Regulation of peripheral tert-butyl position: Approaching efficient blue OLEDs based on solution-processable hole-transporting materials

Article abstract of DOI:10.1016/j.orgel.2019.05.005

Solution-processable hole transporting materials (HTMs)play a vital role in organic light emitting diodes (OLEDs)with the purpose of reducing costs and improving productivity. Herein, two twisted HTMs N₂,N₂,N₇,N₇-tetrakis(4-(3,6-di-tert-butyl-9H- carbazol-9-yl)phenyl)-9,9-dimethyl-9H-fluoren-2,7-diamine (p-BCz-F)and N₂,N₂,N₇, N₇-tetrakis(4-(2,7-di-tert-butyl-9H-carbazol-9-yl)phenyl)-9,9-dimethyl-9H-fluoren-2,7-diamine (m-BCz-F), have been successfully synthesized. The two compounds are endowed with superior solubility in common organic solvents, facilitating solution-processed OLEDs. By simply changing position of peripheral tert-butyl moiety, the material performance could be regulated. The HTMs p-BCz-F exhibits superior glass transition temperature (T_g)of 168 °C. Particularly, in terms of mobility, 1.42-fold higher could be also found for p-BCz-F (4.35 × 10^?4 cm²V^{– 1}S^?1)than m-BCz-F (3.05 × 10^?4 cm²V^{– 1}S^?1). Similarly, the p-BCz-F-based blue devices with BCzVBi as the emitter show better performance with maximum current efficiency (CE)of 7.05 cd/A and luminance (L)of 23365 cd/m², corresponding CE of 6.12 cd/A and L of 21055 cd/m² for m-BCz-F-based devices. In short, the different position of peripheral tert-butyl on carbazolyl moiety will lead to different performance and it provided a simple strategy to design solution-processable HTMs for efficient OLEDs.

Full text of DOI:10.1016/j.orgel.2019.05.005

Accepted Manuscript
Regulation of peripheral tert-butyl position: Approaching efficient blue OLEDs based
on solution-processable hole-transporting materials
Sen Wu, Hongli Liu, Wei Sun, Xianggao Li, Shirong Wang
PII:
S1566-1199(19)30227-7
DOI:
https://doi.org/10.1016/j.orgel.2019.05.005
ORGELE 5240
Reference:
To appear in: Organic Electronics
Received Date: 26 February 2019
Revised Date: 29 March 2019
Accepted Date: 7 May 2019
Please cite this article as: S. Wu, H. Liu, W. Sun, X. Li, S. Wang, Regulation of peripheral tert-butyl
position: Approaching efficient blue OLEDs based on solution-processable hole-transporting materials,
Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.05.005.
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Regulation of Peripheral tert-Butyl Position: Approaching
Efficient Blue OLEDs based on Solution-Processable
Hole-Transporting Materials
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a,b
a,b,*
a,b
a,b
a,b,*
Sen Wu , Hongli Liu
, Wei Sun , Xianggao Li , Shirong Wang
a.
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Tianjin University, School of Chemical Engineering and Technology, Tianjin 300072, China
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Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
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Abstract
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Solution-processable hole transporting materials (HTMs) play a vital role in organic
light emitting diodes (OLEDs) with the purpose of reducing costs and improving
productivity. Herein, two twisted HTMs N ,N ,N ,N -tetrakis(4-(3,6-di-tert-butyl-9H-
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carbazol-9-yl)phenyl)-9,9-dimethyl-9H-fluoren-2,7-diamine (p-BCz-F) and N ,N ,N ,
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N -tetrakis(4-(2,7-di-tert-butyl-9H-carbazol-9-yl)phenyl)-9,9-dimethyl-9H-fluoren-2,
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7-diamine (m-BCz-F), have been successfully synthesized. The two compounds are
endowed with superior solubility in common organic solvents, facilitating
solution-processed OLEDs. By simply changing position of peripheral tert-butyl
moiety, the material performance could be regulated. The HTMs p-BCz-F exhibits
superior glass transition temperature (T ) of 168 °C. Particularly, in terms of mobility,
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1.42-fold higher could be also found for p-BCz-F (4.35×10 cm V S ) than
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m-BCz-F (3.05×10 cm V S ). Similarly, the p-BCz-F-based blue devices with
BCzVBi as the emitter show better performance with maximum current efficiency
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(CE) of 7.05 cd/A and luminance (L) of 23365 cd/m , corresponding CE of 6.12 cd/A
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and L of 21055 cd/m for m-BCz-F-based devices. In short, the different position of
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peripheral tert-butyl on carbazolyl moiety will lead to different performance and it
provided a simple strategy to design solution-processable HTMs for efficient OLEDs.
