Near-unity Photoluminescence Quantum Yield Research Articles

Multichromophore Molecular Design for Thermally Activated Delayed-Fluorescence Emitters with Near-Unity Photoluminescence Quantum Yields.

Three multichromophore thermally activated delayed fluorescence (TADF) molecules, p-di2CzPN, m-di2CzPN, and 1,3,5-tri2CzPN, were synthesized and characterized. These molecules were designed by connecting the TADF moiety 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN) to different positions of a central benzene ring scaffold. Three highly soluble emitters all exhibited near-quantitative photoluminescence quantum yields (ΦPL) in toluene. High ΦPLs were also achieved in doped films, 59 and 70% for p-di2CzPN and m-di2CzPN in 10 wt % DPEPO doped film, respectively, and 54% for 1,3,5-tri2CzPN in 20 wt % doped CBP films. The rate constant of reverse intersystem crossing (kRISC) for p-di2CzPN and m-di2CzPN in DPEPO films reached 1.1 × 105 and 0.7 × 105 s-1, respectively, and kRISC for 1,3,5-tri2CzPN in the CBP film reached 1.7 × 105 s-1. A solution-processed organic light-emitting diode based on 1,3,5-tri2CzPN exhibited a sky-blue emission with CIE coordinates of (0.22, 0.44) and achieved a maximum external quantum efficiency of 7.1%.

Inhibited nonradiative decay at all exciton densities in monolayer semiconductors.

Most optoelectronic devices operate at high photocarrier densities, where all semiconductors suffer from enhanced nonradiative recombination. Nonradiative processes proportionately reduce photoluminescence (PL) quantum yield (QY), a performance metric that directly dictates the maximum device efficiency. Although transition metal dichalcogenide (TMDC) monolayers exhibit near-unity PL QY at low exciton densities, nonradiative exciton-exciton annihilation (EEA) enhanced by van-Hove singularity (VHS) rapidly degrades their PL QY at high exciton densities and limits their utility in practical applications. Here, by applying small mechanical strain (less than 1%), we circumvented VHS resonance and markedly suppressed EEA in monolayer TMDCs, resulting in near-unity PL QY at all exciton densities despite the presence of a high native defect density. Our findings can enable light-emitting devices that retain high efficiency at all brightness levels.

Vacuum-assisted low-temperature growth of perovskite nanocrystals reaching near-unity photoluminescence quantum yield

In this work, luminescent perovskite nanocrystals (PNCs) were grown by the modified hot-injection method under reduced pressure at low temperatures. This modified method utilizing the property of reduction of the boiling point of the solvent under reduced pressure. This proposed method facilitates the growth of high-quality nanocrystals (NCs) without a Schlenk line and inert environment. The synthesis temperature of these PNCs using this method is reduced by 30–40 °C compared to the conventional hot-injection method. Different PNCs starting from nanoplatelets to nanocubes and nanorods were successfully grown using this method. The morphology, structural and luminescence properties of the NCs which are grown by this modified hot-injection method depend on synthesis temperature. The average lifetime of the charge carriers in PNCs is estimated from the time-resolved photoluminescence (TRPL) measurements is of the order of 25–75ns. The nanocubes synthesized at 120 °C reaches nearly 100% photoluminescence quantum yield (PLQY) within the measurement error and also exhibit excellent stability. The aged nanocrystals synthesized at 120 °C retain 83% of their initial PLQY when exposed to an ambient atmosphere for one month. Besides, we have investigated the origin of the instability of these PNCs when exposed to an ambient atmosphere. It is evident from the X-ray diffraction (XRD) measurements the degradation of the PNCs via the formation of the Cs-rich by-product Cs4PbBr6. The main reason for the formation of the Cs-rich by-product is due to the limited solubility of PbBr2 at relatively low temperatures, which results in the vacancy formation of Pb and Br during synthesis. Another reason for the formation of Cs-rich by-product Cs4PbBr6 nanospheres is due to the ligand loss from the surface of the PNC which might cause the Pb and Br defects.

