8 research outputs found
Charge Transport Limitations in Perovskite Solar Cells: The Effect of Charge Extraction Layers
Understanding
the charge transport characteristics and their limiting factors in
organolead halide perovskites is of great importance for the development
of competitive and economically advantageous photovoltaic systems
derived from these materials. In the present work, we examine the
charge carrier mobilities in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> (MAPI) thin films obtained from a one-step synthesis procedure and
in planar n–i–p devices based on these films. By performing
time-of-flight measurements, we find mobilities around 6 cm<sup>2</sup>/V s for electrons and holes in MAPI thin films, whereas in working
solar cells, the respective effective mobility values are reduced
by 3 orders of magnitude. From complementary experiments on devices
with varying thicknesses of electron and hole transport layers, we
identify the charge extraction layers and the associated interfaces
rather than the perovskite material itself as the major limiting factors
of the charge carrier transport time in working devices
Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films
Micro-
and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film
solar cells today reach power conversion efficiencies of over 20%.
We investigate the impact of grain boundaries on charge carrier transport
in large crystal MAPI thin films using time-resolved photoluminescence
(PL) microscopy and numerical model calculations. Crystal sizes in
the range of several tens of micrometers allow for the spatially and
time resolved study of boundary effects. Whereas long-ranged diffusive
charge carrier transport is observed within single crystals, no detectable
diffusive transport occurs across grain boundaries. The observed PL
transients are found to crucially depend on the microscopic geometry
of the crystal and the point of observation. In particular, spatially
restricted diffusion of charge carriers leads to slower PL decay near
crystal edges as compared to the crystal center. In contrast to many
reports in the literature, our experimental results show no quenching
or additional loss channels due to grain boundaries for the studied
material, which thus do not negatively affect the performance of the
derived thin-film devices
Enhanced Single-Photon Emission from Carbon-Nanotube Dopant States Coupled to Silicon Microcavities
Single-walled
carbon nanotubes are a promising material as quantum
light sources at room temperature and as nanoscale light sources for
integrated photonic circuits on silicon. Here, we show that the integration
of dopant states in carbon nanotubes and silicon microcavities can
provide bright and high-purity single-photon emitters on a silicon
photonics platform at room temperature. We perform photoluminescence
spectroscopy and observe the enhancement of emission from the dopant
states by a factor of ∼50, and cavity-enhanced radiative decay
is confirmed using time-resolved measurements, in which a ∼30%
decrease of emission lifetime is observed. The statistics of photons
emitted from the cavity-coupled dopant states are investigated by
photon-correlation measurements, and high-purity single photon generation
is observed. The excitation power dependence of photon emission statistics
shows that the degree of photon antibunching can be kept high even
when the excitation power increases, while the single-photon emission
rate can be increased to ∼1.7 × 10<sup>7</sup> Hz
Fluorescent Carbon Nanotube Defects Manifest Substantial Vibrational Reorganization
Fluorescent defects have opened up
exciting new opportunities to
chemically tailor semiconducting carbon nanotubes for imaging, sensing,
and photonics needs such as lasing, single photon emission, and photon
upconversion. However, experimental measurements on the trap depths
of these defects show a puzzling energy mismatch between the optical
gap (difference in emission energies between the native exciton and
defect trap states) and the thermal detrapping energy determined by
application of the van ’t Hoff equation. To resolve this
fundamentally important problem, here we synthetically incorporated
a series of fluorescent aryl defects into semiconducting single-walled
carbon nanotubes and experimentally determined their energy levels
by temperature-dependent and chemically correlated evolution of exciton
population and photoluminescence. We found that depending on the chemical
nature and density of defects, the exciton detrapping energy is 14–77%
smaller than the optical gap determined from photoluminescence. For
the same type of defect, the detrapping energy increases with defect
density from 76 to 131 meV for 4-nitroaryl defects in (6,5) single-walled
carbon nanotubes, whereas the optical gap remains nearly unchanged
(<5 meV). These experimental findings are corroborated by quantum-chemical
simulations of the chemically functionalized carbon nanotubes. Our
results suggest that the energy mismatch arises from vibrational reorganization
due to significant deformation of the nanotube geometry upon exciton
trapping at the defect site. An unexpectedly large reorganization
energy (on the order of 100 meV) is found between ground and excited
states of the defect tailored nanostructures. This finding reveals
a molecular picture for description of these synthetic defects and
suggests significant potential for tailoring the electronic properties
of carbon nanostructures through chemical engineering
Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces
All-dielectric
metasurfaces made from arrays of high index nanoresonators
supporting strong magnetic dipole modes have emerged as a low-loss
alternative to plasmonic metasurfaces. Here we use oxygen-doped single-walled
carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon
metasurfaces to study effects of the magnetic dipole modes of the
constituent nanoresonators on the photoluminescence (PL) of individual
SWCNTs. We find that when in resonance, the magnetic mode of the silicon
nanoresonators can lead to a moderate average PL enhancement of 0.8–4.0
of the SWCNTs, accompanied by an average increase in the radiative
decay rate by a factor of 1.5–3.0. More interestingly, single
dopant polarization experiments show an anomalous photoluminescence
polarization rotation by coupling individual SWCNTs to silicon nanoresonators.
