In Situ Inorganic Ligand Replenishment Enables Bandgap Stability in Mixed‐Halide Perovskite Quantum Dot Solids

Abstract

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 ...

Elucidating the Role of Antisolvents on the Surface Chemistry and Optoelectronic Properties of CsPbBrxI3-x Perovskite Nanocrystals

“…Cesium lead halide perovskite nanocrystals (NCs) have drawn significant attention due to their superior optoelectronic properties, which include their efficient luminescence and narrow emission peak that is spectrally tunable over the entire visible wavelength range. − Since Kovalenko and co-authors established a simple hot injection synthesis method for CsPbX 3 (X = Cl, Br, or I) NCs in 2015, these all-inorganic perovskite NCs have emerged as an appealing class of materials for ultrahigh-definition display (via light-emitting diodes or LEDs), ,,− lasing, , and photovoltaic applications. ,,, For all of these applications, it is important to be able to fine-tune the optical bandgap, and this is commonly achieved by mixing species in the halide site, such as Br/Cl for blue-emitters or Br/I for red-emitters according to the Rec. 2020 standard. − …”

Metal Halide Perovskites for Red‐Emission Light‐Emitting Diodes

“…Back in the 1990s, 2D structure‐based perovskite materials achieved highly efficient LEDs at low temperatures (77 K) with an external quantum efficiency (EQE) of 2–4% and a maximum luminance of more than 10 000 cd m −2 at a drive voltage of 2.4 V. [ 6,7 ] However, electroluminescence at room temperature was underappreciated due to the occurrence of exciton annihilation. [ 8–11 ] In 2014, Snaith and Friend et al first demonstrated MAPbI 3− x Cl x ‐based NIR perovskite diodes under room temperature. [ 7,12 ] This achievement is a huge milestone, and set perovskite LEDs (PeLEDs) over the last years.…”

“…[ 7,12 ] This achievement is a huge milestone, and set perovskite LEDs (PeLEDs) over the last years. [ 13 ] Currently, the highest EQEs of green PeLEDs [ 14 ] and red PeLEDs [ 9 ] have exceeded 28% and 24.4%, respectively, which are comparable to the conventional OLEDs and QLEDs. However, the red channel emitted at 630 nm for LED displays still lags behind than green analogue.…”

Cu substitution boosts self-trapped exciton emission in zinc-based metal halides for sky-blue light-emitting diodes

“…1–16 Only five years after the first demonstration of room-temperature electroluminescence (EL) from perovskite light-emitting diodes (PeLEDs), lead-based PeLEDs have already achieved an impressive set of achievements, 17 with the highest external quantum efficiency (EQE) of both green and red light-emitting perovskite diodes exceeding 24% to date. 18,19 However, the development of blue LEDs as a key optoelectronic component is lagging far behind that of green and red LEDs. Lead leakage may pose serious environmental and public health hazards due to the inherent toxicity of lead.…”