Doped all-inorganic cesium zirconium halide perovskites with high-efficiency and tunable emission

Doping enables manipulation of both the electrical and optical properties of halide perovskites. Herein, we incorporated Te4+ into Cs2ZrCl6 single crystal, simultaneously preserving the vacancy-ordered structure, to obtain an efficient yellow-emitting perovskite with a near-unity photoluminescence quantum yield (PLQY ≈ 97.6%). Te4+ doping modifies the hue and emission color of pristine Cs2ZrCl6, generates new absorption channels, and successfully extends the excitation energy from < 280 nm to 360–450 nm range. Detailed spectral characterizations, including ultrafast femtosecond transient absorption measurements, reveal that the bright yellow light is derived from triplet self-trapped excitons. Moreover, further tuning doping concentration enables Te-doped Cs2ZrCl6 single crystals to exhibit efficient warm white light emission. This work provides a new perspective for the development and design of stable lead-free perovskites with highly efficient luminescence.

Near-unity blue-orange dual-emitting Mn-doped perovskite nanocrystals with metal alloying for efficient white light-emitting diodes

The tunable dual-color emitting Mn2+ doped CsPbCl3-xBrx nanocrystals (NCs) with near-unity photoluminescence quantum yield (PL QY) were synthesized through post-treatment of metal bromide at room temperature for fabrication of efficient warm white light-emitting diodes (WLEDs). Especially, the CdBr2 treated blue-orange emitting Mn doped NCs with various Mn/Pb molar feed ratios exhibit higher PL QY of 97% and longer Mn2+ PL lifetime of 0.9 ms. It is surprisingly found that the X-ray diffraction peak at 31.9° is almost not changed with increasing Br composition, meaning formation of metal alloying due to incorporation of amount of divalent cation in NCs. The strong and stable Mn2+ PL at temperature ranging from 80 K to 360 K are revealed and the temperature-dependent energy transfer efficiencies in Mn2+ doped CsPbCl1.5Br1.5 NCs are obtained. The enhancement mechanism of Mn2+ PL QY was attributed to improved energy transfer from exciton to Mn2+ d–d transition and suppressed defect state density after post-treatment. The efficient warm WLEDs with color rendering index of 90 and luminous efficacy of 92 lm/W at 10 mA were fabricated by combining blue-orange dual-emitting Mn2+ doped CsPbCl3-xBrx@SiO2 and green emissive CsPbBr3@SiO2 NCs with violet GaN chips.

Tunable Dual-Color Emission Perovskites via Post-Synthetic Modification Strategy for Near-Unity Photoluminescence Quantum Yield.

Lead halide perovskites (LHPs) with excellent performance have become promising materials for optoelectrical devices. However, as for the dual-color emission LHPs (DELHPs), the low photoluminescence quantum yield (PLQY) hinders their applications. Herein, a simple low-cost room-temperature post-synthetic modification strategy is used to achieve a near-unity PLQY of DELHPs. It is proven that ZnBr2 plays an important role as an inorganic ligand in reducing surface defects to induce a 95.4% increase in the radiative decay rate and a 99.5% decrease in the nonradiative decay rate in the treated DELHPs compared with the pristine DELHPs. The performance of the blue emission from the surface lattice is greatly improved via the modification of ZnBr2. DELHPs with different ratios of blue and green emissions are obtained by changing the specific surface area and ZnBr2 concentration. The distribution and mechanism of Zn2+ are discussed using the research model based on these DELHPs. The first example of the single-layer dual-color perovskite electroluminescence device is realized from DELHPs. This work provides a new perspective for improving the performance of DELHPs, which will greatly accelerate the development of emission materials of LHPs.

