Lithium phenolate complexes for an electron injection layer in organic light-emitting diodes

Organic Electronics 10 (2009) 228–232

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Lithium phenolate complexes for an electron injection layer in organic light-emitting diodes Yong-Jin Pu *, Masashi Miyamoto, Ken-ichi Nakayama, Toshiro Oyama, Yokoyama Masaaki, Junji Kido * Department of Organic Device Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan

a r t i c l e

i n f o

Article history: Received 29 July 2008 Received in revised form 7 November 2008 Accepted 7 November 2008 Available online 17 November 2008

PACS: 61.66.Hq 78.55.Fv 78.55.Kz 78.60.Fi

a b s t r a c t We synthesized p-conjugated lithium phenolate complexes, lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(20 , 200 -bipyridine-60 -yl)phenolate (LiBPP), and lithium 2-(isoquinoline-10 -yl)phenolate (LiIQP). These complexes showed lower sublimation temperatures of 305–332 °C compared to 717 °C of LiF. The organic light-emitting devices (OLEDs) using these complexes as an electron injection layer exhibited high efficiencies which are comparable to that of the device using LiF. Especially, a 40-nm thick film of LiBPP or LiPP was effective as an electron injection material, providing low driving voltages, while such a thick film of LiF serves as a complete insulator, resulting in high driving voltages. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Lithium complexes Electron injection Organic light-emitting diode

1. Introduction In organic light-emitting devices (OLEDs), aluminum metal has been widely used for cathode because of its high stability in air and easy processability. However, work function of Al (4.2 eV) as an anode is not low enough to inject electrons into organic layer at low driving voltages. In the first report on an efficient OLED by Tang and Van Slyke, Mg:Ag alloy was used to improve electron injection from cathode [1]. Later, Li:Al alloy and double layer cathode of Li/Al were found to reduce driving voltage drastically [2]. These effects are due to lower work function of Li than that of Mg. Insertion of thin inorganic Li salt layer, such as Li2O [3] or LiF [4], between organic layer and cathode Al was found to be also very effective * Corresponding authors. E-mail addresses: [email protected] (Y.-J. Pu), [email protected]. ac.jp (J. Kido). 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.11.003

to reduce electron injection barrier to the organic layer, and now it is widely used. LiF is expected to be reduced to Li by thermally activated hot Al in successive deposition, and interacts with an electron-transporting material like an electron donor [5]. That is why the thickness of LiF has to be very thin, no thicker than 1 nm, to be fully reacted by Al and not to leave unreduced insulating part of LiF. High temperature is also required to evaporate LiF under vacuum. Such thin thickness and low processability are disadvantages, especially in industrial process and large area deposition. On the other hand, organic pconjugated ligands can be helpful to reduce evaporation temperature in a deposition process, and give chargetransporting ability to the complex. We previously reported that lithium quinolinolate complex (Liq) serves as an excellent electron injection material (EIM), and its thickness is much less sensitive to a performance of the device than that of LiF, because of an electron-transporting ability of Liq [6].

229

Y.-J. Pu et al. / Organic Electronics 10 (2009) 228–232

Br N

i

HO

LiO

ii N

N

1

LiPP

Br N

HO

i

LiO

ii

N

N N N

N 2

LiBPP

Cl N

i

HO

ii

LiO

N

N

3

LiIQP

Scheme 1. Synthetic route of the lithium complexes. Reagents and conditions: (i) 2-hydroxyphenylboronic acid, Pd(PPh3)4, K2CO3 aqueous, toluene, ethanol, reflux, 8 h and (ii) LiOHH2O, methanol, r.t., 1 h.

