Photoacoustic (PA) and modulated reflectance (MR) spectroscopy have been applied to study the indirect and direct band gap for van der Waals (vdW) crystals: dichalcogenides (MoS
, MoSe
, MoTe
, HfS
, ...HfSe
, WS
, WSe
, ReS
, ReSe
, SnS
and SnSe
) and monochalcogenides (GaS, GaSe, InSe, GeS, and GeSe). It is shown that the indirect band gap can be determined by PA technique while the direct band gap can be probed by MR spectroscopy which is not sensitive to indirect optical transitions. By measuring PA and MR spectra for a given compound and comparing them with each other it is easy to conclude about the band gap character in the investigated compound and the energy difference between indirect and direct band gap. In this work such measurements, comparisons, and analyses have been performed and chemical trends in variation of indirect and direct band gap with the change in atom sizes have been discussed for proper sets of vdW crystals. It is shown that both indirect and direct band gap in vdW crystals follow the well-known chemical trends in semiconductor compounds.
Investigation of the inherent field-driven charge transport behaviour of three-dimensional lead halide perovskites has largely remained challenging, owing to undesirable ionic migration effects near ...room temperature and dipolar disorder instabilities prevalent specifically in methylammonium-and-lead-based high-performing three-dimensional perovskite compositions. Here, we address both these challenges and demonstrate that field-effect transistors based on methylammonium-free, mixed metal (Pb/Sn) perovskite compositions do not suffer from ion migration effects as notably as their pure-Pb counterparts and reliably exhibit hysteresis-free p-type transport with a mobility reaching 5.4 cm
V
s
. The reduced ion migration is visualized through photoluminescence microscopy under bias and is manifested as an activated temperature dependence of the field-effect mobility with a low activation energy (~48 meV) consistent with the presence of the shallow defects present in these materials. An understanding of the long-range electronic charge transport in these inherently doped mixed metal halide perovskites will contribute immensely towards high-performance optoelectronic devices.
Perovskite light-emitting diodes (LEDs) have attracted broad attention due to their rapidly increasing external quantum efficiencies (EQEs)
. However, most high EQEs of perovskite LEDs are reported ...at low current densities (<1 mA cm
) and low brightness. Decrease in efficiency and rapid degradation at high brightness inhibit their practical applications. Here, we demonstrate perovskite LEDs with exceptional performance at high brightness, achieved by the introduction of a multifunctional molecule that simultaneously removes non-radiative regions in the perovskite films and suppresses luminescence quenching of perovskites at the interface with charge-transport layers. The resulting LEDs emit near-infrared light at 800 nm, show a peak EQE of 23.8% at 33 mA cm
and retain EQEs more than 10% at high current densities of up to 1,000 mA cm
. In pulsed operation, they retain EQE of 16% at an ultrahigh current density of 4,000 mA cm
, along with a high radiance of more than 3,200 W s
m
. Notably, an operational half-lifetime of 32 h at an initial radiance of 107 W s
m
has been achieved, representing the best stability for perovskite LEDs having EQEs exceeding 20% at high brightness levels. The demonstration of efficient and stable perovskite LEDs at high brightness is an important step towards commercialization and opens up new opportunities beyond conventional LED technologies, such as perovskite electrically pumped lasers.
Abstract
I-V-VI
2
ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have ...slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS
2
nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >10
5
cm
−1
just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS
2
nanocrystals. These unusual features arise because of the localised, non-bonding S
p
character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.
The room temperature synthesis and photoluminescence properties of hexagonal phase NaBiF4 nanocrystals doped with lanthanide ions have been presented. An efficient infrared Stokes emission and ...upconverted photoluminescence from the rare-earth ions (Yb3+, Er3+) codoped into bismuth–fluoride nanomatrix allows for dual modality of temperature and pressure sensing in VIS and NIR spectral regions. Upconversion emission shows nearly quadratic dependence of the photoluminescence intensity on the excitation light power, confirming a two-photon induced process and allowing two-photon sensing by nanocrystals with low-power CW laser diodes. The determined temperature sensitivity in the visible range varied from 1.07%/K at 303 K down to the minimum value of 0.27%/K at 523 K. Both Stokes and anti-Stokes emission bands are affected by matrix compression, with the determined emission wavelength pressure coefficients for the NIR range lines exceeding that of ruby. These novel nanocrystals combine the simplified green chemistry synthesis approach and features of lanthanide ions, providing a unique platform for many nanophotonics and sensing applications.
Reduced dimensionality of structures such as 0D quantum dots, 1D nanorods, and 2D nanoplatelets is predicted to favor the creation of tightly bound excitons stable at room temperature, making ...experimental determination of the exciton binding energy (R x ) crucial for evaluating the performance of semiconductor nanoparticles. We propose a fully optical approach for R x determination based on a complementary combination of photoacoustic and transmission spectra, using 5.5, 4.5, and 3.5 ML CdSe nanoplatelets as a benchmark system. The absence of excitonic features in photoacoustic spectra allows for probing the band-to-band transition, leading to the band gap determination. Such an unusual effect is explained by efficient re-emission of the absorbed radiation typical for high quantum yield structures, keeping the crystal lattice from excess phonon generation. The determined exciton binding energy for CdSe nanoplatelets ranges from 130 to 230 meV, confirming the presence of robust excitons in highly confined 2D systems.
