65 research outputs found
Colloidal quantum dot light-emitting devices
Colloidal quantum dot light-emitting devices (QD-LEDs) have generated considerable interest for applications such as thin film displays with improved color saturation and white lighting with a high color rendering index (CRI). We review the key advantages of using quantum dots (QDs) in display and lighting applications, including their color purity, solution processability, and stability. After highlighting the main developments in QD-LED technology in the past 15 years, we describe the three mechanisms for exciting QDs - optical excitation, Förster energy transfer, and direct charge injection - that have been leveraged to create QD-LEDs. We outline the challenges facing QD-LED development, such as QD charging and QD luminescence quenching in QD thin films. We describe how optical downconversion schemes have enabled researchers to overcome these challenges and develop commercial lighting products that incorporate QDs to achieve desirable color temperature and a high CRI while maintaining efficiencies comparable to inorganic white LEDs (>65 lumens per Watt). We conclude by discussing some current directions in QD research that focus on achieving higher efficiency and air-stable QD-LEDs using electrical excitation of the luminescent QDs
Mapping the Diffusion Tensor in Microstructured Perovskites
Understanding energy transport in semiconductors is critical for design of
electronic and optoelectronic devices. Semiconductor material properties such
as charge carrier mobility or diffusion length are measured in bulk crystals
and determined using models that describe transport behavior in homogeneous
media, where structural boundary effects are minimal. However, most emerging
semiconductors exhibit microscale heterogeneity. Therefore, experimental
techniques with high spatial resolution paired with models that capture
anisotropy and domain boundary behavior are needed. We develop a diffusion
tensor-based framework to analyze experimental photoluminescence (PL) diffusion
maps accounting for material microstructure. Specifically, we quantify both
carrier transport and recombination in single crystal and polycrystalline lead
halide perovskites by globally fitting diffusion maps, with spatial, temporal,
and PL intensity data. We reveal a 29% difference in principal diffusion
coefficients and alignment between electronically coupled grains for CH3NH3PbI3
polycrystalline films. This framework allows for understanding and optimizing
anisotropic energy transport in heterogeneous materials.Comment: 47 pages, 19 figure
Electrical Tuning of Neutral and Charged Excitons with 1-nm Gate
Electrical control of individual spins and photons in solids is key for
quantum technologies, but scaling down to small, static systems remains
challenging. Here, we demonstrate nanoscale electrical tuning of neutral and
charged excitons in monolayer WSe2 using 1-nm carbon nanotube gates.
Electrostatic simulations reveal a confinement radius below 15 nm, reaching the
exciton Bohr radius limit for few-layer dielectric spacing. In situ
photoluminescence spectroscopy shows gate-controlled conversion between neutral
excitons, negatively charged trions, and biexcitons at 4 K. Important for
quantum information processing applications, our measurements indicate gating
of a local 2D electron gas in the WSe2 layer, coupled to photons via trion
transitions with binding energies exceeding 20 meV. The ability to
deterministically tune and address quantum emitters using nanoscale gates
provides a pathway towards large-scale quantum optoelectronic circuits and
spin-photon interfaces for quantum networking.Comment: 21 pages, 11 figure
A Lithographic Process for Integrated Organic Field-Effect Transistors
Abstract-This paper reports a photolithographic process for fabricating organic field-effect transistors which provides two layers of metal with arbitrary via placement, and optionally allows for subtractive lithographic patterning of the transistor active layer. The demonstrated pentacene transistors have a field-effect mobility of 0.1 0.05 cm 2 /(V s). Parylene-C is used both as the gate dielectric and an encapsulation layer which allows for subtractive lithographic patterning. Also demonstrated is a PMOS inverter without level shifting circuitry and level-restoring High and Low . This work demonstrates a high definition, multilayer, integrated photolithographic process which creates organic field effect transistors suitable for use in integrated circuit applications such as a display backplanes
Terahertz-driven Luminescence and Colossal Stark Effect in CdSe:CdS Colloidal Quantum Dots
Unique optical properties of colloidal semiconductor quantum dots (QDs),
arising from quantum mechanical confinement of charge within these structures,
present a versatile testbed for the study of how high electric fields affect
the electronic structure of nanostructured solids. Earlier studies of quasi-DC
electric field modulation of QD properties have been limited by the
electrostatic breakdown processes under the high externally applied electric
fields, which have restricted the range of modulation of QD properties. In
contrast, in the present work we drive CdSe:CdS core:shell QD films with
high-field THz-frequency electromagnetic pulses whose duration is only a few
picoseconds. Surprisingly, in response to the THz excitation we observe QD
luminescence even in the absence of an external charge source. Our experiments
show that QD luminescence is associated with a remarkably high and rapid
modulation of the QD band-gap, which is changing by more than 0.5 eV
(corresponding to 25% of the unperturbed bandgap energy) within the picosecond
timeframe of THz field profile. We show that these colossal energy shifts can
be consistently explained by the quantum confined Stark effect. Our work
demonstrates a route to extreme modulation of material properties without
configurational changes in material sets or geometries. Additionally, we expect
that this platform can be adapted to a novel compact THz detection scheme where
conversion of THz fields (with meV-scale photon energies) to the
visible/near-IR band (with eV-scale photon energies) can be achieved at room
temperature with high bandwidth and sensitivity.Comment: 8 pages, 4 figures, supplementary informatio
Slowed Recombination via Tunable Surface Energetics in Perovskite Solar Cells
Metal halide perovskite semiconductors have the potential to reach the
optoelectronic quality of meticulously grown inorganic materials, but with a
distinct advantage of being solution processable. Currently, perovskite
performance is limited by charge carrier recombination loss at surfaces and
interfaces. Indeed, the highest quality perovskite films are achieved with
molecular surface passivation, for example with n-trioctylphosphine oxide, but
these treatments are often labile and electrically insulating. As an
alternative, the formation of a thin 2D perovskite layer on the bulk 3D
perovskite reduces non-radiative energy loss while also improving device
performance. But, thus far, it has been unclear how best to design and optimize
2D/3D heterostructures and whether critical material properties, such as charge
carrier lifetime, can reach values as high as ligand-based approaches. Here, we
study perovskite devices that have exhibited power conversion efficiencies
exceeding 25% and show that 2D layers are capable of pushing beyond molecular
passivation strategies with even greater tunability. We set new benchmarks for
photoluminescence lifetime, reaching values > 30 {\mu}s, and perovskite/charge
transport layer surface recombination velocity with values < 7 cm s^{-1}. We
use X-ray spectroscopy to directly visualize how treatment with hexylammonium
bromide not only selectively targets defects at surfaces and grain boundaries,
but also forms a bandgap grading extending > 100 nm into the bulk layer. We
expect these results to be a starting point for more sophisticated engineering
of 2D/3D heterostructures with surface fields that exclusively repel charge
carriers from defective regions while also enabling efficient charge transfer.
It is likely that the precise manipulation of energy bands will enable
perovskite-based optoelectronics to operate at their theoretical performance
limits.Comment: Main text: 15 pages, 4 figures. Supporting Information: 31 pages, 19
figure
Photo-induced halide redistribution in organic-inorganic perovskite films.
Organic-inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced 'brightening' of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance
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