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Browsing by Author "Sampath, Walajabad S., advisor"

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    Addressing absorber quality in CD1-xMGxTE wide bandgap solar cells for tandem applications
    (Colorado State University. Libraries, 2018) Reich, Carey L., author; Sampath, Walajabad S., advisor; Sites, James, committee member; Popat, Ketul, committee member
    Tandem photovoltaic (PV) solar cells, which use multiple absorbing layers to convert light into electricity, have the potential to surpass the conversion efficiency limits of PV which uses a single absorber. This has been proven using epitaxially grown III-V semiconductors, but these are expensive and are only commonly used for extra-terrestrial applications. To realize terrestrial, cost effective tandem PV, low cost production of these highly efficient cells is required. Using absorbers which are similar to cost effective, mass produced PV such as CdTe, Si, or CIGS, this is possible. Si and CIGS have appropriate properties for the IR absorbing layer in a tandem cell, but there is no common PV material with the ideal properties for the UV/Visible light absorbing layer, although CdTe is quite close. Even better, CdTe's properties can be altered to those of ideal with the addition of ternary elements such as Mg, Zn, and Mn. Issues still remain however as the quality of solar cells produced using ternary alloys of CdTe is much lower than that of the base material. These quality issues seem to stem from the CdTe bulk passivation process, which involves a thermal treatment in the presence of Cl (commonly CdCl2 is used as a source) to passivate the grain boundaries and catalyze the recrystallization and grain growth process which annihilates detrimental planar crystalline defects in the absorbing material. The work presented in this thesis addresses issues with the absorber quality of solar cells using Cd1-xMgxTe by using concurrent Cl sources with CdCl2, diffusion barriers during CdCl2, and tweaking the absorber material with the addition of quaternary elements or novel layers in the device stack. This work culminated in the production of a 10.6% efficient device, a record for devices using CdMgTe as an absorber, and concludes with paths for future improvements in device performance.
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    Density functional theory and Green's function approach to investigate cadmium telluride based thin film photovoltaics
    (Colorado State University. Libraries, 2020) Nicholson, Anthony P., author; Sampath, Walajabad S., advisor; Weinberger, Chris, advisor; Popat, Ketul, committee member; Sites, James, committee member; Martinez, Umberto, committee member
    In recent years, cadmium telluride (CdTe) based thin film photovoltaics (PV) have exhibited remarkable improvements in overall efficiency and device performance. As the most notable thin film PV technology, CdTe PV is developed and manufactured in the U.S. as the leading cost-competitive option for electricity generation in comparison to other PV technologies such as silicon and CIGS PV. However, CdTe PV faces major challenges that limit its achievable performance during the solar energy conversion process. It has become increasingly evident that to improve PV efficiency, an understanding of surfaces and interfaces is necessary. Therefore, high-fidelity quantum-based atomistic simulations will be used to calculate energy band alignment of CdTe thin film surfaces and interfaces to resolve the issues found in such problematic areas and advance PV efficiency. Ab initio simulation models implement density functional theory (DFT) coupled with Green's function (GF) for investigating the electronic and structural properties of thin film surfaces and interfaces within CdTe PV device configurations. Comprehensive studies on spatially-dependent energy band alignments with respect to plane orientation, terminated surfaces, carrier concentrations and elemental composition were computationally evaluated to determine their possible effect on CdTe solar cell device performance. A total of 14 unique CdTe-based surfaces and 3 different CdTe/Te interfaces were simulated to determine their effect on energy band alignment. A number of key insights were gained that include: 1) the band bending directions dictated by the termination layer based on surface theory; 2) the role of surface reconstruction in flattening the CdTe surface energy band alignments while neutralizing surface states due to the fulfillment of the electron counting rule; 3) the formation of a cusp energy potential feature along the CdTe{111} plane oriented energy band alignments as observed by external literature studies within the CdTe/Te interface. Results to date indicate that the DFT+GF atomistic modeling approach to constructing energy band alignments matches closer to experiments than conventional band alignment methods. State-of-the-art DFT+GF calculations on CdTe-based thin film surfaces and interfaces provide a methodology for studying quantum mechanical effects in thin film PV devices such as high-efficiency single junction CdTe solar cells and tandem solar cells.
