Solar cells

We carry out search for third generation solar cells. Particularly, we seek and characterize such materials where thin-film or nanostructured semiconductors may show enhanced absorption and/or carrier multiplication for increased efficiency.

A new candidate for efficient solar cells: strongly correlated materials

The standard process in conventional semiconductors is shown in the top, while the expected process in strongly correlated insulators is shown in the bottom row.

In collaboration with Manousakis group at Florida State University, we propose[1]Author: J. E. Coulter, E. Manousakis, A. Gali
Doi: 10.1103/PhysRevB.90.165142
Journal: Phys. Rev. B
Pages: 165142
Title: Optoelectronic excitations and photovoltaic effect in strongly correlated materials
Volume: 90
Year: 2014
that strongly correlated materials are strong candidates for realizing efficient photovoltaic cells because the enhanced carrier multiplication rates. The idea is sketched on the figure.

Carrier multiplication rates for VO₂ and Si

We have shown by first-principles calculations that the carrier multiplication rate is two orders of magnitude higher in VO₂ than in Si and much higher than the rate of hot electron/hole decay due to phonons. As VO₂ is a proto-typical material of strongly correlated materials, we think that the family of strongly correlated materials exhibit similar properties. This may lead to a single photon in - two electrons out operation of solar cells in strongly correlated materials that can significantly increase the efficiency of this type solar cells compared to the case of conventional semiconductors. [1]Author: J. E. Coulter, E. Manousakis, A. Gali
Doi: 10.1103/PhysRevB.90.165142
Journal: Phys. Rev. B
Pages: 165142
Title: Optoelectronic excitations and photovoltaic effect in strongly correlated materials
Volume: 90
Year: 2014

Characterization of an efficient light absorber: point defects in tin monosulfide (SnS)

SnS material where the red ball depicts an interstitial position

Tin monosulfide (SnS) is quasi-2D material which is a metastable crystalline form of Sn and S. From solar cell application point of view, the very attractive property of SnS is the strong absorption starting at about 1.3 eV. However, the real SnS material is very defective, and often exhibits unintentional p-type doping. In collaboration with Kaxiras group at Harvard University we found[2]Author: B. D. Malone, A. Gali, E. Kaxiras
Doi: 10.1039/C4CP03010A
Journal: Phys. Chem. Chem. Phys.
Pages: 26176-26183
Title: First principles study of point defects in SnS
Volume: 16
Year: 2014
that for the intrinsic defect, Sn-vacancy acceptor defect is responsible for the intrinsic p-type conductivity of SnS. For the extrinsic defects, we find support for the experimental suggestion that P, under S-rich conditions, prefers to substitutionally occupy the Sn site rather than the S site, and this leads to n-type behavior. Additionally, we support that Sb acts as a donor in Sn. We also show that Cl prefers to substitute for S atoms where it acts as a donor.

Enhanced absorption and carrier multiplication in semiconductor nanoparticles

The effect of adjiont hexyl groups on the absorption intensity of a silicon nanoparticle. The increment of integrated absorption intensity is 126% and 65% in the 0 eV–6 eV and in the 0 eV–9 eV range, respectively.

Surface effects on nanoparticle boundaries in solid-state matrix can help circumvent the limitations of traditional (first and second-generation) photovoltaics. This effect may be enhanced and fine-tuned by modifying the surface. The core aim of this strategy is to produce solid-state systems which absorb visible light, and in which the resulting high-energy exciton generates multiple charge carriers instead of decaying into a low-energy one by heating the solar cells.

Tweaking the absorption of Si nanoparticles by surface engineering

We calculated ab-initio the absorption of Si nanoparticles with various surface terminations [3]Author: M. Vörös, D. Rocca, G. Galli, G. T. Zimanyi, A. Gali
Doi: 10.1103/PhysRevB.87.155402
Journal: Phys. Rev. B
Pages: 155402
Title: Increasing impact ionization rates in Si nanoparticles through surface engineering: A density functional study
Volume: 87
Year: 2013
, [4]Author: A. Gali, M. Vörös, D. Rocca, G. T. Zimanyi, G. Galli
Doi: 10.1021/nl901970u
Journal: Nano Letters
Note: PMID: 19785388
Number: 11
Pages: 3780-3785
Title: High-Energy Excitations in Silicon Nanoparticles
Volume: 9
Year: 2009
. We showed in collaboration with Giulia Galli group at UC Davis that the surface termination of small nanoparticles can seriously change their absorption spectrum in the lowest energy region which can be the most critical in solar cell applications. In addition, we showed that the empty states of the nearby Si nanoparticles interact with each other that also seriously influence their absorption properties [4]Author: A. Gali, M. Vörös, D. Rocca, G. T. Zimanyi, G. Galli
Doi: 10.1021/nl901970u
Journal: Nano Letters
Note: PMID: 19785388
Number: 11
Pages: 3780-3785
Title: High-Energy Excitations in Silicon Nanoparticles
Volume: 9
Year: 2009
. This finding explains previous experimental data on colloid quantum dots, and highlighted the importance of surface engineering and the role of the density of nanoparticles towards the optimization for solar cell applications.

