Cyanopyridine Based Bipolar Host Materials for Green

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Cyanopyridine Based Bipolar Host Materials for Green Electrophosphorescence with Extremely Low Turn-on Voltages and High Power Efficiencies Wei Li, Jiuyan Li, Di Liu, Deli Li, and Fang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04395 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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ACS Applied Materials & Interfaces

Cyanopyridine Based Bipolar Host Materials for Green Electrophosphorescence with Extremely Low Turn-on Voltages and High Power Efficiencies Wei Li, Jiuyan Li,* Di Liu, Deli Li, Fang Wang State Key Laboratory of Fine Chemicals, College of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, 116024, China. E-mail: [email protected]

ABSTRACT Low driving voltage and high power efficiency are basic requirements when practical applications of organic light emitting diodes (OLEDs) in displays and lighting are considered. Two novel host materials m-PyCNmCP and 3-PyCNmCP incorporating cyanopyridine moiety as electron-transporting unit are developed for use in fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) based green phosphorescent OLEDs (PhOLEDs). Extremely low turn-on voltages of 2.01 V and 2.27 V are realized, which are even lower than the theoretical limit of the emitted photon energy (hv)/electron charge (e) (2.37 V) of Ir(ppy)3. High power efficiency of 101.4 lm/W (corresponding to a maximum external quantum efficiency of 18.4%) and 119.3 lm/W (24.7%) are achieved for m-PyCNmCP and 3-PyCNmCP based green PhOLEDs. The excellent EL performance benefits from the ideal parameters of host materials by combining cyano and pyridine to enhance the n-type feature. The energetic favorable alignment of HOMO/LUMO levels of hosts with adjacent layers and the dopant for easy charge injections and direct charge trapping by dopant, their bipolar feature to balance charge transportations, sufficiently high triplet energy and small singlet/triplet energy difference (0.38 eV and 0.43 eV) combine to be responsible for the extremely low driving voltages and high power efficiencies of the green PhOLEDs.

1

ACS Paragon Plus Environment


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Keywords: phosphorescent organic light-emitting diodes (PhOLEDs), pyridine, cyano, host material, low turn-on voltage

INTRODUCTION After decades of development, phosphorescent organic light emitting diodes (PhOLEDs) have demonstrated their great potential in energy-saving flat-panel displays and general lighting. Some devices such as smart phones and high-definition television (HDTV) are beginning to be commercialized. For industrialized application of OLEDs, low operating voltage and high power efficiency are primary requirements. A low operating voltage of OLEDs always means high power efficiency.1 Thus, a key issue to increase power efficiency is to reduce the operating voltage of device. It is generally believed that the theoretical limit of turn-on voltage (defined as the voltage at which the brightness is 1 cd/m2) equals to the emitted photon energy (hv) divided by the electron charge (e).1,2 For example, the famous green phosphor Ir(ppy)3 typically emits at 520-524 nm, corresponding to a photon energy of 2.37-2.38 eV and a turn-on voltage limit of 2.37-2.38 V.1,3 In fact, the turn-on voltages of many Ir(ppy)3 based green PhOLEDs in previous reports were much higher than 2.4 V and only a few samples with well-designed device structure or using excellent host achieved ideal turn-on voltages.1,4,5,6 Several typical strategies are used to reduce the turn-on voltage of OLEDs. The first is to employ the exciplex-forming co-host that is usually composed by the hole transporting material (HTM) and electron transporting material (ETM).4,7 The negligible charge-injection barriers from the charge transporting layers to the emitting layer can remarkably reduce the turn-on voltage and increase the device efficiency. The second is to introduce a p-i-n device structure. The highly conductive p- and n-doped layers enhance the charge injection from the contacts and reduce the ohmic losses in these layers in p-i-n devices, consequently, the turn-on voltage can be reduced.8 The third is to use a bipolar host to align the HOMO/LUMO levels of the 2


