Shorter Photoluminescence Lifetime Research Articles

Aggregation Induced Emission-Based Covalent Organic Frameworks for High-Performance Optical Wireless Communication.

Here, we report the first utilization of covalent organic frameworks (COFs) in optical wireless communication (OWC) applications. In the solid form, aggregation-induced emission (AIE) luminogen often shows promising emissive characteristics that augment radiative decays and improve fluorescence. We have synthesized an AIE-COF through the Knoevenagel condensation reaction by taking advantage of the ability to carefully design and alter the COF structure by integrating an AIE luminogen with linear building blocks. The synthesized AIE-COF exhibited a high solid-state photoluminescence quantum yield (∼39%) and a short photoluminescence lifetime (∼1 ns), crucial for achieving modulation bandwidth for high-speed OWC applications. For comparison, we constructed an aggregation-caused quenching based COF, showing a similar lifetime but almost insignificant quantum yield. The orthogonal frequency-division multiplexing modulation strategy employed by the AIE-COF demonstrates remarkable high-rate data transmission, with a wide -3 dB modulation bandwidth of nearly 200 MHz and achieving high net data rates of 825 Mb/s, outperforming traditional materials. These results open new avenues for the ability to design and finetune new COF materials for their utilization as color converters in developing cutting-edge OWC components, enabling faster and more efficient data transfer.

Microcavity Design Upping Light Extraction Efficiency over 50% in High‐Index Perovskite Light‐Emitting Diodes

AbstractPerovskite light‐emitting diodes (PeLEDs) are promising candidates for lighting and display applications. However, perovskites usually have a relatively high refractive index compared to organic semiconductors, causing a lower optical efficiency due to a narrower escape cone. In this work, the theoretical analysis shows that the microcavity effect enables PeLEDs to achieve a light extraction efficiency of 51%, much higher than the ≈20% estimated by the classical theory. Besides the interference in the microcavity, the efficiency improvement is also attributed to the low surface plasmon loss and a high ratio of horizontal dipoles under the Purcell effect. In a microcavity PeLED, a horizontal dipole shows three times the improvement in the Purcell factor of a vertical dipole, making the horizontal dipole with high efficiency contribute 86% of light output. Moreover, the experiment shows that the microcavity can increase the external quantum efficiency by 70%. A narrower spectrum and shorter photoluminescence lifetime are also observed. These phenomena are attributed to the simultaneous enhancement of light extraction efficiency and internal quantum efficiency. This work reveals the importance of microcavity design and provides guidance to break through the classical efficiency limit and achieve high‐efficiency PeLEDs.

Steady-state and dynamic characteristics of deep UV luminescence in rock salt-structured MgxZn1−xO

Temperature-dependent cathodoluminescence spectra were measured for rock salt-structured MgxZn1−xO films with x = 0.95–0.61. The Mg0.95Zn0.05O film exhibited the shortest deep UV peak wavelength of 199 nm (6.24 eV) at 6 K. Relatively high equivalent internal quantum efficiencies of 0.9%–11% were obtained. The Tauc plots, which were obtained from temperature-dependent optical transmittance measurements, exhibited large Stokes-like shifts of 0.7–0.9 eV at 6–300 K. Time-resolved photoluminescence (PL) signals at 7 K exhibited fast and slow decay components. The fast decay component had PL lifetimes of 2.59–3.08 ns, and the slow decay component far exceeded the measurement time range of 12.5 ns. The fast decay constant reflected the transfer lifetime of the photoexcited carriers to certain trapping centers. These centers were tentatively ascribed to Zn-related isoelectronic trapped-hole centers and may be a cause of the large Stokes-like shifts. The signals at 300 K exhibited very short PL lifetimes of 120–180 ps. The PL lifetimes were mainly attributed to the nonradiative recombination lifetime. Simultaneous decreases in the Zn-related isoelectronic trapped-hole centers and the nonradiative recombination centers were found to be necessary to improve the DUV emission properties of RS-MgxZn1−xO films.