Highly stable CsPbI3:Sr2+ nanocrystals with near-unity quantum yield enabling perovskite light-emitting diodes with an external quantum efficiency of 17.1%

Removing long-chain ligands on the surface of perovskite nanocrystals (NCs) is a promising strategy to facilitate charge transport towards a high-performance perovskite light-emitting diode (PeLED). However, perovskite NCs, especially CsPbI 3 NCs, tend to decompose in the ligand removal procedure due to their poor stability of perovskite NCs and high polarity of solvents. Here, we report Sr 2+ -doped CsPbI 3 NCs that were synthesized by a modified hot injection method. These NCs show enhanced stability and near-unity photoluminescence quantum yield (PLQY, 99.8%). More importantly, the CsPbI 3 :Sr 2+ NCs still maintain a high PLQY (97.5%) after removing surface ligands by a polar solvent (ethyl acetate, EA). Furthermore, a mixture of EA and octane was used to prepare smooth and dense CsPbI 3 :Sr 2+ NC films with an exceedingly high PLQY of 73.2% and a high stability under ambient conditions. Finally, the red-emitting CsPbI 3 :1.8% Sr 2+ PeLEDs demonstrate a world-record peak external quantum efficiency (EQE) of 17.1%. Sr 2+ -doped CsPbI 3 NCs with near-unity photoluminescence quantum yield (PLQY, 99.8%) and enhanced stability were synthesized. By modifying the wetting behavior of NCs solution, uniform and smooth perovskite films were prepared, which shows the highest PLQY of 73.2%. Moreover, by employing the CsPbI 3 :1.8% Sr 2+ NC films as the emission layer, a red PeLED was fabricated with a new record peak EQE of 17.1%. • Sr 2+ -doped CsPbI 3 NCs with near-unity photoluminescence quantum yield and enhanced stability • The perovskite films were optimized by modifying the wetting behavior of NCs solution. • A CsPbI 3 : Sr 2+ NC films based PeLED was fabricated with a new record peak EQE of 17.1%.

Critical role of additive-induced molecular interaction on the operational stability of perovskite light-emitting diodes

Summary Despite rapid improvements in efficiency and brightness of perovskite light-emitting diodes (PeLEDs), the poor operational stability remains a critical challenge hindering their practical applications. Here, we demonstrate greatly improved operational stability of high-efficiency PeLEDs, enabled by incorporating dicarboxylic acids into the precursor for perovskite depositions. We reveal that the dicarboxylic acids efficiently eliminate reactive organic ingredients in perovskite emissive layers through an in situ amidation process, which is catalyzed by the alkaline zinc oxide substrate. The formed stable amides prohibit detrimental reactions between the perovskites and the charge injection layer underneath, stabilizing the perovskites and the interfacial contacts and ensuring the excellent operational stability of the resulting PeLEDs. Through rationally optimizing the amidation reaction in the perovskite emissive layers, we achieve efficient PeLEDs with a peak external quantum efficiency of 18.6% and a long half-life time of 682 h at 20 mA cm−2, presenting an important breakthrough in PeLEDs.

Correlation between the Intrinsic Photophysical Properties of the Spirobifluorene-Derived Monomer.

Spiro-molecules derived from the functional spirobifluorene core play important roles in the frontiers of diverse optoelectronics. The optoelectronics of these molecules have been intensively studied without yielding a knowledge base of precisely parameterized photophysical properties. Here, we report the precisely parameterized photophysics of spiro-OMeTAD, one prototypical optoelectronic spirobifluorene derivative. The use of a preobtained single-crystalline pure spiro-OMeTAD solid for the solution preparation allows for accurate determination of its molar absorption coefficient (ε) in its monomer form. A near-unity photoluminescence quantum yield (ΦL ∼ 99%) was observed from the monomer solution. The monomer’s photoluminescence decay follows a mono-exponential channel that results in a lifetime (τ) of ∼ 1.64 ns. Taken together ε, ΦL, and τ correlate well via the Strickler–Berg equation. The Strickler–Berg relationship among the key photophysical properties determined on spiro-OMeTAD applies for spirobifluorene derivatives, as verified in an extended test on the newly created spiro-mF. Practical issues that may lead to misparameterized photophysical properties of these molecules are emphasized. Our results of the precisely parameterized photophysical properties of the spiro-OMeTAD monomer in dilute solution serve as background references for studying the optoelectronic processes in the technically more useful thin-film form in practical optoelectronic devices.

