This journal is © The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 9761--9769 | 9761 Cite this: J. Mater. Chem. C, 2017, 5, 9761 A correlation between small-molecule dependent nanomorphology and device performance of organic light-emitting diodes with ternary blend emitting layers† Francis Okello Odongo Ngome, a Young-Tae Kim, * a Hyeon-Dong Lee, a Young-Hoon Kim, b Tae-Woo Lee bcd and Chan-Gyung Park* ae The morphology of emitting layers (EMLs) plays a vital role in determining the overall performance of solution processed phosphorescent organic light emitting diodes (PhOLEDs). Herein, the morphology of undoped small molecule binary blend EMLs prepared by blending tris(4-carbazoyle-9-ylphenyl)amine (TCTA) hole transport material (HTM) with a series of electron transport materials (ETMLs) was studied using a transmission electron microscope (TEM). Experimental results show that phase separation of the binary blend EMLs significantly depends on the polarity of the host. The binary blend ELMs were further doped with tris(2-phenylpyridine)- iridium(III) (Ir(ppy) 3 ) to form ternary blend EMLs and the resulting morphology was examined using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS). The results report for the first time the existence of Ir(ppy) 3 needle-like aggregates in small molecule ternary blend EMLs. By comparing the size of the aggregates formed in small molecule ternary blend EMLs with those formed in polymer–small molecule blends, our results showed that small molecule blend EMLs exhibit minimal Ir(ppy) 3 aggregates with a low aspect ratio in contrast to polymer–small molecule blends. The effect of mixed solvent on the distribution of the aggregates was also examined using a TEM and an atomic force microscope (AFM). The disappearance of the aggregates with varying solvent mixture ratios signifies that solvent mix- ture is an effective way to control homogeneous distribution of Ir(ppy) 3 in the emitting layers of PhOLEDs. This was further evidenced by an improvement in light emitting efficiency and current efficiency of OLED devices fabricated using a mixed solvent. 1. Introduction Organic light emitting diodes (OLEDs) have received a great deal of attention since their first introduction into the market in 1987 by C. W. Tang et al. 1 This is due to their superior properties such as high resolution from their self-emissive properties, flexibility, low power consumption, light weight, wide color gamut, wide viewing angle etc. which make them ideal for application in areas that require flat-panel displays and solid state lighting. 1–7 OLEDs are manufac- tured by thermal vacuum deposition and solution processing methods. The latter, however, is increasingly gaining much research interest due to its simplicity, low material waste, scalability and cost- effectiveness in large-area displays. 8–12 Nevertheless, one of the main issues in the solution processing technique is the ability to develop high performance-solution processed OLED materials. To enhance the internal quantum efficiency (IQE) of OLEDs, significant efforts have been made towards broadening the exciton recombination zone to minimize the accumulation of charge density in the emitting layer which would otherwise result in triplet–triplet annihilation (TTA) as well as triplet– polaron annihilation (TPA). 13 This is achieved by employing a binary blend consisting of hole and electron transport materials to make emitting layers (EMLs). 5,14 By using a binary host, the chances of excitons recombining within the emitting layer can be boosted which leads to improved device performance. Furthermore, incorporation of phosphorescent dopants in the emitting layer has been adopted to upgrade the device efficiency. The phosphorescent guest materials can promote intersystem a Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea. E-mail: yt1001@postech.ac.kr, cgpark1@postech.ac.kr b Research Institute of Advanced Materials, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea c BK21PLUS SNU Materials Division for Education Creative Global Leaders, Seoul National University, 1 Gwana-ro, Gwanak-gu, Seoul 08826, Republic of Korea d Department of Material Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea e National Institute for Nanomaterials Technology (NINT), Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc02404e Received 1st June 2017, Accepted 25th August 2017 DOI: 10.1039/c7tc02404e rsc.li/materials-c Journal of Materials Chemistry C PAPER Published on 25 August 2017. Downloaded by Seoul National University on 12/22/2020 7:48:14 AM. View Article Online View Journal | View Issue