Academic literature on the topic 'Photovoltaic nanostructures'

In this PhD thesis, vertical arrays of zinc oxide (ZnO) nanowires (NWs) are synthesized in a CVD system and then deposited on patterned electrodes using dielectrophoresis (DEP). The nanowire devices illustrate 4 orders of magnitude increase in conductivity when exposed to ultra violet (UV) irradiation of 1220 μW/cm². The UV response has a fast component, due to electron-hole generation, as well as a slower component, attributed to the release of oxygen. Moreover, due to the increased electron density in the presence of UV, the type of oxygen species on the surface of ZnO changes to more reactive negative ions. In addition, when the pressure is decreased to 0.05 mBar, the conductivity of the NWs increases ∼ 2 and 3.5 times for NWs with 300-nm and 100-nm diameter, respectively. For the first time, UV irradiation is used to improve the carbon monoxide (CO) sensing properties of ZnO. When exposed to 250 μW/cm² UV irradiation, not only the sensitivity increases more than 75%, but also a repeatable and recoverable response is obtained, which is due to formation of more reactive oxygen ions. For the same reason, when the temperature is elevated, higher sensitivity to CO is achieved. The devices demonstrate exponential sensitivities of more than 5 decades to 60% increase in relative humidity (RH) at room temperature, which is a record for ZnO NW based RH sensors. A novel, low-cost and simple technique is developed for fabrication of sensors based on solution processed ZnO nanoparticles (NPs) by simply sketching the electrode lines and painting the NP ink. Sensors show 2000 times increase in conductivity when exposed to 1220 μW/cm² UV irradiation and more than 200% increase in current when exposed to 5-mins of CO pulse at room temperature. Furthermore, this thesis presents efficient (3.8%) inverted organic photovoltaic devices based on a P3HT:PCBM bulk heterojunction blend with improved charge-selective layers. ZnO NP films with different thicknesses are deposited on the transparent electrodes as a nano-porous electron-selective contact layer. The optimized inverted devices show exceptional short circuit current, which is related to increased quantum efficiency.

Thesis (Ph. D.)--University of California, San Diego, 2009.<br>Title from first page of PDF file (viewed August 20, 2009). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references.

Recently extensive efforts have been spent in order to achieve all silicon based photonic devices exploiting the efficient light emission from nanostructured silicon systems. In this thesis, silicon based nanostructures have been investigated for electro-optical and photovoltaic applications. The thesis focused on three application areas of silicon nanostructures: Light emitting diode (LED), light modulation using quantum confined Stark effect (QCSE) and photovoltaic applications. In the context of LED applications, ZnO nanocrystal/silicon heterojunctions were investigated. Contrary to observation of pure ultraviolet photoluminescence (PL) from ZnO nanocrystals that were synthesized through vapor liquid solidification (VLS) method, visible emissions were observed in the electroluminescence (EL) due to defect states of ZnO. The discrepancy between these emissions could be ascribed to both change in excitation mechanisms and the defect formation on ZnO nanocrystals surface during device fabrication steps. EL properties of silicon nanocrystals embedded in SiO2 matrix were also systematically studied with and without Tb doping. Turn-on voltage of the Tb doped LED structures was reduced below 10 V for the first time. Clear observation of QCSE has been demonstrated for the first time in Si nanocrystals embedded in SiO2 through systematic PL measurements under external electric field. Temperature and size dependence of QCSE measurements were consistently supported by our theoretical calculations using linear combination of bulk Bloch bands (LCBB) as the expansion basis. We have managed to modulate the exciton energy as high as 80 meV with field strength below MV/cm. Our study could be a starting point for fabrication of electro-optical modulators in futures for all silicon based photonic applications. In the last part of the thesis, formation kinetics of silicon nanowires arrays using a solution based novel technique called as metal assisted etching (MAE) has been systematically studied over large area silicon wafers. In parametric studies good control over nanowire formation was provided. Silicon nanowires were tested as an antireflective layer for industrial size solar cell applications. It was shown that with further improvements in surface passivation and contact formation, silicon nanowires could be utilized in very efficient silicon solar cells.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015.<br>Cataloged from PDF version of thesis. Vita. Page 296 blank.<br>Includes bibliographical references.<br>In several electronic, electrochemical and photonic systems, the organization of materials at the nanoscale is critical. Specifically, in nanostructured heterojunction solar cells, active materials with high surface area and continuous shapes tend to improve charge transport and collection, and to minimize recombination. Organizing nanoparticles, quantum dots or organic molecules intro three-dimensional structures can thus improve device efficiency. To do so, biotemplates with a wide variety of shapes and length scales can be used to nucleate nanoparticles and to organize them into complex structures. In this work, we have used microorganisms as templates to assemble metal oxide and metal nano- and microstructures that can enhance the performance of photovoltaic devices. First, we used M13 bacteriophages for their high aspect ratio and ability to bind noble metal nanoparticles, to create plasmonic nanowire arrays. We developed a novel process to assemble bacteriophages into nanoporous thin films via layer-by-layer assembly, and we mineralized the structure with titania. The resulting porous titania network was infiltrated with lead sulfide quantum dots to construct functional solar cells. We then used this system as a platform to study the effects of morphology and plasmonics on device performance, and observed significant improvements in photocurrent for devices containing bacteriophages. Next, we developed a process to magnesiothermally reduce biotemplated and solution-processed metal oxide structures into useful metallic materials that cannot be otherwise synthesized in solution. We applied the process to the synthesis of silicon nanostructures for use as semiconductors or photoactive materials. As starting materials, we obtained diatomaceous earth, a natural source of biotemplated silica, and we also mineralized M13 bacteriophages with silica to produce porous nanonetworks, and Spirulina major, a spiral-shaped algae, to produce micro-coils. We successfully reduced all silica structures to nanocrystalline silicon while preserving their shape. Overall, this work provides insights into incorporating biological materials in energy-related devices, doping materials to create semiconductors, characterizing their morphology and composition, and measuring their performance. The versatility and simplicity of the bottom-up assembly processes described here could contribute to the production of more accessible and inexpensive nanostructured energy conversion devices.<br>by Noémie-Manuelle Dorval Courchesne.<br>Ph. D.

