A Series of Lithium Pyridyl Phenolate Complexes with a Pendant

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A Series of Lithium Pyridyl Phenolate Complexes with a Pendant Pyridyl Group for Electron-Injection Layers in Organic Light-Emitting Devices Satoru Ohisa, Taichiro Karasawa, Yuichiro Watanabe, Tatsuya Ohsawa, Yong-Jin Pu, Tomoyuki Koganezawa, Hisahiro Sasabe, and Junji Kido ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13550 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

A Series of Lithium Pyridyl Phenolate Complexes with a Pendant Pyridyl Group for ElectronInjection Layers in Organic Light-Emitting Devices Satoru Ohisa1,2,3*, Taichiro Karasawa1, Yuichiro Watanabe1, Tatsuya Ohsawa1, Yong-Jin Pu1,2,3, Tomoyuki Koganezawa4, Hisahiro Sasabe1,2,3, and Junji Kido1,2,3* 1

Department of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa,

Yamagata 992-8510, Japan 2

Research Center for Organic Electronics, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata

992-8510, Japan 3

Frontier Center for Organic Materials, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata

992-8510, Japan 4

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-

5198, Japan E-mail: [email protected]; [email protected]

1 ACS Paragon Plus Environment


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Abstract We report a new series of lithium pyridyl phenolate complexes with a pendant pyridyl group, Li2BPP, Li3BPP, and Li4BPP, in which the pendant pyridines are substituted at the 2-, 3-, and 4positions, respectively. The most important difference between these complexes is their molecular planarity; Li3BPP and Li4BPP adopt twisted bipyridine structures, whereas Li2BPP adopts a planar structure owing to the steric hindrance and chelating effect of bipyridine on the Li core. The planar structure leads to crystallization through p–p stacking interactions, and the small differences in the molecular structures of the pendant pyridine rings cause drastic differences in the physical properties of thin solid films of these complexes. We applied these complexes as electron-injection layers (EILs) in Ir(ppy)3-based organic light-emitting devices. When thin EILs were used, Li3BPP and Li4BPP afforded lower driving voltages than Li2BPP; the order of the driving voltages followed the order of their electron affinity values. Moreover, the dependence of driving voltage on the EIL thickness was investigated for each complex. Among the three LiBPP derivatives, Li2BPP-based devices showed almost negligible EIL thickness dependence, which may be attributable to the high crystallinity of Li2BPP. All LiBPP-based devices also showed higher stability than conventional 8-quinolinolato lithium-based devices.

Keywords: substitution positional isomer, Li complexes, chelating effect, p–p stacking, electron injection


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1. Introduction Since the first report of organic light-emitting devices (OLEDs) by Tang and VanSlyke in 1987,1 continuous research and development has led to improvements in OLED characteristics. Nowadays, the device efficiency and lifetime have reached a practical level, and commercial products including smartphone displays, large-area TV displays, and general lighting are widely available; however, further improvement of device characteristics is required, especially for further reduction of power consumption. For low power consumption devices, reductions in driving voltages and increases in external quantum efficiencies (EQEs) are required. One of the most effective ways to reduce the driving voltage is to enhance the electron-injection efficiency. Until now, various types of electroninjection layer (EIL) materials have been developed,2-12 and the most typical EIL materials are alkali metals2 and inorganic or organic salts.4, 7, 11, 12 Unfortunately, such types of EIL materials exhibit high air sensitivity and large thickness dependence on the device characteristics, which limit their applications in OLEDs. An effective way to solve these problems is to use lithium complexes with a semiconducting ligand Li-complex-based EILs have higher resistance to air moisture and oxygen compared with Li metal, and relatively good charge-transporting properties.13-14 Li complexes were first introduced to OLEDs by Endo et al. in 1998.5, 15 They inserted a 1 nm-thick 8-quinolinolato lithium (Liq) film as an EIL between an Al cathode and a tris(8-quinolinolato)aluminum (Alq3) electron-transporting layer (ETL), and the resulting device showed significant driving voltage reduction compared with the device without Liq. Since then, much research on the applications of Li complexes in organic electronic devices has been reported.5, 8, 13-29 Pu et al. synthesized lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(2´, 2´´-bipyridine-6´-yl)phenolate (Li2BPP), and lithium 2-(isoquinoline-1´-yl)phenolate, and these were applied as EILs in OLEDs;13 the insertion of these EILs greatly reduced the driving voltages of 3 ACS Paragon Plus Environment


