This study has been conducted as a Minor Field Study (MFS). It focuses on solar water pumping for small-scale farmers in the Kilimanjaro Region of Tanzania. The purpose is to investigate the possibilities for rural farmers to operate their irrigation with solar power instead of their current option: fossil fuels, primarily petrol. The study was conducted in three phases, starting with pre-study in Sweden, followed by field study in Tanzania from January to March 2015 and finishing with summarizing and calculating in Sweden. Fuel powered water pumping has a cheap capital cost; however, it is expensive and problematic to maintain and operate. Solar powered water pumping is almost completely opposite. It has a higher initial cost; however, it is considerably cheaper to run. The results indicate that the investment in solar power might be too expensive for the farmers, as long as they do not receive external financial and educational support. Assuming that the farmers are able to obtain a solar water pumping system, results show that they will benefit and save a considerably amount of money over a long period of time. Also, solar water pumping is environmentally friendly compared to the systems in Tanzania today.

Transferring organic photovoltaics (OPV) from the laboratory into economically feasible products, requires the fabrication of modules, a series of connected single cells. During this transition, there is typically a drastic decrease in power conversion efficiency (PCE). This thesis reports on the design, fabrication, and characterization of state-of-the-art, high-performance organic photovoltaic modules with a novel geometry that composed of unit cells with alternating electrical polarities. Such configuration is realized by exclusive patterning of the interlayers and electrodes and avoids patterning of the photoactive layer. With this novel architecture, area losses of photovoltaic module can be significantly reduced compared with the conventional configurations. The processing of this new solar cell module is also compatible with large area processing techniques such as slot-die coating. This thesis reports on 4-cell and 8-cell modules, wherein the measured fill-factors (FF) and PCE of the constituent sub-cells and of the modules are almost identical. The 4-cell module, with a total area of 0.8 cm2, exhibits an open-circuit voltage (VOC) of 3.15 V, a short circuit-current density (JSC) of 2.3 mA/cm2 and a FF of 0.69, yielding a PCE of 5.01%. The 8-cell module, with a total area of 1.6 cm2, exhibits a VOC of 6.39 V, a JSC of 1.2 mA/cm2 and a FF of 0.63, yielding a PCE of 5.06%. Similar PCE values between 4-cell and 8-cell module is a demonstration of scalability of this novel geometry without compromising the efficiency.

CdS/CdTe solar cell performance and reproducibility can be improved by integrating a ZTO buffer layer, which interdiffuses into the CdS layer during device fabrication. Reducing the thickness of CdS layer improves the QE in the blue spectral region without affecting the device performance. This buffer layer is expected to prevent the formation of localized TCO/CdTe junction during high temperature processing.The CdS/CdTe Solar Cell was modified by introducing ZTO as a buffer layer between the window layer (CdS) and the absorber layer (CdTe). Studies were performed on different varying ZTO processing parameters like (a) Zn/Sn atomic ratios during sputtering process, (b) ZTO thickness, (c) ZTO heat treatment temperature, and (d) ZTO heat treatment ambient.

This thesis presents audible vibratory mode data obtained by mechanically exciting acoustic modes in mc-Si wafers grown by EFG technique with various levels and distributions of residual stress. Stress maps obtained using scanning infrared polariscopy are presented, illustrating the variation of residual stress.Modal analyses of the wafers are performed using the finite element method and are in remarkably good agreement with the measured frequency response data. The calculated mode shapes were further validated through classic Chladni type patterns.The vibratory data is found to correlate with the residual stress measurements. The data is fit with both linear and quadratic models with correlation coefficients of 0.8. The results reveal a dependence of wafer audible mode frequencies on residual stress level that may be useful for solar cell mechanical quality control and breakage inspection.

