9-Silafluorene and 9-germafluorene: Novel platforms for highly efficient red phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c8tc02851f

Two novel heterofluorene motifs consisting of the group 14 elements Si and Ge were developed as robust molecular platforms for the construction of organic light-emitting diode (OLED) materials. These two compounds exhibited similar photophysical properties, thermal stabilities and electrochemical behaviors except for the charge transport abilities. The red OLED hosted by a 9-silafluorene derivative (DPS) with Ir(MDQ)₂(acac) as the emitter exhibited state-of-the-art current efficiency, power efficiency, and external quantum efficiency (EQE) with maxima of 50.7 cd A^-1, 44.7 lm W^-1, and 28.3%, respectively. In addition, the maximum EQE of a 9-germafluorene derivative (DPG)-based red device could reach 20%. These results indicated the great potential of group 14 element derivatives in the construction of highly efficient OLED materials.

Full text of DOI:10.1039/c8tc02851f

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC02851F
9-Silafluorene and 9-Germafluorene: Novel Platforms for Highly
Efficient Red Phosphorescent Organic Light-Emitting Diodes
Xiang-Yang Liu,† Qi-Sheng Tian,† Danli Zhao, Quan Ran, Liang-Sheng Liao* and
Jian Fan*
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute
of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou,
Jiangsu 215123, China
*E-mail: lsliao@suda.edu.cn, jianfan@suda.edu.cn
† The two authors contribute equally to this paper.
Abstract: Two novel heterofluorene motifs consisting of the group 14 elements Si
and Ge were developed as robust molecular platforms for the construction of organic
light-emitting diode (OLED) materials. These two compounds exhibited similar
photophysical properties, thermal stabilities and electrochemical behaviors except the
charge transport abilities. The red OLED hosted by 9-silafluorene derivative (DPS)
with Ir(MDQ)2(acac) as the emitter exhibited the state-of-the-art current efficiency,
power efficiency, and external quantum efficiency (EQE) with maxima of 50.7 cd A-1,
44.7 lm W-1, and 28.3%, respectively. In addition, the maximum EQE of
9-germafluorene derivative (DPG)-based red device can reach 20%. These results
indicated the great potential of group 14 elements derivatives in the construction of
highly efficient OLED materials.
Introduction
Nowadays organic light-emitting diodes (OLEDs) have achieved remarkable progress
for the practical application due to the high efficiency, color purity and improved
device lifetime.1-3 The development of high performance host materials is crucial to
realize efficient phosphor emission in phosphorescent OLEDs (PHOLEDs). To

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develop novel host materials for PHOLEDs, several criteria have to be considered
such as high triplet energy, good charge transporting ability, suitable highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) level
and good thermal and morphological stability.1 To increase the triplet energy of the
host materials, there are several design strategies: i) to introduce high triplet energy
units such as dibenzofuran,4 dibenzothiophene,5 diphenylamine,6 and carbazole7 into
the molecular structure; ii) to shorten the conjugation length of the host material.
Silicon,8 sp3 carbon,9 and phosphine oxide10 are often used to break intramolecular
π-conjugation. Sterically hindered molecular structure and ortho or meta linking
strategy are also used to reduce the conjugation length. According to these empirical
design principles, many highly efficient host materials have been developed with high
triplet energy over the last decade.11
Recently, fluorene derivatives have been widely used for the applications in OLED
materials due to their excellent physical properties, such as high fluorescence
quantum yields and good thermal stability.12 In addition, fluorene moiety
demonstrated diverse functionality due to its rich chemistry.13 However, fluorene
readily suffered from oxidative degradation of C9-position to yield fluorenone and
intermolecular cross-linking byproducts at high operating temperature and electrical
field, leading to a shift of light emission and a decrease in fluorescence quantum
yield.14-18 Obviously, the introduction of substituent group into the C9-position of
fluorene moiety could effectively suppress the oxidation process.19 On the other hand,
to prevent oxidation in C9-position the carbon atom can be replaced with other
hetero-atom, such as group 13 elements B20 and Ga,21 group 14 elements Si,22 Ge,23
and Sn,24 group 15 elements N,25 P,26 and As;27 and group 16 elements O,28 S,29 Se,30
and Te,27 etc.30-33 It should be noted that the lone pair in N atom at the fluorene
C9-position (also known as carbazole) will affect the electronic structure of overall
molecule and make carbazole moiety a fully aromatic species.25b So far these tricyclic
skeletons have attracted much attention as potential building blocks for novel

