Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Cressler, John D.; Committee Member: Papapolymerou, John; Committee Member: Ralph, Stephen; Committee Member: Shen, Shyh-Chiang; Committee Member: Zhou, Hao Min.

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2007. / Cressler, John, Committee Chair ; Laskar, Joy, Committee Member ; Papapolymerou, Ioannis, Committee Member ; Zhou, Haomin, Committee Member ; Milor, Linda, Committee Member.

Les récents développements dans le domaine du photovoltaïque ont permis l'apparition de cellules utilisant des technologies nouvelles. Parmi elles on trouve les cellules photovoltaïques hybrides, cependant les méthodes de fabrication utilisées actuellement présentent des défauts. Cette thèse a pour but de proposer deux nouvelles approches pour la préparation de cellules photovoltaïques hybrides sous forme d'hétérojonctions en volume d'un oxopolymère de titane et d'un polythiophène. Dans un premier temps la formation de mésostructures vermiculaires dans des couches minces de TiO2 par autoassemblage par évaporation a été étudiée, l'oxopolymère amorphe obtenu a ensuite été cristallisé en conditions douces. Ces résultats ont ensuite été utilisés afin de préparer des matériaux hybrides à partir d'homopolymères de thiophène portant des substituants hexyl et acide, ainsi qu'à partir de copolymères. Des matériaux hybrides ne présentant pas de macroségrégation ont été obtenus pour un polymère portant des fonctions acides et pour les copolymères. Dans un second temps l'électrochimie du ferrocène et du cuivre ont été étudiés dans des films de TiO2 mésostructuré et mésoporeux, puis deux dérivés thiophène : le mot et l'edot ont été électropolymérisés dans ces structures. Des cellules photovoltaïques ont été préparées en utilisant ces matériaux hybrides et caractérisées par des mesures de la courbe I/E ainsi que par l'étude du rendement quantique externe, des facteurs de forme et des rendements ont été calculés. En conclusion, deux nouvelles approches de synthèse de matériaux hybrides ont été proposées et menées à bien les propriétés photovoltaïques et ces matériaux ont été mesurées. / Recent advances in the field of photovoltaics have led to the emergence of new solar cell technologies. Among them can be found the hybrid solar cells, unfortunately the way such cells are built is still a source of problems. The aim of this phd is to develop two new approaches in the synthesis of hybrid materials as bulk heterojunctions. In first place the titanium dioxide component vas prepared by sol-gel process and its mesostructure was studied, low temperature crystallization was also investigated. Those results were used in order to prepare hybrid materials from preformed polymers. The behavior of polythiophènes with hexyl and carboxylic acid functions were used as well as copolymers bearing both functions. Hybrids without macrosegregations phenomena were obtained using acid bearing homopolymers as well as copolymers. The second approach was about investigating the electrochemical behavior of ferrocene and copper ions inside the mesoporosity, this first study was followed by a study of the electropolymerization of mot and edot inside the porosity in order to prepare hybrid materials. The obtained hybrids were studied in solar cells by measuring the I/V curve as well as the external quantum efficiency, fill factors and efficiencies were also obtained. To conclude, both approaches leaded to hybrid materials with measurable photovoltaic properties.

