spectres obtenus en TDDFT, avec le kernel à longue portée, donne de bons résultats. J'ai commencé l'étude de tranches de ces deux matériaux. Des couches épaisses de InP et InSe ont été considérées et une reconstruction de surface (2x2) a été réalisée sur InP pour obtenir une surface semi-conductrice. La structure de bande LDA et les spectres d'absorption ont été calculés. Comme des systèmes d'une telle taille sont hors de portée des calculs de corrections HSE, l'étude s'est concentrés sur des tranches beaucoup plus fine de InSe. / The purpose of the thesis was to simulate the absorption spectrum of meta-materials for photovoltaic applications. By meta-material, we mean an assembly of nanometric size objects at mesoscopic distance. The underlying idea is that by adjusting the size of the nano-object and the geometric arrangement, one could tune the absorption edge. To calculate these quantities, I used state-of-the art formalism, namely ab-initio methods.The first step of the work has been dedicated to the calculation of the absorption of an isolated object (slab of silicon, graphene, hBN). In the framework of periodic codes, one uses a supercell with vacuum to isolate the object, and a method has been developed previously in the Theoretical spectroscopy group at LSI, to provide results independent of vacuum. It is called “Selectd-G” method, and was successfully applied to silicon surfaces. For an isolated slab, a modified expression of the reciprocal space Coulomb potential, called “slab potential”, must be used. To validate the use of the slab potential on the microscopic dielectric matrix, I have simulated Electron Energy Loss spectra for slabs of few graphene layers, and successfully reproduced available experimental data. This has also offered the possibility to study the plasmon dispersion of a single graphene layer, and discuss the nature of electronic excitations in the system (intraband transitions or 2D-plasmon).The second step has been dedicated to the study of the absorption spectrum of an array of interacting slabs. Since it has been evidenced that the supercell formalism acts as an effective medium theory with vacuum, with the spurious effect of having spectra dependent on the size of the supercell, I have reversed the procedure to extract the spectrum of the interacting slab, "cured" from the vacuum problem. First, the feasibility has been demonstrated on slabs of hBN, as their semi-conducting characteristics with a the large gap prevent numerical instabilities. Then, it has allowed us to understand the reason why the absorption of the interacting slab of silicon appears at lower energy than its bulk counterpart: it is due to the presence of surface states in the gap of the bulk band structure. Nevertheless, the difference with the isolated slab must be further investigated.The third part has been dedicated to the study of materials currently used or candidates for photovoltaic applications: InP and InSe. I have first studied the band structures of bulk InP and InSe. To correct for the underestimation of the band gap in the local density approximation (LDA), I have used GW corrections and the Heyd-Scuseria-Ernzerhof (HSE) exchange-correlation functional. The absorption spectrum for bulk InP has been calculated by means of the solution of the Bethe-Salpeter equation to correctly account for the excitonic effects. As expected, the experimental macroscopic function is well reproduced. Since the calculation is numerically demanding, I have also compared the results with the much lighter calculation using TDDFT where I used the long range kernel to mimic the excitonic effects. For bulk InSe, I have calculated the HSE corrections for the eigenvalues and obtained a good agreement with the experimental band gap. The spectrum obtained within TDDFT, with the long range kernel, gives satisfying results. We have started the calculations for slabs of these two materials. Thick slabs of InP and InSe have been considered and a 2x2 reconstruction have been performed for the InP slab to recover the semi-conducting surface. The LDA band structures and absorption spectra have been calculated. Then, such large systems being out of range of HSE corrections calculations, the study has been focused on much thiner slabs in the case of InSe.

