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The neutron star ocean is a plasma of ions and electrons that extends from the base of the neutron star's envelope to a depth where the plasma crystallizes into a solid crust. During an accretion outburst in an X-ray transient, material accumulates in the envelope of the neutron star primary. This accumulation compresses the neutron star's outer layers and induces nuclear reactions in the ocean and crust. Accretion-driven heating raises the ocean's temperature and increases the frequencies of g-modes in the ocean; when accretion halts, the ocean cools and ocean g-mode frequencies decrease. If the observed low-frequency quasi-periodic oscillations on accreting neutron stars are g-modes in the ocean, the observed quasi-periodic oscillation frequencies will increase during outburst---reaching a maximum when the ocean temperature reaches steady state --- and subsequently decrease during quiescence. For time-averaged accretion rates during outbursts between $\langle \dot{M} \rangle = 0.1 \textrm{--} 1.0\, \dot{M}_{\rm Edd}$ the predicted g-mode fundamental $n=1$ $l=2$ frequency is between $\approx 3 \textrm{--} 7 \, \mathrm{Hz}$ for slowly rotating neutron stars. Accreting neutron stars that require extra shallow heating, such as the Z-sources MAXI J0556-332, MXB 1659-29, and XTE J1701-462, have predicted g-mode fundamental frequencies between $\approx 3 \textrm{--} 16 \, \mathrm{Hz}$. Therefore, observations of low-frequency quasi-periodic oscillations between $\approx 8 \textrm{--} 16\, \mathrm{Hz}$ in these sources, or in other transients that require shallow heating, will support a g-mode origin for the observed quasi-periodic oscillations., Comment: Accepted to the Astrophysical Journal

An accretion outburst in an X-ray transient deposits material onto the neutron star primary; this accumulation of matter induces reactions in the neutron star's crust. During the accretion outburst these reactions heat the crust out of thermal equilibrium with the core. When accretion halts, the crust cools to its long-term equilibrium temperature on observable timescales. Here we examine the accreting neutron star transient MAXI J0556-332, which is the hottest transient, at the start of quiescence, observed to date. Models of the quiescent light curve require a large deposition of heat in the shallow outer crust from an unknown source. The additional heat injected is $\approx 4\textrm{-}10\,\mathrm{MeV}$ per accreted nucleon; when the observed decline in accretion rate at the end of the outburst is accounted for, the required heating increases to $\approx 6\textrm{-}16\,\mathrm{MeV}$. This shallow heating is still required to fit the lightcurve even after taking into account a second accretion episode, uncertainties in distance, and different surface gravities. The amount of shallow heating is larger than that inferred for other neutron star transients and is larger than can be supplied by nuclear reactions or compositionally driven convection; but it is consistent with stored mechanical energy in the accretion disk. The high crust temperature ($T_b \gtrsim 10^{9} \, {\rm K}$) makes its cooling behavior in quiescence largely independent of the crust composition and envelope properties, so that future observations will probe the gravity of the source. Fits to the lightcurve disfavor the presence of Urca cooling pairs in the crust., Comment: 6 pages, 4 figures, submitted to ApJL

The theoretical effects of phase separation on encounter-limited charge carrier recombination in organic semiconductor blends are investigated using kinetic Monte Carlo (KMC) simulations of pump-probe experiments. Using model bulk heterojunction morphologies, the dependence of the recombination rate on domain size and charge carrier mobility are quantified. Unifying competing models and simulation results, we show that the mobility dependence of the recombination rate can be described using the power mean of the electron and hole mobilities with a domain size dependent exponent. Additionally, for domain sizes typical of organic photovoltaic devices, we find that phase separation reduces the recombination rate by less than one order of magnitude compared to the Langevin model and that the mobility dependence can be approximated by the geometric mean., Comment: Main article: 6 pages, 4 figures, Supplementary Information: 3 pages, 1 figure

The slow decay of charge carriers in polymer-fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior, including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations., Comment: 9 pages, 4 figures; corrected a few minor typos and grammatical errors

The temperature in the crust of an accreting neutron star, which comprises its outermost kilometer, is set by heating from nuclear reactions at large densities, neutrino cooling, and heat transport from the interior. The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope. Here we report that cycles of electron capture and its inverse, $\beta^-$ decay, involving neutron-rich nuclei at a typical depth of about 150 m, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This Urca mechanism has been studied in the context of white dwarfs and Type Ia supernovae, but hitherto was not considered in neutron stars, because previous models computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. This thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface., Comment: 15 pages with 5 figures. Nature advance online publication, December 1, 2013