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OLEDs. Kong et al. synthesized lithium 2-(2-pyridyl)-3-pyridinolate (LiPPy) and lithium 4-phenyl-2(2-pyridyl)phenolate (LiPPP), and these were also applied as EILs in OLEDs;25 the LiPPy- and LiPPPbased devices showed lower driving voltages and superior luminescent characteristics compared with the Liq- and LiPP-based devices. Despite the development of these effective Li complexes, it is still challenging to further improve the performance of Li complexes. Varying the substitution position of a functional group is a simple but very effective way to modify the physical properties of chemical compounds.30-42 The substitution position effects of functional groups, such as pyridine,30-34, 36, 40 pyrimidine,35 spirobifluorene,41 dihydroindenofluorene,37-38, 43 and [70]fullerene42, on device characteristics have been investigated. Sasabe and Yokoyama et al. reported the effect of the peripheral pyridine rings of a pyrimidine-based ETL.34, 36 The 3- and 4-substituted terminal pyridines contributed to the formation of an intermolecular C–H…N hydrogen bond network, which showed great influences on film properties and electron-transporting characteristics. Sicard et al. reported the structure–property relationships of four phenyl-substituted spirobifluorene (SBF) regioisomers possessing a different linkage.41 Among them, 1-substituted SBF showed the largest optical energy gap (Eg) and the lowest triplet energy due to the localized p-conjugation. When applied as a host material in blue phosphorescent OLEDs, 1-substituted SBF gave the highest performance. Therefore, varying the substitution position of functional groups can drastically improve the performance of materials. In this work, we designed a new series of Li complexes, lithium 2-(2´, 3´´-bipyridine-6´yl)phenolate (Li3BPP) and lithium 2-(2´, 4´´-bipyridine-6´-yl)phenolate (Li4BPP), which are the isomers of Li2BPP (Figure 1). In these complexes, the terminal pyridines are substituted at the 2(Li2BPP), 3- (Li3BPP), and 4-positions (Li4BPP). The small structural modifications induced drastic changes to their molecular planarity, film crystallinity, and physical properties. We applied these Li 4 ACS Paragon Plus Environment

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complexes as EILs in green phosphorescent emitter-based OLEDs, and the electron-injection capability, the dependence of device characteristics on the EIL thickness, the effect of doping on ETLs, and the stability of the devices were investigated.

Figure 1. DFT calculation results of LiBPP derivatives. Optimized structures and single-point energies were calculated at the RB3LYP 6-31G(d) level for the ground state.



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2. Results and Discussion 2.1. Quantum chemical calculations. Quantum chemical calculations of LiBPP derivatives were performed using density functional theory (DFT). The optimized structures and single-point energies were calculated at the RB3LYP 631G(d) level for the ground state (Figure 1 and Table 1). In addition, the dependence of single-point energies on the dihedral angle φ of bipyridine moieties was calculated using optimized structure parameters except for the dihedral angle (Figure S1). Here, a dihedral angle of 0° represents planar bipyridine structures, and the nitrogen atoms in the pendant pyridines face toward the Li core in Li2BPP and Li3BPP. In the optimized structures, all three derivatives adopted almost planar Li– pyridylphenolato moieties. For Li2BPP, the DFT calculation results showed two types of optimized Li2BPP structures, planar (φ = 0°) and twisted (φ = 145°) structures. Here, we neglect the enantiomer having a dihedral angle of −145° to simplify the discussion, and this is also the case in the subsequent discussion. The planar structure was more stable by 25 kcal/mol than the twisted structure, so the structure of Li2BPP should be planar. This significant stabilization results from the chelation of the pendant pyridine to the Li core. For Li3BPP, the DFT calculation results showed two types of optimized structures with different dihedral angles (φ = 34° and 152°), and the structure of the bipyridine is twisted. The twisted structure with the dihedral angle of 34° is slightly more stable by 0.3 kcal/mol than the structure with the dihedral angle of 152°. For Li4BPP, the DFT calculation results showed one type of optimized structure with a dihedral angle of 35°. As seen in Li3BPP and Li4BPP, the twisted structure is common in pairs of connected aromatic rings.44-46 For the most stable structures, the calculated highest occupied molecular orbital energy levels of the LiBPP derivatives were −4.45 eV for Li2BPP, −4.71 eV for Li3BPP, and −4.80 eV for Li4BPP, and the lowest unoccupied molecular orbital energy levels were –2.01 eV for Li2BPP, –1.76 eV for Li3BPP, and −1.96 eV for Li4BPP. 6 ACS Paragon Plus Environment

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Table 1. Physical properties of LiBPP derivatives and ligands.

a

Tga/Tma/Td5b/Tsc (°C)

Ipd/Ege/Eaf (eV)

HOMOg/LUMOg/ΔEH–Lg (eV)

Li2BPP

112/354/435/274

5.56/2.94/2.62

–4.45/–2.01/2.44

Li3BPP

n.d./333/449/312

5.90/3.00/2.90

–4.71/–1.76/2.95

Li4BPP

n.d./354/453/335

6.01/3.00/3.01

–4.80/–1.96/2.84

2BPP

n.d./107/236/93

n.d./n.d./n.d.

n.d./n.d./n.d.

3BPP

n.d./83/270/101

n.d./n.d./n.d.

n.d./n.d./n.d.

4BPP

n.d./89/284/104

n.d./n.d./n.d.

n.d./n.d./n.d.

Determined by DSC measurements. bDetermined by TG measurements under a nitrogen flow. c Determined by TG measurements under vacuum.

d

Obtained from PYS. eTaken as the point of intersection of the normalized absorption spectra. fCalculated using Ip and Eg. gCalculated at at the

RB3LYP 6-31G(d) levels for the ground state; ΔEH–L = HOMO–LUMO.

2.2. Material syntheses. The ligands, 2-(2,3´-bipyridyl)phenol (3BPP) and 2-(2,4´-bipyridyl)phenol (4BPP), were synthesized via a two-step Suzuki–Miyaura coupling reaction from the corresponding boronic acid and bromine-containing compounds. Complexation reactions of Li3BPP or Li4BPP between LiOH and 3BPP or 4BPP proceeded in methanol solutions at room temperature, and yellow-to-orangecolored precipitates were obtained for both compounds. The synthesized compounds were purified by train sublimation and then characterized by 1H-NMR, mass spectrometry, and/or elemental analysis. The details for the syntheses and characterization are summarized in the experimental section and the Supporting Information (Figures S2–S10).

2.3. Thermal properties and film morphologies. The thermal properties of the LiBPP derivatives and the ligands were investigated using thermal gravimetric analysis under a nitrogen flow or vacuum as well as differential scanning calorimetry (DSC) measurements, and the obtained results are summarized in Table 1 and the Supporting Information (Figures S11–S14). The LiBPP derivatives showed much higher thermal stability than 7 ACS Paragon Plus Environment


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the corresponding ligands. For all three LiBPP derivatives, the 5% decomposition temperatures (Td5s) were more than 100 °C higher than the sublimation temperatures (Tss), which indicates that all three LiBPP derivatives can be successfully evaporated under vacuum without decomposition. The melting points (Tms) of the LiBPP derivatives were 354 °C for Li2BPP, 333 °C for Li3BPP, and 354 °C for Li4BPP. Chemical compounds including terminal 3-pyridines and 4-pyridines are well known to have high Tms owing to the existence of intermolecular C–H…N hydrogen bonding interactions, and the interaction degree of 4-pyridine-containing compounds is larger than that of 3-pyridine-containing compounds.34,

