Crystalline silicon (Si) wafers used as substrates in the semiconductor and photovoltaic (PV) industries are traditionally manufactured using a multi-wire slurry sawing (MWSS) technique. Due to its high productivity potential, the fixed abrasive diamond wire sawing (DWS) technique is of considerable interest to Si wafer producers. Although both sawing techniques are currently used in the industry, a fundamental understanding of the underlying process is still lacking, particularly for diamond wire sawing. Consequently, optimization of the wire sawing process is carried out largely based on experience and trial and error. This thesis aims to develop a systematic fundamental understanding of diamond wire sawing of Si materials used for PV applications. First of all, a comparative analysis of the characteristics of silicon wafers cut by slurry and fixed abrasive diamond wire sawing is presented. The analysis results indicate that fixed abrasive diamond wire sawing may be a viable alternative to slurry wire sawing. Modeling and experimental studies of single grit diamond scribing of Si are proposed to shed light on the basic cutting mechanisms. Although Si is brittle at room temperature, it is possible to properly control the cutting conditions to obtain a completely ductile mode of material removal. The effects of material anisotropy, abrasive grit shape, friction condition and external hydrostatic pressure on the ductile-to-brittle mode transition in cutting of single crystal Si (sc-Si) are systematically investigated. Multicrystalline Si (mc-Si) based solar cells take up the majority of the global PV market. Hard inclusions (Silicon carbide and Silicon nitride) in multicrystalline Si (mc-Si) ingots may cause wire breakage and negatively impact the process, surface/subsurface morphology and mechanical properties of the resulting wafer. Their effects are experimentally studied through the single grit diamond scribing on the mc-Si sample with high density of inclusions. Finally, it is identified that there is a correlation between the high dislocation density and the increase of fracture toughness in mc-Si. The increase in fracture toughness leads to greater capability of ductile mode of cutting and higher specific scribing energy in the brittle fracture regime. Results of these fundamental investigations are expected to generate useful knowledge for optimizing the diamond wire sawing process in order to achieve high productivity and minimum surface/subsurface damage.

The development of organic photovoltaics (OPVs) has attracted a lot of attention due to their potential to create a low cost flexible solar cell platform. In general, an OPV is comprised of a number of layers of thin films that include the electrodes, active layers and barrier films. Thus, with all of the interfaces within OPV devices, the potential for failure exists in numerous locations if adhesion at the interface between layers is inherently low or if a loss of adhesion due to device aging is encountered. To date, few studies have focused on the basic properties of adhesion in organic photovoltaics and its implications on device reliability. In this dissertation, we investigated the adhesion between interfaces for a model multilayer barrier film (SiNx/PMMA) used to encapsulate OPVs. The barrier films were manufactured using plasma enhanced chemical vapor deposition (PECVD) and the interfacial fracture toughness (Gc, J/m2) between the SiNx and PMMA were quantified. The fundamentals of the adhesion at these interfaces and methods to increase the adhesion were investigated. In addition, we investigated the adhesive/cohesive behavior of inverted OPVs with different electrode materials and interface treatments. Inverted OPVs were fabricated incorporating different interface modification techniques to understand their impact on adhesion determined through the interfacial fracture toughness (Gc, J/m2). Overall, the goal of this study is to quantify the adhesion at typical interfaces used in inverted OPVs and barrier films, to understand methods that influence the adhesion, and to determine methods to improve the adhesion for the long term mechanical reliability of OPV devices.

Renewable distributed generation such as wind power and photovoltaics are gaining popularity all over the world. The overall aim of this Master thesis was to gather experience and knowledge regarding small wind power and photovoltaic with both a market based evaluation and technical description. Methods used have been literature review, interviews with market participants, evaluation of a wind mill and a photovoltaic system simulation with PVsyst 5.41. It was found that the main common incentive today for the development and spread of small wind power and photovoltaics for market participants is the symbolic value. It was also discovered that the market situation is complicated for the producing consumer. The spread of small wind power and PV today are a few per mille of a future potential, where politics largely control development and spread of small-scale solutions. The market is unclear and solutions around net charge is still an ongoing debate. Majority of the interviewed persons believes more in PV than in small wind power due to facts such as wind power is size-dependent and not optimal to build in urban areas. Results show that power quality issues are dependent on the network system as a whole and are often a matter of cost and can be prevented with different technical solutions. One conclusion was that bidirectional power flow increase complexity of problems around protection. Major energy companies are involved in projects to gather knowledge how to deal with DG both in technical aspects and how to deal with customers practically.

