Few-Nanometer-Sized α-CsPbI₃ Quantum Dots Enabled by Strontium Substitution and Iodide Passivation for Efficient Red-Light Emitting Diodes

Abstract

Cubic phase CsPbI3 quantum dots (α-CsPbI3 QDs) as a newly emerging type of semiconducting QDs hold tremendous promise for fundamental research and optoelectronic device applications. However, stable and sub-5 nm-sized α-CsPbI3 QDs have rarely been demonstrated so far due to their highly labile ionic structure and low phase stability. Here, we report a novel strontium-substitution along with iodide passivation strategy to stabilize the cubic phase of CsPbI3, achieving the facile synthesis of α-CsPbI3 QDs with a series of controllable sizes down to sub-5 nm. We demonstrate that the incorporation of strontium ions can significantly increase the formation energies of α-CsPbI3 QDs and hence reduce the structure distortion to stabilize the cubic phase at the few-nanometer size. The size ranging from 15 down to sub-5 nm of as-prepared stable α-CsPbI3 QDs allowed us to investigate their unique size-dependent optical properties. Strikingly, the few-nanometer-sized α-CsPbI3 QDs turned out to retain high photoluminescence and highly close packing in solid state thin films, and the fabricated red light emitting diodes exhibited high brightness (1250 cd m–2 at 9.2 V) and good operational stability (L50 > 2 h driven by 6 V). The developed cation-substitution strategy will provide an alternative method to prepare uniform and finely size-controlled colloidal lead halide perovskite QDs for various optoelectronic applications.

Improving Lead‐Free Double Perovskite Cs2NaBiCl6 Nanocrystal Optical Properties via Ion Doping

“…For instance, the metal halide DP Cs 2 AgBiCl 6 and Cs 2 AgBiBr 6 NCs with good stability under ambient conditions have been synthesized, but the optical properties of the pure phase lead‐free double halide perovskite NCs are unsatisfactory (e.g., low PL quantum yield [PLQY]) for many optical applications 9. Doping “impurities” in semiconducting NCs has aroused great interest because it is efficient to control over the photoelectric properties of NCs and have shown potentials in solar cells,10 light emitting diodes,11 and safety markers 12. Recently, impurity dopants including Mn 2+ , In 3+ , Yb 3+ , Er 3+ , and Eu 3+ have been tried in lead‐free halide DP NCs to endow desirable optical properties in these DP NCs 13.…”

Metal Doping/Alloying of Cesium Lead Halide Perovskite Nanocrystals and their Applications in Light‐Emitting Diodes with Enhanced Efficiency and Stability

“…Yao et al demonstrated that stable cubic-phase CsPbI 3 with tunable size from 15 to sub-5 nm through introducing Sr 2 + substitution along with iodide passivation. [85] It was found that the incorporation of Sr 2 + ions significantly increased the formation energies of the perovskite structure and reduced the structure distortion for small-sized CsPbI 3 NCs. The obtained few-nanometer-sized doped CsPbI 3 NCs still retained the high PL emission properties and highly close packing in the deposited thin films, with improved PL stability of the colloidal solution and the thin films.…”

“…The divalent doping strategy of CsPbI 3 NCs can be combined with efficient halide surface passivation to further improve the material properties and the device performance. Yao et al [85] and Lu et al [86] independently demonstrated high performance red-emissive LEDs based on CsPbI 3 NCs with simultaneous strontium (Sr 2 + ) doping and iodide/chlorine surface passivation. Owing to the slightly smaller ion radius of Sr 2 + (118 pm) than the Pb 2 + (119 pm) ions, doping of the Sr 2 + into the CsPbI 3 NCs may cause a slight lattice contraction.…”

“…The obtained devices based on 3.1 % Sr 2 + doped CsPbI 3 NCs (5.38 nm) exhibited a high EQE of 5.92 %. [85] In addition, due to the improved material stability, the resulting LEDs based on Sr 2 + -substituted CsPbI 3 NCs exhibited a L 50 (needed time decay to 50 % of the initial luminance) of 120 min, which is significantly improved as compared to the devices based on undoped ones (10 min). Simultaneously introducing Sr 2 + doping and Cl À surface passivation can also generate a high PLQY of 84 % (Table 1) for CsPbI 3 NCs with further enhanced materials stability.…”

High‐Efficiency Perovskite Quantum Dot Hybrid Nonfullerene Organic Solar Cells with Near‐Zero Driving Force

“…[ 26,27 ] Among them, CsPbI 3 is widely used due to its optimal bandgap ( E g ≈ 1.73 eV), high dielectric constant, large absorption attenuation coefficient, strong fluorescence, and long‐range electron transport. [ 28–31 ] CsPbI 3 perovskite quantum dots (PQDs) were used for perovskite solar cells, [ 32–36 ] light‐emitting diodes, [ 37–39 ] and even fullerene‐based OSCs. [ 40 ]…”