36, 47

Therefore, Li3BPP and Li4BPP are expected to have intermolecular Li…N

interactions with the Li cores of neighboring molecules, and these interactions should contribute to the improvement of their thermal properties. On the other hand, Li2BPP is not expected to have intermolecular hydrogen bonding and chelating interactions; however, Li2BPP had the highest Tm, which suggests that there are other kinds of intermolecular interactions in Li2BPP. Owing to the high planarity of Li2BPP, there are probably strong p–p interactions in Li2BPP. The film morphologies of the LiBPP derivatives were investigated by 2D grazing-incidence wide-angle X-ray diffraction (GIWAXD) (Figures 2(a)–(c)). The measurement samples were prepared by evaporating the three LiBPP derivatives (25 nm) onto 1,3-bis(3,5-dipyrid-3-ylphenyl)benzene (B3PyPB) films (10 nm). As shown by the results in Figures 2(a)–(c), Li2BPP obviously had stronger diffraction peaks than the others, indicating the higher crystallinity of Li2BPP. The isotropic diffraction peak at 15.2 nm−1 corresponds to a d spacing of 4.1 Å. Generally, p–p spacing is between 3.4 Å and 4.2 Å. Therefore, this peak was assigned to p–p stacking. The other isotropic peak at 12.0 nm−1 and the anisotropic peak at 6.1 nm−1 correspond to d spacings of 5.2 Å and 10.3 Å, respectively. Unfortunately, we cannot determine the origins of these peaks and their assignment will be a future task. The surface topology of the three LiBPP derivative films was investigated by atomic force microscopy (AFM). The 8 ACS Paragon Plus Environment

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measurement samples were prepared by evaporating the three LiBPP derivatives onto Si wafer/B3PyPB films (70 nm), and the film thicknesses of the LiBPP derivatives were 1 nm, 2 nm, 5 nm, or 10 nm. The AFM images of LiBPP derivatives are shown in Figures 2(d)–(f) and S15. In the Li2BPP film, many granular aggregates were observed, which were probably Li2BPP crystals; however, in the Li3BPP and Li4BPP films, no such aggregates were observed. Table S1 summarizes the surface roughness (Ra) values of these films. The obtained Ra values were in the range of 0.235– 0.551 nm for Li2BPP, 0.137–0.291 nm for Li3BPP, and 0.139–0.266 nm for Li4BPP, and the bare B3PyPB surface showed an Ra value of 0.114 nm. Li2BPP had obviously larger Ra values than Li3BPP and Li4BPP, which is likely due to the self-aggregation of Li2BPP. Figure 3 summarizes the interactions occurring in the LiBPP derivatives.

Figure 2. 2D GIWAXD profiles of (a) Li2BPP, (b) Li3BPP, and (c) Li4BPP films (25 nm) on B3PyPB (10 nm) films, and surface profiles of (d) Li2BPP, (e) Li3BPP, and (f) Li4BPP films (1 nm) on B3PyPB (70 nm) films obtained by AFM.



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Figure 3. Interaction scheme of LiBPP derivatives.

2.4. Optoelectronic properties. The optoelectronic properties of the LiBPP derivatives were investigated by ultraviolet–visible (UV–Vis) absorption spectroscopy, photoluminescence (PL) spectroscopy, and photoelectron yield spectroscopy (PYS). The obtained results are summarized in Table 1 and the Supporting Information (Figures S16–S18). Figure S16 shows the normalized UV–Vis absorption and PL emission spectra. In the UV–Vis absorption spectra, Li3BPP showed a similar spectrum to Li4BPP, and the spectrum of Li2BPP is red-shifted compared with those of Li3BPP and Li4BPP. The Eg values were obtained from the absorption edges and were 2.94 eV for Li2BPP, 3.00 eV for Li3BPP, and 3.00 eV for Li4BPP. In the emission spectra, the three LiBPP derivatives showed different peak positions, which were at 546 nm for Li2BPP, 498 nm for Li3BPP, and 536 nm for Li4BPP. The ionization potentials (Ips) were obtained by PYS measurements and were 5.56 eV for Li2BPP, 5.90 eV for Li3BPP, and 6.01 eV for 10 ACS Paragon Plus Environment

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Li4BPP. The electron affinity (EA) values were calculated by subtracting the Eg values from the Ip values and were 2.61 eV for Li2BPP, 2.90 eV for Li3BPP, and 3.01 eV for Li4BPP.