CdTe is one of the most promising absorbers for use in inexpensive semiconductor solar cells having achieved a high efficiency of 16.4% in small area cells [1]. One of the most important technological problems in obtaining high efficiencies is to have a good ohmic contact on the CdTe, which is characterized by a very high work function [2]. Cu is used as a dopant in CdTe at the contact to promote quantum mechanical tunneling [3]. But the oversupply of Cu causes the diffusion of Cu through CdTe to the underlying CdS layer resulting in the degradation of the cell performance. It has been reported that Cu was segregated near the CdS/CdTe junction. To avoid the Cu segregation at the junction, Cu supply should be minimized while the ohmic characteristics of p-CdTe contact are maintained [4]. In this thesis, the main objective is to understand the role of Cu at the CdS/CdTe interface. Here the Cu is added at the CdS/CdTe interface and is avoided at the back contact.

Non-destructive monitoring of residual elastic stress in silicon wafers is a matter of strong concern for modern photovoltaic industry. The excess stress can generate cracks within the crystalline structure, which further may lead to wafer breakage. Cracks diagnostics and reduction in multicrystalline silicon, for example, are ones of the most important issues in photovoltaics now. The industry is intent to improve the yield of solar cells fabrication. There is a number of techniques to measure residual stress in semiconductor materials today. They include Raman spectroscopy, X-ray diffraction and infrared polariscopy. None of these methods are applicable for in-line diagnostics of residual elastic stress in silicon wafers for solar cells. Moreover, the method has to be fast enough to fit in solar cell sequential production line.

Electronic materials are of great interest for use in photovoltaics, sensors, light-emitting diodes, and molecular electronics. Hybrid Inorganic/Organic materials have been studied for device application due to their unique electronic properties. These properties result from the formation of bulk heterojunctions between inorganic (n-type) and organic (p-type) materials. However, due to incomplete pathways for charge transport and poor interfaces between materials, charge trapping and exciton recombination is often high. In an effort to alleviate these problems, we have developed an approach to fabricate bulk heterojunction materials via a seeded growth process. Electropolymerizable Schiff base complexes have been designed, synthesized, and utilized as precursors for conducting metallopolymers. The embedded metal centers are used as seed points for direct growth of size-controllable semiconductor nanoparticles within the polymer film leading to direct electronic communication between the two materials. The synthesis of CdS, CdSe, Ga₂S₃, CuInS₂, CuInSe₂, CuGaS₂, CuGaSe₂, CuGa[subscript x]In[subscript x]-₁S₂, and CuGa[subscript x]In[subscript x]-₁Se₂ has been seen through TEM and EDX. Devices have been fabricated and current studies have focused on the photovoltaic characterization of these materials which have a PCE of 0.11%. As a second but closely related area, polymers have also been studied as organic semiconductors for device applications. However they are hard to process from solution and their polymeric structure can vary. Both of these problems can be solved by using well-defined solution processable oligomers. Thiophene oligomers have been synthesized and characterized through Single Crystal X-Ray Crystallography, Four Point Probe Conductivity, and Powder Diffraction. These oligomers have a well-defined structure and are solution processable from a variety of solvents which can then be used as models to predict and study the properties of polythiophene. / text

Quantum-scale semiconductors embedded in an electrically-active matrix have the potential to improve photovoltaic (PV) device power conversion efficiencies by allowing the solar spectral absorption and photocarrier transport properties to be tuned through the control of short and long range structure. In the present work, the effects of phase assembly on quantum confinement effects and carrier transport were investigated in CdTe - ZnO nanocomposite thin films for use as a spectrally sensitized n-type heterojunction element. The nanocomposites were deposited via a dual-source, sequential radio-frequency (RF) sputter technique that offers the unique opportunity for in-situ control of the CdTe phase spatial distribution within the ZnO matrix. The manipulation of the spatial distribution of the CdTe nanophase allowed for variation in the electromagnetic coupling interactions between semiconductor domains and accompanying changes in the effective carrier confinement volume and associated spectral absorption properties. Deposition conditions favoring CdTe connectivity had a red shift in absorption energy onset in comparison to phase assemblies with a more isolated CdTe phase. While manipulating the absorption properties is of significant interest, the electronic behavior of the nanocomposite must also be considered. The continuity of both the matrix and the CdTe influenced the mobility pathways for carriers generated within their respective phases. Photoconductivity of the nanocomposite, dependent upon the combined influences of nanostructure-mediated optical absorption and carrier transport path, increased with an increased semiconductor nanoparticle number density along the applied field direction. Mobility of the carriers in the nanocomposite was further mediated by the interface between the ZnO and CdTe nanophases which acts as a source of carrier scattering centers. These effects were influenced by low temperature annealing of the nanocomposite which served to increase the crystallinity of the phases without modification of the as-deposited phase assembly and associated absorption properties. Integration of the nanocomposite as an n-type heterojunction element into a PV device demonstrated the ability to tune device response based on the spectral absorption of the nanocomposite sensitizer film as dictated by the phase assembly. Overall the various phase assemblies studied provided increased opportunity for optimization of the absorption and carrier transport properties of the nanocomposite thin films.

Indium-tin oxide (ITO) is commonly used as the transparent electrode in organic photovoltaic (OPV) devices. ITO's transparent properties come at the expense of less than ideal electrode characteristics arising from insulating over-oxidized surface species. OPVs fabricated on the native ITO surface tend to exhibit poor performance with a high degree of variability from device to device. Aggressive acid etching of the ITO surface removes the majority of the insulating surface species leading to improvements in OPV efficiency with greater reproducibility and increased device to device consistency.Organic light emitting diodes (OLEDs) are planar electroluminescent light sources that naturally couple a portion of their emission into internally reflected modes within the device substrate. Although this coupling property is well known, few attempts have been made to integrate OLEDs as light sources for internal reflection elements. Furthermore, OPVs share the optical coupling properties of OLEDs and therefore can be used as integrated internal reflection detectors. Integrating both an OLED light source and an OPV detector onto the same substrate results in an internal reflection sensing platform that requires no free-space optics, has low power consumption requirements, and can be easily fabricated on substrates occupying an area less than one square inch. In this work we establish a functional prototype design, characterize the fundamental coupling properties, and demonstrate several surface sensing responses of this fully integrated optical sensing platform.The net solar power production from OPVs arises from the interactions between multiple currents through the device. The photocurrent is the only power producing current in the device and understanding the voltage dependent nature of this current is essential in OPV research. Analysis methods of conventional, inorganic photovoltaics do not adequately describe the photocurrent behavior commonly observed in OPVs. OPV analysis is therefore somewhat limited by the methods commonly employed. To improve upon the convention methods we develop a simplified method of OPV photocurrent analysis based on electrochemical methods that accurately describes the voltage dependence of the photocurrent and leads to greater insight into the key parameters involved in solar power production from OPVs.

Understanding organic semiconductor interfaces is critical to developing organic photovoltaics (OPV). OPV interfaces are disordered due to weak intermolecular interactions, resulting in diverse charge transfer micro-environments. I present experimental data isolating high-order intermolecular interactions controlling interfacial energy level alignment and describe new instrumental capabilities providing access to the local electronic and kinetic landscape at organic semiconductor interfaces. Interface formation between vanadyl naphthalocyanine (VONc) and highly ordered pyrolytic graphite (HOPG) is investigated. Ultraviolet photoemission spectroscopy (UPS) shows that the VONc binding energy (BE) decouples from the work function, shifting in an opposite direction and contradicting the standard interface dipole model. This effect is quantitatively described using an electrostatic depolarization model and confirmed by simulations which show an inhomogeneous potential at the interface. New data and literature values suggest orthogonality between polarizability and molecular dipole in polar porphyrazines. Their potential for interface engineering is discussed. The electron-rich Au(111)/VONc interface is investigated. The organic layer induces a large interface dipole in Au(111) which can be fit to a depolarization model. Ionization potential and depolarization data suggest that the second VONc layer on Au(111) adopts a tilted geometry. Electrostatic differences between Au(111)/VONc and HOPG/VONc are discussed, demonstrating that interface dipole contributions are not interchangeable. The surface states of the Au(111)/VONc interface are characterized by angle resolved 2-photon photoemission to determine the magnitude of the perturbation. The measured free-electron-like effective mass and BE destabilization of the Shockley state is attributed to step edges caused by lifting the Au(111) (22 x √3) reconstruction. The Shockley state is accessible primarily through resonance with the n = 1 image state. Another resonance between the image state and a molecular state of VONc is tentatively identified. Design and construction of a confocal fluorescence microscope capable of single molecule detection in ultrahigh vacuum is described. Initial images and fluorescence trajectories demonstrate the ability to measure charge transfer kinetics between an individual organic semiconductor molecule and well-characterized insulating surfaces. Progress towards completion of a scanning photoionization microscope is presented. The microscope demonstrates diffraction-limited imaging capabilities using fs-laser-generated photoelectron current as contrast. Recommendations are given towards achieving spectral resolution and for future experimental systems.

Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.