This thesis addresses the syntheses towards high quality CdSe and CdTe nanoparticles. Therefore, thermodynamic and kinetic aspects of the hot injection method are investigated. By means of the introduction of a thermodynamically less favored nuclei species the nucleation event of CdSe quantum dot synthesis is affected. Utilizing highly reactive tin or lithium silylamides, primarily formed SnSe or Li2Se nuclei undergo a cation exchange to the demanded CdSe particles. The further growth proceeds without the incorporation of the so called quasi-seed species. In this manner, the mechanism of the cation exchange-mediated nucleation is proven and optimized with respect to the required amount of the quasi-seed species. Furthermore, this protocol is applied to up-scaling attempts to reduce the efforts for optimization to a minimum. Following this, a successful laboratory batch up-scaling is achieved by increasing flask size as well as precursor concentrations by factors of 2 and 10, respectively. A further possibility to thermodynamically influence the hot injection synthesis is the activation of the precursor species. By altering the injection pathway, as compared to the standard synthesis, the precursor species are differently coordinated and hence possess different thermodynamic stabilities. Investigations on the system of CdTe quantum dots lead to the result of a cation activation by the use of the thermodynamically less stable carboxylate ligands instead of phosphonates. Additionally, anion activation is suggested due to a kind of aging of the phosphine ligands via their oxidation by phosphonic acids. Furthermore, it is found that the ratio of Cd-to-Te strongly influences the formation of so called magic-sized clusters. Following the results, the smallest detectable species is determined as a cluster species with a size of 1.8 nm. The role of the magic-sized clusters is not fully resolved, but the initial growth is assumed to occur via monomer deposition onto or the fusion of the observed clusters. On the other hand, cluster dissolution is thermodynamically forced by the decreasing monomer concentration and can simply be explained by the process of Ostwald ripening via the creation of a smaller cluster species. Mechanistically this is explained by the formation of configurational deviations from the ideal closed-shell structure. Finally the inorganic coating of the core quantum dots in investigated. Therefore, homoepitaxial coating is employed to overcome the limit in particle size by introducing additional monomer supply. As a result, following the classical crystallization theory, defined injections of precursor material during the diffusion limited growth regime allow a fine tuning of the final particle size. Nevertheless, homoepitaxial coating inevitably leads to photoluminescence quenching, whereas heteroepitaxial growth usually improves the optical quality. By means of a type I structure, CdSe/CdS/ZnS, the successive ion layer adsoption and reaction mechanism is discussed. Furthermore, alloy structures of CdSe/ZnSe with a radially gradated intermediate shell of CdZnSe are achieved by postsynthetic high temperature treatments. This annealing induces internal diffusion processes and allows exactly adjusting the emission wavelength due to defined shrinkage of the initial core size during the alloying process.

<p>In this work we investigated fabrication of semiconductor nanomaterials and evaluated their potential for photo-chemical and photovoltaic applications. We investigated two different II-VI semiconductor nanomaterial systems; (i) ZnO oriented nanowire arrays non-epitaxially grown from a substrate; and (ii) colloidal CdTe nanotetrapods synthesized by solution-based thermal decomposition of organo-metallic precursors. In both the cases our main focus has been optimizing material synthesis for improving potential applications based on photon-electron interactions.</p><p>We have studied the synthesis of vertically aligned ZnO nanowire arrays (NWA), by a wet chemical process on various substrates. The synthesis is based on epitaxial growth of ZnO seed-layer on a substrate in a chemical bath consisting of an aqueous solution of zinc nitrate and hexamethylenetetramine (HMT). We have suggested an additional role played by HMT during the synthesis of ZnO nanowire arrays. We have also extended this synthesis method to fabricate hierarchical nanostructures of nanofibers of poly-L-lactide acting as a substrate for the radially oriented growth of ZnO nanowires. The combination of high surface area of the nanofibrous substrate with the flexibility of the PLLA-ZnO hierarchical nanostructure enabled the proof-of-principle demonstration of a ‘continuous-flow’ water treatment system that could effectively decompose single and combination of known organic pollutants in water, as well as render common waterborne bacteria nonviable.