Reexamination of the Giant Oscillator Strength Effect in CdSe Nanoplatelets

CdSe nanoplatelets have large extinction coefficients and, at low temperatures, very short photoluminescence lifetimes. To explain these observations, the giant oscillator strength (GOST) effect has been hypothesized to exist in such semiconductor nanoplatelets. In principle, suppression of phonon scattering can increase the area of coherent exciton motion up to the full size of the nanoplatelet, increasing oscillator strength proportional to its size. Yet at high temperatures, the measured area of exciton motion as estimated from various photophysical methods is much smaller than the size of nanoplatelets and insensitive to nanoplatelet size. To examine the emergence of this discrepancy, this work uses temperature-dependent measurements of steady-state absorption, transient optical bleaching, and the optical Stark effect. Although the excitonic oscillator strength (and size) does increase somewhat at reduced temperature, a large difference remains between the area of coherent exciton motion and the entire nanoplatelet area. These measurements indicate that the area of coherent exciton center-of-mass motion does not increase sufficiently to explain observed rapid radiative lifetimes in CdSe nanoplatelets at 3 K. Instead, the data are consistent with localization of excitons to areas much smaller than whole NPLs.

Alkyl-promoted iridium complex for high-performance deep-red phosphorescent organic light-emitting diodes

In this work, a novel deep-red (642 nm) iridium complex, Ir(bp4b)2(dend), is developed. The introduction of alkyl groups in ligands weakens the intermolecular interactions and endows Ir(bp4b)2(dend) with favorable physical properties: short photoluminescence lifetime (271.6 ns), low concentration sensitivity (10−6 to 10−4 M) and balanced charge-transporting ability. Ir(bp4b)2(dend)-doped phosphorescent device displays high efficiencies (9.62 cd/A, 12.25 lm/W, 15.29%), low roll-off of efficiencies (from 10 to 1000 cd/m2: no decrease; from 1000 to 5000 cd/m2: 10.6%), relatively low concentration sensitivity (from 5% to 15%), long device lifetime (T80: 5225 h at 100 cd/m2) and very stable electroluminescent spectra (from 1 cd/m2 to maximum luminance). This performance is comparable to those of previous record deep-red (around 640 nm) phosphorescent emitters. More importantly, compared with previous record emitters, Ir(bp4b)2(dend) reports more kinds of excellent parameters and such comprehensive deep-red emitter is rare at present.

Recent Advances in Colloidal Quantum Dots or Perovskite Quantum Dots as a Luminescent Downshifting Layer Embedded on Solar Cells.

The solar cell has a poor spectral response in the UV region, which affects its power conversion efficiency (PCE). The utilization of a luminescent downshifting (LDS) layer has been suggested to improve the spectral response of the photovoltaics in the short wavelength region through photoluminescence (PL) conversion and antireflection effects, which then enhance the PCE of the solar cell. Recently, colloidal quantum dots (CQDs) or perovskite quantum dots (PQDs) have been gaining prime importance as an LDS material due to their eminent optical characteristics, such as their wide absorption band, adjustable visible emission, short PL lifetime, and near-unity quantum yields. However, the instability of QDs that occurs under certain air, heat, and moisture conditions limits its commercialization. Thus, in this review, we will focus on the physical and optical characteristics of QDs. Further, we will discuss different synthesis approaches and the stability issues of QDs. Different approaches to improve the stability of QDs will be discussed in detail alongside the recent breakthroughs in QD-based solar cells for various applications and their current challenges. We expect that this review will provide an effective gateway for researchers to fabricate LDS-layer-based solar cells.

Short Photoluminescence Lifetimes Linked to Crystallite Dimensions, Connectivity, and Perovskite Crystal Phases.