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emitting layer with adjacent charge transporting layers..9,10,11 As the charge carrier injection into the emitting layer typically occurs into the HOMO and LUMO of the host, the host material with a small Eg, which approximately corresponds to the singlet excited state energy (Es), can make the driving voltage of OLEDs low.12 It is obvious that the bipolar host method is advantageous over the other two in terms of easy device fabrication. However, simultaneous shallow HOMO and deep LUMO usually imply moderate or low singlet and triplet energies, which is undesired since the high triplet energy of host is necessary for efficient energy transfer to dopant. Therefore, it always remains as a challenge to develop host materials possess sufficiently high triplet energy, small ∆EST, and appropriate HOMO/LUMO levels at the same time by skillful molecular designing.11,12 Cyano (CN) group and pyridine moiety are widely-used electron-transport units because of their strong electron deficiency characteristic. For example, Kido et al.13 reported two pyridine-containing bipolar host materials 26DCzPPyand 35DCzPPy for blue PhOLEDs. An external quantum efficiency (ηext) of 24% and power efficiency (ηp) of 46 lm/W were obtained for 26DCzPPy hosted blue PhOLEDs. Wong et al.14 reported one CN-modified bipolar host mCPCN. Highly efficient blue PhOLEDs with the maximum current efficiency (ηc), ηp, and ηext of 58.6 cd A–1, 57.6 lm W–1, and 26.4% were achieved using mCPCN as the host material and FIrpic as the triplet emitter. In this report, we incorporated two electron deficient moieties, i.e. cyano (CN) and pyridine, to construct two bipolar host materials m-PyCNmCP and 3-PyCNmCP. With the combination of pyridine and CN as electron-deficient units in these hosts, excellent electroluminescence performance was achieved for their hosted green PhOLEDs. In particular, the extremely low turn-on voltages of 2.01 eV and 2.27 V were achieved in m-PyCNmCP and 3-PyCNmCP based devices, respectively, which are even lower than the theoretical limit of the energy gap voltage of the phosphor emitter Ir(ppy)3 (2.4 V). High power efficiency of 101.4 lm/W and 119.3 lm/W were achieved for m-PyCNmCP and 3-PyCNmCP based devices, respectively. Such low turn-on voltages and high power efficiencies are 3



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attributed to the appropriate device structures and ideal parameters of these host materials, such as good aligment of HOMO/LUMO levels with adjacent layers, sufficiently high ET values to confine the exciton within the emitting layer, bipolar properties to balance charge transportation, and low singlet and triplet energy difference ∆EST (0.38 eV for m-PyCNmCP and 0.43 eV for 3-PyCNmCP). The structure-property-performance correlation of these host materials will be emphasized to discuss.

EXPERIMENTAL SECTION Instruments and Methods. The instruments and measuring methods for 1H NMR and 13C NMR spectra, mass spectra, photoluminescence (PL) and UV-vis absorption spectra and LT PL spectra, thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC), cyclic voltammetry (CV) and OLED fabrication, and the density functional theory (DFT) calculations details have been reported in our previous publications.9,15 Compounds

Syntheses.

The

important

intermediates

9,9'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9H-carbazole)

(1)

9-(3-(9H-carbazol-9-yl)phenyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole

and (2)

were synthesized according to the literature methods.16,17 Synthesis of 5-(3,5-di(9H-carbazol-9-yl)phenyl)nicotinonitrile (m-PyCNmCP)18 A mixture of 5-bromonicotinonitrile (183 mg, 1 mmol), 1 (561 mg, 1.05 mmol), toluene (10 mL), ethanol (2 mL), aqueous sodium carbonate (2 M, 2.5 mL, 5 mmol), and tetrakis(triphenylphosphino)palladium(0) (57 mg, 0.05 mmol) was refluxed at 80 ℃ under nitrogen atmosphere overnight. After cooling and filtrating, the filtrate was evaporated under reduced pressure. The residue was purified by column chromatography over silica using petroleum ether/ethyl acetate (4:1) as eluent, followed by repeated recrystallization in methanol/chloroform to give pure m-PyCNmCP as a white solid (347 mg, 68% 4


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yield). Mp: 252 ℃. 1H NMR (500 MHz, CDCl3) δ: 9.18 (s, 1H), 8.95 (s, 1H), 8.27 (s, 1H), 8.18 (d, J = 7.7 Hz, 4H), 7.97 (s, 1H), 7.91 (s, 2H), 7.58 (d, J = 8.2 Hz, 4H), 7.48 (t, J = 7.7 Hz, 4H), 7.35 (t, J = 7.5 Hz, 4H). 13C NMR (126 MHz, CDCl3) δ: 151.75, 151.44, 140.79, 140.36, 139.03, 137.43, 126.43, 125.67, 124.13, 123.87, 120.83, 120.69, 116.15, 111.12, 110.19, 109.46. TOF-EI-MS (m/z): 510.1855 [M]+. Anal.calcd for C36H22N4: C, 84.68; H, 4.34; N, 10.97; Found: C, 84.66; H, 4.35; N, 10.96. 5-(9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)nicotinonitrile (3-PyCNmCP): Reaction of intermediate 2 with 5-bromonicotinonitrile following the same procedure for synthesis of m-PyPzmCP generated the pure 3-PyCNmCP as a white solid. Yield: 78%. No melting point was detected before 280 ℃. 1H NMR (500 MHz, CDCl3) δ: 9.13 (s, 1H), 8.82 (s, 1H), 8.31 (s, 1H), 8.22 – 8.18 (m, 2H), 8.15 (d, J = 7.7 Hz, 2H), 7.87 (t, J = 7.9 Hz, 1H), 7.81 (s, 1H), 7.74 (d, J = 8.1 Hz, 1H), 7.69 (d, J = 8.0 Hz, 1H), 7.63 – 7.58 (m, 2H), 7.57 – 7.51 (m, 3H), 7.50 (t, J = 7.7 Hz, 1H), 7.44 (t, J = 7.7 Hz, 2H), 7.37 (t, J = 7.4 Hz, 1H), 7.31 (t, J = 7.5 Hz, 2H).

