High Photoluminescence Quantum Efficiency Research Articles

Introductory lecture: origins and applications of efficient visible photoluminescence from silicon-based nanostructures.

A variety of silicon-based nanostructures with dimensions in the 1-5 nm range now emit tunable photoluminescence (PL) spanning the visible range. Achievement of high photoluminescence quantum efficiency (PLQY) relies critically on their surface chemistry passivation and an impressive "tool box" of options have been developed. Two distinct PL bands are dominant. The "S-Band" (red-green emission with Slow microsecond decay) has PLQY that has steadily improved from ∼3% in 1990 to 65 ± 5% by 2017. The "F-Band" (blue-yellow with Fast nanosecond decay) has reported PLQY values that have improved from ∼0.1% in 1994 to as high as ∼90% by 2016. The vast literature on both bands is surveyed and for the S-band, size-structure-PL correlations and selective photo-excitation studies are highlighted. Resonant photoexcitation and single quantum dot studies have revealed the key role of quantum confinement and the excitonic phonon-assisted nature of the radiative transitions. For the F-band, in contrast, specific phenomenological studies are highlighted that demonstrate similar emission without the presence of silicon nanostructures. Low PLQY F-band emission from pure silicon-silica core shell systems is probably associated with oxide-related defects, but ultrahigh PLQY from many lower temperature synthesis routes is likely to be from carbon-based nanostructures or chromophores, not silicon nanostructures. Potential applications for both PL bands include sensing, medical imaging, theranostics, photovoltaics, LED colour converters and nano-thermometry. Emerging "green" synthesis routes are mentioned. If scalability and cost are significantly improved then a number of other proposed uses of ultra-efficient PL from "nano-Si" could become viable in cosmetics, catalysis, security and forensics.

Fabrication of CsPbBr3 Perovskite Quantum Dots/Cellulose-Based Colorimetric Sensor: Dual-Responsive On-Site Detection of Chloride and Iodide Ions

Cesium lead bromide perovskite quantum dots (PQDs) have recently attracted much attention because of their remarkable optoelectronic properties, such as high photoluminescence quantum efficiency, n...

Effects of Material Dimensionality on the Optical Properties of CsPbBr3 Nanomaterials

Inorganic perovskites such as CsPbBr3 have attracted intensive research attention due to their excellent thermal stability and high photoluminescence quantum efficiency. With the decrease of the material’s dimension, quantum confinement effects will take place, which will induce many interesting properties. Here, quasi two-dimensional perovskite nanoplatelets (NPLs) and three-dimensional perovskite nanocrystals (NCs) have been grown, and their optical properties have been compared and discussed. It is interesting to find that the photon energy of CsPbBr3 NPLs hardly changes with temperature due to strong electron–phonon interaction. At the same time, the strong quantum confinement effect results in a smaller Stokes shift and a faster fluorescence lifetime. In addition, although the NPLs have many surface defects, they exhibit a relatively strong negative thermal quenching effect of emission. These results demonstrate the unique optical properties of perovskite NPLs that can pave the way for future nanosca...

Mononuclear Copper(I) complexes based on phenanthroline derivatives P^N^N^P tetradentate ligands: Syntheses, crystal structure, photochemical properties

Two novel heteroleptic Cu(I) complexes with the single tetradentate ligands have been designed and synthesized from phenanthroline. The complexes having one copper structure were fully characterized by NMR, ESI-MS and the single-crystal X-ray structures. Their photoelectric chemical properties were investigated by using cyclic voltammetry, UV/vis and fluorescence spectroscopy. Compared with the classic Cu(P^P)(N^N) complexes, this distorted tetrahedral coordination structure of Cu(P^N^N^P) complexes showed a wider excitation wavelength and a red-shifted about 100 nm emission peak, due to the link function between phenanthroline and phosphorus ligands. Moreover, the copper complex B having the naphthyl link groups displayed high red-emissive photoluminescence quantum efficiency (PLQY), which was up to 3.8%.

