695 research outputs found
Charge Transport Properties of a Metal-free Phthalocyanine Discotic Liquid Crystal
Discotic liquid crystals can self-align to form one-dimensional
semiconducting wires, many tens of microns long. In this letter, we describe
the preparation of semiconducting films where the stacking direction of the
disc-like molecules is perpendicular to the substrate surface. We present
measurements of the charge carrier mobility, applying temperature-dependent
time-of-flight transient photoconductivity, space-charge limited current
measurements, and field-effect mobility measurements. We provide experimental
verification of the highly anisotropic nature of semiconducting films of
discotic liquid crystals, with charge carrier mobilities of up to
2.8x10cm/Vs. These properties make discotics an interesting choice
for applications such as organic photovoltaics.Comment: 5 pages, 5 figure
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
Spectroscopy of electronic defect states in Cu(In, Ga)(S, Se)-based heterojunctions and Schottky diodes under damp-heat exposure
The changes of defect characteristics induced by accelerated lifetime tests
on the heterostructure n-ZnO/i-ZnO/CdS/Cu(In, Ga)(S, Se)/Mo relevant for
photovoltaic energy conversion are investigated. We subject heterojunction and
Schottky devices to extended damp heat exposure at 85C ambient
temperature and 85% relative humidity for various time periods. In order to
understand the origin of the pronounced changes of the devices, we apply
current--voltage and capacitance--voltage measurements, admittance
spectroscopy, and deep-level transient spectroscopy. The fill factor and
open-circuit voltage of test devices are reduced after prolonged damp heat
treatment, leading to a reduced energy conversion efficiency. We observe the
presence of defect states in the vicinity of the CdS/chalcopyrite interface.
Their activation energy increases due to damp heat exposure, indicating a
reduced band bending at the Cu(In, Ga)(S, Se) surface. The Fermi-level
pinning at the buffer/chalcopyrite interface, maintaining a high band bending
in as-grown cells, is lifted due to the damp-heat exposure. We also observe
changes in the bulk defect spectra due to the damp-heat treatment.Comment: 4 pages, 5 figure
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
Temperature-Dependent Charge-Transfer-State Absorption and Emission Reveal the Dominant Role of Dynamic Disorder in Organic Solar Cells
The energetic landscape of charge-transfer (CT) states at the interface of electron donating and electron accepting domains in organic optoelectronic devices is crucial for their performance. Central questions - such as the role of static energetic disorder and vibrational effects - are under ongoing dispute. This study provides an in-depth analysis of temperature-dependent broadening of the spectroscopic absorption and emission features of CT states in devices with small molecule-fullerene blends. We confirm the validity of the electro-optical reciprocity relation between the photovoltaic external quantum efficiency and electroluminescence, enabling us to validate the device temperature during the experiment. The validated temperature allows us to fit our experimental data with several models, and compare extracted CT state energies with the corresponding open-circuit voltage limit at 0 K. Our findings reveal that the absorption and emission characteristics are usually not symmetric, and dominated by temperature-activated broadening (vibrational) effects instead of static disorder
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
High-Sensitivity Measurement of 3He-4He Isotopic Ratios for Ultracold Neutron Experiments
Research efforts ranging from studies of solid helium to searches for a
neutron electric dipole moment require isotopically purified helium with a
ratio of 3He to 4He at levels below that which can be measured using
traditional mass spectroscopy techniques. We demonstrate an approach to such a
measurement using accelerator mass spectroscopy, reaching the 10e-14 level of
sensitivity, several orders of magnitude more sensitive than other techniques.
Measurements of 3He/4He in samples relevant to the measurement of the neutron
lifetime indicate the need for substantial corrections. We also argue that
there is a clear path forward to sensitivity increases of at least another
order of magnitude.Comment: 11 pages, 10 figure
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
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