An integrated computational and experimental study of FeS2 pyrite reveals that phase coexistence is an important factor limiting performance as a thin‐film solar absorber. This phase coexistence is suppressed with the ternary materials Fe2SiS4 and Fe2GeS4, which also exhibit higher band gaps than FeS2. Thus, the ternaries provide a new entry point for development of thin‐film absorbers and high‐efficiency photovoltaics.

A plot of electron affinity (EA) and ionization potential (IP) versus energy band gap (E(G)) for 69 binary closed-shell inorganic semiconductors and insulators reveals that E(G) is centered about the hydrogen donor/acceptor ionization energy ε(+/-). Thus, ε(+/-), or equivalently the standard hydrogen electrode (SHE) energy, functions as an absolute energy reference, determining the tendency of an atom to be either a cation or anion in a compound. This empirical trend establishes the basis for defining a new solid state energy (SSE) scale. This SSE scale makes possible simple approaches for quantitatively assessing electronegativity, chemical hardness, and ionicity, while also providing new insight into the periodic trends of solids.

Atomic solid state energy scale: Universality and periodic Trends in oxidation state, Journal of Solid State Chemistry, http://dx. ABSTRACTThe atomic solid state energy (SSE) scale originates from a plot of the electron affinity (EA) and ionization potential (IP) versus band gap (E G ). SSE is estimated for a given atom by assessing an average EA (for a cation) or an average IP (for an anion) for binary inorganic compounds having that specific atom as a constituent. Physically, SSE is an experimentally-derived average frontier orbital energy referenced to the vacuum level. In its original formulation, 69 binary closed-shell inorganic semiconductors and insulators were employed as a database, providing SSE estimates for 40 elements.In this contribution, EA and IP versus E G are plotted for an additional 92 compounds, thus yielding SSE estimates for a total of 64 elements from the s-, p-, d-, and f-blocks of the periodic table. Additionally, SSE is refined to account for its dependence on oxidation state. Although most cations within the SSE database are found to occur in a single oxidation state, data are available for nine d-block transition metals and one p-block main group metal in more than one oxidation state. SSE is deeper in energy for a higher cation oxidation state. Two p-block main group non-metals within the SSE database are found to exist in both positive and negative oxidation states so that they can function as a cation or anion.SSEs for most cations are positioned above -4.5 eV with respect to the vacuum level, and SSEs for all anions are positioned below. Hence, the energy -4.5 eV, equal to the hydrogen donor/acceptor ionization 2 energy ε(+/-) or equivalently the standard hydrogen electrode energy, is considered to be an absolute energy reference for chemical bonding in the solid state.

used to guide the selection and design of new absorbers using computational techniques. This metric captures the leading physics of absorption relevant to PV efficiency, and it improves upon the simple Shockley-Queisser bandgap model [ 7 ] by considering the full absorption spectrum of the absorber. Hence, SLME should be effective for guiding the selection and design of new ultrathin absorber materials for study and use in drift-aided cells. We have applied the SLME approach to the analysis of all ternary chalcogenides containing Cu, including materials in the system Cu-V-VI. [ 9 ] Several materials were found to exhibit SLME values much higher than those of conventional thin-fi lm absorbers.Subsequently, two general principles were formulated to guide the selection and design of new high-performance absorbers. [ 9 ] In the fi rst case, structural isolation of a Group-V atom in its 5+ oxidation state, e.g., Sb 5+ , within a Cu-rich matrix leads to a narrow Group-V-derived s-orbital band at the conduction band minimum (CBM). In combination with a narrow Cu d band at the valence band maximum (VBM), a high joint density of states and a high absorption coeffi cient ensue. In the second case, if the Group-V atom exists in the lower 3+ oxidation state, e.g., Sb 3+ , it adopts an ns 2 valence electron confi guration. In this 3+ oxidation state, highly asymmetric coordination environments can lead to long distances between the Group-V atoms, resulting in fl at bands and attendant high joint densities of states. These features contribute to an abrupt absorption onset and a high absorption coeffi cient. Strong absorption is reinforced by the parity-allowed nature of the electric-dipole V s or Cu d → V p transition. Hence, the second design principle focuses on structurally isolating the lower oxidation state Group-V element in a semiconducting matrix. Results and DiscussionThese considerations have led us to consider the mineral tetrahedrite, i.e., Cu 12 Sb 4 S 13 , as the basis for a new family of solar absorbers. This material was not revealed as a candidate in the initial SLME search [ 9 ] of Cu-V-VI materials because it was computed to have a zero bandgap, i.e., it is a metal and not a semiconductor. This metallic behavior can readily be appreciated by considering the mixed oxidation-state nature of Cu 12 Sb 4 S 13 . A mixture of formal oxidation states Cu 2+ (d 9 ) and Cu 1+ (d 10 ) is required for charge neutrality, i.e., 10 of the 12 Cu atoms in the chemical formula Cu 12 Sb 4 S 13 are monovalent, while the remaining two Cu atoms are divalent. An incompletely fi lled d-valence band is expected to give rise to the observed degenerate (metallic) behavior, [10][11][12][13] consistent with the Computational inverse design and consequent experimental results allow for the identifi cation of new tetrahedrite-mineral compositions as promising absorber candidates in drift-based thin-fi lm solar cells. In device simulations, cell effi ciencies above 20% are modeled with absorber layers as thin as 250 nm. These new compositio...

High-frequency (optical) and low-frequency (static) dielectric constant versus band gap trends, as well as index of refraction versus band gap trends are plotted for 107 inorganic semiconductors and insulators. These plots are describable via power-law fitting. Dielectric screening trends that emerge from this analysis have important optical and electronic implications. For example, barrier lowering during Schottky emission, phonon-assisted or Fowler-Nordheim tunneling, or Frenkel-Poole emission from a trap is found to be significantly more pronounced with increasing band gap due to a reduction in the optical dielectric constant with increasing band gap. The decrease in the interface state density with increasing band gap is another optical dielectric constant trend. The tendency for a material with a wider band gap to be more difficult to dope is attributed to an increase in the ionization energy of the donor or acceptor dopant, which in turn, depends on the optical dielectric constant and the effective mass. Since the effective mass for holes is almost always larger than that for electrons, p-type doping is more challenging than n-type doping in a wide band gap material. Finally, the polar optical phonon-limited mobility depends critically upon the reciprocal difference of the optical and the static dielectric constant. Consequently, electron and hole mobility tend to decrease with increasing band gap in a polar material.