Numerical simulations indicate that this is caused by modification
of near-field polarization distribution at certain areas in the proximity
of the silicon nanoresonators at the excitation wavelength, thus presenting
an approach to control emission polarization. These findings indicate
silicon nanoresonators as potential building blocks of quantum photonic
circuits capable of manipulating PL intensity and polarization of
single photon sources
Solvent- and Wavelength-Dependent Photoluminescence Relaxation Dynamics of Carbon Nanotube sp<sup>3</sup> Defect States
Photoluminescent
sp<sup>3</sup> defect states introduced to single
wall carbon nanotubes (SWCNTs) through low-level covalent functionalization
create new photophysical behaviors and functionality as a result of
defect sites acting as exciton traps. Evaluation of relaxation dynamics
in varying dielectric environments can aid in advancing a more complete
description of defect-state relaxation pathways and electronic structure.
Here, we exploit helical wrapping polymers as a route to suspending
(6,5) SWCNTs covalently functionalized with 4-methoxybenzene
in solvent systems including H<sub>2</sub>O, D<sub>2</sub>O, methanol,
dimethylformamide, tetrahydrofuran, and toluene, spanning a range
of dielectric constants from 80 to 3. Defect-state photoluminescence
decays were measured as a function of emission wavelength and solvent
environment. Emission decays are biexponential, with short lifetime
components on the order of 65 ps and long components ranging from
around 100 to 350 ps. Both short and long decay components increase
as emission wavelength increases, while only the long lifetime component
shows a solvent dependence. We demonstrate that the wavelength dependence
is a consequence of thermal detrapping of defect-state excitons to
produce mobile E<sub>11</sub> excitons, providing an important mechanism
for loss of defect-state population. Deeper trap states (i.e., those
emitting at longer wavelengths) result in a decreased rate for thermal
loss. The solvent-independent behavior of the short lifetime component
is consistent with its assignment as the characteristic time for redistribution
of exciton population between bright and dark defect states. The solvent
dependence of the long lifetime component is shown to be consistent
with relaxation via an electronic to vibrational energy transfer mechanism,
in which energy is resonantly lost to solvent vibrations in a complementary
mechanism to multiphonon decay processes
Low-Temperature Single Carbon Nanotube Spectroscopy of sp<sup>3</sup> Quantum Defects
Aiming
to unravel the relationship between chemical configuration
and electronic structure of sp<sup>3</sup> defects of aryl-functionalized
(6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature
single nanotube photoluminescence (PL) spectroscopy studies and correlate
our observations with quantum chemistry simulations. We observe sharp
emission peaks from individual defect sites that are spread over an
extremely broad, 1000–1350 nm, spectral range. Our simulations
allow us to attribute this spectral diversity to the occurrence of
six chemically and energetically distinct defect states resulting
from topological variation in the chemical binding configuration of
the monovalent aryl groups. Both PL emission efficiency and spectral
line width of the defect states are strongly influenced by the local
dielectric environment. Wrapping the SWCNT with a polyfluorene polymer
provides the best isolation from the environment and yields the brightest
emission with near-resolution limited spectral line width of 270 μeV,
as well as spectrally resolved emission wings associated with localized
acoustic phonons. Pump-dependent studies further revealed that the
defect states are capable of emitting single, sharp, isolated PL peaks
over 3 orders of magnitude increase in pump power, a key characteristic
of two-level systems and an important prerequisite for single-photon
emission with high purity. These findings point to the tremendous
potential of sp<sup>3</sup> defects in development of room temperature
quantum light sources capable of operating at telecommunication wavelengths
as the emission of the defect states can readily be extended to this
range <i>via</i> use of larger diameter SWCNTs
Giant PbSe/CdSe/CdSe Quantum Dots: Crystal-Structure-Defined Ultrastable Near-Infrared Photoluminescence from Single Nanocrystals
Toward a truly photostable PbSe quantum
dot (QD), we apply the
thick-shell or “giant” QD structural motif to this notoriously
environmentally sensitive nanocrystal system. Namely, using a sequential
application of two shell-growth techniquespartial-cation exchange
and successive ionic layer adsorption and reaction (SILAR)we
are able to overcoat the PbSe QDs with sufficiently thick CdSe shells
to impart new single-QD-level photostability, as evidenced by suppression
of both photobleaching and blinking behavior. We further reveal that
the crystal structure of the CdSe shell (cubic zinc-blende or hexagonal
wurtzite) plays a key role in determining the photoluminescence properties
of these giant QDs, with only cubic nanocrystals sufficiently bright
and stable to be observed as single emitters. Moreover, we demonstrate
that crystal structure and particle shape (cubic, spherical, or tetrapodal)
and, thereby, emission properties can be synthetically tuned by either
withholding or including the coordinating ligand, trioctylphosphine,
in the SILAR component of the shell-growth process