Solution to the quantum confinement dilemma: enhanced carrier multiplication rates in the solar spectrum in Si and Ge metastable nanoparticles

Different phases of silicon as nanoparticles. Of them BC8 is the prospective candidate for photovoltaics.

Multiple Exciton Generation (MEG) in nanoparticle-based solar cells promises to increase the cell-efficiency above the Shockley–Queisser limit. However, utilizing MEG is hampered by the Quantum Confinement Dilemma (QCD): quantum confinement advantageously increases the effective Coulomb interaction, but at the same time disadvantageously increases the electronic gap. Using ab initio calculations we showed in collaboration with Giulia Galli group at UC Davis that germanium nanoparticles with core structures of high pressure phases of bulk Ge can transcend the QCD, by simultaneously lowering gaps and increasing the MEG rates above those of NPs with a cubic diamond core. Synthesis routes to obtain Ge colloidal ST12 core structures are available and hence we propose that exploring ST12 Ge NPs for MEG solar cells is a promising research effort [5]Author: M. Voros, S. Wippermann, B. Somogyi, A. Gali, D. Rocca, G. Galli, G. T. Zimanyi
Doi: 10.1039/C4TA01543F
Journal: J. Mater. Chem. A
Pages: 9820-9827
Title: Germanium nanoparticles with non-diamond core structures for solar energy conversion
Volume: 2
Year: 2014
. We also showed that Si nanoparticles with a BC8 core structure exhibit significantly lower optical gaps and MEG thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size [6]Author: S. Wippermann, M. Vörös, D. Rocca, A. Gali, G. Zimanyi, G. Galli
Doi: 10.1103/PhysRevLett.110.046804
Journal: Phys. Rev. Lett.
Pages: 046804
Title: High-Pressure Core Structures of Si Nanoparticles for Solar Energy Conversion
Volume: 110
Year: 2013
. This BC8 phase of Si can be fabricated by various techniques, thus our proposal opens the window for efficient solar cell absorbers from environment friendly and widely accessible materials and materials processing.

Efficient electron-hole separation for Si nanoparticles embedded into a matrix

Electronic structure of Si nanoparticles in a-ZnS matrix

Solar cell application of the very promising silicon or germanium nanoparticles (NP) requires such a matrix where the photo-generated carriers can be efficiently separated and can be lifted to the contacts. The strong Coulomb interaction between the carriers in nanoparticles mediate the MEG process, however, this strong Coulomb interaction may also disadvantageously inhibit the efficient separation of the two types of carriers, electrons and holes, needed for photo-voltaics operation.

We propose that embedding silicon NP into amorphous, nonstoichiometric zinc-sulfide (ZnS) leads to promising nanocomposites for solar energy conversion. Using ab initio molecular dynamics simulations we show in collaboration with Giulia Galli group that, upon high temperature amorphization of the host chalcogenide, sulfur atoms are drawn to the NP surface. We find that the sulfur content may be engineered to form a type II heterojunction, with complementary charge transport channels for electrons and holes, and that sulfur capping is beneficial to lower the nanoparticle gap, with respect to that of NPs embedded in oxide matrices [7]Author: S. Wippermann, M. Vörös, A. Gali, F. Gygi, G. T. Zimanyi, G. Galli
Doi: 10.1103/PhysRevLett.112.106801
Journal: Phys. Rev. Lett.
Pages: 106801
Title: Solar Nanocomposites with Complementary Charge Extraction Pathways for Electrons and Holes: Si Embedded in ZnS
Volume: 112
Year: 2014
.

Bibliography

[1]	J. E. Coulter, E. Manousakis, A. Gali: Phys. Rev. B, 90, 165142 (2014). Optoelectronic excitations and photovoltaic effect in strongly correlated materials
[2]	B. D. Malone, A. Gali, E. Kaxiras: Phys. Chem. Chem. Phys., 16, 26176-26183 (2014). First principles study of point defects in SnS
[3]	M. Vörös, D. Rocca, G. Galli, G. T. Zimanyi, A. Gali: Phys. Rev. B, 87, 155402 (2013). Increasing impact ionization rates in Si nanoparticles through surface engineering: A density functional study
[4]	A. Gali, M. Vörös, D. Rocca, G. T. Zimanyi, G. Galli: Nano Letters, 9, 3780-3785 (2009). High-Energy Excitations in Silicon Nanoparticles
[5]	M. Voros, S. Wippermann, B. Somogyi, A. Gali, D. Rocca, G. Galli, G. T. Zimanyi: J. Mater. Chem. A, 2, 9820-9827 (2014). Germanium nanoparticles with non-diamond core structures for solar energy conversion
[6]	S. Wippermann, M. Vörös, D. Rocca, A. Gali, G. Zimanyi, G. Galli: Phys. Rev. Lett., 110, 046804 (2013). High-Pressure Core Structures of Si Nanoparticles for Solar Energy Conversion
[7]	S. Wippermann, M. Vörös, A. Gali, F. Gygi, G. T. Zimanyi, G. Galli: Phys. Rev. Lett., 112, 106801 (2014). Solar Nanocomposites with Complementary Charge Extraction Pathways for Electrons and Holes: Si Embedded in ZnS