Context. The processes leading to the birth of high-mass stars are poorly understood. The key first step to reveal their formation processes is characterising the clumps and cores from which they form. Aims. We define a representative sample of massive clumps in different evolutionary stages selected from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), from which we aim to establish a census of molecular tracers of their evolution. As a first step, we study the shock tracer, SiO, mainly associated with shocks from jets probing accretion processes. In low-mass young stellar objects (YSOs), outflow and jet activity decreases with time during the star formation processes. Recently, a similar scenario was suggested for massive clumps based on SiO observations. Here we analyse observations of the SiO (2−1) and (5−4) lines in a statistically significant sample to constrain the change of SiO abundance and the excitation conditions as a function of evolutionary stage of massive star-forming clumps. Methods. We performed an unbiased spectral line survey covering the 3-mm atmospheric window between 84−117 GHz with the IRAM 30 m telescope of a sample of 430 sources of the ATLASGAL survey, covering various evolutionary stages of massive clumps. A smaller sample of 128 clumps has been observed in the SiO (5−4) transition with the APEX telescope to complement the (2−1) line and probe the excitation conditions of the emitting gas. We derived detection rates to assess the star formation activity of the sample, and we estimated the column density and abundance using both an LTE approximation and non-LTE calculations for a smaller subsample, where both transitions have been observed. Results. We characterise the physical properties of the selected sources, which greatly supersedes the largest samples studied so far, and show that they are representative of different evolutionary stages. We report a high detection rate of >75% of the SiO (2−1) line and a >90% detection rate from the dedicated follow-ups in the (5−4) transition. Up to 25% of the infrared-quiet clumps exhibit high-velocity line wings, suggesting that molecular tracers are more efficient tools to determine the level of star formation activity than infrared colour criteria. We also find infrared-quiet clumps that exhibit only a low-velocity component (FWHM ∼ 5−6 km s −1 ) SiO emission in the (2−1) line. In the current picture, where this is attributed to low-velocity shocks from cloud-cloud collisions, this can be used to pinpoint the youngest, thus, likely prestellar massive structures. Using the optically thin isotopologue ( 29 SiO), we estimate that the (2−1) line is optically thin towards most of the sample. Furthermore, based on the line ratio of the (5−4) to the (2−1) line, our study reveals a trend of changing excitation conditions that lead to brighter emission in the (5−4) line towards more evolved sources. Our models show that a proper treatment of non-LTE effects and beam dilution is necessary to constrain trends in the SiO column den...
Thin oxide interlayers are commonly added to the back reflector of thin-film silicon solar cells to increase their current. To gain more insight in the enhancement mechanism, we tested different back reflector designs consisting of aluminium-doped zinc oxide (ZnO:Al) and/or hydrogenated silicon oxide (SiO x :H) interlayers with different metals (silver, aluminium, and chromium) in standard p-i-n a-Si:H solar cells. We use a unique inverse modeling approach to show that in most back reflectors the internal metal reflectance is lower than expected theoretically. However, the metal reflectance is increased by the addition of an oxide interlayer. Our experiments demonstrate that SiO x :H forms an interesting alternative interlayer because unlike the more commonly used ZnO:Al it can be deposited by plasma-enhanced chemical vapour deposition and it does not reduce the fill factor. The largest efficiency enhancement is obtained with a double interlayer of SiO x :H and ZnO:Al.
In this paper we report the analysis of thermal stability of High Brightness Light Emitting Diode subjected to thermal and bias ageing. The degradation mechanisms of several families of commercial available devices were investigated. In the first part of the work we estimated thermal resistance and thermal behaviour under dc bias condition. After this thermal characterisation two different ageing tests were carried out on devices: thermal aging at high temperature levels without biasing the devices and accelerated dc stress at nominal current value (400mA). At each step a complete electrical and optical characterisation of aged devices was performed, in order to find a correlation between different aging and a better understanding of degradation mechanism. This characterisation included I-V measurements, optical power vs current characteristics and spectral analysis. During thermal stress we observed the increase of forward voltage at nominal current and the degradation of optical power with nearly exponential kinetics. We found that lifetimes were well correlated with stress temperature: therefore it was possible to find an activation energy of degradation mechanism of about 1.5eV. Moreover, modifications of spectral properties during electrical and thermal stress were found. Thus, a package level analysis was carried out in order to clarify the role of modification in optical properties of reflector cup and the efficiency of phosphors. Finally, evaluation of differential structure functions indicated that stress induces also the worsening of the properties of the chip-to-package thermal path: this phenomenon has been attributed to the partial detachment and degradation of the ohmic contacts.
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