13

C NMR (126 MHz,

CDCl3) δ: 151.57, 149.90, 141.27, 140.96, 140.53, 139.58, 138.80, 137.60, 137.08, 131.41, 127.61, 127.04, 126.32, 126.19, 125.74, 125.28, 125.14, 124.57, 123.65, 123.16, 120.99, 120.63, 120.52, 120.45, 119.24, 116.79, 110.74 (s), 110.11, 109.58. TOF-EI-MS (m/z): 510.1853 [M]+. C36H22N4: C, 84.68; H, 4.34; N, 10.97; Found: C, 84.65; H, 4.36; N, 11.00.

RESULTS AND DISCUSSION Synthesis and Thermal Properties. The chemical structures and synthetic routes of compounds m-PyCNmCP

and

3-PyCNmCP

are

depicted

in

Scheme

1.

The

key

9,9'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9H-carbazole)

intermediates (1)

and

9-(3-(9H-carbazol-9-yl)phenyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole

(2)

were

(a)

synthesized

from

9,9'-(5-bromo-1,3-phenylene)bis(9H-carbazole) 5



9-(3-(9H-carbazol-9-yl)phenyl)-3-bromo-9H-carbazole (b) using the literature methods.16,17 The target compounds m-PyCNmCP and 3-PyCNmCP were prepared at yields of 68% and 78% through a Suzuki cross-coupling reaction between intermediate 5-bromonicotinonitrile and the corresponding borate esters 1 and 2.18 All these compounds have a good solubility in common organic solvents, such as dichloromethane, tetrahydrofuran and ethyl acetate, so that they could be thoroughly purified by column chromatography and repeated recrystallization to reach a high purity for OLED applications. Their chemical structures were fully characterized by 1H NMR, 13C NMR spectroscopy, mass spectrometry and elemental analysis.

O O B

Br

N N

N

N

N

N

Br

O O B B O O

a

1

N

N

O B O

N

N

b

CN m-PyCNmCP

Suzuki Coupling

Br N

CN N

Pd(dppf)Cl 2,CH3COOK 1,4-dioxane

2

N N

N

CN

3-PyCNmCP

Scheme 1 Chemical structures and synthetic routes of m-PyCNmCP and 3-PyCNmCP.

Endothermic

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3-PyCNmCP 104℃

m-PyCNmCP

20

40

60

112℃

80 100 120 140 Temperature (℃)

160

Figure 1. DSC traces (at the second heating cycle) of m-PyCNmCP and 3-PyCNmCP, recorded at a heating rate of 10 ℃ min–1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed 6