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    Experimental and theoretical investigations of selenium graded cadmium telluride-based solar cells
    (Colorado State University. Libraries, 2022) Shah, Akash, author; Sampath, Walajabad S., advisor; Weinberger, Chris, advisor; Sites, James, committee member; Martinez, Umberto, committee member
    In the past few years, cadmium telluride (CdTe) based solar cells have emerged as an important photovoltaic (PV) technology for electricity generation. The optimal bandgap of the absorber material and high throughput manufacturing methods make CdTe PV a leading cost-competitive alternative to silicon solar cells. However, the performance of CdTe PV-devices strongly depends on the processing of the absorber layer. This work uses experiments in conjunction with two-probe atomistic models based on density functional theory (DFT) to understand the processing effects on the electronic properties of heterostructures in CdTe PV-devices and improve the efficiencies of CdTe solar cells. Various processing conditions such as cadmium chloride (CdCl2) treatment and selenium (Se) alloying of CdTe absorber layer were found to improve the PV-device efficiencies (from 0.22% to 18.3%). Atomistic simulation models utilized DFT coupled with Green's function to investigate the electronic properties of thin film surfaces, grain boundaries and interfaces within theCdSeTe/CdTe PV-device configuration. Structural and electronic properties of bulk CdSexTe1-x and CdSe0.25Te0.75 surfaces were calculated and compared with the experimental and theoretical literature to establish the modeling parameters. The results from the elemental characterization done on the CdCl2 treated CdSeTe/CdTe films formed the basis of respective CdSeTe/CdTe interface and grain-boundary atomistic models. The electronic properties calculated for Se-graded CdSeTe/CdTe interfaces showed a pick up in the conduction band (CB) energy level (creating electron reflector effect) and was verified experimentally by Ultraviolet Photoelectron Spectroscopy (UPS). DFT models also suggested that higher p-doping is required in CdSeTe-only absorber to achieve similar electron reflector effect at the CdSeTe/Te interface. The grain-boundary models further showed that presence of Se and Cl at the CdTe grain-boundary passivates the critical acceptor defect states and leads to n-type inversion at the grain-boundary. The band engineering and defect state passivation in the Cl and Se-alloyed CdTe solar cells efficiently extracted the charge carriers, thereby producing high-performing CdSeTe/CdTe PV-devices ( 19.4%). In comparison to unalloyed CdTe absorbers, Se alloying passivated the defect states in CdTe grain boundaries. It has also been observed that Alumina/CdSeTe/Alumina double heterostructures showed higher minority carrier lifetime indicating better passivation in the bulk and the oxide/absorber interface. The electronic properties calculated via DFT suggested Te/O interface formation at the CdSeTe/Alumina interface led to reduction in the interfacial defect states. This combination of lower density of defect states at the CdSeTe grain boundaries and CdSeTe/oxide interface produces high lifetime (430 ns) in the oxide/CdSeTe double heterostructures. Following this, TeOx was also used as the back passivation layer of unalloyed CdSeTe/CdTe bilayer to produce 19% efficient CdTe PV-devices. The introduction of TeOx as the potential passivation layer eliminates the requirement of copper doping in the CdTe PV-devices, thereby increasing the longevity of CdTe solar panels in the field.
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    Improving thin-film polycrystalline CdSeTe/CdTe solar cell efficiencies through statistical design of experiments
    (Colorado State University. Libraries, 2022) Lustig, Zachary F., author; Sampath, Walajabad S., advisor; Sites, James R., committee member; Popat, Ketul C., committee member
    In recent decades, cadmium telluride (CdTe) solar photovoltaic (PV) technology has become increasingly popular to meet global energy demands. Its high throughput industrial fabrication methods, low material usage, recyclability, longevity, and theoretical maximum efficiency have led to its widespread integration in the PV sector. Most of the CdTe PV research reported in literature utilizes one-factor-at-a-time (OFAT) experiments. This work leverages statistical design of experiments (DOE) and statistical analysis of data to study the relationships between multiple processing factors and solar cell performance metrics. OFAT only indicates the primary effect of the chosen variable, whereas DOE determines the primary effect as well as the interaction effects. DOE determines both critical and insignificant factors, whereas OFAT assumes everything is a critical factor. DOE also requires fewer experiments, has more sophisticated predictive capabilities, and streamlines process optimization in comparison to OFAT. Since DOE is most effective with large data sets, the unique high throughput capability of the Advanced Research Deposition System (ARDS) at Colorado State University makes our lab a perfect candidate to utilize DOE for CdTe solar cell research. In this study, DOE and statistical analysis were used to investigate copper (Cu) doping, electrode painting, absorber deposition rate and temperature, p-doping of CdSe0.4Te0.6 (CST40) through arsenic (As) incorporation and tellurium (Te) overpressure, and oxide deposition at the back of the cell. Multiple linear regression (MLR) and analysis of variance (ANOVA) were conducted on all DOE's. An improved process was identified for the baseline high efficiency Cu-doped solar cells in which total process time was reduced by 33%. A thick 6µm structure of 18.5%+ efficiency was developed following statistical model suggestions. A standard procedure for electrode painting was developed. As a result of DOE, several 19%+ cells were fabricated achieving the highest efficiency of 19.44%. The best performing As doped CST40 graded CdTe cells of 18.5%+ were also fabricated using these methods. Carrier concentration versus voltage plots indicated successful p-doping of CST40 with As. Annealing the absorber with cadmium arsenide (Cd3As2) and depositing tellurium oxide (TeOx) at the back of the cell improved performance, yielding 80%+ fill factors. Decreasing thickness of CdTe behind CST40:As increased short-circuit current density to 30 mA/cm2+. Lastly, thinner absorbers yielded higher performance when backed with NiO:Cu.
