6 research outputs found
Water-Induced Dimensionality Reduction in Metal-Halide Perovskites
Metal-halide
perovskite materials are highly attractive materials
for optoelectronic applications. However, the instability of perovskite
materials caused by moisture and heat-induced degradation impairs
future prospects of using these materials. Here we employ water to
directly transform films of the three-dimensional (3D) perovskite
CsPbBr<sub>3</sub> to stable two-dimensional (2D) perovskite-related
CsPb<sub>2</sub>Br<sub>5</sub>. A sequential dissolution–recrystallization
process governs this water-induced transformation under PbBr<sub>2</sub> rich conditions. We find that these postsynthesized 2D perovskite-related
material films exhibit excellent stability against humidity and high
photoluminescence quantum yield. We believe that our results provide
a new synthetic method to generate stable 2D perovskite-related materials
that could be applicable for light emitting device applications
Water-Induced Dimensionality Reduction in Metal-Halide Perovskites
Metal-halide
perovskite materials are highly attractive materials
for optoelectronic applications. However, the instability of perovskite
materials caused by moisture and heat-induced degradation impairs
future prospects of using these materials. Here we employ water to
directly transform films of the three-dimensional (3D) perovskite
CsPbBr<sub>3</sub> to stable two-dimensional (2D) perovskite-related
CsPb<sub>2</sub>Br<sub>5</sub>. A sequential dissolution–recrystallization
process governs this water-induced transformation under PbBr<sub>2</sub> rich conditions. We find that these postsynthesized 2D perovskite-related
material films exhibit excellent stability against humidity and high
photoluminescence quantum yield. We believe that our results provide
a new synthetic method to generate stable 2D perovskite-related materials
that could be applicable for light emitting device applications
Tetrathienodibenzocarbazole Based Donor–Acceptor Type Wide Band-Gap Copolymers for Polymer Solar Cell Applications
Two
new donor–acceptor alternating wide band gap copolymers, <b>PTTDBC-PhQC8</b> and <b>PTTDBC-BTC12</b>, based on an electron-rich
tetrathienodibenzocarbazole (<b>TTDBC</b>) donor have been designed
and synthesized, wherein two additional thienyl rings are fused into
a 2,7-dithienylcarbazole skeleton to reinforce the structural coplanarity
and rigidity of polymers. The quinoid thiophene in <b>TTDBC</b> can endow not only a wide optical band gap (∼1.9 eV) but
also a deep HOMO (∼5.3 eV) for the resulting polymers. The
conventional configuration solar cells based on <b>PTTDBC-BTC12</b> exhibit a large <i>V</i><sub>oc</sub> of 0.91 V and a
power conversion efficiency of 4.30% (with a <i>J</i><sub>sc</sub> of 9.27 mA cm<sup>–2</sup> and a FF of 0.51), while
a slightly higher PCE of 4.50% can be achieved for the inverted structure
devices. We believe that these wide band gap polymers have potential
to be used for tandem cell applications
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>–</sup>, I<sup>–</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 (∼5–10
μm). 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 device’s 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
Double Charged Surface Layers in Lead Halide Perovskite Crystals
Understanding defect
chemistry, particularly ion migration, and its significant effect
on the surface’s 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