Browsing by Subject "Solar cell"
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Item The application of light trapping structures and of InGaAs/GaAs quantum wells and quantum dots to improving the performance of single-junction GaAs solar cells(2012-05) McPheeters, Claiborne Ott; Yu, Edward T.; Alu, Andrea; Bank, Seth R.; Chen, Ray T.; Zhang, John X.High efficiency photovoltaic solar cells are expected to continue to be important for a variety of terrestrial and space power applications. Solar cells made of optically thick materials often cannot meet the cost, efficiency, or physical requirements for specialized applications and, increasingly, for traditional applications. This dissertation investigates improving the performance of single-junction GaAs solar cells by incorporating InGaAs/GaAs quantum wells and quantum dots to increase their spectral response bandwidth, and by incorporating structures that confine light in the devices to improve their absorption of it. InGaAs/GaAs quantum dots-in-wells extend the response of GaAs homojunction devices to wavelengths >1200 nm. Nanoparticles that are randomly deposited on the top of optically thick devices scatter light into waveguide modes of the device structures, increasing their absorption of electromagnetic energy and improving their short-circuit current by up to 16%. Multiply periodic diffractive structures have been optimized using rigorous software algorithms and fabricated on the back sides of thin film quantum dot-in-well solar cells, improving their spectral response at wavelengths 850 nm to 1200 nm, where only the quantum dot-in-well structures absorb light, by factors of up to 10. The improvement results from coupling of diffracted light to waveguide modes of the thin film device structure, and from Fabry-Perot interference effects. Simulations of absorption in these device structures corroborate the measured results and indicate that quantum well solar cells of ~2 µm in thickness, and which are equipped with optimized backside gratings, can achieve 1 Sun Airmass 0 short-circuit current densities of up to ~5 mA/cm2 (15%) greater than GaAs homojunction devices, and of up to >2 mA/cm2 (7%) greater than quantum well devices, with planar back reflectors. A combination of Fabry-Perot interference and diffraction into waveguide modes of the thin devices is shown to dominate the simulated device response spectra. Simulations also demonstrate the importance of low-loss metals for realizing optimal light trapping structures. Such device geometries are promising for reducing the cost of high efficiency solar cells that may be suitable for a variety of traditional and emerging applications.Item Design and production of an energy harvesting wireless sensor(2013-05) Bar, Farris Ahmad; Abraham, Jacob A.The widespread deployment of wireless sensors in our homes, offices, factories and infrastructure has opened the door for system designers to create novel approaches for powering wireless sensor nodes. In recent years, energy harvesting has emerged as the power supply of choice for embedded system designers, enabling wireless sensors to be used in applications that previously were not feasible with conventional battery-powered designs. This report details the design and development of an energy harvesting wireless sensor from concept to production. Design constraints included the requirement to operate reliably in a wide variety of environments, the use of commercially available components, and a visually appealing form factor. The result is a very power-efficient, solar-powered wireless sensor that measures temperature, voltage, and illumination level at the solar cell and has an ultra slim form factor.Item Extension of tetrathiafulvalene conjugation through pyrrollic-based dyes : ExTTF porphyrin and ExTTF BODIPY(2013-12) Bill, Nathaniel Lloyd; Sessler, Jonathan L.The research and development of organic electron donors is essential in the discovery of photodynamic therapy photosensitizers and catalysts, as well as in the fabrication of organic-based electronic devices. Recently, [pi]-extended tetrathiafulvalenes (exTTFs) have emerged as important organic donors due to their superb electronic properties. However, in general, exTTFs lack significant absorption in the visible and near-infrared portions of the electromagnetic spectrum, thereby limiting their utility. This doctoral dissertation depicts the author's efforts to address this inherent drawback of exTTFs by extending the electronic conjugation of tetrathiafulvalene moieties through pyrrole-based chromophores. The reported findings describe the design, synthesis, properties and potential applications of exTTFs with greatly enhanced absorption profiles. The first Chapter provides a brief historical overview on the history and development of [pi]-extended tetrathiafulvalenes. The various conjugated linkers utilized in exTTF systems are reviewed. In the latter part of the Chapter, emphasis is given to the applications in which exTTFs find use. Chapter 2, as the major focus of the dissertation, details the synthesis and characteristics of a quinoidal porphyrin-bridged exTTF, termed MTTFP. Several metalated complexes, including the Zn, Co, Cu, and Ni derivatives of MTTFP are reported. Additionally, the electrochemical, photophysical, and structural properties of MTTFPs are discussed. We also detail our efforts to synthesize and characterize both the one- and two-electron oxidized forms of MTTFPs. Finally, we discuss our efforts to reversibly switch thermodynamic electron transfer from ZnTTFP to Li@C₆₀ through coordination of axial ligands. Chapter 3 describes the formation of a 2:1 supramolecular ionic porphyrin complex between the two-electron