Low Photoluminescence Quantum Yield Research Articles

N-Functionalized Graphene Quantum Dots with Ultrahigh Quantum Yield and Large Stokes Shift: Efficient Downconverters for CIGS Solar Cells

Copper indium gallium selenide (CIGS) is the most promising thin film solar cell technology. However, its high performance is hampered by its poor short-wavelength response. The short-wavelength response can be enhanced via photon downconversion using quantum dots. Unfortunately, most graphene quantum dots (GQDs) are not suitable as downconverters in CIGS cells owing to their low photoluminescence quantum yield (PL QY) and/or low Stokes shift. Herein, an ultrahigh PL QY (99%) and a large Stokes shift (98 nm) are achieved for N-doped GQDs via a novel method. The performance of a CIGS solar cell is enhanced via photon downconversion and the light-trapping effect using the NGQDs. The effectiveness of the NQGDs is manifested in a conversion efficiency (η) of 15.31%. In addition, improvements in the short-circuit current density from 30.69 mA/cm2 to 31.77 mA/cm2 and fill factor from 71.25% to 73.09% are observed. The n and J0 values are decreased by insertion of NGQDs, indicating a reduction in the recombinati...

Photoluminescence quantum yields for atomically thin-layered ReS2: Identification of indirect-bandgap semiconductors

Rhenium dichalcogenides have attracted considerable attention as new members of group VII layered semiconductor transition-metal dichalcogenides (TMDs) with respect to fundamental physics and potential applications. In this study, room-temperature photoluminescence (PL) spectra, as well as PL quantum yields (QYs) of thin-layer rhenium disulfide (ReS2), were evaluated. Low PL QYs of ∼10–4 were determined from a monolayer thickness to seven layers (1–7L) of ReS2 regardless of the layer number. These low PL QYs strongly suggest that the ReS2 is an indirect-bandgap semiconductor from a monolayer limit to the bulk, which is in contrast to those observed for group VI TMDs (MX2: M = Mo and W; X = S and Se). Our experimental findings will provide valuable information for the electronic and optical device applications in atomically thin-layered ReS2.

Lead-Free Highly Efficient Blue-Emitting Cs3 Cu2 I5 with 0D Electronic Structure.

Halide perovskites, including CsPbX3 (X = Cl, Br, I), have gained much attention in the field of optoelectronics. However, the toxicity of Pb and the low photoluminescence quantum yield (PLQY) of these perovskites hamper their use. In this work, new halide materials that meet the requirements of: (i) nontoxicity, (ii) high PLQY, and (iii) ease of fabrication of thin films via the solution process are explored. In particular, copper(I) halide compounds with low-dimensional electronic structures are considered. Cs3 Cu2 I5 has a 0D photoactive site and exhibits blue emission (≈445 nm) with very high PLQYs of ≈90 and ≈60% for single crystals and thin films, respectively. The large exciton binding energy of ≈490 meV explains well the 0D electronic nature of Cs3 Cu2 I5 . Blue electroluminescence of Pb-free halides is demonstrated using solution-derived Cs3 Cu2 I5 thin films.

Click-Derived Triazolylidenes as Chelating Ligands: Achievement of a Neutral and Luminescent Iridium(III)-Triazolide Complex.

Versatility in the synthesis of triazole derivatives was exploited to obtain convenient mesoionic carbenes working as chelating or cyclometalating ligands for the preparation of cationic or neutral iridium(III) complexes. We present the synthesis and characterization of three new cationic cyclometalating iridium(III) complexes (1-3-BF4) and a neutral one (4), equipped with functionalized triazolylidene ligands. All the complexes are obtained in good yields, present irreversible or quasi-reversible oxidation and reduction processes, and display good photophysical stability. The complexes emit from 3MLCT or 3LC states, depending on the nature of the ancillary ligand. Compounds 1-3-BF4 display very low photoluminescence quantum yields (PLQY ≈ 1% in acetonitrile solution). Density functional theory calculations show that the luminescence of these three complexes is quenched by the presence of low-lying 3MC states, leading to a reversible detachment of the neutral ancillary ligands from the metal coordination sphere. On the contrary, this nonradiative deactivation pathway is not present in the case of the neutral complex 4, which in fact shows PLQYs above 10% and is the best emitter of the series. Moreover, complex 4 represents the first reported example of a photochemically and thermally stable neutral triazolide iridium(III) complex.

