Quantum dot intermediate band solar cells (S. Sanguinetti, M.Acciarri, S. Binetti)

Intermediate band solar cells (IBSC) are part of a new third generation approach for increasing the efficiency of photovoltaic devices. The so-called first generation devices are made of a p-n semiconductor junction which can absorb light with energy above the semiconductor band gap and convert it into electrical current. This design has an intrinsic efficiency limit mainly due to the fact that low energy photons are not absorbed and high energy photons generate electron-hole pairs that thermalize in the semiconductor crystal losing energy as heat. With a low gap solar cell more current is produced but at lower voltage. The main goal of IBSCs is a current increase due to sub-gap photon absorption without voltage degradation .
This can be done with a material with an intermediate band (IB), that is a band of allowed states inside the main semiconductor band gap. If this IB is half filled sub-gap photon absorption is possible: in addition to the traditional valence-to-conduction band transition, also valence-band-to-IB and IB-to-conduction-band transitions are possible.
Quantum dots superlattices are a good candidate for IB material, since the confined energy levels of the quantum dots can overlap leading to the formation of a miniband inside the energy gap of the matrix.
Such a structure has already been realized with InAs quantum dots in a GaAs matrix, and the key mechanisms of sub-gap and two-photon absorption have been demonstrated. Unfortunately the InAs/GaAs system shows a great density of dislocations that heavily degrade the output voltage of the device.
We have chosen to develop an IB material with GaAs quantum dots in AlGaAs matrix: this system has nearly no strain, so no dislocations should appear in the device. Our samples are grown via droplet modified molecular beam epitaxy an innovative MBE technique which can achieve a very high quantum dot surface density even with lattice matched semiconductors.

Post-doctoral fellows: Sergio Bietti (device growth), Andrea Scaccabarozzi (device modeling and characterization)


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