9 research outputs found
Thermochromic Perovskite Inks for Reversible Smart Window Applications
Thermochromic Perovskite Inks for Reversible Smart
Window Application
Zero-Dimensional Cs<sub>4</sub>PbBr<sub>6</sub> Perovskite Nanocrystals
Perovskite
nanocrystals (NCs) have become leading candidates for
solution-processed optoelectronics applications. While substantial
work has been published on 3-D perovskite phases, the NC form of the
zero-dimensional (0-D) phase of this promising class of materials
remains elusive. Here we report the synthesis of a new class of colloidal
semiconductor NCs based on Cs<sub>4</sub>PbBr<sub>6</sub>, the 0-D
perovskite, enabled through the design of a novel low-temperature
reverse microemulsion method with 85% reaction yield. These 0-D perovskite
NCs exhibit high photoluminescence quantum yield (PLQY) in the colloidal
form (PLQY: 65%), and, more importantly, in the form of thin film
(PLQY: 54%). Notably, the latter is among the highest values reported
so far for perovskite NCs in the solid form. Our work brings the 0-D
phase of perovskite into the realm of colloidal NCs with appealingly
high PLQY in the film form, which paves the way for their practical
application in real devices
Formamidinium Lead Halide Perovskite Crystals with Unprecedented Long Carrier Dynamics and Diffusion Length
State-of-the-art perovskite solar
cells with record efficiencies
were achieved by replacing methylammonium (MA) with formamidinium
(FA) in perovskite polycrystalline films. However, these films suffer
from severe structural disorder and high density of traps; thus, the
intrinsic properties of FA-based perovskites remain obscured. Here
we report the detailed optical and electrical properties of FAPbX<sub>3</sub> (where X = Br<sup>ā</sup> and I<sup>ā</sup>) single crystals. FAPbX<sub>3</sub> crystals exhibited markedly
enhanced transport compared not just to FAPbX<sub>3</sub> polycrystalline
films but also, surprisingly, to MAPbX<sub>3</sub> single crystals.
Particularly, FAPbBr<sub>3</sub> crystals displayed a 5-fold longer
carrier lifetime and 10-fold lower dark carrier concentration than
those of MAPbBr<sub>3</sub> single crystals. We report long carrier
diffusion lengthsīømuch longer than previously thoughtīøof
6.6 Ī¼m for FAPbI<sub>3</sub> and 19.0 Ī¼m for FAPbBr<sub>3</sub> crystals, the latter being one of the longest reported values
in perovskite materials. These findings are of great importance for
future integrated applications of these perovskites
Perovskite Nanocrystals as a Color Converter for Visible Light Communication
Visible light communication
(VLC) is an emerging technology that
uses light-emitting diodes (LEDs) or laser diodes for simultaneous
illumination and data communication. This technology is envisioned
to be a major part of the solution to the current bottlenecks in data
and wireless communication. However, the conventional lighting phosphors
that are typically integrated with LEDs have limited modulation bandwidth
and thus cannot provide the bandwidth required to realize the potential
of VLC. In this work, we present a promising light converter for VLC
by designing solution-processed CsPbBr<sub>3</sub> perovskite nanocrystals
(NCs) with a conventional red phosphor. The fabricated CsPbBr<sub>3</sub> NC phosphor-based white light converter exhibits an unprecedented
modulation bandwidth of 491 MHz, which is ā¼40 times greater
than that of conventional phosphors, and the capability to transmit
a high data rate of up to 2 Gbit/s. Moreover, this perovskite-enhanced
white light source combines ultrafast response characteristics with
a high color rendering index of 89 and a correlated color temperature
of 3236 K, thereby enabling dual VLC and solid-state lighting functionalities
Efficient Photon Recycling and Radiation Trapping in Cesium Lead Halide Perovskite Waveguides
Cesium
lead halide
perovskite materials have attracted considerable
attention for potential applications in lasers, light-emitting diodes,
and photodetectors. Here, we provide the experimental and theoretical
evidence for photon recycling in CsPbBr<sub>3</sub> perovskite microwires.
