10 research outputs found
Quantum-Dot-Sensitized Solar Cells with Water-Soluble and Air-Stable PbS Quantum Dots
The sensitization of dispersed P25
TiO<sub>2</sub> nanoparticles
(NPs) and macroporous TiO<sub>2</sub> films with water-soluble and
air-stable PbS quantum dots (QDs) capped with l-glutathione
(GSH) ligands was investigated. Optimum sensitization was achieved
by careful adjustment of the surface charges of TiO<sub>2</sub> and
PbS QDs by controlling the pH of the QD solution. Efficient electron
transfer from photoexcited PbS QDs via the GSH ligands into the conduction
band of TiO<sub>2</sub> was demonstrated by photoluminescence (PL)
spectroscopy of PbS-sensitized P25 nanoparticles. The PbS QD-sensitized
porous TiO<sub>2</sub> electrodes were used to prepare quantum-dot-sensitized
solar cells (QDSSCs) utilizing a Cu<sub><i>x</i></sub>S<sub><i>y</i></sub> counter electrode and aqueous polysulfide
electrolyte. Cells with up to 64% injection efficiency, 1.1% AM 1.5
conversion efficiency, and short circuit current density of 7.4 mA
cm<sup>ā2</sup> were obtained. The physical parameters of the
cells were investigated using impedance spectroscopy
Epitaxial Halide Perovskite Lateral Double Heterostructure
Epitaxial
IIIāV semiconductor heterostructures are key components
in modern microelectronics, electro-optics, and optoelectronics. With
superior semiconducting properties, halide perovskite materials are
rising as promising candidates for coherent heterostructure devices.
In this report, spinodal decomposition is proposed and experimentally
implemented to produce epitaxial double heterostructures in halide
perovskite system. Pristine epitaxial mixed halide perovskites rods
and films were synthesized via van der Waals epitaxy by chemical vapor
deposition method. At room temperature, photon was applied as a knob
to regulate the kinetics of spinodal decomposition and classic coarsening.
By this approach, halide perovskite double heterostructures were created
carrying epitaxial interfaces and outstanding optical properties.
Reduced FroĢhlich electronāphonon coupling was discovered
in coherent halide double heterostructure, which is hypothetically
attributed to the classic phonon confinement effect widely existing
in IIIāV double heterostructures. As a proof-of-concept, our
results suggest that halide perovskite-based epitaxial heterostructures
may be promising for high-performance and low-cost optoelectronics,
electro-optics, and microelectronics. Thus, ultimately, for practical
device applications, it may be worthy to pursue these heterostructures
via conventional vapor phase epitaxy approaches widely practised in
IIIāV field
Motional Narrowing Effects in the Excited State Spin Populations of Mn-Doped Hybrid Perovskites
Spināorbit coupling in the electronic states of
solution-processed
hybrid metal halide perovskites forms complex spin-textures in the
band structures and allows for optical manipulation of the excited
state spin-polarizations. Here, we report that motional narrowing
acts on the photoexcited spin-polarization in CH3NH3PbBr3 thin films, which are doped at percentage-level
with Mn2+ ions. Using ultrafast circularly polarized broadband
transient absorption spectroscopy at cryogenic temperatures, we investigate
the spin population dynamics in these doped hybrid perovskites and
find that spin relaxation lifetimes are increased by a factor of 3
compared to those of undoped materials. Using quantitative analysis
of the photoexcitation cooling processes, we reveal increased carrier
scattering rates in the doped perovskites as the fundamental mechanism
driving spin-polarization-maintaining motional narrowing. Our work
reports transition-metal doping as a concept to extend spin lifetimes
of hybrid perovskites
Ultrafast Dynamics of Polariton Cooling and Renormalization in an Organic Single-Crystal Microcavity under Nonresonant Pumping
Microcavity systems with organic
luminescent materials have a hot
prospect for room-temperature cavity-polariton devices. The polariton
dispersion relation of organic microcavities is significantly different
from that of inorganic microcavities due to the strong localization
of Frenkel excitons. Also photoexcited particles will undergo a different
cooling mechanism until they reach the polariton ground state. In