Time-correlated single photon counting has been conducted to gain further insights into the short photoluminescence lifetimes (nanosecond) of lead iodide perovskite (MAPbI3) thin films (∼100 nm). We analyze three different morphologies, compact layer, isolated island, and connected large grain films, from 14 to 300 K using a laser excitation power of 370 nJ/cm2. Lifetime fittings from the Generalized Berberan-Santos decay model range from 0.5 to 6.5 ns, pointing to quasi-direct bandgap emission despite the three different sample strains. The high energy band emission for the isolated-island morphology shows fast recombination rate centers up to 4.8 ns–1, compared to the less than 2 ns–1 for the other two morphologies, similar to that expected in a good quality single crystal of MAPbI3. Low-temperature measurements on samples reflect a huge oscillator strength in this material where the free exciton recombination dominates, explaining the fast lifetimes, the low thermal excitation, and the thermal escape obtained.

Mechano-optical Modulation of Excitons and Carrier Recombination in Self-Assembled Halide Perovskite Quantum Dots.

Mechanically modulating optical properties of semiconductor nanocrystals and organic molecules are valuable for mechano-optical and optomechanical devices. Halide perovskites with excellent optical and electronic properties are promising for such applications. We report the mechanically changing excitons and photoluminescence of self-assembled formamidinium lead bromide (FAPbBr3) quantum dots. The as-synthesized quantum dots (3.6 nm diameter), showing blue emission and a short photoluminescence lifetime (2.6 ns), form 20-300 nm 2D and 3D self-assemblies with intense green emission in a solution or a film. The blue emission and short photoluminescence lifetime of the quantum dots are different from the delayed (ca. 550 ns) green emission from the assemblies. Thus, we consider the structure and excitonic properties of individual quantum dots differently from the self-assemblies. The blue emission and short lifetime of individual quantum dots are consistent with a weak dielectric screening of excitons or strong quantum confinement. The red-shifted emission and a long photoluminescence lifetime of the assemblies suggest a strong dielectric screening that weakens the quantum confinement, allowing excitons to split into free carriers, diffuse, and trap. The delayed emission suggests nongeminate recombination of diffusing and detrapped carriers. Interestingly, the green emission of the self-assembly blueshifts by applying a lateral mechanical force (ca. 4.65 N). Correspondingly, the photoluminescence lifetime decreases by 1 order of magnitude. These photoluminescence changes suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination. The mechanically changing emission color and lifetime of halide perovskite are promising for mechano-optical and optomechanical switches and sensors.

Alleviation of π–π* Transition Enabling Enhanced Luminescence in Emerging TpyInClx (x = 3, 5) Perovskite Single Crystals

AbstractOrganic–inorganic hybrid metal halide perovskites have played an important role in optoelectronic and photonic community owing to their remarkably high photoluminescence quantum yields and color tunability with ultrafast emission performance. Herein, two kinds of novel perovskite single crystals, i.e., TpyInCl3 and TpyInCl5, grown by means of a solution‐processed hydrothermal method are innovatively designed. Regulating the polarity of the precursor solution allows to fine‐tune the light emission from pink to blue with an extremely short photoluminescence lifetime of 2.5 ns. The density functional theory calculations reveal that the unique luminescence characteristics are associated with the contribution of the terpyridine π–π* transition and the exciton quenching in the inorganic framework, that is, InCl3/[In2Cl10]4−, wherein the reduction of the recombination path and the increase of exciton quenching can effectively increase the luminous intensity. The current study provides a new paradigm toward designing multifunctional perovskite materials with tunable structure and electron dimension in emitter devices.

Tin-based all-inorganic perovskite photodetectors fabricated by chemical vapor deposition

Tin-based perovskites are the promising alternative applications in environment-friendly optoelectronic devices when compared with Pb-based perovskites. All-inorganic tin-based perovskite (CsSnBr 3 ) films were prepared by chemical vapor deposition (CVD) method. The film deposited at 500 °C shows high crystalline quality with the grain size close to 1 μm and uniform surface coverage. The optical properties of CsSnBr 3 film were investigated by steady-state and time-resolved photoluminescence spectra in detail. The short photoluminescence lifetime may be attributed to the defects at the molecular level rather than the grain boundaries. Consequently, the photodetector based on CsSnBr 3 film exhibits good stability and high on/off ratio about 320. • Tin-based perovskite films were fabricated by chemical vapor deposition. • CsSnBr 3 film shows high crystalline quality and uniform surface coverage. • Short PL lifetime may be attributed to the defects at the molecular level. • Photodetector based on CsSnBr 3 film shows good stability and high on/off ratio.

