No description available.

No description available.

No description available.

One-dimensional vertical nanostructures, nanowire arrays, are investigated for applications in photovoltaics. Specifically, III-V core-shell p-i-n nanowire arrays are grown by molecular beam epitaxy on silicon substrates, using the self-assisted vapour-liquid-solid growth method. GaAs1-xPx nanowires are grown with an optimized composition to maximize the potential efficiency of a GaAsP nanowire-on-silicon tandem solar cell under AM1.5G illumination. Photovoltaic devices are fabricated and assessed by optical and electrical characterization techniques, to identify areas for refinement of device design and processing. Combining the unique properties of nanowire arrays, the quality and tunability of III-V materials, and the economics and infrastructure of silicon-based device fabrication, this work examines a novel approach to affordable renewable energy. Methods of substrate removal via etching are investigated for optical characterization of nanowire arrays, and an improved technique for electrical characterization of ITO contacts is explored. The first nanowire-on-silicon tandem device utilizing a radial p-n junction nanowire structure is reported, achieving an open circuit voltage of 1.2 V, a short circuit current density of 7.6 mA/cm2, a fill factor of 40%, and an efficiency of 3.5%. Finally, projects for future improvements to the work described herein are suggested. / Thesis / Master of Applied Science (MASc)

Gallium antimonide (GaSb) is a promising low-bandgap binary substrate for the fabrication of various infrared-based optoelectronic devices, particularly thermophotovoltaics (TPV). In order to make GaSb-based technologies like TPV more widely available, non-epitaxial dop- ing methods for GaSb must be pursued. Ion implantation is relatively unexplored for GaSb, and can offer advantages over the more common method of zinc diffusion, including higher flexibility with regards to substrate type and control over the resulting doping profile. Pre- vious work has shown beryllium (Be+) implantation to be a suitable method for fabricating a diode in an n-type GaSb substrate, opening the possibility for other ions to be considered for implanting into both n-type and p-type substrates. This work identifies sulfur (S+) as another species to investigate for this purpose. To do so, material and electrical characterization was done on S+ and beryllium implanted GaSb films grown onto a semi-insulating gallium arsenide (GaAs) substrate. X-ray Diffraction spectroscopy (XRD) and Atomic Force Microscopy (AFM) indicate that the post-implant anneal of 600 for 10 s repaired the implant damage in the bulk material, but left behind a damaged surface layer composed of coalesced vacancies. While the beryllium implant resulted in moderate doping concentrations corresponding to an activation percentage near 15 %, Hall Effect data showed that implanting S+ ions induced a strongly p-type behavior, with hole concentrations above 1 × 19 cm^3 and sheet hole densities 3.5 times higher than the total implanted dose. This strong p-type behavior is attributed to the remaining lattice damage caused by the implant, which induces a large density of acceptor-like defect states near the valence band edge. This technique was used on an unintentionally-doped p-type GaSb substrate to create a + /p junction. The implant process succeeded in producing a potential barrier similar to that of a hole-majority camel diode with a thin delta-doped region suitable for collecting diffused carriers from the p-type substrate. A post-fabrication etching process had the effect of strongly increasing the short circuit current density to as high as 41.8 mA/cm^2 and the open circuit voltage as high as 0.21 V by simultaneously removing a high carrier recombination surface layer. This etching process resulted in a broadband spectral response, giving internal quantum efficiencies greater than 90 %. / Doctor of Philosophy / Thermophotovoltaics (TPV) is a technology that converts light and other forms of electromagnetic energy into electrical power, much like a typical solar panel. However, instead of sunlight, the energy source used in a TPV system is a terrestrial heat source at a temperature range of 1250–1750 ◦C, whose radiation is primarily infrared (IR). The IR-absorbing qualities and commercial availability of the compound semiconductor gallium antimonide (GaSb) have made it a key component in the development of absorber devices for TPV-related systems. GaSb-based devices have most often been fabricated using epitaxy, a method in which layer(s) of material are ‘grown’ in a layer-by-layer fashion atop a substrate GaSb wafer to induce an interface between negatively-charged (n-type) and positively-charged (p-type) regions. In order to improve upon the scalability of TPV production, device fabrication methods for GaSb that avoid the use of epitaxy are sought after as a lower-cost alternative. In this work, sulfur ion implantation is examined as one of these methods, in which elemental sulfur ions are injected at a high energy into a p-type GaSb substrate. The implanted ions then alter the charge characteristics at the surface of the material, producing an electric field from which a photovoltaic (PV) device can be fabricated. The results of this study showed that by implanting sulfur ions, an extremely p-type (p++) layer was formed at the surface of the GaSb substrate, which was attributed to residual damage induced by the implant process. The resulting interface between the p++ surface and the moderately p-type GaSb substrate was found to induce an electric field suitable for a PV device. Removing the excess surface damage away from the device’s metal contacts resulted in an improvement in the output electrical currents, with measured values being significantly higher than that of other devices made using more common non-epitaxial fabrication methods. The success of this work demonstrates the advantages of using a p-type GaSb substrate in place of an n-type substrate, and could help diversify the types of TPV-related devices that can be produced.