The 33S(p,\gamma)34Cl reaction is important for constraining predictions of certain isotopic abundances in oxygen-neon novae. Models currently predict as much as 150 times the solar abundance of 33S in oxygen-neon nova ejecta. This overproduction factor may, however, vary by orders of magnitude due to uncertainties in the 33S(p,\gamma)34Cl reaction rate at nova peak temperatures. Depending on this rate, 33S could potentially be used as a diagnostic tool for classifying certain types of presolar grains. Better knowledge of the 33S(p,\gamma)34Cl rate would also aid in interpreting nova observations over the S-Ca mass region and contribute to the firm establishment of the maximum endpoint of nova nucleosynthesis. Additionally, the total S elemental abundance which is affected by this reaction has been proposed as a thermometer to study the peak temperatures of novae. Previously, the 33S(p,\gamma)34Cl reaction rate had only been studied directly down to resonance energies of 432 keV. However, for nova peak temperatures of 0.2-0.4 GK there are 7 known states in 34Cl both below the 432 keV resonance and within the Gamow window that could play a dominant role. Direct measurements of the resonance strengths of these states were performed using the DRAGON recoil separator at TRIUMF. Additionally two new states within this energy region are reported. Several hydrodynamic simulations have been performed, using all available experimental information for the 33S(p,\gamma)34Cl rate, to explore the impact of the remaining uncertainty in this rate on nucleosynthesis in nova explosions. These calculations give a range of ~ 20-150 for the expected 33S overproduction factor, and a range of ~ 100-450 for the 32S/33S ratio expected in ONe novae., Comment: 12 pages, 8 figures, Accepted for publication in Physical Review C

As for its inorganic counterparts the future developments in organic electronics are driven by an advanced device miniaturization. Therefore, the opto-electronic behavior of up-to-date devices is progressively governed by the local structural environment. However, there is a lack of organic semiconductor materials providing access to the fundamental structure-functionality relation, either due to limitations by their inherent growth or their optical characteristics. In this work we present a systematic investigation of the optical states, so-called excitons, and their temporal evolution in the prototypical organic semiconductor rubrene by means of time and temperature dependent photoluminescence studies. This material offers the unique possibility of preparing well-defined morphologies with adjustable degree of confinement. By this approach we are able to confirm the direct influence on the temperature dependent optical processes with picosecond resolution already for a spatial localization of excitation on the \mu m length scale. While in bulk single crystals the exciton decay dynamics are governed by thermally activated singlet fission, excitons created in microcrystals are trapped by dark states localized on the surface and leading to a pronounced enhancement of their average lifetime. Our results highlight the impact of the local environment on the excitonic states and their dynamics in organic semiconductors. With respect to the spatial dimensions of organic thin film devices, this correlation and the reported effects emerging by the confinement have to be considered upon further miniaturization and in the development of innovative device concepts, such as photovoltaic cells based on triplet-harvesting., Comment: 12 pages, 3 figures, 1 table, supplementary: 6 pages, 4 figues

If the observed quasi-periodic oscillations in magnetar flares are partially confined to the crust, then the oscillation frequencies are unique probes of the nuclear physics of the neutron star crust. We study crustal oscillations in magnetars including corrections for a finite Alfven velocity. Our crust model uses a new nuclear mass formula that predicts nuclear masses with an accuracy very close to that of the finite range droplet model. This mass model for equilibrium nuclei also includes shell corrections and an updated neutron-drip line. We perturb our crust model to predict axial crust modes and assign them to observed giant flare quasi-periodic oscillation frequencies from the soft gamma-ray repeater SGR 1806-20. We find magnetar crusts that match observations for various magnetic field strengths, entrainment of the free neutron gas in the inner crust, and crust-core transition densities. We find that observations can be reconciled with smaller values of the symmetry energy slope parameter, L, if there is a significant amount of entrainment of the neutrons by the superfluid or if the crust-core transition density is large. We also find neutron star masses and radii which are in agreement with expectations from what is known about low-density matter from nuclear experiment. Matching observations with a field-free model we obtain the approximate values of M =1.35 Msun and R = 11.9 km. Matching observations using a model with the surface dipole field of SGR 1806-20 (B=2.4x10^15 G) we obtain the approximate values of M = 1.25 Msun and R = 12.4 km., Comment: 10 pages, 7 figures. Accepted for publication in Phys. Rev. C

