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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Low‐Temperature Synthesis of Stable CaZn 2 P 2 Zintl Phosphide Thin Films as Candidate Top Absorbers

Abstract The development of tandem photovoltaics and photoelectrochemical solar cells requires new absorber materials with bandgaps in the range of ≈1.5–2.3 eV, for use in the top cell paired with a narrower‐gap bottom cell. An outstanding challenge is finding materials with suitable optoelectronic and defect properties, good operational stability, and synthesis conditions that preserve underlying device layers. This study demonstrates the Zintl phosphide compound CaZn 2 P 2 as a compelling candidate semiconductor for these applications. Phase‐pure, ≈500 nm‐thick CaZn 2 P 2 thin films are prepared using a scalable reactive sputter deposition process at growth temperatures as low as 100 °C, which is desirable for device integration. Ultraviolet‐visible spectroscopy shows that CaZn 2 P 2 films exhibit an optical absorptivity of ≈10 4 cm −1 at ≈1.95 eV direct bandgap. Room‐temperature photoluminescence (PL) measurements show near‐band‐edge optical emission, and time‐resolved microwave conductivity (TRMC) measurements indicate a photoexcited carrier lifetime of ≈30 ns. CaZn 2 P 2 is highly stable in both ambient conditions and moisture, as evidenced by PL and TRMC measurements. Experimental data are supported by first‐principles calculations, which indicate the absence of low‐formation‐energy, deep intrinsic defects. Overall, this study shall motivate future work integrating this potential top cell absorber material into tandem solar cells.

14 SOLAR ENERGY↗
DOE OSTI2020

Materials Data on CaZn by Materials Project

CaZn crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. Ca is bonded in a 7-coordinate geometry to seven equivalent Zn atoms. There are a spread of Ca–Zn bond distances ranging from 3.20–3.31 Å. Zn is bonded in a 9-coordinate geometry to seven equivalent Ca and two equivalent Zn atoms. Both Zn–Zn bond lengths are 2.63 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaZn by Materials Project

CaZn is Tetraauricupride structured and crystallizes in the orthorhombic Pmma space group. The structure is three-dimensional. Ca is bonded in a body-centered cubic geometry to eight equivalent Zn atoms. There are a spread of Ca–Zn bond distances ranging from 3.17–3.23 Å. Zn is bonded in a body-centered cubic geometry to eight equivalent Ca atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaZn by Materials Project

CaZn is Tungsten Carbide structured and crystallizes in the hexagonal P-6m2 space group. The structure is three-dimensional. Ca is bonded in a 6-coordinate geometry to six equivalent Zn atoms. All Ca–Zn bond lengths are 3.13 Å. Zn is bonded to six equivalent Ca atoms to form a mixture of distorted edge, face, and corner-sharing ZnCa6 pentagonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaZn by Materials Project

CaZn is Tungsten Carbide-like structured and crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Ca is bonded to six equivalent Zn atoms to form a mixture of distorted edge and corner-sharing CaZn6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Ca–Zn bond lengths are 3.12 Å. Zn is bonded to six equivalent Ca atoms to form a mixture of distorted edge and corner-sharing ZnCa6 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaZn(GeO3)2 by Materials Project

CaZnGe2O6 is Esseneite structured and crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. Ca2+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Ca–O bond distances ranging from 2.37–2.72 Å. Zn2+ is bonded to six O2- atoms to form ZnO6 octahedra that share corners with six equivalent GeO4 tetrahedra and edges with two equivalent ZnO6 octahedra. There are a spread of Zn–O bond distances ranging from 2.10–2.22 Å. Ge4+ is bonded to four O2- atoms to form GeO4 tetrahedra that share corners with three equivalent ZnO6 octahedra and corners with two equivalent GeO4 tetrahedra. The corner-sharing octahedra tilt angles range from 44–60°. There are a spread of Ge–O bond distances ranging from 1.73–1.84 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent Ca2+ and two equivalent Ge4+ atoms. In the second O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to one Ca2+, one Zn2+, and one Ge4+ atom. In the third O2- site, O2- is bonded to one Ca2+, two equivalent Zn2+, and one Ge4+ atom to form a mixture of distorted edge and corner-sharing OCaZn2Ge tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaZn(SiO3)2 by Materials Project

CaZnSi2O6 is Esseneite structured and crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. Ca2+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Ca–O bond distances ranging from 2.34–2.82 Å. Zn2+ is bonded to six O2- atoms to form ZnO6 octahedra that share corners with six equivalent SiO4 tetrahedra and edges with two equivalent ZnO6 octahedra. There are a spread of Zn–O bond distances ranging from 2.09–2.21 Å. Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with three equivalent ZnO6 octahedra and corners with two equivalent SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 36–59°. There are a spread of Si–O bond distances ranging from 1.61–1.70 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent Ca2+ and two equivalent Si4+ atoms. In the second O2- site, O2- is bonded in a distorted T-shaped geometry to one Ca2+, one Zn2+, and one Si4+ atom. In the third O2- site, O2- is bonded in a 4-coordinate geometry to one Ca2+, two equivalent Zn2+, and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Map of the Zintl AM 2 Pn 2 Compounds: Influence of Chemistry on Stability and Electronic Structure

The AM 2 Pn 2 (A= Ca, Sr, Ba, Yb, Mg; M = Zn, Cd, Mg; and Pn = N, P, As, Sb, Bi) family of Zintl phases has been known as thermoelectric materials and has recently gained much attention for highly promising materials for solar absorbers in single-junction and tandem solar cells. In this paper, we will, from first principles, explore the entire family of AM 2 Pn 2 compounds in terms of their ground-state structure, thermodynamic stability, and electronic structure. We also perform photoluminescence spectroscopy on bulk powder and thin film samples to verify our results, including the first measurements of the band gaps of SrCd 2 P 2 and CaCd 2 P 2 . The AM 2 Pn 2 compounds exhibit broad stability, are mostly isostructural to CaAl 2 Si 2 (P$\overline{3}$m1), and cover a wide range of band gaps from 0 to beyond 3 eV. This could make them useful for a variety of purposes, for which we propose several candidates, such as CaZn 2 N 2 for tandem top cell solar absorbers and SrCd 2 Sb 2 and CaZn 2 Sb 2 for infrared detectors. By examining the band structures of the AM 2 Pn 2 , we find that Mg 3 Sb 2 has the most promise as a thermoelectric material due to several off-Γ valence band pockets, which are unique to it among the compositions studied here.

