Charge-Carrier Mobility and Localization in Semiconducting Cu2AgBiI6 for Photovoltaic Applications

Lead-free silver–bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu2AgBiI6 using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu2AgBiI6 are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm2 V–1 s–1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ∼29 meV and a direct band gap near 2.1 eV, these findings highlight Cu2AgBiI6 as an attractive lead-free material for photovoltaic applications

Understanding the degradation of methylenediammonium and its role in phase-stabilizing formamidinium lead triiodide

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal–halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl–. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Direct observation of phase transitions between delta- and alpha-phase FAPbI<sub>3</sub><i>via</i> defocused Raman spectroscopy

Defocused Raman spectroscopy is used to assess the phase stability of semiconducting perovskites, preventing laser-induced damage and simultaneously improving the signal-to-noise ratio

AgBiI4 as a Lead-Free Solar Absorber with Potential Application in Photovoltaics

AgBiI₄ powder, crystals, and polycrystalline films were synthesized by sealed tube solid state reactions, chemical vapor transport (CVT), and solution processing, respectively, and their structural, optical and electronic properties are reported. The structure of AgBiI₄ is based unambiguously upon a cubic close packed iodide sublattice, but it presents an unusual crystallographic problem: we show that the reported structure, a cubic defect-spinel, cannot be distinguished from a metrically cubic layered structure analogous to CdCl₂ using either powder or single crystal X-ray crystallography. In addition, we demonstrate the existence a noncubic CdCl₂-type polymorph by isolation of nontwinned single crystals. The indirect optical band gap of AgBiI₄ is measured to be 1.63(1) eV, comparable to the indirect band gap of 1.69(1) eV measured for BiI₃ and smaller than that reported for other bismuth halides, suggesting that structures with a close-packed iodide sublattice may give narrower band gaps than those with perovskite structures. Band edge states closely resemble those of BiI₃; however, the p-type nature of AgBiI₄ with low carrier concentration is more similar to MAPbI₃ than the n-type BiI₃. AgBiI₄ shows good stability toward the AM1.5 solar spectrum when kept in a sealed environment and is thermally stable below 90 °C

Chemical Control of the Dimensionality of the Octahedral Network of Solar Absorbers from the Cul-Agl-Bil(3) Phase Space by Synthesis of 3D CuAgBil(5)

[Image: see text] A newly reported compound, CuAgBiI(5), is synthesized as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag(+)/Bi(3+) network as in spinel, but occupancy of the tetrahedral interstitials by Cu(+) differs from those in spinel. The 3D octahedral network of CuAgBiI(5) allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI–AgI–BiI(3) phase field, giving the ability to chemically control structural dimensionality. To investigate composition–structure–property relationships, we compare the basic optoelectronic properties of CuAgBiI(5) with those of Cu(2)AgBiI(6) (which has a 2D octahedral network) and reveal a surprisingly low sensitivity to the dimensionality of the octahedral network. The absorption onset of CuAgBiI(5) (2.02 eV) barely changes compared with that of Cu(2)AgBiI(6) (2.06 eV) indicating no obvious signs of an increase in charge confinement. Such behavior contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in photoluminescence spectra and lifetimes between the two compounds mostly derive from the difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI(5) powder samples are found to be more sensitive to degradation under solar irradiation compared to Cu(2)AgBiI(6)

Crypt stem cells as the cells-of-origin of intestinal cancer

Intestinal cancer is initiated by Wnt- pathway- activating mutations in genes such as adenomatous polyposis coli ( APC). As in most cancers, the cell of origin has remained elusive. In a previously established Lgr5 ( leucine- rich- repeat containing G- protein-coupled receptor 5) knockin mouse model, a tamoxifen- inducible Cre recombinase is expressed in long- lived intestinal stem cells(1). Here we show that deletion of Apc in these stem cells leads to their transformation within days. Transformed stem cells remain located at crypt bottoms, while fuelling a growing microadenoma. These microadenomas show unimpeded growth and develop into macroscopic adenomas within 3-5 weeks. The distribution of Lgr5(+) cells within stem- cell- derived adenomas indicates that a stem cell/ progenitor cell hierarchy is maintained in early neoplastic lesions. When Apc is deleted in short- lived transit- amplifying cells using a different cre mouse, the growth of the induced microadenomas rapidly stalls. Even after 30 weeks, large adenomas are very rare in these mice. We conclude that stem- cell- specific loss of Apc results in progressively growing neoplasia

Solvent-free method for defect reduction and improved performance of p-i-n vapor-deposited perovskite solar cells

As perovskite-based photovoltaics near commercialization, it is imperative to develop industrial-scale defect-passivation techniques. Vapor deposition is a solvent-free fabrication technique that is widely implemented in industry and can be used to fabricate metal-halide perovskite thin films. We demonstrate markably improved growth and optoelectronic properties for vapor-deposited [CH(NH2)2]0.83Cs0.17PbI3 perovskite solar cells by partially substituting PbI2 for PbCl2 as the inorganic precursor. We find the partial substitution of PbI2 for PbCl2 enhances photoluminescence lifetimes from 5.6 ns to over 100 ns, photoluminescence quantum yields by more than an order of magnitude, and charge-carrier mobility from 46 cm2/(V s) to 56 cm2/(V s). This results in improved solar-cell power conversion efficiency, from 16.4% to 19.3% for the devices employing perovskite films deposited with 20% substitution of PbI2 for PbCl2. Our method presents a scalable, dry, and solvent-free route to reducing nonradiative recombination centers and hence improving the performance of vapor-deposited metal-halide perovskite solar cells

Factors influencing magmatism during continental breakup: New insights from a wide-angle seismic experiment across the conjugate Seychelles-Indian margins

We present a model of the northern Seychelles continental margin derived from controlled source, wide-angle seismic traveltime inversion and teleseismic receiver functions. This margin has been widely cited as a classic example of rifting in association with a continental flood basalt province, the Deccan Traps. However, we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts, such as those of the north Atlantic. The oceanic crust formed immediately after breakup and throughout the first 3 Ma of seafloor spreading is just 5.2 km thick, less than half that typically seen at other volcanic margins. The continent-ocean transition zone is narrow and while two packages of seaward-dipping reflectors are imaged within this transition they are weakly developed. Beneath the thinned continental crust there is an approximately 4 km thick layer of high-velocity material (7.5–7.8 km/s) that we interpret as mafic material intruded and underplating the lower crust. However, we believe that this underplating most likely happened prior to the breakup. Overall the observations show that the rifting of India from the Seychelles was characterized by modest magmatism. The spatial extent of the Deccan flood basalt province is therefore smaller than previously thought. We speculate that either the lateral flow of Deccan-related hot material beneath the breakup region was hampered, perhaps as the rifted margins did not intersect the center of the Deccan source, or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates. If the latter explanation is correct, then the rate of plate separation, as indicated by the initial seafloor-spreading rate, is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized