IP: 37.9.40.107 On: Mon, 17 Dec 2018 14:54:29 Copyright: American Scientific Publishers Delivered by Ingenta Copyright © 2019 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 19, 1420–1424, 2019 www.aspbs.com/jnn Optimization of a Solution-Processed Quantum-Dot Light-Emitting-Diode with an Inverted Structure Nam-Hoon Baek and Woon-Seop Choi ∗ School of Display Engineering, Hoseo University, Asan, Chungnam, 31499, Korea Colloidal quantum-dot based light-emitting diodes (QD-LEDs) are attractive for use in display devices because of the remarkable electrical and optical characteristics of colloidal quantum dots. An inverted structure may be one method to achieve the necessary multilayer device structures in QD-LEDs. In this study, each layer of an inverted-structure QD-LED was optimized. The effect of the solvent on the hole transfer layer was investigated, along with the effect of the concentration of the electron transfer layer, the effect of the co-solvent on the hole transfer layer, and the effect of the concentration and solvent of quantum dot layer. The quantum dots and ZnO NPs were syn- thesized as the emitting layer and carrier transporting layer using a solution-mediated process. The inverted QD-LED device showed a luminance of 3,762 cd/m 2 , current efficiency of 1.86 cd/A, and EQE of 1.18%. Keywords: Quantum Dot Light Emitting Diode (QD-LED), Inverted Structure, Solution-Process. 1. INTRODUCTION Colloidal quantum-dot-based light-emitting diodes (QD-LEDs) have been considered attractive electronic devices because of the remarkable electrical and optical characteristics of colloidal quantum dots, such as their wide-range color tenability, good color purity, high bright- ness, and narrow emission bandwidth. 1–2 There are various colloidal QDs based on II–VI, III–V, and IV–VI com- pounds. There has been considerable progress in II–VI QDs, which have high color purity (full width at half maximum: 20–40 nm) and photoluminescence quantum efficiency (PL QE, above 70%) in the visible range. 2 QD-LEDs can be fabricated by simple solution pro- cesses, such as spin coating, inkjet coating, and other coating or printing methods to make low-cost, large size, and flexible displays. To obtain high-efficiency QD- LEDs, it is important to optimize each layer’s formulation and coating conditions. Most of the reported QD-LEDs have a normal device structure, such as anode/HIL/ HTL/QDs/ETL/EIL/cathode. However, inverted QD-LED structures are more attractive for display applications because they can be integrated with low-cost n-type metal- oxide or amorphous silicon thin-film transistors. 3 Inverted QD-LED architectures with solution processing have ∗ Author to whom correspondence should be addressed. rarely been reported. 3 4 The first all-solution-processed inverted-structure QD-LEDs had very poor properties (the maximum current efficiency was 0.35 cd/A). 4 Improved inverted QD-LEDs was reported using polyoxyethylene tridecyl ether as a surfactant to improve the deposition of PEDOT:PSS on poly(4-butylphenyl-diphenyl-amine), resulting in a maximum efficiency of 12,510 cd/m 2 and current efficiency of 0.69 cd/A. 3 To achieve a multilayer structure in QD-LEDs, the orthogonal solvents must be used to avoid damaging the underlying layers when depositing the upper layers. 5 Another important thing is that the upper layer should have good wettability on the underlying layer; otherwise, it is difficult to control the deposition and the uniformity of the layer. 3 Therefore, for all-solution-processed inverted QD- LEDs, each layer, the solvents, concentrations, and coating process were optimized to achieve improved electrolumi- nescence performance. 2. EXPERIMENTAL DETAILS 2.1. Synthesis of CdSe/CdS/ZnS QDs Red QDs with core/shell/shell type multi-structure red QDs were synthesized using the procedures reported elsewhere. 6–7 Briefly, 1.6 mmol of CdO powder and 6.4 mmol of oleic acid (OA) were added to 40 mL of 1420 J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 3 1533-4880/2019/19/1420/005 doi:10.1166/jnn.2019.16221