Ultrafast dynamics of a highly-emissive zero-dimensional organic tin bromide perovskite

A zero-dimensional (0D) yellow emissive tin bromide perovskite with near-unity photoluminescence quantum yield (PLQY) was synthesized. The ultrafast excited-state dynamics of the yellow emissive perovskite was investigated by using femtosecond transient absorption spectroscopy. Two ultrafast dynamics processes, intersystem crossing (ISC) from a dark singlet self-trapped exciton (STE) state to a triplet STE state with time constant of 571 fs, and a cooling of a hot triplet STE state with time constant of 28.6 ps were unveiled. A clear mechanism depicting the growth and decay pathways of STEs in the 0D assembly was provided.

Size-tunable and stable cesium lead-bromide perovskite nanocubes with near-unity photoluminescence quantum yield.

Stable cesium lead bromide perovskite nanocrystals (NCs) showing a near-unity photoluminescence quantum yield (PLQY), narrow emission profile, and tunable fluorescence peak in the green region can be considered the ideal class of nanomaterials for optoelectronic applications. However, a general route for ensuring the desired features of the perovskite NCs is still missing. In this paper, we propose a synthetic protocol for obtaining near-unity PLQY perovskite nanocubes, ensuring their size control and, consequently, a narrow and intense emission through the modification of the reaction temperature and the suitable combination ratio of the perovskite constituting elements. The peculiarity of this protocol is represented by the dissolution of the lead precursor (PbBr2) as a consequence of the exclusive complexation with the bromide anions released by the in situ SN2 reaction between oleylamine (the only surfactant introduced in the reaction mixture) and 1-bromohexane. The obtained CsPbBr3 nanocubes exhibit variable size (ranging from 6.7 ± 0.7 nm to 15.2 ± 1.2 nm), PL maxima between 505 and 517 nm, and near-unity PLQY with a narrow emission profile (fwhm of 17–19 nm). Additionally, the NCs synthesized with this approach preserve their high PLQYs even after 90 days of storage under ambient conditions, thus displaying a remarkable optical stability. Through the rationalization of the obtained results, the proposed synthetic protocol provides a new ground for the direct preparation of differently structured perovskite NCs without resorting to any additional post-synthetic treatment for improving their emission efficiency and stability.

Poly(vinylidene fluoride)-passivated CsPbBr3 perovskite quantum dots with near-unity photoluminescence quantum yield and superior stability

A passivation strategy was demonstrated for CsPbBr3 QDs with an α-PVDF ligand, which could suppress phase transformation and enhance optical performance.

Pushing the Band Gap Envelope of Quasi-Type II Heterostructured Nanocrystals to Blue: ZnSe/ZnSe 1- X Te X /ZnSe Spherical Quantum Wells

Quasi-type II heterostructured nanocrystals (NCs) have been of particular interest due to their great potential for controlling the interplay of charge carriers. However, the lack of material choices for quasi-type II NCs restricts the accessible emission wavelength from red to near-infrared (NIR), which hinders their use in light-emitting applications that demand a wide range of visible colors. Herein, we demonstrate a new class of quasi-type II nanoemitters formulated in ZnSe/ZnSe 1- X Te X /ZnSe seed/spherical quantum well/shell heterostructures (SQWs) whose emission wavelength ranges from blue to orange. In a given geometry, ZnSe 1- X Te X emissive layers grown between the ZnSe seed and the shell layer are strained to fit into the surrounding media, and thus, the lattice mismatch between ZnSe 1- X Te X and ZnSe is effectively alleviated. In addition, composition of the ZnSe 1- X Te X emissive layer and the dimension of the ZnSe shell layer are engineered to tailor the distribution and energy of electron and hole wave functions. Benefitting from the capabilities to tune the charge carriers on demand and to form defect-free heterojunctions, ZnSe/ZnSe 1- X Te X /ZnSe/ZnS NCs show near-unity photoluminescence quantum yield ( PL QY &gt; 90 % ) in a broad range of emission wavelengths (peak PL from 450 nm to 600 nm). Finally, we exemplify dichromatic white NC-based light-emitting diodes (NC-LEDs) employing the mixed layer of blue- and yellow-emitting ZnSe/ZnSe 1- X Te X /ZnSe/ZnS SQW NCs.

