Status and perspectives of the CAST experiment

Arık, Metin (Dogus Author) -- The conference paper was firstly submitted to 12th International Conference on Topics in Astroparticle and Underground Physics.The CERN Axion Solar Telescope (CAST) is currently the most sensitive axion helioscope designed to search for axions produced by the Primakoff process in the solar core. CAST is using a Large Hadron Collider (LHC) test magnet where axions could be converted into X-rays with energies up to 10 keV. During the phase I, the experiment operated with vacuum inside the magnet bores and covered axion masses up to 0.02 eV. In the phase II, the magnet bores were filled with a buffer gas (first 4He and later 3He) at various densities in order to extend the sensitivity to higher axion masses (up to f .18 eV). The phase II data taking was completed in 2011. So far, no evidence of axion signal has been found and CAST set the most restrictive experimental limit on the axion-photon coupling constant over a broad range of axion masses. The latest CAST results with 3He data in the mass range 0.39 eV < m a < 0.64 eV will be presented

The effect of temperature on resistive ZnO layers and the performance of thin film CdTe solar cells

The use of a highly resistive transparent (HRT) layer has been shown to increase the efficiency of thin film CdTe heterostructure solar cells incorporating a thin CdS layer. In this study ZnO HRT layers were deposited at different substrate temperatures on soda lime glass and on fluorine-doped tin oxide-coated glass to enable structural, optical and electrical characterization. The performance of equivalent films was tested within CdS/CdTe solar cells. The ZnO thickness was limited to 150 nm, whilst the substrate temperature was varied from 20 °C to 400 °C during deposition. X-ray diffraction patterns and transmission electron microscopy of the cross-sectional microstructure of completed devices showed that the growth of the ZnO is improved when the films are deposited at higher temperatures. Film resistivity was lowest at 100 °C and highest at 400 °C, ranging from 10− 2 Ω·cm to 0.33 Ω·cm. The high temperature deposited ZnO exhibits improved micro-structural growth and an improvement in device efficiency

Novel hydrophobic coatings for soiling mitigation in the PV industry: durability and anti-soiling demonstrations

Soiling, the buildup of debris on the front-side of a solar module, can significantly reduce the power production due to a reduction in transmitted light to the underlying solar cell. A potential solution to soiling mitigation involves the use of hydrophobic coatings. In this study commercially available hydrophobic coatings found in non-PV sectors of industry are investigated. Through the use of anti-soiling and standardized durability testing procedures, the suitability of these existing hydrophobic coatings for use in the PV industry is quantitatively analyzed. </div

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

Latest results and prospects of the CERN Axion Solar Telescope

Çetin, Serkant Ali (Dogus Author) -- Ezer, C. (Dogus Author) -- Yıldız, Süleyman Cenk (Dogus Author)The CERN Axion Solar Telescope (CAST) experiment searches for axions from the Sun converted into few keV photons via the inverse Primakoff effect in the high magnetic field of a superconducting Large Hadron Collider (LHC) decommissioned test magnet. After results obtained with vacuum in the magnet pipes (phase I of the experiment) as well as with 4He the collaboration is now immersed in the data taking with 3He, to be finished in 2011. The status of the experiment will be presented, including a preliminary exclusion plot of the first 3He data. CAST is currently sensitive to realistic QCD axion models at the sub-eV scale, and with axion-photon couplings down to the ∼ 2 × 10-10 GeV -1, compatible with solar life limits. Future plans include revisiting vaccuum and 4He configurations with improved sensitivity, as well as possible additional search for non-standard signals from chamaleons, paraphotons or other WISPs. For the longer term, we study the feasibility of an altogether improved version of the axion helioscope concept, with a jump in sensitivity of about one order of magnitude in gaγ beyond CAST

Effect of varying process parameters on CdTe thin film device performance and its relationship to film microstructure

The performance of CdTe thin film photovoltaic devices are sensitive to process parameters. In this study, efforts are made to further understand the effects of process parameters like process temperature and variation in cadmium chloride passivation treatment on CdTe films deposited using a sublimation based deposition system. The effects on film microstructure are studied using advanced microstructural characterization methods like TEM, SEM, EDS and SIMS while electrical performance is studied using various electrical measurements such as current density vs. voltage and electroluminescence. The aim of this study is to provide new insight into the understanding of relationship between fabrication process, device performance and thin film microstructure