Surface Passivation of Bismuth-Based Perovskite Variant Quantum Dots To Achieve Efficient Blue Emission.

Metal halide perovskite quantum dots (QDs) recently have attracted great research attentions. However, blue-emitting perovskite QDs generally suffer from low photoluminescence quantum yield (PLQY) because of easily formed defects and insufficient surface passivation. Replacement of lead with low toxicity elements is also preferred toward potential commercial applications. Here, we apply Cl-passivation to boost the PLQY of MA3Bi2Br9 QDs to 54.1% at the wavelength of 422 nm, a new PLQY record for blue emissive, lead-free perovskite QDs. Because of the incompatible crystal structures between MA3Bi2Br9 and MA3Bi2Cl9 and the careful kinetic control during the synthesis, Cl- anions are engineered to mainly locate on the surface of QDs acting as passivating ligands, which effectively suppress surface defects and enhance the PLQY. Our results highlight the potential of MA3Bi2Br9 QDs for applications of phosphors, scintillators, and light-emitting diodes.

Doping-Enhanced Short-Range Order of Perovskite Nanocrystals for Near-Unity Violet Luminescence Quantum Yield.

All-inorganic perovskite nanocrystals (NCs) have emerged as a new generation of low-cost semiconducting luminescent system for optoelectronic applications. The room-temperature photoluminescence quantum yields (PLQYs) of these NCs in the green and red spectral range approach unity. However, their PLQYs in the violet are much lower, and an insightful understanding of such poor performance remains missing. We report a general strategy for the synthesis of all-inorganic violet-emitting perovskite NCs with near-unity PLQYs through engineering local order of the lattice by nickel ion doping. A broad range of experimental characterizations, including steady-state and time-resolved luminescence spectroscopy, X-ray absorption spectra, and magic angle spinning nuclear magnetic resonance spectra, reveal that the low PLQY in undoped NCs is associated with short-range disorder of the lattice induced by intrinsic defects such as halide vacancies and that Ni doping can substantially eliminate these defects and result in increased short-range order of the lattice. Density functional theory calculations reveal that Ni doping of perovskites causes an increase of defect formation energy and does not introduce deep trap states in the band gap, which is suggested to be the main reason for the improved local structural order and near-unity PLQY. Our ability to obtain violet-emitting perovskite NCs with near-perfect properties opens the door for a range of applications in violet-emitting perovskite-based devices such as light-emitting diodes, single-photon sources, lasers, and beyond.

Highly efficient and stable blue-emitting CsPbBr3@SiO2 nanospheres through low temperature synthesis for nanoprinting and WLED

Inorganic perovskite quantum dots (QDs) have attracted wide attention in display and solid-state lighting because of their easily tunable band-gaps and high photoluminescence quantum yields (PLQY) of green light emission. However, some drawbacks limit their practical applications, including the low PLQY of blue light emission and the instability in the moisture environment. In this work, efficient blue-light emitting CsPbBr3 perovskite QDs with PLQY of 72% were developed through a bandgap engineering approach. The achieved blue-light emitting PLQY is much higher than the values acquired in the inorganic perovskite QDs in the literature. And the emission color of the as-prepared QDs can be facially tuned by only adjusting the reaction temperature. Further, the mono-dispersed perovskite QDs@SiO2 composites were constructed benefiting from the low temperature synthesis. The optical performance of the QDs could be well persisted even in the moisture environment. Finally, the as-prepared QDs@SiO2 composite was fabricated as the QD ink on the anti-counterfeit printing technology, from which the obtained pattern would emit varied color under UV lamp. And the as-prepared composites was also applied for fabricating WLED, with Commission Internationale de l’Eclairage (CIE) color coordinates of (0.33, 0.38) and power efficiency of 32.5 lm W−1, demonstrating their promising potentials in solid-state lighting.