Today, there is an ever-growing demand for compact, and energy autonomous, implantable biomedical sensors. These devices, which continuously collect in vivo physiological data, are imperative in the next generation patient monitoring systems. One of the fundamental challenges in their implementation, besides the obvious size constraints and the tissue-to-electronics biocompatibility impediments, is the efficient means to wirelessly deliver power to them. This work addresses this challenge by demonstrating an energy-autonomous and fully-integrated implantable sensor chip which takes advantage of the existing on-chip photodiodes of a standard CMOS process as photovoltaic (PV) energy-harvesting cells. This 2.5 mm × 2.5 mm chip is capable of harvesting [mu]W’s of power from the ambient light passing through the tissue and performing real-time sensing. This system is also MRI compatible as it includes no magnetic material and requires no RF coil or antennae. In this dissertation, the optical properties of tissue and the capabilities of the CMOS integrated PV cells are studied first. Next, the implementation of an implantable sensor using such PV devices is discussed. The sensor characterizing and the in vitro measurement results using this system, demonstrate the feasibility of monolithically integrated CMOS PV-driven implantable sensors. In addition, they offer an alternative method to create low-cost and mass-deployable energy autonomous ICs in biomedical applications and beyond. / text

A pulse-laser based time-resolved photocurrent (TRPC) and photoluminescence (TRPL) system with a programmable Boxcar integrator/averager system incorporated was implemented to investigate the optical properties and charge carrier dynamics in a GaInP/GaAs single-junction photovoltaic device for the purposes of understanding fundamental optoelectronic processes in the solar cell. The implementation of whole system was realized by integrating the instrument of a Boxcar averager system with a pulse laser source + spectroscopic facilities. The delay time control and data acquisition were organized by the software code. The effects of the hardware configurations and the software parameters on the performance of the system were particularly addressed for the optimization of measurement conditions and precisions. Two main functions of TRPC and TRPL with a wide time range were demonstrated for the system. The system was employed to measure temperature- and bias voltages-dependent TRPC and TRPL spectra of a GaInP/GaAs single-junction photovoltaic device. The spectral data show a lot of information about the transient dynamic behaviors of photogenerated charge carriers in the device, including both the rise and decay processes. Interestingly, the measured time-resolved photocurrent curves are characterized by a fast rising edge followed by a relatively slow decay process as the temperature increases. Relevant theoretical calculations and analysis to the experimental curves were also carried out to understand diffusion and transport processes of charge carriers inside the device. The results show that the variation in temperature and reverse biases results in the structural change in the space charge region of the P-N junction and therefore affects the rise and decay time constants of the time-resolved photocurrent. The TRPL spectral data give information of mid-way radiative recombination of charge carriers in the device. / published_or_final_version / Physics / Master / Master of Philosophy