Water-stable CsPbBr3 perovskite quantum-dot luminous fibers fabricated by centrifugal spinning for dual white light illumination and communication

Lead halide perovskite quantum dots (PQDs) display remarkable photoelectric performance. However, defects such as weak stability in air and water environments limit the development of lead halide PQDs in solid-state light applications. Herein, centrifugal spinning is used for the fabrication of stable luminous CsPbBr 3 PQD nanofibers. After immersion in water for 11 months, the PQD fibers still maintained considerable photoluminescence quantum yield, showing high stability in hostile environments. The water-stability mechanism of the fibers can be explained by the changing defect density, crystal growth of PQDs, and the molecular transformation at the fiber surface. The white LED based on the CsPbBr 3 fibers exhibits satisfying color gamut performance (128% of National Television System Committee). Due to the short photoluminescence lifetime of CsPbBr 3 PQDs, the communication potential is also considered. The CsPbBr 3 fibers obtained by centrifugal spinning present a bandwidth of 11.2 MHz, showing promising performance for solid-state light and visible light communication applications.

Red-emitting improvement of CaAlSiN3:Eu2+ phosphor-in-glass: Insight into the effect of atmospheric pressure preparation on photoluminescence properties and thermal degradation

The red-emitting phosphor-in-glass (PiG)s have been successfully prepared by a facile one-step co-sintering method on the basis of ultra-low melting Sn-P-F-O glass and CaAlSiN3: Eu2+ red phosphors synthesized under atmospheric-pressure (denoted as CASN-AP) and high-pressure (denoted as CASN-HP), respectively. The crystal structure, particle morphology, photoluminescence (PL) properties and surface states of the phosphors were investigated detail by X-ray powder diffraction (XRD), field emission electron microscopy (FESEM), fluorescence spectroscopy and X-ray photoelectron spectroscopy (XPS). Under excitation of blue light (460 nm), CASN-AP phosphor with red emission at 659 nm shows a higher luminescent intensity by 23 % than that of CASN-HP phosphor. Moreover, a shorter photoluminescence lifetime and red shift of 9 nm for emission peak can be also found for the CASN-APphosphor, which is ascribed to the increased Si-N bonds and surface Si/N ratio (1.86) demonstrated by the XPS analysis. Besides, the quantum yield and thermal degradation of both phosphors have been investigated. CASN-AP phosphor exhibits higher emission intensity because of its more emitting photons, but a lower quantum yields than CASN-HP phosphor due to its more absorbed photons. Based on the above, the low-temperature melting process for Sn-P-F-O glass does not damage the phosphor and the enhanced (≥35%) red-emitting CASN-AP PiG will be a promising candidate for essential red component in high quality white LEDs.

Accelerating the Screening of Perovskite Compositions for Photovoltaic Applications through High‐Throughput Inkjet Printing

AbstractThe exploration and optimization of numerous mixed perovskite compositions are causing a strong demand for high‐throughput synthesis. Nevertheless high‐throughput fabrication of perovskite films with representative film properties, which can efficiently screen the perovskite compositions for photovoltaic applications, has rarely been explored. A high‐throughput inkjet printing approach that can automatically fabricate perovskite films with various compositions with high reproducibility and high speed is developed. The automatic sequential printing of four precursors forms 25 mixed films in a fast and reproducible manner. The obtained bandgaps, photoluminescence (PL) peak positions, and PL lifetimes allow for the efficient screening of perovskite compositions for photovoltaic applications. To exemplify this concept, among 25 tested films, two compositions CH3NH3PbBr0.75I2.25 (MA) and (HC(NH2)2)0.75(CH3NH3)0.25PbBr0.75I2.25 (FA0.75MA0.25) with a long (237 ns) and short (49.0 ns) PL lifetime, respectively, are screened out for device investigations. As expected, the MA‐based device exhibits a much higher efficiency (19.0%) than that (15.3%) of the FA0.75MA0.25 counterpart. This efficiency improvement is mainly ascribed to a smaller dark saturate current density, a lower level of energetic disorder, more efficient charge transfer and decreased charge recombination losses, which are consistent with the much longer PL lifetime in the database.