Au cours de cette thèse, j’ai étudié la possibilité et les avantages d’utiliser des contacts nanométriques au-dessous de 1 µm. Des simulations analytiques et numériques ont montré que ces contacts nanométriques sont avantageux pour les cellules en silicium cristallin comme ils peuvent entrainer une résistance ohmique négligeable. Mon travail expérimental était focalisé sur le développement de ces contacts en utilisant des nanoparticules de polystyrène comme un masque. En utilisant la technique de floating transfert pour déposer les nanosphères, une monocouche dense de nanoparticules s’est formée. Cela nécessite une gravure par plasma de O2 afin de réduire la zone de couverture des NPs. Cette gravure était faite et étudiée en utilisant la technique de plasmas matriciels distribués à résonance cyclotronique électronique (MD-ECR). Une variété de techniques de créations de trous nanométriques était développée et testée dans des structures de couches minces et silicium cristallin. Des trous nanométriques étaient formés dans la couche de passivation, de SiO2 thermique, du silicium cristallin pour former des contacts nanométriques dopés. Un dopage local de bore était fait, à travers ces trous nanométriques par diffusion thermique et implantation ionique. En faisant la diffusion, le dopage local était observé par CP-AFM en mesurant des courbes de courant-tension à l’intérieur et à l’extérieur des zones dopées et en détectant des cellules solaires nanométriques. Par contre le processus de dopage local par implantation ionique a besoin d’être améliorer afin d’obtenir un résultat similaire à celui de diffusion<br>The use of point contacts has made the Passivated Emitter and Rear Cell design one of the most efficient monocrystalline-silicon photovoltaic cell designs in production. The main feature of such solar cell is that the rear surface is partially contacted by periodic openings in a dielectric film that provides surface passivation. However, a trade-off between ohmic losses and surface recombination is found. Due to the technology used to locally open the contacts in the passivation layer, the distance between neighboring contacts is on the order of hundreds of microns, introducing a significant series resistance.In this work, I explore the possibility and potential advantages of using nanoscale contact openings with a pitch between 300 nm to 10 µm. Analytic and numerical simulations done during the course of this thesis have shown that such nanoscale contacts would result in negligible ohmic losses while still keeping the surface recombination velocity Seff,rear at an acceptable level, as long as the recombination velocity at the contact (Scont) is in the range from 103-105 cm/s. To achieve such contacts in a potentially cost-reducing way, my experimental work has focused on the use of polystyrene nanospheres as a sacrificial mask.The thesis is therefore divided into three sections. The first section develops and explores processes to enable the formation of such contacts using various nanosphere dispersion, thin-film deposition, and layer etching processes. The second section describes a test device using a thin-film amorphous silicon NIP diode to explore the electrical properties of the point contacts. Finally, the third section considers the application of such point contacts on crystalline silicon by exploring localized doping through the nanoholes formed.In the first section, I have explored using polystyrene nanoparticles (NPs) as a patterning mask. The first two tested NPs deposition techniques (spray-coating, spin-coating) give poorly controlled distributions of nanospheres on the surface, but with very low values of coverage. The third tested NPs deposition technique (floating transfer technique) provided a closely-packed monolayer of NPs on the surface; this process was more repeatable but necessitated an additional O2 plasma step to reduce the coverage area of the sphere. This was performed using matrix distributed electron cyclotron resonance (MD-ECR) in order to etch the NPs by performing a detailed study.The NPs have been used in two ways; by using them as a direct deposition mask or by depositing a secondary etching mask layer on top of them.In the second section of this thesis, I have tested the nanoholes as electrical point-contacts in thin-film a-Si:H devices. For low-diffusion length technologies such as thin-film silicon, the distance between contacts must be in the order of few hundred nanometers. Using spin coated 100 nm NPs of polystyrene as a sacrificial deposition mask, I could form randomly spaced contacts with an average spacing of a few hundred nanometers. A set of NIP a-Si:H solar cells, using RF-PECVD, have been deposited on the back reflector substrates formed with metallic layers covered with dielectrics having nanoholes. Their electrical characteristics were compared to the same cells done with and without a complete dielectric layer. These structures allowed me to verify that good electrical contact through the nanoholes was possible, but no enhanced performance was observed.In the third section of this thesis, I investigate the use of such nanoholes in crystalline silicon technology by the formation of passivated contacts through the nanoholes. Boron doping by both thermal diffusion and ion implantation techniques were investigated. A thermally grown oxide layer with holes was used as the doping barrier. These samples were characterized, after removing the oxide layer, by secondary electron microscopy (SEM) and conductive probe atomic force microscopy (CP-AFM)

