With the development of new generations of optoelectronic devices that combine high performance and novel functionalities (e.g., flexibility/bendability, adaptability, semi or full transparency), ...several classes of transparent electrodes have been developed in recent years. These range from optimized transparent conductive oxides (TCOs), which are historically the most commonly used transparent electrodes, to new electrodes made from nano‐ and 2D materials (e.g., metal nanowire networks and graphene), and to hybrid electrodes that integrate TCOs or dielectrics with nanowires, metal grids, or ultrathin metal films. Here, the most relevant transparent electrodes developed to date are introduced, their fundamental properties are described, and their materials are classified according to specific application requirements in high efficiency solar cells and flexible organic light‐emitting diodes (OLEDs). This information serves as a guideline for selecting and developing appropriate transparent electrodes according to intended application requirements and functionality.
The most relevant transparent electrodes developed to date are reviewed and classified according to specific application requirements, with a focus on solar cells and organic light‐emitting diodes. Current challenges and future directions for the development of transparent electrodes with properties that enable high efficiency and functionality of optoelectronic devices are discussed.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of ...solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH
NH
PbI
hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment (r
= 0.93) improves significantly on results obtained using a constant relaxation time approximation (r
= 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power.
The thermophysical properties of lunar regolith have been thoroughly investigated for temperatures higher than 100 K. For the near‐equatorial thermal measurements of the Apollo era, this temperature ...range was sufficient to generate appropriate models. However, recent measurements from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment have revealed polar temperatures as low as 20 K, with apparently lower thermal inertia than explainable by existing theory. In the absence of comprehensive laboratory measurements of regolith thermal properties at low temperatures (<100 K), we investigate solid state theory and fits to lunar simulant materials to derive a semiempirical model of specific heat and thermal conductivity in lunar regolith in the full range 20–400 K. The primary distinctions between these previous models are (1) the temperature dependence of the solid conduction component of thermal conductivity at low temperatures, (2) the focus on regolith bulk density as the primary variable, and (3) the concept that the composition and modal petrology of grains could significantly influence thermal properties of the bulk regolith. This model predicts that at low temperatures, thermal conductivity is as much as an order of magnitude lower and specific heat is likely higher than expected from current models. The thermal conductivity at low temperature should vary depending on the constituent grain materials, their crystallinity, contributions from phonon scattering modes, bulk porosity, and density. To demonstrate the impact of our finding, we extrapolate the effects of our conductivity model on temperature variations in permanently shadowed regions on the Moon. This work motivates experimental confirmation of thermophysical properties of lunar regolith at low temperature.
Key Points
Thermal conductivity or lunar soil drops at low temperature
This could have profound effects on lunar cold trap temperatures
New lab measurements of low‐temperature thermal conductivity should be pursued
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The growth of materials databases has yielded significant quantities of data to mine for new energy materials using high-throughput screening methodologies. One application of interest to energy and ...optoelectronics is the prediction of new high performing p-type transparent conductors (TCs). However, screening methods for such materials have never been validated over the breadth of computed materials properties. In this study, we compile an experimental data set of 74 bulk crystal structures corresponding to known state-of-the-art n-type and p-type TCs and compute a series of corresponding computational descriptor properties. Our goals are to (1) compare computational descriptors to experimentally