The charge carrier dynamics of organic solar cells are strongly influenced by trapping and allow to draw conclusions on the loss mechanisms limiting the photovoltaic performance. In this study we derive the recombination order \Delta\ of mobile charge carriers. For annealed P3HT:PCBM solar cells, it allows us to pinpoint the dominant recombination of mobile with trapped charge carriers in tail states. While the characteristic tail state energy of about 40 meV rises to about 100 meV for 30 h oxygen exposure under illumination, \Delta\ decreases only weakly from 1.70 to 1.62: This corresponds to a slight shift towards trap-assisted recombination., Comment: 4 pages, 3 figures. Corrected one reference and one technical term

The 18F(p,\alpha)15O reaction rate is crucial for constraining model predictions of the \gamma-ray observable radioisotope 18F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, 19Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between J\pi=3/2+ states near the proton threshold (Sp = 6.411 MeV) and a broad J\pi=3/2+ state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the 19F(3He,t)19Ne reaction. In stark contrast with previous assumptions we find at least 3 resonances between the proton threshold and Ecm=50 keV, all with different angular distributions. None of these are consistent with J\pi= 3/2+ angular distributions. We find that the main uncertainty now arises from the unknown proton-width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects 18F production by at least a factor of two in the model considered., Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev. Lett

The pairing properties of the neutrinoless double beta decay $(0\nu2\beta)$ candidate $^{100}$Mo have been studied, along with its daughter $^{100}$Ru, to provide input for nuclear matrix element calculations relevant to the decay. The $(p,t)$ two-neutron transfer reaction was measured on nuclei of $^{102,100}$Ru and $^{100,98}$Mo. The experiment was designed to have particular sensitivity to $0^{+}$ states up to excitation energies of $\sim 3$ MeV with high energy resolution. Measurements were made at two angles and L=0 transitions identified by the ratio of yields between the two angles. For the reactions leading to and from $^{100}$Ru, greater than 95% of the L=0 $(p,t)$ strength was in the ground state, but in $^{100}$Mo about 20% was in excited $0^{+}$ states. The measured $(p,t)$ data, together with existing $(t,p)$ data, suggest that $^{100}$Mo is a shape-transitional nucleus while $^{100}$Ru is closer to the spherical side of that transition. Theoretical calculations of the $0\nu2\beta$ nuclear matrix element may be complicated by this difference in shape., Comment: 5 pages, 2 figures

The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations., Comment: 22 pages, 12 figures

The charge carrier drift mobility in disordered semiconductors is commonly graphically extracted from time-of-flight (ToF) photocurrent transients yielding a single transit time. However, the term transit time is ambiguously defined and fails to deliver a mobility in terms of a statistical average. Here, we introduce an advanced computational procedure to evaluate ToF transients, which allows to extract the whole distribution of transit times and mobilities from the photocurrent transient, instead of a single value. This method, extending the work of Scott et al. (Phys. Rev. B 46, 8603), is applicable to disordered systems with a Gaussian density of states (DOS) and its accuracy is validated using one-dimensional Monte Carlo simulations. We demonstrate the superiority of this new approach by comparing it to the common geometrical analysis of hole ToF transients measured on poly(3-hexyl thiophene-2,5-diyl) (P3HT). The extracted distributions provide access to a very detailed and accurate analysis of the charge carrier transport. For instance, not only the mobility given by the mean transit time, but also the mean mobility can be calculated. Whereas the latter determines the macroscopic photocurrent, the former is relevant for an accurate determination of the energetic disorder parameter $\sigma$ within the Gaussian disorder model (GDM). $\sigma$ derived by using the common geometrical method is, as we show, underestimated instead., Comment: 10 pages, 6 figures

Transient photovoltage (TPV) and voltage dependent charge extraction (CE) measurements were applied to poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells to analyze the limitations of solar cell performance in pristine and annealed devices. From the determined charge carrier decay rate under open circuit conditions and the voltage dependent charge carrier densities n(V) the nongeminate loss current jloss of the device is accessible. We found that jloss alone is sufficient to describe the j-V characteristics across the whole operational range, for annealed and, not yet shown before, also for the lower performing pristine solar cells. Even in a temperature range from 300 K to 200 K nongeminate recombination is found to be the dominant and, therefore, performance limiting loss process. Consequently, charge photogeneration is voltage independent in the voltage range studied., Comment: 3 pages, 3 figures; Rapid Research Letter 2012

