Monochromatic LEDs based on perovskite quantum dots: Opportunities and challenges

Water-stable all-inorganic CsPb1-XSnXBr2I perovskite quantum dots encapsulated in paraffin for white light-emitting diodes

Enhancing air-stability of CH3NH3PbBr3 perovskite quantum dots by in-situ growth in metal-organic frameworks and their applications in light emitting diodes

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.

Colloidal semiconductor quantum dots (QDs) have strong light absorption, continuously adjustable narrowband emission, and high photoluminescence quantum yields, thereby making them promising materials for light-emitting diodes, solar cells, detectors, and lasers. Single-QD photoluminescence spectroscopy can remove the ensemble average to reveal the structure information and exciton dynamics of QD materials at a single-particle level. The study of single-QD spectroscopy can provide guidelines for rationally designing the QDs and giving the mechanism basis for QD-based applications. We can also carry out the research of the interaction between light and single QDs on a nanoscale, and prepare QD-based single-photon sources and entangled photon sources. Here, we review the recent research progress of single-QD photoluminescence spectroscopy and exciton dynamics, mainly including photoluminescence blinking dynamics, and exciton and multi-exciton dynamics of single colloidal CdSe-based QDs and perovskite QDs. Finally, we briefly discuss the possible future development trends of single-QD spectroscopy and exciton dynamics.

I-III-VI quantum dots (QDs) and derivatives (I, III, and VI are Ag+/Cu+, Ga3+/In3+, and S2-/Se2-, respectively) are the ideal candidates to replace II-VI (e.g., CdSe) and perovskite QDs due to their nontoxicity, pure color, high photoluminescence quantum yield (PLQY), and full visible coverage. However, the chaotic cation alignment in multielement systems can easily lead to the formation of multiple surface vacancies, highlighted as VI and VVI, leading to nonradiative recombination and nonequilibrium carrier distribution, which severely limit the performance improvement of materials and devices. Here, based on Zn-Ag-In-Ga-S QDs, we construct an ultrathin indium sulfide shell that can passivate electron vacancies and convert donor/acceptor level concentrations. The optimized In-rich 2-layer indium sulfide structure not only enhances the radiative recombination rate by preventing further VS formation but also achieves the typical DAP emission enhancement, achieving a significant increase in PLQY to 86.2% at 628 nm. Moreover, the optimized structure can mitigate the lattice distortion and make the carrier distribution in the interior of the QDs more balanced. On this basis, red QD light-emitting diodes (QLEDs) with the highest external quantum efficiency (EQE; 5.32%) to date were obtained, providing a novel scheme for improving I-III-VI QD-based QLED efficiency.

Lead halide perovskite quantum dots (PeQDs) are triggering a revolution in display technologies because of their near-unity photoluminescence quantum yield (PLQY), narrow band emission, wide tunable bandgap and facile synthesis. Therefore, they have been extensively investigated as color converters or emissive layers for light-emitting diode (LED) displays. However, the poor stabilities in the phase and structure of PeQDs make them hard to be practically applie, and the ultimate goal is therefore to improve the long-term stability of PeQDs. The poor stability of PeQDs originates from their inherent ionic nature. Under external conditions (such as light, heat, water and oxygen), PeQDs will decompose through interfacial reactions, leading to obvious fluorescence quenching. Although many efforts have been made to prepare robust PeQDs that are, to some extent, resistant to water, air, light, or heat exposure, he overall reliability of PeQDs shows limited improvement under the synergistic effects of multiple factors, which are usually the case in practical LED applications. We report ultrastable CsPbBr3:Sr/PbBr(OH)/molecular sieve composites (Fig. 1) with a high photoluminescence quantum yield of 75% and high overall stability when exposed to water, blue light and heat, owing to (i) the increased formation energy of CsPbBr3 PeQDs by Sr doping and (ii) good encapsulation by PbBr(OH) and molecular sieves. These composites maintain 54% of their initial intensity after being aged at 85°C and 150 mW/cm2 for 720 min, which is superior to commercial green CdSe-based QDs. These promising composites are then used to produce Mini-LED backlights with a high luminous efficacy of 86 lm/W and a color area of ~124% of the National Television Standards Committee or 93% of the Rec. 2020 standards by combining them with red CdSe/CdS/ZnS QDs and a blue Mini-LED chip. We believe that highly reliable PeQDs can be achieved by building well-designed composite structures, thus enabling them to be practically applied in emerging lighting and displays. Figure 1

The past decade has witnessed the increasing interest in cesium lead halide perovskite quantum dots (PQDs) for their excellent optical properties with higher photoluminescence efficiency and tunable emission wavelengths widely applied in white LED, photovoltaic devices, etc. Here we report the preparation of CsPbBr3 PQDs by a facile anti-solvent method using conventional quaternary ammonium bromide (CTAB) as a double modifier-both proper alkyl group protection and bromine source donator. The as-formed PQDs are well-monodispersed cubes with a size of 10-15 nm and high photoluminescence quantum yield (PLQY) of up to 43 %. To enhance the stability of PQDs, CsPbBr3 @PS microspheres were formed by electrospraying process. The microspheres not only show excellent luminous properties, but exhibit much higher stability against air and UV light irradiation due to the super hydrophobic property of polystyrene.

Complementary ligand decorated cesium lead bromide nanoplatelets for high performance light-emitting diodes

Light-emitting diodes (LEDs) based on perovskite quantum dots have shown external quantum efficiencies (EQEs) of over 23% and narrowband emission, but suffer from limited operating stability1. Reduced-dimensional perovskites (RDPs) consisting of quantum wells (QWs) separated by organic intercalating cations show high exciton binding energies and have the potential to increase the stability and the photoluminescence quantum yield2,3. However, until now, RDP-based LEDs have exhibited lower EQEs and inferior colour purities4-6. We posit that the presence of variably confined QWs may contribute to non-radiative recombination losses and broadened emission. Here we report bright RDPs with a more monodispersed QW thickness distribution, achieved through the use of a bifunctional molecular additive that simultaneously controls the RDP polydispersity while passivating the perovskite QW surfaces. We synthesize a fluorinated triphenylphosphine oxide additive that hydrogen bonds with the organic cations, controlling their diffusion during RDP film deposition and suppressing the formation of low-thickness QWs. The phosphine oxide moiety passivates the perovskite grain boundaries via coordination bonding with unsaturated sites, which suppresses defect formation. This results in compact, smooth and uniform RDP thin films with narrowband emission and high photoluminescence quantum yield. This enables LEDs with an EQE of 25.6% with an average of 22.1 ±1.2% over 40 devices, and an operating half-life of two hours at an initial luminance of 7,200 candela per metre squared, indicating tenfold-enhanced operating stability relative to the best-known perovskite LEDs with an EQE exceeding 20%1,4-6.