An in depth study of the performance of thermally interdiffused concentration gradient polymer photovoltaic devices is carried out with particular attention to the effect of the thickness and the thermal treatments on the power conversion efficiency, short circuit current, open circuit voltage and other key electrical properties. Bilayer films of sequentially spin-cast donor and acceptor materials are exposed to various heat treatments in order to induce the interdiffusion. The depth profiles show concentration gradients in the donor and acceptor as a result of interdiffusion and these devices show an order of magnitude increase in the device performance compared to the bilayer devices. Dual spin-cast poly (3-octylthiophene-2,5-diyl) (P3OT)- [6,6] phenyl C61 butyric acid methyl ester (PCBM) and poly (3-hexylthiophene-2,5-diyl) (P3HT)-PCBM interdiffused devices are studied in detail by varying the thickness of the donor and acceptor layers as well as the annealing conditions for initial polymer layer and the time and temperature of the interdiffusion process. Auger spectroscopy and X-ray photoelectron spectroscopy along with ion beam milling are used to investigate the concentration gradient formed as a result of the interdiffusion. The sulfur signal present in the P3OT and P3HT backbone is detected to identify the concentration profiles in the P3OT-PCBM and P3HT-PCBM devices. The interdiffusion conditions and thickness of the active layers have been optimized to obtain the highest power conversion efficiency. The best device performance of the P3OT-PCBM interdiffused devices is achieved when the interdiffusion is carried out at 150°C for 20 minutes and the P3OT thickness is maintained at 70 nm and the PCBM thickness at 40-50 nm. The highest efficiency achieved for P3OT-PCBM interdiffused devices is 1.0% under AM1.5G solar simulated spectrum. In order to further increase the efficiency, P3OT is replaced by (P3HT) which has higher hole mobility. P3HT- PCBM based concentration gradient devices show improved device performance over P3OT-PCBM devices. Power conversion efficiency of the order of ~3.0% is obtained for P3HT-PCBM interdiffused devices when the interdiffusion is carried out at 150°C for 20 minutes. For both P3OT:PCBM and P3HT:PCBM devices, the optimum performance occurs when the concentration gradient extends across the entire film and is correlated with an increase in the short circuit current density and fill factor as well as a decrease in the series resistance. The results demonstrate that an interdiffused bilayer fabrication approach is a novel and efficient approach for fabrication of polymer solar cell devices. In addition, porphyrin derivative 5, 10, 15, 20-Tetraphenyl-21H, 23H-porphine zinc (ZnTPP) is studied as a new donor material for organic solar cells. ZnTPP: PCBM blend devices are investigated in detail by varying the weight ratio of the donor and acceptor materials in blend devices. The devices with ZnTPP: PCBM in 1:9 ratios showed the best device performance and the efficiency of the order of 0.2% is achieved under AM1.5G solar simulated conditions. Trimetallic Nitride Tempelated (TNT) endohedral fullerenes are also examined in this thesis as the novel acceptor materials. Bulk heterojunction or blend devices are fabricated with P3HT as the donor material and several TNT endohedral fullerenes as the acceptor material. Y3N@C₈₀PCBH based devices which are annealed both before and after the electrode deposition show improvement in the device performance compared to devices that are only annealed before the electrode deposition. The highest power conversion efficiency achieved for TNT endohedral fullerene devices is only 0.06%, suggesting that substantial additional work must be done to optimize the compatibility of the donor and acceptor as well as the device fabrication parameters. / Ph. D.

