1,094 research outputs found
Degradation Kinetics of Inverted Perovskite Solar Cells
We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA)0.83Cs0.17Pb(I0.8Br0.2)3, CH3NH3PbI3, and CH3NH3PbI3-xClx perovskite solar cells. We continuously test the devices in-situ and in-operando with current-voltage sweeps, transient photocurrent, and transient photovoltage measurements, and find that degradation in the CH3NH3PbI3-xClx solar cells due to oxygen exposure occurs over shorter timescales than FA0.83Cs0.17Pb(I0.8Br0.2)3 mixed-cation devices. We attribute these oxygen-induced losses in the power conversion efficiencies to the formation of electron traps within the perovskite photoactive layer. Our results highlight that the formamidinium-caesium mixed-cation perovskites are much less sensitive to oxygen-induced degradation than the methylammonium-based perovskite cells, and that further improvements in perovskite solar cell stability should focus on the mitigation of trap generation during ageing
Conjugation-Length Dependence of Spin-Dependent Exciton Formation Rates in Pi-Conjugated Oligomers and Polymers
We have measured the ratio, r = of the formation cross
section, of singlet () and triplet () excitons
from oppositely charged polarons in a large variety of -conjugated
oligomer and polymer films, using the photoinduced absorption and optically
detected magnetic resonance spectroscopies. The ratio r is directly related to
the singlet exciton yield, which in turn determines the maximum
electroluminescence quantum efficiency in organic light emitting diodes (OLED).
We discovered that r increases with the conjugation length, CL; in fact a
universal dependence exists in which depends linearly on ,
irrespective of the chain backbone structure. These results indicate that
-conjugated polymers have a clear advantage over small molecules in OLED
applications.Comment: 5 pages, 4 figure
Weak charge form factor and radius of 208Pb through parity violation in electron scattering
We use distorted wave electron scattering calculations to extract the weak
charge form factor F_W(q), the weak charge radius R_W, and the point neutron
radius R_n, of 208Pb from the PREX parity violating asymmetry measurement. The
form factor is the Fourier transform of the weak charge density at the average
momentum transfer q=0.475 fm. We find F_W(q) =0.204 \pm 0.028 (exp) \pm
0.001 (model). We use the Helm model to infer the weak radius from F_W(q). We
find R_W= 5.826 \pm 0.181 (exp) \pm 0.027 (model) fm. Here the exp error
includes PREX statistical and systematic errors, while the model error
describes the uncertainty in R_W from uncertainties in the surface thickness
\sigma of the weak charge density. The weak radius is larger than the charge
radius, implying a "weak charge skin" where the surface region is relatively
enriched in weak charges compared to (electromagnetic) charges. We extract the
point neutron radius R_n=5.751 \pm 0.175 (exp) \pm 0.026 (model) \pm 0.005
(strange) fm$, from R_W. Here there is only a very small error (strange) from
possible strange quark contributions. We find R_n to be slightly smaller than
R_W because of the nucleon's size. Finally, we find a neutron skin thickness of
R_n-R_p=0.302\pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm, where
R_p is the point proton radius.Comment: 5 pages, 1 figure, published in Phys Rev. C. Only one change in this
version: we have added one author, also to metadat
The alteration history of the Jbilet Winselwan CM carbonaceous chondrite: An analog for C‐type asteroid sample return
Jbilet Winselwan is one of the largest CM carbonaceous chondrites available for study. Its light, major, and trace elemental compositions are within the range of other CM chondrites. Chondrules are surrounded by dusty rims and set within a matrix of phyllosilicates, oxides, and sulfides. Calcium‐ and aluminum‐rich inclusions (CAIs) are present at ≤1 vol% and at least one contains melilite. Jbilet Winselwan is a breccia containing diverse lithologies that experienced varying degrees of aqueous alteration. In most lithologies, the chondrules and CAIs are partially altered and the metal abundance is low (<1 vol%), consistent with petrologic subtypes 2.7–2.4 on the Rubin et al. (2007) scale. However, chondrules and CAIs in some lithologies are completely altered suggesting more extensive hydration to petrologic subtypes ≤2.3. Following hydration, some lithologies suffered thermal metamorphism at 400–500 °C. Bulk X‐ray diffraction shows that Jbilet Winselwan consists of a highly disordered and/or very fine‐grained phase (73 vol%), which we infer was originally phyllosilicates prior to dehydration during a thermal metamorphic event(s). Some aliquots of Jbilet Winselwan also show significant depletions in volatile elements such as He and Cd. The heating was probably short‐lived and caused by impacts. Jbilet Winselwan samples a mixture of hydrated and dehydrated materials from a primitive water‐rich asteroid. It may therefore be a good analog for the types of materials that will be encountered by the Hayabusa‐2 and OSIRIS‐REx asteroid sample‐return missions