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to investigate the thermal properties of m-PyCNmCP and 3-PyCNmCP, and all the pertinent data are summarized in Table 1. As shown by the TGA thermograms in Figure S1 (a), Both compounds exhibit high thermal decomposition temperatures (Td, corresponding to a 5% weight loss) of 408 and 386 ℃, respectively, indicating good thermal stabilities. The DSC traces during the first heating cycle is given in Figure S1 (b). A melting point of 252 ℃ was detected for m-PyCNmCP and no distinct melting was observed for 3-PyCNmCP before 280 ℃. As shown by the DSC traces in Figure 1, the DSC curves of m-PyCNmCP and 3-PyCNmCP during the second heating scans reveal well-defined glass transition temperatures (Tg) of 112 and 104 ℃, respectively. Based on the high decomposition temperatures and high Tg values, good stability can be expected for OLEDs containing these compounds as host materials. Photophysical Properties. Figure 2 illustrates the electronic absorption and fluorescence spectra of m-PyCNmCP and 3-PyCNmCP in dilute toluene solutions. The absorption at around 292 nm for both compounds could be assigned to the carbazole-centered π-π* transition. The longer wavelength absorption at 325−339 nm could be attributed to the n–π* transitions of the carbazole moiety.3,10,15 Upon optical excitation at the absorption maxima, m-PyCNmCP and 3-PyCNmCP emit purple-blue fluorescence with emission peaks at 433 and 420 nm, respectively. The low-temperature photoluminescence (LT PL) spectra were measured in both the frozen 2-methyltetrahydrofuran glass (Figure 2) and neat films (Figure S2 in supporting information) at 77 K. The triplet energies were estimated from the highest-energy vibronic sub-band of the phosphorescence spectra as ca. 2.82 eV and 2.76 eV for m-PyCNmCP and 3-PyCNmCP, respectively, both of which are high enough as hosts for the typical green emitter Ir(ppy)3 (2.37 eV, corresponding to an emission peak at 524 nm). For an accurate understanding the electronic properties of these host materials in optoelectronic devices, the singlet and triplet energy differences ∆EST for m-PyCNmCP and 3-PyCNmCP were estimated from the LT PL spectra of their films (Figure S2) to be 0.38 eV and 0.43 eV, respectively. 7


ACS Applied Materials & Interfaces 1.25

0.50

0.50

0.25

0.25

0.00 300

350 400 450 500 Wavelength (nm)

550

0.00 600

Absorbance (a.u.)

0.75

1.00 PL LT PL 0.75

PL Intensity (a.u.)

(b) 3-PyCNmCP

1.00

250

1.25

1.25

(a) m-PyCNmCP

PL LT PL

1.00

1.00

0.75

0.75

0.50

0.50

0.25

0.25

PL Intensity (a.u.)

1.25 Absorbance (a.u.)

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0.00

0.00 250

300

350 400 450 500 Wavelength (nm)

550

600

Figure 2 UV-vis absorption, room-temperature photoluminescence (PL) spectra in dilute toluene solution, and LT PL spectra in frozen 2-methyltetrahydrofuran matrix at 77 K of m-PyCNmCP (a) and 3-PyCNmCP (b).

Table 1 Physical Data of m-PyCNmCP and 3-PyCNmCP. Compound

λmax em

λabs [nm] a

[nm] a

ET (eV) b

Eg (eV) c

HOMO /LUMO (eV) c

Td (℃) d

Tg (℃)

112 –5.70/–2.67 408 104 –5.65/–2.57 386 a b Absorption and fluorescence peak wavelengths in dilute toluene solutions. Measured in c 2-Me-THF at 77 K. Determined from electrochemical measurements; Eg: the electrochemical band gap that is determined as the potential difference between oxidation onset and reduction onset multiplied by the electron charge (e). d Td is the thermal decomposition temperature corresponding to 5% weight loss. m-PyCNmCP 3-PyCNmCP

292,325,338 293,326,339

433 420

2.82 2.76

3.03 3.08

Electrochemical Properties and Theoretical Calculations. The electrochemical redox properties of these two compounds were investigated by cyclic voltammetry (CV) using 0.1 M tetra(n-butyl)ammonium hexa-fluorophosphate (n-Bu4NPF6) as the supporting electrolyte in deoxygenated CH2Cl2 and N,N-dimethylformamide (DMF) solutions. As shown in Figure 3, m-PyCNmCP and 3-PyCNmCP have distinct oxidation and reduction behaviors, which should arise from the electron-donating carbazole unit and electron-withdrawing pyridine and CN units, respectively. It should be noted that these two compounds underwent two one-electron oxidations during the anodic scan, while only one higher-current reduction wave was detected. This is probably 8


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because the two-step reductions are too fast relative to the scan speed to be well dissolved in spectral profile in our experiment. The additional reduction wave at about 0.95 V for both compounds should be because of the instability of radical cations of nonprotected carbazole derivatives at 3,6-sites. The HOMO energies were determined from the onset potential of the first oxidation wave ( E oxonset ) according to the equation of EHOMO = -e( E oxonset + 4.4)3,10 as –5.70 eV and –5.65 eV for m-PyCNmCP and 3-PyCNmCP, respectively. The HOMO levels of these compounds are close to that of the widely used hole transport material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, –5.50 eV),10,15 In a similar way, the LUMO energies were calculated from the onset potential of the first reducation onset onset wave ( E rex ) according to the equation of ELUMO = -e( E rex + 4.4)15 to be ca. –2.67 eV and –2.57