</p><p>We have studied another chemical synthesis that is commonly used for size controlled synthesis of colloidal quantum dots, which was modified to obtain anisotropic nanocrystals mainly for CdE (E=S, Se, Te) compositions. In this work we demonstrate by use of oleic acid (instead of alkylphosphonic acids) it is possible to synthesize CdTe and CdSe nanotetrapods at much lower temperatures (~180 ºC) than what is commonly reported in the literature, with significantly different  formation mechanism in the low-temperature reaction.</p><p>Finally, we have performed preliminary photoconduction measurements with CdTe nanotetrapods using gold ‘nanogap’ electrodes fabricated in-house, and obtain up to 100 times enhancement in current levels in the <em>I–V </em>measurements under illumination with a white light source.</p> / QC20100607

Gallium arsenide (GaAs), cadmium selenide (CdSe), and cadmium telluride (CdTe) are semiconductor materials with remarkable opto-electronic properties that make them suitable for a wide variety of applications including, light emitting diodes (LEDs), mobile phones, tablets, and solar panels. Due to the growing demand and the short lifespan of these electronic devices, a remarkable amount of electronic waste (e-waste) has been produced in the last decades. An important fate of e-waste is landfill disposal; therefore, there is an increasing concern about the release of toxic elements into the landfill environment and the potential risks for human health and the environment. Among the elements constituting GaAs, CdTe, and CdSe, tellurium (Te) has gained increasing attention in recent years. Tellurium is a scarce element on the earth’s crust, and a shortage in its supply might compromise the development of new advanced technology, particularly in the energy and defense fields. For these reasons, the US Department of Energy and the European Union regard Te as a critical element, and have urged the need to develop efficient and cost-effective processes to recover Te from waste streams. This thesis dissertation explored different aspects related to the fate and impact of the widely used semiconductor materials, GaAs, CdSe and CdTe in municipal mixed solid waste (MSW) landfills. Furthermore, it investigated the removal of the Te oxyanions, tellurite (Teᴵⱽ, TeO₃²⁻) and tellurate (Teⱽᴵ, TeO₄²⁻), from aqueous streams and the recovery of this strategic metalloid as biogenic, elemental tellurite (Te⁰) nanoparticles (NPs). In the first part this work, the dissolution of GaAs was evaluated under a range of redox conditions, pH levels, ionic strength, and the presence of organic constituents commonly found in landfills. Our results indicated that aerobic conditions and mildly alkaline pH conditions favor the dissolution and release of high levels of soluble arsenic (As) and gallium (Ga) to the synthetic leaching solutions. The rate of As and Ga dissolution in long-term exposure experiments was initially constant but later progressively decreased due to the formation of a passivating layer on the surface of GaAs. The leaching behavior of CdSe and CdTe was also investigated under different pH and redox conditions in solutions simulating landfill leachates. CdTe and CdSe were subjected to two different standardized leaching tests, the federal Toxicity Characteristic Leaching Procedure (TCLP) and the California Waste Extraction Test (WET). CdTe showed a very high leaching potential in both tests and the concentrations of Cd released were 1500- and 260-times higher than the regulatory limit (1 mg Cd L⁻¹), respectively. In contrast, CdSe was relatively stable and dissolved selenium (Se) in both leaching tests was below the regulatory threshold (1 mg Se L⁻¹). Tests performed under different pH and redox conditions confirmed a marked enhancement in CdTe and CdSe dissolution both under acidic pH and aerobic conditions, which is consistent with thermodynamic predictions. Due to the high leaching potential observed for CdTe in the previous batch experiments, leaching studies were designed to investigate the potential release of soluble Cd and Te from a commercially available