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Concept and Enabling Technologies

The maximum theoretical efficiency of single-junction solar cells is limited by the inability to absorb photons with energy lower than the material bandgap and by the thermalisation of electron-hole pairs produced by photons with energies larger than the bandgap. For GaAs based solar cells, this turns into a Shockley-Queisser efficiency limit of about 33.5% and 40% under unconcentrated and full concentrated light, respectively.

The TFQD project will develop lightweight GaAs-based thin-film quantum dot solar cells exceeding the Shockley-Queisser limit, based on the following cornerstones:
Thin-film design to enable low weight, flexibility and advanced design for efficiency optimization
– Use of Quantum Dots (QD) to overcome the efficiency limit of single-junction cells by harvesting a larger portion of the solar spectrum with respect to the host bulk semiconductor.
– Implementation of light trapping techniques to increase the photocurrent generation at the GaAs band edge and at the QD wavelengths and of photon recycling to suppress radiative recombination and maximize the open circuit voltage.
– Cost-effective and scalable fabrication process, suitable for further industrialization, by adopting epitaxial lift-off (ELO) technology and NanoImprint Lithography (NIL).

Device technology development will be sustained by an extensive design activity, supported by advanced numerical simulation tools.

The target efficiency of 30%, together with a reduction of weight close to 90%, will make the thin-film light-trapping enhanced quantum dot solar cell to rank record power-to-weight ratios with the potentiality to become a cost-effective, attractive alternative to next generation multi-junction solar cells.

Quantum Dots

Quantum Dots (QDs) are semiconductor crystals of a few nanometers that provide confinement of electrons and holes in the three spatial dimensions. They can be seen as “artificial atoms” because, due to spatial confinement, charge carriers can occupy only a discrete set of energy levels, just like electrons in an atom. III-V QDs can be …

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Light-trapping

The light-trapping concept exploits total internal reflection to confine the incident light within the cell, thus making the effective optical path length longer than the cell physical thickness. Nanophotonic gratings pattered over the bottom or top layers of the cell and the use of suitable reflectors and AntiReflection Coating (ARC) layers may drastically increase the …

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ELO thin-film cells

Epitaxial Lift-Off (ELO) is a patented [1] technology approach pioneered at RBU, where the expensive Ge or GaAs substrate wafer is removed from the III-V photo-sensitive layers in such a way that it can be reused. For this purpose a few nm thick release layer is deposited before the actual III-V cell structure. This sacrificial …

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NanoImprint Lithography

The fabrication technology of the nanophotonic gratings based on replication by Nanoimprint Lithography (NIL) allows to pattern even subwavelength period gratings over large areas. NIL provides cost efficient mass production possibility for the final product. The main part of the nanolithography costs when employing NIL technique, besides the labour and capital expenses, is the cost …

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Modelling and Simulation

Advanced in-house and commercial simulation tools available at POLITO and TUT will help to advance the understanding of the physics governing the devices developed in TFQD and support device development by extensive simulation studies. The peculiarity of QD solar cells physics will be addressed by an ad-hoc numercial simulator [1] that exploits a self-consistent description …

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