Highly Simplified Tandem Organic Light-Emitting Devices ...

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Highly Simplified Tandem Organic Light-Emitting Devices Incorporating a Green Phosphorescence Ultrathin Emitter within a Novel Interface Exciplex for High Efficiency Ting Xu,†,‡ Jun-Gui Zhou,† Chen-Chao Huang,† Lei Zhang,† Man-Keung Fung,† Imran Murtaza,§,∥ Hong Meng,*,‡,§ and Liang-Sheng Liao*,† †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China ‡ School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China § Key Laboratory of Flexible Electronics and Institute of Advanced Materials, Jiangsu National Synergistic Innovation Centre for Advanced Materials, Nanjing Tech University, Nanjing 211816, China ∥ Department of Physics, International Islamic University, Islamabad 44000, Pakistan ABSTRACT: Herein we report a novel design philosophy of tandem OLEDs incorporating a doping-free green phosphorescent bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)2(acac)) as an ultrathin emissive layer (UEML) into a novel interface-exciplex-forming structure of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TmPyPB). Particularly, relatively low working voltage and remarkable efficiency are achieved and the designed tandem OLEDs exhibit a peak current efficiency of 135.74 cd/A (EQE = 36.85%) which is two times higher than 66.2 cd/A (EQE = 17.97%) of the device with a single emitter unit. This might be one of the highest efficiencies of OLEDs applying ultrathin emitters without light extraction. Moreover, with the proposed structure, the color gamut of the displays can be effectively increased from 76% to 82% NTSC if the same red and blue emissions as those in the NTSC are applied. A novel form of harmonious fusion among interface exciplex, UEML, and tandem structure is successfully realized, which sheds light on further development of ideal OLED structure with high efficiency, simplified fabrication, low power consumption, low cost, and improved color gamut, simultaneously. KEYWORDS: organic light-emitting diodes, tandem structure, interface exciplex, ultrathin emissive layers, current efficiency

1. INTRODUCTION After the invention of the first succinct organic light-emitting device (OLED) with two-layer structure in 1987 by C. W. Tang,1 flat-panel displays and illumination applications based on OLED technology have developed sharply due to their attractive features such as simple fabrication process, devisible smart structure, impressive color rendering, ultrathin structure, wide viewing angles, abundant organic materials, lightweight, and high compatibility with flexible substrates.2−9 OLED stacking by two or more emitter units has drawn enormous attention due to impressive current efficiency (CE), brightness, external quantum efficiency (EQE), luminous efficiency, and device lifetime compared to conventional OLEDs.10−14 However, stacking multiple electroluminescence (EL) units in tandem OLEDs increases driving voltage and complicates the fabrication process relative to their standard single-unit counterparts. Moreover, commercial OLED products are still expensive for consumers.15 Therefore, it is urgently needed to simplify the OLED structure and fabrication technology to reduce the cost of products.15 To be specific, in typical tandem OLEDs, the doping technique requires careful selection of © 2017 American Chemical Society

appropriate hosts for different dopants in the host−guest emitter system.16 Furthermore, the control of codeposition rate and dopant concentration in the fabrication procedure is complex and not absolutely accurate, which tends to lower the yield in the production line. Moreover, the physical vapor deposition equipment needs to be remolded with adequate sensors and evaporator sources to realize complex device fabrication of tandem OLEDs, which increase their manufacturing costs. To loosen the bottlenecks, the doping-free technology applying ultrathin emitting nanolayers (