demonstrated properties of real materials in the data set, (2) determine the ability of ground state, density functional theory (DFT)-based computational screening methodologies to identify these experimentally realized TCs, and (3) use this understanding to estimate reasonable screening cutoffs for four commonly used descriptors. First, stability calculations demonstrate that most materials in the data set have an energy above the convex hull (E hull) of <100 meV/atom, and we also propose a Pourbaix analysis technique to estimate moisture stability. Second, we find a lenient cutoff for the DFT PBE band gap of 0.5 eV is sufficient to include a majority of the wide gap candidates and exclude narrow gap compounds. Next, the effective mass, m*, is found to correlate weakly to conductivity in the p-type materials as compared with n-type materials, which may motivate an increase in the m* cutoff as well. Lastly, we perform an uncertainty analysis and literature comparison for the branch point energy (BPE), a qualitative descriptor for dopability. We find the BPEs of most n-type materials to lie near the conduction band and those of most p-type materials to lie at midgap; this is a significant distinction, suggesting BPE to be a more definitive descriptor for n-type TC materials. By assessing the validity of this simple screening methodology via comparing experimental data to computational descriptors, we aim to motivate and strengthen future materials discovery efforts in the field of transparent conductors and beyond.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Nitride materials feature strong chemical bonding character that leads to unique crystal structures, but many ternary nitride chemical spaces remain experimentally unexplored. The search for ...previously undiscovered ternary nitrides is also an opportunity to explore unique materials properties, such as transitions between cation-ordered and -disordered structures, as well as to identify candidate materials for optoelectronic applications. Here, we present a comprehensive experimental study of MgSnN2, an emerging II–IV–N2 compound, for the first time mapping phase composition and crystal structure, and examining its optoelectronic properties computationally and experimentally. We demonstrate combinatorial cosputtering of cation-disordered, wurtzite-type MgSnN2 across a range of cation compositions and temperatures, as well as the unexpected formation of a secondary, rocksalt-type phase of MgSnN2 at Mg-rich compositions and low temperatures. A computational structure search shows that the rocksalt-type phase is substantially metastable (>70 meV/atom) compared to the wurtzite-type ground state. Spectroscopic ellipsometry reveals optical absorption onsets around 2 eV, consistent with band gap tuning via cation disorder. Finally, we demonstrate epitaxial growth of a mixed wurtzite-rocksalt MgSnN2 on GaN, highlighting an opportunity for polymorphic control via epitaxy. Collectively, these findings lay the groundwork for further exploration of MgSnN2 as a model ternary nitride, with controlled polymorphism, and for device applications, enabled by control of optoelectronic properties via cation ordering.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Inorganic nitrides with wurtzite crystal structures are well-known semiconductors used in optical and electronic devices. In contrast, rocksalt-structured nitrides are known for their superconducting ...and refractory properties. Breaking this dichotomy, herewe report ternary nitride semiconductors with rocksalt crystal structures, remarkable electronic properties, and the general chemical formula MgₓTM
1−xN (TM = Ti, Zr, Hf, Nb). Our experiments show that these materials form over a broad metal composition range, and that Mg-rich compositions are nondegenerate semiconductors with visible-range optical absorption onsets (1.8 to 2.1 eV) and up to 100 cm² V−1·s−1 electron mobility for MgZrN₂ grown on MgO substrates. Complementary ab initio calculations reveal that these materials have disorder-tunable optical absorption, large dielectric constants, and electronic bandgaps that are relatively insensitive to disorder. These ternary MgₓTM
1−xN semiconductors are also structurally compatible both with binary TMN superconductors and main-group nitride semiconductors along certain crystallographic orientations. Overall, these results highlight MgₓTM
1−xN as a class of materials combining the semiconducting properties of main-group wurtzite nitrides and rocksalt structure of superconducting transition-metal nitrides.
