Employing Photo-Assisted Ligand Exchange Technique in Layered
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Employing Photo-Assisted Ligand Exchange Technique in Layered Quantum Dot LEDs Wenjia Hu1, 2, Shuai Gao1, 2, Paras N. Prasad3, Jingkang Wang2 and Jian Xu1 1
Engineering Science and Mechanics, the Pennsylvania State University, University Park, Pennsylvania; 2
Chemical Engineering and Technology, Tianjin University, Tianjin, China;
3
Institute for Lasers, Photonics and Biophotonics, University at Buffalo, The State University of New York, Buffalo, New York Abstract We presented in this paper a photo-assisted ligand exchange approach whereby light will be introduced to facilitate the replacement of oleic acid (OA) ligand molecules over PbSe quantum dots (QDs). The ligand-exchanged QDs were used to fabricate quantum dot light-emitting-diodes (QD-LEDs), which outperform the devices comprising the QDs without ligand-replacement. Introduction
Colloidal semiconductor quantum dots (QDs), by virtue of the large tunability in their bandgap, high luminance efficiency, narrow spectral emission, and high photo-stability, are promising lumophores and sensitizers in next-generation optoelectronic devices, such as light emitting diodes (LEDs)[1], photo detectors[2], and solar cells[3]. However, the surface of the QDs is often capped with long-chain ligand molecules, which are electrically insulating and thus hinder the electronic applications of the QDs[4]. Ligand exchange with shorter and more conductive surfactant molecules is necessary for improved device performance.i We reported in this communication the use of a photo-assisted ligand exchange approach to facilitate the dot-to-dot electron transport, which results in a significant improvement in the output emission of QD-LEDs. Experiment details PbSe QDs were synthesized following the noncoordinating solvent technique developed by Yu et al[5]. The as prepared PbSe QDs, 4.5nm in diameter and nearly monodisperse, were stabilized with a capping layer of oleate molecules coordinated to the Pb atoms (Fig.1). Ligand exchange processing of the PbSe QDs was based on the method proposed by kim et al[6]. 3ml tert-butyl N-(2-mercaptoethyl)carbamate(t-BOC) and 15mg/ml PbSe QDs in hexane solution
(5ml) were mixed in centrifuge tubes, and then vortexed overnight under nitrogen. Because the –S-Rn in t-BOC has higher polarity than –O-CO-Rn in oleic acid, the surface ligands of PbSe QDs will be replaced by t-BOC. Following ligand replacement, the QDs were washed twice with methanol using a centrifuge and then redispersed in chloroform. By this simple treatment the 18-carbon-chain oleate molecules overcoating the PbSe QDs were replaced with 4-carbon- chain 2-Mercaptoethylamine surfactants.
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Figure 1. (A)Absorption and photoluminous spectra of as prepared PbSe QDs; (B) TEM image of PbSe QDs with an average diameter of 4.5nm. The design of the QD-LEDs was illustrated in Fig.2. A multilayer structure was employed in the device fabrication, consisting of ITO/poly(3,4-ethylene
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