Human-induced climate change, with its potentially catastrophic impacts on weather patterns, water resources, ecosystems, and agricultural production, is the toughest global problem of modern times. Impeding catastrophic climate change necessitates the widespread deployment of renewable energy technologies for reducing the emissions of heat-trapping gases, especially carbon di-oxide (CO₂). However, the deployment of renewable energy technologies is plagued by various market failures, such as environmental externalities from conventional energy sources, learning-by-doing, innovation spillover effects, and peer effects. In efforts to begin to address these market failures, several governments at all levels—city, state, regional, and national—have instituted various subsidies for promoting the adoption of renewable energy technologies. Public resources are limited and have competing uses. So, it is important to ask: how cost-effective are renewable energy subsidies? Are the subsidies even reaching the intended subjects—the potential adopters of renewable energy technologies? In this empirically-driven dissertation, I analyze these important policy design and evaluation questions with a focus on the solar subsidy programs in California. All programs to incentivize the adoption of renewable energy technologies run into the same key question: what is the optimal (maximum capacity inducing) rebate schedule in the face of volatile product prices and the need for policy certainty? Answering this question requires careful attention to both supply-side (learning-by-doing) and demand-side (peer effects) market dynamics. I use dynamic programming to analyze the effectiveness of the largest state-level solar photovoltaic (PV) subsidy program in the U.S. – the California Solar Initiative (CSI) – in maximizing the cumulative PV installation in California under a budget constraint. I find that previous studies overestimated learning-by-doing in the solar industry. Consistent with other studies, I also find that peer effects are a significant demand driver in the California solar market. The main implication of this empirical finding in the dynamic optimization context is that it forces the optimal solution towards higher subsidies in earlier years of the program, and, hence, leads to a lower program duration (for the same budget). In particular, I find that the optimal rebate schedule would start not at $2.5/W as it actually did in CSI, but instead at $4.2/W; the effective policy period would be only three years instead of the realized period of six years. This optimal (i.e., most cost effective) solution results in total PV adoption of 32.2 MW (8.1%) higher than that installed under CSI, using the same budget. Furthermore, I find that the optimal rebate schedule starts to look like the actual CSI in a ‘policy certainty’ scenario where the variation of periodic subsidy-level changes is constrained. Finally, introduction of stochastic learning-by-doing as a way to better capture the dynamic nature of learning in markets for new products does not yield significantly different results compared to the deterministic case. Another, still-unanswered, redistribution question related to the CSI program is: to what degree have the direct PV incentives in California been passed through from installers to consumers? I address this question by carefully examining the residential PV market in California by applying multiple methods. Specifically, I apply a structural-modeling approach, a reduced-form regression analysis, and regression discontinuity designs to estimate the incentive pass-through rate in California’s solar program. The results consistently suggest a high average pass-through rate of direct incentives of nearly 100%, though with regional differences among California counties and utilities. While these results could have multiple explanations, they suggest a relatively competitive market and a smoothly operating subsidy program. Combining evidence from the optimal subsidy policy design and the incentive pass-through analysis, this dissertation lends credibility to the cost-effectiveness of CSI given CSI’s design goal of providing policy certainty and also finds a near-perfect incidence in CSI. Long-term credible commitment as reflected through CSI’s capacity-triggered step changes in rebates along with policy and data transparency are important factors for CSI’s smooth and cost-effective functioning. Though CSI has now wound down because final solar capacity targets have been reached, the historical performance of CSI is relevant not only as an ex-post analysis in California, but potentially has broader policy implications for other solar incentive programs both nationally and internationally. / text

Colloidal nanocrystal assemblies offer an attractive opportunity for designer metamaterials. The ability to permute chemical composition, size, shape, and arrangement of nanocrystals leads to an astounding number of unique materials properties that find use in an extensive array of applications---ranging from solar cells to medicine. However, to take full advantage of these materials in useful applications, the nature of their assembly and their behavior under external stimuli must be well understood. Additionally, the assembly of colloidal nanocrystals into thin films provides a promising pathway to the solution-processing of inorganic materials that are prohibitively too expensive and/or difficult to deposit by conventional methods. Nanocrystal superlattices (NCSLs) of sterically stabilized nanocrystals were assembled by slow evaporation of colloidal dispersions on various substrates. Detailed analysis of the NCSL structures was carried out using transmission and scanning electron microscopy (TEM and SEM) and small-angle x-ray scattering (SAXS). Body-centered cubic (bcc) NCSLs, in particular, were studied in detail and ligand packing frustration was proposed as a significant driving force for their assembly. The behavior of NCSLs was also studied by SAXS under mild heating and solvent vapor exposure revealing several remarkable order-order, order-disorder, and amorphous-crystalline structural transitions. Colloidal Cu(In [subscript 1-x] Ga [subscript x])Se₂ (CIGS) nanocrystals were synthesized by arrested precipitation and formulated into inks. These inks were spray deposited into thin films under ambient conditions to serve as the active light absorbing material in printed low-cost photovoltaic (PV) devices. These devices, which were fabricated without the need for high temperature processes, have achieved power conversion efficiencies above 3 % under AM1.5 illumination. While the efficiencies of these devices are still too low for commercial viability, this work does provide a proof of concept that reasonable efficient solar cells can be created with a low-cost printable process using nanocrystal inks. Since high temperatures are not used to form the light-absorbing layer, nanocrystal-based solar cells were built on flexible light weight plastic substrates. The main obstacle to achieving high power conversation efficiencies was found to be the ability to extract the photo induced charge carriers. Nanocrystal films suffer from poor transport that leads to high recombination rates in thicker films. To date, the best efficiencies have been achieved with thin light absorber layers that only absorb a fraction of the incident light. / text