L’objectif de ce travail de thèse a été d’étudier une alternative à la cellule photovoltaïque à hétérojonction classique de silicium amorphe/cristallin avec un matériau (GaP) qui permettrait une amélioration de rendement grâce à ses propriétés optiques et électriques. L’étude du potentiel des hétérojonctions GaP/Si pour des applications PV nous a amené à étudier chacun des aspects critiques inhérents à leur réalisation. La préparation chimique de la surface des substrats et les mécanismes qui contrôlent la structuration de la surface de Si(100) ont été étudiés afin d’obtenir une surface de silicium mono-domaine (à marches diatomiques) et faiblement rugueuse par homoépitaxie (dépôts par UHV-CVD). Cette étude a été complétée par l’étude de l’influence de la préparation de surface (préparation chimique et homoépitaxie) du substrat sur la qualité cristalline du GaP déposé en deux étapes par MEE et MBE. La croissance de GaP par MEE a par la suite été effectuée sur des substrats de Si(100) ayant uniquement subi une préparation chimique de surface. Les paramètres de la séquence de croissances MEE ont été étudiés et ajustés afin d’optimiser la phase de nucléation du GaP. La qualité structurale des dépôts a été évaluée par des caractérisations par AFM et DRX. Les couches minces de faibles épaisseurs (20nm) présentent une faible rugosité de surface équivalente à une homoépitaxie et une fraction volumique de MTs inférieure à la limite de détection. La croissance MEE permet d’assurer une nucléation 2D. Cependant les caractérisations par TEM et STM révèlent la présence de parois d’antiphase. En parallèle, la simulation de structures HET GaP/Si (effectuée grâce au programme AFORS-HET) et la réalisation de diodes et de démonstrateurs cellules GaP/Si ont permis de démontrer les optimisations apportées par l’utilisation d’un émetteur de GaP. Ces composants ont été étudiés par caractérisations optiques et électriques. Nous avons constaté une limitation des performances due à la présence de pièges à l’interface et dans le volume. Ces différentes études ont donc permis d’identifier les verrous technologiques à lever pour exploiter pleinement les cellules à hétérojonctions GaP sur silicium afin d’obtenir des hauts rendements photovoltaïques. / The main objective of this thesis is to study an alternative to conventional amorphous/crystalline silicon heterojunction solar cell using gallium phosphide (GaP) as an emitter layer. This would allow a performance improvement because of its optical and electrical properties. The potential of GaP/Si heterojunction solar cells have been evaluated by studying each of the critical issues inherent to their fabrication process. The chemical preparation of the substrates surface and the mechanisms controling the structure of the Si (100) surface have been studied in order to obtain a single domain silicon surface (with diatomic steps) and slightly roughened by homoepitaxy (UHV-CVD). This work was completed by the study of the impact of surface preparation (chemical preparation and homoepitaxy) of the substrate on the crystalline quality of GaP deposited in two steps by MBE and MEE. The growth of GaP by MEE was subsequently carried out on Si(100) substrates having only undergone a chemical surface preparation. MEE growth sequence parameters were studied and adjusted to optimize GaP nucleation. The structural quality of the thin films was evaluated by AFM and XRD characterizations. Thin films of 20 nm have lower surface roughness equivalent to an homoepitaxy and a volume fraction of MTs below the detection limit. The MEE growth ensures a 2D nucleation. However, TEM and STM characterizations reveal the presence of antiphase boundaries. In parallel, simulations of the structure HET GaP/Si (with AFORS-HET) have been performed to evaluate the potential of the structure. First, diodes and demonstrator cells with GaP/Si junction have been fabricated and optically/electrically characterized. Limitations in performance due to the presence of traps at the interface and silicon volume degradation have been observed. All this work has allowed us to identify the technological issues to overcome in order to fully exploit the GaP/Si heterojunction cells to improve solar cell performance.

We present oxide bipolar heterojunction diodes consisting of p-type ZnCo2O4 and n-type ZnO fabricated by pulsed-laser deposition. Hole conduction of ZnCo2O4 (ZCO) was evaluated by Hall and Seebeck effect as well as scanning capacitance spectroscopy. Both, ZCO/ZnO and ZnO/ZCO type heterostructures, showed diode characteristics. For amorphous ZCO deposited at room temperature on epitaxial ZnO/Al2O3 thin films, we achieved current rectification ratios up to 2x1010, ideality factors around 2, and long-term stability.