eV, respectively. The detailed electrochemical and electronic data of the two molecules are listed in Table 1. The energy gap (Eg) of m-PyCNmCP and 3-PyCNmCP were determined to be 3.03 eV and 3.08 eV, respectively, by calculating the energy difference between the LUMO and HOMO. Apparently these gap values are identical to the lowest singlet excited state energies (ES) obtained from LT PL of their films (Figure S2). This is consistent with the general belief that the singlet excited state energy (ES) of organic molecule approximately corresponds to its energy gap Eg, also confirms the reliability of the small ∆EST values of these hosts. Therefore, it can be expected that the small ∆EST values of these hosts can help to reduce the turn-on voltage when used as host in OLEDs.4,11,12

Current (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


m-PyCNmCP

3-PyCNmCP

2000

1000 0 -1000 Potential ( mV vs SCE)

-2000 9



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Figure 3. Cyclic voltammograms of m-PyCNmCP and 3-PyCNmCP measured in CH2Cl2 and DMF at a scan rate of 100 mV s–1.

Figure 4. Geometry optimized structures, HOMO and LUMO distribution, and the spin-density distribution in T1 states of m-PyCNmCP and 3-PyCNmCP. In order to understand the optical and electrochemical properties of these compounds, the geometry optimization and spatial distribution analyzing of HOMO and LUMO was performed with the Gaussian 03 package at the B3LYP/6-31G(d) level using the Density Function Theory (DFT). The optimized molecular structures and HOMO/LUMO distribution for m-PyCNmCP and 3-PyCNmCP are given in Figure 4. The HOMO of m-PyCNmCP and 3-PyCNmCP were mainly localized on their hole-transporting carbazole moieties, while the LUMO are mainly distributed on electron-accepting pyridine and CN fragments. The spatial separation of HOMO and LUMO is believed to be the origin of the potential bipolar carrier transporting characteristics that can balance the carriers in emitting layer (EML) and thus favor to improve the electroluminescence (EL) performance of devices. Besides, for m-PyCNmCP and 3-PyCNmCP, each carbazole has a large dihedral angle of 56.3° or 53.6° with the directly linked phenyl unit, and the dihedral angle between the cyanopyridine ring and the phenyl or carbaozle ring is still 39.3° or 36.7°. These large dihedral angles should be caused by the meta-substitution of the two aromatic groups on each discussed ring 10


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and thus the steric hindrance effect. The twist configuration of these molecules and their nonplanar three-dimensional structures can effectively inhibit the unwanted intermolecular interaction in solid states, which is usually desired in OLEDs. The spin-density distributions (SDDs) of the two molecules were also simulated to locate the T1 excited states (Figure 4). For both molecules, the spin density distribution of T1 is mainly located on the cyanopyridine and the neighbouring phenyl ring or the benzene part on the carbazole ring, with small contribution from the carbazole units, and this can explain similar triplet energies of both compounds. 200

2

)

(a) hole only devices

25 m-PyCNmCP 3-PyCNmCP

20

(b) electron only devices

2

Current density (mA/cm )

30

Current density(mA/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 10 5

150

m-PyCNmCP 3-PyCNmCP

100

50

0

0 0

5

10 15 Voltage(V)

20

25

0

5

10 15 Voltage (V)

20

25

Figure 5 The current density versus voltage curves of the hole–only (a) and electron–only devices (b) for the compounds m-PyCNmCP and 3-PyCNmCP. Carrier Transport Property. Bipolar charge transport property of host materials is one of most critical factors for achieving the high efficiency of PhOLEDs. The above electrochemical results and DFT calculation have indirectly demonstrated that m-PyCNmCP and 3-PyCNmCP have bipolar carrier transporting characteristics. The single-carrier devices with simple structures can reveal the intrinsic hole-transporting and electron-transporting capabilities of the major functional layer. Herewith, hole-only devices with configuration of ITO/PEDOT:PSS (40 nm)/TAPC (5 nm)/host (100 nm)/TAPC (5 nm)/Al (200 nm) and electron-only devices with configuration of ITO/TmPyPB (5 nm)/host (100 nm)/TmPyPB (5 nm)/Li (1 nm)/Al (200 nm) were fabricated, respectively, to study the charge transporting properties of m-PyCNmCP and 3-PyCNmCP. The chemical structures of 11



