An electron-transporting host material compatible with diverse triplet
emitters used for highly efficient red- and green-electrophosphorescent
devicesw
Tsyr-Yuan Hwu,
aTsung-Cheng Tsai,
bWen-Yi Hung,*
bSheng-Yuan Chang,
cYun Chi,*
cMei-Hsin Chen,
dChih-I Wu,
dKen-Tsung Wong*
aand
Liang-Chen Chi
aReceived (in Cambridge, UK) 9th May 2008, Accepted 14th July 2008 First published as an Advance Article on the web 2nd September 2008 DOI: 10.1039/b807954d
A bifluorene analogue, T2N, containing a pyridyl moiety serves as both a host and an efficient electron-transporting material that is compatible with various heavy metal-containing red (Ir, Ru, Os, and Pt) and green (Ir) phosphors for highly efficient phosphorescent OLEDs possessing simple device architectures.
Dispersing transition metal-containing phosphors into suita-ble host materials can reduce the quenching processes asso-ciated with the relatively long lifetimes and triplet–triplet annihilations of triplet excited states. Using this approach, both singlet and triplet excitons can be harvested efficiently, making it possible to prepare organic light-emitting devices (OLEDs) exhibiting 100% internal efficiency.1 Most of the efficient host materials reported were hole-transporting (HT) compounds—in particular, carbazole-based organoaryls;2
electron-transporting (ET) host materials are relatively rare.3 Electrophosphorescent devices incorporating HT-type host materials normally require sophisticated configurations, in-corporating various functional materials, including transport-ing and blocktransport-ing layers, to facilitate efficient exciton confinement on the phosphors. Such devices are not suffi-ciently cost-effective to compete with other contemporary flat-panel display technologies. In contrast, ET-type host materials are capable of being fabricated into efficient OLEDs with simplified device structures. Thus, the development of efficient ET-type host materials is in high demand for providing extra flexibility in designing device configurations. In previous stu-dies, we used spiro-configured bifluorenes as hosts for red electrophosphorescent devices;4the maximum electrolumines-cence (EL) quantum efficiencies observed were substantially higher than that obtained when using the carbazole-based host
4,40-bis(9-carbazolyl)-2,20-biphenyl (CBP).2a,3a We suspected
that if we implanted a p-electron-deficient heteroarene into the bifluorene backbone, the resulting molecule would have a relatively higher electron affinity, which would facilitate injec-tion and transport of electrons from the cathode. In this paper, we report a new 4-azafluorene-based host, T2N (Scheme 1), in which a nitrogen atom has been implanted into the backbone of a bifluorene unit, exhibiting modified electronic properties. This new material serves as a suitable ET host for a range of distinctive green and red emitters possessing wide structural features, giving electrophosphorescent devices with low turn-on voltages and remarkable efficiencies.
Scheme 1 depicts our synthetic pathway toward T2N. Suzuki coupling of the pyridine-containing amide 1 and the fluorene boronic ester 2 afforded the amide 3 in 68% isolated yield. The azafluorenone 4 was obtained in 77% yield through intramole-cular cyclization upon treating 3 with LDA. The Grignard reaction of 4 with p-tolylmagnesium bromide and subsequent Friedel–Crafts alkylation provided T2N in 42% yield.
The introduction of a pyridyl moiety as a constituent of the p-conjugated backbone had an interesting effect on the physical properties of T2N relative to those of its pure-hydrocarbon counterpart, T2.5 The fluorescence and absorption spectra of
T2N in CH2Cl2solution (Fig. 1) exhibited limited red shifting
relative to those of the parent bifluorene T2. The phosphores-cence of T2N was also measured at 77 K (in EtOH). The difference between the fluorescence (377 nm) and phosphores-cence (506 nm) spectra corresponds to an exchange energy of ca. 1.2 eV between the singlet and triplet states of T2N. The triplet energy of 2.45 eV (defined as the 0–0 transition in the
Scheme 1 Synthesis of T2N. aDepartment of Chemistry, National Taiwan University, Taipei,
Taiwan 106. E-mail: [email protected]; Fax: +886 2 33661667; Tel: +886 2 33661665
b
Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung, Taiwan 202.
E-mail: [email protected]
cDepartment of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 300. E-mail: [email protected]
d
Department of Electrical Engineering and Graduate Institute of Electro-optical Engineering, National Taiwan University, Taipei, Taiwan 106
w Electronic supplementary information (ESI) available: Synthesis and characterization of new compounds; UPS traces of T2 and T2N. See DOI: 10.1039/b807954d
4956 | Chem. Commun., 2008, 4956–4958 This journal isc The Royal Society of Chemistry 2008
phosphorescent spectrum) of T2N is similar to that of T2 (2.48 eV).4These results indicate that the photophysical properties are slightly altered upon introducing the pyridyl moiety onto the conjugated chromophore.
