42 research outputs found
Core/Shell Quantum Dot Based Luminescent Solar Concentrators with Reduced Reabsorption and Enhanced Efficiency
CdSe/CdS core/shell quantum dots
(QDs) have been optimized toward
luminescent solar concentration (LSC) applications. Systematically
increasing the shell thickness continuously reduced reabsorption up
to a factor of 45 for the thickest QDs studied (with ca. 14 monolayers
of CdS) compared to the initial CdSe cores. Moreover, an improved
synthetic method was developed that retains a high-fluorescence quantum
yield, even for particles with the thickest shell volume, for which
a quantum yield of 86% was measured in solution. These high quantum
yield thick shell quantum dots were embedded in a polymer matrix,
yielding highly transparent composites to serve as prototype LSCs,
which exhibited an optical efficiency as high as 48%. A Monte Carlo
simulation was developed to model LSC performance and to identify
the major loss channels for LSCs incorporating the materials developed.
The results of the simulation are in excellent agreement with the
experimental data
Improved Precursor Chemistry for the Synthesis of IIIâV Quantum Dots
The synthesis of IIIâV quantum dots has been long
known to be more challenging than the synthesis of other types of
inorganic quantum dots. This is attributed to highly reactive group-V
precursors. We synthesized molecules that are suitable for use as
group-V precursors and characterized their reactivity using multiple
complementary techniques. We show that the size distribution of indium
arsenide quantum dots indeed improves with decreased precursor reactivity
Media 1: Multispectral imaging via luminescent down-shifting with colloidal quantum dots
Originally published in Optical Materials Express on 01 August 2013 (ome-3-8-1167
Effect of Trace Water on the Growth of Indium Phosphide Quantum Dots
We report that trace amounts of water
impurities in indium myristate
precursors can negatively impact indium phosphide nanoparticle growth
by limiting its size tunability. Without water, the growth can be
effectively tuned by growth temperature and time with the first absorption
peak reaching 620 nm; with water, the growth presents a âfocusedâ
behavior with the first absorption peak remaining around 550 nm. The
results imply that water impurities, either from indium acetate derived
indium precursors or generated in situ during nanoparticle growth,
may be the cause of the currently observed inhibited growth behavior
of indium phosphide quantum dots. We use multistage microfluidic reactors
to show that this inhibiting effect occurs at the late stage of particle
growth, following precursor depletion. We extend our study by showing
that trace amounts of free hydroxide can also inhibit nanoparticle
growth. We attribute the inhibited growth behavior to the hydroxylation
effect of water or free hydroxide
The Dominant Role of Exciton Quenching in PbS Quantum-Dot-Based Photovoltaic Devices
We
present a quantitative measurement of the number of trapped
carriers combined with a measurement of exciton quenching to assess
limiting mechanisms for current losses in PbS-quantum-dot-based photovoltaic
devices. We use photocurrent intensity dependence and short-wave infrared
transient photoluminescence and correlate these with device performance.
We find that the effective density of trapped carriers ranges from
1 in 10 to 1 in 10â000 quantum dots, depending on ligand treatment,
and that nonradiative exciton quenching, as opposed to recombination
with trapped carriers, is likely the limiting mechanism in these devices
Imaging Schottky Barriers and Ohmic Contacts in PbS Quantum Dot Devices
We fabricated planar PbS quantum dot devices with ohmic
and Schottky
type electrodes and characterized them using scanning photocurrent
and photovoltage microscopies. The microscopy techniques used in this
investigation allow for interrogation of the lateral depletion width
and related photovoltaic properties in the planar Schottky type contacts.
Titanium/QD contacts exhibited depletion widths that varied over a
wide range as a function of bias voltage, while the gold/QD contacts
showed ohmic behavior over the same voltage range
Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics
Lead sulfide quantum
dots (PbS QDs) are an attractive material
system for the development of low-cost photovoltaics (PV) due to their
ease of processing and stability in air, with certified power conversion
efficiencies exceeding 11%. However, even the best PbS QD PV devices
are limited by diffusive transport, as the optical absorption length
exceeds the minority carrier diffusion length. Understanding minority
carrier transport in these devices will therefore be critical for
future efficiency improvement. We utilize cross-sectional electron
beam-induced current (EBIC) microscopy and develop methodology to
quantify minority carrier diffusion length in PbS QD PV devices. We
show that holes are the minority carriers in tetrabutylammonium iodide
(TBAI)-treated PbS QD films due to the formation of a pân junction
with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction
with n-type ZnO forms a weaker n<sup>+</sup>ân junction. This
indicates that modifying the standard device architecture to include
a p-type window layer would further boost the performance of PbS QD
PV devices. Furthermore, quantitative EBIC measurements yield a lower
bound of 110 nm for the hole diffusion length in TBAI-treated PbS
QD films, which informs design rules for planar and ordered bulk heterojunction
PV devices. Finally, the low-energy EBIC approach developed in our
work is generally applicable to other emerging thin-film PV absorber
materials with nanoscale diffusion lengths
Measurement of Emission Lifetime Dynamics and Biexciton Emission Quantum Yield of Individual InAs Colloidal Nanocrystals
The
understanding of the photophysics of visible-emitting colloidal
nanocrystals (NCs) has long been aided by single-molecule studies
of their emission. Until recently, no suitable detection technologies
have existed for corresponding studies of shortwave-infrared (SWIR)
emitters. Now, the use of superconducting nanowire single-photon detectors
(SNSPDs) enables the detailed study of SWIR NC emission dynamics at
the single-emitter level. Here, we report a detailed analysis of the
emission dynamics of individual InAs/CdZnS NCs emitting in the SWIR
region. We observe blinking akin to the type A and type B blinking
previously observed in visible-emitting CdSe NCs. We determine the
intrinsic radiative lifetime of several InAs/CdZnS NCs and find examples
ranging from 50â200 ns, indicative of a quasi-type-II electronic
structure. We also measure <i>g</i><sub>0</sub><sup>(2)</sup> for several of these NCs and
find that their biexciton emission quantum yields vary from <1%
up to 43%
Low-Temperature Solution-Processed Solar Cells Based on PbS Colloidal Quantum Dot/CdS Heterojunctions
PbS colloidal quantum dot heterojunction solar cells
have shown
significant improvements in performance, mostly based on devices that
use high-temperature annealed transition metal oxides to create rectifying
junctions with quantum dot thin films. Here, we demonstrate a solar
cell based on the heterojunction formed between PbS colloidal quantum
dot layers and CdS thin films that are deposited via a solution process
at 80 °C. The resultant device, employing a 1,2-ethanedithiol
ligand exchange scheme, exhibits an average power conversion efficiency
of 3.5%. Through a combination of thickness-dependent current densityâvoltage
characteristics, optical modeling, and capacitance measurements, the
combined diffusion length and depletion width in the PbS quantum dot
layer is found to be approximately 170 nm
Quantum-Dot Size and Thin-Film Dielectric Constant: Precision Measurement and Disparity with Simple Models
We study the dielectric constant
of lead sulfide quantum dot (QD)
films as a function of the volume fraction of QDs by varying the QD
size and keeping the ligand constant. We create a reliable QD sizing
curve using small-angle X-ray scattering (SAXS), thin-film SAXS to
extract a pair-distribution function for QD spacing, and a stacked-capacitor
geometry to measure the capacitance of the thin film. Our data support
a reduced dielectric constant in nanoparticles