A Kinetic Monte Carlo Study of Mesoscopic Perovskite Solar Cell Performance Behavior

Perovskite solar cells have received considerable attention in recent years due to their low processing cost and high energy conversion efficiency. However, the mechanisms of perovskite solar cell performance are not fully understood. Models based on probabilistic and statistical approaches can be used to simulate, optimize, and predict perovskite solar cell photovoltaic performance, and they can also guide experimental processing and fabrication conditions to achieve higher photovoltaic efficiency. This work developed a 3D model based on the kinetic Monte Carlo (KMC) approach to simulate 3D morphology of perovskite-based solar cells and predict their photovoltaic performance. The model incorporated the physical behavior of perovskite cells with respect to their charge generation, transport, and recombination characteristics. KMC simulation results showed that perovskite films with the pin holes-free and a homogenous perovskite capping layer of 400 nm thickness produced a maximum photovoltaic efficiency of 20.85%, resulting in minimal charge transport time (τt) and maximum charge carrier recombination lifetime (τr). Photovoltaic performance from the fabricated device has been used to validate this simulation model. This model provides significant conceptual advances in identifying current performance constraints and guiding novel device designs that enhance overall perovskite photovoltaic performance

Mass transport and electrochemical properties of La2Mo2O9 as a fast ionic conductor

La2Mo2O9, as a new fast ionic conductor, has been investigated widely due to its high ionic conductivity which is comparable to those of the commercialized materials. However, little work has been reported on the oxygen transport and diffusion in this candidate electrolyte material. The main purpose of this project was to investigate oxide ion diffusion in La2Mo2O9 and also the factors which could affect oxygen transport properties. Oxygen isotope exchange followed by Secondary Ion Mass Spectrometry (SIMS) measurements were employed to obtain oxygen diffusion profiles. A correlation between oxygen ion transport and the electrochemical properties such as ionic conductivity was built upon the Nernst Einstein equation relating the diffusivity to electrical conductivity. In-situ neutron diffraction and AC impedance measurements were designed and conducted to investigate the correlation between crystal structure and oxygen transport in the bulk materials. Other techniques, such as synthesis, microstructure studies, and thermal analysis were also adopted to study the electrochemical properties of La2Mo2O9. The results of the study on the effects of microstructure on oxygen diffusion in La2Mo2O9 revealed that the grain boundary component played a significant role in electrochemical performance, although the grain size seemed to have little influence on oxygen transport. The oxygen isotope exchange in 18O2 was successfully carried out by introducing a silver coating on the sample surface, which solved the main difficulty in applying oxygen isotope exchange on pure ionic conductors. The ionic conductivity obtained from the diffusion coefficients was consistent with the result from AC impedance spectroscopy. The number of mobile oxygen ions was estimated to be 5 per unit cell. There was a difference of oxygen self diffusion coefficient when the isotope exchange was conducted in 18O2 and H2 18O. The activation energy of oxygen diffusion in humidified atmosphere was higher than that measured in dry atmosphere. It indicated that the humidified atmosphere had affected oxygen transport in the material. The studies on hydroxyl incorporation and transport explained the decreased oxygen diffusion coefficients in wet atmosphere and also suggested proton conductivity in La2Mo2O9, which leads to further investigation on applications of La2Mo2O9 as a proton conductor. In-situ neutron diffraction and AC impedance measurement revealed a close relationship between crystal structure and ionic conductivity. The successful application of this technique provides a new method to simultaneously investigate crystal structure and electrical properties in electro-ceramics in the future

Material inspection using new electromagnetic testing technology : coplanar capacitive sensing technique