Functionalized coumarin derivatives containing aromatic-imidazole unit as organic luminescent materials

Three new coumarin derivatives containing aromatic-imidazole unit (i.e., benzo[d]imidazole, phenanthro[9,10-d]imidazole and pyreno[4,5-d]imidazole), 3-(1-phenyl-1H-benzo[d]imidazole-2-yl)-7-(diethylamino)coumarin (BI–C), 3-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole-2-yl)-7-(diethylamino)coumarin (PI–C) and 3-(9-(4-(tert-butyl)phenyl)-9H-pyreno[4,5-d]imidazole-10-yl)-7-(diethylamino)coumarin (PyI-C), were successfully prepared for organic light-emitting devices. These compounds show good thermal properties and strong emissions from greenish-blue to green lights owing to the extended π-conjugation of the rigid polyaromatic imidazole skeleton, and have high photoluminescence quantum efficiency of 94%, 97% and 98% in chloroform solutions. Among these compounds, the doped organic light-emitting device fabricated from the compound PI-C exhibited the maximum luminous efficiency of 6.07 cd/A and maximum EQE of 2.78%.

Multiple-Quantum-Well Perovskites for High-Performance Light-Emitting Diodes.

Light-emitting diodes (LEDs) based on solution-processed metal halide perovskites have shown great application potential in energy-efficient lighting and displays. Multiple-quantum-well (MQW) perovskites simultaneously possess high photoluminescence quantum efficiency and good film morphology and stability, making it attractive for high-performance perovskite LEDs. Here, merits of MQW perovskites and the progress in MQW perovskite LEDs are reviewed. Challenges and future directions of perovskite LEDs are also discussed.

Lead-Free Hybrid Metal Halides with a Green-Emissive [MnBr4] Unit as a Selective Turn-On Fluorescent Sensor for Acetone.

Organic-inorganic hybrid metal halides with zero-dimensional (0D) structure has emerged as a new class of light-emitting materials. Herein, a new lead-free compound (C9NH20)2MnBr4 has been discovered and a temperature-dependent phase transition has been identified for two phases (space group P21/c and C2/c) in which individual [MnBr4]2- anions connect with organic cations, (C9NH20+) (1-buty-1-methylpyrrolidinium+), forming periodic structure with 0D blocks. A green emission band, peaking at 528 nm with a high photoluminescence quantum efficiency (PLQE) of 81.08%, has been observed at room temperature, which is originated from the 4T1(G) to 6A1 transition of tetrahedrally coordinated Mn2+ ions, as also elaborated by density functional theory calculation. Accordingly, a fast, switchable, and highly selective fluorescent sensor platform for different organic solvents based on the luminescence of (C9NH20)2MnBr4 has been developed. We believe that the hybrid metal halides with high PLQE and the exploration of these as a fluorescence sensor will expand the applications scope of bulk 0D materials for future development.

Electrogenerated Chemiluminescence of Perovskite Quantum Dots

Metal halide perovskites have attracted immense scientific interest owing to their unique optoelectronic properties, such as high absorption coefficient, band gap tunable in the entire visible range, and high photoluminescence quantum efficiency. Perovskites have been applied to applications in solar cells, photodetectors, nanolasers, and light emitting diodes (LED) because of these unique properties. Light emitting diodes are based on the luminescence of the material. Tunable photoluminescence (PL) and electrogenerated chemiluminescence (ECL) can be obtained from perovskite quantum dots (QDs) by varying their size and shapes because of quantum confinement effect. In this study, ligand assisted precipitation method is used to synthesize lead based mixed chloride and bromide perovskite quantum dots. The absorbance, fluorescence at 450nm, lifetime and quantum yield results will be presented. The average diameter of 3.5±1nm and the d-spacing of 0.3nm was determined using HR-TEM. ECL can be obtained from a transparent electrode coated with perovskite QDs using tripropylamine (TPrA) as the co-reactant in acetonitrile as solvent, although the ECL performance of dry-coated perovskite QDs suffers from decomposition and/or dissolution of the dots into electrolyte. The ECL stability issue can be partially addressed by coating the QDs along with polystyrene film accompanied with decrease in ECL intensity due to the sluggish charge transfer through polystyrene film. A dual ECL peak spectrum with emissions at 480 nm and 580 nm is observed over the period of ECL generation and collection. Further experiments need to be done to fully understand the mechanism behind this shift.

