2435wileyonlinelibrary.com IntroductionQuantum dots (QDs), combined with unique optical, electrical properties and solution processed functional applications, have been studied intensively for decades. [ 1,2 ] Recently, Kovalenko and co-workers and Li and co-workers developed CsPbX 3 (X = Cl, Br, I) inorganic perovskite quantum dots (IPQDs), which exhibited ultrahigh photoluminescence (PL) quantum yields (QYs), lowthreshold lasing, and multicolor electroluminescence. However, the usual synthesis needs high temperature, inert gas protection, and localized injection operation, which are severely against applications. Moreover, the so unexpectedly high QYs are very confusing. Here, for the fi rst time, the IPQDs' roomtemperature (RT) synthesis, superior PL, underlying origins and potentials in lighting and displays are reported. The synthesis is designed according to supersaturated recrystallization (SR), which is operated at RT, within few seconds, free from inert gas and injection operation. Although formed at RT, IPQDs' PLs have QYs of 80%, 95%, 70%, and FWHMs of 35, 20, and 18 nm for red, green, and blue emissions. As to the origins, the observed 40 meV exciton binding energy, halogen self-passivation effect, and CsPbX 3 @X quantumwell band alignment are proposed to guarantee the excitons generation and high-rate radiative recombination at RT. Moreover, such superior optical merits endow them with promising potentials in lighting and displays, which are primarily demonstrated by the white light-emitting diodes with tunable color temperature and wide color gamut.

Recently, the pursuit of high photoluminescence quantum yields (PLQYs) for blue emission in perovskite nanocrystals (NCs) has attracted increased attention because the QY of blue NCs lags behind those of green and red ones severely, which is fatal for three-primary-color displays. Here, we propose an in situ PbBr 6 4− octahedra passivation strategy to achieve a 96% absolute QY for the ultrapure (line width = 12 nm) blue emission from CsPbBr 3 nanoplatelets (NPLs), and both values rank first among perovskite NCs with blue emission. From the aspect of constructing intact PbBr 6 4− octahedra, additional Br − was introduced to drive the ionic equilibrium to form intact Pb−Br octahedra. The reduced Br vacancy and inhibited nonradiative recombination processes are well proved by reduced Urbach energy, increased Pb−Br bonds, and slower transient absorption delay. Blue light-emitting diodes (LEDs) using NPLs were fabricated, and a high external quantum efficiency (EQE) of 0.124% with an emission line width of ∼12 nm was realized. This work will provide good references to break the "blue-wall" in perovskite NCs.

The recent success of organometallic halide perovskites (OHPs) in photovoltaic devices has triggered lots of corresponding research and many perovskite analogues have been developed to look for devices with comparable performance but better stability. Upon the preparation of all inorganic halide perovskite nanocrystals (IHP NCs), research activities have soared due to their better stability, ultrahigh photoluminescence quantum yield (PL QY), and composition dependent luminescence covering the whole visible region with narrow line-width. They are expected to be promising materials for next generation lighting and display, and many other applications. Within two years, a lot of interesting results have been observed. Here, the synthesis of IHPs is reviewed, and their progresses in optoelectronic devices and optical applications, such as light-emitting diodes (LEDs), photodetectors (PDs), solar cells (SCs), and lasing, is presented. Information and recent understanding of their crystal structures and morphology modulations are addressed. Finally, a brief outlook is given, highlighting the presently main problems and their possible solutions and future development directions.