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these materials and the energy level diagram of the devices are shown in Figure S3.10,15 In hole-only devices, PEDOT:PSS/TAPC were inserted between anode and host layer to facilitate hole injection and the TAPC/Al interface with large electron barrier of 2.3 eV was designed to prevent electron injection. In electron-only devices, the presence of TmPyPB between ITO anode and the host layer was to inhibit hole injection due to too deep HOMO level (–6.68 eV) of TmPyPB and the resultant large hole barrier of 1.88 eV. The studied m-PyCNmCP or 3-PyCNmCP layer in these single-carrier devices was controlled absolutely thicker than the adjacent layers with expectation to reveal the intrinsic charge-transporting feature of these host materials by reducing the influence from the ancillary layers. As seen in Figure 5, both the hole-only and electron-only devices of these two materials exhibited reasonable hole currents and electron currents in the typical voltage range that is suitable for OLEDs, confirming the appropriate hole-transporting and electron-transporting abilities of m-PyCNmCP and 3-PyCNmCP and their bipolar charge transporting nature. It is interesting that both the hole-only and electron-only devices of 3-PyCNmCP exhibited higher current densities than those of m-PyCNmCP at a given voltage (Figure 5). Considering their similar HOMO/LUMO levels and similar charge injections, the higher current densities in 3-PyCNmCP based single-carrier devices imply that the incorporation of cyanopyridine at the 3-site of carbazole in 3-PyCNmCP probably leads to certain molecular conformation and intermolecular stacking style that are favorable for more rapid charge transportation. In spite of different device structures for the hole-only device and electron-only device, the apparently higher electron current densities (100-150 mA/cm2 at 25 V) than the hole current densities (10-20 mA/cm2 at 25 V) in the entire voltage range for both m-PyCNmCP and 3-PyCNmCP still indicate that m-PyCNmCP and 3-PyCNmCP are good at electron transporting based on the present device structures. We suppose the good electron transportation ability of m-PyCNmCP and 3-PyCNmCP should benefit from the combination of pyridine and cyano groups as n-type units and their direct linking mode, which definitely enhance 12


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both electron injection and transportation. It is expected that the good electron transporting abilities of m-PyCNmCP and 3-PyCNmCP can help to balance the positive and negative charges in the emitting layer when they are used as host materials in OLEDs. Electroluminescent Devices. The Ir(ppy)3-based green electrophosphorescent devices G1 and G2 with the configuration of ITO/PEDOT:PSS (40 mn)/TAPC (20 nm)/host:10wt% Ir(ppy)3 (30 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (200 nm) were fabricated to evaluate the capability of m-PyCNmCP and 3-PyCNmCP as host materials in PhOLEDs. In these devices, TAPC was used as the hole-transport layer (HTL) and TmPyPB as electron-transport (ETL) and hole-blocking layer (HBL) (Figure S3), PEDOT:PSS and LiF were used as hole- and electron-injecting layers, respectively. The current density–voltage–brightness (J–V–B) characteristics and efficiency curves of these devices are shown in Figure 6 and Figure S4 (a) and the electroluminescence (EL) data are summarized in Table 2. Optimization studies showed that the best EL performance was achieved with 10 wt% of dopant. Devices G1 and G2 emitted green phosphorescence from the Ir(ppy)3 dopant with CIE coordinates of (0.30, 0.64) and (0.30, 0.63), respectively. As shown in Figure 6 and Table 2, an extremely low turn-on voltages (to realize a brightness of 1 cd/m2) of 2.01 V and 2.27 V were achieved for m-PyCNmCP based G1 and 3-PyCNmCP based G2. Both turn-on voltages are below a theoretical limit of the band gap (Eg) voltage of 2.37 V that is determined as the emitted photo energy hv (1240/524=2.37 eV) at the emission peak wavelength (524 nm in present study, Figure S4 b) divided by the electron charge (e).1,2 At the same time, high power efficiency of 101.4 lm/W and 119.3 lm/W were achieved for G1 and G2, respectively. The maximum external quantum efficiency (ηext) of each device is consistent with the calculated value from the PL quantum yield of doped Ir(ppy)3:host film if assuming the outcoupling efficiency of these devices are in the range of 20∼30%, as shown in Table 2. Like the case in single-carrier devices, 3-PyCNmCP hosted green device G2 exhibited much higher current density than m-PyCNmCP device G1 in almost the whole detected 13



voltage range (except at the extremely low voltage point such at turn-on voltage), as shown in Figure 6a. This is reasonable since the single-carrier devices results revealed that 3-PyCNmCP is better at both hole transporting and electron transporting due to appropriate molecular arrangement than m-PyCNmCP. 5

4

10

G1 G2

150

3

120

10

90

10

2

60

1

10

30

120

0

(b)

