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Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH₃NH₃PbBr₃, CH(NH₂)₂PbBr₃, and CsPbBr₃. We observed hot fluorescence emission from energetic carriers with ~10²-picosecond lifetimes in CH₃NH₃PbBr₃ or CH(NH₂)₂PbBr₃, but not in CsPbBr₃. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

Impressive performance of hybrid perovskite solar cells reported in recent years still awaits a comprehensive understanding of its microscopic origins. In this work, the intrinsic Hall mobility and photocarrier recombination coefficient are directly measured in these materials in steady-state transport studies. The results show that electron-hole recombination and carrier trapping rates in hybrid perovskites are very low. The bimolecular recombination coefficient (10 −11 to 10 −10  cm 3  s −1 ) is found to be on par with that in the best direct-band inorganic semiconductors, even though the intrinsic Hall mobility in hybrid perovskites is considerably lower (up to 60 cm 2  V −1  s −1 ). Measured here, steady-state carrier lifetimes (of up to 3 ms) and diffusion lengths (as long as 650 μm) are significantly longer than those in high-purity crystalline inorganic semiconductors. We suggest that these experimental findings are consistent with the polaronic nature of charge carriers, resulting from an interaction of charges with methylammonium dipoles. Hybrid perovskites exhibit long carrier diffusion lengths and lifetimes. Here, Chen et al . show experimentally that carrier recombination in perovskites is far from Langevin and closer to the best direct-bandgap semiconductors, which can be explained by the dipolar polaronic nature of charge carriers.

Kiyoshi Miyata and X.-Y. Zhu analyse the ferroelectric-like dielectric response of lead halide perovskites in the terahertz region and discuss the potential role of polar nanodomains in accounting for the defect tolerance and low recombination rates of these materials.

Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells. In organic photovoltaics, the best-performing devices are often based on fullerene derivatives as the electron acceptor counterpart. Here, the authors present new molecular electron acceptors with a helical structure and achieve 8.3% power conversion efficiency.

Lead halide perovskites are emerging as an excellent material platform for optoelectronic processes. There have been extensive discussions on lasing, polariton formation, and nonlinear processes in this material system, but the underlying mechanism remains unknown. Here we probe lasing from CsPbBr 3 perovskite nanowires with picosecond (ps) time resolution and show that lasing originates from stimulated emission of an electron-hole plasma. We observe an anomalous blue-shifting of the lasing gain profile with time up to 25 ps, and assign this as a signature for lasing involving plasmon emission. The time domain view provides an ultra-sensitive probe of many-body physics which was obscured in previous time-integrated measurements of lasing from lead halide perovskite nanowires. Lead-halide perovskite are an interesting material platform for light-emitting devices but the underlying lasing mechanism is still disputed. Here, Schlaus et al . use time-resolved spectroscopy of CsPbBr 3 nanowires to show that lasing results from stimulated emission of an electron-hole plasma.

Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.

The aim of this study was to develop a widely accepted prognostic nomogram for extranodal NK/T-cell lymphoma, nasal-type (NKTCL). The clinical data from 1383 patients with NKTCL treated at 10 participating institutions between 2000 and 2011 were reviewed. A nomogram was developed that predicted overall survival (OS) based on the Cox proportional hazards model. To contrast the utility of the nomogram against the widely used Ann Arbor staging system, the International Prognostic Index (IPI) and the Korean Prognostic Index (KPI), we used the concordance index (C-index) and a calibration curve to determine its predictive and discriminatory capacity. The 5-year OS rate was 60.3% for the entire group. The nomogram included five important variables based on a multivariate analysis of the primary cohort: stage; age; Eastern Cooperative Oncology Group performance status; lactate dehydrogenase; and primary tumor invasion. The calibration curve showed that the nomogram was able to predict 5-year OS accurately. The C-index of the nomogram for OS prediction was 0.72 for both cohorts, which was superior to the predictive power (range, 0.56–0.64) of the Ann Arbor stage, IPI and KPI in the primary and validation cohorts. The proposed nomogram provides an individualized risk estimate of OS in patients with NKTCL.

In typical semiconductor solar cells, photons with energies above the semiconductor bandgap generate hot charge carriers that quickly cool before all of their energy can be captured, a process that limits device efficiency. Although fabricating the semiconductor in a nanocrystalline morphology can slow this cooling, the transfer of hot carriers to electron and hole acceptors has not yet been thoroughly demonstrated. We used time-resolved optical second harmonic generation to observe hot-electron transfer from colloidal lead selenide (PbSe) nanocrystals to a titanium dioxide (TiO₂) electron acceptor. With appropriate chemical treatment of the nanocrystal surface, this transfer occurred much faster than expected. Moreover, the electric field resulting from sub-50-femtosecond charge separation across the PbSe-TiO₂ interface excited coherent vibrations of the TiO₂ surface atoms, whose motions could be followed in real time.

The formability and mechanical properties of many engineering alloys are intimately related to the formation and growth of twins. Understanding the structure and chemistry of twin boundaries at the atomic scale is crucial if we are to properly tailor twins to achieve a new range of desired properties. We report an unusual phenomenon in magnesium alloys that until now was thought unlikely: the equilibrium segregation of solute atoms into patterns within fully coherent terraces of deformation twin boundaries. This ordered segregation provides a pinning effect for twin boundaries, leading to a concomitant but unusual situation in which annealing strengthens rather than weakens these alloys. The findings point to a platform for engineering nano-twinned structures through solute atoms. This may lead to new alloy compositions and thermomechanical processes.

A bstract We compute the production rate of the energy density carried by gravitational waves emitted by a Standard Model plasma in thermal equilibrium, consistently to leading order in coupling constants for momenta k ∼ πT . Summing up the contributions from the full history of the universe, the highest temperature of the radiation epoch can be constrained by the so-called N eff parameter. The current theoretical uncertainty ∆ N eff ≤ 10 − 3 corresponds to T max ≤ 2 × 10 17 GeV. In the course of the computation, we show how a subpart of the production rate can be determined with the help of standard packages, even if subsequently an IR subtraction and thermal resummation need to be implemented.