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Key words: Current efficiency, Hole transporting-material, OLEDs,
Solution-processable, Tert-butyl carbazole
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Introduction
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Organic light emitting diodes (OLEDs) have been widely used in portable display
devices, and even large area display panels owing to their splendid merits, such as
self-luminous, wide viewing angle, short response time and flexible preparation [1-5].
Vacuum evaporation and solution-process methods are commonly employed in the
preparation of OLEDs. Although vacuum evaporation method (dry process) can form
homogeneously smooth film, there still exists some deficiencies, such as complicated
fabrication process and low materials utilization (~20%) [6]. Compared with dry
process, all-solution deposition or solution-vacuum-mixed deposition technology
renders simple OLEDs preparation process and lower manufacturing cost, which
meets the requirements of commercial production [7].
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Hole transporting layer can smooth energy level, balance the carrier and confine
excitons in emitting layer, playing an important role in improving OLEDs
performance [8-10]. (N,N’-Bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine) NPB,
(N,N’-bis(3-me-thylphenyl)-N,N’-bis(phenyl)benzidine) TPD and (1,1-bis((di-4-tolyl-
amino)-phenyl)cyclohexane) TAPC are the most widely used small molecular HTMs
because of high hole mobility and easy sublimation [11-14]. However, their poor film
formation, low morphological stability, and easy crystallization at low temperature
confines their application in solution-process OLEDs [15, 16]. Thus, much effort
should be devoted to exploring efficient HTMs for solution-processable OLEDs.
Carbazole is widely regarded as an excellent building block for HTMs, host
materials and light emitting materials owing to good hole transporting properties, high
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singlet or triplet energy, good thermal and photochemical stability [17, 18].
Particularly, dendrimer carbazole or tert-butyl-substituted carbazole derivatives
contains several electron-donating nitrogen atom and large conjugated plane, resulting
in good hole transporting properties. What’s more, the tert-butyl on carbazolyl group
could improve the solubility in common solvents, eventually guarantee the solution-
processable OLEDs. Yang et al. [19] reported that the resultant solution-processable
OLEDs exhibited a maximum current efficiency of 2.5 cd/A when employed
1,3,5,7-tetrakis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)- adamantine as HTMs,
which is a dendritic molecule with carbazole as terminal group. Li et al. [20]
synthesized several π-conjugated dendrimers. Using three carbazole based twisted
HTM G1 as HTL, the device shows a maximum luminescence efficiency of 1.70 cd/A.
Promarak et al. [21] prepared an OLEDs device hiring multistage carbazole
dendrimer compound as THL, the resultant OLEDs device demonstrated a current
efficiency of 4.47 cd/A, which gives the credit to the perfect hole transporting ability
of HTMs. A maximum current efficiency of 7.7 cd/A was obtained in green OLED by
Sariciftci [22] in the case of a fluorene-carbazole dendrimer served as HTM. All these
carbazole-based dendritic HTMs are synthesized with multistage carbazole as
branches and applied to green OLEDs. As a consequence, carbazolylarylamine as
branches can be introduced to developing a series strongly twisted HTMs, which may
further inhibit crystallization, improve the film formation and deepen HOMO level.
In this paper, two twisted HTMs p-BCz-F and m-BCz-F (Fig. 1) with carbazolyl-
arylamine as peripheral group and 9,9-dimethylfluorene as central core [23, 24] were
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designed and investigated. Tert-butyl groups are introduced to para- (3,6) and meta-
(2,7) position of carbazole. By this simple way, the energy level, mobility, and
molecular steric configuration are expected to be slightly tuned. Meanwhile, the
thermal properties, hole mobility, film properties, ect. of two materials have been
systematically studied. p-BCz-F exhibited outstanding thermal stability (T 541°C, T
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168 °C) and higher hole mobility (4.35×10 cm V S ). Multilayer OLED devices
were fabricated to investigate their hole-transporting properties. Similarly, the
p-BCz-F-based blue device with BCzVBi as emitter shows the better performance
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with CEmax of 7.05 cd/A and Lmax of 23365 cd/m .