100

G1 G2

100 80

10

60 40 20

Power efficiency (lm/W)

10

(a)

Current efficiency (cd/A)

180

Brightness (cd/m2)

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 0

2

4

6 8 Voltage (V)

0

10

10000

20000

30000

1 40000

Brightness (cd/m2)

Figure 6 (a) J–V–L characteristics and (b) efficiency curves for m-PyCNmCP and 3-PyCNmCP based green PhOLEDs G1 and G2. Kido, et al.1 has realized an extremely low turn-on voltage of 1.97 V by elaborately selecting hole-injection material hexacarbonitrile (HATCN), host materials BCzTPA20, electron-transporter B4PYPPM and electron-injection layer Libppy,21 utilizing a high doping level of 17 wt% Ir(ppy)3 to fabricate green device of ITO (130 nm)/HATCN (1 nm)/TAPC (50 nm)/17 wt% Ir(ppy)3-doped host (10 nm)/B4PyPPM (50 nm)/Libpp (1 nm)/Al (80 nm). As far as we know, that was the lowest turn-on voltage for Ir(ppy)3 based PhOLEDs reported so far. Kim, et al.4 reported a turn-on voltage of 2.4 V for Ir(ppy)3-doped green OLED with exciplex-forming co-host. In this case, the emitting layer was obtained by vacuum co-deposition of three components, i.e. the phosphor dopant, the electron donor, and the electron acceptor, the later two of which were used to form the exciplex host. It should be noted that our present devices are advantageous over the aforementioned literature cases in terms of much simpler device structure. Apparently the turn-on voltage of 2.01 V achieved by m-PyCNmCP hosted G1 is quite close to the 1.97 V obtained in Kido’s work with well-designed 14


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device structure, and much lower than the turn-on voltage in Kim’s work where the complicate exciplex-forming co-host was employed. We attribute such low turn-on voltages and high power efficiencies of m-PyCNmCP and 3-PyCNmCP hosted green devices to the following reasons. Firstly, the energetic alignment of HOMO/LUMO levels of these hosts with both the dopant and adjacent layers should be the most important reason for the low driving voltages. By combining two electron-withdrawing units, i.e. pyridine and cyano, the LUMO levels of m-PyCNmCP and 3-PyCNmCP are as deep as -2.67 eV and -2.57 eV, both which have small electron barriers of only 0.06 eV and 0.16 eV from the electron transporting layer (TmPyPB), as shown by the energy diagram in Figure S3. At the same time, just because of the direct linking of these two electron-transporting units, the HOMOs of these hosts are kept moderately high at -5.70 eV and -5.65 eV, respectively, which also form small hole barriers of 0.20 eV and 0.15 eV with the hole transporting layer (TAPC). These tiny electron barriers and hole barriers can unambiguously guarantee charge injections into the emitting layer and light emission at very low driving voltages. However, the LUMOs of these two hosts are still slightly higher than that of the Ir(ppy)3 dopant (-2.8 eV), together with the slightly deeper HOMOs of these hosts than that of the dopant to guarantee efficient forward Förster energy transfer from host to dopant. In addition, just because the HOMO and LUMO of the doped Ir(ppy)3 are sandwiched between those of the host, the direct charge trapping by the dopant should be another excitation way in addition to energy transfer from host to dopant. This can be proved by our experimental fact that the similar devices with lower doping concentrations (6 wt%) turned on at higher voltages. For example, the control device G11 has identical device structure and same host with G1 but with lower doping concentration (6 wt% Ir(ppy)3) in the emitting layer. As shown by Figure S5 and the data in Table S1, G11 turned on at slightly higher voltage of 2.22 V (relative to 2.01 V for G1) and exhibited decreased efficiencies (52.4 cd A–1 relative to 68.2 cd A–1 for G1). In the same way, the control device G21 with lower 15