Quasi-2D Hybrid Organic-Inorganic Perovskites: DFT Modeling Approach

Hybrid organic-inorganic perovskites (HOIPs) have developed to become highly promising solution-processable direct-bandgap semiconductors, which have exhibited high solar conversion efficiencies due to their outstanding charge carrier mobilities. HOIPs exhibit superior optical properties due to their relatively high photoluminescence quantum efficiency and high brightness emission. Furthermore, HOIPs have garnered much interest due to their tunable bandgaps; which are controlled by modifying the dimensionality and composition of the perovskites. In this study, we utilize DFT methods to investigate the emission of HOIPs in order to achieve reliable tunability of bandgap. We investigate quasi-2D perovskites which are considered to form multiple quantum wells, in which the inorganic layers act as wells and the bulky organic spacers as barriers. It is well known that the bandgap of these layered perovskites can be tuned by varying the “quantum well” thickness (corresponding to n in Aʹ2An-1BnX3n+1). While it has been shown that the size of the organic cations occupying the cuboctahedral cavity of 3D perovskites indirectly affect bandgap by altering B-X orbital overlap through octahedral distortions, the effect of the spacer size (in the interlayer region) has not been fully understood in 2D perovskites. In this study, we investigate the effects of aliphatic and aromatic spacers on the bandgap and formation energies of HOIPs. Furthermore, we characterize a correlation between the steric size of the spacer cation and band gap, which is a key characteristic for the design of optoelectronics. Overall, we find that the choice of the spacer cation can play a crucial role in tuning the emission of perovskite light-emitting diodes.

High‐gravity‐assisted synthesis of aqueous nanodispersions of organic fluorescent dyes for counterfeit labeling

AbstractOrganic fluorescent dyes have attracted wide interest because of their high photoluminescence quantum efficiencies. However, there are several application limitations arising from their hydrophobicity, poor dispersity and large particle sizes. These problems can be improved by preparing nanoparticles with a small size. Herein, we present a continuous approach to efficiently prepare an aqueous nanodispersion of water‐insoluble organic fluorescent dye Nile red (NR) with monodispersed and uniform nanoparticles (35 nm) by high‐gravity antisolvent precipitation in a rotating packed bed (RPB). In contrast, NR nanodispersions prepared using a traditional batch stirred tank (ST) had a broad size distribution (20–150 nm). Due to its small size effect and good dispersity in water, the RPB nanodispersion displayed significantly increased saturation solubility and much stronger fluorescent intensity compared to raw NR, and was obviously superior to the ST counterpart. Furthermore, NR nanodispersions were mixed with ink to draw fluorescent patterns on paper for counterfeit labeling.

The dibenzothiophene-S,S-dioxide and spirobifluorene based small molecules promote Low roll-off and Blue organic light-emitting diodes

Blue organic light-emitting diodes (OLEDs) are of great significance in the fields of flat-panel display and solid-state lighting sources. Herein, two blue small molecules, consisting of spirobifluorene and dibenzothiophene-S,S-dioxide units (FSO-37SF and FSO-28SF) were successfully synthesized by the simple palladium-catalyzed Suzuki coupling reaction. The resulting molecules exhibited excellent thermal stability with thermal decomposition temperatures over 480 °C and high photoluminescence quantum efficiency up to 95.4% in solution. Ascribing to the introduction of unique spirobifuorene in dibenzothiophene-S,S-dioxide backbone, both of FSO-37SF and FSO-28SF showed the weak intermolecular interaction. Through the electroluminescent characterization, the donor-acceptor (D-A) typed FSO-37SF in the neat device exhibited a low roll-off and stable blue emission with Commission International de L’Eclairge (CIE) coordinates of (0.19, 0.14). Besides, the FSO-28SF in the neat device obtained a low turn-on voltage of 3.0 V in spite of broad emission. However, in the doping device with mCP as host, the FSO-28SF displayed higher luminance efficiency with CIE coordinates of (0.16, 0.06) and a narrow full-width at half maximum (FWHM) of 60 nm, which was very close to the CIE of the high-definition television (HDTV) standard blue.

