This dissertation reports the fabrication and characterization of thin films from the water soluble polymer sodium poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate] (PTEBS) by electrospray deposition (ESD). Contiguous thin films were created by adjusting the parameters of the electrospray apparatus and solution properties to maintain a steady Taylor cone for uniform nanoparticle aerosolization and controlling the particle water content to enable coalescence with previously deposited particles. The majority of deposited particles had diameters less than 52 nm. A thin film of 64.7 nm with a root mean square surface roughness of 20.2 nm was achieved after 40 minutes of ESD. Hybrid Solar Cells (HSCs) with PTEBS thin films from spin coating and electrospray deposition (ESD) were fabricated, tested, and modeled. A single device structure of FTO/TiO2/PTEBS/Au was used to study the effects of ESD of the PTEBS layer on device performance. ESD was found to double the short circuit current density (Jsc) by a factor of 2 while decreasing the open circuit voltage (Voc) by half compared to spin coated PTEBS films. Comparable efficiencies of 0.009% were achieved from both device construction types. Current-Voltage curves were modeled using the characteristic solar cell equation showed a similar increase in generated photocurrent with a decrease of two orders of magnitude in the saturation current in devices from ESD films. Increases in Jsc are attributed to increased interfacial contact area between the TiO2 and PTEBS layers, while decreases in Voc are from poor film quality from ESD. Polymer solar cells (PSCs) with water-soluble active layers deposited by ESD were fabricated and tested. The water soluble, bulk heterojunction active layers consisted of PTEBS and the fullerene C60 pyrrolidine tris-acid. A single device structure of ITO/PEDOT:PSS/bulk(PTEBS+C60)/Al was used to study the effect of PTEBS to C60 tris-acid ratio on photovoltaic performance. An active layer ratio of PTEBS:C60 tris-acid (1:2) achieved the highest power conversion efficiency (0.0022%), fill factor (0.25), and open circuit voltage (0.56 V). The percolation threshold of C60 was achieved between 1 part PTEBS and 2 to 3 parts C60. Increasing the C60 tris-acid ratio (1:3) improved short circuit current, but reduced the open circuit voltage enough to lower efficiency.

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In order to limit global warming to 1.5-2 °C deployed solar photovoltaic (PV) power must increase from today's 0.228 terawatts to 2-10 terawatts installed by 2030, depending on demand. These goals require increasing manufacturing capacity, which, in turn, requires lowering the cost of electricity produced by PV. However, high demand scenarios will require greater cost reductions in order to make PV generated electricity <i> as competitive </i> as it needs to be to enable this growth. It is unclear whether established PV technologies — silicon, CdTe, GaAs, or CuInxGa1-xSe2 — can achieve the necessary breakthroughs in efficiency and price. A newer technology known as the &apos;perovskite solar cell&apos; (PSC) has recently emerged as promising contender. <br>     In the last seven years the efficiency of PSCs increased by the same amount covered by established technologies in the last thirty. However, PSCs suffer from chemical instability under operating conditions and hysteresis in current-voltage measurements used to characterize power output. Characterizing the defect structures formed by this material and how they interact with device performance and degradation may allow stabilization of PSCs. To that end, this work investigates defects in perovskite solar cells, the impact of these defects on performance, and the effect of alloying and degradation on the electronically active defect structure. Chapter I gives a brief introduction, motivating research in solar cells generally and perovskites in particular as well as introducing some challenges the technology faces. Chapter II gives some background in semiconductors and the device physics of solar cells. Chapter III introduces the performance and defect characterization methods employed. Chapter IV discusses results of these measurements on methylammonium lead triiodide cells correlating defects with device performance. Chapter V applies the some of the same techniques to a series of CH3NH3Pb(I1-xBrx)3 based perovskites aged for up to 2400 hours to explore the impact of alloying and aging on the defect structure. Chapter VI discusses implications for perovskite development and directions for future research. <br>     This dissertation includes previously published co-authored material.

The as-produced isomer mixture of the organic photovoltaic device acceptor material bisPC62BM has been purified into its constituents by peak-recycling HPLC, and those individual isomers were characterised by UV-Vis absorption spectroscopy and cyclic voltammetry. A total of 18 isomers were purified from the mixture to a standard exceeding 99.5% with respect to other isomers. The HOMOs, LUMOs, and HOMO-LUMO gaps of the purified isomers vary from (-5.673 to -5.402 eV), (-3.901 to -3.729 eV), and (1.664 to 1.883 eV), respectively. We also find a correlation between HPLC retention time and the relative positions of the addends; in that generally the closer the addends are to each other the longer the retention time of the isomer, and vice versa. The OPV acceptor molecule PC71BM was also purified into its constituent isomers to a standard of at least 99%. The total three purified isomers were each characterized by 13C NMR and UV-vis spectroscopy, and cyclic voltammetry. These characterizations were supported by HF/DFT ab-initio calculations. All three isomers are methano-fullerenes. The most abundant isomer (85% of the mixture) exists as a racemate involving the 8-25 bond of C70. The other two isomers both involved the 9-10 bond of C70, but are distinguished by opposing orientations of the addend with r and s pseudo-asymmetry about carbon atom 71. The r and s isomers comprised 9% and 6% of the as-produced of the mixture, respectively. In order of decreasing abundance, the LUMO levels of the isomers were -3.9316, -3.9194 and -3.9197 eV and the HOMO-LUMO gaps were 1,772, 1.754 and 1.748 eV.