CdTe thin-film solar panel under different chemical and biogeochemical conditions commonly found in landfills. The solar panel was subjected to two standardized batch leaching tests (i.e., TCLP and WET), and to a continuous column test designed to investigate the dissolution of CdTe under conditions simulating the acidic- and the methanogenic circumneutral phases of a MSW landfill. A negligible amount of Cd and Te was measured in the synthetic leachate of both batch tests. On the other hand, a significant amount of Cd and Te was released from the panel to the synthetic leachate of the column simulating an acid landfill after 30 days (73% and 21% of the total Cd and Te, respectively). In contrast, the amount of Cd or Te detected in the effluent of the column operated at circumneutral pH values was negligible. The marked difference in the leaching behavior of CdTe in both columns is related to the different aqueous pH and redox conditions promoted by the microbial communities in the columns. The microbial toxicity of the soluble species that can be released from CdTe and CdSe was also assessed as a part of this work. The toxicity of cadmium (Cdᴵᴵ), selenite (Seᴵⱽ), selenate (Seⱽᴵ), Teᴵⱽ, and Teⱽᴵ was evaluated in bioassays with different microbial targets, including acetoclastic and hydrogenotrophic methanogenic populations in a mixed microbial culture, similar to those involved in the stabilization of organic waste stabilization in a landfill, and the bioluminescent marine bacterium, 𝐴𝑙𝑖𝑖𝑣𝑖𝑏𝑟𝑖𝑜 𝑓𝑖𝑠𝑐ℎ𝑒𝑟𝑖 (Microtox® test). The acetoclastic methanogens were most sensitive to the presence of the various soluble species, with the toxicity decreasing in the following order: Cdᴵᴵ, Teᴵⱽ, Teⱽᴵ, Seᴵⱽ, while Seⱽᴵ was only toxic at non-environmentally relevant concentration. Hydrogenotrophic methanogens were highly inhibited by Cdᴵᴵ and Seᴵⱽ, but Teᴵⱽ and Teⱽᴵ only had a moderate toxic impact. The bacterium 𝐴. 𝑓𝑖𝑠𝑐ℎ𝑒𝑟𝑖 was very sensitive to inhibition by Cdᴵᴵ and Seᴵⱽ, and Teᴵⱽ. In the last part of this work, the potential recovery of insoluble, elemental Te⁰ NPs from aqueous solutions containing soluble Teᴵⱽ or Teⱽᴵ was investigated in batch- and continuous flow bioreactors inoculated with a methanogenic granular mixed culture. In the batch experiments, the capacity of the culture to catalyze the reduction of the Te oxyanions and to produce Te⁰ NPs internally and externally to the cells was demonstrated. The granular sludge was found to contain enough endogenous substrates to provide the electron equivalents required to reduce both Te oxyanions and the reduction rates were only modestly increased by an exogenous electron-donor (e-donor) such as H₂. The effect of several redox mediators (RM), namely, anthraquinone-2,6-disulfonate (AQDS), hydroxocobalamin, riboflavin, and lawsone, was also tested. Riboflavin and lawsone caused a remarkable increase of the rate of Teᴵⱽ and Teⱽᴵ reduction, respectively, and also enhanced the fraction of Te recovered as extracellular Te⁰ NPs. The morphology and localization of the Te⁰ NPs were also impacted by the presence of a particular RM and e-donor in the system, suggesting that NP production can be tailored for a particular application. Finally, the feasibility of utilizing upflow anaerobic sludge bed (UASB) bioreactors to reduce Teᴵⱽ oxyanions to non-toxic Te⁰ NPs was also investigated. Two reactors were supplied with ethanol as the external e-donor source to promote the biological reduction of Teᴵⱽ. Riboflavin, a redox mediator, was supplied to one of the reactors to enhance Teᴵⱽ bioreduction. Continuous formation of Te⁰ NPs using an UASB was found to be feasible and remarkably improved by addition of riboflavin at the low Teᴵⱽ:riboflavin molar ratio of 4:1. This flavonoid enhanced the conversion rate of Teᴵⱽ and reduced the toxic impact of Teᴵⱽ towards the methanogenic consortium. Overall, the evidence found in this work indicates that recycling of decommissioned devices containing GaAs, CdTe, or CdSe is desirable to prevent the potential environmental release of toxic metals and metalloids in MSW landfills, but also to allow the recovery of critical resources. Microbial processes offer potential for the removal and recovery of soluble metals and metalloid ions leached from decommissioned semiconductor materials. In particular, this study demonstrated the feasibility of utilizing continuous UASB bioreactors for the removal of Teᴵⱽ from aqueous streams and the recovery of this valuable metalloid as biogenic Te⁰ NPs.