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1. Results and discussion
2.1 Synthesis
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Scheme 1. Molecular structures and synthetic routes of p-BCz-F and m-BCz-F
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The synthetic routes of p-BCz-F and m-BCz-F are shown in Scheme 1. The
intermediates p-1, p-2, p-3 and p-4 as well as the target molecules were synthesized as
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follows. 4-(3,6-di-tert-butyl-9H-carbazol-9-yl)aniline (p-2) was obtained by a simple
two-step reaction. First, p-1 was prepared with 4-fluoro-nitrobenzene and
3,6-di-tert-butyl-9H-carbazole by Nucleophilic Substitution reaction with K CO in
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DMF. Then p-1 undergoes a nitro reduction reaction with SnCl 2H O in refluxed
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anhydrous ethanol to obtain p-2 [25]. 9-(4-bromophenyl)-3,6-di-tert-butyl-
9H-carbazole (p-3) was synthesized using 3,6-di-tert-butyl-9H-carbazole and
1-bromo-4-iodobenzene by
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copper-catalyzed Ullmann reaction [26].
Bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)amine (p-4) was obtained by a
monosubstituted C-N coupling reaction between p-2 and p-3. Finally, p-BCz-F was
acquired through palladium-catalyzed Buchwald-Harting reaction between p-4 and
2,7-dibromo-9,9-dimethyl-fluorene [27]. The synthesis method of p-BCz-F can be
absolutely applied to that of m-BCz-F. All synthesized structures were characterized
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by H NMR, and the target molecules were proved the structure by H NMR, C
NMR and MALDY-TOF-MS (dipitcted in Fig. S4–S16, ESI†).
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1.2 Theoretical calculation and structural analysis
Density functional theory (DFT) calculations were performed at B3LYP/6-31G(d)z
level (see the ESI for full computational details†) to reveal the effect of tert-butyl
substitution positions on electronic properties and molecular space configuration. Fig.
1 depicts the optimized ground state molecular structure. As anticipated, the
peripheral tert-butyl position on carbazolyl group endows an appreciable influence on
the molecular steric configuration. The larger twist angle between carbazole and
benzene ring can be detected on meta-substituted m-BCz-F (59.9°) instead of
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para-substituted p-BCz-F (56.4°). Meta-position tert-butyl could increase
intermolecular steric hindrance, consequently give rise to more distorted molecular
configuration and low conjugation degree (m-BCz-F). The more extended
π-conjugated plane of p-BCz-F is in agreement with the hole transport hopping
mechanism and assists in the hole transport.
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Fig. 1 Optimized geometries and HOMO and LUMO distribution for p-BCz-F and m-BCz-F.
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The electronic structure of the two molecules were calculated, as displayed in Fig.
1, HOMO and LUMO distributions of the two molecules are alike, mainly because of
their similar framework. HOMO localizes at the whole molecule, while LUMO
mainly distributes on central fluorene core. The abundant overlap between HOMO
and LUMO facilitate the formation of excitons and hole migration. The HOMO and
LUMO energy levels of p-BCz-F and m-BCz-F are calculated to be -4.77/-1.21 eV
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and -4.85/-1.27 eV, respectively, corresponding the band gap (E ) are 3.56 eV for
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p-BCz-F and 3.58 eV for m-BCz-F. The slightly higher E (m-BCz-F) might be
g
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para-position tert-butyl, which is consistent with the optimized ground geometry [28].
Reorganization energy (λ) calculated at B3LYP/6-31G(d)z level was employed to
evaluate the hole transporting properties of p-BCz-F, m-BCz-F and NPB (see the ESI
for full computational details†). Reorganization energies of p-BCz-F (λ ), m-BCz-F
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(λ ), and NPB (λ ) were calculated to be 0.185, 0.211 and 0.261 eV, respectively. The
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result, λ < λ < λ , suggests that p-BCz-F possesses the best hole-transporting
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property. Compared to m-BCz-F, the smaller λ of p-BCz-F might result from more
extend π-conjugation area.