In this paper, we report new series of Li complexes, such as lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(20 , 20 0 -bipyridine-60 -yl)phenolate (LiBPP), and lithium 2-(isoquinoline-10 -yl)phenolate (LiIQP), and their improved electron-transporting ability in OLED due to their more extended p-conjugated ligand than that of Liq. 2. Results and discussion 2.1. Synthesis, thermal and optoelectrochemical properties Phenol ligands 1–3, substituted 2-pyridyl or 2-quinolyl group on ortho position, were synthesized by Suzuki coupling of 2-hydroxyphenylboronic acid and the corresponding bromopyridine or chloroquinoline. Complexation reactions of the phenols and lithium hydroxide in methanol were straightforward, and the yields were quantitative (Scheme 1). All these complexes were further purified by train sublimation, and characterized with 1H NMR and elemental analysis. Melting points of the complexes were relatively high (>300 °C) although their molecular weight is only around 200, probably because of their strong molecular interaction derived from their ionic property (Table 1). The complexes were sublimable, and the sublimation temperatures Tss of the complexes were estimated from 10 wt% loss temperature in thermal gravimetric analysis under vacuum (Fig. 1). The lithium phenolate complexes showed much lower Tss around 300 °C, while LiF showed a high Ts of 717 °C. These Tss are nearly 100 °C lower than their decomposition temperatures, suggesting that the

complexes are not decomposed and do not release lithium ion or neutral atom during the thermal evaporation process. UV–vis and photoluminescence (PL) spectra of the film on quartz substrates are shown in Fig. 2. LiPP showed a widest p–p* energy gap derived from the ligand 1, which is estimated from the absorption edge, and LiBPP showed the narrowest gap, derived from the ligand 2. The photoluminescence of the complexes was blue to green, and the order of kmax is consistent with the order of energy gap from UV spectra. The highest occupied molecular orbital (HOMO) levels of the complexes, or the ionization potentials (Ips), determined from photoelectron spectrometer surface analysis, were 5.56–5.70 eV, and the lowest unoccupied molecular orbital (LUMO) levels, estimated by the difference of Ip and optically obtained energy gap, were 2.49–2.68 eV.

Table 1 Thermal and optoelectrochemical properties of the lithium phenolate complexes.

LiF LiPP LiBPP LiIQP a b c

Tma (°C)

Tdb (°C)

Tsc (°C)

Eg (eV)

PL (nm)

Ip (eV)

Ea (eV)

845 331 361 351

– 411 431 455

717 317 305 332

– 3.18 2.95 3.02

– 430 546 508

– 5.67 5.56 5.70

– 2.49 2.61 2.68

Melting point. Decomposition temperature: 5 wt% loss at atmosphere. Sublimation temperature: 10 wt% loss at 2.0  105 Torr.

230

Y.-J. Pu et al. / Organic Electronics 10 (2009) 228–232

0

Weight loss (wt%)

-20 LiF LiPP LiBPP LiIQP

-40 -60 -80 -100 0

200

400

600

800

Temperature (°C) Fig. 1. Thermal gravimetric analysis of the complexes under vacuum.

2.5

Intensity (a. u.)

2 LiPP LiBPP LiIQP

1.5

1

0.5

0 200

300

400

500

600

700

Wavelength (nm) Fig. 2. UV–vis and PL spectra of the complexes in the film.

2.2. OLED performance The OLEDs with the configuration of an ITO/N,N0 bis(naphthalen-1-yl)-N, N0 -bis(phenyl)benzidine (a-NPD) (50 nm)/tris(8-quinolinolato) aluminum (Alq3) (70 nm)/ EIM (0.5–7.0 nm)/Al (100 nm) were fabricated. The device performance is summarized in Table 2. The devices without EIMs:ITO/a-NPD (50 nm)/Alq3 (70 nm)/Al (100 nm) showed much higher driving voltage and much lower efficiencies compared with those with EIMs. These results demonstrated that the electron injection from Al cathode (4.2 eV) was not blocked by the higher LUMO levels of the lithium complexes, but was rather facilitated, although the LUMO levels of the EIMs (2.5–2.7 eV) are higher than that of Alq3 (3.2 eV). This improvement of the electron injection is probably because of the low barrier height for the electron injection from the Al cathode, which are the results from the lithium metal doping of the EIL materials

