Spectroscopic and electrical characteristics of highly efficient tetraphenylsilane-carbazole organic compound as host material for blue organic light emitting diodes Taiju Tsuboi a, * , Shun-Wei Liu b , Min-Fei Wu b , Chin-Ti Chen b a Faculty of Computer Science and Engineering, Kyoto Sangyo University, Kamigamo, Kyoto 603-8555, Japan b Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC article info Article history: Received 7 June 2009 Received in revised form 24 July 2009 Accepted 24 July 2009 Available online 4 August 2009 PACS: 33.20.Wr 33.50.Dq 73.61.Ph 78.55.Kz 78.60.Fi 78.66.Qn Keywords: Blue organic light emitting diodes Tetraphenylsilane-carbazole compound Carrier mobility Photoluminescence Fluorescence Phosphorescence abstract Thermal, electrical and spectroscopic properties have been studied for bis(3,5-di(9H-car- bazol-9-yl) phenyl)diphenylsilane (SimCP2) which has exhibited high external quantum efficiency of 17.7% and power efficiency of 24.2 lm/W when it is used as host material for iridium bis(4,6-difluorophenypyridinato)picolate (FIrpic) blue emitter. They are com- pared with 1,3-bis (9-carbazolyl) benzene (mCP) and 3,5-bis (9-carbazolyl) tetraphenylsi- lane (SimCP) which have been also used as host for blue emitters. SimCP2 exhibits a highest glass transition temperature (148 °C) and is morphologically more stable. The elec- tron and hole mobilities are higher (4.8 Â 10 À4 and 2.7 Â 10 À4 cm 2 V À1 s À1 , respectively, at electric field of 9 Â 10 4 V cm À1 ) than those of mCP and SimCP. The zero-phonon S 1 emis- sion band is observed at 344 nm, while the T 1 emission band at 412 nm, i.e., this material preserves the characteristics of wide band-gap of 3.56 eV and high T 1 triplet energy of 3.01 eV. From the intensity ratio of the T 1 emission to the S 1 emission, it is suggested that the intersystem crossing rate is smaller for SimCP2 than for mCP and SimCP. From these results, we clarify the reasons why SimCP2 is superior to mCP and SimCP as the host mate- rial for blue phosphorescence emitter in organic light emitting diodes. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Organic light emitting diodes (OLEDs) generate electro- luminescence (EL) under application of low bias voltage less than 10 V. Of various OLEDs, blue OLED is important for full color displays with primary RGB OLED emitters. The blue OLED is also important to obtain green and red light emission by combining with color changing media (CCM). The EL with high efficiency has been achieved by using a guest–host system in the emitting layer of OLED device. This system is made by dispersing emitting dopant in host matrix. In blue OLEDs, host materials with large triplet gap energy (>2.8 eV) is requested by the following reasons. One is to avoid triplet excitons of the dopant from quenching by back energy transfer from dopant to host, and the other is to confine the electro-generated triplet excitons in the dopant molecules. Arylamino-containing organic substances such as 4,4 0 -bis(9-carbazolyl)-2,2 0 -biphenyl (CBP) have been usu- ally chosen as host materials and are proved working rea- sonably well for phosphorescent green or red emitters. CBP, however, has a low glass transition temperature (62 °C [1]), resulting in morphological instability [2]. Addi- 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.07.020 * Corresponding author. Tel./fax: +81 75 705 1899. E-mail address: tsuboi@cc.kyoto-su.ac.jp (T. Tsuboi). Organic Electronics 10 (2009) 1372–1377 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel