Binary Solvent Effects on Thermally Crosslinked Small Molecular Thin Films for Solution Processed Organic Light-Emitting
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ORIGINAL ARTICLE - ELECTRONICS, MAGNETICS AND PHOTONICS
Binary Solvent Effects on Thermally Crosslinked Small Molecular Thin Films for Solution Processed Organic Light-Emitting Diodes Robert Bail1 · Joon Won Kang2 · You Jung Kang2 · Byung Doo Chin2 Received: 12 June 2020 / Accepted: 10 November 2020 © The Korean Institute of Metals and Materials 2020
Abstract Solution processing of crosslinkable small molecular thin films is potentially attractive for the fabrication of large-area display panels based on organic light-emitting diodes (OLEDs). The characteristics of a wet-processed film made of a thermally crosslinked small-molecular material can be controlled by simply adding a cosolvent to its solution. The surface morphology of a nanometer-scale thin film made of N4,N4′-di(naphthalen-1-yl)-N4,N4′-bis(4-vinylphenyl)biphenyl-4,4′-diamine (VNPB), dissolved in chlorobenzene, improved dramatically after the addition of 20% cyclohexanone. At the same time, better resistance of the crosslinked layer to a number of solvents frequently used in the fabrication of the adjacent emitting layer was achieved. Moreover, the maximum current efficiency in the binary solvent device increased significantly to 22.2 Cd/A, compared to 9.7 Cd/A in the reference device without VNPB, and the device lifetime was extended by up to 74.8%. This illustrates a simple and additive-free strategy to prevent the formation of defects in solution processed thin-film stacks made of small-molecular materials. The insights gained will be helpful in the solution-based manufacture of large-area OLEDs and other optoelectronic components. Graphic Abstract
Keywords Organic light-emitting diode · Crosslinkable small molecule · Binary solvent · Film morphology · Solvent resistivity * Robert Bail [email protected] Extended author information available on the last page of the article
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1 Introduction Solution processing of various photoactive materials enables the rapid deposition of multilayer stacks required in organic light-emitting diodes (OLEDs), which could potentially enable a more cost-effective production of large-area display panels [1–4]. However, high efficiency and long lifetime in solution-processed, multi-layer OLED devices can only be achieved if partial dissolution of the hole transport layer (HTL) and intermixing with the subsequently deposited layer—often the emitting layer (EML)— is prevented. To address this challenge, a number of photocurable or thermally crosslinkable hole transport materials (x-HTMs) have been developed [5–8]. Thermal x-HTMs with high purity can be obtained by functionalization of conventional small molecular materials such as N,N′-(3methyl-phenyl)-1,1′-biphenyl-4,4′-diamine (TPD), N,N′di(1-naphthyl)-N,N′-diphenylbenzidine (NPD), or tri(Ncarbazolyl)-triphenylamine (TCTA), for example, with two styryl groups [9–13]. These materials possess high triplet energy, suitable HOMO levels, chemical stability and, once crosslinked, good resistance to solvents commonly used in the deposition of th
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