Solution-processed bipolar small molecular host materials for single-layer blue phosphorescent organic light-emitting diodes

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Vol. 2; no. 2; pp. 382 - 391
• Main Authors: Lee, Yi-Ting, Chang, Yung-Ting, Lee, Meng-Ting, Chiang, Po-Hsuan, Chen, Chin-Ti, Chen, Chao-Tsen
• Format: Journal Article
• Language: English
• Published: 01.01.2014
• Subjects: Charge transport; Current efficiency; Devices; Energy gap; Organic light-emitting diodes; Oxides; Pyrimidines; Transporting
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/C3TC31641F

Three new solution processable small molecular host materials based on the bis-[3,5-di(9H-carbazol-9-yl)phenyl] structural moiety have been developed for blue phosphorescent (FIrpic dopant) organic light-emitting diodes. All three host materials have been characterized as having high glass transition temperatures (T sub(g)s), 155-175 degree C, indicative of good morphological stability of their amorphous thin films prepared from the solution process. Whereas N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]methylamine (CzPAMe) has the highest solid state triplet energy gap (E sub(T)) of 2.73 eV, tetrakis-[3,3',5,5'-(9H-carbazol-9-y l)]triphenylphosphine oxide (CzPPO) and N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]pyrimidin-2-ami ne (CzPAPm) are two host materials which are potentially bipolar for charge transport due to the electron deficient units of phenylphosphine oxide and pyrimidine, respectively. Due to the insufficient E sub(T) (2.56 eV) of CzPAPm, CzPPO or CzPAMe devices are significantly better than CzPAPm devices with or without a 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phe nylene (OXD-7) co-host. Particularly, having no OXD-7 co-host and no vacuum thermal-deposited extra electron transporting layer, single-layer devices of CzPPO surpass CzPAMe devices and reach current efficiencies as high as 9.32 cd A super(-1) (or power efficiency of 4.97 lm W super(-1)), one of the highest efficiencies among small molecular devices with the same fabrication process and same device configuration.