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the results of a study of the galvanomagnetic properties of indium arsenide grown by liquid-phase epitaxy are presented. It is shown that the use of the rare earth element holmium in the growth of InAs epitaxial layers makes it possible to reduce the electron concentration by two orders of magnitude to n = 2 . 1 × 10 15 cm –3 at T = 77 K. This effect is due to the gettering of shallow background impurities with the formation of their compounds in the melt. With an increase in the holmium content of more than 0.12 mol % the concentration of current carriers in the material begins to increase, while mobility decreases due to the influence of V As –Ho donor centers. This method of gettering is promising for obtaining A 3 B 5 materials with a low concentration of current carriers, which are in demand in the optoelectronic industry.

A set of Ag x Cu 1 –  x GaSe 2 (0 ≤ x ≤ 1) solid solution powders has been prepared by solid-state synthesis. Using a combination of X-ray diffraction analysis and Raman spectroscopy, it has been found that the samples have a single-phase tetragonal structure (space group I -42d). It has been shown that their crystal lattice parameters do not follow Vegard’s law up to x ≈ 0.4. It has been revealed that the band gap of the samples also changes nonlinearly: initially it decreases and then increases. Studies of the low-temperature luminescence and microwave photoconductivity decay spectra have shown that a set of samples with x of 0 to ~0.4 and further in the region with x > 0.4 is characterized by an increase in the photogenerated current carrier lifetime in Ag x Cu 1 –  x GaSe 2 powders. The observed effect is apparently attributed to the replacement of deep charge carrier traps, such as selenium vacancies, by shallower cation vacancies.

The electrical resistivity and the Hall effect of topological insulator Bi2Te3 and Bi2Se3 single crystals were studied in the temperature range from 4.2 to 300 K and in magnetic fields up to 10 T. Theoretical calculations of the electronic structure of these compounds were carried out in density functional approach, taking into account spin–orbit coupling and crystal structure data for temperatures of 5, 50 and 300 K. A clear correlation was found between the density of electronic states at the Fermi level and the current carrier concentration. In the case of Bi2Te3, the density of states at the Fermi level and the current carrier concentration increase with increasing temperature, from 0.296 states eV−1 cell−1 (5 K) to 0.307 states eV−1 cell−1 (300 K) and from 0.9 × 1019 cm−3 (5 K) to 2.6 × 1019 cm−3 (300 K), respectively. On the contrary, in the case of Bi2Se3, the density of states decreases with increasing temperature, from 0.201 states eV−1 cell−1 (5 K) to 0.198 states eV−1 cell−1 (300 K), and, as a consequence, the charge carrier concentration also decreases from 2.94 × 1019 cm−3 (5 K) to 2.81 × 1019 cm−3 (300 K).

Although much effort has been devoted to improving photoelectrochemical water splitting of hematite (α-Fe 2 O 3 ) due to its high theoretical solar-to-hydrogen conversion efficiency of 15.5%, the low applied bias photon-to-current efficiency remains a huge challenge for practical applications. Herein, we introduce single platinum atom sites coordination with oxygen atom (Pt-O/Pt-O-Fe) sites into single crystalline α-Fe 2 O 3 nanoflakes photoanodes (SAs Pt:Fe 2 O 3 -Ov). The single-atom Pt doping of α-Fe 2 O 3 can induce few electron trapping sites, enhance carrier separation capability, and boost charge transfer lifetime in the bulk structure as well as improve charge carrier injection efficiency at the semiconductor/electrolyte interface. Further introduction of surface oxygen vacancies can suppress charge carrier recombination and promote surface reaction kinetics, especially at low potential. Accordingly, the optimum SAs Pt:Fe 2 O 3 -Ov photoanode exhibits the photoelectrochemical performance of 3.65 and 5.30 mA cm −2 at 1.23 and 1.5 V RHE , respectively, with an applied bias photon-to-current efficiency of 0.68% for the hematite-based photoanodes. This study opens an avenue for designing highly efficient atomic-level engineering on single crystalline semiconductors for feasible photoelectrochemical applications. The achievable photocurrent of hematite, α-Fe 2 O 3 , is typically limited far below its theoretical limit. Here, the authors engineer single Pt atomic sites with surface oxygen vacancies into hematite photoanodes, which leads to enhanced photoelectrochemical water splitting.

