Perovskite solar cells are remarkably efficient; however, they are prone to degradation in water, oxygen and ultraviolet light. Cation engineering in 3D perovskite absorbers has led to reduced degradation. Alternatively, 2D Ruddlesden–Popper layered perovskites exhibit improved stability, but have not delivered efficient solar cells so far. Here, we introduce n-butylammonium cations into a mixed-cation lead mixed-halide FA0.83Cs0.17Pb(IyBr1−y)3 3D perovskite. We observe the formation of 2D perovskite platelets, interspersed between highly orientated 3D perovskite grains, which suppress non-radiative charge recombination. We investigate the relationship between thin-film composition, crystal alignment and device performance. Solar cells with an optimal butylammonium content exhibit average stabilized power conversion efficiency of 17.5 ± 1.3% with a 1.61-eV-bandgap perovskite and 15.8 ± 0.8% with a 1.72-eV-bandgap perovskite. The stability under simulated sunlight is also enhanced. Cells sustain 80% of their ‘post burn-in’ efficiency after 1,000 h in air, and close to 4,000 h when encapsulated. Various strategies are developed to combine high efficiency and stability in perovskite solar cells. Here, Wang et al. mix 2D and 3D mixed-cation and mixed-halide perovskite phases in solar cells with stabilized efficiencies up to 19.5% and improved stability under full illumination and ambient air.

Efficient ambient-air-stable solar cells with 2D–3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites

The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

Transition metal-doped zinc chalcogenides: Spectroscopy and laser demonstration of a new class of gain media

Spintronics increases the functionality of information processing while seeking to overcome some of the limitations of conventional electronics. The spin-injected field effect transistor, a lateral semiconducting channel with two ferromagnetic electrodes, lies at the foundation of spintronics research. We demonstrated a spin-injected field effect transistor in a high-mobility InAs heterostructure with empirically calibrated electrical injection and detection of ballistic spin-polarized electrons. We observed and fit to theory an oscillatory channel conductance as a function of monotonically increasing gate voltage.

https://science.sciencemag.org/content/sci/325/5947/1515.full.pdf

Control of spin precession in a spin-injected field effect transistor.

We review pioneering and recent studies of the conductivity of solid state systems at terahertz frequencies. A variety of theoretical formalisms that describe the terahertz conductivity of bulk, mesoscopic and nanoscale materials are outlined, and their validity and limitations are given. Experimental highlights are discussed from studies of inorganic semiconductors, organic materials (such as graphene, carbon nanotubes and polymers), metallic films and strongly correlated electron systems including superconductors.

A Review of the Terahertz Conductivity of Bulk and Nano-Materials

Two-dimensional (2D) perovskites have attracted considerable interest for their promising applications for solar cells and other optoelectronics, such as light-emitting diodes, spintronics, and photodetectors. Here, we review the recent achievements of 2D perovskites for various optoelectronic applications. First, we discuss the basic structure and optoelectronic properties of 2D perovskites, including band structure, optical properties, and charge transport. We then highlight recent achievements using 2D perovskites in solar cells and beyond solar cells, including progress on various synthesis strategies and their impact on structural and optoelectronic properties. Finally, we discuss current challenges and future opportunities to further develop 2D perovskites for various applications.

Advances in two-dimensional organic–inorganic hybrid perovskites

We report on a specific growth procedure combining low-temperature growth of ZnMgO and postgrowth annealing at intermediate temperatures. Despite the large lattice misfit induced by the sapphire substrate, layer-by-layer growth is accomplished up to the phase-separation limit found at a c-lattice constant of 0.5136nm and Mg mole fraction of 0.40. The procedure allows us to grow quantum wells with atomically smooth interfaces in a wide range of structural designs exhibiting prominent emission features up to room temperature.

Growth of high-quality ZnMgO epilayers and ZnO/ZnMgO quantum well structures by radical-source molecular-beam epitaxy on sapphire

We discuss the growth by molecular-beam epitaxy, and studies of the low-temperature electrical properties, of undoped InAs/AlSb quantum wells. The two-dimensional electron gas realized in the wells exhibits high mobility at low temperatures, and an analysis of its Shubnikov–de Haas oscillations suggests this mobility is limited by scattering from remotely located unintentional dopants. Spin splitting of the oscillations is clearly resolved at 4.2 K, revealing a g-factor as large as −60 at high magnetic fields. The size of this enhancement increases with decreasing electron density, and is thought to reflect the associated increase in the strength of the effective Coulomb interaction.

Large g-Factor Enhancement in High-Mobility InAs/AlSb Quantum Wells

Measurements of the transition energies of GaAsSb quantum well samples with different barrier configurations reveal that the conduction band offset of the coherently strained GaAs{sub 1-y}Sb{sub y}/GaAs heterojunction grown on GaAs has a zero crossing at a Sb mole fraction of y=0.43{+-}0.07. A type-I band alignment is formed for lower Sb mole fractions and a type-II band alignment is formed for higher Sb mole fractions. This occurs as a consequence of a considerable amount (58%) of the -1.58 eV bandgap bowing being distributed to the conduction band. As a suitable active material for 1.3 {mu}m emission, pseudomorphic GaAs{sub 0.643}Sb{sub 0.357} grown on GaAs is determined to have a weak, 23{+-}23 meV, type-I conduction band offset and a bandgap energy of 928{+-}4 meV.

Band edge alignment of pseudomorphic GaAs1-ySby on GaAs

GaAsSb/GaAs band alignment evaluation for long-wave photonic applications

The evolution of the Shubnikov-de Haas oscillations in InAs/AlSb heterostructures with twodimensional electron gas in InAs quantum wells 12–18 nm wide with considerable variation in the electron concentration (3–8) × 1011 cm−2 due to the effect of negative persistent photoconductivity is studied. The values of the effective Lande factor for electrons g* = −(15–35) are determined. It is shown that the value of the g* factor increases as the quantum well width increases.

Yu. G. Sadofyev

Papers

Growth of high-quality ZnMgO epilayers and ZnO/ZnMgO quantum well structures by radical-source molecular-beam epitaxy on sapphire

Large g-Factor Enhancement in High-Mobility InAs/AlSb Quantum Wells

Band edge alignment of pseudomorphic GaAs1-ySby on GaAs

GaAsSb/GaAs band alignment evaluation for long-wave photonic applications

Exchange enhancement of the g factor in InAs/AlSb heterostructures