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optoelectronic materials.
Small molecular organosilicon compounds as good building blocks for OLED
materials have been widely studied.8,34 For instance, Thompson et al. reported a series
of tetraaryl silicon compounds as host materials with ultrahigh triplet energies (~3.5
eV).8a Yang and coworkers applied organosilicon derivatives as hosts in deep blue and
white PHOLEDs.8d Furthermore, it has been reported that the utility of Si atom can
reduce the conformational disorder and lower the HOMO energy level as compared
with C atom.8 On the other hand, the group 14 element Ge has been extensively used
in inorganic semiconductor filed.35 However, the application of small molecular
organogermanium compounds in OLEDs has been rarely reported till now.36 Herein,
two
novel
Si-
and
Ge-containing
fluorene
derivatives,
10,10'-(5,5-diphenyl-5H-dibenzo[b,d]silole-2,8-diyl)bis(10H-phenothiazine) (DPS)
and 10,10'-(5,5-diphenyl-5H-dibenzo[b,d]germole-2,8-diyl)bis(10H-phenothiazine)
(DPG), were designed and synthesized. The further functionalization at 3,6-positions
of 9-silafluorene and 9-germafluorene backbones will not only restrict the conjugation
degree between phenothiazine and heterofluorenes, but also lead to high triplet energy
level of whole molecule.37 The introduction of heteroatom has minor effect on the
photophysical properties of DPS and DPG. In contrast, DPS exhibited excellent
electroluminescence performance as compared with DPG. The red PHOLED hosted
by DPS with bis(2-methyldibenzo-[f,h]-quinoxaline) iridium (III) (acetylacetonate)
(Ir(MDQ)2(acac)) as dopant demonstrated the state-of-the-art device performance
with external quantum efficiency of 28.3%. This remarkable result indicate the great
potentials of silafluorene derivatives for highly efficient OLEDs.
Experimental Section
Materials Synthesis
The
synthesis
of
2,2'-dibromo-1,1'-biphenyl
(1)38
and
2,2'-dibromo-5,5'-diiodo-1,1'-biphenyl (2)39 are prepared by reported methods.

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2,2'-dibromo-1,1'-biphenyl (1)
1
White solid. H NMR (400 MHz, DMSO-d6) δ 7.74 (d, J = 8.0 Hz, 2H), 7.48 (t, J =
7.4 Hz, 2H), 7.37 (t, J = 7.8 Hz, 2H), 7.30 (d, J = 7.5 Hz, 2H) ppm.
2,2'-dibromo-5,5'-diiodo-1,1'-biphenyl (2)
White solid. 1H NMR (400 MHz, DMSO-d6) δ 7.75 – 7.65 (m, 4H), 7.51 (d, J = 8.4
Hz, 2H) ppm.
10,10'-(6,6'-dibromo-[1,1'-biphenyl]-3,3'-diyl)bis(10H-phenothiazine) (3)
K2CO3 (1.93 g, 14 mmol) was added to a solution of 2 (2.0 g, 3.5 mmol),
10H-phenothiazine (1.55 g, 7.8 mmol) and Cu (90 mg, 1.4 mmol) in
1,2-dichlorobenzene (o-DCB, 30 mL) under N2. The reaction mixture was heated at
o
180 C for 36 hours. After cooling down to the room temperature, the mixture was
filtered and washed with dichloromethane, and then the solvent was removed under a
reduced pressure. The crude product was purified by column chromatography with
petroleum ether/dichloromethane (2/1, v/v) as eluent to yield a white solid (1.5 g,
1
60%). H NMR (400 MHz, Chloroform-d) δ 7.89 (d, J = 8.5 Hz, 2H), 7.40 (s, 2H),
7.32 (d, J = 8.5 Hz, 2H), 7.06 (d, J = 7.4 Hz, 4H), 6.95 – 6.80 (m, 8H), 6.38 (d, J =
8.0 Hz, 4H) ppm. MALDI-TOF-MS: m/z: calcd for C36H22Br2N2S2: 706.514, found:
705.938. Anal. Calcd for C36H22Br2N2S2 (%): C 61.20, H 3.14, N 3.97; found: C 61.23,
H 3.22, N 4.02.
10,10'-(5,5-diphenyl-5H-dibenzo[b,d]silole-2,8-diyl)bis(10H-phenothiazine) (DPS)
o
To a solution of 3 (2 g, 2.8 mmol) in THF (150 mL) under argon at -78 C, n-butyl
lithium (3.88 mL, 6.2 mmol, 1.6 M) was added dropwise via a syringe. After stirring
o
at -78 C for 1 h, dichlorodiphenylsilane (0.71 g, 2.8 mmol) was added within 1 min.
Then, the mixture was gradually warmed up to room temperature and allowed to stir
for 16 hours. Then the reaction was eventually quenched by adding water (5 mL). The
resulting mixture was placed into 150 mL of water, and the desired product was
extracted with dichloromethane (3×50 mL). The organic layer was then separated,
dried over sodium sulfate (Na2SO4), filtered and evaporated under reduced pressure to
give a crude product which was further purified by column chromatography using

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petroleum ether/dichloromethane (3/1, v/v) to obtain the final product (1.5 g, 73%).
1H NMR (400 MHz, Chloroform-d) δ 8.02 (d, J = 7.6 Hz, 2H), 7.78 – 7.73 (m, 6H),
7.52 – 7.43 (m, 6H), 7.38 – 7.33 (m, 2H), 7.04 (d, J = 7.2, 1.8 Hz, 4H), 6.93 – 6.76 (m,
8H), 6.40 (d, J = 8.0 Hz, 4H) ppm. 13C NMR (151 MHz, Chloroform-d) δ 151.03,
135.96, 135.53, 135.42, 132.02, 130.50, 128.33, 126.90, 117.08 ppm.
MALDI-TOF-MS: m/z: calcd for C48H32N2S2Si: 729.003, found: 728.893. Anal.
Calcd for C48H32N2S2Si (%): C 79.08, H 4.42, N 3.84; found: C 79.17, H 4.48, N
3.91.
10,10'-(5,5-diphenyl-5H-dibenzo[b,d]germole-2,8-diyl)bis(10H-phenothiazine)
(DPG)
1
DPG was prepared with similar method as for DPS (68%). H NMR (600 MHz,
Chloroform-d) δ 8.01 (d, J = 7.5 Hz, 2H), 7.85 (s, 2H), 7.70 (d, J = 7.3 Hz, 4H), 7.49
– 7.42 (m, 6H), 7.39 (d, J = 7.6 Hz, 2H), 7.01 (d, J = 7.5 Hz, 4H), 6.84 (t, J = 7.8 Hz,
13
4H), 6.79 (t, J = 8.1 Hz, 4H), 6.33 (d, J = 8.2 Hz, 4H) ppm. C NMR (151 MHz,
Chloroform-d) δ 149.52, 143.92, 143.44, 137.87, 135.72, 134.69, 134.38, 130.52,
129.95, 128.72, 126.84, 123.96, 122.67, 120.79, 116.50 ppm. MALDI-TOF-MS: m/z:
calcd for C48H32N2S2Ge: 773.548, found: 774.052. Anal. Calcd for C48H32N2S2Ge (%):
C 74.53, H 4.17, N 3.62; found: C 74.57, H 4.24, N 3.68.
Results and Discussion
Preparation and Characterization
The synthetic routes of two novel materials, DPS and DPG, were outline in Scheme 1.
2,2'-dibromo-1,1'-biphenyl (1) was obtained in good yield with the reported
procedure.38 2,2'-dibromo-5,5'-diiodo-1,1'-biphenyl (2) was prepared at a relatively
lower yield (~40%) by iodination of 1 with the mixture of I2 and NaIO4 in H2SO4.39
Then
the
key
intermediate
of
10,10'-(6,6'-dibromo-[1,1'-biphenyl]-3,3'-diyl)bis(10H-phenothiazine)
(3)
was
synthesized via the Cu-catalyzed Ullmann coupling reaction between 2 and
10H-phenothiazine in moderate yield. At last, the target materials of DPS and DPG

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were readily prepared by cyclization reaction between the double lithiation reagent of
3 and the respective dichloride compounds.27a The structure of the precursors and
target products were fully characterized by nuclear magnetic resonance spectroscopy
(1H and 13C NMR), mass spectrometry (MS) and elemental analysis (EA).
The thermal properties of DPS and DPG were examined by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC). As shown in Fig. 1, both
DPS and DPG exhibited high thermal stability with decomposition temperature (Td, 5%
weight loss) of 430 and 416 oC, respectively. Due to the rigidity and robustness of the
structures of DPS and DPG, they showed high glass transition temperature (Tg) over
o
240 C. DPS and DPG demonstrated similar thermal properties due to their similar
molecular configurations. The excellent thermal properties of these two compounds
will be beneficial for their OLEDs applications. The Si-aryl and Ge-aryl substituents
and phenothiazine groups render the molecular structures rather bulky, providing an
effective steric hindrance. As a result, these compounds could form stable and
homogeneous amorphous films when co-deposited with dopants,12h which were
generally featured with smooth and uniform surfaces.12g
Photophysical properties
The UV-vis absorption, photoluminescence (PL) spectra in toluene solution at room
temperature, and low temperature (77 K) phosphorescence (Phos) spectra in toluene
matrix of DPS and DPG are shown in Fig. 2. The maxima absorption bands of both
DPS and DPG were around 300 nm, which could be assigned to intramolecular π-π*
transitions. Si-bridged DPS (379 nm) showed a small red-shift of the absorption onset
as compared with Ge-bridged DPG (372 nm), which could be attributed to their
different electronic effects caused by the mutual overlapping between d- or p-orbitals
of heteroatoms and the π-orbitals of the peripheral phenyl rings.40,41 Therefore, the
optical energy gap (Eg) of DPS (3.27 eV) is slightly narrower than that of DPG (3.33
eV). DPS and DPG showed similar structureless PL spectra with the maximum
emission peaks at 491 nm and 487 nm, respectively. The Phos spectra of DPS and

