Efficient carrier- and exciton-confining device structure that enhances blue PhOLED efficiency and reduces efficiency roll-off Wen-Yi Hung a,⇑ , Zhong-Wen Chen a , Hong-Wei You a , Fu-Chuan Fan b , Hsiao-Fan Chen b , Ken-Tsung Wong b,⇑ a Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan b Department of Chemistry, National Taiwan University, Taipei 106, Taiwan article info Article history: Received 16 November 2010 Received in revised form 11 January 2011 Accepted 11 January 2011 Available online 31 January 2011 Keywords: Phosphorescent OLEDs Device structure Time-of-flight Charge mobility abstract In this study we used a known host material diphenylbis[4-(9-carbazoyl)phenyl]silane (SiCa) to realize a highly efficient bis[4,6-(difluorophenyl)-pyridinato-N,C 2 0 ]picolinate (FIr- pic)–based blue phosphorescent device exhibiting high efficiencies of up to 18.3%, 41.5 cd/ A, and 31 lm/W in a carrier- and exciton-confined device structure. At a practical bright- ness of 1000 cd/m 2 , the device performance (g ext ) remained as high as 16.7%. The enhanced efficiency and reduced roll-off resulted mainly from the introduction of (a) DTAF as an HTL that efficiently blocked and confined excitons within the FIrpic-doped emissive layer, and (b) a thin layer of DPPS between the EML and TAZ, serving as an exciton- and hole-blocking layer to suppress exciton quenching by FIrpic at the EML–ETL interface. In addition, we incorporated a red phosphorescent dopant (Mpq 2 Iracac) into the blue device to fabricate a white phosphorescent organic light-emitting device, which exhibited satisfactory effi- ciencies (11.6%, 21.2 cd/A, 14.2 lm/W) with the Commission Internationale de L’éclairage (CIE) coordinates of (0.35, 0.38). Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Phosphorescent organic light-emitting devices (PhOL- EDs) featuring emissive layers comprising transition me- tal–centered dyes doped into appropriate host materials are attracting much research attention because they can achieve internal efficiencies as high as 100% [1]. Unlike the reported efficient green- and red-PhOLEDs, the realiza- tion of highly efficient blue PhOLEDs remains a great chal- lenge because the triplet energy (E T ) of host molecules must be sufficiently high (i.e., E T P 2.7 eV) to confine the triplet excitons within the emitting layer [2–5]. In addition, the tendency of triplet excitons to diffuse into the non- radiative levels of the neighboring hole transport layer (HTL) and/or electron transport layer (ETL) must also be avoided. In this regard, Zheng et al. demonstrated highly efficient blue PhOLEDs using 1,1-bis[(di-4-tolylami- no)phenyl]cyclohexane (TAPC) as the HTL, obtaining a maximum external quantum efficiency (g ext ) of 18.1%, which is approximately 50% higher than the value obtained (12%) for a previously reported device featuring bis[N-(1- naphthyl)-N-phenylamino]biphenyl (NPB) as the HTL [6]. The Kido group recently reported several ETLs exhibiting hole-blocking ability that improve the performance of bis[4,6-(difluorophenyl)-pyridinato-N,C 2 0 ]picolinate (FIr- pic)–based blue PhOLEDs [7–9]. For example, a device incorporating diphenylbis[4-(pyridin-3-yl)phenyl]silane (DPPS) as the ETL provided a maximum value of g ext of 22%, which is much higher than the value obtained (15.3%) when using 3-(4-biphenylyl)-4-phenyl-5-(4-tert- butylphenyl)-1,2,4-triazole (TAZ) as the ETL [9]. It appears that achieving highly efficient blue PhOLEDs requires improvements in the charge recombination efficiency by 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.01.012 ⇑ Corresponding authors. Tel.: +886 2 24622192x6718; fax: +886 2 24634360 (W.-Y. Hung). E-mail addresses: wenhung@mail.ntou.edu.tw (W.-Y. Hung), kenwong @ntu.edu.tw (K.-T. Wong). Organic Electronics 12 (2011) 575–581 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel