A thermally cross-linkable hole-transporting small-molecule for efficient solution-processed organic light emitting diodes

Article abstract of DOI:10.1016/j.orgel.2019.06.011

Fabrication of multilayered organic light-emitting diodes (OLEDs) through solution process involves several challenges, especially in preventing dissolution of prior layers during subsequent coating. To overcome, extensive efforts had been made in developing cross-linkable materials. In this work, a thermally cross-linkable hole-transporting material, (9,9′-bis(4-vinylbenzyl)-9H,9′H-3,3′-bicarbazole) (VyPyMCz), is synthesized, characterized and successfully applied to multilayered OLEDs via solution-process. After cross-linking, the hole-transporting material forms robust, smooth and solvent-resistant network, enabling a subsequent spin-coating without deteriorating its film integrity. The measured energy level suggests that VyPyMCz facilitates the injection of hole and effectively blocks electron to realize high efficiency, especially at high luminance. At 1000 cd m^?2 for example, the power efficiency of a studied red device is increased from 7.5 to 11.9 lm W^?1, an increment of 58%, and the maximum brightness improved from 7724 to 13,560 cd m^?2, an increment of 75%, as this electron confining, hole transporting material is incorporated. Remarkably, VyPyMCz also works for a high band gap (2.90 eV) with a high triplet energy (2.80 eV) blue emitter containing OLED device, the power efficiency is increased from 6.6 to 11.8 lm W^?1, an increment of 78%, and the maximum luminance enhanced from 5260 to 6857 cd m^?2, an increment of 30%, because of its higher triplet energy (2.87 eV).

Full text of DOI:10.1016/j.orgel.2019.06.011

Accepted Manuscript
A thermally cross-linkable hole-transporting small-molecule for efficient solution-
processed organic light emitting diodes
Deepak Kumar Dubey, Sujith Sudheendran Swayamprabha, Rohit Ashok Kumar
Yadav, Daiva Tavgeniene, Dmytro Volyniuk, Saulius Grigalevicius, Jwo-Huei Jou
PII:
S1566-1199(19)30306-4
DOI:
https://doi.org/10.1016/j.orgel.2019.06.011
ORGELE 5299
Reference:
To appear in: Organic Electronics
Received Date: 9 April 2019
Revised Date: 29 May 2019
Accepted Date: 6 June 2019
Please cite this article as: D.K. Dubey, S.S. Swayamprabha, R.A. Kumar Yadav, D. Tavgeniene, D.
Volyniuk, S. Grigalevicius, J.-H. Jou, A thermally cross-linkable hole-transporting small-molecule for
efficient solution-processed organic light emitting diodes, Organic Electronics (2019), doi: https://
doi.org/10.1016/j.orgel.2019.06.011.
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ACCEPTED MANUSCRIPT
Graphical Abstract

ACCEPTED MANUSCRIPT
A Thermally Cross-Linkable Hole-Transporting Small-Molecule
for Efficient Solution-Processed Organic Light Emitting Diodes
a
a
a
Deepak Kumar Dubey, Sujith Sudheendran Swayamprabha, Rohit Ashok Kumar Yadav,
b
b
b,*
a,*
Daiva Tavgeniene, Dmytro Volyniuk, Saulius Grigalevicius, Jwo-Huei Jou
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu
0013, Taiwan
a
3
b
Department of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu plentas 19, LT50254, Kaunas, Lithuania
E-mail address of corresponding authors:
[
[
1] Prof. Jwo-Huei Jou: jjou@mx.nthu.edu.tw
2] Prof. Saulius Grigalevicius: saulius.grigalevicius@ktu.lt

