The microstructure of thin film cadmium telluride photovoltaic materials

In this work cadmium telluride thin film photovoltaic devices have successfully been produced using a novel closed-field magnetron sputtering technique. This technique offers the possibility of producing cells in an all-in-one vacuum process with the potential to provide a new lower cost production route. The sputtered cadmium telluride layers were characterised in detail using a range of advanced microscopy based techniques both in the as deposited and after the cadmium chloride treated state, a treatment that is necessary to produce a working cell. In the as deposited condition the cadmium telluride layer was seen to have a fine-grained columnar structure containing a high density of stacking faults. After the cadmium chloride treatment these grains recrystallized and the new grains were equiaxed with a much lower density of intragranular defects. Similar effects were also observed in samples prepared using close space sublimation. To understand this recrystallization behaviour during the cadmium chloride treatment, the key treatment parameters were systematically varied. Chemical analysis in Scanning Transmission Electron Microscopy (STEM) showed that chlorine travelled down the cadmium telluride grain boundaries and accumulated adjacent to the cadmium telluride/cadmium sulphide interface. This interface is where the cadmium telluride grains were found to recrystallise first during interrupted cadmium chloride treatments. The nature of the stacking faults was examined using High Resolution Transmission Electron Microscopy (HR-TEM). This showed that in localised regions up to one plane of atoms per sequence was missing based on the expected zinc blende structure. This changed the packing of the atoms such that a local change in crystal structure occurred. This local change in phase was successfully mapped using Electron Backscatter Diffraction in planar section produced using Focused Ion Beam milling. This was subsequently studied in more detail using Transmission Electron Backscatter Diffraction in the Scanning Electron Microscope, where the intra-granular arrangement of the phases was observed. HR-TEM was used to quantitatively measure the linear defects in the cadmium telluride layer after thermal annealing with and without the cadmium chloride present. This showed that annealing alone resulted in only a modest reduction in the density of linear defects and grain recrysallisation only occurred in the presence of cadmium chloride. Cadmium magnesium telluride (CMT) was successfully grown epitaxially onto the cadmium telluride as an electron reflector layer to improve cell performance. During deposition the cell experienced high temperatures and this caused the stacking faults to return in a cell that had been previously cadmium chloride treated. This resulted in a reduction in cell efficiency, providing another link between linear defects and a degradation in cell performance

Intragranular defects in As-deposited and Cadmium Chloride treated Polycrystalline Cadmium Telluride solar cells

Atomic-scale defects limit the open circuit Voltage, and the conversion efficiency of thin film polycrystalline cadmium telluride solar cells. Using state of the art aberration corrected high resolution transmission electron microscopy, the type, density and atomic structure of intragranular defects present in cadmium chloride treated and untreated CdTe has been established. The cadmium chloride activation process dramatically reduces defect density but faults do remain. Characterizing the defects in both materials is an essential first step to determining their potential electrical effects, and to understanding how the cadmium chloride treatment reduces their density. Improving our knowledge of the mechanisms involved can lead to further process improvements

Electrodeposition of CdTe films on CdS layers deposited using magnetron sputtering and chemical bath deposition

This paper compares the structural and optical properties of electrodeposited Cadmium Telluride films grown on Cadmium Sulphide films prepared by two different methods: pulsed DC Magnetron Sputtering and Chemical Bath deposition (CBD). The films were characterised using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS) and Spectrophotometry. The SEM and TEM characterisation revealed that the microstructure of the CdTe film was influenced by the deposition method used for the underlying thin film CdS. The CdTe films deposited on the CBD CdS films showed development of bigger crystallites compared to the films grown on the sputtered CdS layer. Thickness measurements showed that the substrate had significant influence on the growth rate of the CdTe, with the material grown on sputtered CdS having 3 times higher deposition rate. Transmission measurements showed that the material deposited on the sputtered CdS had a narrower band gap, 1.41eV, compared to that deposited on CBD CdS, 1.46eV

Activation of thin film CdTe solar cells using a cadmium bromide treatment

The activation of CdTe with a cadmium chloride annealing treatment is a vital step in the fabrication of high efficiency solar cells. Thin film MZO/CdTe cells have been activated using CdBr2 instead of CdCl2 with a lower activation process temperature. Using this method, CdBr2 does activate the cell as revealed by J-V and EQE measurements. TEM and EDX elemental maps from device cross-sections confirm that bromine is present in the grain boundaries. TEM shows that the treatment removes stacking faults at 425 °C. CdBr2 treatment resulted in a relatively modest conversion efficiency of 5.49% when treated at 375 °C. Nevertheless, the experiments shed further light on the mechanisms involved in the activation

Optimization-Based Design and Selection of Working Fluids for Heat Transfer: Case Study in Heat Pipes

In this paper, an in silico methodology for optimizing/designing working fluids for heat transfer is presented. The method is tested for heat pipe fluids using a validated model of an evacuated tube solar water heater as a case study. Two model modes are assessed: one simplistic temperature-independent mode and one more complex temperature-dependent mode. The method demonstrates successful optimization of working fluid properties rapidly leading to optimal design of real working fluid mixtures, thus avoiding laborious and time-consuming trial-and-error experimentation. Simulations of solar water heaters with optimized working fluids lead to overall performance improvements in the order of 40%

Intragranular defects in As-deposited and cadmium chloride-treated polycrystalline cadmium telluride solar cells

Atomic-scale defects limit the open circuit Voltage, and the conversion efficiency of thin film polycrystalline cadmium telluride solar cells. Using state of the art aberration corrected high resolution transmission electron microscopy, the type, density and atomic structure of intragranular defects present in cadmium chloride treated and untreated CdTe has been established. The cadmium chloride activation process dramatically reduces defect density but faults do remain. Characterizing the defects in both materials is an essential first step to determining their potential electrical effects, and to understanding how the cadmium chloride treatment reduces their density. Improving our knowledge of the mechanisms involved can lead to further process improvements

Reduction in percentage of number of deaths.

<p>YLL, year of life lost.</p

Three core parameters used to create impacts of cycling tool scenarios.

<p>Three core parameters used to create impacts of cycling tool scenarios.</p

Mode share baseline compared with scenario 25%, equity off, e-bike on/off.

<p>Mode share baseline compared with scenario 25%, equity off, e-bike on/off.</p

Impacts of cycling tool online interface.

<p>Impacts of cycling tool online interface.</p