Light-emitting diodes (LEDs) based on metal halide perovskite quantum dots (QDs) have achieved impressive external quantum efficiencies; however, the lack of surface protection of QDs, combined with efficiency droop, decreases device operating lifetime at brightnesses of interest. The epitaxial incorporation of QDs within a semiconducting shell provides surface passivation and exciton confinement. Achieving this goal in the case of perovskite QDs remains an unsolved challenge in view of the materials’ chemical instability. Here, we report perovskite QDs that remain stable in a thin layer of precursor solution of perovskite, and we use strained QDs as nucleation centers to drive the homogeneous crystallization of a perovskite matrix. Type-I band alignment ensures that the QDs are charge acceptors and radiative emitters. The new materials show suppressed Auger bi-excition recombination and bright luminescence at high excitation (600 W cm–2), whereas control materials exhibit severe bleaching. Primary red LEDs based on the new materials show an external quantum efficiency of 18%, and these retain high performance to brightnesses exceeding 4700 cd m–2. The new materials enable LEDs having an operating half-life of 2400 h at an initial luminance of 100 cd m–2, representing a 100-fold enhancement relative to the best primary red perovskite LEDs.
unite solution processing with desirable optoelectronic properties such as tunable light emission and long carrier lifetimes and diffusion lengths. [9][10][11] The rapid development of mixed-halide CsPbBr x /I 3−x nanocrystals via compositional tuning has enabled an EQE of 20.3% in the red with a full-width at half maximum (FWHM) of 40 nm. [12] Unfortunately, they have yet to rise to match the operating stability of organic [13,14] and inorganic quantum dot [15,16] LEDs: device operating stability (T 50 ) to date, at an initial luminance of 140 cd m -2 , has been limited to hours. [12] The organic ligands used in synthesis, and the subsequent exchange, provide colloidal stability in solution. However, these surface ligands are labile, exhibit limited surface binding affinity, and provide high surface coverage only when present in excess in solution. [18,19] They are readily desorbed upon dilution and washing, inducing incomplete passivation of surface sites.Introducing inorganic ligands in order to better passivate surfaces has shown promising progress in single halide composition in our previous work in both blue and red LEDs. [10,27] However, these strategies rely on a highly polar solvent (DMF) to carry out the inorganic salts. The introduction of the highly polar solvent compromises perovskite nanocrystal structural stability and harms the operating stability of LEDs.Here, we report a strategy wherein we introduce inorganic ligands in the antisolvent used in nanocrystal purification. We show that working with a mildly polar antisolvent, such as ethyl acetate used in this work, allows a gentler processing of the vulnerable perovskite quantum dots. Only by introducing ultrasonication during the introduction of the antisolvent were we able to develop an exchange that was successful, and substantially complete. The inorganic ligands replace, in situ, the organic ligands that are detached from the dot surface (Figure 1a): the small inorganic cations provide a rich surface coverage superior to that offered by long-chain organic ligands, and thus prevent trap formation. The inorganic ligands fill surface defects and improve material conductivity and charge-carrier injection in LEDs. The strategy improves bandgap stability, resulting in MHP solids with a storage stability of 1 year in ambient conditions (25 °C and 40% humidity), in comparison to controls that show phase changes after 7 days under the same conditions.Instability in mixed-halide perovskites (MHPs) is a key issue limiting perovskite solar cells and light-emitting diodes (LEDs). One form of instability arises during the processing of MHP quantum dots using an antisolvent to precipitate and purify the dots forming surface traps that lead to decreased luminescence, compromised colloidal stability, and emission broadening. Here, the introduction of inorganic ligands in the antisolvents used in dot purification is reported in order to overcome this problem. MHPs that are colloidally stable for over 1 year at 25 °C and 40% humidity are demonstrated and films ...
Metal halide perovskites have emerged as promising candidates for solution-processed blue light-emitting diodes (LEDs). However, halide phase segregationand the resultant spectral shiftat LED operating voltages hinders their application. Here we report true-blue LEDs employing quasi-two-dimensional cesium lead bromide with a narrow size distribution of quantum wells, achieved through the incorporation of a chelating additive. Ultrafast transient absorption spectroscopy measurements reveal that the chelating agent helps to control the quantum well thickness distribution. Density functional theory calculations show that the chelating molecule destabilizes the lead species on the quantum well surface and that this in turn suppresses the growth of thicker quantum wells. Treatment with γ-aminobutyric acid passivates electronic traps and enables films to withstand 100°C for 24 h without changes to their emission spectrum. LEDs incorporating γ-aminobutyric acid-treated perovskites exhibit blue emission with Commission Internationale de l'Éclairage coordinates of (0.12, 0.14) at an external quantum efficiency of 6.3%.
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