In 2012 the net world electricity generation was 21.56 trillion kilowatt hours. Photovoltaics only accounted for only 0.1 trillion kilowatt hours, less than 1 % of the total power. Recently there has been a push to convert more energy production to renewable sources. In recent years a great deal of interest has been shown for dye sensitized solar cells. These devices use inexpensive materials and have reported efficiencies approaching 12% in the lab. Here methods have been studied to improve upon these, and other, devices. Different approaches for the addition of gold nanoparticles to TiO2 films were studied. These additions acted as plasmonic and light scattering enhancements to reported dye sensitized devices. These nanoparticle enhancements generated a 10% efficiency in device performance for dye sensitized devices. Quantum dot (QD) sensitized solar cells were prepared by successive ionic layer adsorption and reaction (SILAR) synthesis of QDs in mesoporous films as well as the chemical attachment of colloidal quantum dots using 3-mercaptopropionic acid (3-MPA). Methods of synthesizing a copper sulfide (Cu2S) counter electrode were investigated to improve the device performance. By using a mesoporous film of indium tin oxide nanoparticles as a substrate for SILAR growth of Cu2S catalyst, an increase in device performance was seen over that of devices using platinum. These devices did suffer from construction drawbacks. This lead to the development of 3D nanostructures for use in Schottky photovoltaics. These high surface area devices were designed to overcome the recombination problems of thin film Schottky devices. The need to deposit a transparent top electrode limited the success of these devices, but did lead to the development of highly ordered metal nanotube arrays. To further explore these nanostructures depleted heterojunction devices were produced. Along with these devices a new approach to depositing lead sulfide quantum dots was developed. This electrophoretic deposition technique uses an applied electric field to deposit nanoparticles onto a substrate. This creates the possibility for a low waste method for depositing nanocrystals onto nanostructured substrates.

