CDS-CU(_x)S single crystal and thin film solar cells

The work presented in this thesis is concerned with photovoltaic cells formed by plating CdS single crystals and thin films, and Cd(_y) Zn(1 _ y)S single crystals, with copper sulphide. An electroplating technique has been used to control the phase of copper sulphide by changing the electric field during its formation. Different phases of Cu(_x)S have been identified directly using Reflection High Energy Diffraction (RHEED), and indirectly from spectral response measurements. A dramatic change in the spectral response accompanying the reduction in the covellite response associated with an increase in that from chalcocite following argon heat treatment has been achieved. The change from the djurleite phase to that of chalcocite has also been obtained by using argon heat treatment for 5 minutes at 200 C. This effect was found to be reversible in that layers of chalcocite were converted to djurleite when air was used as the ambient for the heat treatment. C-V measurements have demonstrated that with increasing plating bias the donor concentration decreases at first before it assumes a constant value. This led to the effect of decreasing the junction capacitance as the width of the depletion region changed. The problem of the stability of the CdS-Cu(_2)S photovoltaic devices formed by wet plating" is addressed by studying the combined effects of the substrate onto which the CdS is deposited and the ambient used during annealing. Thin film cells have been prepared on both Ag/Cr and SnO substrates, and the device characteristics for each have been investigated as a function of annealing ambient. The results have shown that devices formed on Ag/Cr substrates were more stable following annealing in air than in argon, while the converse was true for cells fabricated on SnO(_x) substrates. The degradation effects of CdS-Cu(_2) S photovoltaic cells have been investigated. While devices stored in the dark showed little or no degradation, those maintained under illumination exhibited a significant deterioration in all operational parameters over a four week period. As far as the combined effect of temperature and ambient on the stability of cells are concerned, it was found that the ageing of devices in argon at room temperature in the dark was negligible, and moreover the fill factor was observed to improve marginally. When the devices were stored in the same ambient conditions at 50 C, they showed a significant improvement in the fill factor, but simultaneously exhibited a considerable reduction in the short circuit current. This process was reversible, since the sensitivity of degraded devices could be restored by annealing them in a hydrogen/nitrogen mixture. By comparing Electron Spectroscopy for Chemical Analysis (ESCA) studies with solar cell device characteristics, it has been shown that the formation of copper oxide on the Cu(_2)S surface plays a significant role in the degradation of CdS-Cu(_2) S devices. The extent of the cross-over between the dark and light J-V characteristics is a function of the period of etching used prior to junction formation. The variation of current and diode factor has been established as a function of the bias value. The dependence of forward current on the temperature at fixed forward voltage has also been investigated. Finally this work has shown that an increase in V(_oc) can be achieved when Cd(_0◦8)Zn(_0◦2)S is used as a base material for solar cells instead of CdS. Different traps were identified through a photocapacitance investigation. An important trap was found at 0.78eV below the conduction band. It has been demonstrated that the effect of this level was found to be diminished much more slowly when the annealing was carried out in argon rather than in air. This level may play an important role in the Cd(0◦8) Zn(0◦2)S-Cu(_2)S solar cell properties

Annual Report: Photovoltaic Subcontract Program FY 1991

This report summarizes the progress of the Photovoltaic (PV) Subcontract Program of the Solar Energy Research Institute (SERI) from October 1, 1989 through September 30, 1990. The PV Subcontract Program is responsible for managing the subcontracted portion of SERI's PV Advanced Research and Development Project. In fiscal year 1990, this included more than 54 subcontracts with a total annualized funding of approximately $11.9 million. Approximately two-thirds of the subcontracts were with universities at a total funding of nearly$3.3 million. The six technical sections of the report cover the main areas of the subcontract program: the Amorphous Silicon Research Project, Polycrystalline Thin Films, Crystalline Silicon Materials Research, High-Efficiency Concepts, the New Ideas Program, and the University Participation Program. Technical summaries of each of the subcontracted programs provide a discussion of approaches, major accomplishments in FY 1990, and future research directions. Another section introduces the PVMaT project and reports on its progress

Photodetectors based on colloidal QDs

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.Vita.Includes bibliographical references (p. 193-212).Inspired by recent work demonstrating photocurrent enhancement in quantum-dot (QD) solids via post-deposition chemical annealing and by recent successes incorporating single monolayers of QDs in light-emitting devices (QD-LEDs), we set out to develop thin-film, layered photodetectors in which the active layer is a chemically annealed QD solid. This thesis reports initial steps in this development. Chapters 1 and 2 contain introductory material. In Chapter 3, we demonstrate a layered QD photodetector (QD-PD) in which the active layer is a 200-nm thick film of CdSe QDs annealed with n-butylamine. These "thick-slab" devices, active in the visible spectrum, represent a minimal transformation from the transverse channel test structures employed for more fundamental studies to a layered, sandwich geometry. The thick-slab design exhibits a number of desirable characteristics, including near-unity internal quantum efficiency and a 50-kHz bandwidth. More complex QD-PD designs, allowing the successful incorporation of much thinner QD solids, are discussed in Chapter 4, and these devices provide insight into the zero-bias operation of thick-slab QD-PDs.(cont.) The final three chapters of the thesis focus on PbSe QDs, this group's initial material of choice for accessing the short-wavelength-IR spectral window. Initial study of PbSe QDs as a material for QD-PDs is described in Chapter 5. Chapter 6 discusses an 31P-based NMR investigation of the mechanism of PbSe monomer formation, allowing for rational "doping" of syntheses to increase chronically low yields. Finally, Chapter 7 discusses a brief assessment of the suitability of PbSe QDs for in vivo imaging.by David C. Oertel.Ph.D

Cadmium sulphide transducers : thick vacuum-deposited films for ultrasonic shear-mode low-frequencey operation

The primary aim of this research project was to deposist thick-film (low-frequency) s-mode CdS piezoelectric transducers directly onto copper or aluminium rods which formed part of the welding-electrode of a spot-welding machine. These transducers were to replace the discrete transducers used in a set-up for the "on-machine" evaluation of spot-welds. A secondary aim was to deposit very thin CdS films on glass slides for use as microwave resonators. The dependence of film adhesion on film thickness, and crystallographic orientation changes with thickness imposed an upper limit on the thickness of transducers with adequate properties for applications. The final goal, established through experience on the project, was to determine how thick piezoelectric films could be deposited to make useful transducers. Highly stoichiometric (deviations from stoichiometry of the order of 1 part in 10^13 ) and highly resistive (> 10^10 Ω.m.), and highly oriented films up to 100 μm thick have been successfully deposited on Ae rod substrates. Two deposition techniques were used : CdS/S electron beam bombardment evaporation and Cd/S isothermal cells. Provided that the temperature of the vapour molecules was less than 400 degrees C and that the pumping speed could be increased at will , then, the faster the deposition rate, the sharper the oblique c-axis preferred orientation, and the better the piezoelectric performance of the films. The pumping speed limited the deposition rate to 10 μm.h^- 1. Appreciable thermal stresses in the films gave rise to large forces which induced the thick-films to flake off or disintegrate. The dependence of film adhesion on film thickness is explained in terms of the inequality between the forces which bind the film to the substrate (independent of thickness) and the forces which induce the film to flake off (proportional to thickness). Thick CdS films were made to adhere to the substrate by making the substrate surface rouqher so that the films "keyed-in". No appreciable temperature gradients existed in the CdS films during growth, either across their thicknesses or along their surfaces. No changes in temperature gradients occurred in the films due to changes in film orientation, and vice versa. Up to a certain critical thickness, the c-axes of most CdS film crystallites aligned themselves with the direction of the vapour beam. When the thickness of the film exceeded the critical thickness, the growth of oblique crystallites was stifled and the film's c-axis tilted towards the substrate-normal and eventually became parallel to it. This was confirmed by etching-back a thick CdS film which was deposited at oblique vapour incidence. A model is presented for the "stifling process” which gives the relation between the critical thickness, the grain size and the deposition angle of the film. For a given deposition environment, the stifling process imposed an upper limit on the thickness of an s-mode transducer. The use of copper substrates, and of copper parts inside the deposition chamber, was abandoned because of the corrosive action of sulphur on copper. Cu/CdS junctions were nearly ohmic, and the anomalous behaviour of these junctions is explained in terms of the reaction between Cu and S to form Cu2S. CdS s-mode transducers with untuned two-way insertion loss of 35 dB in a 50 ohm system have been successfully deposited on glass slides for operation at frequencies down to 20 MHz. The stress in CdS films on glass slides was much less than that on Ae rods. It is possible that the higher stress in films on Ae rods weakened their piezoelectric performance

Proceedings of the 4th International Conference and Exhibition: World Congress on Superconductivity, volume 1

The papers presented at the 4th International Conference Exhibition: World Congress on Superconductivity held at the Marriott Orlando World Center, Orlando, Florida, are contained in this document and encompass the research, technology, applications, funding, political, and social aspects of superconductivity. Specifically, the areas covered included: high-temperature materials; thin films; C-60 based superconductors; persistent magnetic fields and shielding; fabrication methodology; space applications; physical applications; performance characterization; device applications; weak link effects and flux motion; accelerator technology; superconductivity energy; storage; future research and development directions; medical applications; granular superconductors; wire fabrication technology; computer applications; technical and commercial challenges, and power and energy applications

Raman and FTIR studies of the distribution and dynamics of active molecules in polymer films.