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2.3 Photophysical properties
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Photophysical properties of the compounds were studied by ultraviolet-visible
(UV-Vis) absorption and photoluminescence (PL) spectra. The strong absorption
around 236 nm should be originated from π-π* transitions of benzene ring [29];
weaker absorption around 300 nm should be attributed to the electron transition of
carbazole [26, 30]. The weak absorption peaks in the range of 338~381 nm could
assign to n-π* transition of fluorene core [31]. Because of the better planarity and
larger electron delocalization, the E of p-BCz-F (3.05 eV) is smaller than m-BCz-F
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(3.06 eV). PL spectra of p-BCz-F and m-BCz-F in THF and solid films are measured
and depictedin Fig. 2. In solution, the two compounds p-BCz-F and m-BCz-F both
display purple-blue emission with maximum emission peak (λmax) at 407 and 409 nm,
respectively. Comparably, in solid film, their PL spectra exhibit an obvious red shift,
which can be interpreted by the weak intermolecular force in solid state.
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Fig. 2 The ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectra of (a) p-BCz-F and
(b) m-BCz-F. All the data were measured by dissolving the HTMs in dilute THF and in neat film.
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2.4 Electrochemical properties
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Electrochemical properties of the compounds were studied by cyclic voltammetry
(CV) and differential pulse voltammetry (DPV). As depicted in Fig. 3, the CV profile
of the two HTMs are semblable because of their similar molecular framework.
According to the HOMO distribution, two oxidation couples were observed in the
anodic scan, which is stemming from carbazolyldiphenylamine and the first peak may
originate from diphenylamine and the second one from carbazolyl. To further study
the oxidation process, DPV measurement was carried out. As shown in Fig. S2,
p-BCz-F and m-BCz-F both showed two oxidation peaks in DPV curves, which is in
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accordance with the CV curves. The initial oxidation potentials (Eonset) of the two
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compounds were 0.76 and 0.80 eV, respectively. The higher Eonset of m-BCz-F is
mainly ascribed to the small π-conjugation degree and less electron-donating ability
of meta-position tert-butyl. Based on Eonset, HOMO energy levels of p-BCz-F and
m-BCz-F were calculated to be -5.50 and -5.54 eV, respectively. Then, in combination
with E and HOMO energy level, the LUMO energy levels were calculated as -2.45
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and 2.48 eV (listed in Table 1).
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Fig. 3 Cyclic voltammetry curves of (a) p-BCz-F and (b) m-BCz-F in THF.
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2.5 Thermal properties and film morphology
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The thermal properties of p-BCz-F and m-BCz-F were carried out with thermo-
gravimetric (TGA) and differential scanning calorimetry (DSC) analysis (Fig. 4). The
decomposition temperature (T ) of p-BCz-F and m-BCz-F are 541 and 547 °C (5%
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weight loss in nitrogen atmosphere). The excellent thermostability is caused by rigid
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carbazole and fluorene unit and is conducive to inhibit the decomposition of HTMs
during device operation as well as maintain the material purity. What’s more, highly
twisted and rigid structure of m-BCz-F should be responsible for the higher T . The
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glass transition temperature (T ) of p-BCz-F is 168 °C. However, the glass transition
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endothermic peak was scarcely observed in DSC curve of m-BCz-F. This
phenomenon may be ascribed to the superior crystallinity degree of m-BCz-F powder.
As known, only materials that exist as amorphous state or the mixture of amorphous
and crystalline state can perform glass transition process. As depicted in Figure 5(a),
powder XRD of p-BCz-F show indistinct peaks, while m-BCz-F show sharp
crystalline peaks. It is demonstrated that most m-BCz-F powder is tend to exist as
crystalline state. Thus, no obvious glass transition endothermic peak was detected.
Besides, Better degree of crystallinity may attribute to its more rigidity structure.
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Fig. 4 TGA and DSC (inset) curves of (a) p-BCz-F and (b) m-BCz-F.
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Fig. 5 (a) XRD spectra of p-BCz-F and m-BCz-F powder. (b) XRD spectra of ITO substrate,
p-BCz-F and m-BCz-F film before annealing, annealing at 80 ℃ , 120 ℃ and 160 ℃ for 12 h. (c)
Surface morphologies of films and molecular structures of HTMs. All the films were spin-coated
from chlorobenzene solutions onto the PEDOT:PSS layer.