at the cathode interface. As reported [6], the lithium ion in the organic lithium complexes can be reduced by thermally activated Al to release lithium metal, which dopes the EIL materials at the interface to form the gap states. When a thin layer, <3.0 nm, of the lithium complexes was used as an EIM, the turn-on voltages at 1 cd/m2 in the all devices were around 3–4 V, and the driving voltages at 100 cd/m2 were 6–8 V. The efficiencies are comparable to those of the device using LiF as an EIM. On the other hand, the life time of the devices using these lithium phenolate complexes was the same as that of the device using LiF, and so there was no negative effect of the phenolate ligand for the device stability. These results reveal that the three lithium complexes can be alternative to LiF as an EIM. In contrast to LiF, these complexes have a p-conjugated ligand, which may act as a charge-transporting moiety. To investigate the electron-transporting ability of the complexes, the devices with a thick EIM layer, having a structure of an ITO/NPD (50 nm)/Alq3 (70  x nm)/EIM (x = 20 or 40 nm)/Al (100 nm), were fabricated. Fig. 3 shows the dependency of driving voltage on the thickness of EIM. As the thickness of LiPP increased from 3.0 to 40 nm, the turn-on voltage at 1 cd/m2 increased by 8.8–13.2 V. On the other hand, the turn-on voltage with LiBPP and LiIQP increased only by 2.1 and 2.6 V, respectively, with increase of the thickness from 3.0 to 40 nm. At 100 cd/m2, LiBPP and LiIQP also showed less increase in the driving voltage compared with that of LiPP. These results show that LiBPP and LiIQP may have higher electron-transporting mobility than LiPP, resulting from their wider p-conjugation. The EL emissions of the devices were green and ascribed to the emission of Alq3. However, the EL spectra with a thick LiPP layer exhibited a shoulder peak around 430 nm derived from LiPP emission (Fig. 4). The shoulder peak was increased as the thickness of LiPP increased from 20 to 40 nm. These shoulder peaks indicate that all injected holes were not consumed through recombination in Alq3 because of a poorer electron-transporting ability of LiPP, and some of the holes reached into LiPP layer to give a blue emission of LiPP (Scheme 1). When BCP (bathocuproine), which has a more crystalline feature than Alq3, was used as 0.5 nm of thin ETL between Alq3 and EIL (device: ITO/NPD (50 nm)/Alq3 (60 nm)/BCP (10 nm)/EIM (LiF or LiPP) (0.5 nm)/Al (100 nm)), LiPP showed a slightly poorer electron injection compared with LiF. This result is probably because LiF having smaller molecular size than LiPP could make a better contact with rough BCP surface, while there were no differences between LiF and LiPP for the electron injection to amorphous Alq3 layer. Further investigation on the effect of roughness of ETL and on the size of the lithium complexes is ongoing.

3. Conclusion

p-Conjugated lithium phenolate complexes, lithium 2(2-pyridyl)phenolate (LiPP), lithium 2-(20 , 200 -bipyridine60 -yl)phenolate (LiBPP), and lithium 2-(isoquinoline-10 yl)phenolate (LiIQP), were synthesized. The complexes

231

Y.-J. Pu et al. / Organic Electronics 10 (2009) 228–232 Table 2 OLED performances with the various thicknesses of the EIM. EIMa (nm)

