Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes

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

Reversible and Irreversible Interactions of Poly(3-hexylthiophene) with Oxygen Studied by Spin-Sensitive Methods

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

Charge Carrier Extraction by Linearly Increasing Voltage:Analytic framework and ambipolar transients

Up to now the basic theoretical description of charge extraction by linearly increasing voltage (CELIV) is solved for a low conductivity approximation only. Here we present the full analytical solution, thus generalize the theoretical framework for this method. We compare the analytical solution and the approximated theory, showing that especially for typical organic solar cell materials the latter approach has a very limited validity. Photo-CELIV measurements on poly(3-hexyl thiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester based solar cells were then evaluated by fitting the current transients to the analytical solution. We found that the fit results are in a very good agreement with the experimental observations, if ambipolar transport is taken into account, the origin of which we will discuss. Furthermore we present parametric equations for the mobility and the charge carrier density, which can be applied over the entire experimental range of parameters.Comment: 8 pages, 5 figure

Detailed study of N,N'-(diisopropylphenyl)- terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for solar cells application

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 tabl

Is \gamma-ray emission from novae affected by interference effects in the 18F(p,\alpha)15O reaction?

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. Let

Unbound states of 32Cl and the 31S(p,\gamma)32Cl reaction rate

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 Metal-Insulator Transition in \u3cem\u3eVO\u3csub\u3e2\u3c/sub\u3e\u3c/em\u3e Studied using Terahertz Apertureless Near-Field Microscopy

We have studied the metal-insulator transition in a vanadium dioxide (VO2) thin film using terahertz apertureless near-field optical microscopy. We observe a variation of the terahertz amplitude due to the phase transition induced by an applied voltage across the sample. The change of the terahertz signal is related to the abrupt change of the conductivity of the VO2 film at the metal-insulator transition. The subwavelength spatial resolution of this near-field microscopy makes it possible to detect signatures of micron-scale metallic domains in inhomogeneous VO2 thin films

An experimental study of the rearrangements of valence protons and neutrons amongst single-particle orbits during double {\beta} decay in 100Mo

The rearrangements of protons and neutrons amongst the valence single-particle orbitals during double {\beta} decay of 100Mo have been determined by measuring cross sections in (d,p), (p,d), (3He,{\alpha}) and (3He,d) reactions on 98,100Mo and 100,102Ru targets. The deduced nucleon occupancies reveal significant discrepancies when compared with theoretical calculations; the same calculations have previously been used to determine the nuclear matrix element associated with the decay probability of double {\beta} decay of the 100Mo system.Comment: 18 pages, 13 figures, 37 pages of supplemental informatio