Les matériaux diélectriques jouent un rôle important dans les applications industrielles et les domaines de la recherche scientifique et leur utilisation a augmenté ces dernières années. Leurs applications concernent l'industrie moderne des circuits intégrés et les réseaux d'antennes compacts. De plus, les composites structuraux légers dans l'industrie aérospatiale, les armures corporelles en Kevlar et les composites à matrice céramique pour la stabilité thermique dans les environnements chauds des moteurs sont des exemples de certaines des applications récemment développées des matériaux diélectriques. Par conséquent, la détection des défauts de ces matériaux diélectriques devient très importante pour le contrôle du processus de fabrication, l'optimisation de la conception et des performances des appareils électriques, et la surveillance et le diagnostic du système. Par conséquent, le besoin de tests de contrôle non destructifs (CND) précis des matériaux structurels et fonctionnels diélectriques a également augmenté. Cependant, le CND de ces matériaux n'est pas aussi développé que celui des métaux et de nouvelles approches pour évaluer la qualité de ces matériaux lors de la fabrication et de la maintenance n'ont pas encore été développées. Par conséquent, il sera utile de développer de nouvelles méthodes telles que des techniques de détection capacitive qui peuvent surmonter certaines des restrictions associées à d'autres techniques d'évaluation des matériaux diélectriques. La simulation numérique utilisant la modélisation par éléments finis (FEM) tridimensionnelle (3D) est utilisée dans le logiciel COMSOL Multiphysics pour simuler la distribution du champ électrique à partir d'un capteur capacitif coplanaire et la façon dont il interagit avec divers échantillons composés de différents types de défauts. Une analyse détaillée FEM est fournie pour étudier les paramètres de conception, y compris la forme/taille/distance des électrodes coplanaires pour évaluer et identifier les caractéristiques importantes des électrodes capacitives coplanaires, telles que la pénétration et la force du champ électrique en fonction du capteur propriétés géométriques. De plus, l'influence des différentes fréquences, du décollement et de la présence ou de l'absence d'une plaque de blindage métallique et d'une électrode de garde sur le résultat de sortie est analysée par la même méthode. En outre, la distribution du champ électrique, en fonction du nombre d'électrodes, à partir d'un capteur capacitif coplanaire multi-électrodes avec différents agencements d'électrodes d'entraînement et de détection, et comment ce champ peut être modifié en changeant l'agencement est simulé et illustré par le MEF 3D. Des expériences physiques sont réalisées avec plusieurs capteurs capacitifs coplanaires pour vérifier les résultats de la simulation et évaluer les performances de la sonde. Dans ces expériences, les performances d'imagerie du capteur, l'effet des paramètres de conception sur les performances du capteur, l'impact des divers matériaux testés et la faisabilité de la sonde capacitive coplanaire multi-électrodes seront pris en compte. La comparaison des résultats de simulation numérique et d'expériences physiques montre qu'ils sont en bon accord qualitatif.Dielectric materials have an extensive role in both industrial applications and scientific research areas and their use has increased in recent years. Furthermore, lightweight structural composites in the aerospace industry, Kevlar body-armour and ceramic-matrix composites for thermal stability in hot engine environments are examples of some of the recently developed applications of dielectric materials. Therefore, the flaw detection of these dielectric materials becomes markedly important for the process control in manufacturing, optimization of electrical apparatus design and performance, and system monitoring and diagnostics. Consequently, the need for accurate non-destructive testing (NDT) of dielectric structural and functional materials has also been increased. However, the NDT of such materials is not as well developed as those for metals and new approaches to evaluate the quality of these materials during manufacturing and maintenance have not yet been expanded. Therefore, it will be valuable to develop new methods such as capacitive sensing techniques which can overcome some of the restrictions associated with other techniques for assessing dielectric materials. The numerical simulation using three dimensional (3 D) Finite Element Modelling (FEM) is employed in COMSOL Multiphysics software to simulate the electric field distribution from a coplanar capacitive sensor and the way it interacts with various specimens composed of different types of defects. A detailed analysis FEM is provided to study the design parameters including the shape/size/distance of the coplanar electrodes to assess and identify the important features of the coplanar capacitive electrodes, such as the penetration and strength of the electric field as a function of sensor geometrical properties. In addition, the influence of the different frequencies, lift-off, and the presence or absence of a metal shielding plate and guard electrode on the output result is analyzed by the same method. Besides, the electric field distribution, as a function of the number of electrodes, from a multi-electrode coplanar capacitive sensor with different arrangements of driving and sensing electrodes, and how this field may be altered by changing the arrangement is simulated and illustrated by the 3D FEM. Physical experiments are carried out by several coplanar capacitive sensors to verify the simulation results and evaluate the performance of the probe. In these experiments, the imaging performance of the sensor, the effect of design parameters on the sensor performance, the impact of various materials under test, and the feasibility of the multi-electrode coplanar capacitive probe will be considered. Comparison of the numerical simulation results and physical experiments illustrate that they are in good qualitative agreement

Novel processing approaches for thin film solar and related technologies.