1 cd/m2

100 cd/m2

1000 cd/m2

No EIMb

(V) 9.1

(V) 14.8

(lm/W) 0.4

(cd/A) 0.8

(%) 0.6

(V) 18.2

(lm/W) 0.4

(cd/A) 2.1

(%) 0.6

LiPP

0.5 1.0 2.0 3.0 5.0 20 40

3.9 3.1 3.4 4.4 7.5 9.1 13.2

7.6 5.9 6.8 8.9 14.2 14.8 19.1

1.5 1.6 1.7 1.0 0.1 0.1 0.2

3.6 3.1 3.8 3.0 0.3 0.3 0.9

1.1 1.0 1.2 0.9 0.1 0.1 0.3

10.4 8.6 9.8 12.3 – – –

1.3 1.4 1.4 0.9 – – –

4.4 3.7 4.5 3.5 – – –

1.4 1.2 1.5 1.1 – – –

LiBPP

0.5 2.0 3.0 5.0 7.0 20 40

4.0 4.0 2.9 3.4 4.5 4.7 5.0

7.7 7.3 5.6 7.3 9.2 9.7 10.6

1.5 1.4 1.7 0.6 0.9 1.0 0.7

3.6 3.3 3.0 1.4 2.5 3.1 2.5

1.2 1.0 1.0 0.4 0.8 1.0 0.8

10.6 10.1 8.2 10.8 12.6 13.1 14.1

1.2 1.2 1.4 0.5 0.8 0.8 0.6

4.2 3.9 3.6 1.7 3.0 3.4 2.6

1.3 1.3 1.1 0.5 0.9 1.1 0.8

LiIQP

0.5 2.0 3.0 5.0 20 40

3.0 3.1 3.0 3.6 6.0 5.6

6.3 6.6 6.1 7.6 11.0 11.0

1.8 1.2 1.5 1.1 0.8 0.7

3.6 2.6 2.9 2.6 2.8 2.6

1.2 0.8 0.9 0.8 0.8 0.8

9.1 9.5 8.8 10.8 14.2 14.6

1.5 1.0 1.2 0.9 0.7 0.6

4.3 3.2 3.4 3.0 3.2 3.0

1.4 1.0 1.1 0.9 1.0 0.9

a EIM, 0.5–7.0 nm: ITO/NPD (50 nm)/Alq3 (70 nm)/EIM (0.5–7.0 nm)/Al (100 nm) and EIM, 20 or 40 nm: ITO/NPD (50 nm)/Alq3 (70  x nm)/EIM (x nm)/Al (100 nm). b ITO/NPD (50 nm)/Alq3 (70 nm)/Al (100 nm).

20 1

0.8

Intensity (a.u.)

Voltage (V)

15

10

5

0.6

0.4

0.2

0

0

10

20

30

40

Thickness of EIM (nm) Fig. 3. Driving voltage at 1 cd/m2 (solid line) and 100 cd/m2 (dotted line) of LiPP (circle), LiBPP (triangle) and LiIQP (square). Device structures: ITO/ NPD (50 nm)/Alq3 (70 nm)/EIM (0.5–7.0 nm)/Al (100 nm); ITO/NPD (50 nm)/Alq3 (50 nm)/EIM (20 nm)/Al (100 nm) and ITO/NPD (50 nm)/ Alq3 (30 nm)/EIM (40 nm)/Al (100 nm).

were readily sublimated under vacuum, compared with LiF. The organic light-emitting diodes (OLEDs) using these complexes as an electron injection layer exhibited a facilitated electron injection from Al cathode to Alq3 emitting layer, as well as the device using LiF, and even in 40 nm of thick film, LiBPP and LiPP were still effective because of their electron-transporting p-conjugated ligand.

0 300

400

500

600

700

800

WaveLength (nm) Fig. 4. EL spectra of the OLED devices with LiPP at 20 mA/cm2. Circle: ITO/ NPD (50 nm)/Alq3 (30 nm)/LiPP (40 nm)/Al (100 nm); square: ITO/NPD (50 nm)/Alq3 (50 nm)/LiPP (20 nm)/Al (100 nm) and triangle: ITO/NPD (50 nm)/Alq3 (70 nm)/LiF (0.5 nm)/Al (100 nm).

4. Experimental 4.1. Materials 6-Bromo-2, 20 -dipyridyl was synthesized according to the literature. 2-Hydroxyphenylboronic acid was purchased from Wako Chemical.

