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The concept: INTERMEDIATE BAND SOLAR CELLS via QDs (QDSC)
Intermediate band (IB) solar cells have been proposed as a way to overcome one of the fundamental limitations intrinsic to the generation of an electric current by absorption of light in semiconductors, that is the threshold energy that photons must have in order to generate electron-hole pairs in semiconductors. The concept is basically that if a half-full energy band (the IB) is present in the middle of the bandgap of a semiconductor, sub-gap optical transitions involving the IB are enabled, thus increasing the generation of carriers in a solar cell. Radiative only recombination effects should not be detrimental for device performance if the system can be described by three independent quasi-Fermi levels, for conduction, valence, and intermediate band respectively. If these conditions are fulfilled, the short-circuit current of the device will increase and the open circuit voltage will remain the same of a device with the same bandgap, thus leading to a net increase of efficiency, that has been demonstrated to reach 63% under maximal concentration.
QD or QW superlattice (SL) incorporated in the active region of p-i-n single-junction solar cells has been considered as one of the most promising candidates to realize the IBSCs.In the last decade, there has been an extensive effort to demonstrate the Quantum Dot Solar Cells with a central focus on III–V compound semiconductors.
QDSC made by InAs/GaAs QDs posses large number of excited states compared to the ideal situation which can couple with the barrier states and increase thermal or tunneling escape from the QDs. This mechanism is clearly competitive with the optical extraction of carriers from the QDs that constitute the intermediate band states, and leads to to the decrease of open circuit voltage in QD devices.
Figure:
Top Panel: Band diagram of a IBSC highlighting the presence of a two sequential absorption of sub-gap photons that permits to promote a valence electron to the conduction band [1].
Bottom Panel: Limiting efficiency of an IBSC calculated assuming 6000 K blackbody radiation from the sun. The calculation has been performed as a function of the widest bandgap (host) material and for maximum concentration as well as 1 sun conditions [2]
References
[1] A. Luque and A. Marti, Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels, Phys. Rev. Lett. 78, 5014 (1997).
[2] Y. Okada, ET AL. Intermediate Band Solar Cells: Recent Progress and Future Directions, Appl. Phys. Rev. 2, 021302 (2015).
OUR APPROACH
A careful design the QD states, not only of the ground state, which defines the absorption threshold, but rather of the entire energy density of states of the QDs could overvecome the reduction of the open circuit voltage problematics in QDSC. The possibility to tune the size of the QDs, as well as their aspect ratio, having a direct and important impact on the QD energy density of states, would provide fundamental degree of freedom in the quantum design of the IB solar cells.
Droplet Epitaxy allows the realization of lattice matched nanostructures without the presence of wetting layers and relief of strain by introduction of dislocations in the epitaxial film.
We exploit the capability to control the actual shape of the QDs to show that it is possible to finely tune the electronic states of the QDs to 1) decrease the sub-gap absorption wavelength threshold in intermediate band solar cells, for a better spectral match with sunlight; 2) engineer the thermalization and the extraction process of carriers from the IB by reducing the phonon assisted transition probability and increasing the thermal extraction barrier energy.
Light-trapping techniques can be exploited to further increase 1st and 2nd photon optical transitions, increasing the short circuit current and preserving the open circuit voltage thanks to the improved ratio between optical and thermal extraction rates from the IB.
Figure:
Top Panel: Schematics of the QDSC fabricated in our laboratory
Bottom Panel: Shape enginnered QDs by droplet epitaxy (see AFM on the left panels) permits to reduce the open voltage loss.
Relevant Publications:
A. Scaccabarozzi et al. Evidence of two-photon absorption in strain-free quantum dot GaAs/AlGaAs solar cells. Phys. Stat. Sol. RRL 3, 173 (2013) DOI:10.1002/pssr.201206518 (cover article)
A. Scaccabarozzi et al. Enhancing intermediate band solar cell performances through quantum engineering of dot states by droplet epitaxy. Progress in Photovoltaics 21, 637 (2023) DOI: 10.1002/pip.3672
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