1041-1135 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2016.2578643, IEEE Photonics Technology Letters 1 Abstract— We report efficient quantum dot light-emitting diodes (QLEDs) using sizable 13-nm green-emission quantum dots (QDs) as the emissive layer and solution-processable MoOx as the hole injection layer (HIL). The MoOx HIL was prepared by the decomposition of a solution of ammonium molybdate tetrahydrate at 80 C under ambient conditions, further spin-coated onto an ITO substrate to facilitate hole injection. Compared to the reference sample with a polymeric hole injection material poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the sMoOx film showed a higher work function of 5.6 eV, better transparency, and smoother surface morphology. The stable QLED with optimized MoOx film thickness achieved a higher maximum current efficiency of 10.8 cd/A at a lower turn-on voltage of 2.2 V versus the one with a PEDOT:PSS HIL, 9.9 cd/A and 2.9 V, respectively. Moreover, a small leakage current in sMoOx-device was found, which was attributed to the better surface morphology of sMoOx. Index Terms—Quantum dots (QDs), giant QDs, quantum dot light-emitting diodes (QLED), molybdenum oxides, MoOx. I. INTRODUCTION uantum dot light-emitting diodes (QLEDs) is a potential candidate for the next-generation display due to various favorable properties such as band-gap tunability, excellent color purity, high electroluminescence (EL) performance, and solution processability [1-3]. Recently, many studies have put more efforts to cost-down while still remaining highly efficient performance of QLEDs. The architecture design of QLEDs have been focused as an intrinsically important role to achieve simple and cost-effective fabrication process allowing devices with high performance and long lifetime [4-5]. While ZnO nanoparticles (NPs) have been the outstanding electron injection/transport material due to the suitable band-gap offset, high mobility, air stability, and solution-processability, both in QLEDs and polymer LEDs (PLEDs) [1,6], the hole injection materials of similar quality are still in demand. PEDOT:PSS with high conductivity and water solubility is widely used as one of the best hole transport material in QLEDs [2.3,5]. The acidic and hygroscopic nature of PEDOT:PSS induces the corrosion of ITO surface and concerns in long-term stability [7]. Consequently, the fast degradation and short lifetime are observed in QLEDs with a PEDOT:PSS HIL [8]. On the other hand, inorganic hole injection materials with low-lying valence bands that match with QDs are the desirable candidates for stable and long-lived devices [1,5]. Sputtered metal oxides including NiO, WO3, or Zn2SnO4 (ZTO) have been successfully utilized to replace PEDOT:PSS in initially [9-11]. Though these materials are efficient p-type semiconductors, the process is generally not a cost-effective fabrication. Following these ideas, recently solution-processable WO3 NPs, and NiO via sol-gel processes [12-14] have also been applied and highly appreciated as potential stable hole injection materials for QLEDs. However, their EL performance is generally far from that of the conducting polymer-based device. Furthermore, the reported work function (WF) of WO3 NPs was 5.15 eV, lower than the highest occupied molecular orbital (HOMO) level of the polymeric hole transport poly[N,N’-bis(4-butylphenyl)-N,N’- bis(phenyl)-benzidine] (poly-TPD) [12]. Molybdenum oxide, possessing high work function and well-developed solution-processable precursors, is investigated as one of the promising candidates for electronic applications. It was demonstrated that solution MoOx (sMoOx) has been an excellent anode buffer layer for solar cells [7] as well as promising hole injection material for organic light-emitting diodes [15], but no studies about the QLEDs with a sMoOx HIL were ever reported. In this report, to demonstrate the validity of sMoOx HIL in QD-based device, we prepared the sMoOx HIL utilizing a simple, non-toxic process via the decomposition of (NH4)6Mo7O24.4H2O precursor solution. The sMoOx film was optimized to achieve high transparency, efficient current injection, and smooth surface morphology. The results show that the sMoOx–device has comparable performance with the PEDOT:PSS-based QLEDs. II. EXPERIMENTAL PROCESS All devices were fabricated by spin-coating along with thermal evaporation on commercially pre-coated indium tin oxide (ITO)-glass substrates. Schematic structure of all QLEDs and those energy band-diagrams are shown in Figs. 1(a) and 1(b). The giant CdSe@ZnS/ZnS QDs with a composition gradient of CdSe-rich core and ZnS-rich shell were synthesized following the method we previously reported and have an average size of ~13 nm [16]. It is shown that the Solution-processable MoO x for efficient light-emitting diodes based on giant quantum dots Hoang-Tuan Vu, Yan-Kuin Su*, IEEE Fellow, Ray-Kuang Chiang, Chun-Yuan Huang*, Chih-Jung Chen, and Hsin-Chieh Yu Q _________________________________________________ The authors would like to thank the Ministry of Science and Technology of Taiwan, ROC for the financial support under Contract No. MOST 103-2221-E-006-001. The authors would also like to thank Industrial Technology Research Institute for the financial support and assistance in device characterization. Y.-K. Su, H.-T. Vu, and H.-C. Yu are with the Institute of Microelectronics, Department of Electrical Engineering and Advanced Optoelectronic Technology Center, National Cheng-Kung University, Tainan 701, Taiwan. E-mail : yksu@mail.ncku.edu.tw Y.-K. Su is with the Department of Electrical Engineering, Kun-Shan University, Tainan 710, Taiwan. C.-Y. Huang is with the Department of Applied Science, National Taitung University, Taitung 950, Taiwan. E-mail : laputa@nttu.edu.tw C.-J. Chen and R.-K. Chiang are with the Nanomaterials Laboratory, Far East University, Hsing-Shih, Tainan 74448, Taiwan.