The resonant interaction of light with metal nanoparticles can result in extraordinary optical effects in both the near and far fields. Plasmonics, the study of this interaction, has the potential to enhance performance in a wide range of applications, including sensing, photovoltaics, photocatalysis, biomedical imaging, diagnostics, and treatment. However, the mechanisms of plasmonic enhancement often remain poorly understood, limiting the design and effectiveness of plasmonics for advanced applications. This dissertation focuses on evaluating the mechanisms of plasmonic enhancement and distinguishing between near and far field effects using simulations and experimental results. Thorough characterization of metal nanoparticle colloids shows that electromagnetic simulations can be used to accurately predict the optical response of nanoparticles only if the true shapes and size distributions are taken into account. By coupling these optical interaction calculations with heat transfer models, experimental limits for the maximum optical power before nanoparticle melting can be found. These limits are important for plasmonic multiphoton luminescence imaging applications. Subsequently, we demonstrate ultrafast laser plasmonic nanoablation of silicon substrates using gold nanorods to identify the near-field enhancement and mechanism of plasmon-assisted ablation. The experimentally observed shape of the ablation region and reduction of the ablation threshold are compared with simulations to show the importance of the enhanced electromagnetic fields in near-field nanoablation with plasmonic nanoparticles. The targeted use of plasmonic nanoparticles requires narrow size distribution colloids, because wide size distributions result in a blurring and weakening of the optical response. A new synthesis method is presented for the seeded-growth of nearly monodisperse metal nanoparticles ranging from 10 to 100 nm in diameter, both with and without dielectric shells of controlled thickness. This method is used to acquire fine control over the position and width of the plasmonic peak response. We also demonstrate self-assembled sub-monolayers of these particles with controllable concentrations, which is ideal for looking at plasmonic effects in surface and layered geometries. Finally, we present results for the spatial distribution of absorption around plasmonic nanoparticles. We introduce field-based definitions for distinguishing near-field and far-field regions and develop a new set of equations to determine the point-by-point enhanced absorption in a medium around a plasmonic nanoparticle. This set of equations is used to study plasmon-enhanced optical absorption for thin-film photovoltaic cells. Plasmonic nanoparticle systems are identified using simulations and proof-of-concept experiments are used to demonstrate the potential of this approach.

The goal of this research was to investigate an experimental infrared transmission technique and associated analysis tools that extract the in-plane residual stresses in thin single and poly-crystalline silicon sheet, and try to relate the residual stresses to physical parameters associated with silicon growth and cell processing. Previous research has suggested this concept, but many engineering and analytical details had not been addressed. In this research, a system has been designed and built. A fringe multiplier was incorporated into the system to increase the sensitivity. The error was analyzed and the resolution of the system was found to be 1.2~MPa. To convert the experimental results to residual stresses, the stress-optic coefficients of (001), (011) and (111) silicon were analyzed analytically and calibrated using a four-point bending fixture. Anisotropy in (001) and (011) silicon was found to be 33%, and the coefficient of EFG silicon is 1.7 times larger than that of (001) silicon. The polariscope together with other techniques was applied to silicon wafers after various processing steps in the manufacture of photovoltaic cells. The influence of the processing on residual stress was investigated and positive correlations between residual stresses, PL and efficiency were obtained.