Highly luminescent CH3NH3PbBr3 quantum dots with 96.5% photoluminescence quantum yield achieved by synergistic combination of single-crystal precursor and capping ligand optimization

Inorganic–organic lead halide perovskite quantum dots (PQDs) are promising for application in a wide range of optoelectronic devices due to their high photoluminescence quantum yield (PLQY), tunable band gap, and narrow emission width. Despite intense research effort being devoted to the synthesis of highly luminescent PQDs, the achievement of defect-mediated low PLQY is still challenging. For PQDs processed by the well-known ligand-assisted reprecipitation technique, defects are inevitably formed on the surface, possibly due to the highly ionic precursor solution containing disordered elemental ions. To overcome this problem, in this work, CH3NH3PbBr3 single crystals with low defect densities were prepared using the inverse temperature crystallization method and were applied as sources for PQDs. Furthermore, capping ligands with different alkyl chain lengths were utilized to reprecipitate uniform and stable PQDs. Through the synergistic effect achieved by the use of the single-crystalline precursor and capping ligand optimization, we achieved a near-unity PLQY of 96.5% and a strong green emission. In addition, the obtained highly luminescent PQDs displayed few metallic Pb surface defects and reduced Stokes shifts that will be beneficial for future device applications.

Ni2+-doped CsPbI3 perovskite nanocrystals with near-unity photoluminescence quantum yield and superior structure stability for red light-emitting devices

Room temperature phase instability of cubic CsPbI3 perovskite nanocrystals is one of the notorious limitations for practical applications in optoelectronic fields. Herein, the incorporation of Ni2+ ions into CsPbI3 lattice was successfully achieved by a modified hot-injection method using nickel acetate as doping precursor. The as-prepared Ni2+ (3.3 mol%): CsPbI3 nanocrystals exhibited an improved, near-unity (95%~100%) photoluminescence quantum yield owing to the enhanced radiative decay rate and the decreased non-radiative decay rate. Additionally, Ni-doping was demonstrated to stabilize CsPbI3 lattice and the Ni2+: CsPbI3 film and colloidal solution can retain their red luminescence up to 15 days and 7 months in atmosphere, respectively. First-principle calculations verified that the significantly improved optical performance and stability of Ni2+: CsPbI3 nanocrystals arose primarily from the increased formation energy due to the successful doping of Ni2+ in CsPbI3. Benefiting from such an effective doping strategy, the as-prepared Ni2+-doped CsPbI3 perovskite nanocrystals can function well as efficient red-light emitter toward the fabrication of high-performance perovskite LED with a peak external quantum efficiency of ~7%.

Highly efficient and air-stable Eu(II)-containing azacryptates ready for organic light-emitting diodes

Divalent europium 5d-4f transition has aroused great attention in many fields, in a way of doping Eu2+ ions into inorganic solids. However, molecular Eu2+ complexes with 5d-4f transition are thought to be too air-unstable to explore their applications. In this work, we synthesized four Eu2+-containing azacryptates EuX2-Nn (X = Br, I, n = 4, 8) and systematically studied the photophysical properties in crystalline samples and solutions. Intriguingly, the EuX2-N8 complexes exhibit near-unity photoluminescence quantum yield, good air-/thermal-stability and mechanochromic property (X = I). Furthermore, we proved the application of Eu2+ complexes in organic light-emitting diodes (OLEDs) with high efficiency and luminance. The optimized device employing EuI2-N8 as emitter has the best performance as the maximum luminance, current efficiency, and external quantum efficiency up to 25470 cd m−2, 62.4 cd A−1, and 17.7%, respectively. Our work deepens the understanding of structure-property relationship in molecular Eu2+ complexes and could inspire further research on application in OLEDs.