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Film surface morphology and stability were characterized by XRD and AFM,
respectively. XRD spectra (Fig. 5 (b)) displayed that the diffraction peaks of the two
HTMs film are completely identical to ITO before and after annealing at 80 °C,
120 °C and 160 °C for 12 h, indicating the formation of thermally stable amorphous
film. Smooth film of p-BCz-F and m-BCz-F with surface roughness (RMS) of 0.84
and 0.95 nm was obtained, respectively (Fig. 5). Obviously, the quality of
p-BCz-F-based film is better compared to m-BCz-F-based film. Such results should
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be ascribed to the better solubility (~20 mg/mL for m-BCz-F, ~100 mg/mL for
p-BCz-F in chlorobenzene). And the different tert-butyl substitution should undertake
the difference. From the optimized structure, para-position tert-butyl is more
peripheral, which tends to increase the solubility, while the meta-position tert-butyl
points to the core of the molecule, leading to inferior solubility for m-BCz-F. Such
high film stability contributes to retaining amorphous film and prohibiting joule heat
during device operation process [32].
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2.6 Hole mobility
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Fig. 6 Mobility of p-BCz-F- and m-BCz-F-based hole-only devices.
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Single carrier devices were fabricated to estimate the hole mobility of the materials
with space charge limited current (SCLC) (Fig. 6). Device structure is designed as
ITO/PEDOT:PSS (40 nm)/p-BCz-F or m-BCz-F (60 nm)/MoO (10 nm)/Al (100 nm).
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No lighting phenomena was detected during measurement, implying these are singlet
carrier device. Hole mobility is calculated by the following formula [33].
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ꢃꢄ
ꢀꢁ
=
ꢂꢂꢁ ꢀꢁꢆꢇꢈ ꢉ( ꢊ√ꢃ)
ꢅ
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Where E is electric field, ɛ and ɛ are relative dielectric constant and permittivity of
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the free space, respectively, L is the thickness of organic layer, µ is zero-field
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mobility, and β is Frenkel factor.
From Fig.6, the hole mobility of p-BCz-F (4.35×10 cm V S ) is 1.42-fold higher
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than that of m-BCz-F (3.05×10 cm V S ). More extend π-conjugation system and
better film quality can account for the superior hole mobility for p-BCz-F. Such result
indicates the p-BCz-F and m-BCz-F could be excellent HTMs and bring out good
device performance, as well as significant influence of the tert-butyl substituted
positions in molecular structure on the hole mobility
Table 1. Photophysical, electrochemical, and thermal characteristics of the two HTMs.
λabs
λem
HOMO LUMO
Eg
Td/Tg
Hole mobility
HTM
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(nm)
(eV)
(eV)
(eV)
(°C)
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m-BCz-F
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-2.48
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2.7 EL performance
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Fig.7 (a) Schematic energy level diagram of the fabricated OLED devices. (b) The device
structure of OLEDs.
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In view of the energy level, the conventional emitter BCzVBi doped blue
fluorescence devices are fabricated: ITO/PEDOT:PSS (40 nm)/p-BCz-F or m-BCz-F
(25 nm)/TCTA (5 nm)/CBP:BCzVBi (mass 5%) (30 nm)/TPBi (20 nm)/LiF(1 nm)/Al
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(100 nm) [34]. PEDOT:PSS serves as hole injecting layer; TPBi as electron
transporting layer (ETL) and LiF/Al as anode. Because the lower LUMO level of the
both HTMs, the exciton energy is prone to transfer from emitting layer to hole
transporting layer. So TCTA is employed as electron blocking layer (EBL) to inhibit
this phenomenon. The OLEDs was fabricated by solution process and HTL p-BCz-F
and m-BCz-F were spin-coated from chlorobenzene solutions. Fig. 7 plots the device
structure and energy levels alignment of materials used in OLEDs. The relevant
parameters of fluorescent OLEDs are summarized in table. 2.