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Abstract
Fabrication of multilayered organic light-emitting diodes (OLEDs) through solution process
involves several challenges, especially in preventing dissolution of prior layers during
subsequent coating. To overcome, extensive efforts had been made in developing cross-
linkable materials. In this work, a thermally cross-linkable hole-transporting material, (9,9'-
bis(4-vinylbenzyl)-9H,9'H-3,3'-bicarbazole) (VyPyMCz), is synthesized, characterized and
successfully applied to multilayered OLEDs via solution-process. After cross-linking, the
hole-transporting material forms robust, smooth and solvent-resistant network, enabling a
subsequent spin-coating without deteriorating its film integrity. The measured energy level
suggests that VyPyMCz facilitates the injection of hole and effectively blocks electron to
-2
realize high efficiency, especially at high luminance. At 1,000 cd m for example, the power
-1
efficiency of a studied red device is increased from 7.5 to 11.9 lm W , an increment of 58%,
-2
and the maximum brightness improved from 7,724 to 13,560 cd m , an increment of 75%, as
this electron confining, hole transporting material is incorporated. Remarkably, VyPyMCz
also works for a high band gap (2.90 eV) with a high triplet energy (2.80 eV) blue emitter
-1
containing OLED device, the power efficiency is increased from 6.6 to 11.8 lm W , an
-2
increment of 78%, and the maximum luminance enhanced from 5,260 to 6,857 cd m , an
increment of 30%, because of its higher triplet energy (2.87 eV).
Keywords: Thermal crosslinking, hole transporting materials, solution-processed OLEDs.

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1
. Introduction
Organic light-emitting diodes (OLEDs) have drawn enormous attention in academia and
industries because of their amazing applications in both next generation full-color flat-panel
display and solid-state lighting, owing to their potential for fabrication over a large area, light
weight, rapid response, low power consumption, and wide viewing angle [1-10]. There are
two general approaches to fabricate OLEDs, one is thermal evaporation process under high
vacuum whereas another one is based on solution process feasible deposition [5,10].
Typically, the former methodology straightforwardly enables the successive deposition of
multiple layers such as charge transport, multiple emissive layer or carrier blocking layers,
which are highly crucial to improve the device performance and lifetime. Nevertheless, this
process suffering with several serious downsides, amongst, low material utilization rates,
poor scalability, high capital cost, and difficulty in patterning are the main [5,10]. The latter
one seems to be more superior and attracted by the academia and industry because of its
descriptive features such flexible, roll-to-roll manufacturability on larger area and cost
effective with high efficiency if one can prevent the blending or dissolution of formerly
deposited layers throughout the consecutive deposition of multilayer stacks [5,10-14].
Previously, in order to avoid the aforementioned issues, three major competitive approaches
have been adopted by the scientists, i.e. (1) the usage of orthogonal solvents for successive
layer deposition [15] (2) change of polarity of a precursor in a certain layer, and (3) usage of
cross-linkable polymers [16-17]. In 2012, by using the first approach and a caesium
carbonate (CsCO ) doped 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene (TmPyPB) electron
3
transporting layer (ETL), Jenekhe and co-workers developed a fully solution-processed blue
-1
-
phosphorescent OLED with a maximum current efficiency (CE) of 37.7 cd A at 1,300 cd m
2
-1
-2
[
18]. In same year, they further reported a CE of 53.8 cd A at 5, 900 cd m for a green
device with a 4,7-diphenyl-1,10-phenanthroline (BPhen) ETL [19]. In 2014, Bradley,
Anthopoulos, Perumal and co-workers successfully develped a solution processed copper
thiocyanate as hole injection layer (HIL) or hole transport layer (HTL) function layer for
-1
-1
-2
OLED and obtained a maximum efficacy (PE) of 22 lmW (CE 47 cd A ) at 1,000 cd m for
a green device [20]. Nevertheless, only a limited range of materials can readily be made
compatible with this solubility criterion, critically diminishing the applicability of this
method. The second approach has also proven to be of rather low importance, due to the
difficulty of finding classes of materials, the polarity of which can be altered after deposition.