Enhancing silicon solar cells with singlet fission: the case for Förster resonant energy transfer using a quantum dot intermediate

One way for solar cell efficiencies to overcome the Shockley–Queisser limit is downconversion of high-energy photons using singlet fission (SF) in polyacenes like tetracene (Tc). SF enables generation of multiple excitons from the high-energy photons, which can be harvested in combination with Si. In this work, we investigate the use of lead sulfide quantum dots (PbS QDs) with a band gap close to Si as an interlayer that allows Förster resonant energy transfer (FRET) from Tc to Si, a process that would be spin-forbidden without the intermediate QD step. We investigate how the conventional FRET model, most commonly applied to the description of molecular interactions, can be modified to describe the geometry of QDs between Tc and Si and how the distance between QD and Si, and the QD bandgap affects the FRET efficiency. By extending the acceptor dipole in the FRET model to a 2-D plane, and to the bulk, we see a relaxation of the distance dependence of transfer. Our results indicate that FRET efficiencies from PbS QDs to Si well above 50% are possible at very short but possibly realistic distances of around 1 nm, even for QDs with relatively low photoluminescence quantum yield.

Rubidium Doping for Enhanced Performance of Highly Efficient Formamidinium-Based Perovskite Light-Emitting Diodes.

Organometal halide perovskites (OHPs) have become the most promising optoelectronic material in the past few years with a myriad of applications in the photovoltaic, light-emitting, and laser fields. However, for light-emitting applications, the low photoluminescence quantum yield (PLQY) of OHP film is critical to hinder the efficiency improvement of OHP-film-based light-emitting diodes (PeLEDs). Herein, we study the effects of rubidium incorporation on the crystal growth, structure, and photoelectric and optical properties of formamidinium-lead-bromide-based (FAPbBr3-based) perovskite films and light-emission performance of PeLEDs. It is found that rubidium incorporation can significantly enhance the PLQY of FAPbBr3 film by suppressing the trap density and thus improve the withstand voltage as well as the performance of PeLEDs. When FAPbBr3 film with optimal Rb doping ratio is employed as the light emitter of PeLEDs, the maximum luminance and current efficiency is enhanced by ∼10-fold and ∼5-fold to 66 353 cd/m2 and 24.22 cd/A compared to the controlled device, respectively, the record performance based on FAPbBr3 PeLEDs so far. The enhanced performance can be chiefly attributed to the increase of PLQY and decrease of trap defect density of perovskite film with rubidium incorporation. Our research is expected to stimulate the development of OHPs for the next-generation lighting and display fields.

Plasmonic Oleylamine-Capped Gold and Silver Nanoparticle-Assisted Synthesis of Luminescent Alloyed CdZnSeS Quantum Dots.

We report on a novel strategy to tune the structural and optical properties of luminescent alloyed quantum dot (QD) nanocrystals using plasmonic gold (Au) and silver (Ag) nanoparticles (NPs). Alloyed CdZnSeS QDs were synthesized via the organometallic synthetic route with different fabrication strategies that involve alternative utilization of blends of organic surfactants, ligands, capping agents, and plasmonic oleylamine (OLA)-functionalized AuNPs and AgNPs. Ligand exchange with thiol l-cysteine (l-cyst) was used to prepare the hydrophilic nanocrystals. Analysis of the structural properties using powder X-ray diffraction revealed that under the same experimental condition, the plasmonic NPs altered the diffractive crystal structure of the alloyed QDs. Depending on the fabrication strategy, the crystal nature of OLA-AuNP-assisted CdZnSeS QDs was a pure hexagonal wurtzite domain and a cubic zinc-blende domain, whereas the diffraction pattern of OLA-AgNP-assisted CdZnSeS QDs was dominantly a cubic zinc-blende domain. Insights into the growth morphology of the QDs revealed a steady transformation from a heterogeneous growth pattern to a homogenous growth pattern that was strongly influenced by the plasmonic NPs. Tuning the optical properties of the alloyed QDs via plasmonic optical engineering showed that the photoluminescence (PL) quantum yield (QY) of the AuNP-assisted l-cyst-CdZnSeS QDs was tuned from 10 to 31%, whereas the PL QY of the AgNP-assisted l-cyst-CdZnSeS QDs was tuned from 15 to 90%. The low PL QY was associated with the surface defect state, while the remarkably high PL QY exhibited by the AgNP-assisted l-cyst-CdZnSeS QDs lends strong affirmation that the fabrication strategy employed in this work provides a unique opportunity to create single ensemble, multifunctional, highly fluorescent alloyed QDs for tailored biological applications.

Ultrasmall Pb:Ag2 S Quantum Dots with Uniform Particle Size and Bright Tunable Fluorescence in the NIR-II Window.

Ag2 S quantum dots (QDs) are well-known near-infrared fluorophores and have attracted great interest in biomedical labeling and imaging in the past years. However, their photoluminescence efficiency is hard to compete with Cd-, Pb-based QDs. The high Ag+ mobility in Ag2 S crystal, which causes plenty of cation deficiency and crystal defects, may be responsible mainly for the low photoluminescence quantum yield (PLQY) of Ag2 S QDs. Herein, a cation-doping strategy is presented via introducing a certain dosage of transition metal Pb2+ ions into Ag2 S nanocrystals to mitigate this intrinsic shortcoming. The Pb-doped Ag2 S QDs (designated as Pb:Ag2 S QDs) present a renovated crystal structure and significantly enhanced optical performance. Moreover, by simply adjusting the levels of Pb doping in the doped nanocrystals, Pb:Ag2 S QDs with bright emission (PLQY up to 30.2%) from 975 to 1242 nm can be prepared without altering the ultrasmall particle size (≈2.7-2.8 nm). Evidently, this cation-doping strategy facilitates both the renovation of crystal structure of Ag2 S QDs and modulation of their optical properties.

Facile access to deep red/near-infrared emissive AIEgens for efficient non-doped OLEDs.

Notwithstanding the huge demand in bio-imaging and optoelectronics, the construction of highly emissive deep red/near infrared (DR/NIR) organic luminogens is still a big challenge because a narrow energy gap generally leads to low photoluminescence quantum yield. It is even more difficult to afford DR/NIR emitters in the solid state due to the aggregation caused quenching (ACQ) effect. In this work, we found that the direct attachment of a tetraphenylethylene substituted arylamine to the electron accepting 2,1,3-benzothiadiazole produces DR/NIR AIE luminogens with bright emission facilely and efficiently. And the emission wavelengths could be tuned from the red to the DR/NIR region by regulating the variety of the substituents. The long emission wavelength and high photoluminescence quantum yield of these AIEgens are ascribed to the effective intramolecular charge transfer and the suppressed intramolecular motion. Furthermore, non-doped OLEDs based on one of the AIEgens showed an EL emission at 684 nm with a large radiance of 5772 mW Sr-1 m-2 and an impressive external quantum efficiency (EQE) of 1.73%.

All‐Inorganic Bismuth‐Based Perovskite Quantum Dots with Bright Blue Photoluminescence and Excellent Stability

AbstractLead halide perovskite quantum dots (QDs) possess color‐tunable and narrow‐band emissions and are very promising for lighting and display applications, but they suffer from lead toxicity and instability. Although lead‐free Bi‐based and Sn‐based perovskite QDs (CsSnX3, Cs2SnX6, and (CH3NH3)3Bi2X9) are reported, they all show low photoluminescence quantum yield (PLQY) and poor stability. Here, the synthesis of Cs3Bi2Br9 perovskite QDs with high PLQY and excellent stability is reported. Via a green and facile process using ethanol as the antisolvent, as‐synthesized Cs3Bi2Br9 QDs show a blue emission at 410 nm with a PLQY up to 19.4%. The whole series of Cs3Bi2X9 (X = Cl, Br, and I) QDs by mixing precursors can cover the photoluminescence emission range from 393 to 545 nm. Furthermore, Cs3Bi2Br9 QDs show excellent photostability and moisture stability due to the all‐inorganic nature and the surface passivation by BiOBr, which enables the one‐pot synthesis of Cs3Bi2Br9 QD/silica composite. A lead‐free perovskite white light‐emitting diode is fabricated by simply combining the composite of Cs3Bi2Br9 QD/silica with Y3Al5O12 phosphor. As a new member of lead‐free perovskite QDs, Cs3Bi2Br9 QDs open up a new route for the fabrication of optoelectronic devices due to their excellent stability and photophysical characteristics.