Electron correlation effects in electron-hole recombination in organic light-emitting diodes
We develop a general theory of electron--hole recombination in organic light
emitting diodes that leads to formation of emissive singlet excitons and
nonemissive triplet excitons. We briefly review other existing theories and
show how our approach is substantively different from these theories. Using an
exact time-dependent approach to the interchain/intermolecular charge-transfer
within a long-range interacting model we find that, (i) the relative yield of
the singlet exciton in polymers is considerably larger than the 25% predicted
from statistical considerations, (ii) the singlet exciton yield increases with
chain length in oligomers, and, (iii) in small molecules containing nitrogen
heteroatoms, the relative yield of the singlet exciton is considerably smaller
and may be even close to 25%. The above results are independent of whether or
not the bond-charge repulsion, X_perp, is included in the interchain part of
the Hamiltonian for the two-chain system. The larger (smaller) yield of the
singlet (triplet) exciton in carbon-based long-chain polymers is a consequence
of both its ionic (covalent) nature and smaller (larger) binding energy. In
nitrogen containing monomers, wavefunctions are closer to the noninteracting
limit, and this decreases (increases) the relative yield of the singlet
(triplet) exciton. Our results are in qualitative agreement with
electroluminescence experiments involving both molecular and polymeric light
emitters. The time-dependent approach developed here for describing
intermolecular charge-transfer processes is completely general and may be
applied to many other such processes.Comment: 19 pages, 11 figure
A Precision Measurement of the Neutron Twist-3 Matrix Element : Probing Color Forces
Double-spin asymmetries and absolute cross sections were measured at large
Bjorken (0.25 0.90), in both the deep-inelastic and resonance
regions, by scattering longitudinally polarized electrons at beam energies of
4.7 and 5.9 GeV from a transversely and longitudinally polarized He target.
In this dedicated experiment, the spin structure function on He was
determined with precision at large , and the neutron twist-three matrix
element was measured at \left of 3.21 and 4.32
GeV/, with an absolute precision of about . Our results are
found to be in agreement with lattice QCD calculations and resolve the
disagreement found with previous data at \left = 5 GeV/.
Combining and a newly extracted twist-four matrix element, , the
average neutron color electric and magnetic forces were extracted and found to
be of opposite sign and about 30 MeV/fm in magnitude.Comment: Corrected a typo in the author list and Figure 1 legend. 6 pages, 2
figures, 2 table
Electroexcitation of the at low momentum transfer
We report on new p measurements at the
resonance at the low momentum transfer region. The mesonic
cloud dynamics is predicted to be dominant and rapidly changing in this
kinematic region offering a test bed for chiral effective field theory
calculations. The new data explore the low dependence of the resonant
quadrupole amplitudes while extending the measurements of the Coulomb
quadrupole amplitude to the lowest momentum transfer ever reached. The results
disagree with predictions of constituent quark models and are in reasonable
agreement with dynamical calculations that include pion cloud effects, chiral
effective field theory and lattice calculations. The reported measurements
suggest that improvement is required to the theoretical calculations and
provide valuable input that will allow their refinements
Measurement of the Neutron Radius of 208Pb Through Parity-Violation in Electron Scattering
We report the first measurement of the parity-violating asymmetry A_PV in the
elastic scattering of polarized electrons from 208Pb. A_PV is sensitive to the
radius of the neutron distribution (Rn). The result A_PV = 0.656 \pm 0.060
(stat) \pm 0.014 (syst) ppm corresponds to a difference between the radii of
the neutron and proton distributions Rn - Rp = 0.33 +0.16 -0.18 fm and provides
the first electroweak observation of the neutron skin which is expected in a
heavy, neutron-rich nucleus.Comment: 6 pages, 1 figur
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