Full text
Available for:
BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Achieving stable operation of photoanodes used as components of solar water splitting devices is critical to realizing the promise of this renewable energy technology. It is shown that p-type ...transparent conducting oxides (p-TCOs) can function both as a selective hole contact and corrosion protection layer for photoanodes used in light-driven water oxidation. Using NiCo2O4 as the p-TCO and n-type Si as a prototypical light absorber, a rectifying heterojunction capable of light driven water oxidation was created. By placing the charge separating junction in the Si using a np+ structure and by incorporating a highly active heterogeneous Ni–Fe oxygen evolution catalyst, efficient light-driven water oxidation can be achieved. In this structure, oxygen evolution under AM1.5G illumination occurs at 0.95 V vs RHE, and the current density at the reversible potential for water oxidation (1.23 V vs RHE) is >25 mA cm–2. Stable operation was confirmed by observing a constant current density over 72 h and by sensitive measurements of corrosion products in the electrolyte. In situ Raman spectroscopy was employed to investigate structural transformation of NiCo2O4 during electrochemical oxidation. The interface between the light absorber and p-TCO is crucial to produce selective hole conduction to the surface under illumination. For example, annealing to produce more crystalline NiCo2O4 produces only small changes in its hole conductivity, while a thicker SiO x layer is formed at the n-Si/p-NiCo2O4 interface, greatly reducing the PEC performance. The generality of the p-TCO protection approach is demonstrated by multihour, stable, water oxidation with n-InP/p-NiCo2O4 heterojunction photoanodes.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
All transparent conducting materials (TCMs) of technological practicality are n‐type; the inferior conductivity of p‐type TCMs has limited their adoption. In addition, many relatively high‐performing ...p‐type TCMs require synthesis temperatures >400 °C. Here, room‐temperature pulsed laser deposition of copper‐alloyed zinc sulfide (CuxZn1‐xS) thin films (0 ≤ x ≤ 0.75) is reported. For 0.09 ≤ x ≤ 0.35, CuxZn1‐xS has high p‐type conductivity, up to 42 S cm−1 at x = 0.30, with an optical band gap tunable from ≈3.0–3.3 eV and transparency, averaged over the visible, of 50%–71% for 200–250 nm thick films. In this range, synchrotron X‐ray and electron diffraction reveal a nanocrystalline ZnS structure. Secondary crystalline CuyS phases are not observed, and at higher Cu concentrations, x > 0.45, films are amorphous and poorly conducting. Within the TCM regime, the conductivity is temperature independent, indicating degenerate hole conduction. A decrease in lattice parameter with Cu content suggests that the hole conduction is due to substitutional incorporation of Cu onto Zn sites. This hole‐conducting phase is embedded in a less conducting amorphous CuyS, which dominates at higher Cu concentrations. The combination of high hole conductivity and optical transparency for the peak conductivity CuxZn1‐xS films is among the best reported to date for a room temperature deposited p‐type TCM.
A promising p‐type transparent conducting material, Cu‐alloyed ZnS (CuxZn1‐xS), is synthesized at room temperature using pulsed laser deposition. Cu substitutes for Zn in nanocrystalline ZnS domains, leading to p‐type behavior. Its combined high hole conductivity (up to 42 S cm−1) and high optical band gap (>3 eV) place CuxZn1‐xS among the best performing p‐type TCMs deposited at room temperature.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Ternary metal‐oxide material systems commonly crystallize in the perovskite crystal structure, which is utilized in numerous electronic applications. In contrast to oxides, there are no known nitride ...perovskites, likely due to the competition with oxidation, which makes the formation of pure nitride materials difficult and synthesis of oxynitride materials more common. While deposition of oxynitride perovskite thin films is important for many electronic applications, it is difficult to control oxygen and nitrogen stoichiometry. Lanthanum tungsten oxynitride (LaWN3−δOδ) thin films with varying La:W ratio are synthesized by combinatorial sputtering and characterized for their chemical composition, crystal structure, and microstructure. A three‐step synthesis method, which involves co‐sputtering, capping layer deposition, and rapid thermal annealing, is established for producing crystalline thin films while minimizing the oxygen content. Elemental depth profiling results show that the cation‐stoichiometric films contain approximately one oxygen for every five nitrogen (δ = 0.5). Synchrotron‐based diffraction indicates a tetragonal perovskite crystal structure. These results are discussed in terms of the perovskite tolerance factors, octahedral tilting, and bond valence. Overall, this synthesis and characterization is expected to pave the way toward future thin film property measurements of lanthanum tungsten oxynitrides and eventual synthesis of oxygen‐free nitride perovskites.
Oxynitride perovskites have many useful applications; however, unlocking their potential in certain areas is limited by the need to grow thin films while controlling anion stoichiometry. A method is demonstrated for synthesis of lanthanum tungsten oxynitride thin films. The control of oxygen sources produces films with ≈1:5 oxygen substitution of nitrogen, which crystallize in a tetragonal perovskite structure.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
PDF
