The search for cheap, Earth abundant materials for solar cells, and for photo-electrodes for water splitting, calls for detailed investigations of the efficiency of light absorption in materials and nanostructures. Theoretical frameworks and computer simulations are used to help interpret a growing body of complex measurements, and to predict optimal systems for harvesting sun light.
Novel silicon for solar energy conversion
Silicon exhibits a large variety of different bulk phases, allotropes and composite structures, such as e. g. clathrates or nanostructures, at both higher and lower densities compared to diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, that are non-toxic and earth-abundant, with properties tailored precisely towards specific applications, including solar energy conversion devices.
"Novel Silicon phases and nanostructures for solar energy conversion",
Stefan Wippermann, Yuping He, Marton Voros, and Giulia Galli,
Appl. Phys. Rev. (2016), accepted
Solids of nanoparticles
The Intermediate Band (IB) solar cell concept is a promising idea to transcend the Shockley–Queisser limit. Using the results of first-principles calculations, we propose that colloidal nanoparticles (CNPs) are a viable and efficient platform for the implementation of the IB solar cell concept. We focused on CdSe CNPs and we showed that intragap states present in the isolated CNPs with reconstructed surfaces combine to form an IB in arrays of CNPs, which is well separated from the valence and conduction band edges. We demonstrated that optical transitions to and from the IB are active. We also showed that the IB can be electron doped in a solution, e.g., by decamethylcobaltocene, thus activating an IB-induced absorption process. Our results, together with the recent report of a nearly 10% efficient CNP solar cell, indicate that colloidal nanoparticle intermediate band solar cells are a promising platform to overcome the Shockley–Queisser limit.
"Colloidal Nanoparticles for Intermediate Band Solar Cells",
Márton Vörös, Giulia Galli, and Gergely Zimanyi ,
ACS Nano9, 6882 (2015)
Excitation spectra of Si and Ge nanoparticles
We carried out density functional and many body perturbation theory calculations of the electronic, optical, and impact ionization properties of Si and Ge nanoparticles (NPs), including core structures based on high-pressure bulk Si and Ge phases. Si particles with a BC8 core structure exhibit significantly lower optical gaps and multiple exciton generation (MEG) thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size. Hence BC8 nanoparticles may be promising candidates for MEG-based solar energy conversion. High pressure structures of Ge nanoparticles (ST12) show as well promising MEG properties.
"Ab initio optoelectronic properties of silicon nanoparticles: Excitation energies, sum rules, and Tamm-Dancoff approximation",
Dario Rocca, Márton Vörös, Adam Gali and Giulia Galli,
J. Comp. Theor. Chem.10, 3290 (2014)
"Germanium nanoparticles with non-diamond core structures for solar energy conversion",
Márton Vörös, Stefan Wippermann, Bálint Somogyi, Adam Gali, Dario Rocca, Giulia Galli, and Gergely T. Zimanyi,
J. Mater. Chem. A2, 9820 (2014)
"High-Pressure Core Structures of Si Nanoparticles for Solar Energy Conversion",
S. Wippermann, M. Vörös, D. Rocca, A. Gali, G. Zimanyi and G. Galli,
Phys. Rev. Lett., 110, 046804 (2013)
"Increasing impact ionization rates in Si nanoparticles through surface engineering: A density functional study",
M. Vörös, D. Rocca, G. Galli, G.T. Zimanyi and A. Gali,
Phys. Rev. B, 87, 155402 (2013)
"High energy excitations in silicon nanoparticles",
A. Gali, M. Vörös, D. Rocca, G. Zimanyi and G. Galli,
Nano Lett., 9, 3780 (2009)
Embedded Si Nanocrystals
Si nanocrystals (NCs) are often synthesized in oxide or nitride matrices. Our coupled classical and quantum simulations of 1-2 nm Si nanoparticles embedded in amorphous-SiO2 has shown that by tuning the density of the oxide , one may form nanoscale heterojunctions with either straddling (type I) or staggered (type II) energy gaps. We have also found that interfacial strain plays a key role in determining the variation of the nanaoparticle gap as a function of size, as well as conduction band offsets. NCs extracted from matrices with their oxide shell have allowed us to study the origin of blinking. In addition to oxides we have studied Si NPs in ZnS matrices and found that upon high temperature amorphization of the host chalcogenide, sulfur atoms are drawn to the NP surface. Sulfur content may be engineered to form a type II heterojunction, with complementary charge transport channels for electrons and holes.