the characterization of efficient polariton condensates, therefore,
the polariton cooling dynamics as well as the kinetics of the polariton
eigenstate should be measured. Here we present experimental studies
on ultrafast dynamics of cavity polaritons in an organic single-crystal
microcavity under nonresonant pumping. In time-resolved photoluminescence
measurements we observed, for the first time, an ultrafast dynamics
of stimulated cooling of the organic cavity polariton. Transient transmission
measurement enabled us to investigate the detailed renormalization
dynamics of the polariton eigenstate. The results clearly demonstrated
the prospect of organic microcavities for room-temperature polaritonic
devices
Photon Transport in One-Dimensional Incommensurately Epitaxial CsPbX<sub>3</sub> Arrays
One-dimensional nanoscale epitaxial
arrays serve as a great model in studying fundamental physics and
for emerging applications. With an increasing focus laid on the Cs-based
inorganic halide perovskite out of its outstanding material stability,
we have applied vapor phase epitaxy to grow well aligned horizontal
CsPbX<sub>3</sub> (X: Cl, Br, or I or their mixed) nanowire arrays
in large scale on mica substrate. The as-grown nanowire features a
triangular prism morphology with typical length ranging from a few
tens of micrometers to a few millimeters. Structural analysis reveals
that the wire arrays follow the symmetry of mica substrate through
incommensurate epitaxy, paving a way for a universally applicable
method to grow a broad family of halide perovskite materials. The
unique photon transport in the one-dimensional structure has been
studied in the all-inorganic Cs-based perovskite wires via temperature
dependent and spatially resolved photoluminescence. Epitaxy of well
oriented wire arrays in halide perovskite would be a promising direction
for enabling the circuit-level applications of halide perovskite in
high-performance electro-optics and optoelectronics
Preparation of Single-Phase Films of CH<sub>3</sub>NH<sub>3</sub>Pb(I<sub>1ā<i>x</i></sub>Br<sub><i>x</i></sub>)<sub>3</sub> with Sharp Optical Band Edges
Organometallic lead-halide perovskite-based
solar cells now approach
18% efficiency. Introducing a mixture of bromide and iodide in the
halide composition allows tuning of the optical bandgap. We prepare
mixed bromideāiodide lead perovskite films CH<sub>3</sub>NH<sub>3</sub>PbĀ(I<sub>1ā<i>x</i></sub>Br<sub><i>x</i></sub>)<sub>3</sub> (0 ā¤ <i>x</i> ā¤ 1) by
spin-coating from solution and obtain films with monotonically varying
bandgaps across the full composition range. Photothermal deflection
spectroscopy, photoluminescence, and X-ray diffraction show that following
suitable fabrication protocols these mixed lead-halide perovskite
films form a single phase. The optical absorption edge of the pure
triiodide and tribromide perovskites is sharp with Urbach energies
of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for
CH<sub>3</sub>NH<sub>3</sub>PbI<sub>1.2</sub>Br<sub>1.8</sub>. We
demonstrate a bromideāiodide lead perovskite film (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>1.2</sub>Br<sub>1.8</sub>) with an optical
bandgap of 1.94 eV, which is optimal for tandem cells of these materials
with crystalline silicon devices
Atmospheric Influence upon Crystallization and Electronic Disorder and Its Impact on the Photophysical Properties of OrganicāInorganic Perovskite Solar Cells
Recently, solution-processable organicāinorganic metal halide perovskites have come to the fore as a result of their high power-conversion efficiencies (PCE) in photovoltaics, exceeding 17%. To attain reproducibility in the performance, one of the critical factors is the processing conditions of the perovskite film, which directly influences the photophysical properties and hence the device performance. Here we study the effect of annealing parameters on the crystal structure of the perovskite films and correlate these changes with its photophysical properties. We find that the crystal formation is kinetically driven by the annealing atmosphere, time and temperature. Annealing in air produces an improved crystallinity and large grain domains as compared to nitrogen. Lower photoluminescence quantum efficiency (PLQE) and shorter photoluminescence (PL) lifetimes are observed for nitrogen annealed perovskite films as compared to the air-annealed counterparts. We note that the limiting nonradiative pathways (<i>i.e</i>., maximizing PLQE) is important for obtaining the highest device efficiency. This indicates a critical impact of the atmosphere upon crystallization and the ultimate device performance