2.5. OLEDs. 2.5.1. Comparison of OLED characteristics between different Li-complex-based EILs. We fabricated OLEDs with a structure of [ITO (130 nm)/triphenylamine-containing polymer: 4isopropyl-4-methyldiphenyl-iodonium tetrakis(pentafluorophenyl)borate (PPBI)48 (HIL) (20 nm)/4,4ʹcyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) phenylpyridinato)iridium(III)

(Ir(ppy)3)

doped

(30

nm)/8wt%

4,4'-bis(carbazol-9-yl)biphenyl

(CBP)

tris(2(20

nm)/B3PyPB (50 nm)/EIL (2 nm)/Al (100 nm)], where the three LiBPP derivatives and Liq were used as EILs (Figures S19 and S20). Figure 4 shows the current density (J)–voltage (V), luminance (L)–V, L–EQE, and L–power efficiency characteristics and the electroluminescence (EL) spectra of the fabricated OLEDs. These characteristics are summarized in Table 2. At a luminance of 100 cd/m2, the driving voltages were 3.65 V for Li2BPP-, 3.31 V for Li3BPP-, 3.30 V for Li4BPP-, and 3.52 V for Liq-based devices. The driving voltages of the Li3BPP- and Li4BPP-based devices were nearly the same and were the lowest among the devices. The EA value of Liq was reported to be 2.8 eV. Therefore, the order of the driving voltages followed the order of the EA values of the Li complexes. The Li3BPPand Li4BPP-based devices showed higher EQE values (around 22%) than that of the Li2BPP-based device (18.4%). The power efficiencies of the Li3BPP- and Li4BPP-based devices were 74.3 lm/W and 75.3 lm/W, and these values were about 1.3 times larger than that of the Li2BPP-based device (56.6 lm/W). In addition, 4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2-phenylpyrimidine (B4PyPPM), which has a larger EA value (3.4 eV) than B3PyPB (2.6 eV), was also applied as an ETL instead of B3PyPB (Figure S21 and Table S2). Unlike the case with a B3PyPB ETL, the devices with the three LiBPP 11 ACS Paragon Plus Environment


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derivatives showed nearly the same driving voltages. This is probably because the larger EA value of B4PyPPM masked the differences between the electron-injection capabilities of each Li complex.

Figure 4. OLED characteristics. (a) Current density–voltage (V), (b) luminance (L)–V, (c) L–external quantum efficiency, and (d) L–power efficiency characteristics. The inset is the EL spectra. The EILs used here are Li2BPP, Li3BPP, Li4BPP, and Liq (2 nm).

Table 2. OLED characteristics with 2 nm-EILs. Von/ηp,on/ηc,on/ηEQE,on a)

V100/ηPE,100/ηCE,100/ηEQE,100 b)

V1000/ηPE,1000/ηCE,1000/ηEQE,1000 c)

[V/lm W–1/cd A–1/ %]

[V/lm W–1/cd A–1/ %]

[V/lm W–1/cd A–1/ %]

Li2BPP

2.77/82.8/73.2/20.4

3.65/56.6/65.7/18.4

4.71/37.5/56.2/15.7

Li3BPP

2.73/94.1/82.1/23.1

3.31/74.3/78.5/22.1

4.02/56.6/72.4/20.4

Li4BPP

2.72/94.1/81.7/22.9

3.30/75.3/79.3/22.1

4.02/57.4/73.4/20.6

Liq

2.84/84.8/76.7/21.5

3.52/65.0/72.8/20.4

4.21/50.7/68.0/19.1

EIL

a) Voltage (V), power efficiency (PE), current efficiency (CE) and external quantum efficiency (EQE) at 1 cd m–2. b) V, PE, CE, V and EQE at 100 cd m–2. c) V, PE, CE and EQE at 1,000 cd m–2.


