π-π stacking: a strategy to improve the electron mobilities of bipolar hosts for TADF and phosphorescent devices with low efficiency roll-off

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ISSN 2050-7526

PAPER

~

Nguyên T. K. Thanh, Xiaodi Su et al.

Fine-tuning of gold nanorod dimensions and plasmonic properties using

the Hofmeister eﬀects

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πꢀπ stacking: a strategy to improve the electron mobilities of

bipolar hosts for TADF and phosphorescent devices with low

efficiency roll-off

Received 00th January 20xx,

Accepted 00th January 20xx

Minghan Cai,^aXiaozeng Song,^aDongdong Zhang,^aJuan Qiao^aand Lian Duan*^a

DOI: 10.1039/x0xx00000x

A series of D-π-A type bipolar hosts based on triphenylene/carbazole were designed and synthesized. π-π stacking of

triphenylene units between two adjacent molecules renders these hosts high electron mobilities above 1×10^-4cm²V^-1s^-1

and their electron and hole mobilities can be regulated through varying the connection position and π moieties. Due to

more balanced charge mobilities, D1 based green thermally activated delayed fluorescence (TADF) device achieved the

highest external quantum efficiency (EQE) of 16.3% and its EQE could still maintain 15.3% and 13.8% at high luminance of

5000 cd m^-2and 10000 cd m^-2, respectively. D1 based green phosphorescent devices also exhibited a highest EQE of 18.6%

www.rsc.org/

with

reduced

roll-off

to

17.6%

at

5000

cd

m^-2

and

16.2%

at

10000

cd

m^-2.

energy transfer to emitters, suitable highest occupied

molecular orbital (HOMO) and lowest unoccupied molecular

orbital (LUMO) energy levels.^9-14Besides, balanced hole and

electron mobilities of hosts are also crucial for high efficiency

and long lifetime devices.^15-17Bonding electron-donating and

electron-accepting unit is a commonly used strategy for

designing bipolar hosts with balanced electron and hole

mobilities.^18-25For example, Lee and his group synthesized

Introduction

Nowadays, organic light emitting diodes (OLEDs) have already

become the most promising technology in display field due to

their characteristics such as self-luminous, low power

consumption, high resolution and high flexibility.¹The

utilization of excitons plays an important role in making OLED

devices more efficient.²Compared with fluorescent materials

which can merely use singlets to emit light, phosphorescent

and TADF materials can utilize both singlets and triplets.^3-7

Recently developed phosphorescent and TADF materials can

even achieve 100% internal quantum efficiencies (IQEs).

However, the efficiency roll-off of most phosphorescent

organic light emitting diodes (PHOLEDs) and TADF OLEDs is

serious, especially in high luminance of 5000 cd m^-2and 10000

cd m^-2. Efficiency roll-off will not only increase power

consumption but also shorten the lifetime of devices. Previous

studies showed that efficiency roll-off could be ascribed to

triplet-triplet annihilation (TTA) and triplet-polaron

annihilation (TPA) caused by the long-lived triplets of emitters

under high driving voltage and aggregation induced quenching

(ACQ) phenomenon caused by high doping concentration of

dopants.⁸

bipolar

host

3’,5’-di(carbazol-9-yl)-[1,1’-biphenyl]-3,5-

dicarbonitrile (DCzDCN) for both TADF and phosphorescent

OLEDs. Efficiency roll-off of these devices had been

significantly suppressed.²⁶

Interestingly, bipolar hosts with equal electron and hole

mobilities may not be the best choice. Adachi’s group found

that PHOLEDs using electron transporting layer materials as

hosts exhibited higher efficiencies than reference device using

bipolar host 4,4’-Bis-(9-carbazolyl)-biphenyl (CBP) as host.²⁷

Recently, they also confirmed that blue TADF OLEDs could be

more stable if the emitters are doped into electron-transport

materials instead of hole-transport materials as a result of

reduced electrochemical oxidation process and a photo-

oxidation process.²⁸Besides, hole mobilities of most hole

transporting materials (HTMs) are about two to three orders of

magnitude higher than the electron mobilities of classical

electron transporting materials (ETMs).²⁹Bipolar hosts with

relatively higher electron mobilities will be more compatible

with adjacent charge transport layers. Thus, new strategies to

further improve electron mobilities of bipolar hosts are highly

desired.

To suppress efficiency roll-off, phosphorescent and TADF

materials are often dispersed in hosts to limit their

intermolecular interaction. Our previous researches showed

that hosts should possess sufficient high triplets, effective

Previous studies showed that interaction between

adjacent molecules can significantly improve the electron

mobilities.^30-32π-π stacking is a kind of intermolecular

interaction widely found in polycyclic aromatic derivatives.

Besides, their HOMOs and LUMOs can be easily modulated by

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changing the fragments attaching to them. For example, against ferrocene/ ferrocenium. The oxidation potentials were

triphenylene based materials usually tend to pile up by π-π measured in dichloromethane solution containing 0.1 M n-

stacking and show one-dimensional charge transporting Bu₄NPF₆as supporting electrolyte at a scan rate of 100 mV s^-1.

properties. Conductivity along the columns is much higher The UV/Vis absorption spectra were recorded by an Agilent

than that in the perpendicular direction.^33-34By attaching to 8453 spectrophotometer. The quantum chemical calculations

pyridine, Adachi designed triphenylene-based ETM 2,7- were performed by using the Gaussian 09 program package.

Bis(2,2’-bipyridine-5-yl)triphenylene

(Bpy-TP2).

OLEDs The geometries of materials in the ground stat were optimized

containing Bpy-TP2 as ETM showed significantly lower driving via density functional theory (DFT) calculations at the B3LYP/6-

voltages, lower power consumption and equal lifetime when 31G(d) level. Triplet states and singlet states of these materials

compared with devices using conventional ETMs.³⁵Besides, were calculated by using time-dependent density functional

triphenylene derivatives can also be used as hosts.^36-38By theory (TD-DFT) calculations at B3LYP/6-31G(d) level. The

attaching to carbazole, Lee’s group prepared triphenylene HOMO and LUMO orbitals were visualized by using Gaussview.

based

host

4-(3-(triphenylen-2- The surface morphologies of films were measured by SPA400

yl)phenyl)dibenzo[b,d]thiophene (DBTTP1) for green TADF (Seiko instruments Inc.). Single crystals were grown from slow

OLED and it achieved a high EQE above 20% with a long evaporation of dichloromethane/petroleum ether (1:4)

lifetime.³⁶Lee and Jeon synthesised three triphenylene solutions. The room temperature single-crystal X-ray

derivatives for the use as hosts for green PHOLEDs and experiments were performed on a RIGAKU SATURN 724+ CCD

achieved a high EQE up to 20.3%^.37However, the relationship diffractometer equipped with a graphite monochromatized

between intermolecular interaction, electron and hole Mo Kα radiation. The structures were solved by direct

mobilities and good device performance was still unclear in the methods and refined with a full-matrix least-squares technique

above researches.

based on F2 with the SHELXL-97 crystallographic software

Here, we report six hosts D1-D6 for both TADF and package. The corresponding CCDC reference numbers (CCDC:

phosphorescent OLEDs. Triphenylene was selected as n-type 1524760 and 1524763) and the data can be obtained free of

unit and carbazole was selected as p-type unit. Due to various charge from The Cambridge Crystallographic Data Centre.

connection position and π moieties, six hosts exhibit different

hole and electron mobilities. We fabricated green TADF and

Syntheses

phosphorescent devices using 1,2,3,5-tetrakis(carbazol-9-yl)-

4,6-dicyanobenzene (4CzIPN) and tris[2-phenylpyridinato-C²,N]

Materials. 2-bromotriphenylene was purchased from the

Shanghai

Dibai

Technology

Co.

Ltd.

2-(3-

iridium(III) [Ir(ppy)₃] as the dopants, respectively. Due to more

balanced charge transport properties, D1 based TADF and

phosphorescent devices exhibited the highest EQEs up to

16.3% and 18.6%, respectively. These values are higher than

the EQEs of reference TADF device (11.5%) and

phosphorescent device (15.8%) using CBP as the host. The

efficiency roll-off of D1 based devices is also lower than that of

CBP based device.

Bromophenyl)triphenylene was purchased from the Suzhou

ge’ao New Material Co. Ltd. and all of the boronic acid

ramifications were purchased from Beijing Pure Chem. Co. Ltd.

All of these reagents were received and used without further

purification.

General procedure for synthese of 9-(3-(triphenylen-11-

yl)phenyl)-9H-carbazole

bromotriphenylene (2.15 g, 7.0 mmol), 3-(9H-Carbazol-9-

yl)phenylboronic acid (2.00 g, 7.0 mmol),

and

other

products.

2-

Experimental section

tetrakis(triphenylphosphine)palladium(0) (0.24 g, 0.21 mmol),

2M Na₂CO₃(20 ml), ethanol (3 ml) and toluene (30 ml) were

sequentially added to a 100 ml round-bottom flask. The

mixture was heated to 80 ^oC and refluxed under nitrogen

atmosphere for 48 hours. After the reaction completed, the

mixture was extracted with 500 ml dichloromethane for two

times. The organic layer was stored and evaporated under

reduced pressure. Then, the crude product was isolated by

General information

The ¹H NMR spectra were measured by a JEOLAL-600 MHz

spectrometer at ambient temperature with tetramethylsilane

as the internal standard. The mass spectra were recorded on

MALDI-TOF MS, Performance (Shimadzu, Japan). The

thermogravimetric analysis (TGA) was performed on a STA

409PC thermogravimeter at a heating rate of 10 ^oC min^-1from

ambient temperature to 600 ^oC under nitrogen atmosphere. silica gel column chromatography and further purified by

The differential scanning calorimetry (DSC) measurements

were performed on a DSC 2910 modulated calorimeter at a

heating rate of 10 ^oC min^-1from ambient temperature to the

temperature before decomposition under nitrogen

atmosphere. The electrochemical measurements were

vacuum sublimation to give pure product as white solid.

9-[3-(2-triphenylenyl)phenyl]-9H-Carbazole (D1). Yield: 76%.

¹H NMR (600 MHz, CDCl₃, TMS): δ= 8.89 (s, 1H), 8.71 (t, 2H),

8.66 (d, J=8Hz, 3H), 8.19 (d, J=8Hz, 2H), 8.02 (s, 1H), 7.92 (t,

2H), 7.76 (t, 1H), 7.70-7.65 (m, 3H), 7.63 (t, 2H), 7.54 (d, J=8Hz,

2H), 7.45 (t, 2H), 7.32 (t, 2H). ¹³C NMR (150 MHz, CDCl₃): δ=

143.17, 141.05, 138.76, 138.52, 130.59, 130.31, 130.17,

130.01, 129.73, 129.56, 129.47, 127.62, 127.55, 127.48,

127.45, 126.52, 126.28, 126.23, 126.19, 126.06, 124.25,

performed with

a Potentiostat/ Galvanostat Model 283

(Princeton Applied Research) electrochemical workstation by

using Pt as working electrode, platinum wire as auxiliary

electrode, and a Ag wire as reference electrode standardized

2 | J. Name., 2012, 00, 1-3

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123.57, 123.53, 123.49, 123.45, 121.95, 120.54, 120.17, C₄₂H₂₇N: C, 92.45; H, 4.99; N, 2.57; found: C, 92.33; H, 5.01; N,

109.99. MS (MALDI-TOF): m/z 469.2710 [M]⁺. Elemental 2.66.

analysis: calcd. for C₃₆H₂₃N: C, 92.08; H, 4.94; N, 2.98; found: C, 9-phenyl-3-[3-(2-triphenylenyl)phenyl]-9H-Carbazole

(D6).

Yield: 80%. ¹H NMR (600 MHz, CDCl₃, TMS): δ= 8.96 (s, 1H),

91.98; H, 4.90; N, 3.12.

9-[4-(2-triphenylenyl)phenyl]-9H-Carbazole (D2). Yield: 83%. 8.81-8.75 (m, 2H), 8.73-8.66 (m, 3H), 8.49-8.47 (s, 1H), 8.26-

¹H NMR (600 MHz, CDCl₃, TMS): δ= 8.95 (s, 1H), 8.82-8.79 (m, 8.22 (d, J=8Hz, 1H), 8.16-8.13 (s, 1H),8.03-7.99 (d, J=9Hz, 1H),

1H), 8.77(d, J=8Hz, 1H), 8.73-8.66 (m, 3H), 8.18 (d, J=8Hz, 2H), 7.82-7.76 (m, 3H), 7.72-7.60 (m, 9H), 7.55-7.48 (m, 2H), 7.47-

8.03 (d, J=8Hz, 2H), 7.99 (d, J=8Hz, 1H), 7.73 (d, J=8Hz, 2H), 7.42 (m, 2H), 7.36-7.31 (m, 1H). ¹³C NMR (150 MHz, CDCl₃): δ=

7.72-7.67 (m, 4H), 7.54 (d, J=8Hz, 2H), 7.46 (t, 2H), 7.32 (t, 2H). 142.90, 141.90, 141.50, 140.62, 140.16, 137.74, 133.49,

¹³C NMR (150 MHz, CDCl₃): δ= 141.00, 140.37, 139.04, 137.24, 130.24, 130.16, 130.08, 129.93, 129.74, 129.53, 129.11,

130.35, 130.25, 130.02, 129.86, 129.67, 129.35, 128.94, 127.69, 127.50, 127.44, 127.41, 127.39, 127.21, 126.74,

127.64, 127.54, 127.49, 126.37, 126.16, 124.27, 123.61, 126.71, 126.65, 126.34, 125.87, 125.75, 124.07, 123.57,

123.55, 123.49, 121.99, 120.52, 120.18, 110.01. MS (MALDI- 123.51, 123.49, 122.06, 120.59, 120.26, 119.14, 110.26,

TOF): m/z 469.3142 [M]⁺. Elemental analysis: calcd. for 110.09. MS (MALDI-TOF): m/z 545.3032 [M]⁺. Elemental

C₃₆H₂₃N: C, 92.08; H, 4.94; N, 2.98; found: C, 91.88; H, 5.01; N, analysis: calcd. for C₄₂H₂₇N: C, 92.45; H, 4.99; N, 2.57; found: C,

3.11.

92.40; H, 4.98; N, 2.62.

9-phenyl-3-(2-triphenylenyl)-9H-Carbazole (D3). Yield: 80%.

¹H NMR (600 MHz, CDCl₃, TMS): δ= 8.96 (s, 1H), 8.82 (d, J=8Hz,

1H), 8.72 (d, J=8Hz, 1H), 8.69 (t, 3H), 8.55 (s, 1H), 8.28 (d,

J=8Hz, 1H), 8.02 (d, J=8Hz, 1H), 7.86 (d, J=8Hz, 1H), 7.73-7.66

(m, 4H), 7.66-7.61 (m, 4H), 7.55 (d, J=8Hz, 1H), 7.53-7.48 (m,

1H), 7.46 (d, J=4Hz, 2H), 7.38-7.32 (m, 1H). ¹³C NMR (150 MHz,

CDCl₃): δ= 141.57, 140.88, 140.69, 137.78, 133.52, 130.32,

130.12, 128.51, 127.74, 127.45, 127.40, 127.26, 127.23,

126.89, 126.39, 125.87, 124.20, 124.03, 123.62, 123.57,

Device fabrication and measurements

Besides the synthesized materials, all other reagents used in

fabricating devices were purchased from Jilin Optical and

Electronic Materials Co. Ltd. and Xi’an Polymer Light

Technology Co. Ltd. The organic layers were deposited

consecutively on the precleaned ITO-coated glass substrates in

a vacuum chamber with a pressure of 2×10^-4Torr. Next, the

cathode was fabricated with thermal evaporation of a 0.5 nm

123.50, 123.46, 121.91, 120.65, 120.30, 119.26, 110.36, LiF layer followed by a 150 nm aluminum layer. The deposition

rates of all organic materials and aluminum were 1-2 Å s^-1,

while that of the LiF layer was 0.1 Å s^-1. The electrical

characteristics of the devices were measured with Keithley

2400 source meter. The electroluminescence spectra and

luminance of the devices were obtained on a PR650

spectrometer. Characteristics of all the OLED devices were

carried out in ambient laboratory conditions at room

temperature.

110.14. MS (MALDI-TOF): m/z 469.3012 [M]⁺. Elemental

analysis: calcd. for C₃₆H₂₃N: C, 92.08; H, 4.94; N, 2.98; found: C,

92.02; H, 4.88; N, 3.10.

9-[3'-(2-triphenylenyl)[1,1'-biphenyl]-3-yl]-9H-Carbazole (D4).

Yield: 68%. ¹H NMR (600 MHz, CDCl₃, TMS): δ= 8.94 (s, 1H),

8.91-8.77 (m, 1H), 8.76 (d, J=8Hz, 1H), 8.72-8.66 (m, 3H), 8.17

(d, J=8Hz, 2H), 8.11 (s, 1H), 7.99 (d, J=8Hz, 1H), 7.94 (d, J=8Hz,

2H), 7.85 (d, J=8Hz, 1H), 7.74 (d, J=8Hz, 1H), 7.72-7.65 (m, 7H),

7.52 (d, J=8Hz, 2H), 7.44 (t, 2H), 7.31 (t, 2H). ¹³C NMR (150

MHz, CDCl₃): δ= 142.12, 141.22, 140.98, 140.37, 139.83,

137.24, 130.30, 130.22, 129.99, 129.90, 129.73, 129.27,

128.87, 127.59, 127.56, 127.50, 127.45, 126.92, 126.58,

126.50, 126.14, 124.16, 123.59, 123.58, 123.54, 123.52,

122.07, 120.50, 120.16, 109.99. MS (MALDI-TOF): m/z

545.3247 [M]⁺. Elemental analysis: calcd. for C₄₂H₂₇N: C, 92.45;

H, 4.99; N, 2.57; found: C, 92.20; H, 4.91; N, 2.89.

Results and discussion

Synthesis and characterization

Bipolar hosts D1-D6 were synthesized by Suzuki coupling

reaction. The synthetic routes and chemical structures of all

hosts are shown in Scheme 1. All reagents were commercially

available and used as obtained. ¹H nuclear magnetic resonance

(¹H NMR) spectrometer and mass spectrometry were used to

characterize the molecular structures.

9-[3'-(2-triphenylenyl)[1,1'-biphenyl]-4-yl]-9H-Carbazole (D5).

Yield: 73%. ¹H NMR (600 MHz, CDCl₃, TMS): δ= 8.89 (s, 1H),

8.76-8.73 (m, 1H), 8.71 (d, J=8Hz, 1H), 8.69-8.64 (m, 3H), 8.18

(d, J=8Hz, 2H), 8.05 (s, 1H), 7.93 (d, J=7Hz, 2H), 7.82 (d, J=8Hz,

2H), 7.73 (t, 1H), 7.71-7.59 (m, 7H), 7.53 (d, J=8Hz, 2H), 7.45 (t,

J=2Hz, 2H), 7.32 (t, J=2Hz, 2H). ¹³C NMR (150 MHz, CDCl₃): δ=

143.25, 142.11, 141.08, 140.99, 139.68, 138.44, 130.53,

130.27, 130.17, 129.96, 129.86, 129.73, 129.67, 129.24,

127.56, 127.47, 127.11, 126.54, 126.48, 126.17, 126.02,

124.14, 123.56, 123.52, 122.00, 120.52, 120.16, 109.97. MS

(MALDI-TOF): m/z 545.2889 [M]⁺. Elemental analysis: calcd. for

Thermal and morphology properties

We employed TGA and DSC tests to investigate the thermal

properties of D1-D6. The results are shown in Fig. 1 and Fig. S1

(ESI†). Compared with traditional host CBP, D1-D3 show higher

glass-transition temperatures (Tg) of 102, 111 and 107 ^oC,

respectively. Among D1-D3, the liner shape D2 demonstrates

the highest Tg. By introducing an extra phenyl group, the glass-

transition temperatures of D1-D3 are further improved to 124,

114 and 120 ^oC for D4-D6, respectively. The thermal-

decomposition temperatures (Td, corresponding to 5% weight

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loss) of D1-D3 are 399, 374 and 369 ^oC, respectively. Similar to of toluene by more polar solvents cannot influence the

o

Tg, Td of D4-D6 increase to 409, 425 and 421 C, respectively. location but merely change the relative strength of two peaks

Surface morphology of vacuum-deposited D1-D6 films were for all compounds except for D2. This result suggests that,

also measured by atomic force microscopy (AFM) (Fig. S2, although there exist CT states in D1, D4, D5 and D6. Locally

ESI†). We observed no aggregation and crystallization for all of excited (LE) states still dominate. In film, D1, D2 and D3 all

six hosts and the root-mean-square (RMS) roughness values of peak at 401 nm (Fig. S4, ESI†). For D4, D5 and D6 the including

D1-D6 are 0.94, 1.05, 0.94, 1.01, 0.93 and 1.18 nm, of another phenyl introduces large steric hindrance and

respectively. Good thermostabilities and film-forming increases the distance between two adjacent molecules. This

properties of D1-D6 can improve the stability and results in the blue shift of PL emission peaks from 401 nm of

performances of devices.

D1, D2 and D3 to 388, 378 and 390 nm of D4, D5 and D6,

respectively. The triplet energies of D1-D6 were determined by

their first phosphorescence peaks recorded in toluene solution

at 77 K. Similar to their fluorescence spectra in toluene, D1,

D4, D5 and D6 show a much higher triplet energies of 2.66,

2.67, 2.66 and 2.65 eV, respectively. And the triplet energies of

D2 and D3 are 2.60 and 2.61 eV. High triplet energies make

D1-D6 suitable hosts for green dopants.

DFT simulation

The HOMOs and the LUMOs of D1-D6 are simulated at

B3LYP/6-31G(d) level by Gaussian 09 and presented in Fig. 2.

DFT calculations confirm that the LUMOs of six hosts mainly

localize on their triphenylene groups while the HOMOs of D1,

D4, D5 and D6 mainly localize on their carbazole groups.

Whereas, for D2, compared with D1, attaching the carbazole at

para-position extends the location of HOMO from carbazole to

triphenylene through the central phenyl. This phenomena is

further strengthened by directly attaching carbazole at 3-

position to triphenylene in D3, whose HOMO disperses to the

whole molecular. Separately location of LUMOs and HOMOs

usually induce intramolecular charge transfer (ICT). Thus,

Compared with D2 and D3, D1, D4, D5 and D6 possess special

absorption and fluorescence characters. This will be discussed

in the following part.

Electrochemical properties

The electrochemical properties of D1-D6 were investigated by

cyclic voltammetry (CV). As shown in Fig. 4, all of these

compounds show excellent oxidation behaviors. This is due to

the introduction of carbazole group. The HOMO levels of D1-

D6 are determined from the onset of the 1st oxidation

potentials with regard to the energy level of ferrocene and the

LUMO levels are calculated by adding the optical band gap to

the HOMO levels. The estimated HOMO levels of D1, D2 and

D3 are 5.67, 5.63 and 5.54 eV, respectively. Compare with D1,

the enlarged distribution of HOMO shifts the HOMO levels of

D2 and D3 to shallower levels. These values are similar to

previously reported triphenylene and carbazole based host

materials.^{36, 37}As for D4, D5 and D6, their HOMO distribution is

similar to D1. The introducing of extra phenyl causes slight

increase of HOMO levels (5.65 eV for D4 and D5, 5.63 eV for

D6). Optical band gaps of D1-D6 are estimated by the onset of

their absorption spectra in Fig. 4. They are 3.57, 3.50, 3.44,

3.55, 3.75 and 3.45 eV for D1-D6, respectively. LUMO levels

are calculated by HOMO levels and optical band gaps. The

LUMO levels of D1-D6 are 2.10, 2.13, 2.10, 2.10, 2.08 and 2.18

eV, respectively. Except for D6, all hosts show almost identical

LUMO levels. This is because the LUMOs of six hosts mainly

localize on their triphenylene groups and are less more likely

to be affected by the groups attaching to them. The variation

trend of HOMOs and LUMOs of D1-D6 is accord with that of

the calculated values (Fig. S5, ESI†). This result also validates

the accuracy of our theoretical calculations.