bonds. [18,19] The formation of hydrogen bonds reduces the coulombic interactions between lead and bromide ions, and then ammonium ligands will fall off from the QD surface along with bromide ions (Scheme 1a left). [20] On the other hand, the deprotonated ammonium ligands cannot interact with the CsPbBr 3 QDs.Therefore, the use of a single ligand without amine groups and reversible processes could overcome the purification stability problem, as shown recently by Yassitepe et al, who developed an aminefree method using only oleic acid (OA) as ligands. [21] The resulting CsPbBr 3 QDs could be washed several times without size increase or emission shift (Scheme 1a middle); however, the QY of these QDs was relatively low and the long-term stability was poor. The low QY results from the presence of enormous surface bromide vacancies (V Br ), since V Br exhibited obvious negative exciton trapping effect (Scheme 1b middle). [22] The groups of both Xia and Alivisatos recently confirmed the effect of V Br in their very recent works and near 100% QY can be achieved swimmingly by compensating Br ions with any bromides during or after synthesis (Scheme 1b left). [23,24] Hence, it seems contradictory that considering the QY, surface V Br should be avoided, while on account of good stability, ammonium which interacts with Br atoms should be excluded. To reduce the V Br density and reversible protonation, Kovalenko et al. introduced several kinds of zwitterionic ligands. [15,25] CsPbBr 3 QDs maintained high QY values of ≈80% and exhibited good purification stability. However, the preparation procedures were complicated, and even required precursor synthesis. Tan et al. reported the preparation of highly luminescent and stable CsPbBr 3 QDs prepared with addition of octylphosphonic acid (OPA). [26] However, in addition to OPA, the preparation also required the introduction of OA and trioctylphosphine oxide (TOPO), complicating the mechanism because TOPO and OA can also passivate and efficiently stabilize the CsPbBr 3 surface. [27,28] Additionally, OPA is insoluble in octadecene and is unable to dissolve PbBr 2 without TOPO. [29] In this work, we propose a concept of equivalent ligand, hypothesizing that if a ligand can play a role similar to Br ions to a certain extent and form strong interaction with lead ions (surfactant), then the problems of V Br and weak interaction can be solved simultaneously. The simplest source of Br ions is HBr, andThe stability and optoelectronic device performance of perovskite quantum dots (Pe-QDs) are severely limited by present ligand strategies since these ligands exhibit a highly dynamic binding state, resulting in serious complications in QD purification and storage. Here, a "Br-equivalent" ligand strategy is developed in which the proposed strong ionic sulfonate heads, for example, benzenesulfonic acid, can firmly bind to the exposed Pb ions to form a steady binding state, and can also effectively eliminate the exciton trapping probability due to bromide vacancies. From these two aspects, the s...

wileyonlinelibrary.comRecently, a new semiconductor nanomaterial system, all inorganic halide perovskite nanocrystals (IPNCs, CsPbX 3 , X = Cl, Br, I), have been reported to possess high stability, [ 1 ] ultrahigh photoluminescent quantum yield (PL QY), composition dependent luminescence covering the whole visible region withThe strong ionic character endows all-inorganic CsPbX 3 (X = Cl, Br, I) perovskite nanocrystals (NCs) with different chemical features from classical Cd-based NCs, especially when considering their interaction with polar solvents and surfactants. This has aroused intensive interest, but is still short of comprehensive understanding. More signifi cantly, above characteristic may be used to improve the quality of perovskite thin fi lms, which is crucial for the carrier transport inside optoelectronic devices. Here, an interesting recyclable dissolution-recyrstallization phenomenon of all-inorganic pervoskite, as well as its application on room temperature (RT) self-healing of compact and smooth carrier channels in ambient atmosphere for high-performance PDs with high stability is reported. First, according to solubility equilibrium principle, the size of CsPbBr 3 crystals can be reversibly tuned in the range of 10 nm-1 µm through washing with polar solvent or stirring with assistance of surfactants at RT. Second, such phenomenon is applied for signifi cant fi lm quality improvement by forming a liquid circumstance within fi lms, which can transport matter at surface and sharp parts into the gaps, healing themselves at RT. This strategy results in large-area, crack-free, low-roughness perovskite thin fi lms. Obviously, such improvement facilitates transport and extraction of carriers in the channels of devices, which has been evidenced by the improvement of performances of the corresponding PDs at ambient condition.