Thin-film inorganic solar cells, such as CdTe, have demonstrated the most promise to date for a viable low-cost renewable energy resource. Their current performance, however, is far from the theoretical limit suffering from significant charge recombination losses due to grain boundaries and point defects. It is likely that the microscopic compositions of grain bulk and grain boundaries are significantly different and not optimal for the overall device performance. Good understanding of charge transport along and across the grain boundaries and other microscopic interfaces is lacking, preventing the development of reliable and predictive device models. The insufficient microscopic understanding hinders efficient characterization of photovoltaic materials and also holds back the development of process control techniques. We first show preliminary results for a novel technique, quantum-dot electron-beam induced current to characterize semiconductors in the near-field. We also propose the use of near-field optical scanning microscopy for high precision optical excitation and for local, high-resolution characterization. These imaging techniques are examined with the goal of synthesizing information obtained by both methods, of material phenomena at the relevant length scales, to other measurement methods. The most important nanoscale phenomena being the separation of compositional and electrical effects.

Este trabalho foi desenvolvido em duas etapas: i) síntese, caracterização e aplicação de pontos quânticos de CdS, CdSe e CdTe em células solares. ii) modificação da superfície dos pontos quânticos de CdSe através de troca de ligante, seguida de caracterização e aplicação em células solares. Os pontos quânticos foram sintetizados utilizando acetatos de cádmio, selênio, telúrio e enxofre como precursores e ácido oleico como agente de estabilização. Na segunda etapa o ácido oleico foi substituído por ligantes com maior afinidade eletrônica pelos sítios de Cd2+: ácido 3-mercaptopropiônico, 4-ácido-mercaptobenzóico e ácido 11-mercaptoundecanóico. As amostras foram caracterizadas por UV-Vis, fluorescência, microscopia eletrônica de transmissão, difratometria de raios-X e voltametria cíclica. Adicionalmente, testes de solubilidade, análises de TGA e de RMN foram realizadas para confirmar a troca de ligante. Através dos resultados, verificou-se que todos os pontos quânticos sintetizados são adequados para sensibilização de TiO2 em dispositivos fotovoltaicos. No entanto, os pontos quânticos de CdSe e CdTe apresentaram fatores que evidenciam maior confinamento quântico, sendo que a maior estabilidade do éxciton foi obtida para o CdSe. Através das análises de RMN foi possível verificar que não existe apenas uma confirguração espacial preferencial para a adsorção do ligante sobre a superfície deste ponto quântico enquanto que curvas de corrente versus potencial e de eficiência de conversão de fóton incidente mostraram que a eficiência do dispositivo é fracamente dependente do ligante. Contudo, a troca de ligantes favorece a solubilidade em solventes com diferentes polaridades, inclusive água, o que amplia as possibilidades de aplicação dos pontos quânticos sintetizados neste trabalho. / This work was developed in two stages: i) synthesis, characterization and application of CdS, CdSe and CdTe quantum dots to assemble solar cells, ii) surface modification, characterization and application of CdSe quantum dots to assemble solar cells. The quantum dots were synthesized by using cadmium acetate, Se, S or Te as precursors and oleic acid as stabilizing agent. In the second stage the oleic acid capping layer was replaced by other ligands with higher electron affinity to Cd2+: 3-mercaptopropionic acid, 4-mercaptobenzoic acid and 11-mercaptoundecanoic acid. The samples were characterized by UV-Vis, fluorescence, transmission electron microscopy, x-ray diffractometry and cyclic voltammetry. Additionally, solubility tests, TGA analysis and NMR were performed to evaluate the CdSe surface modification. The results showed that all quantum dots synthesized are adequate to sensitize TiO2 in photovoltaic devices. However, CdSe and CdTe quantum dots presented better quantum confinement and the exciton generated in CdSe presented the higher stability. NMR analysis provided information about the non-preferential orientation for adsorption of the ligands on the CdSe surface, meanwhile measurements of current vs. potential and incident photon current efficiency showed a weak dependence of photovoltaic device efficiency with the nature of the ligand. On the other side, the surface modification favors the solubility in solvents with different polarizabilities, including water, widening the range for applications of the quantum dots synthesized in this work.

