Chiral Twist Interface Modulation Enhances Thermoelectric Properties of Tellurium Crystal.

Manipulating the grain boundary and chiral structure of enantiomorphic inorganic thermoelectric materials facilitates a new degree of freedom for enhancing thermoelectric energy conversion. Chiral twist mechanisms evolve by the screw dislocation phenomenon in the nanostructures; however, contributions of such chiral transport have been neglected for bulk crystals. Tellurium (Te) has a chiral trigonal crystal structure, high band degeneracy, and lattice anharmonicity for high thermoelectric performance.

Morphology and Performance Enhancement through the Strong Passivation Effect of Amphoteric Ions in Tin-based Perovskite Solar Cells.

Despite the optoelectronic similarities between tin and lead halide perovskites, the performance of tin-based perovskite solar cells remains far behind, with the highest reported efficiency to date being ≈14%. This is highly correlated to the instability of tin halide perovskite, as well as the rapid crystallization behavior in perovskite film formation. In this work, l-Asparagine as a zwitterion plays a dual role in controlling the nucleation/crystallization process and improving the morphology of perovskite film.

Fabrication of Colloidal Cesium Metal Halide (CsMX: M = Fe, Co, and Ni) Nanoparticles and Assessment of Their Thermodynamic Stability by DFT Calculations.

We synthesized colloidal cesium metal halide CsMX (M = Fe, Co, Ni; X = Cl, Br) nanoparticles (NPs) and assessed their crystal stability by density functional theory (DFT) calculations. We successfully synthesized CsFeCl, CsFeBr, CsCoCl, CsCoBr, CsNiCl, and CsNiBr NPs. CsMX NPs with Fe and Co exhibited CsMX and CsMX structures depending on the reaction conditions; however, CsNiX NPs exhibited only the CsNiX structure.

High-throughput screening of perovskite inspired bismuth halide materials: toward lead-free photovoltaic cells and light-emitting diodes.

Toxicity is the main bottleneck for the commercialization of Pb halide perovskites. Bi has been considered a promising metal cation to replace Pb because of its comparable electronic structures with Pb and better stability. Although experimental and theoretical studies have proposed various Bi-based halides, the present achievements in photovoltaic cells and other photoelectronic device fields do not compete with Pb analogs.

Enhanced Light Emission through Symmetry Engineering of Halide Perovskites.

Metal-halide perovskites (MHPs) have attracted tremendous attention as active materials in optoelectronic devices. For light-emitting diode (LED) applications, nanostructuring of MHPs is considered to be inevitable, but its light-enhancement mechanism is still elusive because the particle (or grain) size is often beyond the quantum confinement regime. As motivated by the experimental finding that the nanostructuring can change the preferred crystalline symmetry from the orthorhombic phase to the high-symmetric cubic phase, we here investigated the carrier dynamics in various polymorphic phases of CsPbBr using quantum dynamics simulation.

Simultaneous Enhanced Efficiency and Stability of Perovskite Solar Cells Using Adhesive Fluorinated Polymer Interfacial Material.

For enhancing the performance and long-term stability of perovskite solar cell (PSC) devices, interfacial engineering between the perovskite and hole-transporting material (HTM) is important. We developed a fluorinated conjugated polymer PFPT3 and used it as an interfacial layer between the perovskite and HTM layers in normal-type PSCs. Interaction of perovskite and PFPT3 via Pb-F bonding effectively induces an interfacial dipole moment, which resulted in energy-level bending; this was favorable for charge transfer and hole extraction at the interface.

Phase Selection of Cesium Lead Triiodides through Surface Ligand Engineering.

The cesium lead triiodide (CsPbI) perovskite is a promising candidate for stable light absorbers and red-light-emitting sources due to its outstanding stability. Phase engineering is the most important approach for the commercialization of CsPbI because the optically inactive nonperovskite structure is more stable than three-dimensional (3-D) perovskite lattices at ambient temperature. This study presents an in-depth evaluation to find the optimum surface ligand and to reveal the mechanism of phase stabilization by surface ligands.

Dual-site mixed layer-structured FA Cs SbICl Pb-free metal halide perovskite solar cells.

Structure engineering of trivalent metal halide perovskites (MHPs) such as ASbX (A = a monovalent cation such as methyl ammonium (MA), cesium (Cs), and formamidinium (FA) and X = a halogen such as I, Br, and Cl) is of great interest because a two dimensional (2D) layer structure with direct bandgap has narrower bandgap energy than a zero dimensional (0D) dimer structure with indirect bandgap. Here, we demonstrated 2D layer structured FACsSbICl MHP by dual-site (A and X site) mixing. Thanks to the lattice-symmetry change by I-Cl mixed halide, the shortest ionic radius of Cs, and the lower solution energy due to dual-site mixing, the FACsSbICl MHP had 2D layer structure and thereby the MHP solar cells exhibited improved short-circuit current density.

Data on lateral collection length of charge carriers depending on pre-white-light soaking process for metal mesh transparent electrode based Cu(In,Ga)Se solar cells.

The authors have recently reported silver nanowire based Cu(In,Ga)Se solar cells [1,2]. Metal mesh based transparent electrodes other than the silver nanowire can be also employed or have a potential to provide a better performance for the solar cells. To select a suitable electrode for a solar cell among metal meshes, it is required to have data on the lateral collection length of charge carriers in the targeted cell.

Role of Quantum Confinement in 10 nm Scale Perovskite Optoelectronics.