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability.1 For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus low photoluminescence quantum yield (PL QY). Halide vacancy in perovskites has been widely accepted to be the main factor preventing near-unity PL QY by creating uncoordinated Pb octahedra and the subsequent localized deep trap states.2 Very recently, we have succeeded in synthesis of phase-stable and less-defect perovskite QDs, including FAPbI3 QDs, CsPbI3 QDs and Sn-Pb alloyed QDs.3-6 We have demonstrated that a high room-temperature PL QY of up to 100% can be obtained in FAPbI3 and CsPbI3 perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. This is realized by the modulation of halide precursor involved in the synthesis, in which trioctylphosphine (TOP) is used to dissolve halide salts, allowing sufficient releasing of halide ions as well as improved surface passivation. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidences the negligible electron or hole trapping pathways in our QDs, which explains such a high quantum efficiency. For Sn-Pb alloyed QDs, although the alloyed QDs possess much improved structural stability than that of the pure Pb- and Sn-based one, they still suffer from an extremely low PL QY of ~0.3%, indicative of the severe charge trapping in these Sn-containing perovskites. Such low PL QY is believed to result from the widely known facile oxidation of Sn2+ to Sn4+. To address this issue, we have demonstrated very recently that the deep-level trap states in Sn-based perovskites can be well suppressed by Na+ doping, which gives rise to a dramatically improved PL QY of ~28% without alerting their favorable electronic structure.7 X-ray photoelectron spectroscopy (XPS) studies suggest the formation of a stronger chemical interaction between I- and Sn2+ ions upon Na doping, which is in line with the results obtained by DFT calculations. On the other hand, though the reported TOP-based route has demonstrated efficacy in producing CsSnxPb1-xI3 QDs (x = 0~0.6) with superior structural stability, such synthetic protocol can only find success in making these I-related QDs, preparing those pure Cl- and Br-based analogues remains challenging because of the insufficient solubility of PbBr2 and PbCl2 in TOP. Therefore, by taking advantage of the strong coordinating ability of trioctylphosphine oxide (TOPO), we have recently reported the first synthesis of ASnxPb1-xX3 QDs (A = Cs, FA, MA) with excellent tunability in both Sn/Pb ratio and a full range of halide covering. The ability to generate all possible, unique combinations has offered great material versatility for the perovskite QD family. Further, by employing ultrafast spectroscopic techniques, we have resolved the fundamental photoexcited charge carrier dynamics in these alloyed QDs including trapping states-involved recombination, hot carrier relaxation, and charge transfer dynamics, which shall pave the way for future study for enhancing their usefulness in optoelectronic applications.8 In addition, photoexcited hot and cold carrier dynamics as well as charge transfer at the heterojunction of QD/metal oxide were systematically investigated.9 Solar cells based on these high-quality perovskite QDs exhibit power conversion efficiency of over 12%, showing great promise for practical application. We expect the successful synthesis of the “ideal” perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices in the near future. References Swarnkar, A. et al., Science 2016, Vol. 354, pp. 92-95.Alivisatos et al., J. Am. Chem. Soc. 2018, Vol. 140, pp. 17760F. Liu and Q. Shen et al., ACS Nano , Vol. 11, No. 10, pp. 10373-10383, 2017.F. Liu and Q. Shen et al., J. Am. Chem. Soc. , Vol. 139, No. 46, pp. 16708-16719, 2017.F. Liu and Q. Shen et al., J. Phys. Chem. Lett. , Vol. 9, No. 2, pp. 294-297, 2018.F. Liu and Q. Shen et al., Chem. Mater. , Vol. 31, No. 3, pp. 798-807, 2019.F. Liu and Q. Shen et al., Angew. Chem. Int. Ed. , Vol. 59, pp. 1-5, 2020.F. Liu and Q. Shen et al., Chem. Mater. , Vol. 32, No. 3, pp. 1089-1100, 2020.C. Ding and Q. Shen et al., Nano Energy 2020, 67, 104267.

Perovskite nanocrystals exhibit high photoluminescence quantum yields (PLQYs) and tunable bandgaps from ultraviolet to infrared. However, blue perovskite light‐emitting diodes (LEDs) suffer from color instability under applied bias. Developing narrow‐bandwidth deep‐blue emitters will maximize the color gamut of display technologies. Mixed anion approaches suffer from halide segregation that leads to their spectral instability. Here instead, a mixed cation strategy is employed whereby Rb+ is directly incorporated during synthesis into CsPbBr3 nanocrystals. Blue‐emitting perovskite quantum dots (QDs) with stable photoluminescence, PLQYs greater than 60%, tunable emission from 460 to 500 nm, and narrow emission linewidths (<25 nm) are reported. The strategy retains a pure bromine crystal structure resulting in color‐pure stable electroluminescence at operating voltages of up to 10 V, peak external quantum efficiencies (EQEs) of 0.87% and 0.11% for sky‐blue (490 nm), and deep‐blue (464 nm) devices. The sky‐blue devices exhibit the highest combined luminance of 93 cd m−2 at an EQE of 0.75%, the best reported to date of perovskite QD LEDs.