La limite théorique d’efficacité d’une cellule solaire simple jonction est de ~29 %. Afin de dépasser cette limite, une des moyens les plus prometteurs est de combiner le silicium avec des matériaux III-V. Alors que la plupart des solutions proposées dans la littérature proposent de faire croître directement le matériau III-V sur substrat silicium, ce travail présente une approche innovante de fabriquer ces cellules solaires tandem. Nous proposons une approche inverse métamorphique, où le silicium cristallin ou SiGe est cru directement sur le matériau III-V par PECVD. La faible température de dépôt (< 200 °C) diminue les problèmes de différence de dilatation thermique, et le fait de croître le matériau IV sur le matériau III-V élimine les problèmes de polarité.La réalisation de la cellule tandem finale en SiGe/AlGaAs passe par le développement et l’optimisation de plusieurs briques technologiques. Tout d’abord, nous développons l’épitaxie à 175 °C de Si(Ge) sur des substrats de Si (100) dans un réacteur de RF-PECVD industriel. La réalisation de cellules solaires à hétérojonction à partir de ce matériau Si(Ge) crû par PECVD montre que ses performances électriques s’avèrent prometteuses. Nous obtenons pour un absorbeur de 1.5 µm des Voc qui atteignent 0.57 V. L’incorporation de Ge permet d’augmenter le JSC de 15.4 % jusqu’à 16.6 A/cm2 pour Si0.72Ge0.28.En parallèle, la croissance de cellules solaires AlGaAs a été développée, ainsi que sa fabrication technologique. Nous obtenons une efficacité de 17.6 % pour une cellule simple en Al0.22Ga0.78As. Nous développons aussi des jonctions tunnel, parties essentielles d’une cellule tandem dans une configuration à deux terminaux. Nous développons notamment le dopage n du GaAs en utilisant le précurseur DIPTe, et obtenons des jonctions tunnel ayant des courants pic atteignant jusqu’à 3000 A/cm2, rejoignant ainsi les résultats de l’état de l’art.Ensuite, nous étudions l’hétéro-épitaxie de Si sur GaAs par PECVD. Le c-Si montre d’excellentes propriétés structurales. Les premiers stades de croissance sont étudiés par diffraction des rayons X avec rayonnement synchrotron. Nous trouvons un comportement inattendu : le Si est relâché dès les premiers nanomètres, mais sa maille est tétragonale. Alors que le GaAs a un paramètre de maille plus grand que le Si, le paramètre hors du plan (a⏊) du Si est plus élevé que son paramètre dans le plan (a//). Nous trouvons une forte corrélation entre cette tétragonalité et la présence d’hydrogène dans la couche de silicium. D’autre part, nous montrons que le plasma d’hydrogène présent lors du dépôt PECVD affecte les propriétés du GaAs : son dopage diminue d’environ un ordre de grandeur lorsque le GaAs est exposé au plasma H2, dû à la formation de complexes entre le H et le dopant (C, Te ou Si). Le dopage initial peut être retrouvé après un recuit à 350 °C.Enfin, nous étudions la dernière étape de fabrication de la cellule tandem : le collage. Nous avons pu reporter une cellule simple inversée en AlGaAs sur un substrat hôte (en Si), retirer le substrat GaAs et effectuer les étapes de microfabrication sur un substrat 2 pouces. Des couches épaisses de Si (>1 µm) ont été crues avec succès sur une cellule AlGaAs inversée suivie d’une jonction tunnel. Le collage de cette cellule tandem, et la processus de fabrication technologique du dispositif final sont ensuite étudiés, afin de pouvoir caractériser électriquement la première cellule solaire tandem fabriquée par croissance inverse métamorphique de Si sur III-V. / Combining Silicon with III-V materials represents a promising pathway to overcome the ≈29% efficiency limit of a single c-Si solar cell. While the standard approach is to grow III-V materials on Si, this work deals with an innovative way of fabricating tandem solar cells. We use an inverted metamorphic approach in which crystalline silicon or SiGe is directly grown on III-V materials by PECVD. The low temperature of this process (<200 °C) reduces the usual thermal expansion problems, and growing the group IV material on the III-V prevents polarity issues.The realization of the final tandem solar cell made of SiGe/AlGaAs requires the development and optimization of various building blocks. First, we develop the epitaxy at 175°C of Si(Ge) on (100) Si substrates in an industrial standard RF-PECVD reactor. We prove the promising electrical performances of such grown Si(Ge) by realizing PIN heterojunction solar cells with 1.5µm epitaxial absorber leading to a Voc up to 0.57 V. We show that the incorporation of Ge in the layer increases the Jsc from 15.4 up to 16.6 A/cm2 (SiGe28%).Meanwhile, we develop the growth of AlGaAs solar cells by MOVPE and its process flow. We reach an efficiency of 17.6 % for a single Al0.22GaAs solar cell. We then develop the tunnel junction (TJ), essential part of a tandem solar cell with 2-terminal integration. We develop the growth of n-doped GaAs with DIPTe precursor to fabricate TJs with peak tunneling currents up to 3000 A/cm2, reaching state-of-the art TJs.Then, the hetero-epitaxy of Si on GaAs by PECVD is studied. c-Si exhibits excellent structural properties, and the first stages of the growth are investigated by X-ray diffraction with synchrotron beam. We find an unexpected behavior: the grown Si is fully relaxed, but tetragonal. While the GaAs lattice parameter is higher than silicon one, we find a higher out-of-plane Si parameter (a⏊) than in-plane (a//), contradicting the common rules of hetero-epitaxy. We find a strong correlation between this tetragonal behavior and the presence of hydrogen in the Si layer. We furthermore show that hydrogen also plays a strong role in GaAs: the doping level of GaAs is decreased by one order of magnitude when exposed to a H2 plasma, due to the formation of complexes between H and the dopants (C, Te, Si). This behavior can be recovered after annealing at 350°C.Finally, the last step of device fabrication is studied: the bonding. We successfully bonded an inverted AlGaAs cell, removed it from its substrate, and processed a full 2” wafer. We succeeded in growing our first tandem solar cells by growing thick layers (>1 µm) of Si on an inverted AlGaAs solar cells followed by a TJ. The bonding and process of this final device is then performed, leading, as a next step, to the electrical measurement of the very first tandem solar cell grown by inverted metamorphic growth of Si on III-V.

abstract: Silicon photonic technology continues to dominate the solar industry driven by steady improvement in device and module efficiencies. Currently, the world record conversion efficiency (~26.6%) for single junction silicon solar cell technologies is held by silicon heterojunction (SHJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). These solar cells utilize the concept of carrier selective contacts to improve device efficiencies. A carrier selective contact is designed to optimize the collection of majority carriers while blocking the collection of minority carriers. In the case of SHJ cells, a thin intrinsic a-Si:H layer provides crucial passivation between doped a-Si:H and the c-Si absorber that is required to create a high efficiency cell. There has been much debate regarding the role of the intrinsic a-Si:H passivation layer on the transport of photogenerated carriers, and its role in optimizing device performance. In this work, a multiscale model is presented which utilizes different simulation methodologies to study interfacial transport across the intrinsic a-Si:H/c-Si heterointerface and through the a-Si:H passivation layer. In particular, an ensemble Monte Carlo simulator was developed to study high field behavior of photogenerated carriers at the intrinsic a-Si:H/c-Si heterointerface, a kinetic Monte Carlo program was used to study transport of photogenerated carriers across the intrinsic a-Si:H passivation layer, and a drift-diffusion model was developed to model the behavior in the quasi-neutral regions of the solar cell. This work reports de-coupled and self-consistent simulations to fully understand the role and effect of transport across the a-Si:H passivation layer in silicon heterojunction solar cells, and relates this to overall solar cell device performance. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

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