Browsing by Author "Sites, James, advisor"
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Item Open Access Comparative analysis of Cu(InGa)Se2 solar cells(Colorado State University. Libraries, 2016) Counts, Kahl, author; Sites, James, advisor; Sampath, W. S., committee member; de la Venta, Jose, committee memberCu(InGa)Se2, often abbreviated CIGS, photovoltaics have proven to be a commercially viable solar-energy conversion technology. Diverse processes have been employed in the manufacture, with varying end products, most resulting in high efficiency. A collaborative project was undertaken with several CIGS labs and industrial partners to explore the different electrical and spatial characteristics of CIGS solar cells relative to one another. Characterization methods utilized include, current-voltage measurements, quantum efficiency, capacitance-frequency and capacitance-voltage, electroluminescence, light-beam-induced current and Auger profling. Specific parameters for each cell were extracted from the measurements. Together the methods used are a tool for understanding device performance and optimization. Efforts were made to identify strengths, similarities and differences and to connect processing details with observed characteristics.Item Open Access Development and advancement of thin CdTe-based solar cells for photovoltaic performance improvements(Colorado State University. Libraries, 2020) Bothwell, Alexandra, author; Sites, James, advisor; Krueger, David, committee member; Gelfand, Martin, committee member; Sampath, Walajabad, committee member; Topič, Marko, committee memberPhotovoltaic technologies, with an essentially infinite energy source, large total capacity, and demonstrated cost competitiveness, are well-positioned to meet growing global demand for clean energy. Cadmium-telluride (CdTe) thin-film photovoltaics is advantageous primarily for its direct optical band gap (approximately 1.48 eV) which is well-matched to the standard AM 1.5G solar spectrum, and its high absorption coefficient. These advantages, in tandem with innovations in fabrication and photovoltaic design in the past decade, have significantly increased CdTe photovoltaic device performance and reduced cost. Major advances in CdTe device performance have been achieved through improved current collection and fill factor, however, the open-circuit voltage (VOC) of CdTe devices remains limited compared to the band gap-determined maximum achievable VOC. The voltage deficit could be minimized through various approaches, and this work addresses it through progressive structural changes to a thin CdTe device. Absorbers of less than 2 µm were pursued for ultimate electron-reflector devices which incorporate a wide band-gap material behind the absorber to induce a back-surface field via a back-side conduction-band offset for improved VOC. An optimized and stable base structure is necessary to quantify characteristics and improvements in progressive devices with additional material layers. Thin, 0.4-1.2 µm CdTe absorber devices were optimized and demonstrated respectable and repeatable performance parameters, and a maximum efficiency of 15.0% was achieved with only 1.2 µm CdTe. Capacitance measurements also showed that thinner devices had fully-depleted absorbers into forward bias. To improve device performance through increased current collection, a 1.4-eV band gap CdSeTe layer was introduced as an additional absorber material preceding CdTe. Prior understanding of the effects of the additional CdSeTe material was incomplete, and this work deepens and expands this understanding. Performance improvement was achieved for thin, 1.5-µm absorber devices with no intentional interdiffusion of the CdSeTe and CdTe. The importance of the CdSeTe thickness was demonstrated, where performance was consistently reduced for CdSeTe thickness greater than CdTe thickness, independent of CdSe composition in the close-space sublimation (CSS) CdSeTe source material. Longer time-resolved photoluminescence (TRPL) tail lifetimes in CdSeTe/CdTe devices compared to CdTe devices suggested better bulk properties, and current loss analysis showed that CdSeTe is the dominant absorber in 0.5-µm CdSeTe/1.0-µm devices. 1.5-µm CdSeTe/CdTe devices demonstrated increased current collection and 30-mV voltage deficit reduction due to the 100-meV narrower band gap of CdSeTe compared to CdTe and passivating effects of selenium, for an ultimate efficiency improvement to 15.6%. Lattice-constant matching to CdTe and wide, ~1.8-eV band-gap requirements directed the selection of CdMgTe as the electron-reflector layer. CdMgTe was incorporated into the CdSeTe/CdTe device structure first through CSS, but sputter deposition was found to be more favorable to address the material complexities of CdMgTe (temperature-induced magnesium diffusion and CdCl2 passivation loss, doping, and MgO formation), and produced higher performing CdMgTe electron-reflector devices. Low substrate temperature achievable in sputtered CdMgTe deposition proved the greatest advantage over CSS-CdMgTe: CdCl2 passivation and magnesium can be appropriately maintained with a corresponding maintenance of device performance, whereas temperature-induced CdCl2 passivation loss or magnesium loss will occur for CSS-deposited CdMgTe with incumbent performance reduction. Through low-temperature depositions, doping optimization, and small structural adjustments, 16.0% efficiency was achieved with CdMgTe sputtered on 0.5-µm CdSeTe/1.0-µm CdTe absorbers, the highest-known CdMgTe electron-reflector device performance. The CdMgTe and non-CdMgTe-containing device VOC's suggested that electron reflection was enacted with partial success for the sputter CdMgTe-incorporated structure, but the significant improvements expected based on simulation have not been realized due to MgO formation and a negative valence-band offset which somewhat impedes hole transport to the back contact. Suggestions to overcome or circumvent these limitations are presented and discussed in the context of progressed understanding of CdMgTe electron-reflector devices.Item Open Access Device characterization of cadmium telluride photovoltaics(Colorado State University. Libraries, 2014) Geisthardt, Russell M., author; Sites, James, advisor; Gelfand, Martin, committee member; Topič, Marko, committee member; Sampath, W. S., committee memberThin-film photovoltaics have the potential to make a large impact on the world energy supply. They can provide clean, affordable energy for the world. Understanding the device physics and behavior will enable increases in efficiency which will increase their impact. This work presents novel approaches for evaluating efficiency, as well as a set of tools for in-depth whole-cell and uniformity characterization. The understanding of efficiency losses is essential for reducing or eliminating the losses. The efficiency can be characterized by a breakdown into three categories: solar spectrum, optical, and electronic efficiency. For several record devices, there is little difference in the solar spectrum efficiency, modest difference in the optical efficiency, and large difference in the electronic efficiency. The losses within each category can also be further characterized. The losses due to the broad solar spectrum and finite temperature are well understood from a thermodynamic physics perspective. Optical losses can be fully characterized using quantum efficiency and optical measurements. Losses in fill factor can be quantified from series and shunt resistance, as well as the expected fill factor from the measured V oc and A. Open-circuit voltage losses are the most significant, but are also be the hardest to understand, as well as the most technology-dependent. Characterization of the whole cell helps to understand the behavior, performance, and properties of the cell. Several different tools can be used for whole-cell characterization, including current-voltage, quantum efficiency, and capacitance measurements. Each of these tools give specific information about the behavior of the cell. When combined, they can lead to a more complete understanding of the cell performance than when taken individually. These tools were applied to several specific CdTe experiments. They have helped to characterize the baseline performance of both the deposition tool and the measurement systems. Characterization of plasma-cleaned cells show an improvement in performance, even at thinner CdS layer thickness. Measurements of thinning CdTe samples reveal additional optical losses, likely caused by the increasing importance of the back diode. Characterization of Cd(S,O) devices show improved performance, both from improved optical properties and theorized improvement in band alignment properties. Uniformity can have an effect on whole-cell performance, but can also be an important parameter to characterize on its own. Light-beam-induced current is a powerful tool for characterizing uniformity. The LBIC tool was upgraded to improve its accuracy, functionality, and speed. The improved LBIC system aids in the collection of uniformity data. A number of parameters can be varied to provide in-depth uniformity information and help identify causes of nonuniformity. The wavelength can be varied to provide information on different layers. This can help identify variations in CdS thickness and local CdTe band gap. An applied voltage bias can be used to identify locations with weak diode properties. The resolution can also be varied to provide information on nonuniformities at different scales, from variations across the whole cell to variations on the size of several grains. LBIC can also be paired with electroluminescence to create a powerful nonuniformity characterization suite. The two can be paired with EL used as a screening tool to identify cells or areas which need further characterization from LBIC.Item Open Access Electroluminescence of thin-film CdTe solar cells and modules(Colorado State University. Libraries, 2015) Raguse, John Michael, author; Sites, James, advisor; Gelfand, Martin, committee member; Topič, Marko, committee member; Sampath, W.S., committee member; de la Venta, Jose, committee memberThin-film photovoltaics has the potential to be a major source of world electricity. Mitigation of non-uniformities in thin-film solar cells and modules may help improve photovoltaic conversion efficiencies. In this manuscript, a measurement technique is discussed in detail which has the capability of detecting such non-uniformities in a form useful for analysis. Thin-film solar cells emit radiation while operating at forward electrical bias, analogous to an LED, a phenomena known as electroluminescence (EL). This process relatively is inefficient for polycrystalline CdTe devices, on the order of 10⁻⁴%, as most of the energy is converted into heat, but still strong enough for many valuable measurements. A EL system was built at the Colorado State University Photovoltaics Laboratory to measure EL from CdTe cells and modules. EL intensity normalized to exposure time and injection current density has been found to correlate very well with the difference between ideal and measured open-circuit voltage from devices that include a GaAs cell, an AlGaAs LED, and several CdTe cells with variations in manufacturing. Furthermore, these data points were found to be in good agreement when overlaid with calibrated data from two additional sources. The magnitude of the inverse slope of the fit is in agreement with the thermal voltage and the intercept was found to have a value near unity, in agreement with theory. The expanded data set consists of devices made from one of seven different band gaps and spans eight decades of EQELED efficiencies. As expected, cells which exhibit major failure of light-dark J-V superposition did not follow trend of well-behaved cells. EL images of selected defects from CdTe cells and modules are discussed and images are shown to be highly sensitive to defects in devices, since the intensity depends exponentially on the cells' voltages. The EL technique has proven to be a useful high-throughput tool for screening of cells. In addition to EL images, other opto-electronics characterization techniques were used to analyze defects in cells and modules such as weak-diode areas, cell delineation near substrate edge, non-uniform chlorine passivation, holes in back contact, high-resistance foreign layer, high back-contact sheet resistance, a discontinuous P3 line scribe (intercell shunt) and shunt through a cell (intracell shunt). Although EL images are proficient at illustrating the location and severity of defects with potentially high spatial resolution and short measurement times, their ability to identify the cause of such defects is limited. EL in concert with Light-Beam-Induced Current (LBIC), however, makes for a powerful ensemble as LBIC can probe different film layers at arbitrary voltage bias conditions, albeit with increased measurement times and potentially reduced spatial resolution.Item Open Access Measurement of cadmium telluride bilayer solar cells(Colorado State University. Libraries, 2024) Chime, Chinecherem Agnes, author; Sites, James, advisor; Buchanan, Kristen, committee member; Sampath, Walajabad, committee memberPhotovoltaic (PV) technology is a green technology that uses devices and semiconducting materials to generate power by converting the absorbed energy from solar to electrical energy. Understanding the performance and behavior of a fabricated device is essential for enhancing their efficiency for future commercialization. Cadmium-telluride (CdTe) technology is a PV technology that uses CdTe as the semiconductor layer for absorbing and converting sunlight into electricity. Incorporating a bilayer of cadmium selenium telluride (CdSexTe1-x) alloy and CdTe into solar cell devices have shown particularly good performance, enhanced passivation, and higher efficiency. In this research, cadmium telluride solar cells were fabricated with a focus on improving the performance of the absorber layers. Radio frequency (RF) magnetron sputtering and close-space sublimation were adopted in preparing the front and back contact layers respectively. The fabricated device comprises of Tec-10 glass/100-nm magnesium-doped zinc oxide (MZO)/0.5-μm CST40/2.5-µm CdTe/ cadmium-chloride passivation/ Cu-doping/ 40-nm Te/ carbon and nickel paint back contact. As part of the performance improvement measures, the bilayer surface was passivated with cadmium chloride (CdCl2) and doped afterwards with copper. The fabricated CdSexTe1-x/CdTe device was subjected to room temperature and low temperature current density-voltage (J-V), capacitance, phase angle, quantum efficiency (QE), reflectance, electroluminescence (EL), and photoluminescence (PL) measurements. The J-V characteristics gave 15% device efficiency and showed diode curves which rolled over at lower temperatures, but were more ideal at higher temperatures. Capacitance measurements gave a hole density of 4x1014 cm-3 and a phase angle of 88o. The cells recorded high quantum efficiency of about 85% which is indicative of reduced recombination rate. Few defects were observed from the EL images while the PL emission peaks were obtained at 875 nm corresponding to an approximate energy band gap value of 1.42 eV. The measurement results show good performance for use in commercial solar cells for energy sustainability. Future implications encompass module fabrication, flexible devices, and