23 research outputs found
Size-Independent Energy Transfer in Biomimetic Nanoring Complexes
Supramolecular
antenna-ring complexes are of great interest due
to their presence in natural light-harvesting complexes. While such
systems are known to provide benefits through robust and efficient
energy funneling, the relationship between molecular structure, strain
(governed by nuclear coordinates and motion), and energy dynamics
(arising from electronic behavior) is highly complex. We present a
synthetic antenna-nanoring system based on a series of conjugated
porphyrin chromophores ideally suited to explore such effects. By
systematically varying the size of the acceptor nanoring, we reveal
the interplay between antenna-nanoring binding, local strain, and
energy dynamics on the picosecond time scale. Binding of the antenna
unit creates a local strain in the nanoring, and this strain was measured
as a function of the size of the nanoring, by UV–vis-NIR titration,
providing information on the conformational flexibility of the system.
Strikingly, the energy-transfer rate is independent of nanoring size,
indicating the existence of strain-localized acceptor states, spread
over about six porphyrin units, arising from the noncovalent antenna-nanoring
association
Breaking the Symmetry in Molecular Nanorings
Because
of their unique electronic properties, cyclic molecular
structures ranging from benzene to natural light-harvesting complexes
have received much attention. Rigid π-conjugated templated porphyrin
nanorings serve as excellent model systems here because they possess
well-defined structures that can readily be controlled and because
they support highly delocalized excitations. In this study, we have
deliberately modified a series of six-porphyrin nanorings to examine
the impact of lowering the rotational symmetry on their photophysical
properties. We reveal that as symmetry distortions increase in severity
along the series of structures, spectral changes and an enhancement
of radiative emission strength occur, which derive from a transfer
of oscillator strength into the lowest (<i>k</i> = 0) state.
We find that concomitantly, the degeneracy of the dipole-allowed first
excited (<i>k </i>= ±1) state is lifted, leading to
an ultrafast polarization switching effect in the emission from strongly
symmetry-broken nanorings
Morphology-Dependent Energy Transfer Dynamics in Fluorene-Based Amphiphile Nanoparticles
Nanoparticles are interesting systems to study because of their large range of potential uses in biological imaging and sensing. We investigated molecular nanoparticles formed by fast injection of a small volume of molecularly dissolved fluorene-derivative amphiphilic molecules into a polar solvent, which resulted in solid spherical particles of ∼80 nm diameter with high stability. Energy transfer studies were carried out on two-component nanoparticles that contained mixtures of donor and acceptor amphiphiles of various fractions. We conducted time-resolved photoluminescence measurements on the two-component nanoparticles in order to determine whether the fundamental donor–acceptor interaction parameter (the Förster radius) depends on the acceptor concentration. The Förster radius was found to be large for very low incorporated acceptor fractions (<0.1%), but it declined with increasing concentration. These changes were concomitant with shifts in the acceptor emission and absorption circular dichroism spectra that indicated an increasing clustering of acceptors into domains as their fraction was raised. In addition, for acceptor fractions below 2% the extracted Förster radii were found to be significantly larger than predicted from donor–acceptor spectral overlap calculations, in accordance with efficient excitation diffusion within the donor matrix, aiding the overall transfer to acceptors. We conclude that energy transfer in two-component nanoparticles shows a complex interplay between phase segregation of the constituent donor and acceptor molecules and excitation diffusion within their domains
Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>
The organic–inorganic hybrid
perovskites methylammonium
lead iodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) and the
partially chlorine-substituted mixed halide CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> emit strong and broad photoluminescence (PL) around their
band gap energy of ∼1.6 eV. However, the nature of the radiative
decay channels behind the observed emission and, in particular, the
spectral broadening mechanisms are still unclear. Here we investigate
these processes for high-quality vapor-deposited films of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> using time- and excitation-energy dependent photoluminescence
spectroscopy. We show that the PL spectrum is homogenously broadened
with a line width of 103 meV most likely as a consequence of phonon
coupling effects. Further analysis reveals that defects or trap states
play a minor role in radiative decay channels. In terms of possible
lasing applications, the emission spectrum of the perovskite is sufficiently
broad to have potential for amplification of light pulses below 100
fs pulse duration
Photocurrent Spectroscopy of Perovskite Solar Cells Over a Wide Temperature Range from 15 to 350 K
Solar
cells based on metal halide perovskite thin films show great
promise for energy generation in a range of environments from terrestrial
installations to space applications. Here we assess the device characteristics
of the prototypical perovskite solar cells based on methylammonium
lead triiodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) over
a broad temperature range from 15 to 350 K (−258 to 77 °C).
For these devices, we observe a peak in the short-circuit current
density and open-circuit voltage at 200 K (−73 °C) with
decent operation maintained up to 350 K. We identify the clear signature
of crystalline PbI<sub>2</sub> contributing directly to the low-temperature
photocurrent spectra, showing that PbI<sub>2</sub> plays an active
role (beyond passivation) in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells. Finally we observe a blue-shift in the photocurrent
spectrum with respect to the absorption spectrum at low temperature
(15 K), allowing us to extract a lower limit on the exciton binding
energy of 9.1 meV for CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>
Radiative Monomolecular Recombination Boosts Amplified Spontaneous Emission in HC(NH<sub>2</sub>)<sub>2</sub>SnI<sub>3</sub> Perovskite Films
Hybrid
metal-halide perovskites have potential as cost-effective
gain media for laser technology because of their superior optoelectronic
properties. Although lead-halide perovskites have been most widely
studied to date, tin-based perovskites have been proposed as a less
toxic alternative. In this Letter, we show that amplified spontaneous
emission (ASE) in formamidinium tin triiodide (FASnI<sub>3</sub>)
thin films is supported by an observed radiative monomolecular charge
recombination pathway deriving from its unintentional doping. Such
a radiative component will be active even at the lowest charge-carrier
densities, opening a pathway for ultralow light-emission thresholds.
Using time-resolved THz photoconductivity analysis, we further show
that the material has an unprecedentedly high charge-carrier mobility
of 22 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> favoring
efficient transport. In addition, FASnI<sub>3</sub> exhibits strong
radiative bimolecular recombination and Auger rates that are over
an order of magnitude lower than for lead-halide perovskites. In combination,
these properties reveal that tin-halide perovskites are highly suited
to light-emitting devices
Effect of Structural Phase Transition on Charge-Carrier Lifetimes and Defects in CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub> Perovskite
Methylammonium tin triiodide (MASnI<sub>3</sub>) has been successfully
employed in lead-free perovskite solar cells, but overall power-conversion
efficiencies are still significantly lower than for lead-based perovskites.
Here we present photoluminescence (PL) spectra and time-resolved PL
from 8 to 295 K and find a marked improvement in carrier lifetime
and a substantial reduction in PL line width below ∼110 K,
indicating that the cause of the hindered performance is activated
at the orthorhombic to tetragonal phase transition. Our measurements
therefore suggest that targeted structural change may be capable of
tailoring the relative energy level alignment of defects (e.g., tin
vacancies) to reduce the background dopant density and improve charge
extraction. In addition, we observe for the first time an above-gap
emission feature that may arise from higher-lying interband transitions,
raising the prospect of excess energy harvesting
High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires
InAsSb
nanowires are promising elements for thermoelectric devices,
infrared photodetectors, high-speed transistors, as well as thermophotovoltaic
cells. By changing the Sb alloy fraction the mid-infrared bandgap
energy and thermal conductivity may be tuned for specific device applications.
Using both terahertz and Raman noncontact probes, we show that Sb
alloying increases the electron mobility in the nanowires by over
a factor of 3 from InAs to InAs<sub>0.65</sub>Sb<sub>0.35</sub>. We
also extract the temperature-dependent electron mobility via both
terahertz and Raman spectroscopy, and we report the highest electron
mobilities for InAs<sub>0.65</sub>Sb<sub>0.35</sub> nanowires to date,
exceeding 16,000 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> at 10 K
Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide
Metal halide perovskites
have proven to be excellent light-harvesting
materials in photovoltaic devices whose efficiencies are rapidly improving.
Here, we examine the temperature-dependent photon absorption, exciton
binding energy, and band gap of FAPbI<sub>3</sub> (thin film) and
find remarkably different behavior across the β–γ
phase transition compared with MAPbI<sub>3</sub>. While MAPbI<sub>3</sub> has shown abrupt changes in the band gap and exciton binding
energy, values for FAPbI<sub>3</sub> vary smoothly over a range of
100–160 K in accordance with a more gradual transition. In
addition, we find that the charge-carrier mobility in FAPbI<sub>3</sub> exhibits a clear <i>T</i><sup>–0.5</sup> trend
with temperature, in excellent agreement with theoretical predictions
that assume electron–phonon interactions to be governed by
the Fröhlich mechanism but in contrast to the <i>T</i><sup>–1.5</sup> dependence previously observed for MAPbI<sub>3</sub>. Finally, we directly observe intraexcitonic transitions
in FAPbI<sub>3</sub> at low temperature, from which we determine a
low exciton binding energy of only 5.3 meV at 10 K
Nanoengineering Coaxial Carbon Nanotube–Dual-Polymer Heterostructures
We describe studies of new nanostructured materials consisting of carbon nanotubes wrapped in sequential coatings of two different semiconducting polymers, namely, poly(3-hexylthiophene) (P3HT) and poly(9,9′-dioctylfluorene-<i>co</i>-benzothiadiazole) (F8BT). Using absorption spectroscopy and steady-state and ultrafast photoluminescence measurements, we demonstrate the role of the different layer structures in controlling energy levels and charge transfer in both solution and film samples. By varying the simple solution processing steps, we can control the ordering and proportions of the wrapping polymers in the solid state. The resulting novel coaxial structures open up a variety of new applications for nanotube blends and are particularly promising for implementation into organic photovoltaic devices. The carbon nanotube template can also be used to optimize both the electronic properties and morphology of polymer composites in a much more controlled fashion than achieved previously, offering a route to producing a new generation of polymer nanostructures