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2.5.2. Dependence of device characteristics on EIL thickness. In the aforementioned devices adopting a B3PyPB ETL, the layer thicknesses of the LiBPP EILs were varied to 1 nm, 5 nm, or 10 nm, and the thickness dependence of the driving voltage was investigated. The driving voltages are summarized in Figure 5 and in the Supporting Information (Figures S22–S24 and Tables S3–S5). The driving voltages of the Li2BPP-based devices showed almost negligible dependence on the Li2BPP EIL thickness, whereas the driving voltages of the Li3BPP- and Li4BPP-based devices increased slightly with increasing EIL thickness. For example, at a luminance of 100 cd/m2, the driving voltages were in the range of 3.54–3.78 V, 3.31–4.55 V, and 3.30–5.09 V for Li2BPP-, Li3BPP-, and Li4BPP-based devices, respectively. We also fabricated electron-only devices (EODs) with a structure of [ITO (130 nm)/B3PyPB (140 nm)/LiBPP derivatives (1 nm, 2 nm, or 5 nm)/Al (100 nm)], and the J–V characteristics of these devices are shown in Figure S25. As in the OLEDs, the Li2BPP-based EODs showed almost negligible changes in the driving voltages, and the Li3BPP- and Li4BPP-based EODs showed increases in the driving voltages when their thickness was 5 nm. A weak thickness dependence of the driving voltage in Li2BPP-based devices has been previously reported,13 and in our case, we considered that the almost negligible thickness dependence originates from the high crystallinity of Li2BPP.



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Figure 5. Dependence of OLED voltages on the thickness of LiBPP derivative-based EILs at luminances of 1 cd/m2, 100 cd/m2, and 1000 cd/m2.

2.5.3. Effects of doping ETLs with Li complexes on device characteristics. We doped the B3PyPB ETLs with the LiBPP derivatives, and the doping effects on device characteristics were investigated. In the aforementioned OLED structure, we changed the ETL/EIL structure from [undoped B3PyPB (50 nm)/LiBPP derivative EIL] to [B3PyPB (45 nm)/ B3PyPB:30wt% LiBPP derivatives (doped ETL) (5 nm)] or [B3PyPB (45 nm)/doped ETL (5 nm)/LiBPP derivative EIL (2 nm)]. The device characteristics of the OLEDs are shown in Figures S26–S29 and Tables S6–S9. The OLEDs without EILs showed larger driving voltages than those with EILs, indicating that the insertion of EILs is indispensable for reducing driving voltages. Among the 14 ACS Paragon Plus Environment

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devices with EILs, the Li2BPP-doped device showed a greatly reduced driving voltage and an increased external quantum efficiency of 22.1% at 100 cd/m2 compared with the undoped device. The achieved power efficiency of the Li2BPP-doped device was 78.3 lm/W, which was larger than those of the undoped Li3BPP- and Li4BPP-based devices. On the other hand, the devices doped with Li3BPP and Li4BPP showed increases in the driving voltages compared with those undoped devices. We then fabricated a device with the structure of [B3PyPB (45 nm)/Li2BPP-doped B3PyPB (5 nm)/Li4BPP (2 nm)] (Figures 6 and S29). The device characteristics were almost as good as those of the device with a Li2BPP-doped ETL and a Li2BPP EIL, which proved that the Li2BPP doping was effective for reducing the driving voltage. In the Li3BPP- and Li4BPP-doped devices, Li3BPP and Li4BPP act as electron trap sites in the doped B3PyPB layers because the EA values of Li3BPP and Li4BPP are larger than that of B3PyPB.

Figure 6. OLED characteristics. (a) Current density–voltage and (b) luminance–power efficiency characteristics. The ETL/EIL combinations were Li2BPP-doped B3PyPB/Li2BPP (doped + Li2BPP), Li2BPP-doped B3PyPB/Li4BPP (doped + Li4BPP), and B3PyPB/Li4BPP (undoped + Li4BPP).



