Degradation Studies of Colloidal Quantum Dot Light-Emitting Diodes
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Degradation Studies of Colloidal Quantum Dot Light-Emitting Diodes Jie Liu1, Chuang Xie1,2,3, Yu Zhang1,4, Wenjia Hu1,3, Shawn Pickering1, Guanjun You1, Andrew Y. Wang2, Jian Xu1 1 Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802, U.S.A. 2 Ocean NanoTech LLC., 2143 Worth Lane, Springdale, Arkansas 72764, U.S.A. 3 State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China 4 State Key Laboratory on Integrated Optoelectronics, and College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China ABSTRACT The degradation mechanism of CdSe/ZnS quantum dots (QDs) light-emitting diodes (LEDs) was investigated with steady-state and time-resolved photoluminescence measurements. Our study reveals that the degradation is associated with the decreasing quantum efficiency of the CdSe/ZnS QDs in the devices. Two mechanisms that cause the efficiency reduction were verified in the experiments: i.e., thermal instability and luminescence quenching. INTRODUCTION Recent studies suggest that the low-cost, colloidal semiconductor quantum dots (QDs) can be used to replace organic emitting materials in solution-processed light-emitting diodes (LEDs) by virtue of their superior optical properties, including high quantum yield, narrow emission bandwidth, and size-tunable wavelength over a broad spectral regime [1-4]. Nevertheless, there have been few reports to date about studies on the stability and degradation mechanisms of QDLEDs when compared to their organic counterparts [5-7]. In the present work, we took the first step towards understanding the degradation of CdSe/ZnS QD LEDs through photoluminescence (PL) measurements. Our results suggest that the device degradation is associated with a decrease in the quantum efficiency of CdSe/ZnS QDs. EXPERIMENT Synthesis of CdSe/ZnS core/shell QDs The core/shell CdSe/ZnS QDs used in this work were synthesized with an organometallic approach modified from that reported in [8] Structure of the QD LEDs Figure 1 shows the schematic structure of the QD LEDs under study, consisting of indium tin oxide (ITO)/Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS ) (25 nm)/poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (poly-TPD)(45 nm)/QDs
(~3ML)/Al(150 nm). Device fabrication started with spin cast of PEDOT/PSS on UV-ozone treated ITO substrates, over which Poly-TPD was then solution cast from the chlorobenzene as the hole transport layer (HTL). A QD layer was then spin coated from the toluene solution as the emission layer (EML). Finally, the Al cathode, was fabricated by shadow-mask-evaporation at a pressure of
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