Sunlight harvesting

Fig. 25. Comparison of the sunlight harvesting features of tris(2,2 -bipyridyl)ruthenium (9) and a copper-based multielectron transfer photosensitizer (138) carrying a 7i-acceptor ligand of the BIAN-type (13)...

Typically, the optimization of sunlight harvesting in PV devices by means of spectral managing coatings can be performed via three different luminescence processes upconversion (UC), downconversion (DC) (or quantum-cutting (QC)) and downshifting (DS) (see Figure 11.1). As a mle of thumb, UC layers are placed at the rear of the PV cell so as to collect transmitted photons, while DC and DS layers are placed on the front surface. 

The sensitizers display a crucial role in harvesting of sunlight. To trap solar radiation efficiently in the visible and the near IR region of the solar spectrum requires engineering of sensitizers at a molecular level (see Section 9.16.3).26 The electrochemical and photophysical properties of the ground and the excited states of the sensitizer have a significant influence on the charge transfer (CT) dynamics at the semiconductor interface (see Section 9.16.4). The open-circuit potential of the cell depends on the redox couple, which shuttles between the sensitizer and the counter electrode (for details see Section 9.16.5). 

Timber can be viewed as a classic renewable material. Trees absorb carbon dioxide and utilize water and sunlight to produce a material that can be used in construction, to produce paper or to provide chemical feedstocks, with the production of oxygen as a byproduct. Furthermore, at the end of a product life cycle, the material constituents can be combusted, or composted to return the chemical constituents to the grand cycles . In essence, timber use represents a classic example of a cyclic materials flow, mimicking the flows of materials through natural cycles. Provided that we manage our forests well and do not harvest beyond the capacity of the planet to provide timber, we have at our disposal an inexhaustible resource available in perpetuity. 

Herbs harvested for drying need to be collected with as little moisture on them as possible. A dry, warm, but not too sunny day provides ideai conditions. The ideal time is after overnight moisture has dissipated but before strong sunlight has caused the voiatiie oiis to evaporate. In most cases this is around midmorning, depending on the climate and weather conditions. 

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