Thick-shell CdZnSe/ZnSe/ZnS quantum dots for bright white light-emitting diodes

Quantum dots (QDs) have become promising light sources for next-generation lightings and displays. Realizing their applications in optoelectronic devices requires that QDs should retain their superior optical performances in condense QD films. However, most QD systems suffer intense photoluminescence (PL) quenching after film formation. Here, we report device-grade thick-shell CdZnSe/ZnSe/ZnS QDs with near-unity PL quantum yield (QY) in QD solution and 82% PL QY in QD film. We demonstrate that the thick shell functions well in restraining undesired inter-dot energy transfer by separating the emitting cores in condense QD film, and therefore preserving their superior emission properties. White light-emitting diodes (WLEDs) integrated with 5 different CdZnSe/ZnSe/ZnS QDs cover the visible spectrum, delivering a color rendering index (CRI) Ra as high as 91.4, a luminous efficiency (LE) of 68.5 lm/W and a warm bright sun light with correlated color temperature (CCT) of 4138 K. In particular, the champion CdZnSe/ZnSe/ZnS based WLED exhibits a special color index R9 as high as 93.1.

Unraveling the Near-Unity Narrow-Band Green Emission in Zero-Dimensional Mn2+-Based Metal Halides: A Case Study of (C10H16N)2Zn1-xMnxBr4 Solid Solutions.

Zero-dimensional (0D) Mn2+-based metal halides are potential candidates as narrow-band green emitters, and thus it is critical to provide a structural understanding of the photophysical process. Herein, we propose that a sufficiently long Mn-Mn distance in 0D metal halides enables all Mn2+ centers to emit spontaneously, thereby leading to near-unity photoluminescence quantum yield. Taking lead-free (C10H16N)2Zn1-xMnxBr4 (x = 0-1) solid solution as an example, the Zn/Mn alloying inhibits the concentration quenching that is caused by the energy transfer of Mn2+. (C10H16N)2MnBr4 exhibits highly thermal stable luminescence even up to 150 °C with a narrow-band green emission at 518 nm and a full width at half maximum of 46 nm. The fabricated white light-emitting diode device shows a high luminous efficacy of 120 lm/W and a wide color gamut of 104% National Television System Committee standard, suggesting its potential for liquid crystal displays backlighting. These results provide a guidance for designing new narrow-band green emitters in Mn2+-based metal halides.

K2CuX3 (X = Cl, Br): All-Inorganic Lead-Free Blue Emitters with Near-Unity Photoluminescence Quantum Yield

Recently, copper(I) halides have been gaining increased attention as highly luminescent nontoxic alternatives to lead halide perovskites for optoelectronic applications. Here, we report preparation...

Quantum Dots with Highly Efficient, Stable, and Multicolor Electrochemiluminescence.

Outstanding photoluminescence (PL) and electroluminescence properties of quantum dots (QDs) promise possibilities for them to meet challenging expectations of electrochemiluminescence (ECL), which at present relies on inefficient and spectral-irresolvable emitters based on transition-metal complexes (such as Ru(bpy)32+). However, ECL is reported to be extremely sensitive to the surface traps on the QDs likely because of the spatially and temporally separated electrochemical charge injections. Results here reveal that, by engineering the interior inorganic structure (CdSe/CdS/ZnS core/shell/shell structure) and inorganic–organic interface using new synthetic methods, the trap-insensitive QDs with near-unity PL quantum yield and monoexponential PL decay dynamics in water generated narrow band-edge ECL with efficiencies about six orders of magnitude higher than that of the standard Ru(bpy)32+. The band-edge and spectrally resolved ECL from CdSe/CdS/ZnS core/shell/shell QDs demonstrated a new readout scheme using electrochemical potential. Excellent ECL performance of QDs uncovered here offer opportunities to realize the full potential of ECL for biomedical detection and diagnosis.