III-V compound Gallium Antimonide (GaSb), with a low bandgap of 0.72 eV at room temperature, is an attractive candidate for a variety of potential applications in optoelectronic devices. Ion implantation, among non-epitaxial methods, is a common and reliable doping technique to achieve local doping and obtain high-performance ohmic contacts in order to form a pn junction in such devices. An advantage of this technique over the diffusion method is the ability to perform a low-temperature process leading to accurate control of the dopant profile and avoiding Sb evaporation from GaSb surface occurring at 370 C. In this work, the effect of protons and sulfur ions as two implant species on the electrical behavior of MBE-grown undoped GaSb on semi-insulating (SI) GaAs was investigated via the Hall Effect. Protons and sulfur ions were implanted at room temperature (27 C) and 200 C, respectively, and rapid thermal annealing (RTA) was implemented at various temperatures and durations upon encapsulated GaSb. The damage induced by protons enhanced the hole density of GaSb up to around 10 times, whereas mobilities showed both increase and decrease compared to the un-implanted one, depending on the dose. While the activation of sulfur donors at an elevated temperature was anticipated after annealing sulfur implanted GaSb, instead it led to increase in p-type concentration, as the residual damage originated from sulfur implantation dominated substitutional doping. Furthermore, GaSb p/n photovoltaic devices were fabricated by applying sulfur implantation through silicon nitride layer at RT into an n-GaSb wafer (n-type base, p-type emitter). The device showed a rectifying current and photovoltaic characteristic. The J-V plot under AM1.5G illumination conditions, before and after an etch-back optimizing process, indicated lower short circuit current density J_sc, the same open circuit voltage V_oc, and higher fill factor FF, compared to the photovoltaic device with a p-type base. Also, both normalized series R_s and shunt R_p resistances in p/n diode indicated lower and higher values, respectively, as opposed to a GaSb p++/p diode, indicative of higher quality and lower manufacturing defects. / Master of Science / Generally, the photovoltaic effect is a process by which voltage or electric current is generated in a photovoltaic cell when exposed to light. A solar cell is a photovoltaic device, typically consisting a pn junction, that converts incident photon power into electrical power and delivered to a load to do electrical work for variety of applications. There are variety of methods to form a pn junction and fabricate such devices, among which ion implantation is a reliable doping technique. In this process, dopant ions are accelerated and smashed into a perfect semiconductor lattice, creating a cascade of damage that may displace a thousand atoms for each implanted ion and become activated after an annealing process. The ions themselves can act as either electron donors, make the semiconductor n-type, or electron acceptors, make it p-type. In this work, sulfur ions and protons, as two implant species, were implanted into separate Gallium Antimonide (GaSb) substrates and the effect of each on the electrical behavior of GaSb was investigated by Hall effect experiment. Both species raised hole carrier concentration. This behavior was not expected for sulfur ions as they would be assumed to act as electron donors after activation and convert the GaSb surface to an n-type semiconductor. It was identified that this behavior is due to the domination of created defects during implantation over the number of activated sulfur donors. The same characteristics were predicted and verified for proton implantation as well, the effect of which is just leaving damage in the lattice. Furthermore, to verify this method for converting n-type GaSb to p-type and fabricating a pn junction in GaSb for photovoltaic application, sulfur implantation into an n-type GaSb wafer was performed and optimized by removing the excess surface damage away from the device's metal contacts using wet etching. The device showed a diode-like rectifying current and photovoltaic characteristic. Some parameters such as short circuit current density J_sc, open circuit voltage V_oc, fill factor FF, and resistances (shunt and series) were measured and calculated using J-V plot under dark and illuminated conditions.