The resonant interaction of light with metal nanoparticles can result in extraordinary optical effects in both the near and far fields. Plasmonics, the study of this interaction, has the potential to enhance performance in a wide range of applications, including sensing, photovoltaics, photocatalysis, biomedical imaging, diagnostics, and treatment. However, the mechanisms of plasmonic enhancement often remain poorly understood, limiting the design and effectiveness of plasmonics for advanced applications. This dissertation focuses on evaluating the mechanisms of plasmonic enhancement and distinguishing between near and far field effects using simulations and experimental results. Thorough characterization of metal nanoparticle colloids shows that electromagnetic simulations can be used to accurately predict the optical response of nanoparticles only if the true shapes and size distributions are taken into account. By coupling these optical interaction calculations with heat transfer models, experimental limits for the maximum optical power before nanoparticle melting can be found. These limits are important for plasmonic multiphoton luminescence imaging applications. Subsequently, we demonstrate ultrafast laser plasmonic nanoablation of silicon substrates using gold nanorods to identify the near-field enhancement and mechanism of plasmon-assisted ablation. The experimentally observed shape of the ablation region and reduction of the ablation threshold are compared with simulations to show the importance of the enhanced electromagnetic fields in near-field nanoablation with plasmonic nanoparticles. The targeted use of plasmonic nanoparticles requires narrow size distribution colloids, because wide size distributions result in a blurring and weakening of the optical response. A new synthesis method is presented for the seeded-growth of nearly monodisperse metal nanoparticles ranging from 10 to 100 nm in diameter, both with and without dielectric shells of controlled thickness. This method is used to acquire fine control over the position and width of the plasmonic peak response. We also demonstrate self-assembled sub-monolayers of these particles with controllable concentrations, which is ideal for looking at plasmonic effects in surface and layered geometries. Finally, we present results for the spatial distribution of absorption around plasmonic nanoparticles. We introduce field-based definitions for distinguishing near-field and far-field regions and develop a new set of equations to determine the point-by-point enhanced absorption in a medium around a plasmonic nanoparticle. This set of equations is used to study plasmon-enhanced optical absorption for thin-film photovoltaic cells. Plasmonic nanoparticle systems are identified using simulations and proof-of-concept experiments are used to demonstrate the potential of this approach. / text

A computational investigation of some optoelectronic applications of carbon nanotubes (CNT) is presented, including CNT-based solar cells and biosensors. The results could be used to evaluate the performance of CNT devices and clarify the necessity of further experimental research in this area. A coaxially-gated CNT field-effect transistor (CNFET) forms the basic structure of the devices modeled in this thesis. Diffusive transport is present in long-channel devices, as in our case, while the quantum mechanical effects are mainly present in the form of tunneling from Schottky-barrier contacts at the metal-CNT interfaces. Band-to-band recombination of electron-hole pairs (EHP) is assumed to be the source of electroluminescence. In a first-order approximation, protein-CNT interactions are modeled as the modification of the potential profile along the longitudinal axis of CNTs due to electrostatic coupling between partial charges, in the oxide layer of the CNFET, and the nanotube. The possibility of electronic detection is evaluated. The electroluminescence of the CNT is proposed as an optical detection scheme due to its sensitivity to the magnitude and the polarity of the charge in the oxide. The validity of the model is argued for the given models. A value for the minimum required size of a computational window in a detailed simulation is derived. The structure of an electrostatically gated p-i-n diode is simulated and investigated for photovoltaic purposes. The absorbed power from the incident light and the interaction between the nanotubes is modeled with COMSOL. The results are interpreted as a generation term and introduced to the Drift-Diffusion Equation (DDE). We have observed behavior similar to that in an experimentally-realized device. The performance of CNT-based solar cells under standard AM 1.5 sunlight conditions is evaluated in the form of an individual solar cell and also in an array of such devices.

Half of the sun’s power lies in the infrared. As a result, the optimal bandgaps for solar cells in both the single-junction and even the tandem architectures lie beyond 850 nm. However, progress in low-cost, large-area, physically-flexible solar cells has instead been made in organic and polymer materials possessing absorption onsets in the visible. Recent advances have been achieved in solution-cast infrared photovoltaics through the use of colloidal quantum dots. Here we report stable solution-processed photovoltaic devices having 3.6% power conversion efficiency in the infrared. The use of a strongly-bound bidentate linker, benzenedithiol, ensures device stability over weeks. We investigate in detail the physical mechanisms underlying the operation of this class of device. We find that diffusion of electrons and holes over hundreds of nanometers through our PbSe colloidal quantum dot solid is chiefly responsible for the high external quantum efficiencies obtained in this new class of devices.

Half of the sun’s power lies in the infrared. As a result, the optimal bandgaps for solar cells in both the single-junction and even the tandem architectures lie beyond 850 nm. However, progress in low-cost, large-area, physically-flexible solar cells has instead been made in organic and polymer materials possessing absorption onsets in the visible. Recent advances have been achieved in solution-cast infrared photovoltaics through the use of colloidal quantum dots. Here we report stable solution-processed photovoltaic devices having 3.6% power conversion efficiency in the infrared. The use of a strongly-bound bidentate linker, benzenedithiol, ensures device stability over weeks. We investigate in detail the physical mechanisms underlying the operation of this class of device. We find that diffusion of electrons and holes over hundreds of nanometers through our PbSe colloidal quantum dot solid is chiefly responsible for the high external quantum efficiencies obtained in this new class of devices.