Ligand Effects on Photoluminescence and Electroluminescence of Silicon Quantum Dots for Light-Emitting Diodes

Abstract

Colloidal silicon quantum dots (SiQDs) may potentially minimize the environmental impact of commercial LEDs and advance next-generation light sources. Many studies have investigated the optical properties of SiQDs prepared by chemical synthesis, but the essential features of surface ligands have not fully been understood. Characterizing surface ligands should have a significant impact on optoelectronic research and ensuing applications. In this study, colloidal SiQDs were synthesized by pyrolyzing hydrogen silsesquioxane, followed by thermal hydrosilylation with 1-decene. Decyl-terminated SiQDs exhibited photoluminescence (PL) in a wavelength of 730 nm and PL quantum yields (QYs) of up to 38%. Seven decyl-terminated SiQDs with different ligand coverages were synthesized by varying the reaction time of hydrosilylation between 10 min and 9 h, and then these SiQDs were assembled into LEDs. The PL spectra, PLQYs, and performance of the SiQD LEDs were evaluated as a function of the decyl-ligand coverage. The PL properties (i.e., peak wavelength and PLQY) were insensitive to changes in decyl-ligand coverage, whereas the LED performance changed significantly. In particular, a 2-fold difference in decyl-ligand coverage exhibited a 4-fold difference in electroluminescence (EL) turn-on voltage and a 17-fold difference in EL external quantum efficiencies. In addition, the LED performance was characterized by quantifying the relationship between ligand coverage, the number of bonding sites, and the surface areas of the ligands. At greater than 25% coverage, the total surface area of the decyl-ligands was significantly larger than that of a single SiQD, and when decyl-ligands and Si–O groups covered 50% of the surface, the insulation effect impaired the LED performance. Therefore, ligand coverage significantly affected the performance of SiQD LEDs. Although this study was limited to decyl-terminated SiQDs, the same method can be applied to other ligands to further improve LED efficiency of next-generation light sources in displays, lighting, and biomedical imaging.

Stability of Silicon Quantum Dots Against Solar Light/Hot Water: RGB Foldable Films and Ligand Engineering

“…Three different methods were used to synthesize colloidal SiQDs with PL bands corresponding to the three primary colors. Briefly, SiQDs with red (R-SiQD), green (G-SiQD), and blue (B-SiQD) PL colors were synthesized via the pyrolysis of hydrogen silsesquioxane (HSQ), ,, a reduction reaction of SiBr 4 , and a reduction reaction of SiCl 4 , respectively. The surfaces of the three SiQDs were terminated by different ligands: the R-SiQDs were terminated with 1-decene, the G-SiQDs with a carbazole compound (1,2,3,4-tetrahydrocarbazole-4-one), and the B-SiQDs with siloxane groups (Si–O–Si) using octylsilane.…”

“…Details of SiQD synthesis, QD film preparation, and characterization are described in the Supporting Information. Briefly, the red luminescent SiQDs (R-SiQDs) were decyl-passivated SiQDs derived via hydrogen silsesquioxane (HSQ) pyrolysis. ,, HSQ was placed in a quartz boat and heated in a tube furnace at 1100 °C. The obtained matrices were chemically etched in an acid mixture, and hydrogen-passivated SiQDs were obtained.…”

“…Three notable applications of SiQDs rely on solution processing (Table ). ,− The first of these comprises light-emitting diodes (LEDs) based on the electroluminescence (EL) of SiQDs. SiQD LEDs display red, − NIR, and white-blue EL, and demonstrate external quantum efficiencies of up to 8.6% .…”

“… ,− The first of these comprises light-emitting diodes (LEDs) based on the electroluminescence (EL) of SiQDs. SiQD LEDs display red, − NIR, and white-blue EL, and demonstrate external quantum efficiencies of up to 8.6% . Second, solar concentrators (solar windows) − ,, dependent on SiQD PL have been demonstrated.…”

Postproduction Approach to Enhance the External Quantum Efficiency for Red Light-Emitting Diodes Based on Silicon Nanocrystals

“…At present, QLEDs that meet the benchmark of 20% external quantum efficiency (EQE), which is comparable to the energy conversion efficiency of a mercury lamp, are limited to models that use indium phosphide QDs as an optically active layer [ 1 ]. In case of red-QLEDs, the possible QD alternatives are copper indium sulfide (CuInS 2 ) [ 2 ], ZnS-AgInS 2 (ZAIS) [ 3 ], Pb-free perovskite nanocrystals [ 4 ], and silicon (Si) [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ], but the EQE values reported for these QLEDs are still low, such as 7.8% for CuInS 2 [ 2 ], 2.2% for ZAIS [ 3 ], 0.3% for Pb-free perovskite [ 4 ], and 6.2% for Si [ 17 ].…”