14 SOLAR ENERGY↗
DOE OSTI2025

Phase formation in the CaO–Al 2 O 3 –ZnO system as an analogue to CaO–Al 2 O 3 –MgO in spinel containing refractories

Recently, gahnite (ZnAl 2 O 4 ) is gaining attraction as a potential refractory ceramic because of the similarity of its structure and properties with those of magnesium aluminate (MgAl 2 O 4 ) spinel refractories. Formation of MgO and hibonite solid solution (CaMg x Al 12−x O 19−0.5x ; 0 ≤ x ≤ 0.18), CAM-I (Ca 2 Mg 2−3x Al 28+2x O 46 (0 ≤ x ≤ 0.3), and CAM-II (CaMg 2−3x Al 16+2x O 27 , 0 ≤ x ≤ 0.2) phases with platelet and interlocking microstructure in the CaO–Al 2 O 3 –MgO ternary system significantly enhances the high temperature mechanical properties of refractory castables. The CaO–Al 2 O 3 –ZnO ternary system has been studied, for the first time to our knowledge, in a selected compositional range with reference to the CaO–Al 2 O 3 –MgO system from 1650°C to 1700°C. The formation of ZnO and hibonite solid solution (CaZn x Al 12−x O 19−0.5x ; 0 < x < 0.18), CAZ-I (Ca 2 Zn 2−3x Al 28+2x O 46 ; 0 ≤ x ≤ 0.3), and CAZ-II (CaZn 2−3x Al 16+2x O 27 ; 0 ≤ x ≤ 0.2) phases with platelet and interlocking morphology have been found. The crystal structures and lattice parameters of ZnO and hibonite solid solution, CAZ-I, and CAZ-II are comparable, respectively, with MgO and hibonite solid solution, CAM-I, and CAM-II. Furthermore, CAZ-I and CAZ-II phases also form due to reaction between hibonite (CaO·6Al 2 O 3 ) and ZnAl 2 O 4 .

calcium aluminate cement↗
DOE OSTI2025

Microstructure and bonding between calcium aluminate cement‐containing gahnite–alumina matrix and refractory aggregates

Calcium aluminate cement enhances the thermomechanical properties of refractory castables through the formation of acicular calcium hexaluminate (CaO·6Al 2 O 3 ), Ca 2 Mg 2 Al 28 O 46 (CAM-I), and CaMg 2 Al 16 O 27 (CAM-II) phases in MgO- or MgAl 2 O 4 -containing castables. The compatibility of CA 6 with gahnite (ZnAl 2 O 4 ), and acicular Ca 2 Zn 2 Al 28 O 46 (CAZ-I) and CaZn 2 Al 16 O 27 (CAZ-II) phases formation have been previously reported. Here, in this work, the interaction between a CAC binder containing ZnAl 2 O 4 -Al 2 O 3 matrix with commonly used refractory aggregates such as tabular alumina (TA), alumina-rich (AR90, AR78) and stoichiometric (SM72) MgAl 2 O 4 spinels, and fused and dead-burned magnesia (FM, DBM, respectively) were investigated at 1650°C for 5 h. Microstructural analysis, using digital microscopy, scanning electron microscopy, and energy dispersive spectroscopy, revealed the formation of acicular CaZn 0.18 Al 11.82 O 18.91 , CAZ-I and CAZ-II grains, and strong interfacial bonding between the matrix and TA and spinel aggregates. FM and DBM were found to debond from the matrix. Thick interface layers were observed between the matrix and all the aggregates but TA. Null hypothesis significance testing (NHST) shows that the difference in the number of acicular grains between the interface zone and the bulk matrix (Z) is statistically significant for AR90/Z, SM72/Z, FM/Z, and DBM/Z interfaces, but not significant for TA/Z and AR78/Z. The role of the aggregates’ chemistry on the interfacial bonding and microstructure evolution is discussed.

Ramteke, Rajat Durgesh [Univ. of Alabama, Birmingh↗
DOE OSTI2021

When band convergence is not beneficial for thermoelectrics

Abstract Band convergence is considered a clear benefit to thermoelectric performance because it increases the charge carrier concentration for a given Fermi level, which typically enhances charge conductivity while preserving the Seebeck coefficient. However, this advantage hinges on the assumption that interband scattering of carriers is weak or insignificant. With first-principles treatment of electron-phonon scattering in the CaMg 2 Sb 2 -CaZn 2 Sb 2 Zintl system and full Heusler Sr 2 SbAu, we demonstrate that the benefit of band convergence can be intrinsically negated by interband scattering depending on the manner in which bands converge. In the Zintl alloy, band convergence does not improve weighted mobility or the density-of-states effective mass. We trace the underlying reason to the fact that the bands converge at a one k-point, which induces strong interband scattering of both the deformation-potential and the polar-optical kinds. The case contrasts with band convergence at distant k-points (as in the full Heusler), which better preserves the single-band scattering behavior thereby successfully leading to improved performance. Therefore, we suggest that band convergence as thermoelectric design principle is best suited to cases in which it occurs at distant k-points.

42 ENGINEERING↗