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DPG are nearly identical, which means that the introduction of Si and Ge has less
affected their triplet-excited states. In addition, the triplet energies (ETs) of DPS and
DPG (2.48 eV) could be determined by the short-wavelength emission peak in the
Phos spectra. So, we expected that the two compounds could be used as hosts for red
PHOLEDs.
Electrochemical properties and theoretical calculation
The electrochemical properties of DPS and DPG were investigated through cyclic
voltammetry (CV) method. As shown in Fig. S1, both DPS and DPG showed
irreversible oxidation profiles during the anodic scan. The HOMO energy levels were
estimated based on the onsets of oxidation curves, and the detailed results were listed
in Table 1. It is worth noting that both DPS and DPG exhibited the same HOMO
energy levels (-4.99 eV). In addition, the LUMO energy levels calculated from their
HOMO energy levels and the corresponding Egs were -1.72 and -1.66 eV for DPS and
DPG, respectively. These results indicated the heteroatom of Si and Ge in DPS and
DPG showed minor effect on their frontier molecular orbitals (FMOs) energy levels.
Moreover, the electronic properties were investigated by density functional theory
(DFT) calculations at a B3LYP/6-31G(d) level to further understanding of the
structure-property relationship of these two materials. As shown in Fig. 3, the
Si-bridged DPS and Ge-bridged DPG exhibited very similar HOMO/LUMO
distributions. The HOMOs were mainly localized on the electron-rich phenothiazine
units for both DPS and DPG. In addition, the LUMOs of the both new materials were
dominantly distributed on the heterofluorene cores. Therefore, the HOMO and LUMO
energy levels of DPS and DPG are quite similar, which was consistent well with
previously
reported
results.42
Furthermore,
C-bridged
(DPC) was
10,10'-(9,9-diphenyl-9H-fluorene-3,6-diyl)bis(10H-phenothiazine)
selected as the reference material to deepen understanding the effect of heteroatoms
on the FMOs of whole molecule (Table S1 and Fig. 4). It is worth noting that although
the electronegativity of C is larger than those of Si and Ge, the introduction of
heteroatom atoms (Si and Ge) led to the low LUMO energy levels as compared with

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the C linker (Table S1 and Fig. 4), which might be attributed to the σ*−π*
hyperconjugation effect42,43 between the exocyclic σ bonds and the adjacent phenyl π
system.
Electroluminescent properties
To explore the potential of DPS and DPG as host materials in PHOLEDs, the red
devices were fabricated with the configuration of ITO/HAT-CN (10 nm)/TAPC (55
nm)/TCTA (10 nm)/Host: 6 wt.% Ir(MDQ)2(acac) (20 nm)/TmPyPB (35 nm)/Liq (2
nm)/Al (120 nm). The red emitter Ir(MDQ)2(acac) were doped into the hosts of DPS
and
DPG
to
constitute
the
emitting
layer
(EML).
Dipyrazino[2,3-f:2',3'-h]-quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) and
8-hydroxyquinolinolatolithium (Liq) serve as hole and electron injection layer (HIL
and
EIL),
respectively.
In
addition,
and
1,1-bis[4-[N',N'-di(p-tolyl)amino]-phenyl]cyclohexane
(TAPC)
1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (TmPyPB) act as hole and electron transport
layer (HTL and ETL) as well as electron and hole blocking layer (EBL and HBL),
respectively. Furthermore, a thin layer of tris(4-(9H-carbazol-9-yl)phenyl)amine
(TCTA) was used as another hole transport layer due to its relative low HOMO
energy level (-5.90 eV) as compared with that of TAPC (-5.50 eV), which could favor
hole transport and improve the recombination efficiency of hole and electron. The
detailed electroluminescence (EL) data were listed in Table 2.
As depicted in Fig. 5(a), both devices exhibited relatively lower turn-on voltages of
3.6 V and 3.7 V at 200 cd m-2, and the maximum luminance of DPS-based device was
over 12300 cd m-2, which could be comparable with the best reported results.44 The
relatively lower driving voltage and very high brightness of DPS-based device
suggests the efficient charge injection, transport, recombination and the formation of
exciton, as well as the radiation transition process of exciton. As shown in Fig. 5(c),
DPS and DPG-based devices displayed the characteristic emission of Ir(MDQ)2(acac)
with the Commission Internationale de lʹEclairage (CIE) coordinates of (0.61, 0.39)