affordability for enhancing green energy production and minimizing environmental pollution. Prospects envisage fabricating CdTe devices with higher efficiencies which would continue to compete successfully with other solar cell technologies.Item Open Access Metal oxides as buffer layers in polycrystalline CdTe thin-film solar cells(Colorado State University. Libraries, 2021) Pandey, Ramesh, author; Sites, James, advisor; Sampath, W.S., committee member; Ross, Kate, committee member; Harton, John, committee memberThe optical band-gap of 1.5 eV and absorption coefficient the order of 105 cm-1 makes CdTea very attractive absorber for thin-film solar cells. This dissertation explores methods to improve both the front, or emitter, part of the cell and the back contact to the CdTe-based thin-film solar cells. The choice of an n-type emitter partner for CdTe based solar cells is crucial to the overall power conversion efficiency. In comparison to the traditional CdS emitter, metal oxides such as ZnO, MgO, and the ternary alloy MgxZn1-xO have large optical band-gaps making them transparent to most of the solar spectrum and an ideal emitter layer adjacent to light-facing side of the absorber in a superstrate configuration. The optical and electrical properties of MgxZn1-xO emitters can be modulated by varying the elemental ratio of x = Mg:(Mg + Zn) in the ternary alloy. Tracing the variation of the conversion efficiency as a function of Mg fraction in MgxZn1-xO emitter, an optimal Mg fraction of x = 0.15 was found to produce highest efficiency for the CdTe-based thin-film solar cells. Photoelectron spectroscopy demonstrated the conduction band offset at the emitter/absorber interface transitions from a cliff like -0.1 eV for x = 0.00 to a spike like 0.2 eV at the optimal x = 0.15. Photoluminescence and low-temperature current-voltage measurements showed that the interface between MgZnO and the CdSeTe is well passivated for x = 0.15. Further increase in the Mg fraction however increases the band offset between the emitter/absorber leading to distortions of J-V curves under various illumination conditions. Light soaking experiments and numerical simulations show that an insufficient density of carriers in the MgZnO due to the compensating defects causes these distortions: a failure of superposition of light and dark curves referred to as cross over, and distortion from normal current voltage behavior under spectra filtered illumination. An extrinsic doping of the emitter is critical to rectify these distortions and Ga-doped MgZnO was employed to experimentally demonstrate a cure to these J-V distortions characteristic of an undoped MgZnO emitter. It paves pathway to increase the n-type carrier density in the MgZnO emitter. The group-V doping of CdTe has shown potential to improve open circuit voltage, with level of doping in absorber the order of 1016 cm-3 and lifetimes of hundreds of ns. Numerical device simulations demonstrate that doping the emitter layer is essential and a particular challenge if the doping in the absorber is high. The results find the carrier concentration in emitter should be higher than the doping in the absorber to attain high open-circuit voltage in the highly doped CdTe-absorbers possible with arsenic doping. Various back contact metals like Ag, Co, Pt and metalloids like Te, and Se with different work functions were used to make an ohmic contact with the CdTe back surface. The use of a buffer layer behind the bare CdTe surface is found to be critical to the device performance. A thin 30-nm layer of Te have become the preferred choice of back buffer layer. Metal oxides like TeOx has been introduced as back buffer between the CdTe absorber and Te back contact to study their effect in device performance. The study finds that a double CdCl2 passivation procedure before and after the deposition of oxides is critical to the performance of these solar cells. Devices with the TeOx and the Te layer as back buffer demonstrated a power conversion efficiency in excess of 17 % without the incorporation of dopant in the absorber. Such a result is significant, as extrinsic dopants in CdTe-based absorbers often introduce defects in the absorber leading to increased recombination and degradation of cell performance particularly if the absorber is doped with Cu. Spectral and time resolved photoluminescence measurements carried out with illumination from front glass side show such cells have improved minority carrier lifetimes. The rear TRPL illumination to probe a CdTe/TeOx surface measured lifetimes of few ns indicative of the TeOx as a back buffer layer to mitigate the effects of large defects on a free CdTe surface. These results demonstrate metal oxides as a promising candidates for back buffer layers, and passivating back contact for hole selectivity in the CdTe-based solar cells.Item Open Access Performance and metastability of CdTe solar cells with a Te back-contact buffer layer(Colorado State University. Libraries, 2017) Moore, Andrew, author; Sites, James, advisor; Krueger, David, committee member; de la Venta, Jose, committee member; Sampath, W. S., committee memberThin-film CdTe photovoltaics are