oxidized form of ZnTTFP and a tetranionic sulfonated porphyrin. The association constants and the X-ray crystal structure of the complex are reported. A brief discussion outlining the photophysical characteristics (performed in Prof. Shunichi Fukuzumi and Prof. Dongho Kim's group) of the porphyrin donor-acceptor complexes are included. Chapter 4 details the synthesis, photophysical properties, and spectroelectrochemistry of a difluoroboradiazaindacene (BODIPY) bridged exTTF. This compound is referred to as ex-BODIPY. A singlet oxygen generation study provides initial evidence that ex-BODIPY could potentially serve as a photosensitizer. All of the experimental procedures, characterization data, and X-ray crystallographic data tables are reported in Chapter 5.Item Low cost processing of CuInSe2 nanocrystals for photovoltaic devices(2015-05) Stolle, Carl Jackson; Korgel, Brian Allan, 1969-; Mullins, Charles B; Manthiram, Arumugam; Vanden Bout, David A; Markert, John TSemiconductor nanocrystal-based photovoltaics are an interesting new technology with the potential to achieve high efficiencies at low cost. CuInSe2 nanocrystals have been synthesized in solution using arrested precipitation and dispersed in solvent to form a “solar ink”. The inks have been deposited under ambient conditions to fabricate photovoltaic devices with efficiency up to 3%. Despite the low cost spray coating deposition technique, device efficiencies remain too low for commercialization. Higher efficiencies up to 7% have been achieved using a high temperature selenization process, but this process is too expensive. New nanocrystal film treatment processes are necessary which can improve the device efficiency at low cost. To this end, CuInSe2 nanocrystals were synthesized using a diphenyl phosphine:Se precursor which allows for precise control over the nanocrystal size. The size is controlled by changing the temperature of the reaction. The smallest size nanocrystals demonstrated extremely high device open circuit voltage. Ligand exchange procedures were used to replace the insulating oleylamine capping ligand used during synthesis with more conductive halide ions or inorganic chalcogenidometallate cluster (ChaM) ligands. These ligands led to improved charge transport in the nanocrystal films. A high-intensity pulsed light processing technique known as photonic curing was used which allows for high temperature sintering of nanocrystal films on temperature-sensitive substrates. High energy pulses cause the nanocrystals to sinter into large grains, primarily through melting and resolidification. The choice of metal back contact has a dramatic effect on the final film morphology, with Au and MoSe2 back contacts providing much better adhesion with the CuInSe2 than Mo back contacts. Nanocrystal sintering without melting can be achieved by replacing the oleylamine ligands with ChaM ligands prior to photonic curing. Low energy photonic curing pulses vaporize the oleylamine ligands without inducing sintering or grain growth. This greatly improved nanocrystal coupling and interparticle charge transport. Multiexcitons were successfully extracted from these nanocrystal films and external quantum efficiencies over 100% were observed. Transient absorption spectroscopy was used to study the multiexciton generation process in CuInSe2 nanocrystal films and colloidal suspensions. The multiexciton generation efficiency, threshold, and Auger lifetimes for CuInSe2 compare well with other nanocrystal materials.Item Scanning probe microscopy study of thin film solar cells(2014-08) Li, Huan, Ph. D.; Shih, Chih-Kang; de Lozanne, Alejandro; Markert, John; Sitz, Greg; Tutuc, EmanuelThin film solar cells, such as CdTe, CuIn [subscript x] Ga [subscript 1-x] Se₂ (CIGS), Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe), have been intensively studied for their unique features and excellent prospect of mass production in industry. The p-n junction is the most critical part of the thin film solar cell and greatly influences the performance. In this thesis work, the p-n junctions and the device layers of multiple kinds of thin film solar cells have been studied by using scanning probe microscopy based techniques. The scanning spreading resistance microscopy (SSRM) has been developed on the cross-section of CdTe solar cells to study the resistance and carrier concentration distribution in different layers of the device. The CdTe sample was cleaved and milled with the argon ion beam to get a flat cross-section. The multiple device layers of the device were identified by the resistance mapping. A high-resistance region around the junction on the CdTe side due to carrier depletion was measured. With the AFM laser illumination, the resistance in the deep depletion region dropped and the resistance across the entire CdTe layer became relatively uniform due to domination of photo-excited carriers. With carriers injected by applying a forward-bias voltage to the working device, the resistance in the deep depletion region decreased and the region moved toward the CdS/CdTe interface. These observed trends and observations are consistent with device physics. We also measured the surface potential and the electric field across the junction using scanning Kelvin probe force microscopy (SKPFM) in the cross-section of the standard CIGS, ZnS(O,OH)/CIGS and the standard CZTSe devices. Both the heterojunction and homojunction situations of the three solar cells were simulated using the PC1D software. The simulation results were compared with the experimental results to analyze the properties of the junction. The comparison results provided the possible ranges of the thickness and carrier concentration of n-CIGS/n-CZTSe layer.