A growing population along with developing nations are increasing the demand for energy. The International Energy Agency forecasts a global electricity demand increase of 70 percent by 2040. This is an increase from nearly 18 TW to over 30 TW. The sun can be a great clean source of achieving this energy demand. Despite the large solar industry development, the market is still growing as solar energy only accounted for 0.87% of the global energy production in 2013. The opportunity exists to manufacture more affordable solar energy that can penetrate more of the global energy market. In this dissertation, a photonic-based manufacturing technique called intense pulsed light (IPL) was investigated to enhance the photovoltaic properties of CdTe, better understand the CdCl2 treatment used to create higher efficiency CdTe solar devices, enable the first sintering and efficiency enhancement of perovskite solar cell (PSC), and study the possible conversion of a stable 2D perovskite to a 3D perovskite. CdTe thin films grown by low temperature electrodeposition were treated for the first time with IPL. The low temperature electrodeposition growth resulted in films consisting of nanoparticles, with reduced melting point temperatures. In combination with the high temperature rise produced by the pulses of light, the lower melting temperature resulted in pores/voids being filled as well as enhanced grain growth. As a result, pin-holes and grain boundary recombination were diminished. Subsequently the fill factors of PV devices created using this technology significantly increased. In addition, the IPL also successfully improved the crystallinity in the CdTe films by photonically initiating the popular CdCl2 treatment. To understand the mystery behind the mechanism of the CdCl2 treatment, low temperature PL was utilized and new electrodeposition precursors resulting from the study improved device efficiencies. Photoactive perovskite CH3NH3PbI3 layers were successfully sintered with a novel IPL treatment with efficiencies exceeding 12%. The processing time was reduced to 2 ms, which was significantly faster than those from previous reports. Additionally, the average performance of the IPL-processed samples showed an improvement compared to the hot- plate-processed samples. This advance creates an exciting new method to quickly create dense layers of perovskite, eliminating the rate-limiting annealing step detrimental to industry adoption, and shows the first known occurrence of sintering in CH3NH3PbI3 perovskite particles. Lastly, the fast photonic processing of the IPL enabled the first conversion of a stable 2D perovskite structure into a 3D structure. This caused an band gap shift from 2.0 eV to 1.6 eV and showed the capabilities of band gap tuning enabled by the IPL. While this work is the first documentation of band gap tuning enabled by a photonic effect, it presents a possible inexpensive manufacturing technique that could use one material to create several different colors for the future development of pixel-based LED displays

Optical and Mechanical Investigation of InAs /GaAs Quantum Dots Solar Cells and InAs Nanowires for the Application of Photovoltaic Device

Self-assembled quantum dots (QDs) and nanowires (NWs) are currently the subjects of extensive study due to their promising applications in optoelectronic devices. In order to enhance understanding of the short circuit current improvement in InAs/GaAs quantum dots solar cell (QDSC), the mechanisms of carrier escape by thermal activation and tunneling from InAs quantum dots (QDs) confinements in InAs/GaAs QDSCs are investigated. The fitted activation energy of electrons from temperature dependent photoluminescence (TDPL) is 114 meV. Using this fitted activation energy, calculated thermal escape time and tunneling time of electrons from the ground state of the QDs are 10-12 seconds and 10-6 seconds at 300K, respectively. These results indicate that at room temperature thermal escape is dominant for electrons escape from ground state. At low temperature (8K), tunneling mainly affects the electrons escape from ground state, since thermal energy cannot support electrons to overcome the fitted activation energy (barrier, 114 meV). In addition, in order to describe the new physics and achieve the final success in nanowire device for photovoltaic applications, the first step is to develop high-quality semiconductor nanowires on the selected substrate. Morphological and crystal structure characterizations were performed via SEM and TEM for InAs nanowire samples grown with and without Au seed on GaAs substrate using metal organic vapor phase expitaxy (MOVPE). Several major factors affect the NW growth in terms of shape, density, etc. For nanowire growth with Au seed, its growth direction mainly depends on the substrate, while its uniformity is initially related to the Au seed coverage. III/V ratio affects the NW aspect ratio (length/bottom width), ranged from 12.00 to 38.93. Increasing temperature accelerates the growth rate in both axial and radial directions. NWs grown without Au seed using a pattern mask show no tapering along the growth direction with an average diameter of 26 nm. All defects stop in the buffer layer when InAs nanowires grown with an Au seed, but a mix of ZB and WZ crystal phases were observed along the growth direction of nanowire. InAs NWs grown without Au seeds also show a mixture of different crystal phases along the growth direction. The diameter of InAs nanowire should be further reduced to 3-6 nm as to achieve PL response between 1000~1300 nm