Surface passivation has been developed as an effective strategy to reduce trap-state density and suppress non-radiation recombination process in perovskite solar cells. However, passivation agents usually own poor conductivity and hold negative impact on the charge carrier transport in device. Here, we report a binary and synergistical post-treatment method by blending 4- tert -butyl-benzylammonium iodide with phenylpropylammonium iodide and spin-coating on perovskite surface to form passivation layer. The binary and synergistical post-treated films show enhanced crystallinity and improved molecular packing as well as better energy band alignment, benefiting for the hole extraction and transfer. Moreover, the surface defects are further passivated compared with unary passivation. Based on the strategy, a record-certified quasi-steady power conversion efficiency of 26.0% perovskite solar cells is achieved. The devices could maintain 81% of initial efficiency after 450 h maximum power point tracking. The poor conductivity of passivators often impacts the charge carrier transport in perovskite solar cells. Here, the authors report a binary and synergistical post-treatment method to form the passivation layer, achieving certified quasi-steady power conversion efficiency of 26% for stable devices.

The Berry phase provides a modern formulation of electric polarization in crystals. We extend this concept to higher electric multipole moments and determine the necessary conditions and minimal models for which the quadrupole and octupole moments are topologically quantized electromagnetic observables. Such systems exhibit gapped boundaries that are themselves lower-dimensional topological phases. Furthermore, they host topologically protected corner states carrying fractional charge, exhibiting fractionalization at the boundary of the boundary. To characterize these insulating phases of matter, we introduce a paradigm in which “nested” Wilson loops give rise to topological invariants that have been overlooked. We propose three realistic experimental implementations of this topological behavior that can be immediately tested. Our work opens a venue for the expansion of the classification of topological phases of matter.

The Shockley-Queisser limit for solar cell efficiency can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot-carrier migration in methylammonium lead iodide (CH₃NH₃PbI₃) thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites.

The chemical structure of donors and acceptors limit the power conversion efficiencies achievable with active layers of binary donor-acceptor mixtures. Here, using quaternary blends, double cascading energy level alignment in bulk heterojunction organic photovoltaic active layers are realized, enabling efficient carrier splitting and transport. Numerous avenues to optimize light absorption, carrier transport, and charge-transfer state energy levels are opened by the chemical constitution of the components. Record-breaking PCEs of 18.07% are achieved where, by electronic structure and morphology optimization, simultaneous improvements of the open-circuit voltage, short-circuit current and fill factor occur. The donor and acceptor chemical structures afford control over electronic structure and charge-transfer state energy levels, enabling manipulation of hole-transfer rates, carrier transport, and non-radiative recombination losses. Efficiency of organic solar cells is determined by the physical properties of donors and acceptors in bulk heterojunction film. The authors optimise quaternary blends to realize a double cascading energy level alignment enabling efficient carrier dissociation and transport, achieving 18% efficiency.

The formation of a dense and uniform thin layer on the substrates is crucial for the fabrication of high-performance perovskite solar cells (PSCs) containing formamidinium with multiple cations and mixed halide anions. The concentration of defect states, which reduce a cell’s performance by decreasing the open-circuit voltage and short-circuit current density, needs to be as low as possible. We show that the introduction of additional iodide ions into the organic cation solution, which are used to form the perovskite layers through an intramolecular exchanging process, decreases the concentration of deep-level defects. The defect-engineered thin perovskite layers enable the fabrication of PSCs with a certified power conversion efficiency of 22.1% in small cells and 19.7% in 1-square-centimeter cells.

Wide–band gap metal halide perovskites are promising semiconductors to pair with silicon in tandem solar cells to pursue the goal of achieving power conversion efficiency (PCE) greater than 30% at low cost. However, wide–band gap perovskite solar cells have been fundamentally limited by photoinduced phase segregation and low open-circuit voltage. We report efficient 1.67–electron volt wide–band gap perovskite top cells using triple-halide alloys (chlorine, bromine, iodine) to tailor the band gap and stabilize the semiconductor under illumination. We show a factor of 2 increase in photocarrier lifetime and charge-carrier mobility that resulted from enhancing the solubility of chlorine by replacing some of the iodine with bromine to shrink the lattice parameter. We observed a suppression of light-induced phase segregation in films even at 100-sun illumination intensity and less than 4% degradation in semitransparent top cells after 1000 hours of maximum power point (MPP) operation at 60°C. By integrating these top cells with silicon bottom cells, we achieved a PCE of 27% in two-terminal monolithic tandems with an area of 1 square centimeter.