In the field of photovoltaics (PV), an important trend is to increase the aesthetic and creative design aspects of solar cells towards more attractive and customized devices for integration in for instance architecture (e.g. Building Integrated Photovoltaics BIPV). Recent evolutions in this domain are mainly situated in the class of emerging PV such as organic solar cells (OPV), dye sensitized solar cells (DSSC) and perovskite solar cells. These solar cell technologies provide additional degrees of design freedom, as they can be processed by printing and allow the realization of semi-transparent solar cells and the use of various colors. Here we aim to go a step further. In this contribution we report on our aim to develop semi-transparent solar cells with integrated images (photographs, paintings, geometric and graphical patterns, text,..) that generate electricity when illuminated, a concept we termed as “Photovoltaic Photographs”. The proposed concept consists of semi-transparent solar cells with an integrated image (functional as photoactive layer), allowing creative applications such as photovoltaic photographs, paintings, posters, etc. The approach proposed here to obtain a patterned 2D-photoactive layer, is by using direct photo-induced patterning process, i.e. one-step photolithography (with mask / without resist) and direct light writing (i.e. maskless / without resist). Encouraged by our recent realization of a proof-of-principle demonstrator using dye sensitized solar cells, in this project we pursue a thorough understanding and control of the proposed light-induced patterning processes and underlying physicochemical (“bleaching”) mechanisms, and their effects on nanoscale material properties, device characteristics, and stability. These insights will also help us in the exploration of combining this concept and these processes with other PV technologies.
Plant leaves possess microscopic valves, called stomata, that enable control of transpirational water loss. In case of water shortage, stomata close, resulting in decreased transpirational cooling. The ensuing temperature increase is readily visualized by thermography. Salicylic acid, a central compound in the defense of plants against pathogens, also closes stomata in several species. In previous work, thermography permitted to monitor an increase in temperature after infection of resistant tobacco by tobacco mosaic virus, before visual symptoms appeared. Furthermore, cell death was visualized with high contrast in both tobacco and Arabidopsis. In addition to transpiration, photosynthetic assimilation is a key physiological parameter. If the amount of light absorbed by chlorophyll exceeds the capacity of the photosynthetic chain, the surplus is dissipated as light of longer wavelength. This phenomenon is known as chlorophyll fluorescence. If a plant leaf is affected by stress, photosynthesis is impaired resulting in a bigger share of non-utilized light energy emitted as fluorescence. The potential of an automated imaging setup combining thermal and fluorescence imaging was shown by monitoring spontaneous cell death in tobacco. This represents a first step to multispectral characterization of a wide range of emerging stresses, which likely affect one or both key physiological parameters.
Chlorophyll fluorescence has been widely applied as a non-invasive technique for the in vivo analysis of plant stress. In this work, the two-dimensional image analysis of the fluorescence signal was used to evaluate the physiological status of heavy metal stressed leaves, based on their photosynthetic capacity. Chlorophyll fluorescence (greater than 650 nm) emission of control and heavy metal treated plants registered at different times during the blue light illumination of the leaves show abnormal patterns of non- homogeneous spatial distribution of the fluorescence emission from metal-treated plants. This is correlated to an altered photosynthetic functionality in different parts of the leaves. Quantitative evaluation of the photosynthetic activity can be made on data extracted after simple arithmetical pixel-point processing of fluorescence images taken at different time during the illumination process. The altered fluorescence emission was observed in absence of other visual symptoms that could testify problems at the level of the photosynthetic apparatus. This indicates that chlorophyll fluorescence imaging is a suitable tool for the early, pre-visual detection of plant stress also in the case of heavy metal stress.
The work aims to validate the laser-induced fluorescence imaging method for detecting nutrient deficiency of fruit- trees and testing the storage ability of the fruits. Measurements concerned apple-trees (Malus x domestica Borkh.cv.Jonagold 2361) submitted or not to nitrogen fertilization (60 kg/ha) via roots. Besides recordings of fluorescence images of fruits and of leaves at the characteristic emission wavelengths, images which always showed an effect of the nitrogen, chemical and physiological analysis have been performed. The essential results were: (1) For rosette leaves, with a total chlorophyll content significantly lower for nitrogen depleted leaves, and a Chl a/b ratio as well as (phi) po (PS II efficiency of open reaction centers) independent of the treatment, images recorded in the red and in the far-red (690 and 740 nm chlorophyll a emissions) showed red/far-red intensities ratios higher in the absence of fertilization, in agreement with the lower chlorophyll a content. (2) For leaves of one year shoots, having all similar chlorophyll content and PS II efficiency, nitrogen supply led to a slight decrease of the red/far-red ratio value for 532 nm excitation, and for 355 nm excitation to an important decrease of the blue fluorescence/chlorophyll emission ratio, that was not observed for rosette leaves. (3) For apple fruits, presenting a high K/Ca ratio (approximately equals 42) i.e. a bad storage ability, the chlorophylls content of the green face skin as well as (phi) po were the same for both samplings, with a dramatic decrease of (phi) po (0.68 till to 0.45) during conservation (6 months). Under 355 nm excitation, the fluorescence ratios the most sensitive to the nitrogen deficiency were for the green face the blue/red ratios which decreased with nitrogen supply and increased with time, and the blue/green ratio for the apple red face.
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