Solution-processed CsPbBr quantum-dot light-emitting diodes with a 50-fold external quantum efficiency improvement (up to 6.27%) are achieved through balancing surface passivation and carrier injection via ligand density control (treating with hexane/ethyl acetate mixed solvent), which induces the coexistence of high levels of ink stability, photoluminescence quantum yields, thin-film uniformity, and carrier-injection efficiency.

Highly efficient perovskite QLEDs can be realized when QD films possess two crucial synergistic parameters: highly luminescent features and effective electric transport properties. Regarding the emissive properties of QD films, although long organic ligands perfectly passivated the QD's surface and endowed ink with the near-unity luminescent properties with a PLQY approaching 100%, [11,12] the films generally exhibited a relatively low PLQY of about 40% due to the formation of nonradiative recombination centers. This phenomenon results from the dynamic characteristic of the bonding between the QD's surface and organic capping ligands, leading to the mismatched ligands during the film-forming process. [13,14] Meanwhile, these ligands act as electrically insulating layers on the QD's surface resulting in inefficient carrier injection and transportation, [15,16] which are detrimental to device performance. To enhance the electric properties of QD films, much attention [17,18] has been devoted to the development of ligand strategies that minimize the interparticle spacing. For example, Li et al. demonstrated an effective enhancement in electrical properties and EQE of CsPbBr 3 QLEDs through the control of surface ligand density. [9] Through ligand-exchange strategies, [8,19] a relatively short (C12) ligand, didodecyl dimethyl ammonium bromide (DDAB), was used to enhance device performance, obtaining an EQE of 8.73% under an effective washing process. Unfortunately, these methods are still based on long organic ligands, which cannot render the QD solid with ideal carrier injection and transportation features. Thus, it is significantly crucial to find an effective and feasible strategy to control the surface state of perovskite QDs, which could guarantee the high exciton recombination and carrier injection in constructing high-performance electroluminescent (EL) devices.Inorganic ligands with less space separation among particles could effectively enhance the electrical properties of QD films. [20,21] Meanwhile, they also improved the PL features through the reduce of the defect-related nonradiative recombination, which has been proven in traditional QDs. [22][23][24][25] For example, the halide-related ligands have improved the luminescent feature and radiative recombination in Cd-based QDs, which was realized by the ligand-exchange process. [20,26,27] But such a strategy is not feasible for perovskite QDs because they Perovskite quantum dots (QDs) with high photoluminescence quantum yields (PLQYs) and narrow emission peak hold promise for next-generation flexible and high-definition displays. However, perovskite QD films often suffer from low PLQYs due to the dynamic characteristics between the QD's surface and organic ligands and inefficient electrical transportation resulting from long hydrocarbon organic ligands as highly insulating barrier, which impair the ensuing device performance. Here, a general organic-inorganic hybrid ligand (OIHL) strategy is reported on to passivate perovskite QDs for highly efficient el...

Inorganic perovskite CsPbBr nanocrystals (NCs) are emerging, highly attractive light emitters with high color purity and good thermal stability for light-emitting diodes (LEDs). Their high photo/electroluminescence efficiencies are very important for fabricating efficient LEDs. Here, we propose a novel strategy to enhance the photo/electroluminescence efficiency of CsPbBr NCs through doping of heterovalent Ce ions via a facile hot-injection method. The Ce cation was chosen as the dopant for CsPbBr NCs by virtue of its similar ion radius and formation of higher energy level of conduction band with bromine in comparison with the Pb cation to maintain the integrity of perovskite structure without introducing additional trap states. It was found that by increasing the doping amount of Ce in CsPbBr NCs to 2.88% (atomic percentage of Ce compared to Pb) the photoluminescence quantum yield (PLQY) of CsPbBr NCs reached up to 89%, a factor of 2 increase in comparison with the native, undoped ones. The ultrafast transient absorption and time-resolved photoluminescence (PL) spectroscopy revealed that Ce-doping can significantly modulate the PL kinetics to enhance the PL efficiency of doped CsPbBr NCs. As a result, the LED device fabricated by adopting Ce-doped CsPbBr NCs as the emitting layers exhibited a pronounced improvement of electroluminescence with external quantum efficiency (EQE) from 1.6 to 4.4% via Ce-doping.

Developing low-cost and high-quality quantum dots (QDs) or nanocrystals (NCs) and their corresponding efficient light-emitting diodes (LEDs) is crucial for the next-generation ultra-high-definition flexible displays. Here, there is a report on a room-temperature triple-ligand surface engineering strategy to play the synergistic role of short ligands of tetraoctylammonium bromide (TOAB), didodecyldimethylammonium bromide (DDAB), and octanoic acid (OTAc) toward "ideal" perovskite QDs with a high photoluminescence quantum yield (PLQY) of >90%, unity radiative decay in its intrinsic channel, stable ink characteristics, and effective charge injection and transportation in QD films, resulting in the highly efficient QD-based LEDs (QLEDs). Furthermore, the QD films with less nonradiative recombination centers exhibit improved PL properties with a PLQY of 61% through dopant engineering in A-site. The robustness of such properties is demonstrated by the fabrication of green electroluminescent LEDs based on CsPbBr QDs with the peak external quantum efficiency (EQE) of 11.6%, and the corresponding peak internal quantum efficiency (IQE) and power efficiency are 52.2% and 44.65 lm W , respectively, which are the most-efficient perovskite QLEDs with colloidal CsPbBr QDs as emitters up to now. These results demonstrate that the as-obtained QD inks have a wide range application in future high-definition QD displays and high-quality lightings.

Novel fluorescence with highly covert and reliable features is quite desirable to combat the sophisticated counterfeiters. Herein, we report a simultaneously triple-modal fluorescent characteristic of CsPbBr@CsPbBr/SiO by the excitation of thermal, ultraviolet (UV) and infrared (IR) light for the first time, which can be applied for the multiple modal anti-counterfeiting codes. The diphasic structure CsPbBr@CsPbBr nanocrystals (NCs) was synthesized via the typical reprecipitation method followed by uniformly encapsulation into silica microspheres. Cubic CsPbBr is responsible for the functions of anti-counterfeiting, while CsPbBr crystalline and SiO are mainly to protect unstable CsPbBr NCs from being destroyed by ambient conditions. The as-prepared CsPbBr@CsPbBr/SiO NCs possess improved stability and are capable of forming printable ink with organic binders for patterns. Interestingly, the fluorescence of diphasic CsPbBr@CsPbBr/SiO capsule patterns can be reversibly switched by the heating, UV, and IR light irradiation, which has been applied as triple-modal fluorescent anti-counterfeiting codes. The results demonstrate that the perovskite@silica capsules are highly promising for myriad applications in areas such as fluorescent anti-counterfeiting, optoelectronic devices, medical diagnosis, and biological imaging.