Grain size in polycrystalline halide perovskite films is known to have an impact on the optoelectronic properties of the films, but its influence on their soft structural properties and phase ...transitions is unclear. Here, temperature‐dependent X‐ray diffraction, absorption, and macro‐ and micro‐photoluminescence measurements are used to investigate the tetragonal to orthorhombic phase transition in thin methylammonium lead iodide films with grain sizes ranging from the micrometer scale down to the tens of nanometer scale. It is shown that the phase transition nominally at ≈150 K is increasingly suppressed with decreasing grain size and, in the smallest grains, the first evidence of a phase transition is only seen at temperatures as low as ≈80 K. With decreasing grain size, an increasing magnitude of the hysteresis is also seen in the structural and optoelectronic properties when cooling to, and then upon heating from, 100 K. This work reveals the remarkable sensitivity of the optoelectronic, physical, and phase properties to the local environment of the perovskite structure, which will have large ramifications for phase and defect engineering in operating devices.
Temperature‐dependent X‐ray diffraction, absorption and photoluminescence measurements on methylammonium lead iodide thin films with grain sizes ranging from the micrometer to the tens of nanometer scale reveal that the low‐temperature phase transition is increasingly suppressed with decreasing grain size. These results unveil the remarkable sensitivity of optoelectronic and structural properties to the local environment in perovskite thin films.
The family of hybrid organic–inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, from light-emitting diodes to photovoltaics to X-ray detectors. Due ...to the inert nature of most organic molecules, the inorganic sublattice generally dominates the electronic structure and therefore the optoelectronic properties of perovskites. Here, we use optically and electronically active carbazole-based Cz-C i molecules, where C i indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3 to 5, as cations in the two-dimensional (2D) perovskite structure. By investigating the photophysics and charge transport characteristics of (Cz-C i )2PbI4, we demonstrate a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, where photothermal deflection spectroscopy results remarkably reveal an organic–inorganic charge transfer state. Ultrafast transient absorption spectroscopy measurements demonstrate ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-C i molecules, the efficiency of which increases by varying the chain length from i = 5 to i = 3. The charge transfer results in long-lived carriers (10–100 ns) and quenched emission, in stark contrast to the fast (sub-ns) and efficient radiative decay of bound excitons in the more conventional 2D perovskite (PEA)2PbI4, in which phenylethylammonium (PEA) acts as an inert spacer. Electrical charge transport measurements further support enhanced interlayer coupling, showing increased out-of-plane carrier mobility from i = 5 to i = 3. This study paves the way for the rational design of 2D perovskites with combined inorganic–organic electronic properties through the wide range of functionalities available in the world of organics.
Exfoliated CrPS4 with Promising Photoconductivity Budniak, Adam K.; Killilea, Niall A.; Zelewski, Szymon J. ...
Small (Weinheim an der Bergstrasse, Germany),
01/2020, Letnik:
16, Številka:
1
Journal Article
Recenzirano
Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary ...layered semiconductor chromium thiophosphate (CrPS4) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one‐step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS4 is also liquid exfoliated, and in combination with colloidal graphene, an ink‐jet‐printed photodetector is created. This all‐printed graphene/CrPS4/graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 108 (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS4 as active components in light detection, when introduced as ink‐printable moieties with a large benefit for manufacturing.
Chromium thiophosphate (CrPS4) synthesis, characterization, exfoliation, and application are presented. Spectroscopy means describe bulk CrPS4 as a direct semiconductor with 1.30 eV bandgap at room temperature. A simple protocol for mechanical exfoliation directly onto electron microscopy grids is developed, then atomic resolution elemental maps are recorded. Liquid exfoliation of CrPS4 and converting suspension into an ink allows for the development of an all‐printed photodetector.
The phenomenon of dielectric switching has garnered considerable attention due to its potential applications in electronic and photonic devices. Typically, hybrid organic–inorganic perovskites, ...HOIPs, exhibit a binary (low–high) dielectric state transition, which, while useful, represents only the tip of the iceberg in terms of functional relevance. One way to boost the versatility of applications is the discovery of materials capable of nonbinary switching schemes, such as three-state dielectric switching. The ideal candidate for that task would exhibit a trio of attributes: two reversible, first-order phase transitions across three distinct crystal phases, minimal thermal hysteresis, and pronounced, step-like variations in dielectric permittivity, with a substantial change in its real part. Here, we demonstrate a one-dimensional lead halide perovskite with the formula (CH3)2C(H)NH3)PbI3, abbreviated as ISOPrPbI3, that fulfills these criteria and demonstrates three-state dielectric switching within a narrow temperature range of ca. 45 K. Studies on ISOPrPbI3 also revealed the polar nature of the low-temperature phase III below 266 K through pyrocurrent experiments, and the noncentrosymmetric character of the intermediate phase II and low-temperature phase III is confirmed via second harmonic generation measurements. Additionally, luminescence studies of ISOPrPbI3 have demonstrated combined broadband and narrow emission properties. The introduction of ISOPrPbI3 as a three-state dielectric switch not only addresses the limitations posed by the wide thermal gap between dielectric states in previous materials but also opens new avenues for the development of nonbinary dielectric switchable materials.
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