Novel Solar Cells Based on Two-Dimensional Nanomaterials and Recycled Lead Components

To meet the rapidly growing demand for energy and reduce the use of dwindling fossil fuels, the efficient utilization of renewable energy is a constant pursuit globally. Because solar cells convert vastly available sunlight into electricity, developing high-performance and low-cost solar cells is a top strategy for future energy supply. Dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are the most promising choices. In the meantime, highly concentrated sulfuric acids from retired lead-acid batteries become an environmental concern, and lead contamination in drinking water raises concerns in general public. This study addresses both above-mentioned problems by using two-dimensional (2D) nanomaterials and recycled lead for solar cell fabrication. Firstly, 2D nanomaterials are used to improve the performance of solar cells as a counter electrode in DSSCs and a hole transport material (HTM) in PSCs. MoS2/graphene hybrids are hydrothermally prepared at different temperatures. A phase transition is observed from 150 to 210 °C, and the reaction temperature of 180 °C leads to the best performing 2D nanomaterial hybrids for electrochemical catalysis. Semi-transparent PSCs are explored in smart buildings to simultaneously adapt to indoor lighting conditions and convert part of sunlight into electricity. Furthermore, through crumbling graphene oxide with copper thiocyanate at 650 °C, a new HTM is prepared for PSCs with steady, high performance and a low cost. Secondly, the recovery of lead contents from waste acid and contaminated water is studied by using nanocarbon materials. The lead removal in acid is evaluated through adsorption, filtration, and capacitive deionization (CDI). The removal rate is limited, but as-obtained lead wastes are then successfully reformed to produce perovskite precursors. A CDI process through sulfur-treated carbon materials is studied to selectively collect lead contaminants in water. Complete lead recovery and reformation for perovskite precursors is explored to obtain PSCs featuring a low cost and an overall positive environmental impact. This study addresses a full life cycle of solar cell materials to meet sustainable requirements. Taking advantage of outstanding properties of 2D nanomaterials, results from the study will lead to next-generation solar cells featuring enhanced performance, a lower cost, and better environmental sustainability

Investigation and suppression of semiconductor–oxide related defect states : from surface science to device tests

Many present challenges in semiconductor technology are related to utilizing solid structures with atomic scale dimensions and materials with higher charge carrier mobility and/or other readily controllable properties. These include many surface-related problems because the ratio of surface parts of devices to the whole material volume increases all the time in practical device structures. One of the major problems has been oxidation of semiconductor surfaces during the manufacturing of devices. This PhD work deals with the surface and oxide interface properties of different III–V semiconductors induced by the oxidation, the study of which is imperative in realizing devices with desired characteristics. The general goal has been in finding answers to these problematic issues on atomic scale, and whether they can be resolved with simple parameter control of a thermal oxidation treatment. Much of the work leans on a previous novel finding of crystalline oxide phases on indium-containing III–V semiconductor (100) surfaces. Various aspects of applicability of such a structure in real semiconductor devices are considered in this work. Common denominator in all of the experiments and studies is that the initial investigations were carried out in very controlled environment in ultrahigh-vacuum: detailed basics and initial characterizations were carried out with high resolution and precision surface science methods. In particular, this work has resulted in novel crystalline oxide phases observed on GaSb(100) and InSb(111)B semiconductor surfaces. They have been extensively discussed from an applied point of view as well as their fundamental characteristics, relating to their already previously studied counterpart, InSb(100). Furthermore, beneficial passivating characteristics of a stabilizing crystalline InOx interfacial layer beneath an Al2O3 and reasons behind such behavior are demonstrated for InGaAs IR detector device structure. This thesis provides background of semiconductors, their surfaces, interfaces, and semiconductor technology as appropriate, research methods utilized, and briefly summarizes the findings of the work