Deep-red CsPbI3 perovskite quantum dots (PeQDs) are essential for high-efficiency perovskite light-emitting diodes (PeLEDs) because of their high color purity and high photoluminescence quantum yield (PLQY). The synergetic strategy of indium (In) doping and chlorine (Cl) surface passivation not only partially replaced Pb2+ ions with the smaller ionic In3+ but also filled I- vacancies by Cl- on the surface, maintaining the humidity stability for more than 24 days and yielding excellent PLQY. Benefiting from this synergetic strategy, deep-red (approximately at 683 nm) CsPbI3 PeLEDs showed a maximum luminance and external quantum efficiency (EQE) of 311 cd m-2 and 8.32%, respectively.

The class of organic-inorganic lead halides with perovskite crystal structures has recently emerged as promising materials for a variety of practical optoelectronic applications. In particular, hybrid halide perovskite quantum dots possess excellent intrinsic optoelectronic properties such as high color purity (full width at half-maximum of 24.59 nm) and photoluminescence quantum yields (92.7%). In this work, we demonstrate the use of perovskite quantum dot materials as an emissive layer of hybrid light-emitting transistors. To investigate the working mechanism of perovskite quantum dots in light-emitting transistors, we investigated the electrical and optical characteristics under both p-channel and n-channel operation. Using these materials, we have achieved perovskite quantum dot light-emitting transistors with high electron mobilities of up to 12.06 cm2·V-1 s-1, high brightness of up to 1.41 × 104 cd m-2, and enhanced external quantum efficiencies of up to 1.79% operating at a source-drain potential of 40 V.

Currently, perovskite containing organometal halides have issues for limited color range and low photoluminescence (PL). In this regard, we designed split-ligand mediated re-precipitation (Split-LMRP) as a unique synthesis method for improved stability and PL by separating octylamine and oleic acid (OA), compared to a conventional method. [Accepted manuscript] Octylamine adjusted the size of the nucleus as main ligand, while OA acted only as a stabilizer. Especially, the QDs based on Split-LMRP remained PL intensity after 5 days with strong PL emission and a high PL quantum yield (PLQY) of 91.5% due to the removal of most polar solvents during re-precipitation. In addition, the size of the QDs was adjusted constantly in the range of 2-5 nm depending on the concentration of octylamine. When the perovskite QDs were used as the intermediate layer of the perovskite solar cell, performance and reproducibility of the PSC were improved by forming stable phases.

Bright red-emitting phosphors with high color purity and high photoluminescence quantum yield (PLQY) are highly demanded for the fabrication of high-performance warm-white light-emitting diodes (LEDs). Herein, we demonstrated a novel efficient Eu3+-activated Ca2LaHf2Al3O12 garnet phosphor with excellent luminescence properties for near-ultraviolet (near-UV) excited warm-white LEDs. The Ca2LaHf2Al3O12:Eu3+ phosphors exhibited an intense excitation spectrum in the near-UV region with a maximum around 394 nm, and they produced dazzling red luminescence peaking at 592, 614, 659, and 711 nm due to the 5D0 → 7FJ (J = 1-4) transitions of Eu3+ ions when the excitation wavelength was set at 394 nm. Luminescent properties have been studied as a function of Eu3+ doping concentration, and the highest emission intensity was achieved at 50 mol % Eu3+, while the dipole-dipole interaction brought the concentration quenching effect. The Ca2LaHf2Al3O12:50%Eu3+ sample exhibited CIE chromaticity coordinates of (0.6419, 0.3575) with a color purity of 92.7%, and its PLQY was measured to be 64%. The thermal stability and activation energy of Ca2LaHf2Al3O12:50%Eu3+ phosphors were also discussed and analyzed. Finally, we made a near-UV chip-based white LED device in which the Ca2LaHf2Al3O12:50%Eu3+ phosphor was utilized as a red ingredient. A bright warm-white light emission was realized from this LED device under 80 mA driving current, accompanied by a high color rendering index (CRI) of 88.3, a low correlation color temperature of 3853 K, and good CIE chromaticity coordinates of (0.3909, 0.3934). These results revealed that these red-emitting Ca2LaHf2Al3O12:Eu3+ phosphors have promising application prospect in near-UV-excited warm-white LEDs with high a CRI.