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2.5.4. Stability of the OLEDs. Finally, OLEDs with a structure of [ITO (130 nm)/HIL (20 nm)/α-NPD (10 nm)/α-NPD:7wt% rubrene (10 nm)/ tris(8-quinolinolato)aluminum (Alq3):7wt% rubrene (10 nm)/Alq3 (50 nm)/EIL (1 nm)/Al (100 nm) were fabricated, where the three LiBPP derivatives and Liq were used as EILs. All devices showed similar initial characteristics owing to easy electron injection into Alq3 (Figure S30 and Table S10). The driving stability of the devices was tested at constant current densities at an initial luminance of 3000 cd/m2 (Figure S31). LT50 values, the time taken to reach half the initial luminance, were 2200, 2050, 2050, and 1800 h for Li2BPP-, Li3BPP-, Li4BPP-, and Liq-EIL-based devices, respectively. All LiBPP-based devices showed higher stability than the conventional Liq-based device, proving the usefulness of the LiBPP derivatives as EIL materials in OLEDs.

3. Conclusion We reported the effects of the substitution position of the pendant pyridine in LiBPP derivatives on their thermal and optoelectronic properties and the characteristics of LiBPP-based OLEDs. The chelating interaction in Li2BPP caused it to adopt a planar structure, which induced strong intermolecular p–p interactions and affected its crystallization. These interactions caused drastic changes in the physical properties of Li2BPP compared with Li3BPP and Li4BPP. The larger EA values of Li3BPP and Li4BPP led to better performance in thin EIL devices compared with Li2BPP. On the other hand, the OLED performance of highly crystallized Li2BPP exhibited almost negligible thickness dependence. Doping ETLs with Li2BPP was useful for facilitating the electron injection into the ETL. In this work, we succeeded in establishing a new approach for controlling the properties of metal-complex semiconducting films, which simply involves varying the substitution positions in the


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EIL material. Our findings shed new light on the design of novel metal complexes and are expected to lead to further improvements in organic electronic devices.

4. Experimental Section 4.1. Materials. The reagents and solvents for syntheses were purchased from commercial sources and used as received. B3PyPB and B4PyPPM were synthesized according to established procedures.30-31 Li3BPP and Li4BPP were synthesized according to the procedures described in the Supporting Information. Other materials utilized in OLEDs were purchased from e-Ray Optoelectronics Technology Co., Ltd., Chemipro Kasei Kaisha, Ltd., and NARD Institute, Ltd., and were either used after purification by train sublimation or used as received. Glass substrates with ITO electrodes were purchased from Asahi Glass Co., Ltd. The ITO electrodes were patterned by photolithography.

4.2. Characterization methods for material properties. Quantum chemical calculations were performed using the Gaussian 09 program package. Optimized structures and single-point energies were calculated at the RB3LYP 6-31G(d) level for the ground state. The single-point energy on the dihedral angle of bipyridine was calculated at the RB3LYP 6-31G(d) level using optimized structure parameters except for the dihedral angle. The TG measurements under a nitrogen flow were performed using a SEIKO EXSTAR 6000 TG/DTA 6200 unit at a heating rate of 10 °C min−1, and the TG measurements under vacuum were performed using an ULVAC RIKO VAP-9000 unit below 10-4 Pa. DSC measurements were performed using a PerkinElmer Diamond DSC Pyris instrument under a nitrogen flow at a heating rate of 10 °C min–1. GIWAXD measurements were performed at BL46XU in Spring-8. AFM measurements were 17 ACS Paragon Plus Environment


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performed using a Veeco Dimension Icon atomic force microscope with scanning areas of 500 nm × 500 nm. Ip values were determined by PYS under vacuum (∼10−3 Pa). UV−Vis spectra were measured using a Shimadzu UV-3150 UV−vis−NIR spectrophotometer. Eg values were determined from the absorption band edges. EA values were calculated by subtracting the Eg values from the Ip values. PL spectra were measured using a HORIBA JOBIN YVON Fluoromax-4 fluorometer.