Fourier Transform Infrared Attenuated Total Reflection Spectroscopy (FTIR-ATR) and Raman microscopy were employed to study the distribution and leaching of the fungicide fluorfolpet in plasticised poly(vinyl chloride) (pPVC) matrixes, along with the diffusion and perturbation of water molecules in the matrixes. Raman mapping and depth profiling were used to determine the molecular distribution of fluorfolpet on the surface and in the bulk of the films respectively. The films were examined both before and after treatment with water at 25°C, in order to study the effect of the leaching of fluorfolpet and its distribution and that of the plasticiser dioctylphthalate (DOP). It was found that the degree of leaching was strongly dependent upon the concentration of DOP in the films, and that leaching occurred from the surface (i.e. film/solvent interface). The distribution of the additives was determined to be "heterogeneous" on the microscopic scale, but "homogeneous" on the macroscopic scale. Additional information such as the rate of migration of the biocide inside the film was obtained using FTIR spectroscopy and the rotating disc method in conjunction with UV spectroscopy. FTIR-ATR was used to investigate the hydration and dehydration of plasticised PVC films. Monitoring of the time dependent change in the nu(OH) mode of water was used to determine the mode of sorption of water into PVC. A dual mode sorption model was found to fit the data well. Diffusion rates of water were found to be strongly dependent on the DOP concentration in the film. It was shown that the "free volume" and the "number of potential binding sites" were determining factors for the diffusion of water in PVC films. A systematic FTIR-ATR study of the perturbation of water sorbed into PVC polymer as a function of plasticiser content and time is reported. The nu(OH) band of water in the polymer has been fitted to individual components, corresponding to those recently found for pure water itself. A detailed quantitative analysis of the frequency shifts and relative intensities of these bands yields direct evidence for the breaking of the water network in the polymer matrix

Growth and characterisation of single-crystal fibres for sensing applications

The laser heated pedestal growth technique has been successfully employed to grow pure and doped sapphire crystal fibres for characterisation as suitable sensor materials. Source materials used were polycrystalline and crystalline sapphire rods while fibres with typical diameters in the range 80 - 170 mm were grown. Pure sapphire fibres, both a- and c-axis, were found to grow easily with no complications such as melt instability. C-axis fibre growth was readily initiated while a-axis fibres required an appropriate a-axis oriented seed crystal. Dip-coating has been used to prepare suitably coated sapphire source rods for growth into doped fibres. Doped fibres grown included Cr3+:, Er3+:, Er3+:Yb3+: and Yb3+:Er3+:Al2O3. Er3+:Yb3+:Al2O3 fibres have been prepared with approximately equal concentration of both dopants while a 10:1 Yb3+ to Er3+ concentration ratio was used for preparing Yb3+:Er3+:Al2O3 fibres. Ruby fibres were also found to grow easily although brownish-green deposits have been observed on some of these fibres. Large transmission losses have been found in fibres with these deposits. Acid cleaning was not effective in removing these deposits, suggesting that they have diffused beneath the surface of the fibres. This was attributed to the condensation of chromium oxide on the fibre surface during growth. Growth of rare earth-doped fibres was initially problematic due to the constant breaking-off of the crystallising fibres from the melt. This was thought to be due to the flexibility of the small diameter source fibres used as well as the high concentration levels of doping. Replacing these small fibres with larger source rods thus permitted RE-doped fibres with relatively good optical quality to be grown. Fibres were grown with typical growth rates of 0.5 - 1 mm/min

Liquid Phase Chemical Deposition of High Tech ZnSe Thin Flims

Being a promising and potential candidate for optoelectronic device applications, zinc selenide thin films were obtained for the first time on the glass microslides using a liquid phase chemical bath deposition. The deposition conditions, such as bath temperature (70 0C), deposition time (210 mins.), reaction pH (10.5± 0.2) and quantity of hydrazine hydrate (reducing agent) were finalized and the possible growth mechanism and reaction kinetics have been suggested. As the ZnSe films grow well in strong reducing atmosphere, the quantity of hydrazine hydrate was varied from 2 ml to 15 ml and ZnSe samples were obtained. The final product ZnSe thin films are physically hard, tightly adherent, relatively uniform and diffusely reflecting with light brown colored tinge in smoky appearance. An EDS analysis showed that the film stoichiometry depends on quantity of the added reducing agent in the bath. Structural studies were carried out on these films by an X-ray diffraction technique using Cukα radiation. It appeared that the ZnSe films are hexagonal wurtzite in structure with preferred orientation along &lt;101&gt;. The calculated d-values, intensities of reflections and lattice parameters matched with that of the JCPD data. The calculated particle size decreased with quantity of hydrazine hydrate added in the bath (decrease in Zn/Se ratio). The optical absorption measurements in the range of wavelength from 300-1100 nm showed direct type of transitions with an optical gap decreased from 2.71 eV to 2.60 eV for the change of Zn/Se ratio from 0.993 to 0.571. The surface features revealed by SEM showed that the crystallites are nonuniformly distributed and spherical in shape. From micrographs it is seen that crystallite size decreased with increasing amount of hydrazine hydrate. Some overgrowth like structures (of the same size and shapes) has also been observed at high concentration of hydrazine hydrate.DOI: http://dx.doi.org/10.3126/jncs.v30i0.9384Journal of Nepal Chemical Society Vol. 30, 2012 Page: 130-137 Uploaded date: 12/20/2013   </jats:p