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and (0.62, 0.38), respectively. The device hosted by DPG exhibited a moderate
performance with the maximum current efficiency (CE)/power efficiency
(PE)/external quantum efficiency (EQE) of 38.0 cd A-1/33.5 lm W-1/20.0%.
Remarkably, the DPS-based device demonstrated excellent performance with
maximum CE, PE and EQE of 50.7 cd A-1, 44.7 lm W-1 and 28.3%, respectively,
without any light extraction techniques.3a,3b Furthermore, the EQE still can reach 21.8%
at the brightness of 5000 cd m-2. The superior device performance might partially be
due to the high triplet energy of DPS, which could favor the energy transfer process
from the host to the Ir-dopant, and control the drift and diffusion of triplet excitons
and therefore reduce triplet quenching effect in EML. The hole- and electron-only
devices based on DPS and DPG were fabricated (Fig. 6). The good charge carrier
transport ability of DPS resulted in its preferable device performance. On the other
hand, the photoluminescence quantum yield (PLQY) of Ir(MDQ)2(acac) (6 wt.%) in
DPS and DPG thin films were measured in an integrating sphere to be 0.78 and 0.68,
respectively, which could lead to their superior device performance.44b,45 Furthermore,
in order to understand the efficiency roll-off mechanisms of the devices, we chose
triplet-triplet annihilation (TTA) mode (Equation S1)46 and singlet-polaron
annihilation (SPA) mode (Equation S2)44a to simulate the EQE-J relationships of the
devices (Fig. S2). As shown in Fig. S2, the EQE–J curve of DPS-based device
exhibited relatively large deviation from TTA mode and fit well with SPA mode,
which suggested that the singlet-induced quenching process mainly accounted for its
efficiency reduction.47 In contrast, the fine consistency between the EQE–J curve and
the fitting result of TTA mode suggests that TTA mechanism plays an important role
in efficiency roll-off of DPG-based device.48
Conclusion
Novel organosilicon and organogermanium compounds, DPS and DPG, were
designed and synthesized. The influence of Si and Ge in the basic physical and
chemical properties of these compounds were fully studied. The results showed that
the heteroatom of Si and Ge had a tiny effect on their thermal and photophysical

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properties and the electrochemical behaviors. However, there is big differences
between the hole/electron transport abilities and the EL performance of DPS and
DPG. Excellent device performance was achieved for DPS-based device with the
maximum CE, PE and EQE of 50.7 cd A-1, 44.7 lm W-1 and 28.3%, respectively. On
the other hand, the DPG-based device also demonstrated nice performance with the
maximum EQE of 20%. These results revealed the great potentials of organosilicon
and organogermanium derivatives in the exploration of highly efficient OLED
materials.
Acknowledgements
We thank to the financial support from the National Key R&D Program of China
(2016YFB0400703), the National Natural Science Foundation of China (21472135)
and Natural Science Foundation of Jiangsu Province of China (BK20151216). This
project is also funded by Collaborative Innovation Center of Suzhou Nano Science
and Technology (CIC-Nano), Jiangsu Key Laboratory for Carbon-Based Functional
Materials & Devices, Soochow University and by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
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Scheme 1 Synthetic routes of DPS and DPG.

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243 oC
243 oC
100
80
DPS
DPG
150
180
210
240
270
DPS 430 oC
DPG 416 oC
Temperature (oC)
60
40
300
400
500
600
700
Temperature (oC)
Fig. 1 TGA and DSC (inset) curves of DPS and DPG.

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1.0
DPS
DPG
1.0
0.8
0.6
0.4
0.2
0.0
0.8
DPS
DPG
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
UV-Vis
PL
300 350 400 450 500 550 600 650 700
Wavelength (nm)
Fig. 2 Room temperature UV-vis absorption, PL spectra, and low temperature Phos
spectra (inset) in toluene.

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DPS
DPG
-1
-2
-3
-4
-5
-1.52 eV
3.39 eV
-4.91 eV
-1.57 eV
3.35 eV
-4.92 eV
Fig. 3 DFT calculation results of molecule optimized structure, HOMO/LUMO
distributions and energy levels, and energy band gaps of DPS and DPG.

