151 research outputs found
Extended homologous series of Sn-O layered systems: a first-principles study
Apart from the most studied tin-oxide compounds, SnO and SnO2, intermediate
states have been claimed to exist for more than a hundred years. In addition to
the known homologous series (Seko et al., Phys. Rev. Lett. 100, 045702 (2008)),
we here predict the existence of several new compounds with an O concentration
between 50 % (SnO) and 67 % (SnO2). All these intermediate compounds are
constructed from removing one or more (101) oxygen layers of SnO2. Since the
van der Waals (vdW) interaction is known to be important for the Sn-Sn
interlayer distances, we use a vdW-corrected functional, and compare these
results with results obtained with PBE and hybrid functionals. We present the
electronic properties of the intermediate structures and we observe a decrease
of the band gap when (i) the O concentration increases and (ii) more SnO-like
units are present for a given concentration. The contribution of the different
atoms to the valence and conduction band is also investigated.Comment: 8 page
Exceeding the Shockley-Queisser limit within the detailed balance framework
The Shockley-Queisser limit is one of the most fundamental results in the
field of photovoltaics. Based on the principle of detailed balance, it defines
an upper limit for a single junction solar cell that uses an absorber material
with a specific band gap. Although methods exist that allow a solar cell to
exceed the Shockley-Queisser limit, here we show that it is possible to exceed
the Shockley-Queisser limit without considering any of these additions. Merely
by introducing an absorptivity that does not assume that every photon with an
energy above the band gap is absorbed, efficiencies above the Shockley-Queisser
limit are obtained. This is related to the fact that assuming optimal
absorption properties also maximizes the recombination current within the
detailed balance approach. We conclude that considering a finite thickness for
the absorber layer allows the efficiency to exceed the Shockley-Queisser limit,
and that this is more likely to occur for materials with small band gaps.Comment: 6 pages, 3 figure
The decoupled DFT- method for defect excitation energies
The DFT- method is a band gap correction with GW precision at a
DFT computational cost. The method was also extended to correct the gap between
defect levels, allowing for the calculation of optical transitions. However,
this method fails when the atomic character of the occupied and unoccupied
defect levels are similar as we illustrate by two examples, the tetrahedral
hydrogen interstitial and the negatively charged vacancy in diamond. We solve
this problem by decoupling the effect of the occupied and unoccupied defect
levels and call this the decoupled DFT- method for defects.Comment: 10 pages, 9 figures, accepted by Physical Review
Accelerated Discovery of Efficient Solar-cell Materials using Quantum and Machine-learning Methods
Solar-energy plays an important role in solving serious environmental
problems and meeting high-energy demand. However, the lack of suitable
materials hinders further progress of this technology. Here, we present the
largest inorganic solar-cell material search to date using density functional
theory (DFT) and machine-learning approaches. We calculated the spectroscopic
limited maximum efficiency (SLME) using Tran-Blaha modified Becke-Johnson
potential for 5097 non-metallic materials and identified 1997 candidates with
an SLME higher than 10%, including 934 candidates with suitable convex-hull
stability and effective carrier mass. Screening for 2D-layered cases, we found
58 potential materials and performed G0W0 calculations on a subset to estimate
the prediction-uncertainty. As the above DFT methods are still computationally
expensive, we developed a high accuracy machine learning model to pre-screen
efficient materials and applied it to over a million materials. Our results
provide a general framework and universal strategy for the design of
high-efficiency solar cell materials. The data and tools are publicly
distributed at: https://www.ctcms.nist.gov/~knc6/JVASP.html,
https://www.ctcms.nist.gov/jarvisml/, https://jarvis.nist.gov/ and
https://github.com/usnistgov/jarvis
Charge localization, frustration relief, and spin-orbit coupling in UO
Research efforts on the low temperature magnetic order and electronic
properties of UO have been inconclusive so far. Reinterpreting neutron
scattering results, we use group representation theory to show that the ground
state presents collinear out-of-plane magnetic moments, with antiferromagnetic
coupling both in-layer and between layers. Charge localization relieves the
initial geometric frustration, generating a slightly distorted honeycomb
sublattice with N\'eel order. We show, furthermore, that spin-orbit coupling
has a giant effect on the conduction band states and band gap value. Our
results allow a reinterpretation of recent optical absorption measurements.Comment: 12 pages, including supplemental materia
Effect of Zinc Oxide Modification by Indium Oxide on Microstructure, Adsorbed Surface Species, and Sensitivity to CO
Additives in semiconductor metal oxides are commonly used to improve sensing behavior of gas sensors. Due to complicated effects of additives on the materials microstructure, adsorption sites and reactivity to target gases the sensing mechanism with modified metal oxides is a matter of thorough research. Herein, we establish the promoting effect of nanocrystalline zinc oxide modification by 1–7 at.% of indium on the sensitivity to CO gas due to improved nanostructure dispersion and concentration of active sites. The sensing materials were synthesized via an aqueous coprecipitation route. Materials composition, particle size and BET area were evaluated using X-ray diffraction, nitrogen adsorption isotherms, high-resolution electron microscopy techniques and EDX-mapping. Surface species of chemisorbed oxygen, OH-groups, and acid sites were characterized by probe molecule techniques and infrared spectroscopy. It was found that particle size of zinc oxide decreased and the BET area increased with the amount of indium oxide. The additive was observed as amorphous indium oxide segregated on agglomerated ZnO nanocrystals. The measured concentration of surface species was higher on In2O3-modified zinc oxide. With the increase of indium oxide content, the sensor response of ZnO/In2O3 to CO was improved. Using in situ infrared spectroscopy, it was shown that oxidation of CO molecules was enhanced on the modified zinc oxide surface. The effect of modifier was attributed to promotion of surface OH-groups and enhancement of CO oxidation on the segregated indium ions, as suggested by DFT in previous work
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