Low temperature route for synthesis of cadmium selenide quantum dots and their application in fabricating a QD-LED
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Low temperature route for synthesis of cadmium selenide quantum dots and their application in fabricating a QD-LED Menaka Jha1, Michael McCreary2, Sreeram Vaddiraju3 and Delaina A. Amos2* 1 Conn Center for Renewable Energy Research, J.B. Speed School of Engineering, University of Louisville, USA 2 Department of Chemical Engineering, J.B. Speed School of Engineering, University of Louisville, USA 3 Department of Chemical Engineering, Texas A&M University, USA Abstract
The present study describes the effect of ageing time during the synthesis of oleic acid capped cadmium selenide quantum dots synthesized by the hot injection route and their use in the fabrication of a hybrid quantum dot light emitting device (QD-LED). This hot injection process has been carried out at the lower synthesis temperature of 140°C compared to the conventional temperature of ~300°C. Fluorescent monodisperse quantum dots of size 3-5 nm and 8-10 nm have been obtained at an ageing time of 2 and 3 hours respectively. An attempt to fabricate a QD-LED has been carried out. Current versus voltage studies show a turn on voltage at 3.06 V with a current of ~ 87 nA. Introduction
Semiconductor quantum dots (QDs) are of interest largely because of their unique optical properties that originate from the quantum confinement phenomena associated with QDs. These semiconductor QDs have a discrete electronic structure compared to the bulk band structure. This electronic structure leads to a blue shift in the band gap as compared to bulk band gap.1 Due to these interesting properties of QDs, they have a variety of applications including photostable, luminescent biological labels, and light harvesters in photovoltaic devices, as well as emissive materials in solar cells and light emitting devices.2 Cadmium selenide quantum dots are widely used for various applications like optical filters, solar cells, sensor and light emitting devices2. In addition, cadmium selenide based QD-LEDs have been involved in the development of new technology in the flat panel display industry.2-3 The basic device structure required to make a light emitting device is shown in Figure 1a.
Light
Figure 1. (a) Basic OLED device structure and (b) proposed device structure for hybrid polymer QD-LED
Hybrid polymer LEDs have complex band structures (Figure 1) in which indium tin oxide (ITO) acts as the anode layer. The conducting polymer film is sandwiched between the emissive QD layer and ITO.4 Further, the electron transport layer and cathode layer (mostly deposited by a thermal evaporation procedure) composed of an organometallic compound and a low work function metal (typically Al, Au or Ag) respectively. Polymeric QD OLEDs have been reported where conductive and emissive materials are mixed together in a single layer. However, the resulting OLED’s fabricated using the single layer shows very poor performance. The bilayer hybrid OLED performs much better than the single layer one but QD layer still needs to be optimized to facilitate electron transport across the polymer/QD interfac