232

Y.-J. Pu et al. / Organic Electronics 10 (2009) 228–232

4.2. Measurements The 1H NMR spectra were measured in deuterated solvents with JEOL ECX 400 MHz spectrometers. Elemental analyses were carried out in the Elemental Analysis Service, Yamagata University, Japan. Mass spectra were obtained by a JEOL JMS-K9 mass spectrometer. Thermal gravimetric analysis was performed on SII EXSTAR 6000 and TGA/DTA 62000. Ionization potentials were measured with Photoelectron Spectrometer Surface Analyzer (RIKEN KEIKI AC3). UV–visible absorption spectra were recorded with a Shimadzu UV-3150 spectrometer. PL spectra were recorded using a Jobin Yvon Fluoromax-2 fluorometer. The EL devices were fabricated on indium tin oxide (ITO) coated glass substrates, ultrasonicated sequentially in detergent, methanol, 2-propanol, and acetone and exposed under UV–ozone ambient for 20 min. NPD and Alq3 materials were deposited by thermal evaporation under 2  104 Pa. EIM and aluminum were finally deposited under 1  103 Pa as a cathode through a shadow mask, and its active area is 5  5 mm2. Layer thickness calibration was performed using Dektak 3 surface profilometer. The EL spectra were measured on a Hamamatsu photonic multichannel analyzer PMA-11. The current–voltage (I–V) characteristics and luminance were measured using Keithley 2400 Source Meter and Konika Minolta CS-200, respectively. External quantum efficiencies were calculated assuming Lambertian emission pattern and considering all spectral features in the visible. Ligand 1: A mixture of 2-bromopyridine (5.21 g, 33.0 mmol), 2-hydroxyphenylboronic acid (4.14 g, 30.0 mmol), toluene (120 ml), ethanol (60 ml), aqueous potassium carbonate (2.0 mol/l, 30 ml) and tetrakis(triphenylphosphine)palladium (0) was heated at 80 °C for 8 h under nitrogen. The mixture was allowed to cool to room temperature, and then toluene was added. The organic layer was washed with water, dried over magnesium sulfate and filtered. The solvent was removed in vacuo, and the residue was purified by column chromatography over silica using a chloroform–hexane mixture (1:1) as eluent to give 1 (3.4 g, 66%). 1H NMR (400 MHz, d6DMSO, ppm), 8.62 (1H, d, J = 4.1 Hz), 8.21 (1H, d, J = 8.6 Hz), 8.03–8.00 (2H, m), 7.43 (1H, t, J = 6.1 Hz), 7.30 (1H, t, J = 7.5 Hz), 6.91 (2H, t, J = 4.1 Hz). EI–MS (m/z) calcd. 171.2. found 171. Elemental analysis calcd. for C11H9NO: C, 77.17; H, 5.30; N, 8.18. found: C, 77.08; H, 5.39; N, 8.17. Ligand 2: A mixture of 6-bromo-2, 20 -dipyridyl (5.05 g, 21.5 mmol), 2-hydroxyphenylboronic acid (3.27 g, 23.7 mmol), toluene (100 ml), ethanol (50 ml), aqueous potassium carbonate (2.0 mol/l, 24 ml), and tetrakis(triphenylphosphine)palladium (0) was heated at 70 °C for 8 h under nitrogen. The mixture was allowed to cool to room temperature, and then toluene was added. The organic layer was washed with water, dried over magnesium sulfate, and filtered. The solvent was removed in vacuo, and the residue was purified by column chromatography over silica using a chloroform–ethyl acetate mixture (9:1) as eluent to give 2 (4.8 g, 90%). 1H NMR (400 MHz, d6DMSO, ppm) 8.77 (1H, d, J = 4.5 Hz), 8.32–8.26 (2H, m), 8.19–8.13 (2H, m), 8.09–8.03 (2H, m), 7.53 (1H, dd, J = 7.7, 5.0 Hz), 7.34 (1H, t, J = 7.7 Hz), 6.98–6.94 (2H, m). EI–MS (m/z) calcd. 248.3. found 248. Elemental analysis