The effects of morphology on electronic and optical properties in solution processable photovoltaic (PV) materials have been studied through two different approaches. One approach, scanning photocurrent (PC) and photoluminescence (PL) microscopy, involved mapping PC generation and PL in functional PV devices on the length scale of around 250-500 nm. Additionally, local diode characteristics were studied from regions of interest in the PV through local voltage-dependent photocurrent (LVPC) measurements. In a PV made from a Copper Indium Gallium Selenide (CIGS) nanocrystal (NC) "ink", two morphological features were found to cause the spatial heterogeneity in PC generation. Cadmium Sulfide (CdS) aggregates lowered PC generation by blocking incident light to the photoactive layer, and cracks in the CIGS-NC film enhanced PC generation through improved charge carrier extraction. LVPC measurements showed all regions to have similar diode characteristics with the main difference being the PC generated at zero bias voltage. For another PV made from a donor/acceptor blend of poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,Nphenyl- 1,4-phenylenediamine (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole)(F8BT), two incident laser wavelengths were used to selectively illuminate only one or both polymers. The results showed that when F8BT is illuminated, the PFB-rich regions produced the most PC and when both polymers are illuminated (but mostly PFB), the F8BT-rich regions produce the most PC; showing PC generation is more affective when less absorber material is present in the morphology. The other approach to study morphological effects on PV properties was to fabricate particles that mimicked morphological variations known to occur in solution-processable PVs. Through solution processing of an oligothiophene molecule, a range of weakly coupled H-aggregate particles were made. These particles, identifiable by shape, were shown to have a varying degree of energetic disorder (as gauged by the 0-0 vibronic band intensity in the emission spectrum), despite all particles showing a similarly high degree of molecular order from fluorescence dichroism (FD) measurements. A trend was observed correlating a decrease in energetic disorder with an increase in the local contact potential (LCP) difference as measured with Kelvin probe force microscopy (KPFM). The LCP difference was found to range by 70 mV between particles of moderate to low energetic disorder.

This research will strive to establish a design methodology to achieve an ideal balance or ratio between solar energy available at a given site and the electrical energy requirement of a residence in a humid subtropical climate. Solar technology should be considered as an important element of the design and not a mere energy source added after the design has been completed. The introduction of this technology should be established at the conceptual stage and evolve through the whole design process of the project. Solar energy is without doubt the best choice as an alternate to fossil fuels in Florida's humid subtropical climate, however harnessing this readily available energy source requires careful planning. Adding solar energy components during the final design stage will impact negatively on the aesthetics of the design and most likely will not provide the energy necessary to achieve sustainability. Each climate and latitude requires different strategies to maximize available solar energy, thus the design has to adapt to the energy source of the given site. Florida's original vernacular design concepts dealt with solar energy issues in a passive way, by providing shelter from the sun and creating air circulation for evaporative cooling. Today's photovoltaic technology can activate vernacular principles and create new sustainable typologies. Unlike tropical climates, the humid subtropical deals with high relative humidity in the summer months, thus demanding the use of mechanical cooling in order to reach the required comfort zone within the building's envelope. Fortunately solar energy is readily available in Florida during the critical summer season when the largest electrical loads are required by mechanical cooling. An "off the grid" sustainable design could be achieved if electrical energy use is minimized to the essentials, wind technology is used to complement the photovoltaic system and alternate energy sources as gas, alcohol and alike are used for other household energy demands that do not need to be of electrical type. This design criteria will allow Florida residents to experience a more fulfilling existence by interacting with nature in a more dynamic, efficient and intimate way.