Microsecond Carrier Lifetimes, Controlled p-Doping, and Enhanced Air Stability in Low-Bandgap Metal Halide Perovskites.

Mixed lead–tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite–perovskite tandem solar cells. Previous reports on lead–tin perovskites have typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 μs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.

Short Photoluminescence Lifetimes in Vacuum-Deposited CH3NH3PbI3 Perovskite Thin Films as a Result of Fast Diffusion of Photogenerated Charge Carriers.

It is widely accepted that a long photoluminescence (PL) lifetime in metal halide perovskite films is a crucial and favorable factor, as it ensures a large charge diffusion length leading to a high power conversion efficiency (PCE) in solar cells. It has been recently found that vacuum-evaporated CH3NH3PbI3 (eMAPI) films show very short PL lifetimes of several nanoseconds. The corresponding solar cells, however, have high photovoltage (>1.1 V) and PCEs (up to 20%). We rationalize this apparent contradiction and show that eMAPI films are characterized by a very high diffusion coefficient D, estimated from modeling the PL kinetics to exceed 1 cm2/s. Such high D values are favorable for long diffusion length as well as fast transport of carriers to film surfaces, where they recombine nonradiatively with surface recombination velocity S ∼ 104 cm/s. Possible physical origins leading to the high D values are also discussed.

Ultrathin Highly Luminescent Two‐Monolayer Colloidal CdSe Nanoplatelets

AbstractSurface effects in atomically flat colloidal CdSe nanoplatelets (NLPs) are significantly and increasingly important with their thickness being reduced to subnanometer level, generating strong surface related deep trap photoluminescence emission alongside the bandedge emission. Herein, colloidal synthesis of highly luminescent two‐monolayer (2ML) CdSe NPLs and a systematic investigation of carrier dynamics in these NPLs exhibiting broad photoluminescence emission covering the visible region with quantum yields reaching 90% in solution and 85% in a polymer matrix is shown. The astonishingly efficient Stokes‐shifted broadband photoluminescence (PL) emission with a lifetime of ≈100 ns and the extremely short PL lifetime of around 0.16 ns at the bandedge signify the participation of radiative midgap surface centers in the recombination process associated with the underpassivated Se sites. Also, a proof‐of‐concept hybrid LED employing 2ML CdSe NPLs is developed as color converters, which exhibits luminous efficacy reaching 300 lm Wopt−1. The intrinsic absorption of the 2ML CdSe NPLs (≈2.15 × 106 cm−1) reported in this study is significantly larger than that of CdSe quantum dots (≈2.8 × 105 cm−1) at their first exciton signifying the presence of giant oscillator strength and hence making them favorable candidates for next‐generation light‐emitting and light‐harvesting applications.

Excitonic and Confinement Effects of 2D Layered (C10H21NH3)2PbBr4 Single Crystals

Recognition of unusual optoelectronic properties for two-dimensional (2D) layered organic–inorganic lead(II) halide materials (CnH2n+1NH3)2PbX4 (X = I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C10H21NH3)2PbBr4 and pseudo-alloy (C10H21NH3)2PbI2Br2 were grown for photophysical evaluation. A 10-carbon alkylammonium cation was selected for investigation to provide strong dielectric screening in order to highlight quantum confinement effects of the anionic (PbX42–) semiconductor layer. Single crystals of the 2D layered (C10H21NH3)2PbBr4 compound display a characteristic free exciton with a binding energy of ca. 280 meV. Observation of a short photoluminescence lifetime of 2.8 ± 0.2 ns suggests that this electronic transition for the PbBr4-based layered material has only singlet character. Sheets of (C10H21NH3)2PbBr4 with thicknesses of a few layers were fabricated, and the dimensions were verified by AFM experiments. Excitonic emissions from (C10H21NH3)2PbBr4 and (C10H21NH3)2PbI4 exhibit relatively small spectral shifts from the bulk down to a thickness of five layers indicative of the strong confinement effect of the 10-carbon alkylammonium spacers. Single crystals of the pseudo-alloy (C10H21NH3)2PbBr2I2 give an excitonic absorption peak close to that of the tetrabromide (C10H21NH3)2PbBr4 and an emission peak with a large Stokes shift to a position similar to that of the tetraiodide (C10H21NH3)2PbI4.