calcd. for C16H12N2O: C, 77.40; H, 4.87; N, 11.28. found: C, 77.34; H, 4.93; N, 11.32. Ligand 3: A mixture of 1-chloroisoquinoline (5.20 g, 31.8 mmol), 2-hydroxyphenylboronic acid (5.21 g, 37.8 mmol), toluene (100 ml), ethanol (60 ml), aqueous potassium carbonate (2.0 mol/l, 38 ml) and tetrakis(triphenylphosphine)palladium (0) was heated at 70 °C for 8 h under nitrogen. The mixture was allowed to cool to room temperature, and then toluene was added. The organic layer was washed with water, dried over magnesium sulfate, and filtered. The solvent was removed in vacuo, and the residue was purified by column chromatography over silica using a toluene–ethyl acetate mixture (9:1) as eluent to give 3 (5.5 g, 78%). 1H NMR (400 MHz, d6DMSO, ppm) 9.67 (1H, s), 8.53 (1H, d, J = 6.0 Hz), 7.99 (1H, d, J = 8.2 Hz), 7.81 (1H, d, J = 6.0 Hz), 7.76–7.71 (2H, m), 7.57 (1H, t, J = 7.6 Hz), 7.35–7.27 (2H, m), 7.01–6.94 (2H, m). EI–MS (m/z) calcd. 221.3. found 221. Elemental analysis calcd. for C15H11NO: C, 81.43; H, 5.01; N, 6.33. found: C, 81.46; H, 5.07; N, 6.33. 4.3. General procedure of the complexation A methanol solution (2.5 ml) of a ligand (1.0 mmol) was slowly added to a methanol solution (2.5 ml) of lithium hydroxide monohydrate (1.0 mmol), and the mixture was stirred at room temperature. After 30 min, the solvent was evaporated in vacuum to give a yellow solid. The obtained compounds were purified with train sublimation. LiPP: 1H NMR (400 MHz, d6DMSO, ppm) 8.45 (1H, d, J = 7.7 Hz), 8.40 (1H, d, J = 4.5 Hz), 7.68–7.64 (2H, m), 7.06 (1H, t, J = 5.9 Hz), 6.86 (1H, t, J = 7.5 Hz), 6.39 (1H, d, J = 7.7 Hz), 6.15 (1H, s). Elemental analysis calcd. for C11H8NOLi: C, 74.59; H, 4.55; N, 7.91. found: C, 74.64; H, 4.44; N, 7.85. LiBPP: 1H NMR (400 MHz, d6DMSO, ppm) 8.88 (1H, d, J = 7.7 Hz), 8.64 (1H, d, J = 3.6 Hz), 8.43 (1H, d, J = 7.7 Hz), 8.00 (1H, d, J = 7.7 Hz), 7.96–7.92 (2H, m), 7.73 (1H, t, J = 7.9 Hz), 7.40 (1H, t, J = 5.8 Hz), 6.87 (1H, t, J = 7.5 Hz), 6.46 (1H, d, J = 8.2 Hz), 6.18 (1H, t, J = 7.0 Hz). Elemental analysis calcd. for C16H11N2OLi: C, 75.59; H, 4.36; N, 11.02. found: C, 75.58; H, 4.23; N, 11.01. LiIQP: 1H NMR (400 MHz, d6DMSO, ppm) 8.34 (1H, d, J = 5.9 Hz), 8.10 (1H, d, J = 8.2 Hz), 7.85 (1H, d, J = 8.2 Hz), 7.64 (1H, t, J = 7.5 Hz), 7.57 (1H, d, J = 5.9 Hz), 7.46 (1H, t, J = 7.7 Hz), 6.96–6.88 (2H, m), 6.37 (1H, d, J = 8.2 Hz), 6.08 (1H, t, J = 7.3 Hz). Elemental analysis calcd. for C15H10NOLi: C, 79.30; H, 4.44; N, 6.17. Found: C, 79.13; H, 4.31; N, 6.15. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] J. Kido, K. Nagai, Y. Okamoto, IEEE Trans. Electron Devices 40 (1993) 1342. [3] T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada, M. Tsuchida, IEEE Trans. Electron Devices 44 (1997) 1245. [4] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. [5] (a) Q.T. Le, L. Yan, Y. Gao, M.G. Mason, D.J. Giesen, C.W. Tang, J. Appl. Phys. 87 (2000) 375; (b) M.G. Mason, C.W. Tang, L.-S. Hung, P. Raychaudhuri, J. Madathil, D.J. Giesen, L. Yan, Q.T. Le, Y. Gao, S.-T. Lee, L.S. Liao, L.F. Cheng, W.R. Salaneck, D.A.D. Santos, J.L. Brédas, J. Appl. Phys. 89 (2001) 2756; (c) L.S. Hung, R.Q. Zhang, P. He, G. Mason, J. Phys. D: Appl. Phys. 35 (2002) 103; (d) C.-I. Wu, G.-R. Lee, T.-W. Pi, Appl. Phys. Lett. 87 (2005) 212108. [6] J. Endo, T. Matsumoto, J. Kido, Jpn. J. Appl. Phys. 41 (2002) 800.