Les techniques de spectroscopie femtoseconde permettent d’étudier les processus de conversion d’énergie dans les système organiques. Elles permettent d’étudier les populations photo-générées et leur évolution à l’échelle de ces photoréactions. Elles permettent de comprendre les transferts d’énergie et de charge intra- et inter-moléculaires à l’origine du fonctionnement de ces systèmes.La protéine de rétinal Anabaena sensroy Rhodopsin est un photocommutateur naturel, qui est étudié afin de comprendre les paramètres à l’origine de l’efficacité quantique d’isomérisation. Nous avons pu déterminer cette efficacité quantique pour les deux formes stables du rétinal ainsi que leur dynamique d’isomérisation dans les mêmes conditions expérimentales.La génération de charge dans des couches actives pour le photovoltaique organique est étudiée dans un système composé d’un mélange de PCBM et d’un donneur organique dérivé du colorant BODIPY. L’influence de la nanostructuration de la couche active sur la génération de charge est étudiée. La génération de charge est limitée dans ce système par la recombinaison des charges générées et par la diffusion des excition aux interfaces donneur-accepteur. Ces observations indiquent que l’amélioration de la nanostructuration de la couche active peut permettre d’augmenter les rendements de photo-génération de charge<br>Femtosecond transient spectroscopies are used to investigate photonic energy conversion inorganic systems. These techniques allow to observe the ground and excited states of themolecules at the timescale of the photoreactions. It is used to understand the inter- andintramolecular energy and charge transfers leading to the desired photochemical process.The natural photoswiching retinal protein Anabaena sensory Rhodopsin is studied to understand the key parameters ruling the isomerisation quantum yield. We could determine the isomerisation quantum yield of both stable forms and their dynamics in the very same experimental conditions.Charge generation is investigated in small molecule bulk heterojunction active layers for organic solar cells made of PCBM and a BODIPY dye-derivative donor. The influence of the active layer morphology on charge generation is studied. The charge generation is limited by charge recombination but also by exciton diffusion to the donor-acceptor interface. The active layer morphology has to be improved to achieve more efficient organic solar cells with these materials

Zinc oxide (ZnO) is an n-type II-VI semiconductor with a reported band gap of 3.2-3.6 eV [1, 2, 3] and electrical resistivity of ~ 50 Ωcm [4]. Ideal for use in devices such as Photovoltaics (PVs), Light Emitting Diodes (LEDs) and detectors, ZnO has the advantage that it can be electrochemically deposited. This enables the quick and cheap controlled growth of ZnO nanostructures, which can potentially enhance performance in electronic applications over thin films. ZnO doping with a group III element e.g. Aluminium, can increase ZnO conduction by several orders of magnitude whilst having only a subtle effect on its optical properties, therefore further enhancing device performance. For the first time, this thesis presents a unique in-depth study into the potentiostatic electrochemical deposition of well defined zinc oxide nanostructures (nanorods and platelets), their controlled aluminium doping and application in PV devices. This work addresses the mechanism of doping and examines the relationship between the opto-electronic properties, composition, structure, morphology and growth. The results show that arrays of crystalline wurtzite ZnO nanorods with strong (002) preferential orientation can be deposited on ITO and Au using a 1 mM Zn(NO3)2 system. Doping has been successfully carried out using Al(NO3)3 with a doping mechanism confirmed for the first time. This study shows that doped nanorods contain < 5 at. % Al3+, where Al3+ is incorporated in the ZnO lattice as interstitial and/or substitutional ions. This results in a subtle increase in the band gap, and is believed to increase the ZnO conduction by several orders of magnitude. The application of these nanorod arrays in PV devices has improved device efficiency by ~ 1080 %. Furthermore, platelets have been successfully deposited using a 5 mM Zn(NO3)2 system. A critical dopant content ~ 5 at. % Al3+ has been found, above which there is a transition in the doping mechanism towards spontaneous Al2O3 formation in addition to interstitial and substitutional Al3+ ion locations. This results in a gradual decrease in the optical band gap towards that of undoped ZnO. This mechanism occurs in platelets, where at. % Al3+ > 5 %. Platelet formation is associated with small quantities of impurities such as Al2O3, ZnCl2, Zn(ClO4)2 Zn5(OH)8Cl2.H2O and Au3Zn, arising from deposition conditions. Both impurities and dopants result in increased ZnO polycrystallinity and decreased ZnO (002) preferential orientation. The performance of PV devices with nanorod arrays has been shown to be better than previously reported equivalent thin film devices. This work illustrates the significance of electrochemical deposition as a technique for cheap and quick, controlled mass production of high quality tailor-made ZnO semiconductor nanostructures.