Silicium amorphe

Résumé

Le silicium amorphe, généralement abrégé a-Si, est la variété allotropique non cristallisée du silicium, c’est-à-dire dans lequel les atomes sont désordonnés et ne sont pas rangés de façon régulière définissant une structure cristalline. Le silicium amorphe peut être déposé en couches minces à basse température sur un grand nombre de substrats, permettant d'envisager une grande variété d'applications microélectroniques. Ce matériau semi-conducteur est couramment utilisé pour réaliser certains panneaux solaires photovoltaïques. Description L'atome de silicium est tétravalent, il forme donc quatre liaisons covalentes avec ses voisins. Dans sa forme cristalline, le silicium adopte la structure diamant, comme le carbone et le germanium, formant ainsi un cristal ordonné. Le silicium amorphe se caractérise par l'absence d'ordre à longue portée : les atomes forment plutôt un réseau aléatoire continu dans lequel certaines liaisons ne sont pas satisfaites ; on parle alors de lia

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https://fr.wikipedia.org/wiki/Silicium_amorphe

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High-Resolution Photoemission Study of Neutron-Induced Defects in Amorphous Hydrogenated Silicon Devices

Sylvain Dunand, Giulia Rossi

In this paper, by means of high-resolution photoemission, soft X-ray absorption and atomic force microscopy, we investigate, for the first time, the mechanisms of damaging, induced by neutron source, and recovering (after annealing) of p-i-n detector devices based on hydrogenated amorphous silicon (a-Si:H). This investigation will be performed by mean of high-resolution photoemission, soft X-Ray absorption and atomic force microscopy. Due to dangling bonds, the amorphous silicon is a highly defective material. However, by hydrogenation it is possible to reduce the density of the defect by several orders of magnitude, using hydrogenation and this will allow its usage in radiation detector devices. The investigation of the damage induced by exposure to high energy irradiation and its microscopic origin is fundamental since the amount of defects determine the electronic properties of the a-Si:H. The comparison of the spectroscopic results on bare and irradiated samples shows an increased degree of disorder and a strong reduction of the Si-H bonds after irradiation. After annealing we observe a partial recovering of the Si-H bonds, reducing the disorder in the Si (possibly due to the lowering of the radiation-induced dangling bonds). Moreover, effects in the uppermost coating are also observed by spectroscopies.

MDPI2022

Chargement

One of the biggest driving forces in cement research today is the mitigation of the CO2 emissions caused by its production. A potential solution to reduce the environmental impact is to replace cement with supplementary cementitious materials (SCMs). This replacement results in low strength at early ages because they are slow to react. The incorporation of minor elements has shown the potential to improve cement reactivity. A few percent of ZnO in C3S causes a signifi- cant increase in reactivity. This effect was related to the incorporation of zinc into the C-S-H struc- ture, which increases its needle length. This research investigates the effect of ZnO in more realis- tic systems such as alite, C3A-polyclinker, and C4AF-polyclinker. The positive effects observed in C3S are translated in the alite system, but differences were found in the polyclinkers, which have a prolonged induction period. This resulted due concentration of zinc in the interstitial phases and a small amount of zinc in alite phase. An amorphous phase was identified in the C3A polyclinker; it is believed to react rapidly with water, releasing Zn ions into the solution, which delays the reaction of alite. The addition of more gypsum controls the reaction of this phase, and thus the induction period is shortened.This work explores the possibilities of retaining Zn in alite to enhance the reactivity of more realis- tic cementitious systems. The cooling rate was critical, as a slow cooling rate promoted more Zn in the alite, less amorphous, and hence, an enhancement in the hydration of C3A-polyclinker doped with ZnO. The recalcination of this system did not affect the hydration of alite but increased the amount of crystalline C3A, which reacted faster. Thus, the amorphous phase was related to an aluminate phase. Moreover, a C4AF-polyclinker and a system with a lower amount of interstitial phase were investigated. The results showed a higher Zn concentration in alite but also extended induction periods. This was associated with the free ZnO and the amorphous content. Even when the hydration was delayed, the height of the alite peak increased with Zn doping, probably due to the incorporation of Zn into the C-S-H observed in pure C3S.In addition, the interaction between sulfates and zinc in the alite system doped with ZnO was in- vestigated. Severe retardation was observed, and the reason is proposed to be the formation of amorphous Zn compound. The presence of aluminum, ettringite formation, or an amorphous layer were discarded as the cause of the retardation.

EPFL2022

Chargement

Composition and Interface Engineering of High Performing Perovskite Solar Cells

Mousa Abdulla M E Abuhelaiqa

Perovskite solar cells have been established as a disruptive technology in the domain of photo-voltaics. Their facile, low-temperature processing and high-power conversion efficiency make them stand-out among other mature solar technologies. The performance of perovskite solar cells is pro-foundly tied to an interplay of material properties of singular thin films that are assembled to form the photovoltaic device stack. An understanding of optoelectronic, morphological, and other key charac-teristics of each layer and its interfaces is instrumental in advancing the science of perovskite solar cells. Since their inception after just over a decade, much work on perovskite solar cell has been stag-nant to bring the technology to the market. The long-term stability remains foremostly the main hin-drance to facilitating technology transfer. Therefore, the work presented in this thesis explores multi-ple interface and compositional approaches to promote stability and other photovoltaic parameters of perovskite solar cells.In the second chapter of the thesis, a light-soaking exploration was done on perovskite sam-ples that utilize single (TiO2 and SnO2) and bilayered (TiO2/SnO2) electron transporting materials. The work aims to expand consensus on the optoelectronic and morphological features of light-soaked samples and link them with long-term stability. Upon preliminary testing, maximum power point tracking on each sample reveals efficiency preservation of SnO2 and TiO2/SnO2 samples over the course of 1000 hours while prominent degradation was revealed for the TiO2 sample. On a secondary investigation, light-soaking characterizations were performed where fresh and light-soaked samples were characterized to obtain photoluminescence, microscopic, and crystallographic features. The work reveals the added benefit of employing TiO2/SnO2 transporting bilayers to improve both the stability and power conversion efficiency of a perovskite solar cell. Building on an effort to understand TiO2/SnO2 bilayer behaviors, the third chapter investigates the role of halide passivating elements in SnO2 on the performance of perovskite solar cells. The study utilizes three metalorganic precursors based on acetylacetone complexes with chloride and bromide groups to fabricate SnO2 layers at different annealing temperatures. Upon thermal and elemental in-vestigations, the study clearly links the presence of amorphous halide elements in SnO2 to the power conversion efficiency of perovskite solar cells. More-in-depth, the optimal SnO2 annealing tempera-ture for bromide containing samples 250 ï°C compared with 220 ï°C for chloride containing samples. The higher temperature tolerance tendency was correlated to bromideâs lower sublimation sensitivity. Overall, the study highlights the importance of amorphous passivating elements in SnO2 for high per-forming planar perovskite solar cells. Finally, the fourth chapter involves a novel cation mixing approach for 2D perovskites at the interface of the 3D perovskite and the hole transporting layer. Alkyl ammonium halides have been widely used to form light-stable 2D perovskite films upon interaction with excess PbI2. In this study, different alkyl chain cations (propylammonium iodide and octylammonium iodide) were mixed in one solution to form a novel 2D perovskite crystal lattice. Photoluminescence and x-ray diffraction spec-troscopy investigations reveal a unique crystal lattice and a uniform quasi-2

EPFL2022

Chargement

Composition and Interface Engineering of High Performing Perovskite Solar Cells

Mousa Abdulla M E Abuhelaiqa

EPFL2022