Surface-Ligand-Modified CdSe/CdS Nanorods for High-Performance Light-Emitting Diodes

Abstract

Colloidal nanocrystals (NCs) play an important role in the field of optoelectronic devices such as photovoltaic cells, photodetectors, and light-emitting diodes (LEDs). The properties of NC films are strongly affected by ligands attached to them, which constitute a barrier for charge transport between adjacent NCs. Therefore, the method of surface modification by ligand exchange has been used to improve the electrical conductivity of NC films. However, surface modification to NCs in LEDs can also affect emission characteristics. Among NCs, nanorods have unique properties, such as suppression of nonradiative Auger recombination and linearly polarized light emission. In this work, CdSe/CdS nanorods (NRs) were prepared by the hot injection method. To increase the charge transport into CdSe/CdS NRs, we adopted ligand modification to CdSe/CdS NRs. Using this technique, we could shorten the injection barrier length between CdSe/CdS NRs and adjacent layers. It leads to a more balanced charge injection of electron/hole and a greatly increased current efficiency of CdSe/CdS NR-LEDs. In the NR-LEDs, the ligand exchange boosted the electroluminance, reaching a sixfold increase from 848 cd/m 2 of native surfactants to 5600 cd/m 2 of the exchanged n-octanoic acid ligands at 12 V. The improvement of CdSe/CdS NR-LED performance is closely correlated to the efficient control of charge balance via ligand modification strategy, which is expected to be indispensable to the future NR-LED-based optoelectronic system.

Highly Efficient Inverted Light-Emitting Diodes Based on Vertically Aligned CdSe/CdS Nanorod Layers Fabricated by Electrophoretic Deposition

“…Our work presented NR-based inverted LEDs with the best performance found in vertically aligned CdSe/CdS NRs (2 MLs) as the emitters made by the EPD method, with a luminance of 4320 cd/m 2 and an EQE of 6.3%. This performance is comparable to the best-reported values for NR-based LEDs in the conventional architecture under a similar voltage . Such excellent performance is achieved by introducing a conceptually new method for device fabrication resulting from the vertically aligned NR layer between the HTL layer and the oxide EIL.…”

“…This performance is comparable to the best-reported values for NR-based LEDs in the conventional architecture under a similar voltage. 39 Such excellent performance is achieved by introducing a conceptually new method for device fabrication resulting from the vertically aligned NR layer between the HTL layer and the oxide EIL. It is found that the vertically aligned NRs can improve the charge balance and reduce the resistance for carrier transport, which would lead to low-cost, large-area, high-efficiency, high-color-quality, stable electroluminescent devices for both display and solid-state lighting technologies.…”

High-Efficiency and Stable Colloidal One-Dimensional Core/Shell Nanorod Light-Emitting Diodes

“…27−31 Several strategies have been developed to prepare highperformance NR-LEDs, including shell coating to enhance the photoluminescence quantum yield (PLQY) of NRs, ligand modification to improve the electrical conductivity of NR films, and device structure optimization to promote charge injection balance and improve the EQE. 27,32,33 At present, the record EQE of NR-LEDs is 15.7% and the maximum brightness is ∼104 000 cd/m 2 , 27 which are still lower than those of QLEDs (maximum EQE of 35.6%). 25,27,32,34−36 The EQE of NR-LEDs is limited by several issues, including the lower PLQY of traditional CdSe/CdS NRs compared to that of QDs, the poor film morphology of the emission layer, and imbalanced charge injection in NR-LEDs.…”

“…Several strategies have been developed to prepare high-performance NR-LEDs, including shell coating to enhance the photoluminescence quantum yield (PLQY) of NRs, ligand modification to improve the electrical conductivity of NR films, and device structure optimization to promote charge injection balance and improve the EQE. ,, At present, the record EQE of NR-LEDs is 15.7% and the maximum brightness is ∼104 000 cd/m 2 , which are still lower than those of QLEDs (maximum EQE of 35.6%). ,,,− The EQE of NR-LEDs is limited by several issues, including the lower PLQY of traditional CdSe/CdS NRs compared to that of QDs, the poor film morphology of the emission layer, and imbalanced charge injection in NR-LEDs. ,,, Due to the difficulties in obtaining uniform CdSe NRs, most reports have focused on dot-in-rod NRs. However, we note that the rod-in-rod NRs present a degree of polarization ∼1.5 times higher than that of dot-in-rod NRs, which is crucial for improving the photon out-coupling efficiency of NR-LEDs. , In addition to the PLQY, the aspect ratio of NRs also affects the performance of NR-LEDs by affecting the film morphology of the NR emission layer, ,,,,, since it has been reported that defects or the unevenness of the NR film can lead to leakage current and a low EQE. , In addition, the imbalanced carrier injection caused by the high electron mobility and small electron injection barrier of the widely used ZnO electron transport layer further limits the EQE of NR-LEDs. ,,, Similar to QLEDs, NR-LEDs also face the challenge of efficiency droop, where EQE rapidly decreases with an increase in luminance or current density .…”

Management of typical VOCs in air with adsorbents: status and challenges

“…[28][29][30] In general, AC can be divided based on raw materials such as biomass AC (the precursors are wood chips, 31 walnut shells, 32 coconut shells, 33 etc. ), coalbased AC (the precursors are anthracite coal 34 and lignite 35 ) and AC fibers (the precursors are polyacrylonitrile, 36 intermediate phase asphalt, 37 phenolic resin, 38 etc.). The preparation process mainly consists of carbonization and activation.…”