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Dynamical stabilization in delafossite nitrides for solar energy conversion

Szymanski, N. J. and Walters, L. N. and Hellman, O. and Gall, D. and Khare, S. V. (2018) Dynamical stabilization in delafossite nitrides for solar energy conversion. Journal of Materials Chemistry A, 6 (42). pp. 20852-20860. ISSN 2050-7488. doi:10.1039/c8ta07536k.

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Delafossite structured ternary nitrides, ABN_2, have been of recent experimental investigation for applications such as tandem solar and photoelectrochemical cells. We present a thorough first principles computational investigation of their stability, electronic structure, and optical properties. Nine compounds, where A = Cu, Ag, Au and B = V, Nb, Ta, were studied. For three of these compounds, CuTaN_2, CuNbN_2, and AgTaN_2, our computations agree well with experimental results. Optimized lattice parameters, formation energies, and mechanical properties have been computed using the generalized gradient approximation (GGA). Phonon density of states computed at zero-temperature shows that all compounds are dynamically unstable at low temperatures. Including finite-temperature anharmonic effects stabilizes all compounds at 300 K, with the exception of AgVN_2. Analysis of Crystal Orbital Hamiltonian Populations (COHP) provides insight into the bonding and antibonding characters of A–N and B–N pairs. Instability at low temperatures can be attributed to strong A–N antibonding character near the Fermi energy. B–N bonding is found to be crucial in maintaining stability of the structure. AgVN_2 is the only compound to display significant B–N antibonding below the Fermi energy, as well as the strongest degree of A–N antibonding, both of which provide explanation for the sustained instability of this compound up to 900 K. Hybrid functional calculations of electronic and optical properties show that real static dielectric constants in the semiconductors are related to corresponding band gaps through the Moss relation. CuTaN_2, CuNbN_2, AgTaN_2, AgNbN_2, AgVN_2, AuTaN_2, and AuNbN_2 exhibit indirect electronic band gaps while CuVN_2 and AuVN_2 are metallic. Imaginary parts of the dielectric function are characterized by d–d interband transitions in the semiconductors and d–d intraband transitions in the metals. Four compounds, CuTaN_2, CuNbN_2, AgTaN_2, and AgNbN_2, are predicted to exhibit large light absorption in the range of 1.0 to 1.7 eV, therefore making these materials good candidates for solar-energy conversion applications. Two compounds, AuTaN_2 and AuNbN_2, have band gaps and absorption onsets near the ideal range for obtaining high solar-cell conversion efficiency, suggesting that these compounds could become potential candidates as absorber materials in tandem solar cells or for band-gap engineering by alloying.

Item Type:Article
Related URLs:
URLURL TypeDescription Information
Szymanski, N. J.0000-0003-2255-9676
Walters, L. N.0000-0002-9710-9146
Hellman, O.0000-0002-3453-2975
Gall, D.0000-0002-5762-9307
Additional Information:© 2018 The Royal Society of Chemistry. The article was received on 03 Aug 2018, accepted on 28 Sep 2018 and first published on 16 Oct 2018. There are no conflicts of interest. The computing for this project was performed at the Ohio Supercomputer Center (OSC). We thank the National Science Foundation for funding this work through grants 1629230 and 1629239 under the CMMI and 1433467 under the EFRI-2DARE program. The authors also acknowledge funding support from the Air Force Research Laboratory, Space Vehicles Directorate, under Contract # FA9453-11-C-0253.
Funding AgencyGrant Number
Air Force Research LaboratoryFA9453-11-C-0253
Issue or Number:42
Record Number:CaltechAUTHORS:20181029-113116767
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90468
Deposited By: Tony Diaz
Deposited On:30 Oct 2018 01:49
Last Modified:16 Nov 2021 03:32

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