The goals of this project are study Cu free back contact alternatives for CdS/CdTe thin film solar cells, and to research dry etching for CdTe surface preparation before contact application. In addition, an attempt has been made to evaluate the stability of some of the contacts researched. The contacts studied in this work include ZnTe/Cu2Te, Sb2Te3, and Ni-P alloys. The ZnTe/Cu2Te contact system is studied as basically an extension of the earlier work done on Cu2Te at USF. RF sputtering from a compound target of ZnTe and Cu2Te respectively deposits these layers on etched CdTe surface. The effect of Cu2Te thickness and deposition temperature on contact and cell performance will be studied with the ZnTe depositions conditions kept constant. C-V measurements to study the effect of contact deposition conditions on CdTe doping will also be performed. These contacts will then be stressed to high temperatures (70-100 degrees C) and their stability with stress time is analyzed. Sb2Te3 will be deposited on glass using RF sputtering, to study film properties with deposition temperature. The Sb2Te3 contact performance will also be studied as a function of the Sb2Te3 deposition temperature and thickness. The suitability of Ni-P alloys for back contacts to CdTe solar cells was studied by forming a colloidal mixture of Ni2P in graphite paste. The Ni-P contacts, painted on Br-methanol etched CdTe surface, will be studied as a function of Ni-P concentration (in the graphite paste), annealing temperature and time. Some of these cells will undergo temperature stress testing to determine contact behavior with time. Dry etching of CdTe will be studied as an alternative for wet etching processes currently used for CdTe solar cells. The CdTe surface is isotropically etched in a barrel reactor in N2, Ar or Ar:O2 ambient. The effect of etching ambient, pressure, plasma power and etch time on contact performance will be studied.

The CdTe solar cell is a leading candidate for cost-effective thin-film solar cells having demonstrated small area cell effciencies of 16.4%. A Key issue associated with CdTe thin film photovoltaic modules is the analysis of degradation behavior of the device. The analysis is complicated as changes due to degradation may be reversible. Solar cell measurement techniques were used to understand the changes in device parameters after light soaking for 1000 hours. An automated measurement setup was implemented as part of this thesis work. The main objective of this thesis was to study the effect of CdCl2 heat treatment on the device stability. The temperature for this heat treatment was varied from 360oC to 400oC. Cells were stressed under illumination at both short circuit and open circuit conditions. It was found that the increase CdCl2 heat treatment temperature slowed down the degradation rate.This was true for both short and open circuit stress conditions. Also short circuit stress condition slowed down the degradation of the device when compared with the open circuit condition. It became evident that the recombination current mainly got affected when the device was said to be degraded.