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Page 16 of 22

concentration (6 wt%) of Ir(ppy)3 also turned on at higher voltage (2.33 V) and exhibited lower efficiencies (64.8 cd A–1) than the same host based device G2. It has been established that direct charge trapping usually results in quite low driving voltages.1 Therefore, the direct charge trapping in these PhOLEDs is another important excitation channel to contribute to the extremely low turn-on voltages. Secondly, the suitably higher triplet energies of m-PyCNmCP (2.82 eV) and 3-PyCNmCP (2.76 eV) than the dopant (∼2.40 eV) facilitate efficient Dexter energy transfer from host to dopant and confine triplet excitons on dopant molecules, while the small ∆EST values of m-PyCNmCP (0.38 eV) and 3-PyCNmCP (0.43 eV) must contribute to low driving voltages, as verified in many other similar reports.6,11,12,22 In addition, the bipolar charge transport properties of these host materials can definitely enhance charge balance in the emitting layer as demonstrated in single charge devices, which is also an important factor to result in low driving voltages and high power efficiencies. Thirdly, it is generally believed that the thermal energy kT carried by electrons and holes is responsible, at least to some extent, for the extra lower turn-on voltage of OLEDs than the theoretical limit (emitted photo energy hv divided by the electron charge e). The extra thermal energy would make these high-energy carriers more readily diffuse over to the side of opposite type where they recombine than those low-energy ones.1,23,24 Inorganic light-emitting diodes show similar phenomenon, which usually operate at the voltage lower than the minimum value of hv/e by 0.1∼0.2 V. Table 2: Electroluminescence Characteristics of the Devices.a Devices and host

a

Von (V)

Lmax (cd m-2)

G1 m-PyCNmCP

2.01

45510

G2 3-PyCNmCP

2.27

66440

ηc b (cd A–1)

ηp b (lm W–1)

ηext b (%)

68.2, 60.3, 52.7 86.2, 84.9, 58.5

101.4, 62.2, 32.7 119.3, 71.4, 54.5

18.4, 16.3, 14.2 24.7, 24.3, 16.8

CIE c (x, y)

PLQY d (%)

ηext e (%)

0.30, 0.64

75.4

15.1–22.6

0.30, 0.63

92.5

18.5–27.8

Abbreviations: Von, turn–on voltage to give a brightness of 1 cd m−2. Lmax, maximum luminance. 16


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ηext, external quantum efficiency. ηc, current efficiency. ηp, power efficiency. CIE (x,y), Commission International de I’Eclairage coordinates. b Order of measured values: maximum, then at 100 and 1000 cd m−2. c Measured at 8 V. d PLQY, photoluminescence quantum yield of Ir(ppy)3:host films measured by intergrating sphere. e The external quantum efficiency of the corresponding OLED calculated from the PL quantum yield using the equation of ηext =ηint ηout= γ ηr ΦPL ηout, in which both the charge injection ratio γ and the excitation production ratio ηr are supposed as 1, the out-coupling constant ηout is taken as 20∼30%.

CONCLUSION By employing cyanopyridine as electron-transporting moiety, two novel bipolar host materials, namely m-PyCNmCP and 3-PyCNmCP, have been developed for application in green PhOLEDs. Extremely low turn-on of voltages 2.01 and 2.27 V were achieved for m-PyCNmCP and 3-PyCNmCP hosted green devices, respectively, which are even lower than the theoretical limit of the emitted photon energy (hv)/electron charge (e) (2.37 V) of the emitter Ir(ppy)3. The turn-on voltage of 2.01 V is actually comparable with the lowest record (1.97 V) of Ir(ppy)3 devices reported so far. High power efficiencies of 101.4 lm/W (corresponding to an EQE of 18.4%), and 119.3 lm/W (24.7%) were achieved for these green PhOLEDs, respectively. The tiny charge injection barriers on EML interfaces and direct charge trapping mechanism by dopan molecules mainly due to incorporation of cyanopyridine as electron transporting units in host materials is believed as the most important reason responsible for these extremely low turn-on voltages. In addition, the appropriately high singlet and triplet energy levels and small singlet/triplet energy difference, and the bipolar charge transporting feature of these hosts also contribute to the low driving voltages and high power efficiencies of these green PhOLEDs. The excellent performance obtained with so simple device structures indicates these cyanopyridine based host materials can find great potential applications in low-driving and high-performance OLEDs.

ACKNOWLEDGEMENTS 17



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We acknowledge supports from the National Natural Science Foundation of China (21274016, 21374013, and 21421005), the Fundamental Research Funds for the Central Universities (DUT15YQ101 and DUT16ZD221), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R06), and Program for DUT Innovative Research Team (DUT2016TB12).

Supporting Information. The supporting information is available free of charge via the internet at … or from the author. TGA thermograms, LT PL spectra of films, energy level diagram, external quantum efficiency–brightness curves and EL spectra.

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TOC Graphical Abstract

Cyanopyridine based novel bipolar host materials with deep LUMOs and moderately shallow HOMOs are developed to fabricate green phosphorescent organic light-emitting diodes that exhibited extremely low turn-on voltage of 2.01 V and high power efficiency of 119.3 lm/W even with simple device structures.

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Cyanopyridine Based Bipolar Host Materials for Green

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