"Surface Dangling Bonds Are a Cause of B-Type Blinking in Si Nanoparticles",
Nicholas Brawand, Márton Vörös and Giulia Galli,
Nanoscale7, 3737 (2015)
"Solar nanocomposites with complementary charge extraction pathways for electrons and holes: Si embedded in ZnS",
Stefan Wippermann, Márton Vörös, Adam Gali, Francois Gygi, Gergely Zimanyi and Giulia Galli,
Phys. Rev. Lett.112, 068103 (2014)
Bulk MoS2, a prototypical metal dichalcogenide, is an indirect band gap
semiconductor with negligible photoluminescence. When the MoS2 crystal is
thinned to a monolayer, a strong photo-luminescence emerges, indicating an
indirect to direct band gap transition, as predicted by ab-initio calculations. This
observation shows that quantum confinement in layered d-electron materials
such as MoS2 may provide new opportunities for engineering the electronic
structure of matter at the nanoscale.
"Emerging Photoluminescence in Monolayer MoS2",
A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli and F. Wang,
Nano Lett., 10, 1271 (2010)
Optical absorption spectra of thin silicon nanowires have been computed using
many body perturbation theory, by solving the Bethe-Salpeter equation in the static
approximation. We used a technique recently developed in our group, that avoids
explicit calculation of empty electronic states, as well as storage
and inversion of the full dielectric matrix. Establishing numerical error bars of computed
spectra turned out to be critical, in order to draw meaningful comparisons with
experiment and between results obtained within different algorithms. The dependence
of spectra on surface structure has been analyzed in detail.
Ab-initio calculations of absorption spectra of semiconducting nanowires within many body perturbation theory,
Y. Ping, D. Rocca, D. Lu and G. Galli,
Phys. Rev. B85, 035316 (2012)
Silicon exhibits a large variety of different bulk phases, allotropes and composite structures, such as e. g. clathrates or nanostructures, at both higher and lower densities compared to diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, that are non-toxic and earth-abundant, with properties tailored precisely towards specific applications, including solar energy conversion devices.
The Intermediate Band (IB) solar cell concept is a promising idea to transcend the Shockley–Queisser limit. Using the results of first-principles calculations, we propose that colloidal nanoparticles (CNPs) are a viable and efficient platform for the implementation of the IB solar cell concept. We focused on CdSe CNPs and we showed that intragap states present in the isolated CNPs with reconstructed surfaces combine to form an IB in arrays of CNPs, which is well separated from the valence and conduction band edges. We demonstrated that optical transitions to and from the IB are active. We also showed that the IB can be electron doped in a solution, e.g., by decamethylcobaltocene, thus activating an IB-induced absorption process. Our results, together with the recent report of a nearly 10% efficient CNP solar cell, indicate that colloidal nanoparticle intermediate band solar cells are a promising platform to overcome the Shockley–Queisser limit.
We carried out density functional and many body perturbation theory calculations of the electronic, optical, and impact ionization properties of Si and Ge nanoparticles (NPs), including core structures based on high-pressure bulk Si and Ge phases. Si particles with a BC8 core structure exhibit significantly lower optical gaps and multiple exciton generation (MEG) thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size. Hence BC8 nanoparticles may be promising candidates for MEG-based solar energy conversion. High pressure structures of Ge nanoparticles (ST12) show as well promising MEG properties.
Si nanocrystals (NCs) are often synthesized in oxide or nitride matrices. Our coupled classical and quantum simulations of 1-2 nm Si nanoparticles embedded in amorphous-SiO2 has shown that by tuning the density of the oxide , one may form nanoscale heterojunctions with either straddling (type I) or staggered (type II) energy gaps. We have also found that interfacial strain plays a key role in determining the variation of the nanaoparticle gap as a function of size, as well as conduction band offsets. NCs extracted from matrices with their oxide shell have allowed us to study the origin of blinking. In addition to oxides we have studied Si NPs in ZnS matrices and found that upon high temperature amorphization of the host chalcogenide, sulfur atoms are drawn to the NP surface. Sulfur content may be engineered to form a type II heterojunction, with complementary charge transport channels for electrons and holes.
Bulk MoS2, a prototypical metal dichalcogenide, is an indirect band gap
semiconductor with negligible photoluminescence. When the MoS2 crystal is
thinned to a monolayer, a strong photo-luminescence emerges, indicating an
indirect to direct band gap transition, as predicted by ab-initio calculations. This
observation shows that quantum confinement in layered d-electron materials
such as MoS2 may provide new opportunities for engineering the electronic
structure of matter at the nanoscale.
Optical absorption spectra of thin silicon nanowires have been computed using
many body perturbation theory, by solving the Bethe-Salpeter equation in the static
approximation. We used a technique recently developed in our group, that avoids
explicit calculation of empty electronic states, as well as storage
and inversion of the full dielectric matrix. Establishing numerical error bars of computed
spectra turned out to be critical, in order to draw meaningful comparisons with
experiment and between results obtained within different algorithms. The dependence
of spectra on surface structure has been analyzed in detail.