High Quantum Yield Blue InP/ZnS/ZnS Quantum Dots Based on Bromine Passivation for Efficient Blue Light‐Emitting Diodes

Abstract

Due to the biotoxicity and environmental contamination, electronic products containing cadmium are strictly limited. [1][2][3][4] Therefore, the development of environmental-friendly cadmium-free quantum dots (QDs) with competitive performance is one of the frontiers of current QD researches. [5,6] InP QD is considered as the most promising alternative in cadmiumfree QD due to its large exciton Bohr radius and stability. [7][8][9] At present, the performance of red and green InP QDs and devices has been greatly improved. The photoluminescence quantum yield (PLQY) of red InP QDs reached 100%, and the external quantum efficiency (EQE) of red InP quantum dot light-emitting diode (QLED) was 21%. [10] For green light-emitting InP QDs, the QY reached 95% with a QLED EQE of 7.06%. [11] However, the highest QY of blue InP QDs was only 82% and the corresponding QLED EQE was only 2.5%. [12] Thus, the synthesis of high-QY blue InP QDs is of great significance.At present, only a few types of alkylphosphine can be used to synthesize InP QDs due to the difficulty of meeting both the high reactivity and stability of most phosphorus precursors. In earlier studies, the more reactive tri(trimethylsilyl) phosphine [(TMS) 3 P] was used as the precursor of phosphorus. [13,14] Although QDs with good emission performance can be obtained, (TMS) 3 P is expensive and produces highly toxic phosphine gas when exposed to air. In 2016, Tessier et al. [15] proposed a synthesis scheme using indium halide and tri(dimethylamine)phosphine [(DMA) 3 P] as precursors, achieving an economic and large-scale synthesis of InP QDs. By using different halogens, the size of the InP QDs can be controlled, resulting in different emission wavelengths. Also, compared to (TMS) 3 P, (DMA) 3 P is more stable and does not produce toxic gases when exposed to air. Due to its low toxicity and high stability, and its price is only 1/80 times that of (TMS) 3 P, the selection of (DMA) 3 P as the phosphorus precursor to synthesize InP QDs is becoming a promising direction in the future. [16] The lower QY of blue InP QDs is mainly caused by two reasons. First, the small size of blue InP leads to its large specific surface area, and oxidation of the InP surface by water and Red and green InP quantum dots (QDs) already have been demonstrated with excellent luminescence performance closing the gap with CdSe-based QDs. However, the performance of blue InP QDs still lags behind that of red and green QDs. For blue InP QDs synthesized by aminophosphine and zinc iodide, the inherent Ipossesses weak passivation ability. By introducing Brwith a smaller ion radius and larger binding energy, the quantum yield of blue InP QDs is increased from 54% to 93%, which is the highest value reported so far. Meanwhile, the long-chain 1-dodecanethiol is replaced by the short-chain 1-octanethiol through ligand exchange to increase the carrier injection efficiency. The blue quantum dot light-emitting diodes (QLEDs) made of these QDs showed an external quantum efficiency of 2.6%, which is notably th...

Deep Blue and Highly Emissive ZnS-Passivated InP QDs: Facile Synthesis, Characterization, and Deciphering of Their Ultrafast-to-Slow Photodynamics

“…The InP QDs with surface ligand OAC were further characterized by powder X-ray diffraction (PXRD). The reflection peaks of the ultrasmall InP core QDs were broad due to their extremely small size (Figure Ig), which is consistent with the TEM observation. , The strong diffraction peaks located at 2θ of 30.51 and 50.1 correspond to (200) and (311) planes depicting a zinc blend phase. ,, The diffraction peaks were sharper for the InP/ZnS/ZnS QDs compared to InP QDs, which also indicates effective size inflation by epitaxial shell overgrowth. , Minimal contributions around 40°, 46°, and 68° and between 55° and 60° arose, which could be ascribed to ZnS coating . Due to the presence of Zn 2+ in the core and ZnS as the coating, the overall diffraction peak positions also shifted to slightly higher 2θ values …”

“…Minimal shifts in the case of coated InP QDs may be ascribed to the change in the chemical environment . The high-resolution XPS spectra further demonstrated the In 3d 5/2 and In 3d 3/2 peaks at 444.6 and 452.2 eV (Figure II), which are almost identical to the typical values for bulk crystalline InP which further confirms the presence of pure phase InP cores. ,, For the coated QDs, the In 3d 5/2 and In 3d 3/2 peaks appeared exactly at the same positions as in InP cores, i.e., at 444.6 and 452.2 eV (Figure III). In addition, the sharp In 3d 5/2 and In 3d 3/2 peaks of the coated and uncoated InP QDs suggest that the peaks are attributed to a single chemical bonding state of the In–P bond .…”

“…17,28 The strong diffraction peaks located at 2θ of 30.51 and 50.1 correspond to (200) and (311) planes depicting a zinc blend phase. 15,20,29 The diffraction peaks were sharper for the InP/ZnS/ZnS QDs compared to InP QDs, which also indicates effective size inflation by epitaxial shell overgrowth. 28,15 Minimal contributions around 40°, 46°, and 68°and between 55°and 60°arose, which could be ascribed to ZnS coating.…”

“…For blue emissive core/shell QDs, the potential barrier between the core and the shell is very small, which renders easy charge carrier tunneling into surface defects, consecutively decreasing the photoluminescent quantum yield (PLQY) and hindering photonic applications . Most of the InP QDs reported hitherto have shown an emission intensity maximum centered at 480 nm, ,,, with one singular exception where the emission maximum appears at 465 nm . Very recently weak emissive InP clusters with emission maxima 410 nm has been reported .…”

“…15,20,29 The diffraction peaks were sharper for the InP/ZnS/ZnS QDs compared to InP QDs, which also indicates effective size inflation by epitaxial shell overgrowth. 28,15 Minimal contributions around 40°, 46°, and 68°and between 55°and 60°arose, which could be ascribed to ZnS coating. 30 Due to the presence of Zn 2+ in the core and ZnS as the coating, the overall diffraction peak positions also shifted to slightly higher 2θ values.…”