Efficient red phosphorescent OLEDs employing carbazole-based materials as the emitting host Chih-Hao Chang a, * , Raimonda Griniene b , Yu-De Su a , Chia-Chi Yeh a , Hao-Che Kao a , Juozas Vidas Grazulevicius b , Dmytro Volyniuk b , Saulius Grigalevicius b, ** a Department of Photonics Engineering, Yuan Ze University, Chung-Li, 32003, Taiwan b Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu Plentas 19, LT50254, Kaunas, Lithuania article info Article history: Received 5 May 2015 Received in revised form 15 June 2015 Accepted 29 June 2015 Available online 11 July 2015 Keywords: Electro-active Carbazole Thermally stable TCTA Phosphorescent Organic light-emitting diodes abstract We report on the synthesis and characterization of a new series of electro-active carbazole-based compounds. The derivatives are thermally stable amorphous materials with glass transition tempera- tures in the range of 54e93  C. Electron photoemission spectra of thin layers of the materials show ionization potential in the range of 5.4e5.5 eV. The carbazole-based derivatives are fully characterized and their spectroscopic properties are determined by absorption and photoluminescence. All developed materials and commonly-used tris(4-carbazoyl-9-ylphenyl)amine (TCTA) were used as hosts in red phosphorescent organic light-emitting diodes (OLEDs) for comparison. Results indicate that a device with 3-[bis(9-ethylcarbazol-3-yl)methyl]-9-hexylcarbazole exhibited superior performance with peak efficiencies of 8.4%, 5.3 cd/A and 5.5 lm/W. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Applications for organic light-emitting devices (OLED) displays are expanding rapidly due to their superior performance and flexibility as compared with liquid crystal display. Furthermore, phosphorescent OLEDs (PhOLEDs) have attracted research atten- tion because of their intrinsically higher efficiencies as compared to fluorescent OLEDs [1]. To achieve theoretical efficiency levels, the triplet energy should be confined on the phosphorescent dopant without endothermic energy transfer to the host material. Thus, multiple-layer configurations have been widely developed to enhance device efficiency of PhOLEDs, especially in blue and green devices. In contrast, red phosphorescent materials with lower triplet energy gaps allow for simplified device architectures [2]. However, the lower gap of red phosphors usually induces serious carrier trapping, resulting in higher operation voltages. Accord- ingly, realizing red PhOLEDs with low power consumption and high performance requires the development of new host materials. To guarantee exothermic energy transfer, the triplet energy of host materials must be higher than that of the dopant [3]. In addi- tion, a host material with a bulky structure is desirable to favor a spatially-dispersed triplet exciton, which could reduce the inci- dence of tripletetriplet annihilation (TTA) [4]. The carbazole moiety possesses several commendable properties, including chemical stability, easy modification, large triplet energy etc. [5] Conse- quently, many successful host or hole transport materials adopted carbazole-containing designs. For instance, in 2007 our group syn- thesized and characterized carbazole-based aromatic amines with oxetanyl functional groups. The adequate ionization potentials (4.9e5.0 eV) and the charge mobility demonstrated the suitability of these carbazole-based compounds for use in organic electronics [6]. At the same time, we also synthesized a carbazole-based material, 3,6-di(9-carbazolyl)-9-(2-ethylhexyl) carbazole (TCz1), which pos- sesses structurally rigid moieties and a nonplanar molecular configuration, resulting in a morphologically-stable molecule with a wide triplet energy gap [7]. Sky blue PhOLEDs with a TCz1 host were demonstrated with efficiencies of up to 15%, 31 cd/A, and 28 lm/W. In 2011, Chang et al. developed a bipolar carbazole-based material 9- (4,6-diphenyl-1,3,5-triazin-2-yl)-9 0 -phenyl-3,3'-bicarbazole (CzT) comprising a dicarbazole donor linked to an electron deficient 1,3,5- triazine acceptor [8]. Sufficient triplet energy (E T ¼ 2.67 eV) together * Corresponding author. Tel.: þ886 3 4638800x7517; fax: þ886 3 4514281. ** Corresponding author. Tel.: þ370 37 300192; fax: þ370 37 300152. E-mail addresses: chc@saturn.yzu.edu.tw (C.-H. Chang), saulius.grigalevicius@ ktu.lt (S. Grigalevicius). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig http://dx.doi.org/10.1016/j.dyepig.2015.06.038 0143-7208/© 2015 Elsevier Ltd. All rights reserved. Dyes and Pigments 122 (2015) 257e263