One-Step Synthesis of Acidophilic Highly-Photoluminescent Carbon Dots Modified by Ionic Liquid from Polyethylene Glycol.

Acidophilic highly-photoluminescent ionic liquid (IL)-modified carbon dots (CDs) were fabricated directly from polyethylene glycol-2000 (PEG2000N) by a simple one-step hydrothermal method in a system containing an IL (1-butyl-3-methylimidazolium bromide [C4mim]Br) and hydrochloric acid (HCl). In this process, PEG2000N works as the carbon source, [C4mim]Br as the modifier, and HCl as the accelerator. CDs with low photoluminescence (PL) intensity and quantum yields (QYs) were generated in the system without H+, but CDs with high PL intensity and QYs could be prepared after H+ was introduced. Moreover, with the increase of H+ concentration, the QYs of the prepared CDs increase subsequently, and the highest QY reaches up to 43%. The formation mechanism was explored, and the results showed that H+ changes the surface groups of the CDs generated without H+ into those that exist on the CDs generated with H+, which further improves the PL performance of the CDs. Different from most CDs reported in the literature, the as-prepared CDs can still exhibit high PL intensity even under strong acidic condition.

Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals.

With record efficiencies achieved in lead halide perovskite-based photovoltaics, urgency has shifted toward finding alternative materials that are stable and less toxic. Bismuth-based perovskite materials are currently one of the most promising candidates among those alternatives. However, the band structures of these materials, including the nature of the bandgaps, remain elusive due to extremely low photoluminescence quantum yield (PLQY) and scattering issues in their thin-film form. Here, we reveal the specific nature of the material's electronic transitions by realizing monodisperse colloidal nanocrystals (NCs) of hexagonal-phase Cs3Bi2X9 perovskites, which afford well-resolved PL features. Interestingly, the PL profile exhibits a dual-spectral feature at room temperature with comparable intensities, based on which we propose an exciton recombination process involving both indirect and direct transitions simultaneously-an observation further supported by temperature-dependent and density functional theory (DFT) calculations. Our findings provide experimental and theoretical insights into the nature of the bandgaps in bismuth halide materials-essential information for assessing their viability in solar cells and optoelectronics.

Arene‐Inserted Extended Germa[n]pericyclynes: Synthesis, Structure, and Phosphorescence Properties

This report describes the synthesis and characterization of arene-inserted extended (ArEx) germa[n]pericyclynes composed of germanium and 1,4-diethynylbenzene units. These novel cyclic germanium-π unit materials were synthesized with diethynylbenzene and germanium dichloride. X-ray crystallographic analysis revealed their structures, and the planar conformation of ArEx germa[4]pericyclyne along with the regular aromatic rings. UV/Vis absorption spectra and fluorescence emission spectra showed considerably unique and highly improved character compared to previously reported germa[n]pericyclynes. Even in the absence of transition metal components, phosphorescence emissions were observed, and the emission lifetimes were dramatically improved. ArEx germa[n]pericyclynes showed high photoluminescence quantum yields, whereas low photoluminescence quantum yields were observed for acyclic compounds. Density functional theory calculations show delocalized orbitals between skipped alkyne units through a germanium tether, and an increase in the HOMO energy level, leading to a small HOMO-LUMO energy gap.

Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS2/ZnS Core/Shell Colloidal Quantum Dots.

Copper indium sulfide (CIS) quantum dots (QDs) are attractive as labels for biomedical imaging, since they have large absorption coefficients across a broad spectral range, size- and composition-tunable photoluminescence from the visible to the near-infrared, and low toxicity. However, the application of NIR-emitting CIS QDs is still hindered by large size and shape dispersions and low photoluminescence quantum yields (PLQYs). In this work, we develop an efficient pathway to synthesize highly luminescent NIR-emitting wurtzite CIS/ZnS QDs, starting from template Cu2-xS nanocrystals (NCs), which are converted by topotactic partial Cu+ for In3+ exchange into CIS NCs. These NCs are subsequently used as cores for the overgrowth of ZnS shells (≤1 nm thick). The CIS/ZnS core/shell QDs exhibit PL tunability from the first to the second NIR window (750–1100 nm), with PLQYs ranging from 75% (at 820 nm) to 25% (at 1050 nm), and can be readily transferred to water upon exchange of the native ligands for mercaptoundecanoic acid. The resulting water-dispersible CIS/ZnS QDs possess good colloidal stability over at least 6 months and PLQYs ranging from 39% (at 820 nm) to 6% (at 1050 nm). These PLQYs are superior to those of commonly available water-soluble NIR-fluorophores (dyes and QDs), making the hydrophilic CIS/ZnS QDs developed in this work promising candidates for further application as NIR emitters in bioimaging. The hydrophobic CIS/ZnS QDs obtained immediately after the ZnS shelling are also attractive as fluorophores in luminescent solar concentrators.

Borylated Arylamine–Benzothiadiazole Donor–Acceptor Materials as Low-LUMO, Low-Band-Gap Chromophores

Fused and ladder-type benzothiadiazole–arylamine donor–acceptor C,N-chelated boron complexes were synthesized through direct electrophilic C–H borylation. The frontier molecular orbital energy levels of the borylated materials then could be modulated through variation of the exocyclic boron substituents by transmetalation with different diarylzinc reagents. The borylated materials possessed low band gaps and low LUMO energy levels with a number of examples also showing significant absorbance at >700 nm; however, low photoluminescence quantum yields were found for all these borylated compounds.

Highly luminescent silica-coated CdS/CdSe/CdS nanoparticles with strong chemical robustness and excellent thermal stability

We present facile synthesis of bright CdS/CdSe/CdS@SiO2 nanoparticles with 72% of quantum yields (QYs) retaining ca 80% of the original QYs. The main innovative point is the utilization of the highly luminescent CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) as silica coating seeds. The significance of inorganic semiconductor shell passivation and structure design of quantum dots (QDs) for obtaining bright QD@SiO2 is demonstrated by applying silica encapsulation via reverse microemulsion method to three kinds of QDs with different structure: CdSe core and 2 nm CdS shell (CdSe/CdS-thin); CdSe core and 6 nm CdS shell (CdSe/CdS-thick); and CdS core, CdSe intermediate shell and 5 nm CdS outer shell (CdS/CdSe/CdS-SQW). Silica encapsulation inevitably results in lower photoluminescence quantum yield (PL QY) than pristine QDs due to formation of surface defects. However, the retaining ratio of pristine QY is different in the three silica coated samples; for example, CdSe/CdS-thin/SiO2 shows the lowest retaining ratio (36%) while the retaining ratio of pristine PL QY in CdSe/CdS-thick/SiO2 and SQW/SiO2 is over 80% and SQW/SiO2 shows the highest resulting PL QY. Thick outermost CdS shell isolates the excitons from the defects at surface, making PL QY relatively insensitive to silica encapsulation. The bright SiO2-coated SQW sample shows robustness against harsh conditions, such as acid etching and thermal annealing. The high luminescence and long-term stability highlights the potential of using the SQW/SiO2 nanoparticles in bio-labeling or display applications.

Solid Confinement of Quantum Dots in ZIF-8 for Efficient and Stable Color-Conversion White LEDs.

The powder form and low photoluminescence quantum yield (PLQY) of fluorescent metal-organic frameworks (MOFs) present a serious obstacle to fabricating high-efficiency film-like lighting devices. Here, we present a facile way to produce thin films of CdSex S1-x /ZnS quantum dots (QDs)@ZIF-8 with high PLQY by encapsulating red, green, and blue CdSex S1-x /ZnS QDs in ZIF-8 through a one-pot solid-confinement conversion process. The QDs@ZIF-8 thin film emits warm white light with good color quality and presents good thermal stability and long-term durability.