The highest occupied molecular orbital (HOMO) energy level (5.75 eV) of T2N, taken from ultraviolet photoelectron spectroscopy (UPS) measurements (Fig. S-1, ESIw), and the corresponding lowest unoccupied molecular orbital (LUMO) energy (2.50 eV) derived by subtracting the HOMO energy from the UV-Vis absorption edge (3.25 eV). The energy band diagrams of T2 and T2N can be obtained from the UPS data (ESIw). The main features of occupied molecular orbitals of T2N at 4.6 eV, 7 eV, and 9 eV below the Fermi level all shift toward lower binding energy with respect to that of T2. The data indicate that the energy level of HOMO, as well as LUMO, of the T2N is lower than those of T2 (HOMO: 5.65 eV, LUMO: 2.35 eV). Thus, the electronic structure of the parent bifluorene T2 was perturbed after embedding the nitrogen atom into the parent hydrocarbon backbone. This modification decreased both the HOMO and LUMO energy levels; in particular, the low LUMO energy level of T2N is a crucial feature for reducing the energy barrier for electron injection from the cathode. In addition, the presence of the C9 p-tolyl substituents provided high thermal and morphological stability to the thin films of T2N, as indicated by their high decomposition (5% weight loss) temperatures (Td= 340 1C)
and moderately high glass transition temperatures (Tg =
144 1C). The relatively high triplet energy of T2N suggests that it might be a suitable host material for yellow to red phosphors. We employed a relatively simple configuration for fabricating OLED devices: ITO/polyethylenedioxythiophene/
polystyrene sulfonate (PEDOT:PSS, 30 nm)/4,40,400
-tri(N-carbazolyl)triphenylamine (TCTA, 40 nm)/T2N:dopant (25 nm)/T2N (35 nm)/LiF (0.5 nm)/Al; TCTA functioned as the HT layer, while T2N functioned as both host and ET materi-als. The low barrier between the HOMO energy levels of TCTA (5.6 eV) and T2N (5.75 eV) allowed direct hole injection from TCTA into T2N. The electrons injected from the T2N layer were trapped by the dopants dispersed within it, recombining with the holes to generate excitons directly, allowing efficient confinement of the emissive excitons within the emissive layer. We used the heavy metal complexes6 (Scheme 2) Os(fptz)2(PPh2Me)2, Os(fppz)2(PPhMe2)2, Btp2
Ir-(acac), Mpq2Ir(acac), Pt(iqdz)2, and Ru(ifpz)2(PPh2Me)2 as
red dopants and PPy2Ir(acac) as a green dopant for the T2N
layer. Table 1 summarizes the electroluminescence charac-teristics of the resulting devices.
As indicated in Fig. 2(a), these devices turned on at low voltages (2.9–5.7 V at a brightness of 0.1 cd m2), probably because the HOMO and LUMO energy levels of T2N matched well with those of TCTA and cathode, leading devices to have high current density (Fig. 2(b)). All of the devices exhibited high external quantum efficiencies (Zext410%; Fig. 3(a)), and
the EL spectra revealed pure dopant emissions (Fig. 3(b)). These results suggest that our present device structure, incor-porating T2N as an ET-type host, generally provides high-efficiency OLEDs for red phosphors featuring diverse metal centres, and T2N can also accommodate a green emitter [PPy2Ir(acac)] efficiently.
Among these devices, the highest external quantum effi-ciency (15.5% at 0.1 mA cm2) and highest brightness (29 700 cd m2 at 19 V) with saturated red emission of CIE1931 coordinates (0.68, 0.32) were obtained when using 9 wt% Os(fptz)2(PPh2Me)2as the dopant. To the best of our
knowl-edge, this is the highest efficiency reported for a red
Fig. 1 UV-Vis, fluorescence (room temperature in CH2Cl2solution), and phosphorescence (77 K, EtOH) spectra of T2N (black) and T2 (red).
Scheme 2 Structures of the heavy metal complexes used in this study.
Table 1 Electroluminescence characteristics of devices incorporating various red and green dopants
Dopant (wt%) Von
a
/V Lmax/cd m2 Imax/mA cm2 Zext max(%, cd A1) Zp max/lm W1 CIE (x,y)
Btp2Ir(acac) (5%) 3.2 8600 (18 V) 1330 10.4, 8.1 6.8 0.68; 0.32 Mpq2Ir(acac) (9%) 4.7 23 300 (19.5 V) 1180 13.7, 13.3 9.3 0.66; 0.34 Os(fptz)2(PPh2Me)2(9%) 5.5 29 700 (19 V) 1150 15.5, 11.4 6.9 0.68; 0.32 Os(fppz)2(PPhMe2)2(9%) 5.7 22 000 (19.5 V) 1250 13, 9.7 6.8 0.68; 0.32 Pt(iqdz)2(9%) 3.8 15 000 (19.5 V) 1200 10.2, 12.3 9.4 0.64; 0.36 Ru(ifpz)2(PPh2Me)2(6%) 5.6 4200 (19.5 V) 800 11.2, 7.9 6.3 0.68; 0.32 PPy2Ir(acac) (10%) 2.9 69 000 (17 V) 1600 12.3, 45 38 0.48; 0.52 a At a brightness of 0.1 cd m2.
phosphorescent OLED employing an ET-type host material. We note that although the efficiency dropped upon increasing the brightness,7this device retained its high efficiency (10% at 18 mA cm2) at a brightness of 1000 cd m2.
In summary, we have synthesized a novel host material, T2N, featuring an electron-deficient pyridyl moiety embedded into a bifluorene backbone. The electronic behaviour of this chromophore was modulated significantly relative to that of its parent compound, rendering T2N suitable for use as an electron-transporting host material compatible with various heavy metal-containing red and green phosphors. Most im-portantly, taking advantage of the dual functions of T2N, high-efficiency red- and green-phosphorescent OLEDs posses-sing simple device architectures can be fabricated.
This study was supported financially by the National Science Council and Ministry of Economic Affairs of Taiwan.
Notes and references
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Fig. 3 (a) External quantum efficiencies (EQEs) and (b) EL spectra of devices incorporating various dopants.
Fig. 2 (a) Brightness vs. voltage and (b) current density vs. voltage characteristics of devices having an identical structure but incorporat-ing different dopants.