Photophysical properties

UV/Vis

absorption,

photoluminescence

(PL)

and

phosphorescence spectra of D1-D6 in toluene (1×10^-5mol L^-1

)

are shown in Fig. 3 and summarized in Table 1. We first

consider the absorption spectra of these compounds. All six

materials peak at about 290 nm. This is in accordance with

many previous reported carbazole based compounds and can

be assigned to the carbazole centered π-π* transitions.^39-41The

wide featureless emission at 300-320 nm of D1-D6 can be

ascribed to the π-π* transitions between the carbazole units

and the phenyl units connecting to them. Interestingly, weak

absorption peaks can also be observed at 341, 345, 345 and

349 nm for D1, D4, D5 and D6, respectivly. This confirms the

existence of charge transfer (CT) states which are usually

found in D-A type molecules, like TADF materials whose

HOMOs mainly localize on donor groups and LUMOs mainly

localize on acceptor groups. DFT calculation results in Fig. 2

can support this idea. LUMOs of six hosts localize mainly on

triphenylene but the location of their HOMOs expand from

carbazole to triphenylene in the order of D1, D4, D5, D6 to D2

to D3. Thus, only D1, D4, D5 and D6 with significant HOMO and

LUMO separation show this peaks. Upon photoexcitation at

300 nm, we recorded PL spectra of D1-D6 in toluene. The

emission of D1, D4, D5 and D6 in toluene is almost the same

and two peak at about 367 and 378 nm can be recognized. D2

and D3 peak at 371/386 nm and 376/394 nm. We also

recorded spectra of D1-D6 in tetrahydrofuran and

dichloromethane for comparison (Fig. S3, ESI†). Replacement

Charge-transporting properties

We used time of flight (TOF) transient photocurrent

measurements to investigate the charge-transporting

properties of D1-D6. And the results are presented in Fig. 5. All

of the mobility data could be well fitted with the Poole-Frenkel

function of ꢀ = ꢀ_ꢁexp(E^ꢂ/ꢃ/ꢄ). By adopting these equations,

we acquired the hole and electron mobilities of D1-D6 at the

electric field of 6×10⁷V m^-1for comparison (Fig. S6, ESI†). As

we anticipated, electron mobilities of all hosts are high and

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range from 1.02×10^-4to 9.66×10^-4cm²V^-1s^-1. Among these

We optimized the dopant concentrations and the

hosts, D2 shows the highest electron mobility of 9.66×10^-4cm²thickness of different layers. Structure of ITO/HAT-CN

V^-1s^-1. Usually, para-isomer will show enhanced mobility over (20nm)/NPB (40nm)/TCTA (10nm)/HOST: 4CzIPN (10 wt%,

the meta-isomer. This can also be concluded from the single 25nm)/Bphen (35nm)/LiF (1 nm)/Al (150nm) was adopted for

crystal structures and molecular packing data of D1 and D2 green TADF OLEDs and structure of ITO/HAT-CN (20nm)/NPB

(Fig. 6). The triphenylene groups and carbazole groups of D1 (40nm)/TCTA (10nm)/HOST: Ir(ppy)₃(5 wt%, 25nm)/Bphen

and D2 have a dominating influence on the overall molecule (35nm)/LiF (1 nm)/Al (150nm) was adopted for green

stacking and this eventually affect their hole and electron phosphorescent devices.

mobilities. For both of them, their triphenylene groups tend to

The electroluminance (EL) spectra, power efficiencies

pile up along a specific direction. On the one hand, the (PEs), current efficiencies (PEs), EQEs and current density–

carbazole groups of two adjacent D1 moleculars are not in the voltage–luminance (J-V-L) characteristics of TADF OLEDs based

same side and they will rotate to locate on two sides and result on six hosts are shown in Fig. 8 and summarized in Table 2.

in a “stair” like configuration. On the other hand, the carbazole Due to the complete energy transfer, we observed no emission

groups of D2 will align along the direction that triphenylene of hosts for all devices. The EL spectra of device A1, A2, A4 and

groups pile up. Given that carbazole groups can transport A5 peak at around 533 nm. For device A3 and A6, they are 547

holes efficiently, triphenylene can transport electrons nm and 550 nm. The red shift spectra of device A3 and A6 are

efficiently and the distance between two adjacent D2 ascribe to the formation of exciplexes of D3 and D6 with

molecules is merely 3.3 Å. HOMO and LUMO orbitals of nearby 4CzIPN. To confirm this, we measured the PL spectra of 10 wt%

D2 molecules will separately overlap to form two channels to 4CzIPN doped D1-D6 films (Fig. S7, ESI†). 4CzIPN doped D1, D2,

delivery holes and electrons and lead to the high hole and D4 and D5 films all peak at 522 nm. Whereas, the peaks of

electron mobilities of 2.87×10^-4and 9.66×10^-4cm²V^-1s^-1. 4CzIPN doped D3 and D6 films exhibit redshift to 530 nm and

Whereas for D1, the distance between two adjacent 527 nm, respectively. In addition to that, the spectra of 4CzIPN

triphenylene groups are 3.4/2.8 Å and the distance between doped D3 and D6 films are broader than those of 4CzIPN

two adjacent carbazole groups is 4.7 Å. The relatively larger doped D1, D2, D4 and D5 films. This further validates the

distance decrease the change for charges to hope and cause a formation of exciplexes between them. The turn on voltages

lower mobilities of 5.53×10^-5cm²V^-1s^-1for hole and 2.17×10^-4(V_on) of device A1, A2, A4 and A5 are in the range of 3.1-3.6 V

cm²V^-1s^-1for electron. For D3, its hole mobility is 4.82×10^-6and the turn on voltages of A3 and A6 are 2.9 V and 2.7 V. The

cm²V^-1s^-1, which is much lower than that of D1 and D2. This is lower turn on voltages of A3 and A6 are attributed to the

because that the dispersion of HOMO and overlapping with relatively shallower HOMOs of D3 and D6 and the formation of

LUMO usually lead to a decrease in hole mobility. The electron exciplexes (Fig. S8, ESI†). The maximum EQEs of device A1, A2,

mobility of D3 is 3.45×10^-4cm²V^-1s^-1and is between the A4 and A5 are 16.3, 15.9, 15.4 and 15.5% and their maximum

values of D1 and D2. As for D4, D5 and D6, the inducing of power efficiencies are 36.4, 41.4, 37.5 and 35.4 lm/W,

phenyl significantly influence their charge transport properties. respectively. These devices also show small efficiency roll-off

Electron mobilities of D4, D5 and D6 are 2.52×10^-4, 1.74×10^-4to 15.3, 14.2, 14.3, 14.0% and 13.8, 12.4, 12.9, 12.4% at high

and 1.02×10^-4cm²V^-1s^-1, which are much lower than those of luminance of 5000 cd m^-2and 10000 cd m^-2. Whereas, due to

D1, D2 and D3 respectively. This can be ascribe to the inclusion the formation of exciplexes, device A3 and A6 show much

of extra phenyl will increase the steric hindrance and thus lower maximum EQEs of 5.5% and 12.3% and their EQEs

increase the distance of two nearby molecules which can significant decrease to 3.6%, 11.1% and 2.7%, 9.9% at 5000 cd

suppress electrons to hop.

m^-2and 10000 cd m^-2, respectively. Compared with device A1,

A2, A4 and A5, reference device R1 using CBP as the host for

4CzIPN exhibited lower EQE and suffered from more serious

efficiency roll-off.

Devices

When considering the efficiencies of A1 and A2 with A4

and A5, A1 and A2 using D1 and D2 as hosts showed best

performances. This is due to more balanced electron and hole

mobilities of D1 and D2 when compared with other hosts and

their electron mobilities are slightly higher than their hole

mobilities. Given that the hole mobility of TCTA is 3.0×10^-3cm²

V^-1s^-1and electron mobility of Bphen is 5.2×10^-4cm²V^-1s^-1, D1

and D2 with slightly higher electron mobilities will be more

compatible with adjacent layers and enhance the

performances of devices.

To verify the performance of six materials, we fabricated green

TADF OLEDs (A1-A6) and PHOLEDs (B1-B6) containing D1-D6 as

hosts, respectively. Two reference devices using CBP as host

were also fabricated (R1 for TADF device and R2 for PHOLED).

Devices structures are presented in Fig. 7. We used indium tin

oxide (ITO) as an anode, hexaazatriphenylene hexacarbonitrile

(HAT-CN) as a hole-injection layer, N,N’-bis(1-naphthyl)N,N’-

diphenyl-1,1’-biphenyl-4,4’-diamine (NPB) as a hole-transport

layer, TCTA as exciton/electron blocking layers, new

synthesized six materials as host materials for emitting layers

and Ir(ppy)₃and 4CzIPN as dopant materials for emitting

layers. 4,7-diphenyl-1,10-phenanthroline (Bphen) as electron

transport layer and hole bloking layer, lithium fluoride (LiF) as

an electron injection layer, Al as a cathode.

We also fabricated PHOLEDs B1-B6 using D1-D6 as hosts

for green phosphorescent dopant Ir(ppy)₃. All properties are

shown in Fig. 9 and summarized in Table 2. Energy diagram of

green PHOLEDs can be found in Fig. S9 (ESI†). They all show

identical EL spectra with λmax = 517nm. The maximum EQEs of

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Journal Name

devices B1-B6 are 18.6%, 17.3%, 16.2%, 16.8%, 15.7% and

16.4%, respectively. Unlike TADF OLEDs, all PHOLEDs show

9

D. Zhang, L. Duan, C. Li, Y. Li, H. Li, D. Zhang and Y. Qiu,

Advanced Materials, 2014, 26, 5050-5055.

similar roll-off. Their EQEs can still remain 17.6%, 16.5%, 10 D. Zhang, L. Duan, Y. Li, H. Li, Z. Bin, D. Zhang, J. Qiao, G.

15.3%, 15.5%, 14.0%, 16.0% and 16.2%, 15.3%, 14.2%, 14.1%,

12.8%, 15.1% at high luminance of 5000 cd m^-2and 10000 cd

Dong, L. Wang and Y. Qiu, Adv Funct Mater, 2014, 24, 3551-

3561.

m^-2, respectively. Similar to TADF OLEDs, performances of B1 11 D. Zhang, L. Duan, Y. Zhang, M. Cai, D. Zhang and Y. Qiu,

and B2 using D1 and D2 as hosts achieved higher efficiencies

Light Sci Appl, 2015, 4, e232.

than that of CBP based PHOLED device R2. The turn on 12 D. Zhang, L. Duan, D. Zhang and Y. Qiu, Journal of Materials

voltages of six devices range from 3.7-4.4 V.

Chemistry C, 2014, 2, 8983-8989.

13 D. Zhang, L. Duan, D. Zhang, J. Qiao, G. Dong, L. Wang and Y.

Qiu, Organic Electronics, 2013, 14, 260-266.

Conlusion

14 D. Zhang, M. Cai, Z. Bin, Y. Zhang, D. Zhang and L. Duan,

Chemical Science, 2016, 7, 3355-3363.

In summary, we designed and synthesized six

carbazole/triphenylene based hosts. They all show good

thermostability and film-forming property. π-π stacking of

triphenylene units renders these bipolar hosts with high

electron mobilities in the range of 1.02×10^-4to 9.66×10^-4cm²

V^-1s^-1at 6×10⁷V m^-1. Among the six hosts, D1 and D2 show

slightly higher electron mobilities than their corresponding

hole mobilities and this make them promising host candidates.

Compare with CBP and other hosts, TADF and phosphorescent

OLEDs using D1 as the hosts achieved higher maximum EQEs of

16.3% and 18.6%. Even at very high luminance of 10000 cd m^-2,

the efficiencies could still maintain 13.8% and 16.2%. The

result show the great potential that π-π stacking can be used

to improve electron mobilities of bipolar hosts for TADF and

phosphorescent devices with low efficiency roll-off. More

efficient TADF and phosphorescent hosts can be readily

designed by using this stratagem.

15 Y. Sun, L. Duan, D. Zhang, J. Qiao, G. Dong, L. Wang and Y.

Qiu, Adv Funct Mater, 2011, 21, 1881-1886.

16 L. Duan, J. Qiao, Y. Sun and Y. Qiu, Advanced Materials, 2011,

23, 1137-1144.

17 D. Zhang, M. Cai, Y. Zhang, D. Zhang and L. Duan, Acs Appl

Mater Interfaces, 2015, 7, 28693-28700.

18 X. Ban, W. Jiang, K. Sun, X. Xie, L. Peng, H. Dong, Y. Sun, B.

Huang, L. Duan and Y. Qiu, Acs Appl Mater Interfaces, 2015,

7

, 7303-7314.

19 C.-C. Lai, M.-J. Huang, H.-H. Chou, C.-Y. Liao, P. Rajamalli and

C.-H. Cheng, Adv Funct Mater, 2015, 25, 5548-5556.

20 J. Zhao, G.-H. Xie, C.-R. Yin, L.-H. Xie, C.-M. Han, R.-F. Chen,

H. Xu, M.-D. Yi, Z.-P. Deng, S.-F. Chen, Y. Zhao, S.-Y. Liu and

W. Huang, Chem Mater, 2011, 23, 5331-5339.

21 M.-S. Lin, S.-J. Yang, H.-W. Chang, Y.-H. Huang, Y.-T. Tsai, C.-

C. Wu, S.-H. Chou, E. Mondal and K.-T. Wong, J Mater Chem,

2012, 22, 16114-16120.

22 W. Li, J. Li, D. Liu, F. Wang and S. Zhang, Journal of Materials

Chemistry C, 2015, 3, 12529-12538.

Acknowledgements

23 F.-P. Wu, Y.-M. Xie, L.-S. Cui, X.-Y. Liu, Q. Li, Z.-Q. Jiang and L.-

S. Liao, Synthetic Met, 2015, 205, 11-17.

We would like to thank the National Key R&D Program of

China (Grant Nos. 2016YFB0400702 and 2016YFB0401003), the

National Key R&D Program of China (Grant No.

2015CB655002) and the CAS "Interdisciplinary Innovation

Team" for ﬁnancial support.

24 J. Xiao, X.-K. Liu, X.-X. Wang, C.-J. Zheng and F. Li, Organic

Electronics, 2014, 15, 2763-2768.

25 X. Yin, T. Zhang, Q. Peng, T. Zhou, W. Zeng, Z. Zhu, G. Xie, F.

Li, D. Ma and C. Yang, Journal of Materials Chemistry C, 2015,

3

, 7589-7596.

26 Y. J. Cho, K. S. Yook and J. Y. Lee, Advanced Materials, 2014,

26, 4050-4055.

Notes and references

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915.

27 C. Adachi, M. A. Baldo, S. R. Forrest and M. E. Thompson,

Appl Phys Lett, 2000, 77, 904-906.

2

3

A. Chihaya, Jpn J Appl Phys, 2014, 53, 060101.

28 L.-S. Cui, Y.-L. Deng, D. P.-K. Tsang, Z.-Q. Jiang, Q. Zhang, L.-S.

Liao and C. Adachi, Advanced Materials, 2016, 28, 7620-

7625.

Y. Ma, H. Zhang, J. Shen and C. Che, Synthetic Met, 1998, 94

,

245-248.

4

5

6

M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M.

E. Thompson and S. R. Forrest, Nature, 1998, 395, 151-154.

H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi,

Nature, 2012, 492, 234-238.

29 J.-H. Jou, S. Kumar, A. Agrawal, T.-H. Li and S. Sahoo, Journal

of Materials Chemistry C, 2015,

3, 2974-3002.

30 T. Kamata, H. Sasabe, Y. Watanabe, D. Yokoyama, H. Katagiri

and J. Kido, Journal of Materials Chemistry C, 2016, 4, 1104-

Y. Tao, K. Yuan, T. Chen, P. Xu, H. Li, R. Chen, C. Zheng, L.

Zhang and W. Huang, Advanced Materials, 2014, 26, 7931-

7958.

1110.

31 Y. Watanabe, H. Sasabe, D. Yokoyama, T. Beppu, H. Katagiri,

Y.-J. Pu and J. Kido, Advanced Optical Materials, 2015, 3,

7

8

D. Zhang, M. Cai, Y. Zhang, D. Zhang and L. Duan, Materials

769-773.

Horizons, 2016, 3, 145-151.

32 Y. Watanabe, H. Sasabe, D. Yokoyama, T. Beppu, H. Katagiri

C. Xiang, X. Fu, W. Wei, R. Liu, Y. Zhang, V. Balema, B. Nelson

and J. Kido, Journal of Materials Chemistry C, 2016, 4, 3699-

and F. So, Adv Funct Mater, 2016, 26, 1463-1469.

3704.

6 | J. Name., 2012, 00, 1-3

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33 E. Tritto, R. Chico, G. Sanz-Enguita, C. L. Folcia, J. Ortega, S.

Coco and P. Espinet, Inorg Chem, 2014, 53, 3449-3455.

34 J. Shi, Y. Wang, M. Xiao, P. Zhong, Y. Liu, H. Tan, M. Zhu and

W. Zhu, Tetrahedron, 2015, 71, 463-469.

35 K. Togashi, S. Nomura, N. Yokoyama, T. Yasuda and C.

Adachi, J Mater Chem, 2012, 22, 20689-20695.

36 Y. Im, W. Song and J. Y. Lee, Journal of Materials Chemistry C,

2015, 3, 8061-8065.

37 N.-J. Lee, J. H. Jeon, I. In, J.-H. Lee and M. C. Suh, Dyes

Pigments, 2014, 101, 221-228.

38 1. S. N. Park, H. W. Lee, Y. S. Kim, J. Kim, S. E. Lee, H. W. Lee,

Y. K. Kim and S. S. Yoon, Synthetic Met, 2015, 206, 124-130.

39 S. A. Bagnich, A. Rudnick, P. Schroegel, P. Strohriegl and A.

Köhler, Philosophical Transactions of the Royal Society A:

Mathematical, Physical and Engineering Sciences, 2015, 373

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40 S. A. Bagnich, S. Athanasopoulos, A. Rudnick, P. Schroegel, I.

Bauer, N. C. Greenham, P. Strohriegl and A. Köhler, The

Journal of Physical Chemistry C, 2015, 119, 2380-2387.

41 K.-R. Wee, Y.-J. Cho, S. Jeong, S. Kwon, J.-D. Lee, I.-H. Suh and

S. O. Kang, J Am Chem Soc, 2012, 134, 17982-17990.

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Scheme 1 Synthesis routes and chemical structures of D1-D6.

N

HO

B

OH

B

OH

N

D4

D1

D2

D3

HO

B

N

B

N

HO

N

Br

i

Br

D5

N

OH

B

HO

N

D6

i Pd(PPh₃)₄, Na₂CO₃, Ethanol, Toluene

N

D1

1.0

0.8

0.6

0.4

0.2

0.0

D2

D3

D4

D5

D6

100

200

300

400

500

600

Temperature (°C)

Fig. 1 TGA spectra of D1-D6.

8 | J. Name., 2012, 00, 1-3

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Fig. 2 HOMOs and LUMOs of D1-D6.

(a)

D1-Abs

D1-PL

1.0

D1-Phos

D2-Abs

D2-PL

D2-Phos

D3-Abs

D3-PL

0.8

0.6

0.4

0.2

0.0

D3-Phos

300

400

500

600

700

Wavelength (nm)

(b)

D4-Abs

D4-PL

D4-Phos

D5-Abs

D5-PL

D5-Phos

D6-Abs

D6-PL

1.0

0.8

0.6

0.4

0.2

0.0

D6-Phos

300

400

500

600

700

Wavelength (nm)

Fig. 3 UV/Vis absorption, PL and phosphorescence spectra of

(a) D1-D3 and (b) D4-D6 in toluene.

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Journal Name

D1

D2

D3

D4

D5

D6

0.5

1.0

1.5

2.0

Voltage (V)

Fig.

4

Cyclic voltammograms of D1-D6 in ultra-dry

dichloromethane.

-2

(a) ¹⁰

D1 Hole

D1 Electron

D2 Hole

D2 Electron

D3 Hole

10^-3

10^-4

10^-5

10^-6

D3 Electron

2.0x10⁷

4.0x10⁷

6.0x10⁷

8.0x10⁷

Field intensity (V/m)

-2

(b)¹⁰

D4 Hole

D4 Electron

D5 Hole

D5 Electron

D6 Hole

10^-3

10^-4

10^-5

10^-6

D6 Electron

2.0x10⁷

4.0x10⁷

6.0x10⁷

8.0x10⁷

Field intensity (V/m)

Fig. 5 Charge mobilities of (a) D1-D3 and (b) D4-D6 under

different electric fields.

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Fig. 6 Single crystal structures and molecular packing data of

(a) D1 and (b) D2.

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NC

CN

LiF/Al

N

NC

CN

N

NC

CN

35 nm

Bphen

NC

CN

HAT-CN

NPB

HOST: 5 wt% Ir(ppy)₃

or

HOST: 10 wt% 4CzIPN

4CzIPN

25 nm

10 nm

N

TCTA

NPB

Ir

N

3

Ir(ppy)₃

40 nm

20 nm

N

HAT-CN

ITO

N

Bphen

TCTA

Fig. 7 Device structures of TADF and phosphorescent devices and

chemical structures of employed materials.

.

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10⁴

(a)

(b)

A1

A2

A3

A4

A5

A6

R1

A1

A2

A3

A4

A5

A6

R1

6000

4000

2000

1.0

0.8

0.6

0.4

0.2

0.0

10³

10²

10¹

10⁰

0

10

400

500

600

700

0

2

4

6

8

Wavelength (nm)

Voltage (V)

20

15

10

5

50

40

30

20

10

0

A1

A3

A5

A2

A4

A6

R1

(c)

(d)

A1

A3

A5

A2

A4

A6

R1

0

2000

4000

6000

8000

10000

0

2000

4000

6000

8000

10000

Luminance (cd m^-2)

Fig. 8 (a) EL spectra (b) J-V-L characteristics (c) EQEs (d) PEs of

TADF OLEDs

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(a)

Journal Name

10⁴

3000

2000

1000

(b)

B1

B2

B3

B4

B5

B6

R2

B1

B2

B3

B4

B5

B6

R2

1.0

10³

10²

10¹

10⁰

0.8

0.6

0.4

0.2

0.0

0

10

400

500

600

700

0

2

4

6

8

Wavelength (nm)

Voltage (V)

25

20

15

10

5

(c)

(d)

B1

B3

B5

B2

B4

B6

R2

B1

B3

B5

B2

B4

B6

R2

40

30

20

10

0

2000

4000

6000

8000

10000

0

2000

4000

6000

8000

10000

Luminance (cd m^-2)

Fig. 9 (a) EL spectra (b) J-V-L characteristics (c) EQEs (d) PEs of

PHOLEDs.

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a

d

e

T_d

T_g

λ_Abs

λ_Em

HOMO ^b

LUMO ^c

T₁

ꢀ_h

ꢀ_e

Compound

(^oC)

399

374

369

409

425

421

(^oC)

102

111

107

124

114

120

(nm)

293, 341

293

(nm)

(eV)

5.67

5.63

5.54

5.65

5.63

(eV)

2.10

2.13

2.10

2.08

2.18

(eV)

2.66

2.60

2.61

2.67

2.66

2.65

(cm²V^-1s^-1)

5.53×10^-5

2.87×10^-4

4.82×10^-6

5.71×10^-4

1.65×10^-5

8.35×10^-6

(cm²V^-1s^-1)

2.17×10^-4

9.66×10^-4

3.45×10^-4

2.52×10^-4

1.74×10^-4

1.02×10^-4

D1

D2

D3

D4

D5

D6

367, 378

371, 386

376, 394

367, 377

365, 381

290

293, 345

290, 345

290, 349

^aMeasured at toluene solution (1×10^-5mol L^-1), ^bCalculated according to the equation HOMO = 4.8 + onset voltage, ^cEstimated

according to the absorption bandgap and the HOMO energy level, ^dEstimated according to the onset of the phosphorescent spectra

in toluene solution (1×10^-5mol L^-1), ^eAt 6×10⁷V m^-1.

J. Name., 2013, 00, 1-3 | 15

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Journal of Materials Chemistry C

Page 16 of 16

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DOI: 10.1039/C7TC00733G

π-π stacking can improve the electron mobilities of bipolar hosts for TADF and phosphorescent

devices with low efficiency roll-off

Article Doi

π-π stacking: a strategy to improve the electron mobilities of bipolar hosts for TADF and phosphorescent devices with low efficiency roll-off

DOI: 10.1039/c7tc00733g

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Article abstract of DOI:10.1039/c7tc00733g

Full text of DOI:10.1039/c7tc00733g

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