Using two-photon excitation, we recorded photoluminescence (PL) lifetimes
and emission spectra as a function of the lateral distance between
PL excitation and collection positions along the microwire, with separations
exceeding 100 Ī¼m. At longer separations, the PL spectrum develops
a red-shifted emission peak accompanied by an appearance of well-resolved
rise times in the PL kinetics. We developed quantitative modeling
that accounts for bimolecular recombination and photon recycling within
the microwire waveguide and is sufficient to account for the observed
decay modifications. It relies on a high radiative efficiency in CsPbBr<sub>3</sub> perovskite microwires and provides crucial information about
the potential impact of photon recycling and waveguide trapping on
optoelectronic properties of cesium lead halide perovskite materials
The Role of Surface Tension in the Crystallization of Metal Halide Perovskites
The exciting intrinsic
properties discovered in single crystals
of metal halide perovskites still await their translation into optoelectronic
devices. The poor understanding and control of the crystallization
process of these materials are current bottlenecks retarding the shift
toward single-crystal-based optoelectronics. Here we theoretically
and experimentally elucidate the role of surface tension in the rapid
synthesis of perovskite single crystals by inverse temperature crystallization.
Understanding the nucleation and growth mechanisms enabled us to exploit
surface tension to direct the growth of monocrystalline films of perovskites
(AMX<sub>3</sub>, where A = CH<sub>3</sub>NH<sub>3</sub><sup>+</sup> or MA; M = Pb<sup>2+</sup>, Sn<sup>2+</sup>; X = Br<sup>ā</sup>, I<sup>ā</sup>) on the solution surface. We achieve up to
1 cm<sup>2</sup>-sized monocrystalline films with thickness on the
order of the charge carrier diffusion length (ā¼5ā10
Ī¼m). Our work paves the way to control the crystallization process
of perovskites, including thin-film deposition, which is essential
to advance the performance benchmarks of perovskite optoelectronics
Surface Restructuring of Hybrid Perovskite Crystals
Hybrid
perovskite crystals have emerged as an important class of
semiconductors because of their remarkable performance in optoelectronics
devices. The interface structure and chemistry of these crystals are
key determinants of the deviceās performance. Unfortunately,
little is known about the intrinsic properties of the surfaces of
perovskite materials because extrinsic effects, such as complex microstructures,
processing conditions, and hydration under ambient conditions, are
thought to cause resistive losses and high leakage current in solar
cells. We reveal the intrinsic structural and optoelectronic properties
of both pristinely cleaved and aged surfaces of single crystals. We
identify surface restructuring on the aged surfaces (visualized on
the atomic-scale by scanning tunneling microscopy) that lead to compositional
and optical bandgap changes as well as degradation of carrier dynamics,
photocurrent, and solar cell device performance. The insights reported
herein clarify the key variables involved in the performance of perovskite-based
solar cells and fabrication of high-quality surface single crystals,
thus paving the way toward their future exploitation in highly efficient
solar cells
Inside Perovskites: Quantum Luminescence from Bulk Cs<sub>4</sub>PbBr<sub>6</sub> Single Crystals
Zero-dimensional
perovskite-related structures (0D-PRS) are a new
frontier of perovskite-based materials. 0D-PRS, commonly synthesized
in powder form, manifest distinctive optical properties such as strong
photoluminescence (PL), narrow emission line width, and high exciton
binding energy. These properties make 0D-PRS compelling for several
types of optoelectronic applications, including phosphor screens and
electroluminescent devices. However, it would not be possible to rationally
design the chemistry and structure of these materials, without revealing
the origins of their optical behavior, which is contradictory to the
well-studied APbX<sub>3</sub> perovskites. In this work, we synthesize
single crystals of Cs<sub>4</sub>PbBr<sub>6</sub> 0D-PRS, and investigated
the origins of their unique optical and electronic properties. The
crystals exhibit a PL quantum yield higher than 40%, the highest reported
for perovskite-based single crystals. Time-resolved and temperature
dependent PL studies, supported by DFT calculations, and structural
analysis, elucidate an emissive behavior reminiscent of a quantum
confined structure rather than a typical bulk perovskite material
Double Charged Surface Layers in Lead Halide Perovskite Crystals
Understanding defect
chemistry, particularly ion migration, and its significant effect
on the surfaceās optical and electronic properties is one of
the major challenges impeding the development of hybrid perovskite-based
devices. Here, using both experimental and theoretical approaches,
we demonstrated that the surface layers of the perovskite crystals
may acquire a high concentration of positively charged vacancies with
the complementary negatively charged halide ions pushed to the surface.
This charge separation near the surface generates an electric field
that can induce an increase of optical band gap in the surface layers
relative to the bulk. We found that the charge separation, electric
field, and the amplitude of shift in the bandgap strongly depend on
the halides and organic moieties of perovskite crystals. Our findings
reveal the peculiarity of surface effects that are currently limiting
the applications of perovskite crystals and more importantly explain
their origins, thus enabling viable surface passivation strategies
to remediate them