The Shockley equation (SE), originally derived to describe a p--n junction, was frequently used in the past to simulate current--voltage (j/V) characteristics of organic solar cells (OSC). In order to gain a more detailed understanding of recombination losses, we determined the SE parameters, i.e. the ideality factor and the dark saturation current, from temperature dependent static j/V-measurements on poly(3-hexylthiophene-2,5-diyl)(P3HT):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester (PCBM) bulk heterojunction solar cells. As we show here, these parameters are directly related to charge carrier recombination and become also accessible by transient photovoltage and photocurrent methods in the case of field-independent charge carrier generation. Although determined in very different ways, both SE parameters were found to be identical. The good agreement of static and transient approaches over a wide temperature range demonstrates the validity of the Shockley model for OSC based on material systems satisfying the requirement of field-independent polaron-pair dissociation. In particular, we were able to reproduce the photocurrent at various light intensities and temperatures from the respective nongeminate recombination rates. Furthermore, the temperature dependence of the dark saturation current $j_0$ allowed determining the effective band gap of the photoactive blend perfectly agreeing with the literature values of the energy onset of the photocurrent due to charge transfer absorption. We also present a consistent model directly relating the ideality factor to recombination of free with trapped charge carriers in an exponential density of tail states. We verify this finding by data from thermally stimulated current measurements., Comment: 8 pages, 6 figures

Donor-acceptor (D-A) type copolymers show great potential for the application in the active layer of organic solar cells. Nevertheless the nature of the excited states, the coupling mechanism and the relaxation pathways following photoexcitation are yet to be clarified. We carried out comparative measurements of the steady state absorption and photoluminescence (PL) on the copolymer poly[N-(1-octylnonyl)-2,7-carbazole] -alt-5,5-[4',7' -di(thien-2-yl)-2',1',3' -benzothiadiazole] (PCDTBT), its building blocks as well as on the newly synthesized N-(1-octylnonyl)-2,7-bis-[(5-phenyl)thien-2-yl)carbazole (BPT-carbazole) (see Figure 1). The high-energy absorption band (HEB) of PCDTBT was identified with absorption of carbazoles with adjacent thiophene rings while the low-energy band (LEB) originates instead from the charge transfer (CT) state delocalized over the aforementioned unit with adjacent benzothiadiazole group. Photoexcitation of the HEB is followed by internal relaxation prior the radiative decay to the ground state. Adding PC70BM results in the efficient PL quenching within the first 50 ps after excitation. From the PL excitation experiments no evidence for a direct electron transfer from the HEB of PCDTBT towards the fullerene acceptor was found, therefore the internal relaxation mechanisms within PCDTBT can be assumed to precede. Our findings indicate that effective coupling between copolymer building blocks governs the photovoltaic performance of the blends., Comment: 7 pages, 6 figures, 1 table

Current-based deep level transient spectroscopy was used to study trap states in poly(3- hexylthiophene-2,5-diyl) (P3HT), [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and P3HT:PCBM blend. The obtained spectra showed traps of 87 meV activation energy in pure P3HT and 21 meV for PCBM. The blend shows a complex emission rate spectrum consisting of several different emission rate bands in the range of (0.1-30) s^-1, yielding activation energies between about 30 meV and 160 meV., Comment: 4 pages, 3 figures

We investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]- phenyl-C61 butyric acid methyl ester (PC61BM) by means of temperature dependent time delayed collection field (TDCF) measurements. In MDMO-PPV:PC61BM we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scenario of polaron pair dissociation via an intermediate charge transfer (CT) state. In contrast, polaron pair dissociation in P3HT:PC61BM shows only a very weak field dependence, indicating an almost field independent polaron pair dissociation or a direct photogeneration. Furthermore, we found Langevin recombination for MDMO-PPV:PC61BM and strongly reduced Langevin recombination for P3HT:PC61BM., Comment: 4 pages, 3 figures

Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility, and protection of trapped charge carriers from recombination by a donor--acceptor phase separation until reemission from these deep states. In order to clarify which mechanism is dominant we performed temperature and illumination dependent charge extraction measurements under open circuit as well as short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C$_{61}$butyric acid methyl ester (P3HT:PC$_{61}$BM) and PTB7:PC$_{71}$BM (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) solar cells in combination with current--voltage characteristics. We show that the charge carrier density $n$ dependence of the mobility $\mu$ and the recombination prefactor are different for PC$_{61}$BM at temperatures below 300K and PTB7:PC$_{71}$BM at room temperature. Therefore, in addition to $\mu(n)$ a detrapping limited recombination in systems with at least partial donor--acceptor phase separation is required to explain the high recombination orders., Comment: 7 pages, 4 figures

Model predictions of the amount of the radioisotope 26Al produced in hydrogen-burning environments require reliable estimates of the thermonuclear rates for the 26gAl(p,{\gamma})27Si and 26mAl(p,{\gamma})27Si reactions. These rates depend upon the spectroscopic properties of states in 27Si within about 1 MeV of the 26gAl+p threshold (Sp = 7463 keV). We have studied the 28Si(3He,{\alpha})27Si reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with a transfer reaction, we have constrained J{\pi} values for states in 27Si over Ex = 7.0 - 8.1 MeV through angular distribution measurements. Aside from a few important cases, we generally confirm the energies and spin-parity assignments reported in a recent {\gamma}-ray spectroscopy study. The magnitudes of neutron spectroscopic factors determined from shell-model calculations are in reasonable agreement with our experimental values extracted using this reaction., Comment: accepted for publication in Phys. Rev. C

The 31S(p,\gamma)32Cl reaction is expected to provide the dominant break-out path from the SiP cycle in novae and is important for understanding enrichments of sulfur observed in some nova ejecta. We studied the 32S(3He,t)32Cl charge-exchange reaction to determine properties of proton-unbound levels in 32Cl that have previously contributed significant uncertainties to the 31S(p,\gamma)32Cl reaction rate. Measured triton magnetic rigidities were used to determine excitation energies in 32Cl. Proton-branching ratios were obtained by detecting decay protons from unbound 32Cl states in coincidence with tritons. An improved 31S(p,\gamma)32Cl reaction rate was calculated including robust statistical and systematic uncertainties.

The influence of an external electric field on the photoluminescence intensity of singlet excitons and charge transfer complexes is investigated for a poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) diode and a bulk heterojunction of the PPV in combination with [6,6]-phenyl-C61 butyric acid methylester (PCBM), respectively. The experimental data is related to the dissociation probability derived from the Onsager-Braun model. In this way, a lower limit for the singlet exciton binding energy of MDMO-PPV is determined as (327 +- 30) meV, whereas a significantly lower value of (203 +- 18) meV is extracted for the charge transfer complex in a MDMO-PPV:PCBM blend., Comment: 3 pages, 4 figures, published online at physica status solidi (RRL)

Charged polarons in thin films of polymer-fullerene composites are investigated by light-induced electron paramagnetic resonance (EPR) at 9.5 GHz (X-band) and 130 GHz (D-band). The materials studied were poly(3-hexylthiophene) (PHT), [6,6]-phenyl-C61-butyric acid methyl ester (C60-PCBM), and two different soluble C70-derivates: C70-PCBM and diphenylmethano[70]fullerene oligoether (C70-DPM-OE). The first experimental identification of the negative polaron localized on the C70-cage in polymer-fullerene bulk heterojunctions has been obtained. When recorded at conventional X-band EPR, this signal is overlapping with the signal of the positive polaron, which does not allow for its direct experimental identification. Owing to the superior spectral resolution of the high frequency D-band EPR, we were able to separate light-induced signals from P+ and P- in PHT-C70 bulk heterojunctions. Comparing signals from C70-derivatives with different side-chains, we have obtained experimental proof that the polaron is localized on the cage of the C70 molecule.

Understanding of degradation mechanisms in polymer:fullerene bulk-heterojunctions on the microscopic level aimed at improving their intrinsic stability is crucial for the breakthrough of organic photovoltaics. These materials are vulnerable to exposure to light and/or oxygen, hence they involve electronic excitations. To unambiguously probe the excited states of various multiplicities and their reactions with oxygen, we applied combined magneto-optical methods based on multifrequency (9 and 275 GHz) electron paramagnetic resonance (EPR), photoluminescence (PL), and PL-detected magnetic resonance (PLDMR) to the conjugated polymer poly(3-hexylthiophene) (P3HT) and polymer:fullerene bulk heterojunctions (P3HT:PCBM; PCBM = [6,6]-phenyl-C61-butyric acid methyl ester). We identified two distinct photochemical reaction routes, one being fully reversible and related to the formation of polymer:oxygen charge transfer complexes, the other one, irreversible, being related to the formation of singlet oxygen under participation of bound triplet excitons on the polymer chain. With respect to the blends, we discuss the protective effect of the methanofullerenes on the conjugated polymer bypassing the triplet exciton generation.

