J. Phys. Chem. C 2009, 113, 7405–7410
7405
On the Improvement of Blue Emission for All sp2-Hybridized Bistriphenylenyls: Incorporating Phenylenyl Moieties To Enhance Film Amorphism Jian-Yuan Yu,† Min-Jie Huang,‡ Chun-hsien Chen,*,‡ Chang-Sheng Lin,† and Chien-Hong Cheng*,† Department of Chemistry, National Tsing Hua UniVersity, Hsinchu, Taiwan 30013, and Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 10617 ReceiVed: NoVember 18, 2008; ReVised Manuscript ReceiVed: March 11, 2009
The film amorphism of 2,2′-bistriphenylenyl (BTP), a blue-light emitter with all sp2-hybridized polyaromatic, is enhanced by incorporating phenyl groups between the two triphenylene (TP) planes. This series of compounds are BTP, 1,4-di(triphenylen-2-yl)benzene (T1), and 4,4′-di(triphenylen-2-yl)biphenyl (T2), where the latter preserve the characteristics of all sp2 hydrocarbons and improve the optimized electroluminescence (EL) performance. Specifically, the most efficient T2-based device can reach a maximum external quantum efficiency of 5.2% and a very high luminescence of 47776 cd/m2 in the deep-blue region with Commission Internationale d’E´nclairage (CIE) coordinates of (0.15, 0.12). The efficiency approaches the upper theoretical limit of 5-5.5%, estimated for a singlet emitter with a refractive index of 1.7-1.5. For comparison, the optimized BTP-based devices confer the corresponding values of 4.2%, 21 204 cd/m2, and (0.14, 0.11). Such an improvement is ascribed to the effect of introducing the phenylene moiety on the film amorphism, examined by single-crystal X-ray analysis, atomic force microscopy, photoluminescent spectroscopy, and HOMO/LUMO energy levels for the materials involved in EL devices. Introduction
SCHEME 1: Chemical Structures of BTP, T1, and T2
Boosted by the environmental and economical needs of energy efficiency, the progress of OLED development (organic light-emitting diodes) for lighting and flat-panel displays has been prosperous. To strive for efficiency, one of the primary tasks is to minimize the radiationless deactivation of the electrically stimulated fluorophores. The aggregation-induced quenching, arising from intermolecular π-π stacking, appears an unavoidable radiationless pathway even when the thickness of the emission layer is fabricated at the nanometer level.1-6 To preserve the intensity3 and to avoid the associated red-shift4 of the fluorescent emission, the organics are thus engineered to be amorphous molecular materials (molecular glasses)7,8 for both polymers and small molecules where the latter are typically tailored with nonplanarity or bulky derivatives. Although polymeric fluorophores generally exhibit good processability and a strong tendency of preserving the amorphism at the operating temperature9-11 because of the large molecular weights and high Tg (glass transition temperature), small molecules have the advantages of better defined structure, precise molecular formula, and high purity.12-22 To obstruct the intermolecular π-π interaction between the small molecules, it would seem plausible to incorporate steric hindrance by decorating the polyaromatic cores with some aliphatic side chains. However, the thermal stability of corresponding devices deteriorates because such side chains generally lower the Tg of the flourophores.23,24 Furthermore, literature studies indicate that longer aliphatic side chains reduce the hole mobility25 and the charge injection,26 leading to an increase in the turn-on voltage.27,28 * Corresponding author. E-mail:
[email protected] (C.-h.C.); chcheng@ mx.nthu.edu.tw (C.-H.C). † National Tsing Hua University. ‡ National Taiwan University.
Our recent studies of a blue emitter, BTP (2,2′-bistriphenylenyl, Scheme 1), demonstrated that aggregates of polyaromatic hydrocarbons without steric derivatives intriguingly offer excellent OLED performance.29,30 This model compound bears all sp2-hybridized carbons and comprises two triphenylenes (TP), which adopt coplanarity in the crystal packing.29 In the OLED layout in which BTP was thermally evaporated on a hole transporting layer, TM-AFM (tapping mode atomic force microscopy) revealed that the BTP molecules grew granular aggregates with a nominal diameter of ca. 50-60 nm, regardless the BTP film thickness.29 Given the formation of nanoaggregates, instead of undertaking the radiationless decay process as the dominant pathway, BTP emitted brilliant blue luminescence with CIE coordinates of (0.15, 0.06), a maximum luminance of 14954 cd/m2, and an external quantum efficiency (EQE) of 2.8% (2.7 cd/Amp) from the device of ITO/NPB(40 nm)/BTP(40 nm)/ TPBI(40 nm)/Mg:Ag, where ITO, NPB, and TPBI were the anode, the hole transporting layer (HTL), and the electron transportinglayer(ETL),respectively,abbreviatedforindium-tin-oxide, N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine, and 1,3,5-tris(phenyl-2-benzimidazoly)benzene. To examine the effect of the molecular packing structure on the photoluminescent (PL) behaviors, BTP films exhibiting crystal-
10.1021/jp8101223 CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
7406 J. Phys. Chem. C, Vol. 113, No. 17, 2009 line, nanoaggregated, and amorphous features were prepared.29 The spectra of films with nanoaggregates more closely resemble those of crystalline BTP than the spectra of the amorphous ones. Together with simulation results of a small torsion barrier for the rotation of the TP planes (