Les cellules à hétérojonctions de silicium (HET-Si) sont basées sur un substrat de silicium cristallin (c-Si) dopé n (p), une couche très fine de passivation (en général du silicium amorphe (a-Si:H) non dopé), et une couche d’une dizaine de nanomètres de silicium amorphe dopé p (n). Ces cellules atteignent aujourd’hui des rendements de l’ordre de 26% (record de 26,6% par l’entreprise Kaneka en 2017). Un des axes importants de recherche sur les cellules HET-Si porte sur l’étude de l’interface c-Si/a-Si:H qui est un élément clé dans le rendement des cellules. Ce rendement dépend en particulier de la présence d’états recombinants à l’interface c-Si/a-Si:H. Nous nous sommes donc tout particulièrement intéressés aux défauts d’interface en développant un calcul basé sur le modèle du réservoir de défauts (Defect-Pool Model ou DPM) dans le silicium amorphe et en corrélant nos résultats de modélisation avec des résultats expérimentaux de mesure de durée de vie. Afin de déterminer les caractéristiques de l’interface c-Si/a-Si:H, nous avons procédé comme suit : (1) Calcul de la densité d’états (DOS) volumique dans les couches de a-Si:H (dopé et non dopé), en nous appuyant sur le DPM. Dans ce modèle, la DOS varie en fonction notamment de la position du niveau de Fermi par rapport au bord de bande. La courbure des bandes de la jonction a-Si:H/c-Si implique ainsi une variation spatiale de la DOS dans le a-Si:H. (2) Calcul de la DOS surfacique à l'interface par projection des états volumiques présents à l’interface dans le a-Si:H. (3) Calcul des taux de recombinaison puis de la durée de vie effective sur des structures symétriques a-Si:H/c-Si/a-Si:H et comparaison avec des résultats expérimentaux. Nous avons ainsi pu étudier l’impact des paramètres matériaux du a-Si:H sur la durée de vie effective des porteurs minoritaires. L’évolution de la durée de vie avec les paramètres du a-Si:H est parfois contre-intuitive car deux phénomènes de passivation liés à la position du niveau de Fermi à l’interface s’opposent : passivation par la diminution de la densité d’états à l’interface et passivation par effet de champ. Le seul calcul de la DOS à l’interface ne suffit pas toujours à expliquer les variations de durées de vie, un calcul complet sous lumière est nécessaire. Nous avons montré que l’impact de certains paramètres du DPM peut-être grand sur la DOS mais faible sur la durée de vie effective à cause de cette compensation entre les phénomènes de passivation. Nous avons également étudié des structures correspondant aux faces avant : (p)a-Si:H/(i)a-Si:H/(n)c-Si(PIn) et arrière : (n)a-Si:H/(i)a-Si:H/(n)c-Si(NIn) des cellules HET-Si. Nos simulations permettent de montrer que les interfaces NIn sont moins critiques en terme de recombinaisons que les interfaces de type PIn. Nous montrons que la recombinaison aux interfaces PIn est dominée par la capture des électrons par les liaisons brisées de silicium chargées positivement. Nous montrons également que l’énergie d’Urbach est un paramètre qui joue de manière importante dans le calcul de la durée de vie effective et que l’utilisation de valeurs fixes de cette énergie d’Urbach dans la couche de passivation ne permet pas de reproduire les tendances expérimentales dans les structures avec des interfaces PIn. Nous proposons un modèle de variation de l’énergie d’Urbach avec l’épaisseur de la couche de passivation, qui permet de reproduire les tendances expérimentales pour les faibles épaisseurs de la couche de passivation mais qui demande à être complété pour de plus grandes épaisseurs. / Silicon heterojunction (Si- HET) solar cells are based on an n-doped (p-doped) crystalline silicon (c-Si) substrate, a very thin (a few nanometers) passivation layer of undoped hydrogenated amorphous silicon (a-Si:H) and a layer of p-doped (n-doped) a-Si:H, approximately 10 nanometer- thick. These cells currently lead the performance of silicon solar cells with conversion efficiencies in the order of 26% (with a record of 26.6% being achieved by the Kaneka company in 2017). One of the major focal points of research in Si- HET cells is the study of the c-Si/a-Si:H interface, which is a key factor in the cells' efficiency. In particular, this efficiency is strongly dependent on the recombination states at the interface between c-Si and a-Si:H. We therefore focused on developing a model of recombination through interface defects, which were evaluated based on the Defect-Pool Model (DPM) in a-Si:H. We calculated the effective lifetime vs excess carrier density curves and their dependence on the undoped a-Si:H passivation layer thickness and compared them to experimental results.In order to determine the characteristics of the c-Si/a-Si:H interface, we proceeded as follows: (1) Calculation of the volumic density of states (DOS) in a-Si:H layers (doped and undoped) using the DPM. In this model, the DOS varies as a function of the position of the Fermi level in relation to the band edge. The band bending at the a-Si:H/c-Si interface thus implies a spatial variation of the DOS in a-Si:H. (2) Calculation of the surface DOS at the interface by projection from the volumic states present in a-Si:H at the interface. (3) Calculation of the recombination rates and of the effective lifetime curves for symmetrical a-Si:H/c-Si/a-Si:H structures and comparison with experimental results. Thus we were able to study the impact of material parameters of a-Si:H on the effective lifetime curves. The change in lifetime as a function of a-Si:H parameters is sometimes counter-intuitive because two passivation mechanisms, namely passivation by field-effect or by the reduction of the DOS at the a-Si:H/c-Si interface, have opposed behavior in relation to the position of the Fermi level at the interface. A simple calculation of the DOS at the interface is not, therefore, sufficient to explain variations in lifetime, and a complete calculation of effective lifetime under illumination is required and has been performed. We demonstrate that the impact of certain DPM parameters may have a significant effect on the DOS but only a minor effect on the effective lifetime due to the compensation by the field-effect passivation. Moreover we have studied both types of silicon heterojunctions, (p)a-Si:H/(i)a-Si:H/(n)c-Si(PIn), and (n)a-Si:H/(i)a-Si:H/(n)c-Si(NIn) that are used as front emitter and back surface field junctions, respectively, in double-side contacted silicon Si-HET solar cells. Our simulations allowed us to emphasize that NIn interfaces are less critical in terms of recombination than PIn interfaces. We demonstrate that recombination at PIn interfaces is dominated by the capture of electrons by positively charged silicon dangling bonds. We further show that the Urbach energy is the major a-Si:H parameter that determines the effective lifetime in Si-HET solar cells and that the use of fixed values for this Urbach energy in the passivation layer whatever the layer thickness does not permit the experimental trends of PIn interfaces to be reproduced. Instead, we propose a model featuring that the Urbach energy decreases with the thickness of the passivation layer, which does allow experimental trends to be reproduced for very thin passivation layers (< 10 nm), but which requires further elaboration for larger thicknesses, for instance with a combined bandgap variation.

Perovskite solar cells (PSCs) have emerged as a promising renewable energy technology in recent years. However, their path towards industrial production and commercialization presents challenges that demand innovative solutions. This doctoral thesis is dedicated to addressing two pivotal issues in the development of PSCs: (1) the development of a fabrication method compatible with traditional semiconductor industry processes and (2) the exploration of approaches to improve device performance and stabilityThe present thesis separates these two issues into three parts. First, the fabrication method of MA-free perovskites via vacuum vapor deposition process is proposed. CsBr is added to FAPbI3giving FAxCs1-xPbI3-xBrx. to maintain the stable black perovskite phase. Furthermore, the effect of the thermal annealing process on perovskite films with different stoichiometric ratios was explored. It was found that thermal annealing enhances the crystallinity of both FAI-poor and stoichiometric films. For FAI-poor perovskite films, an increase in absorption as well as reduction in defect concentrations was achieved through annealing process. However, the opposite effect was observed for FAI-rich films. By optimizing the fabrication processes, a solar cell device with an efficiency of 16.6% was acquired. However, these vapor-deposited devices still exhibit lower performances compared to those prepared using solution processes, indicating the need for further improvements in perovskite layer composition and interfacial properties to enhance their efficiency. The second part of this dissertation demonstrates the concept of phase heterojunction (PHJ) solar cells by combining two polymorphs of the same material from the evaporation process (γ-CsPbI3) and solution process (β-CsPbI3). It was discovered that the photovoltaic parameters of these PHJ devices significantly surpass those of either single-phase device, resulting in a maximum power conversion efficiency of 20.1%. The enhancement comes from the following three factors: efficient passivation of the β-CsPbI3 by the larger bandgap γ-CsPbI3, an increase in the built-in potential of the PHJ devices enabled by the energetic alignment between the two phases, and enhanced absorption of light resulting from narrower band-gap β-CsPbI3 in the PHJ structure. The approach demonstrated here offers new possibilities for developing photovoltaic devices based on polymorphic materials. In the final part, a 1D-3D dimensional junction formed spontaneously by a two-step process is presented. It was found that the morphology, energy alignment, and defects of the buried interface were improved by creating 1D perovskites. In addition, strip-shaped PbI2 domains and voids are eliminated which ultimately enhances photovoltaic performance and stability. These improvements can be attributed to the passivation effect of the 1D perovskite and better energetic alignment. Furthermore, this dimensional junction strategy also shows potential for use in large-area devices.

