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The high power conversion efficiencies of small-area perovskite solar cells (PSCs) have driven interest in the development of commercial devices. Rong et al. review recent progress in addressing stability, how to allow mass production, and how to maintain uniformity of large-area films. They note that lifetimes exceeding 10,000 hours under 1 sun (1 kW/m 2 ) illumination have been reported for printable triple mesoscopic PSCs. Science , this issue p. eaat8235 Perovskite solar cells (PSCs) have witnessed rapidly rising power conversion efficiencies, together with advances in stability and upscaling. Despite these advances, their limited stability and need to prove upscaling remain crucial hurdles on the path to commercialization. We summarize recent advances toward commercially viable PSCs and discuss challenges that remain. We expound the development of standardized protocols to distinguish intrinsic and extrinsic degradation factors in perovskites. We review accelerated aging tests in both cells and modules and discuss the prediction of lifetimes on the basis of degradation kinetics. Mature photovoltaic solutions, which have demonstrated excellent long-term stability in field applications, offer the perovskite community valuable insights into clearing the hurdles to commercialization.

Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-film absorber materials in photovoltaics but have also proven to be disruptive in the field of colloidal semiconductor nanocrystals (NCs). The most important feature of LHP NCs is their so-called defect-tolerance—the apparently benign nature of structural defects, highly abundant in these compounds, with respect to optical and electronic properties. Here, we review the important differences that exist in the chemistry and physics of LHP NCs as compared with more conventional, tetrahedrally bonded, elemental, and binary semiconductor NCs (such as silicon, germanium, cadmium selenide, gallium arsenide, and indium phosphide). We survey the prospects of LHP NCs for optoelectronic applications such as in television displays, light-emitting devices, and solar cells, emphasizing the practical hurdles that remain to be overcome.

A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This technique is used to guide a procedure capable of making ensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shell quantum dots. We obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.

Thermalization losses limit the photon-to-power conversion of solar cells at the high-energy side of the solar spectrum, as electrons quickly lose their energy relaxing to the band edge. Hot-electron transfer could reduce these losses. Here, we demonstrate fast and efficient hot-electron transfer between lead selenide and cadmium selenide quantum dots assembled in a quantum-dot heterojunction solid. In this system, the energy structure of the absorber material and of the electron extracting material can be easily tuned via a variation of quantum-dot size, allowing us to tailor the energetics of the transfer process for device applications. The efficiency of the transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale. Efficient use of high-energy, or “hot”, carriers could increase the efficiency of solar cells, provided efficient extraction of electrons at a specific energy. Here, the authors show the presence of hot-electron transfer between two quantum dot species, allowing facile optimization of the extraction energy.

The International Roadmap for Devices and Systems (IRDS) forecasts that, for silicon-based metal–oxide–semiconductor (MOS) field-effect transistors (FETs), the scaling of the gate length will stop at 12 nm and the ultimate supply voltage will not decrease to less than 0.6 V (ref.  1 ). This defines the final integration density and power consumption at the end of the scaling process for silicon-based chips. In recent years, two-dimensional (2D) layered semiconductors with atom-scale thicknesses have been explored as potential channel materials to support further miniaturization and integrated electronics. However, so far, no 2D semiconductor-based FETs have exhibited performances that can surpass state-of-the-art silicon FETs. Here we report a FET with 2D indium selenide (InSe) with high thermal velocity as channel material that operates at 0.5 V and achieves record high transconductance of 6 mS μm −1 and a room-temperature ballistic ratio in the saturation region of 83%, surpassing those of any reported silicon FETs. An yttrium-doping-induced phase-transition method is developed for making ohmic contacts with InSe and the InSe FET is scaled down to 10 nm in channel length. Our InSe FETs can effectively suppress short-channel effects with a low subthreshold swing (SS) of 75 mV per decade and drain-induced barrier lowering (DIBL) of 22 mV V −1 . Furthermore, low contact resistance of 62 Ω μm is reliably extracted in 10-nm ballistic InSe FETs, leading to a smaller intrinsic delay and much lower energy-delay product (EDP) than the predicted silicon limit. A two-dimensional field-effect transistor made of indium selenide is shown to outperform state-of-the-art silicon-based transistors, operating at lower supply voltage and achieving record high transconductance and ballistic ratio.