Quantum confinement-driven band structure engineering of metal halide perovskites (MHPs) is examined for realistically sized structures that consist of up to 10 atoms. The structural and compositional effects on band gap energies are simulated for crystalline CHNHPbX (X = I/Br/Cl) with a tight-binding approach that has been well-established for electronic structure calculations of multimillion atomic systems. Solid maps of band gap energies achievable with quantum dots, nanowires, and nanoplatelets concerning sizes, shapes, and halide compositions are presented, which should be informative to experimentalists for band gap designs.

Roles of SnX (X = F, Cl, Br) Additives in Tin-Based Halide Perovskites toward Highly Efficient and Stable Lead-Free Perovskite Solar Cells.

Preserving the stability of Sn-based halide perovskites is a primary concern in developing photovoltaic light-absorbing materials for lead-free perovskite solar cells. Whereas the addition of SnX (X = F, Cl, Br) has been demonstrated to improve the photovoltaic performance of Sn halide perovskite solar cells, the mechanistic roles of SnX in the performance enhancement have not yet been studied appropriately. Here we perform a comparative study of CsSnI films and devices and examine how SnX additives affect their stability, and the results are corroborated by first-principles-based theoretical calculations.

Mixed Valence Perovskite Cs Au I : A Potential Material for Thin-Film Pb-Free Photovoltaic Cells with Ultrahigh Efficiency.

New light is shed on the previously known perovskite material, Cs Au I , as a potential active material for high-efficiency thin-film Pb-free photovoltaic cells. First-principles calculations demonstrate that Cs Au I has an optimal band gap that is close to the Shockley-Queisser value. The band gap size is governed by intermediate band formation.

Understanding of the formation of shallow level defects from the intrinsic defects of lead tri-halide perovskites.

Organic-inorganic hybrid perovskites have unique electronic properties in which deep level defects are rarely formed. This unique defect characteristic is the source of the long carrier diffusion length. This theoretical study shows what causes this characteristic formation of shallow level defects in lead tri-halide perovskites.

Thermoelectric materials by using two-dimensional materials with negative correlation between electrical and thermal conductivity.

In general, in thermoelectric materials the electrical conductivity σ and thermal conductivity κ are related and thus cannot be controlled independently. Previously, to maximize the thermoelectric figure of merit in state-of-the-art materials, differences in relative scaling between σ and κ as dimensions are reduced to approach the nanoscale were utilized. Here we present an approach to thermoelectric materials using tin disulfide, SnS2, nanosheets that demonstrated a negative correlation between σ and κ.

Systematic analysis of the unique band gap modulation of mixed halide perovskites.

Solar cells based on organic-inorganic hybrid metal halide perovskites have been proven to be one of the most promising candidates for the next generation thin film photovoltaic cells. Mixing Br or Cl into I-based perovskites has been frequently tried to enhance the cell efficiency and stability. One of the advantages of mixed halides is the modulation of band gap by controlling the composition of the incorporated halides.

Atomistic study on dopant-distributions in realistically sized, highly P-doped Si nanowires.

The dependency of dopant-distributions on channel diameters in realistically sized, highly phosphorus-doped silicon nanowires is investigated with an atomistic tight-binding approach coupled to self-consistent Schrödinger-Poisson simulations. By overcoming the limit in channel sizes and doping densities of previous studies, this work examines electronic structures and electrostatics of free-standing circular silicon nanowires that are phosphorus-doped with a high density of ∼ 2 × 10(19) cm(-3) and have 12 nm-28 nm cross-sections. Results of analysis on the channel energy indicate that the uniformly distributed dopant profile would be hardly obtained when the nanowire cross-section is smaller than 20 nm.

Retarded dopant diffusion by moderated dopant-dopant interactions in Si nanowires.

The retarded dopant diffusion in Si nanostructures is investigated using the first principles calculation. It is presented that weak dopant-dopant interaction energy (DDIE) in nanostructures is responsible for the suppressed dopant diffusion in comparison with that in bulk Si. The DDIE is significantly reduced as the diameter of the Si nanowire becomes smaller.

The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite.

One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.

Biodegradability engineering of biodegradable Mg alloys: tailoring the electrochemical properties and microstructure of constituent phases.

Crystalline Mg-based alloys with a distinct reduction in hydrogen evolution were prepared through both electrochemical and microstructural engineering of the constituent phases. The addition of Zn to Mg-Ca alloy modified the corrosion potentials of two constituent phases (Mg + Mg2Ca), which prevented the formation of a galvanic circuit and achieved a comparable corrosion rate to high purity Mg. Furthermore, effective grain refinement induced by the extrusion allowed the achievement of much lower corrosion rate than high purity Mg.

A Pathway to Type-I Band Alignment in Ge/Si Core-Shell Nanowires.

We investigate the electronic band structures of Ge/Si core-shell nanowires (CSNWs) and devise a way to realize the electron quantum well at Ge core atoms with first-principles calculations. We reveal that the electronic band engineering by the quantum confinement and the lattice strain can induce the type-I/II band alignment transition, and the resulting type-I band alignment generates the electron quantum well in Ge/Si CSNWs. We also find that the type-I/II transition in Ge/Si CSNWs is highly related to the direct to indirect band gap transition through the analysis of charge density and band structures.

Surface ferromagnetic p-type ZnO nanowires through charge transfer doping.

We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping.

Asymmetric doping in silicon nanostructures: the impact of surface dangling bonds.

We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band.

Strain-driven electronic band structure modulation of si nanowires.

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain.