24 research outputs found
First-principles study of intrinsic and hydrogen point defects in the earth-abundant photovoltaic absorber Zn3P2
Zinc phosphide (Zn3P2) has had a long history of scientific interest largely
because of its potential for earth-abundant photovoltaics. To realize
high-efficiency Zn3P2 solar cells, it is critical to understand and control
point defects in this material. Using hybrid functional calculations, we assess
the energetics and electronic behavior of intrinsic point defects and hydrogen
impurities in Zn3P2. All intrinsic defects are found to act as compensating
centers in p-type Zn3P2 and have deep levels in the band gap, except for zinc
vacancies which are shallow acceptors and can act as a source of doping. Our
work highlights that zinc vacancies rather than phosphorus interstitials are
likely to be the main source of p-type doping in as-grown Zn3P2. We also show
that Zn-poor and P-rich growth conditions, which are usually used for enhancing
p-type conductivity of Zn3P2, will facilitate the formation of certain
deep-level defects (P_Zn and P_i) which might be detrimental to solar cell
efficiency. For hydrogen impurities, which are frequently present in the growth
environment of Zn3P2, we study interstitial hydrogen and hydrogen complexes
with vacancies. The results suggest small but beneficial effects of hydrogen on
the electrical properties of Zn3P2
First principles study of the T-center in Silicon
The T-center in silicon is a well-known carbon-based color center that has
been recently considered for quantum technology applications. Using first
principles computations, we show that the excited state is formed by a
defect-bound exciton made of a localized defect state occupied by an electron
to which a hole is bound. The localized state is of strong carbon \textit{p}
character and reminiscent of the localization of the unpaired electron in the
ethyl radical molecule. The radiative lifetime for the defect-bound exciton is
calculated to be on the order of s, much longer than other quantum defects
such as the NV center in diamond and in agreement with experiments. The longer
lifetime is associated with the small transition dipole moment as a result of
the very different nature of the localized and delocalized states forming the
defect-bound exciton. Finally, we use first principles calculations to assess
the stability of the T-center. We find the T-center to be stable against
decomposition into simpler defects when keeping the stoichiometry fixed.
However, we identify that the T-center is easily prone to (de)hydrogenation and
so requires very precise annealing conditions (temperature and atmosphere) to
be efficiently formed
Designing transparent conductors using forbidden optical transitions
Many semiconductors present weak or forbidden transitions at their
fundamental band gaps, inducing a widened region of transparency. This occurs
in high-performing n-type transparent conductors (TCs) such as Sn-doped In2O3
(ITO), however thus far the presence of forbidden transitions has been
neglected in searches for new p-type TCs. To address this, we first compute
high-throughput absorption spectra across ~18,000 semiconductors, showing that
over half exhibit forbidden or weak optical transitions at their band edges.
Next, we demonstrate that compounds with highly localized band edge states are
more likely to present forbidden transitions. Lastly, we search this set for
p-type and n-type TCs with forbidden or weak transitions. Defect calculations
yield unexplored TC candidates such as ambipolar BeSiP2, Zr2SN2 and KSe, p-type
BAs, Au2S, and AuCl, and n-type Ba2InGaO5, GaSbO4, and KSbO3, among others. We
share our data set via the MPContribs platform, and we recommend that future
screenings for optical properties use metrics representative of absorption
features rather than band gap alone
Discovery of the Zintl-phosphide BaCdP as a long carrier lifetime and stable solar absorber
Thin-film photovoltaics offers a path to significantly decarbonize our energy
production. Unfortunately, current materials commercialized or under
development as thin-film solar cell absorbers are far from optimal as they show
either low power conversion efficiency or issues with earth-abundance and
stability. Entirely new and disruptive materials platforms are rarely
discovered as the search for new solar absorbers is traditionally slow and
serendipitous. Here, we use first principles high-throughput screening to
accelerate this process. We identify new solar absorbers among known inorganic
compounds using considerations on band gap, carrier transport, optical
absorption but also on intrinsic defects which can strongly limit the carrier
lifetime and ultimately the solar cell efficiency. Screening about 40,000
materials, we discover the Zintl-phosphide BaCdP as a potential
high-efficiency solar absorber. Follow-up experimental work confirms the
predicted promises of BaCdP highlighting an optimal band gap for
visible absorption, bright photoluminescence, and long carrier lifetime of up
to 30 ns even for unoptimized powder samples. Importantly, BaCdP
does not contain any critical elements and is highly stable in air and water.
Our work opens an avenue for a new family of stable, earth-abundant,
high-performance Zintl-based solar absorbers. It also demonstrates how recent
advances in first principles computation can accelerate the search of
photovoltaic materials by combining high-throughput screening with experiment
Crystal Structures, Vibrational Spectra, and Fungicidal Activity of 1,5-Diaryl-3-oxypyrazoles
The aryloxypyrazole structure is present in a number of bioactive molecules. Four 1,5-diaryl-3-oxypyrazoles containing benzoyl (I), thiazolidinethione (II and III) or per-O-acetylated glucopyranosyl (IV) moieties were characterized by single-crystal X-ray diffraction. Compounds I and II crystallize in a triclinic P-1 system, whereas III and IV crystallize in an orthorhombic Pbca and a monoclinic P21 space groups, respectively. The dihedral angles between the two benzene rings of the pyrazole are 61.33° (I), 62.87° (II), 57.09° (III) and 70.25° (IV). The structures were stabilized by classical intra- (C-H···S for II and III, C-H···O for IV) and intermolecular (C-H···O for I and IV) H-bonds, as well as intermolecular C-H···π stacking interactions. The theoretical FTIR results showed good agreement with the experimental data. Compounds IV, II and III showed moderate fungicidal activity against Sclerotinia sclerotiorum and Gibberella zeae. The structure-activity relationships were discussed
Crystal Structures, Vibrational Spectra, and Fungicidal Activity of 1,5-Diaryl-3-oxypyrazoles
The aryloxypyrazole structure is present in a number of bioactive molecules. Four 1,5-diaryl-3-oxypyrazoles containing benzoyl (I), thiazolidinethione (II and III) or per-O-acetylated glucopyranosyl (IV) moieties were characterized by single-crystal X-ray diffraction. Compounds I and II crystallize in a triclinic P-1 system, whereas III and IV crystallize in an orthorhombic Pbca and a monoclinic P21 space groups, respectively. The dihedral angles between the two benzene rings of the pyrazole are 61.33° (I), 62.87° (II), 57.09° (III) and 70.25° (IV). The structures were stabilized by classical intra- (C-H···S for II and III, C-H···O for IV) and intermolecular (C-H···O for I and IV) H-bonds, as well as intermolecular C-H···π stacking interactions. The theoretical FTIR results showed good agreement with the experimental data. Compounds IV, II and III showed moderate fungicidal activity against Sclerotinia sclerotiorum and Gibberella zeae. The structure-activity relationships were discussed
Midgap state requirements for optically active quantum defects
Optically active quantum defects play an important role in quantum sensing, computing and communication. The electronic structure and the single-particle energy levels of these quantum defects in the semiconducting host have been used to understand their optoelectronic properties. Optical excitations that are central for their initialization and readout are linked to transitions between occupied and unoccupied single-particle states. It is commonly assumed that only quantum defects introducing levels well within the band gap and far from the band edges are of interest for quantum technologies as they mimic an isolated atom embedded in the host. In this perspective, we contradict this common assumption and show that optically active defects with energy levels close to the band edges can display similar properties. We highlight quantum defects that are excited through transitions to or from a band-like level (bound exciton) such as the T center and Se in silicon. We also present how defects such as the silicon split-vacancy in diamond can involve transitions between localized levels that are above the conduction band or below the valence band. Loosening the commonly assumed requirement on the electronic structure of quantum defects offers opportunities in quantum defects design and discovery especially in smaller band gap hosts such as silicon. We discuss the challenges in terms of operating temperature for photoluminescence or radiative lifetime in this regime. We also highlight how these alternative type of defects bring their own needs in terms of theoretical developments and fundamental understanding. This perspective clarifies the electronic structure requirement for quantum defects and will facilitate the identification and design of new color centers for quantum applications especially driven by first principles computations
Extensive Benchmarking of DFT+U Calculations for Predicting Band Gaps
Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band edge states is a computationally tractable approach to improve the accuracy of band gap predictions beyond that of DFT calculations based on (semi)local functionals. At variance with DFT approximations, which are not intended to describe optical band gaps and other excited-state properties, DFT+U can be interpreted as an approximate spectral-potential method when U is determined by imposing the piecewise linearity of the total energy with respect to electronic occupations in the Hubbard manifold (thus removing self-interaction errors in this subspace), thereby providing a (heuristic) justification for using DFT+U to predict band gaps. However, it is still frequent in the literature to determine the Hubbard U parameters semiempirically by tuning their values to reproduce experimental band gaps, which ultimately alters the description of other total-energy characteristics. Here, we present an extensive assessment of DFT+U band gaps computed using self-consistent ab initio U parameters obtained from density-functional perturbation theory to impose the aforementioned piecewise linearity of the total energy. The study is carried out on 20 compounds containing transition-metal or p-block (group III-IV) elements, including oxides, nitrides, sulfides, oxynitrides, and oxysulfides. By comparing DFT+U results obtained using nonorthogonalized and orthogonalized atomic orbitals as Hubbard projectors, we find that the predicted band gaps are extremely sensitive to the type of projector functions and that the orthogonalized projectors give the most accurate band gaps, in satisfactory agreement with experimental data. This work demonstrates that DFT+U may serve as a useful method for high-throughput workflows that require reliable band gap predictions at moderate computational cost