4.3. Device fabrication and characterization. Glass substrates with ITO electrodes were sequentially cleaned with ultrapure water, acetone, and 2-propanol using an ultrasonic bath sonicator and were then dry-cleaned using a UV/ozone cleaner. A triphenylamine-containing polymer, PPBI, was dissolved in ethyl benzoate and spin-coated onto the pre-cleaned ITO substrates followed by thermal annealing at 200 °C in air. The substrates were then transferred into a vacuum chamber, and organic layers were successively deposited onto the substrates under vacuum (10–5 Pa). Al cathodes were deposited using a shadow mask with patterned openings. The emitting area of the devices was 2 mm × 2 mm. J–V–L characteristics were measured using a current source meter, Keithley 2400, and a luminance meter, Konica Minolta CS-200. EL spectra were measured using a photonic multichannel analyzer, Hamamatsu PMA-11. Quantum efficiencies were calculated based on the Lambertian assumption. Device lifetimes were tested at a constant current density and an initial luminance of 3000 cd/m2.

Supporting Information Dihedral angle dependent DFT calculation results, synthetic procedures, NMR spectra, mass spectra, TGA curves, DSC curves, AFM results, UV-Vis absorption spectra, PL spectra, PYS spectra, OLED characteristics, electron-only device characteristics, OLED lifetime curves are available.


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Acknowledgements Two-dimensional GIWAXD experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, proposal no. 2017A1767), and we thank Seijiro Fukuta and Prof. Tomoya Higashihara for the support of GIWAXD measurements. We thank Yuya Hayasaka for helping with sample preparations. We thank the Center of Innovation (COI) Program from the Japan Science and Technology Agency, JST, and the Japan Society for the Promotion of Science, JSPS KAKENHI Grant Numbers 15J08167 for financial support. References 1. Tang, C. W.; VanSlyke, S. A., Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. 2. Kido, J.; Nagai, K.; Okamoto, Y., Bright Organic Electroluminescent Devices with Double-Layer Cathode. IEEE Transactions on Electron Devices 1993, 40, 1342-1344. 3. Kim, Y.-E.; Park, H.; Kim, J.-J., Enhanced Quantum Efficiency in Polymer Electroluminescence Devices by Inserting a Tunneling Barrier Formed by Langmuir–Blodgett Films. Appl. Phys. Lett. 1996, 69, 599-601. 4. Hung, L. S.; Tang, C. W.; Mason, M. G., Enhanced Electron Injection in Organic Electroluminescence Devices Using an Al/LiF Electrode. Appl. Phys. Lett. 1997, 70, 152-154. 5. Endo, J.; Matsumoto, T.; Kido, J., Ext. Abstr. 9th Int. Workshop on Inorganic & Organic Electroluminescence 1998, 57. 6. Kido, J.; Matsumoto, T., Bright Organic Electroluminescent Devices Having a Metal-Doped Electron-Injecting Layer. Appl. Phys. Lett. 1998, 73, 2866-2868. 7. Le, Q. T.; Yan, L.; Gao, Y.; Mason, M. G.; Giesen, D. J.; Tang, C. W., Photoemission Study of Aluminum/Tris-(8-Hydroxyquinoline) Aluminum and Aluminum/Lif/Tris-(8-Hydroxyquinoline) Aluminum Interfaces. J. Appl. Phys. 2000, 87, 375-379. 8. Lee, J. Y., Efficient Electron Injection in Organic Light-Emitting Diodes Using Lithium Quinolate/Mg:Ag/Al Cathodes. J. Ind. Eng. Chem. 2008, 14, 676-678. 9. Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Brédas, J.-L.; Marder, S. R.; Kahn, A.; Kippelen, B., A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics. Science 2012, 336, 327-332. 10. Pu, Y.-J.; Morishita, N.; Chiba, T.; Ohisa, S.; Igarashi, M.; Masuhara, A.; Kido, J., Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic LightEmitting Devices. ACS Appl. Mater. Interfaces 2015, 7, 25373-25377. 19 ACS Paragon Plus Environment


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Graphical Abstract


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A Series of Lithium Pyridyl Phenolate Complexes with a Pendant

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