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    Investigations to improve CdTe-based solar cell open circuit voltage and efficiency using a passivation and selectivity theoretical framework
    (Colorado State University. Libraries, 2022) Reich, Carey, author; Sampath, Walajabad S., advisor; Holman, Zachary C., committee member; Kuciauskas, Darius, committee member; Sites, James R., committee member
    The voltage of CdTe-based solar cells has remained conspicuously low despite years of efforts focused directly on its improvement. The efforts here have been primarily in increasing the equilibrium carrier concentration of the CdTe or its alloys which are used to absorb the light. This direction has been guided by a theory of solar cells that views the cell only as a single p/n junction. The modelling which has been used to confirm this as an appropriate direction indicated that with a moderate carrier lifetime, relatively small front interface recombination velocity, and large equilibrium carrier concentration in the absorber, efficiencies greater than the current record of 22.1% will be possible with open circuit voltages reaching over 1V. However, cells with these properties have been measured and increases in Voc and efficiency have not been attained. In the c-Si community, notably, the "passivation – selectivity" framework has been developed. In particular, it rejects the view that a singular p/n junction is responsible for the function of a solar cell. Instead, this framework operates with the understanding that the potential in the cell which can be turned into useful electrical energy and an increase in open circuit voltage comes only from the excess carriers generated by sunlight forcing a deviation from the equilibrium condition. As such there are two main components: 1) passivation – which refers to the recombination behavior in the cell and development of a large internal potential difference and 2) selectivity – which refers to the asymmetry of conduction in the cell that allows for production of a unidirectional current and an external voltage approaching that within the cell. This framework tends to break the cell into 3, sometimes overlapping, regions: an absorber region that is used to produce as large a potential difference as possible, and two contact regions in which the transport properties are modified to prefer transport of one carrier or the other. Here this framework is applied to CdTe-based solar cells to determine what limits current cells and how to overcome these limitations. In the investigation of passivation, first the electron contact interface is evaluated, resulting in the determination that this interface is not currently limiting the recombination in the cell. As a result, the current baseline is compared to structures hypothesized to provide improvement in the recombination behavior. It is found that cells with CdSeTe as the only material in the bulk exhibit more ideal recombination behavior when compared to a CdSeTe/CdTe structure as is currently used. This comparison demonstrates a pathway for cells to overcome their current limitation due to recombination, with the possibility of reaching up to 25% efficiency and 970 mV Voc with the material that currently is produced at CSU. A native oxide of TeOx is found to passivate the surface, reducing the rate non-radiative recombination, and forms during dry air exposure, providing a pathway to passivate contacts that would be ideal if not for the recombination at the interface. In the investigation related to selectivity, the electron contact is evaluated and it is demonstrated that MgZnO is appropriately selective when deposited with the correct conditions. It therefore is expected that hole selectivity is the primary loss to open circuit voltage in structures determined to have longer excess carrier lifetimes and large radiative efficiencies. Efforts to investigate novel routes to hole selectivity by use of heterojunction contacts are presented. Such routes did not yield improvements in cell Voc and efficiency, and through this work it was determined that a major source of selectivity losses in these cells is the high resistance to hole transport through the bulk semiconductor. Increasing hole concentration or thinning the absorber provide pathways to overcome this specific limitation, but it is modelled that such cells will require structures with hole selective materials that internally cause a reduction of electron current to see improvement in Voc and efficiency.