Companies say they are close to commercializing cheap perovskite films that could disrupt solar power — but are they too optimistic? Companies say they are close to commercializing cheap perovskite films that could disrupt solar power — but are they too optimistic? A technician working on a commercial sized perovskite-on-silicon tandem solar cell

Hot electrons can dramatically improve the efficiency of solar cells and sensitize energetically-demanding photochemical reactions. Efficient hot electron devices have been hindered by sub-picosecond intraband cooling of hot electrons in typical semiconductors via electron-phonon scattering. Semiconductor quantum dots were predicted to exhibit a “phonon bottleneck” for hot electron relaxation as their quantum-confined electrons would couple very inefficiently to phonons. However, typical cadmium selenide dots still exhibit sub-picosecond hot electron cooling, bypassing the phonon bottleneck possibly via an Auger-like process whereby the excessive energy of the hot electron is transferred to the hole. Here we demonstrate this cooling mechanism can be suppressed in copper-doped cadmium selenide colloidal quantum dots due to femtosecond hole capturing by copper-dopants. As a result, we observe a lifetime of ~8.6 picosecond for 1 P e hot electrons which is more than 30-fold longer than that in same-sized, undoped dots (~0.25 picosecond). Weak electron-phonon scattering that can enable long-lived hot electrons in semiconductors is of interest in hot carrier solar cells. Here, the authors report copper-doped colloidal cadmium-selenide quantum dots with hot electron lifetime extended by more than 30-fold compared to undoped dots.

Large ZnSe nanocrystals are expected to be promising blue-light emitters with an emission peak of 455–475 nm, which is important for the construction of display apparatus. The final size of ZnSe nanocrystals via one-step injection can be varied by the reactivity of the Zn and Se precursors; however, it has a limit of <5 nm. To describe the key factors in determining the final size of ZnSe nanocrystals, we proposed a nuclei number-considered LaMer model based on the Maxwell–Boltzmann distribution of crystal embryos. As a result, a general strategy of reactivity-controlled epitaxial growth was developed to synthesize large ZnSe nanocrystals through sequential injection of high-reactivity and low-reactivity Zn and Se precursors. The resultant ZnSe nanocrystals achieved pure blue emission between 455 and 470 nm. We further fabricated stable, large ZnSe/ZnS core–shell nanocrystals with photoluminescence quantum yields up to approximately 60%. Moreover, the reactivity-controlled epitaxial growth strategy is versatile and could be used to synthesize large ZnSe, CdSe and PbSe nanocrystals with average sizes up to 35 nm, 76 nm and 87 nm, respectively. The control of quantum-confined and classical effects in these large semiconductor nanocrystals will open up new directions for fundamental research and application exploration.Synthesizing Se-based nanocrystals with large diameters remains challenging. Here, a reactivity-controlled epitaxial growth strategy was demonstrated to synthesize nanocrystals of ZnSe, CdSe and PbSe with average diameters of 35 nm, 76 nm and 87 nm, respectively. The large ZnSe nanocrystals emitted pure blue light, which is important for display technology.

Solar electricity is an unlimited source of sustainable fuels, yet the efficiency of solar cells is limited. The efficiency of perovskite solar cells improved from 3.9% to reach 25.5% in just a few years. Perovskite solar cells are actually viewed as promising by comparison with dye-sensitized solar cells, organic solar cells, and the traditional solar cells made of silicon, GaAs, copper indium gallium selenide (CIGS), and CdTe. Here, we review bare and doped metal oxide electron transport layers in the perovskite solar cells. Charge transfer layers have been found essential to control the performance of perovskite solar cells by tuning carrier extraction, transportation, and recombination. Both electron and hole transport layers should be used for charge separation and transport. TiO2 and 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene are considered as the best electron and hole transport layers. Metal oxide materials, either bare or doped with different metals, are stable, cheap, and effective.

Two-dimensional monolayer materials, with thicknesses of up to several atoms, can be obtained from almost every layer-structured material. It is believed that the catalogs of known 2D materials are almost complete, with fewer new graphene-like materials being discovered. Here, we report 2D graphene-like monolayers from monoxides such as BeO, MgO, CaO, SrO, BaO, and rock-salt structured monochlorides such as LiCl, and NaCl using first-principle calculations. Two-dimensional materials containing d-orbital atoms such as HfO, CdO, and AgCl are predicted. Adopting the same strategy, 2D graphene-like monolayers from mononitrides such as scandium nitride (ScN) and monoselenides such as cadmium selenide (CdSe) are discovered. Stress engineering is found to help stabilize 2D monolayers, through canceling the imaginary frequency of phonon dispersion relation. These 2D monolayers show high dynamic, thermal, kinetic, and mechanic stabilities due to atomic hybridization, and electronic delocalization.