Conjugated molecular materials for Photovoltaics and photodetectors. (L.Beverina, A. Papagni)

Luca Beverina and Antonio Papagni are involved in the development of molecular semiconductors both of n- and p-type to be employed in the fabrication of solar cells (mostly Bulk heterojuction cells) and photodetectors. The trademark of the design approach to efficient materials is the use of heteroaromatic and fluorinated building blocks as either intrinsically electron donating or intrinsically electron accepting units. Such approach enables the preparation of easily polarizable, high dielectric constant materials having advantages in terms of excition dissociation and thus photogeneration efficiency. The group is also focusing on the development of straightforward strategies to control the morphology of binary mixtures of acceptors and donors forming the active layer of the solar cells. So far, the most successful approach lies in the post deposition activation of specifically design hydrogen bond interactions.
Finally, the group os also experienced in the preparation of luminescent solar concentrators (LSC) based on organic derivatives, lanthanide chelates and quantum dots. Mostly they work with perylene derivatives.

The LSC concept was introduced to reduce production costs and overcome some limitations of standard silicon-based photovoltaics without changing the basic photon-to-current conversion technology (inorganic semiconductor single junction cells). Moreover, these devices possess building integration opportunities even greater than those of large area Dye Sensitized Solar Cells. In their most common embodiment, LSCs are slabs of transparent, high-quality optical materials doped with luminescent molecules. The host material is typically poly(methylmethacrylate) (PMMA), although in specific cases other acrylic and non-acrylic polymers, as well as sol-gel based glassy materials can be used. The embedded luminescent molecules absorb sunlight and emit light inside the slab. If the refractive index of the slab is significantly higher than that of the air, most of the emitted light is trapped by total internal reflection. The emitted light will mostly travel to the slab edges and will be collected in a small area where a standard silicon solar cell is located. The advantages of such strategy are: 1) The LSC is a light collector where the diffuse light over a large area is concentrated at the slab edges. Silicon PV cells need a certain intensity threshold for converting light into electricity. 2) Strong reduction in the amount of silicon in the cell since it is required only to cover the LCS film edges. 3) The slabs can be easily integrated with buildings enabling color tuning. If properly conceived, a LSC can be at the same time a structural component (for example in sunroofs and windows), an active energy-producing device and a decorative element.

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