Eric S. Toberer

Computational identification of promising thermoelectric materials among known quasi-2D binary compounds

Quasi low-dimensional structures are abundant among known thermoelectric materials, primarily because of their low lattice thermal conductivities. In this work, we have computationally assessed the potential of 427 known binary quasi-2D structures in 272 different chemistries for thermoelectric performance. To assess the thermoelectric performance, we employ an improved version of our previously developed descriptor for thermoelectric performance [Yan et al., Energy Environ. Sci., 2015, 8, 983]. The improvement is in the explicit treatment of van der Waals interactions in quasi-2D materials, which leads to significantly better predictions of their crystal structures and lattice thermal conductivities. The improved methodology correctly identifies known binary quasi-2D thermoelectric materials such as Sb2Te3, Bi2Te3, SnSe, SnS, InSe, and In2Se3. As a result, we propose candidate quasi-2D binary materials, a number of which have not been previously considered for thermoelectric applications.

Thermoelectricity in transition metal compounds: the role of spin disorder

At room temperature and above, most magnetic materials adopt a spin-disordered (paramagnetic) state whose electronic properties can differ significantly from their low-temperature, spin-ordered counterparts. Yet computational searches for new functional materials usually assume some type of magnetic order. In the present work, we demonstrate a methodology to incorporate spin disorder in computational searches and predict the electronic properties of the paramagnetic phase. We implement this method in a high-throughput framework to assess the potential for thermoelectric performance of 1350 transition-metal sulfides and find that all magnetic systems we identify as promising in the spin-ordered ground state cease to be promising in the paramagnetic phase due to disorder-induced deterioration of the charge carrier transport properties. We also identify promising non-magnetic candidates that do not suffer from these spin disorder effects. In addition to identifying promising materials, our results offer insights into the apparent scarcity of magnetic systems among known thermoelectrics and highlight the importance of including spin disorder in computational searches.

Comparison of Cu 2 SnS 3 and CuSbS 2 as potential solar cell absorbers

Earth-abundant chalcogenide thin-film solar cells, in particular Cu2ZnSnS4 (CZTS), have recently attracted a lot of attention in the field of photovoltaics. Further increases in CZTS performance are challenging, in part because of defects caused by the chemical complexity of this quaternary material. Ternary copper chalcogenides, such as Cu2SnS3 and CuSbS2, are chemically simpler, but their performance is still lower than that of CZTS. Here, we compare the physical properties of the Cu-Sn-S and Cu-Sb-S material families using a high-throughput combinatorial approach, with particular focus on Cu2SnS3 and CuSbS2. We find that both materials have similar competing phases, but they differ significantly in terms of their structures, composition stability ranges, optical absorption, and electrical transport properties. The results of this study lead to the conclusion that CuSbS2, with lower conductivity and higher absorption, may be more promising for the development of Earth-abundant thin-film solar cells despite its layered structure and lower phase stability range.

Effects of low temperature annealing on the transport properties of zinc tin nitride

ZnSnN2 has recently garnered increasing interest as a potential solar absorber material due to its direct bandgap that is predicted to be tunable from 1.0-2.1 eV based on cation disorder. One important challenge to the further development of this material for photovoltaics (PV) is to reliably synthesize films with carrier density ≤1017 electrons cm-3. In this work, we perform a systematic annealing study on compositionally-graded Zn-Sn-N thin films to determine the effects on carrier density and transport of such post-growth treatment. We find that annealing up to 6 hr under an activated nitrogen atmosphere results in a reduction in carrier density by ~80% for zinc-rich films, and by ~50% for stoichiometric films. However, we also find that annealing reduces mobility as a function of increasing annealing time. This result suggests that initial film disorder hampers the benefits to film quality that should have been gained through annealing. This finding highlights that carefully managed initial growth conditions will be necessary to obtain PV-quality ZnSnN2 absorber films.