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Fig.8 (a) Luminance-voltage-current density (J-V-L) curves, (b) Current efficiency-current density
(CE-J) curves, (c) Power efficiency-current density (PE-J) curves, and (d) EL spectra of the
fluorescence OLED devices
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Fig. 8(d) displays the electroluminescent (EL) performance of the two devices
under 10 V voltage. All two devices show blue emission with peaks at 445 nm and
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475 nm, belonging to the EL emission peak of BCzVBi [35, 36]. Interestingly, there
are two shoulder peaks at 512 and 560 nm, which should be attributed to the emission
of exciplex. Since the hole transporting property of the host CBP, so the the
recombination zones in devices I and II may shift toward the interface of HTL/EML
or CBP/BCzVBi [37, 38], resulting in the formation of some excimers and/or
exciplexes. The peak at 512 nm may come from the heterojunction CBP/BCzVBi,
which corresponds to the literature data [39]. Besides, the peak at 560nm may
originate from the exciplex EML/ETL. Commission Internationale de’Eclairage (CIE)
coordinates of the two devices are (0.15, 0.19). Especially, at the operated voltage
8-12 V, all devices show excellent spectra stability with stable CIE of (0.15, 0.19) and
the EL spectra at 8-12 V were shown in Fig. S3.
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Promoted by high mobility and good film formation nature, p-BCz-F-based devices
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I performs better performance with CE of 7.05 cd/A and L of 23365 cd/m instead of
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CE of 6.07 cd/A and L of 21055 cd/m for m-BCz-F-based devices II. Significantly,
2
efficiency-off of device I is also relatively small with CE of 6.52 cd/A at 1000 cd/m ,
dropped by 7.4 %. For p-BCz-F, the low efficiency roll-off can be result from its high
hole mobility and film stability. Furthermore, its smooth amorphous film can decrease
the current leakage caused by charge traps, as a result decrease the efficiency roll-off.
These outstanding performance of devices I and II should mainly give the credit to
the low-lying HOMO energy levels and moderate hole mobility of p-BCz-F and
m-BCz-F, which facilitates the effective confinement of electron and hole in emitting
layer especially at high luminance.
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Table 2. EL performance of the fabricated OLED devices
a
b
c
d
e
Von
V)
CE
PE
Lmax
CIE
Compound
Device
2
(
(cd/A)
7.05
(lm/W)
4.07
(cd/cm )
23365
(x, y)
p-BCz-F
m-BCz-F
I
4.92
5.07
(0.15, 0.19)
(0.15, 0.19)
II
6.12
2.96
21055
a
b
c
d
e
-2
2
3
4
5
6
Turn-on voltage (V), recorded at luminance of 1 cd m .
Maximum current efficiency.
Maximum power efficiency.
Maximum luminance.
Commission Internationale de’Eclairage measured at 5 V.
7
2. Conclusion
8
9
0
1
2
3
4
5
6
7
8
9
0
In this study, two novel solution-processable HTMs, i.e. p-BCz-F and m-BCz-F, were
designed and synthesized. The 9,9-dimethylfluorene serve as the central core and
dendrimer carbazoyl-arylamine apply as hole-transporting unit. By changing the
positions of peripheral tert-butyl on carbazoyl group, the energy level, molecular
steric configuration and hole-transporting ability can be tuned simultaneously. The
tert-butyl para-substituted p-BCz-F exhibits better thermal stability (T : 541 °C, T :
1
1
1
1
1
1
1
1
1
1
2
d
g
-4
2
-1 -1
168 °C), hole mobility (4.35×10 cm V s ) as well as suitable HOMO level (-5.50
eV). Besides, thermally stable amorphous film with small RMS (0.84 nm) were
obtained by solution-process. As in return, p-BCz-F-based OLED device exhibits
more brilliant performance with maximum current efficiency of 7.05 cd/A and
2
maximum luminance of 23365 cd/cm . In summary, the different position of
peripheral tert-butyl will lead to different performance and it provided a simple
strategy to design solution-processable HTMs for efficient OLEDs.
2
2
1
2
Acknowledgements
This work is supported by the National Key Research and Development Program of
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China (2016YFB0401303), the National Science Foundation for Young Scientists of
China (No. 61804106) and the Key Projects in Natural Science Foundation of Tianjin
(16JCZDJC37100). The calculation in this work was supported by high performance
computing center of Tianjin University, China.
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. Two novel solution-processable HTMs with superior solubility, mobility and
thermal stability were synthesized.
.
By only changing the positions of peripheral tert-butyl on carbazoyl group,
material properties can be tuned simultaneously.
. Applying the HTMs into BCzVBi doped blue fluorescence devices through
solution-process method, high luminescence and current efficiency were
obtained.