The open circuit voltage Voc and the corresponding charge carrier density were measured in dependence of temperature and illumination intensity by current-voltage and charge extraction measurements for P3HT:PCBM and P3HT:bisPCBM solar cells. At lower temperatures a saturation of Voc was observed which can be explained by energetic barriers at the contacts (metal-insulator-metal model). Such injection barriers can also influence Voc at room temperature and limit the performance of the working solar cell, as was assured by macroscopic device simulations on temperature-dependent IV characteristics. However, under most conditions - room temperature and low barriers - Voc is given by the effective bandgap., Comment: 4 pages, 2 figures

We report on terrylene-3,4:11,12-bis(dicarboximide) (TDI) as electron acceptor for bulk-heterojunction solar cells using poly(3-hexyl thiophene) (P3HT) as complementary donor component. Enhanced absorption was observed in the blend compared to pure P3HT. As shown by the very efficient photoluminescence (PL) quenching, the generated excitons are collected at the interface between the donor and acceptor, where they separate into charges which we detect by photoinduced absorption and electron-spin resonance (ESR). Time-of-flight (TOF) photoconductivity measurements reveal a good electron mobility of 10-3 cm2 V-1 s-1 in the blend. Nevertheless, the photocurrent in solar cells was found to be surprisingly low. Supported by the external quantum efficiency (EQE) spectrum as well as morphological studies by way of X-ray diffraction and atomic force microscopy, we explain our observation by the formation of a TDI hole blocking layer at the anode interface which prevents the efficiently generated charges to be extracted., Comment: Original research article, 9 pages, 10 figures, 1 table

We investigated poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester bulk heterojunction (BHJ) solar cells by means of pulsed photocurrent, temperature dependent current-voltage and capacitance-voltage measurements. We show that a direct transfer of Mott-Schottky (MS) analysis from inorganic devices to organic BHJ solar cells is not generally appropriate to determine the built-in potential, since the resulting potential depends on the active layer thickness. Pulsed photocurrent measurements enabled us to directly study the case of quasi flat bands (QFB) in the bulk of the solar cell. It is well below the built-in potential and differs by diffusion-induced band-bending at the contacts. In contrast to MS analysis the corresponding potential is independent on the active layer thickness and therefore a better measure for flat band conditions in the bulk of a BHJ solar cell as compared to MS analysis., Comment: 4 pages, 5 figures

The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its alpha-decay daughter 142Nd. Based on analyses of 146Sm/147Sm alpha-activity and atom ratios, we determined the half-life of 146Sm to be 68 \pm 7 (1sigma) million years (Ma), which is shorter than the currently used value of 103 \pm 5 Ma. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm_0 = 0.0094\pm0.0005 (2sigma), than previously estimated. Terrestrial, Lunar and Martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)_0 values., Comment: 48 pages, 9 figures, 3 tables. Manuscript revised

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage (VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (JSC) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the JSC in blends containing the high voltage absorber Lu3N@C80-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu3N@C80-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC61BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu3N@C80-PCBEH are responsible for the reduced photocurrent.

The trap states in three fullerene derivatives, namely PC61BM ([6,6]-phenyl C61 butyric acid methyl ester), bisPC61BM (bis[6,6]-phenyl C61 butyric acid methyl ester) and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), are investigated by thermally stimulated current measurements (TSC). Thereby, the lower limit of the trap densities for all studied methanofullerenes exhibits values in the order of 10^22 m^-3 with the highest trap density in bisPC61BM and the lowest in PC61BM. Fractional TSC measurements on PC61BM reveal a broad trap distribution instead of discrete trap levels with activation energies ranging from 15 meV to 270 meV and the maximum at about 75 meV. The activation energies of the most prominent traps in the other two fullerene derivatives are significantly higher, being at 96 meV and 223 meV for PC71BM and 184 meV for bisPC61BM, respectively. The influence of these findings on the performance of organic solar cells is discussed., Comment: 7 pages, 5 figures, 1 table