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0
-1
-2
-3
-4
-5
-6
-7
-1.39 eV
-1.52 eV
-1.57 eV
-4.91 eV
-4.92 eV
-4.94 eV
Fig. 4 HOMO and LUMO energy levels for the carbon-, silicon-, and
germanium-bridged molecules at the B3LYP 6-31G(d) level.

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Table 1. Summary of the physical properties of DPS and DPG.
a
a
b
c
d
e
Abs λmax
[nm]
285
PL λmax
[nm]
491
Tg /Td
Eg
ET
HOMOf
[eV]
LUMOg
[eV]
Molecule
[oC]
[eV] [eV]
3.27 2.48
3.33 2.48
DPS
243/430
243/416
-4.99
-1.72
DPG
285
487
-4.99
-1.66
b
c
aMeasured in toluene solution at room temperature. Tg: Glass transition temperature. Td:
Decomposition temperature. dEg: Optical band gap energies were calculated from the
corresponding absorption onset in toluene solution. eET: Measured in toluene glass matrix at 77 K.
g
fHOMO levels were calculated from CV data. LUMO levels were calculated from the HOMOs
and Egs.

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10k
1k
DPS
DPG
40
30
20
10
0
100
(a)
10
7.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Voltage (V)
60
45
30
15
(b)
0
45
DPS
DPG
30
15
0
30
15
0
0
2000
4000
6000
8000 10000 12000
-2
Luminance (cd m )
(c)
1.0
DPS
DPG
0.8
0.6
0.4
0.2
0.0
-2
@ 5 mA cm
300
400
500
600
700
800
Wavelength (nm)
Fig. 5 (a) J–V–L characteristics, (b) CE–, PE– and EQE–L curves, and (c) EL spectra
of red devices.

Page 25 of 27
Journal of Materials Chemistry C
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DOI: 10.1039/C8TC02851F
200
150
100
50
DPS
DPG
Hole-only devices
0
0
1
2
3
4
5
6
Voltage (V)
50
DPS
DPG
40
30
20
10
0
Electron-only devices
0
2
4
6
8
10
12
14
16
Voltage (V)
Fig. 6 J–V characteristics of hole- and electron-only devices. Device structures:
hole-only: ITO/MoO3 (10 nm)/DPS or DPG (100 nm)/MoO3 (10 nm)/Al (100 nm);
electron-only: ITO/TmPyPB (20 nm)/DPS or DPG (100 nm)/TmPyPB (20 nm)/Liq (2
nm)/Al (100 nm).

Journal of Materials Chemistry C
Page 26 of 27
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DOI: 10.1039/C8TC02851F
Table 2. Summary of electroluminescence performance for red PHOLEDs.
c
c
c
Vb
ηCE
ηPE
ηext
CIEd
Hosta
[V]
3.6
3.7
[cd A-1]
[lm W-1]
[%]
[x, y]
50.7, 48.6, 40.4
38.0, 35.4, 27.5
44.7, 36.4, 24.5
33.5, 25.7, 15.8
28.3, 26.1, 21.8
20.0, 18.7, 14.6
0.61, 0.39
0.62, 0.38
DPS
DPG
a
Device configuration: ITO/HAT-CN (10 nm)/TAPC (55 nm)/TCTA (10 nm)/Host: 6 wt.%
b
c
Ir(MDQ)2(acac) (20 nm)/TmPyPB (35 nm)/Liq (2 nm)/Al (120 nm). Voltages at 200 cd m-2.
d
Efficiencies in the order of the maxima, at 1000 cd m-2 and at 5000 cd m-2. Commission
Internationale de l'Eclairage measured at 5 mA cm-2.

Page 27 of 27
Journal of Materials Chemistry C
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DOI: 10.1039/C8TC02851F
De novo molecular platforms have been applied to build host materials for high
efficient PHOLEDs. 9-Silafluorene derivative was studied for the first time as hosts
and the external quantum efficiency in red PHOLEDs over 28% was achieved
successfully.