No description available.

The objective of the dissertation is to combine the recent Mixed-Mode reliability stress studies into a single text. The thesis starts with a review of silicon-germanium heterojunction bipolar transistor fundamentals, development trends, and the conventional reliability stress paths used in industry, after which the new stress path, Mixed-Mode stress, is introduced. Chapter 2 is devoted to an in-depth discussion of damage mechanisms that includes the impact ionization effct and the selfheating effect. Chapter 3 goes onto the impact ionization effect using two-dimensional calibrated MEDICI simulations. Chapter 4 assesses the reliability of SiGe HBTs in extreme temperature environments by way of comprehensive experiments and MEDICI simulations. A comparison of the device lifetimes for reverse-EB stress and mixed-mode stress indicates different damage mechanisms govern these phenomena. The thesis concludes with a summary of the project and suggestions for future research in chapter 5.

The objective of the proposed work is to study the behavior of SiGe HBTs at cryogenic temperatures and its relation to device scaling and optimization. Not only is cryogenic operation of these devices required by space missions, but characterizing their cryogenic behavior also helps to investigate the performance limits of SiGe HBTs and provides essential information for further device scaling. Technology computer aided design (TCAD) and sophisticated on-wafer DC and RF measurements are essential in this research. Drift-diffusion (DD) theory is used to investigate a novel negative differential resistance (NDR) effect and a collector current kink effect in first-generation SiGe HBTs at deep cryogenic temperatures. A theory of positive feedback due to the enhanced heterojunction barrier effect at deep cryogenic temperatures is proposed to explain such effects. Intricate design of the germanium and base doping profiles can greatly suppress both carrier freezeout and the heterojunction barrier effect, leading to a significant improvement in the DC and RF performance for NASA lunar missions. Furthermore, cooling is used as a tuning knob to better understand the performance limits of SiGe HBTs. The consequences of cooling SiGe HBTs are in many ways similar to those of combined vertical and lateral device scaling. A case study of low-temperature DC and RF performance of prototype fourth-generation SiGe HBTs is presented. This study summarizes the performance of all three prototypes of these fourth-generation SiGe HBTs within the temperature range of 4.5 to 300 K. Temperature dependence of a fourth-generation SiGe CML gate delay is also examined, leading to record performance of Si-based IC. This work helps to analyze the key optimization issues associated with device scaling to terahertz speeds at room temperature. As an alternative method, an fT -doubler technique is presented as an attempt to reach half-terahertz speeds. In addition, a roadmap for terahertz device scaling is given, and the potential relevant physics associated with future device scaling are examined. Subsequently, a novel superjunction collector design is proposed for higher breakdown voltages. Hydrodynamic models are used for the TCAD studies that complete this part of the work. Finally, Monte Carlo simulations are explored in the analysis of aggressively-scaled SiGe HBTs.