Blue-Green Color Tunable Solution Processable Organolead ChlorideāBromide Mixed Halide Perovskites for Optoelectronic Applications
Solution-processed organo-lead halide
perovskites are produced with sharp, color-pure electroluminescence
that can be tuned from blue to green region of visible spectrum (425ā570
nm). This was accomplished by controlling the halide composition of
CH<sub>3</sub>NH<sub>3</sub>PbĀ(Br<sub><i>x</i></sub>Cl<sub>1ā<i>x</i></sub>)<sub>3</sub> [0 ā¤ <i>x</i> ā¤ 1] perovskites. The bandgap and lattice parameters
change monotonically with composition. The films possess remarkably
sharp band edges and a clean bandgap, with a single optically active
phase. These chlorideābromide perovskites can potentially be
used in optoelectronic devices like solar cells and light emitting
diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs
with narrow emission full width at half maxima (FWHM) and low turn
on voltages using thin-films of these perovskite materials, including
a blue CH<sub>3</sub>NH<sub>3</sub>PbCl<sub>3</sub> perovskite LED
with a narrow emission FWHM of 5 nm
Photon Reabsorption in Mixed CsPbCl<sub>3</sub>:CsPbI<sub>3</sub> Perovskite Nanocrystal Films for Light-Emitting Diodes
Cesium lead halide
nanocrystals, CsPbX<sub>3</sub> (X = Cl, Br,
I), exhibit photoluminescence quantum efficiencies approaching 100%
without the coreāshell structures usually used in conventional
semiconductor nanocrystals. These high photoluminescence efficiencies
make these crystals ideal candidates for light-emitting diodes (LEDs).
However, because of the large surface area to volume ratio, halogen
exchange between perovskite nanocrystals of different compositions
occurs rapidly, which is one of the limiting factors for white-light
applications requiring a mixture of different crystal compositions
to achieve a broad emission spectrum. Here, we use mixtures of chloride
and iodide CsPbX<sub>3</sub> (X = Cl, I) perovskite nanocrystals where
anion exchange is significantly reduced. We investigate samples containing
mixtures of perovskite nanocrystals with different compositions and
study the resulting optical and electrical interactions. We report
excitation transfer from CsPbCl<sub>3</sub> to CsPbI<sub>3</sub> in
solution and within a polyĀ(methyl methacrylate) matrix via photon
reabsorption, which also occurs in electrically excited crystals in
bulk heterojunction LEDs
Enhanced Amplified Spontaneous Emission in Perovskites Using a Flexible Cholesteric Liquid Crystal Reflector
Organicāinorganic perovskites
are highly promising solar cell materials with laboratory-based power
conversion efficiencies already matching those of established thin
film technologies. Their exceptional photovoltaic performance is in
part attributed to the presence of efficient radiative recombination
pathways, thereby opening up the possibility of efficient light-emitting
devices. Here, we demonstrate optically pumped amplified spontaneous
emission (ASE) at 780 nm from a 50 nm-thick film of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite that is sandwiched within
a cavity composed of a thin-film (ā¼7 Ī¼m) cholesteric
liquid crystal (CLC) reflector and a metal back-reflector. The threshold
fluence for ASE in the perovskite film is reduced by at least two
orders of magnitude in the presence of the CLC reflector, which results
in a factor of two reduction in threshold fluence compared to previous
reports. We consider this to be due to improved coupling of the oblique
and out-of-plane modes that are reflected into the bulk in addition
to any contributions from cavity modes. Furthermore, we also demonstrate
enhanced ASE on flexible reflectors and discuss how improvements in
the quality factor and reflectivity of the CLC layers could lead to
single-mode lasing using CLC reflectors. Our work opens up the possibility
of fabricating widely wavelength-tunable āmirror-lessā
single-mode lasers on flexible substrates, which could find use in
applications such as flexible displays and friend or foe identification