quickly maturing into a viable clean-energy solution through demonstration of competitive costs and performance stability with existing energy sources. Over the last half decade, CdTe solar technology has achieved major gains in performance; however, there are still aspects that can be improved to progress toward their theoretical maximum efficiency. Perhaps equally valuable as high photovoltaic efficiency and a low levelized cost of energy, is device reliability. Understanding the root causes for changes in performance is essential for accomplishing long-term stability. One area for potential performance enhancement is the back contact of the CdTe device. This research incorporated a thin-film Te-buffer layer into the contact structure, between the CdTe and contact metal. The device performance and characteristics of many different back contact configurations were rigorously studied. CdTe solar cells fabricated with the Te-buffer contact showed short-circuit current densities and open-circuit voltages that were on par with the traditional back-contacts used at CSU. However, the Te-buffer contact typically produced ~2% larger fill-factors on average, leading to greater conversation efficiency. Furthermore, using the Te buffer allowed for incorporation of ~50% less Cu, which is used for p-type doping but is also known to decrease lifetime and stability. This resulted in an additional ~3% fill-factor gain with no change in other parameters compared to the standard-Cu treated device. In order to better understand the physical mechanisms of the Te-buffer contact, electrical and material properties of the Te layer were extracted and used to construct a simple energy band diagram. The Te layer was found to be highly p-type (>1018 cm-3) and possess a positive valence-band offset of 0.35-0.40 eV with CdTe. An existing simulation model incorporating the Te-layer properties was implemented and validated by comparing simulated results of CdTe device performance to experimental values. The Te layer improves performance is attributed to a reduction in the downward energy band bending between the CdTe and typical contact metals. The stability, or rather the metastability, of CdTe solar cells was also studied with a focus on the Te back contact. A metastable device has a series of quasi-stable local energy-minimuma which the device may transition among. This work primarily focused on changes, both beneficial and detrimental, caused by diffusion and drift of atoms in the CdTe lattice. As atoms moved and/or became ionized their defect states were shifted, which resulted in changes in the CdTe doping and recombination. Changes in performance for devices in equilibrium and under stress conditions were analyzed by electrical and material characterization. Mobile impurities and mechanisms responsible for the changes were identified---primarily the migration of interstitial Cu and Cl. The stability of CdTe solar cells with different back contacts were compared. It was found that any contact that included the Te layer was almost always more stable than the traditional contact used at CSU, most likely because of less sensitivity to the impurity profiles in the CdTe. Moreover, the Te contact configuration that introduced the least amount of Cu into the CdTe was discovered to be the most stable, both in storage and under stress conditions.Item Open Access Stability of thin-film CdTe solar cells with various back contacts(Colorado State University. Libraries, 2021) Hill, Taylor D., author; Sites, James, advisor; Sampath, Walajabad, advisor; Bradley, Mark, committee memberWith an increasing reliance on photovoltaic energy comes an ever-increasing demand to understand the mechanisms of failure which lead one to having an under-performing solar module. Recent technological advances have proven CdTe solar cells to be competitive with traditional Si, taking up 5% of the world solar market and reaching efficiency upwards of 22.1% for small area scale and 18.6% for module scale. This thesis explores various back-contact configurations to reduce the contact barrier height as well as how they hold up under accelerated lifetime testing. Various degradation mechanisms, such as diffusion of species, drift within the built-in fields, and formations of various impurities/complexes on the surface and within the bulk were explored. The results of accelerated-lifetime experiments revealed the instability of devices with large amounts of Cu and those containing the colloidal Ni based paint solution as a metallic back contact. Sputtered films of nickel doped with vanadium (Ni:V) and chromium (Cr) demonstrated the capability to produce cells with efficiencies between 12-13% with fill factors up to 75%. Metallic bilayers containing a metallic cap of aluminum (Al) were then evaluated, demonstrating an increase in efficiency up to 15.1%. Buffer layers of NiO revealed the presence of a large back-contact barrier via the rollover effect in forward bias, leading to devices with efficiency of only 3%, but subsequent work revealed that by applying the NiO buffer prior to CdCl2 passivation reduces the back barrier and produces cells with peak efficiency of 14.8%.