Photovoltaics panel (PV) integration with the utility grid has been installed throughout the globe. The fault-monitoring technology for photovoltaics (PV) panels is a method to save energy production losses and become a key contributor to overall cost reduction in variable operating costs for photovoltaics systems. PV researchers today explore factors such as reducing utility energy bills and CO2 emissions, grid voltage stability, peak demand shaving, supply of electric power off-grid areas, and many more. The technology discussed is easy to incorporate, requires no additional hardware, doesn't alter the system’s stability, is implemented at a steady state point, and is helpful to record changes in PV cell operation from forward bias to reverse bias state. PV panel AC impedance can be used as an early-stage fault indicator. Also, comparing AC impedance magnitude and phase at maximum power point (MPP) or near MPP can help identify the nature of the fault in a PV system. The focus of the thesis is proposing the fault detection of 300 W PV panels using online AC impedance measurement, utilizing existing panel-level power optimizers and microinverters in a PV system to actively perturb small signals into the PV panel and compute its small signal impedance. The technology is incorporated in a power optimizer with C2000 MCU and helps identify hot spot faults and short circuit faults in a 300 W rooftop PV panel. Multiple PV panel faults scenarios such as hot spot faults, short circuit faults, junction box faults, and capacitor faults are investigated to deduct further the effectiveness of the online impedance measurement using a small signal. This thesis’s focus areas are, first, modeling the PV panel and power converter and incorporating fault scenarios to identify the fault indicators. Secondly, measuring PV panel impedance under normal and faulty conditions using an equipment-based offline technique. Lastly, measuring PV panel impedance under normal and faulty conditions using a power optimizer. / M.S. / A Photovoltaics panel is a series and parallel combination of many photovoltaics cells to generate electricity from sunlight via a photoelectric process. The fault-monitoring technology for photovoltaics (PV) panels is a method to save energy production losses and become a key contributor to overall cost reduction in variable operating costs for photovoltaics systems. The PV panel, over a period of time, can degrade with fluctuations in temperature and weather. Photovoltaics panel (PV) integration with the utility grid has been installed throughout the globe. PV researchers today explore factors such as reducing utility energy bills and CO2 emissions, grid voltage stability, peak demand shaving, supply of electric power off-grid areas, and many more. The technology discussed is easy to incorporate, requires no additional hardware, doesn't alter the system’s stability, is implemented at a steady state point, and is helpful to record changes in PV cell operation from forward bias to reverse bias state. A PV panel operating at maximum power point (MPP) generates direct current (DC) and maintains a stable voltage across the PV panel load. A small signal injection in PV panel current or voltage is an addition of a sinusoidal signal with an amplitude of 10 % to the operating point of PV panel voltage or current and frequency sweep between 10 Hz to 200 kHz. The PV panel's AC impedance is measured under small signal injection and can be used as an early-stage fault indicator. Also, comparing AC impedance magnitude and phase at maximum power point (MPP) or near MPP can help identify the nature of the fault in a PV system. The focus of the thesis is proposing the fault detection of PV panels using online AC impedance measurement and utilizing existing panel-level power optimizers and microinverters in a PV system to actively perturb small signals into the PV panel and compute its small signal impedance. The technology is incorporated in a power optimizer with C2000 MCU and helps identify hot spot faults and short circuit faults in a 300 W rooftop PV panel. This thesis’s focus areas are, modeling the PV panel and power converter and incorporating fault scenarios to identify the fault indicators. Multiple PV panel faults scenarios such as hot spot fault, short circuit fault, junction box fault, and capacitor fault are investigated to further deduct the effectiveness of the online impedance measurement using a small signal. Secondly, measuring PV panel impedance under normal and faulty conditions using an equipment-based offline technique. Lastly, measuring PV panel impedance under normal and faulty conditions using a power optimizer.

No description available.

Organic photovoltaics (OPVs) that have been studied to date have poor power conversion efficiencies. This dissertation focuses on various molecular designs that could lead to both a fundamental understanding of photoinduced charge separation at a molecular level and also provide a solution to improve bulk properties of organic materials to overcome the poor efficiencies of OPV devices. The effect of molecular architectures on the efficiency of electron transfer, a primary step in OPVs functioning, is evaluated in this work. We have shown that even though dendrimer provides an interesting architecture for efficient electron transfer due to the presence of multiple peripheries around a single core, this architecture leads to trapping of charge at the dendritic core. This results in a decrease in the electron transfer efficiency in solution and also limits the possibility of charge transport to the electron in a photovoltaic device. Non-conjugated polymers containing conductive EDOT units at side chains were also designed and synthesized. The frontier energy levels of these polymers can be easily tuned by changing the conjugation lengths of side chain EDOT oligomers. Moreover, by incorporating crosslinkable units as co-side chains, the absorption bandwidth of these polymers can be manipulated as well.