CdTe films were deposited by sputtering technique and were then carried out by CdCl2 treatment. The SEM micrographs show that the grain sizes of the as-deposited CdTe film were normally ranged from 50 nm to 100 nm, and they were recrystallized after CdCl2 treatment to obtain the grain sizes in the range of 1~3 £gm. A new device structure for CdTe thin-film solar cells has been proposed to exceed the cell efficiency of current record. The superstrate structure with the layer sequence of Glass/AZO/ZnO/CdS/CdTe/CI(G)S/Mo compared with the conventional device structure of Glass/FTO/CdS/CdTe/metal contact would have the following advantages:(1) a highly conductive AZO layer combined with a thin undoped ZnO layer will have higher optical transmission than that of FTO; (2) the use of p-type CIS under the CdTe layer with the same conductivity type can extend the light absorption to longer wavelength range (the band gaps of CdTe and CIS are 1.45eV and 1.04eV, respectively); (3) the proper addition of Ga to CIS may form CIGS quaternary compounds with a bandgap gradient which produce an electric field in the neutral region of a p-n junction to reduce the carrier recombination; (4) the use of Mo contact to CI(G)S is quite stable as compared with the metal contact normally used for p-CdTe. AMPS-1D simulation had been applied to evaluate the newly designed device structure and the results indicated a great improvement in device performance, i.e. the cell efficiency could exceed 20%. The I-V curve of a CdTe solar cell using the new device structure showed a nearly linear characteristic indicating the failure to form a p-n junction. We speculated that Cu might diffuse through the CdTe layer to the depletion region of the p-n junction formed at the CdS/CdTe interface. This would cause the junction failure. Based on the calculation on the Cu diffusion during the deposition of CIS layer at different temperatures even as low as 150˚C, it always had the chance to diffuse through the CdTe layer. An alternate device fabrication process was the use of the substrate structure for preparing CdTe solar cells, i.e. Glass/Mo/CIS/CdTe/CdS/ZnO/AZO/Al. However, the desired diode behavior was not observed until the thickness of CdTe layer was cut down to 10 nm. The electrical properties of that particular solar cell is the following:Voc=0.36V, Isc=4.991mA/cm2, F.F.=25.3%, efficiency=0.472%. It is probably that the lattice mismatch between CIS and CdTe is large that may cause the formation of interfacial defects and the reduction of photo excited carriers through the recombination processes. The annealing processes had been conducted in order to promote the interdiffusion between CdTe and CIS and minimize the lattice mismatch. However, the films peered off after annealing. Further experiments should be done to solve this problem.

This work focuses on the growth of vertically aligned CdTe nanowire arrays of controllable diameter and length using cathodic electro deposition in anodized alumina templates. This step was followed by annealing at 250° C in a reducing environment (95% Ar + 5% H2). AAO template over ITO-glass was used as starting template for the device fabrication. The deposited nanowires showed nanocrystalline cubic phase structures with a strong preference in [111] direction. First gold (Au) was deposited into AAO using cathodic electro deposition. This was followed by CdTe deposition into the pore. Gold was deposited first as it aids the growth of CdTe inside AAO and it makes Schottky contact with the deposited n type CdTe. CdTe was determined to be n-type from the fact that back to back diode was obtained with Au-CdTe-Au test structure. Aluminum (Al) was sputtered on the top to make the ohmic contact to the n type CdTe deposited in AAO. Analysis of Schottky diodes yielded a diode ideality factor of 10.03 under dark and 10.08 under light and reverse saturation current density of 34.9μA/cm2 under dark and 39.7μA/cm2 under light.

Solar cells based on CdTe absorbers are attractive due to the optimal direct band gap energy and large absorption coefficient of CdTe, however, their performance and commercialization is hindered by the lack of reliable p-type contacts. CdTe has a low carrier concentration and a large electron affinity, which results in a requirement of non-realistic work functions for metals to be used as back contacts in the solar cell. Even noble metals such as Ag present a significantly large potential barrier for holes, thereby reducing the hole current through the semiconductor/metal interface. Several attempts to resolve this challenge have been tried, however, many drawbacks have been encountered. Two particular systems, namely Cu2Te thin films and CdTe/ZnTe strained-layer superlattices, are investigated for their potential use as ohmic contacts in CdTe solar cells. A detailed analysis of the optical, electrical, and structural properties of Cu2Te thin films deposited by magnetron sputtering is presented. It is shown that these films have an indirect band gap and highly degenerate semiconductor behavior. The large p-type carrier concentration of Cu2Te films is highly desirable for the application of Cu2Te as a p-type contact to CdTe. In-depth studies of optical transitions and miniband transport in strained-layer CdTe/ZnTe superlattices are presented as well. The band offsets between CdTe and ZnTe were determined by comparison of measured and calculated optical transitions. Superlattice structures that offer best contact performance have been identified by use of tunneling probability simulations. Characterization of CdTe solar cells with above mentioned contacts indicated that contacts based on CdTe/ZnTe superlattices are a viable Cu free option for stable and reliable p-type contacts in CdTe solar cell. The contact performance of Cu2Te thin films was comparable to that of CdTe/ZnTe superlattices and both demonstrated an advantage over contacts based on ZnTe:N thin films which were used a standard.