Efficient Quantum Dot Light-Emitting Diodes Based on Trioctylphosphine Oxide-Passivated Organometallic Halide Perovskites.

Metal halide perovskite quantum dots (QDs) have attracted significant research interest in the next-generation display and solid illumination fields due to their excellent optical properties of high photoluminescence quantum efficiency, high color purity, obvious quantum confinement effect, and large exciton binding energy. A large amount of surface defects and nonradiative recombination induced by these defects are considered as major problems to be resolved urgently for practical applications of perovskite QDs in high-efficiency light-emitting diodes (LEDs). Herein, we report an efficient passivation of green perovskite QD CH3NH3PbBr3 with trioctylphosphine oxide (TOPO). By simply adding the appropriate amount of TOPO into the nonpolar toluene solvent to synthesize CH3NH3PbBr3 QDs, the surface defects of these as-synthesized perovskite QDs are obviously reduced, along with an increased photoluminescence lifetime and suppressed nonradiative recombination. Further investigation indicates that electronegative oxygen from TOPO (Lewis base) bonds with uncoordinated Pb2+ ions and labile lead atoms in perovskite. With TOPO passivation, the green perovskite QD LEDs based on CH3NH3PbBr3 show significant performance improvement factors of 93.5, 161.1, and 168.9% for luminance, current efficiency, and external quantum efficiency, respectively, reaching values of 1635 cd m–2, 5.51 cd A–1, and 1.64% in the eventual optimized devices. Furthermore, the presence of TOPO dramatically improves stabilities of CH3NH3PbBr3 QDs and related devices. Our work provides a robust platform for the fabrication of low-defect-density perovskite QDs and efficient, stable perovskite QD LEDs.

Identifying and Reducing Interfacial Losses to Enhance Color-Pure Electroluminescence in Blue-Emitting Perovskite Nanoplatelet Light-Emitting Diodes.

Perovskite nanoplatelets (NPls) hold promise for light-emitting applications, having achieved photoluminescence quantum efficiencies approaching unity in the blue wavelength range, where other metal-halide perovskites have typically been ineffective. However, the external quantum efficiencies (EQEs) of blue-emitting NPl light-emitting diodes (LEDs) have reached only 0.12%. In this work, we show that NPl LEDs are primarily limited by a poor electronic interface between the emitter and hole injector. We show that the NPls have remarkably deep ionization potentials (≥6.5 eV), leading to large barriers for hole injection, as well as substantial nonradiative decay at the NPl/hole-injector interface. We find that an effective way to reduce these nonradiative losses is by using poly(triarylamine) interlayers, which lead to an increase in the EQE of the blue (464 nm emission wavelength) and sky-blue (489 nm emission wavelength) LEDs to 0.3% and 0.55%, respectively. Our work also identifies the key challenges for further efficiency increases.

Alkali Chlorides for the Suppression of the Interfacial Recombination in Inverted Planar Perovskite Solar Cells

AbstractIn this work, significant suppression of the interfacial recombination by facile alkali chloride interface modification of the NiOx hole transport layer in inverted planar perovskite solar cells is achieved. Experimental and theoretical results reveal that the alkali chloride interface modification results in improved ordering of the perovskite films, which in turn reduces defect/trap density, causing reduced interfacial recombination. This leads to a significant improvement in the open‐circuit voltage from 1.07 eV for pristine NiOx to 1.15 eV for KCl‐treated NiOx, resulting in a power conversion efficiency approaching 21%. Furthermore, the suppression of the ion diffusion in the devices is observed, as evidenced by stable photoluminescence (PL) under illumination and high PL quantum efficiency with alkali chloride treatment, as opposed to the luminescence enhancement and low PL quantum efficiency observed for perovskite on pristine NiOx. The suppressed ion diffusion is also consistent with improved stability of the devices with KCl‐treated NiOx. Thus, it is demonstrated that a simple interfacial modification is an effective method to not only suppress interfacial recombination but also to suppress ion migration in the layers deposited on the modified interface due to improved interface ordering and reduced defect density.

Modulation of Excited State Property Based on Benzo[a, c]phenazine Acceptor: Three Typical Excited States and Electroluminescence Performance.

Throwing light upon the structure-property relationship of the excited state properties for next-generation fluorescent materials is crucial for the organic light emitting diode (OLED) field. Herein, we designed and synthesized three donor-acceptor (D-A) structure compounds based on a strong spin orbit coupling (SOC) acceptor benzo[a, c]phenazine (DPPZ) to research on the three typical types of excited states, namely, the locally-excited (LE) dominated excited state (CZP-DPPZ), the hybridized local and charge-transfer (HLCT) state (TPA-DPPZ), and the charge-transfer (CT) dominated state with TADF characteristics (PXZ-DPPZ). A theoretical combined experimental research was adopted for the excited state properties and their regulation methods of the three compounds. Benefiting from the HLCT character, TPA-DPPZ achieves the best non-doped device performance with maximum brightness of 61,951 cd m−2 and maximum external quantum efficiency of 3.42%, with both high photoluminescence quantum efficiency of 40.2% and high exciton utilization of 42.8%. Additionally, for the doped OLED, PXZ-DPPZ can achieve a max EQE of 9.35%, due to a suppressed triplet quenching and an enhanced SOC.

How β-Phase Content Moderates Chain Conjugation and Energy Transfer in Polyfluorene Films.

Poly(9,9-dioctylfluorene) (PFO) is a blue-light-emitting polymer exhibiting two distinct phases, namely, the disordered "glassy" phase and a more ordered β-phase. We investigate how a systematic increase in the fraction of β-phase present in PFO films controls chain conformation, photoluminescence quantum efficiency (PLQE), and the resonant energy transfer from the glassy to the β-phase. All films are prepared by the same technique, using paraffin oil as an additive to the spin-coating solution, allowing systematic tuning of the β-phase fraction. The PFO films exhibit high PLQE with values increasing to 0.72 for increasing fractions of β-phase present, with the β-phase chain conformation becoming more planar and including more repeat units. Differences in Förster radii calculated from the overlap of steady-state absorptance and emission spectra and from time-resolved ultrafast photoluminescence transients indicate that exciton diffusion within the glassy phase plays an important role in the energy transfer process.

Understanding the atomic and electronic structures origin of defect luminescence of CdSe quantum dots in glass matrix

AbstractWell‐defined density functional theory (DFT) calculations are performed as the first exploratory study for the atomic and electronic mechanism of defect mediated morphology and optical properties of CdnSen (n = 3, 10, 13, and 33) quantum dots (QDs) in inorganic amorphous matrix. The intrinsic defects of pristine CdSe QDs, and the interfacial defects between the QDs and surrounding amorphous matrix, were systematically studied. The calculated electronic structure suggested that the pristine CdSe QDs capped by the structural modifiers or non‐bridging oxygen in the amorphous matrix gave rise to the structure reconstruction and paired defect states at the edge of the valence and conduction bands. The orbital analysis elucidated that the redistribution of the majority of HOMO and LUMO electron density was localized over the bonds formed by capping atoms and QDs. These changes in the electronic structures were further demonstrated by CdSe QDs embedded sodium silicate glasses. It turned out that Se atoms at QDs/glass interface were much more active than those found on the surface of organically passivated CdSe QDs. The results serve as a new paradigm in materials research to explore structural origins of defect emission from QDs and a new strategy to develop glasses containing QDs with high photoluminescence quantum efficiency.

Novel Ir(III) complexes ligated with 2-(2,6-difluoropyridin-3-yl)benzo[d]thiazole for highly efficient OLEDs with mild efficiency roll-off

2-phenylbenzothiazole-based Ir(III) complex [(bt)2Ir(acac)] is a class of typical yellow phosphorescence material, which has been widely used in OLEDs. However, (bt)2Ir(acac) exhibited only mild photoluminescence quantum efficiency (PLQY) of 0.26–0.28. To further improve the luminous efficiency of benzothiazole-based Ir(III) complex and then boost the corresponding electroluminescent performance, (fpbt)2Ir(pic) and (fpbt)2Ir(tftp) were successfully synthesized, where 2-(2,6-difluoropyridin-3-yl)benzo[d]thiazole (fpbt) was applied as C∩N main ligand, and 2-pyridine carboxylic acid (pic), 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine (tftp) acted as different auxiliary ligands. Both (fpbt)2Ir(pic) and (fpbt)2Ir(tftp) presented excellent PLQYs (0.93–0.95) and high thermostability (330–360 °C). The vacuum-deposited devices following the architecture of ITO/TAPC/CBP: dopant/TPBi/Liq/Al demonstrated prominent performances. Of them, (fpbt)2Ir(pic)-doped OLEDs exhibited better efficiencies of 20.6%, 68.8 cd A−1, and 40.8 l m W−1 while maintaining very mild efficiency roll-off. Such outstanding electrophosphorescent performances were further investigated, which was probably attributed to the following factors: 1) high PLQY; 2) good electron mobility; 3) uniform film-forming property.

Synthesis, Photophysical Properties and Two-Photon Absorption Study of Tetraazachrysene-based N-Heteroacenes.

Three novel N-heteroacene molecules (SDNU-1, SDNU-2 and SDNU-3) based on tetraazachrysene units as cores have been designed, synthesized and fully characterized. Their photophysical, electrochemical and fluorescence properties were investigated, and they exhibited blue to green emission in the solid state. Interestingly, SDNU-2 exhibited high solid photoluminescence quantum efficiencies (75.3 %), which is the highest value of N-heteroacenes derivatives to date. Two-photon absorption studies have been conducted by using the open and close aperture Z-san technique. SDNU-3 showed a significant enhancement in the two-photon absorption cross-section with magnitudes as high as about 700 GM (1 GM=1×10-50 cm4 s/photon) when excited with 800 nm light, which is the largest value based on a heteroacene system measured by using a Z-scan experiment so far. We attribute the outcome to sufficient electronic coupling between the strong charge transfer of quadrupolar substituents and the tetraazachrysene core. Our result would provide a new guideline to design novel efficient two-photon materials based on N-heteroacene cores.

Improving performance of Cs-based perovskite light-emitting diodes by dual additives consisting of polar polymer and n-type small molecule

Abstract Organic-inorganic halide perovskites were emerging as promising light emitters due to the high photoluminescence quantum efficiency (PLQE), narrow color spectrum, and facile bandgap tunability. However, the morphological issues of this class of materials remain as a bottleneck in generating high-performance light-emitting devices (LED). In this work, we describe a facile and cost-effective method to improve the morphology of an all-inorganic-perovskite emissive layer by using a dual-additive blending strategy. A polar polymer, polyethylene oxide (PEO), and an n-type small molecule, TPBi, are blended with CsPbBr3 to provide synchronous effect in passivizing the surface defects and reducing the exciton quenching, as evidenced by the formation of smaller uniform grains and improved radiative recombination lifetime. Consequently, the optimized CsPbBr3/PEO/TPBi LED reaches maximum luminance, current efficiency, and external quantum efficiency of 6807 cd m−2 (at 6 V), 0.86 cd A−1 (at 5.5 V) and 0.25% (at 5.5 V), far surpassing the performance of the control CsPbBr3 device. On this basis, we further demonstrated a proof-of-concept LED device with touch-responsivity by using a flexible composite electrode. The merit of dual-additive strategy has been clearly manifested and will facilitate the future development of cost-effective solution processable perovskite LEDs.