Solar axion search with the CAST experiment

Şenel Boydağ, Fatma (Dogus Author), Çetin, Serkant Ali (Dogus Author), Hikmet, İskender (Dogus Author) -- Proceedings of the 34th International Conference in High Energy Physics : (ICHEP08) Philadelphia, Pennsylvania, July 29 - August 5, 2008.The CAST (CERN Axion Solar Telescope) experiment is searching for solar axions by their conversion into photons inside the magnet pipe of an LHC dipole. The analysis of the data recorded during the first phase of the experiment with vacuum in the magnet pipes has resulted in the most restrictive experimental limit on the coupling constant of axions to photons. In the second phase, CAST is operating with a buffer gas inside the magnet pipes in order to extent the sensitivity of the experiment to higher axion masses. We will present the first results on the 4He data taking as well as the system upgrades that have been operated in the last year in order to adapt the experiment for the 3He data taking. Expected sensitivities on the coupling constant of axions to photons will be given for the recent 3He run just started in March 2008

High resolution 3D chemical characterisation of a cadmium telluride solar cell by dynamic SIMS

Impurity elements such as chlorine and sulphur can have significant effects on the electrical performance of cadmium telluride (CdTe) solar cells. Here, the 3D distribution of such elements in a cadmium chloride treated CdTe device has been determined by high resolution dynamic SIMS, a novel technique that has not been applied to thin-film PV cells. It is found that as well as segregating to grain boundaries following treatment, chlorine also segregates to the CdS/CdTe interface. Conversely, sulphur shows a U-shaped diffusion profile. These results have potential implications for the processing thin-film CdTe devices

Testing the durability of anti-soiling coatings for solar cover glass by outdoor exposure in Denmark

The presence of soiling on photovoltaic modules reduces light transmission through the front cover glass to the active absorber, thereby reducing efficiency and performance. Current soiling mitigation techniques are expensive and/or ineffective. However, anti-soiling coatings applied to the solar cover glass have the potential to reduce soiling for long periods of time without continuous maintenance. This paper reports the performance of two transparent hydrophobic coatings (A and B) exposed to the outdoor environment of coastal Denmark for 24 weeks. A comparison was made between the performance of coated and uncoated glass coupons, periodically cleaned coupons, and accelerated laboratory tests. Although initial results were promising, water contact angle and transmittance values were found to decline continuously for all coated and uncoated coupons. Surface blisters, film thickness reduction, changes in surface chemistry (fluorine loss), and abrasion damage following cleaning were observed. Coupons cleaned every 4 weeks showed a restoration in transmittance. Cycles of light rainfall and evaporation combined with a humid and salty environment led to cementation occurring on all coupons. The development of an abrasion-resistant, super-hydrophobic coating with a low roll-off angle and high water contact angle is more likely to provide an anti-soiling solution by reducing the build-up of cementation

Cadmium chloride assisted re-crystallisation of CdTe: The effect on the CdS window layer

© 2015 Materials Research Society. The cadmium chloride annealing treatment is an essential step in the manufacture of efficient thin film CdTe solar cells. In previous work we have shown that the primary effect of the treatment is to remove high densities of stacking faults from the as-deposited material. Use of density functional theory has shown that some of the higher energy stacking faults are hole traps. Removal of these defects dramatically improves cell efficiency. In this study we focus on the effect of the activation treatment on the underlying n-type cadmium sulphide layer. A range of techniques has been used to observe the changes to the microstructure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include High Resolution Transmission Electron Microscopy (HRTEM) for subgrain analysis, EDX for chemical analysis and XPS and SIMS for composition-depth profiling. By studying the effect of increasing the treatment time and temperature, we will show that the cadmium sulphide layer depletes to the point of complete dissolution into the absorber layer. We will also show that chlorine penetrates and decorates the grain boundaries in the cadmium sulphide. In addition we will show that chlorine builds up at the heterojunction and concentrates in voids at the cadmium telluride/cadmium sulphide interface. A combination of these effects damages the electrical performance of the solar cell