Posemètre

vignette|250px|Ancien posemètre inventé par Alfred Watkins vignette|Posemètre Lios Scop, fabriqué en Allemagne vers 1930 Un posemètre est un appareil utilisé en photographie pour mesurer la luminosité d'une scène et ainsi déterminer l'exposition optimum d'une prise de vue. Il permet de calculer la résultante des variables d'exposition en fonction de l'éclairement ou la luminance de la scène, du temps d'exposition, de l'ouverture du diaphragme et de la sensibilité de la surface (film ou capteur). Il est composé d'une cellule photo-électrique et d'un calculateur, manuel (de type règle à calcul) ou électronique. Cette capacité de calcul le différencie d'un simple luxmètre. Les posemètres peuvent fonctionner suivant deux principes différents :

Mesure en lumière incidente : c'est la mesure de l'éclairement (quantité de lumière reçue) à un endroit défini (généralement où se trouve le sujet) indépendamment de la réflectivité du sujet lui-même.
Mesure en lumière réfléchie : c'est la mes

Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions

Jean Cattin

The global photovoltaic market is mostly dominated by solar cells based on crystalline silicon (c-Si), which are covering 95% of the market. This thesis concerns silicon heterojunction (SHJ), a high-efficiency technology with a 2% market share in 2018, but showing a steady global production capacity growth and potential for industry-compatible modules with a conversion efficiency above 21%. This thesis focuses on improving the device efficiency in real operating conditions. More specifically, different effects affecting a cell used in non-standard conditions are assessed, including different temperatures and illuminations, during prolonged exposure to light and the effect of surface inhomogeneities. The main SHJ technology specificity consists of the use of thin hydrogenated amorphous silicon (a-Si) layers that provide excellent c-Si surface passivation. This enables very high operating voltages compared to other c-Si-based technologies, which directly reduces the loss of efficiency induced by higher operating temperatures. The cell's properties are measured at very low temperatures are studied, which amplifies charges transport phenomena. The voltage shows a deviation from its theoretical curve, which we explain to be related to a decrease of the positive contact holes selectivity due to the influence of the transparent conductive oxide layer. An increase in the resistive losses at low temperature is measured. These losses are caused by a resistivity increase in the different a-Si layers and interfaces because of a thermal energy decrease at low temperatures. These effects may also affect cell performances at room temperature and under operating conditions. This implies that optimisation at standard test conditions (STC), generally used to assess the performance of PV modules, does not necessarily result in the same optimum as in real climatic conditions, which generally correspond to a higher temperature and/or a lower illumination. We show that by adding carbon in the front doped layer, a transparency increase, and thus a current gain is observed at the expense of larger ohmic losses, which overall reduces the efficiency at STC. Using simulations and real climate data, we show that using such a layer leads to a relative energy production gain of 0.5 to 0.8%, both in a moderate and arid climate. Another specificity of SHJ cells is a slight efficiency increase when exposed to light for a few days. We show that in case of a too-small positive doped layer charges reserve, cell performance degradation is observed instead of an improvement. In order to avoid such degradation, the positively doped layer should be sufficiently thick and doped to screen the influence of the subsequent transparent conductive oxide layer. A treatment in which a forward bias voltage is applied to the cell allows triggering the gain in efficiency without causing the detrimental effects of light exposure. Finally, the influence of a non-homogeneous surface is studied. Parasitic electric currents are induced between the well-passivated area and poor passivation area through the metallisation. Using dedicated samples, correspondence between these currents, and an often measured drop of the low injection lifetime is described and linked to the detrimental effect of either the sample edges or by surface defects. Concretely, these effects can affect the performances of the cell at low illumination, reducing the performances in real outdoor conditions.

EPFL2020

THIN-FILM BARRIERS FOR DURABLE THIN-FILM PV MODULES

Gianluca Cattaneo, Fanny Sculati-Meillaud

This contribution describes thin-film barrier coatings to enhance the stability of silicon thin-film solar modules. Thin-film barriers are applied to protect the modules from degradation in humid environment. Demonstrator modules were exposed to indoor and outdoor tests to evaluate failure modes by electrochemical corrosion during operation. Test results reveal some major failure modes, but we demonstrate long lifetimes under operational conditions for certain modules without conventional encapsulation. These modules without conventional encapsulation showed less than 1% relative degradation during 1000 hours of damp-heat tests with light exposure at open circuit voltage. These promising results will guide the way to new encapsulation concepts for longer lifetimes and/or new low-cost concepts. This approach might be applied to any thin-film technology and will be an important aspect for consumer electronics with less strict requirements for very long-term operation.

WIP Wirtschaft und Infrastruktur GmbH & Co Planungs KG2016

Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions

Jean Cattin

EPFL2020

Field studies of human light exposure during everyday activities - Methodological aspects and initial results

Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions

THIN-FILM BARRIERS FOR DURABLE THIN-FILM PV MODULES

Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions

Field studies of human light exposure during everyday activities - Methodological aspects and initial results

THIN-FILM BARRIERS FOR DURABLE THIN-FILM PV MODULES