Modification of far-field radiation pattern by shaping InGaN/GaN nanorods

In this work, we report on the fabrication of “golftee,” “castle,” and “pillar” shaped InGaN/GaN nanorod light-emitting diode (LED) arrays with a typical rod diameter of 200 nm based on nanoimprint lithography, dry etching, and wet etching. The photoluminescence (PL) integral intensities per active region area for “golftee,” “castle,” and “pillar” shaped nanorod samples were found to be 2.6, 1.9, and 2.2 times stronger than that of a conventional planar LED. Additionally, the far-field radiation patterns of the three different shaped nanorod samples were investigated based on angular resolved PL (ARPL) measurements. It was found that the sharp lobes appeared at certain angles in the ARPL curve of the “golftee” sample, while broad lobes were observed in the ARPL curves of the “castle” and “pillar” samples. Further analysis suggests that the shorter PL lifetime and smaller spectral width of the “golftee” sample were due to the coupling of photon modes with excitons, which also led to the observed high efficiency and directional emission pattern of the “golftee” sample. Finally, three dimensional finite difference time domain simulations were carried out to study the near-field distribution of the “golftee,” “castle,” and “pillar” shaped nanorods. The simulation results showed not only a strong enhancement of the electric field in the nanocavities of the three nanorod structures but also a reduction of the guided modes into the nanorod substrate for the “golftee” shaped structure.

Enhanced photocatalytic activity of water stable hydroxyl ammonium lead halide perovskites

Hybrid perovskite have shown a potential as structurally and chemically tunable materials with interesting optical properties. Herein, We describe the synthesis and characterization of two new perovskites, hydroxyl ammonium lead iodo chloride, OHNH3PbI2Cl (1) and hydroxyl ammonium lead chloride, OHNH3PbCl3(2) by wet-chemical route. The as-prepared (1) and (2) have shown promising optical properties with suitable band gaps 3.7eV and 3.9eV respectvely. Time resolved Photoluminescence (PL) results showed that hybride pervoskite (2) had a long lived PL whereas (1) exhibited a short PL life-time. Thermo-gravimetrical analysis (TGA) in nitrogen gas environment revealed that (1) and (2) are stable and do not show any sign of decomposition up to ~200oC. Photocatalytical performance of as-prepared materials (1) and (2) under sunlight were mointerted using textile dye direct yellow. The compound (1) degraded dye in 20min while (2) in 55min with superior recyclablity.

Effects of Annealing on the Photoluminescence Properties of Citrate-Capped YVO4:Bi3+,Eu3+ Nanophosphor

We investigated the effects of annealing on the photoluminescence (PL) properties of a citrate-capped YVO4:Bi3+,Eu3+ nanophosphor. The 300 °C annealed nanophosphor exhibited the highest PL intensity at 619.5 nm corresponding to an f–f transition of Eu3+ under near-UV excitation. Fourier transform infrared spectroscopy indicated a decrease in the amounts of adsorbed water and citrate on the nanophosphor surface with annealing. Dehydration and the increase in near-UV absorption contributed to the improved PL intensity, since water is a Eu3+ luminescence quencher. The thermal decomposition of citrate, which photoreduces vanadate under near-UV irradiation, caused the improved photostability. Annealing at ≥400 °C decreased the PL intensity. A color change from white to yellow may be attributed to absorption by byproducts, possibly formed by the segregation of YVO4:Bi3+,Eu3+. A shorter PL lifetime and lower activation energy of thermal quenching at higher annealing temperature were confirmed from PL decay curve...