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The third alternative- approach is the usage of cross-linkable materials [21-22]. Cross-linking
has been used to improve the morphological stability and solubility of formerly spin-coated
layer, hence allowing the successful deposition of the next layer [21-22]. As a result, much
work is currently being dedicated to the development of cross linkable materials that display
the physical properties requisite of a specific OLED layer. On the basis of reactive monomers
present in the molecules and initiation method of cross-linking reaction, crosslinking
mechanism can be classified into two categories, photo cross-linking and thermal cross-
linking. Photo cross-linking assists the fine and thermal stress free patterning of layer at room
temperature in a very short time period with the help of photoinitiators [23-26]. Among
various kinds of photo cross-linking units, oxetanes [27-28], cinnamates [29], chalcones [29],
thiol-ene-click-reactions [30], azides [31], and perfluoro azides [32] have been incorporated
in the OLED devices as hole injecting (HIL) or transporting (HTL) materials. Regrettably,
this process passes one very serious drawback, i.e. formation of undesired contaminations
from the photoinitiators which may affect the operation of the devices [24-26]. Thermal
cross-linking units are usually used for cross-linkable HTL materials because of the relatively
simple deposition procedure. Remarkably, they can be proficiently applied for any type of
material via annealing at high temperatures (>100 °C), with the necessities of cross-linkable
moieties in the material [13,33-35]. Styrenes [36], acrylates [37], azides [38],
trifluorovinylethers [39], phenylethynyl-groups [40], silanes [41], benzocyclobutenes [42], or
alkyne-azide-cycloaddition [43] are the examples of such thermal cross-linking units applied
to HTL materials.
In this study, we have designed and synthesized a thermally cross-linkable HTL material,
VyPyMCz for solution-process based multilayer monochromatic and white OLEDs. The
compound VyPyMCz exhibits high triplet energy and cross-linking function due to the
presence of a carbazolyl moiety and two vinylphenyl groups, respectively. The synthesized
material was examined by various characterization techniques, including thermogravimetry
(TGA), UV-Vis spectroscopy, photoluminescence spectroscopy (PL), electron photoemission
spectroscopy, and time of flight (ToF) method. The morphological behavior of the VyPyMCz
thin films at different annealing temperatures was analyzed by atomic force microscopy
(AFM). OLEDs with the cross-linking layer show low operating voltages and improved
power efficiencies as compared with the corresponding reference devices (without HTL). For
example, the studied red, blue, and green devices showed an increment of 58%, 78% and
-2
4
0% in power efficacy at 1,000 cd m and an increment of 75%, 30% and 20% in maximum
luminance, respectively. The reason why all the fabricated OLED devices show improved

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efficiency at high brightness or voltage may be because of its relatively high lowest
unoccupied molecular orbital (LUMO) energy-level (-2.0 eV). It’s also notable that we did
not observe any detrimental influence of the cross-linking process on the performance of
OLED devices.
2
2
. Result and discussion
.1 Materials
9
H-carbazole (1), iron(III) chloride (FeCl ), chloroform, 4-vinylbenzyl chloride, potassium
3
hydroxide (KOH), tetrabutylmmonium hydrogensulfate (TBAHS), tetrahydrofuran (THF)
and chloroform were purchased from Aldrich and used as received.
9
H,9′H-[3,3′]Bicarbazole (2) was obtained by chemical oxidation of 9H-carbazole (1) in the
presence of FeCl as we described earlier [44].
3
(9,9'-bis(4-vinylbenzyl)-9H,9'H-3,3'-bicarbazole) (3/VyPyMCz) was prepared by the
reaction of 9H,9′H-[3,3′]bicarbazole (2) with an excess of 4-vinylbenzylchloride under basic
conditions in the presence of TBAHS phase transfer catalyst. 2.0 g (6.02 mmol) of compound
2
and 2.02 g (13.24 mmol) of 4-vinylbenzylchloride were heated to reflux in 50 ml of THF at
5 °C. Then 1.01 g (18 mmol) of powdered KOH and a catalytic amount of TBAHS were
6
added to the mixture, and it was refluxed for 24 hours. After thin layer chromatography
TLC) control the reaction mixture was filtered, the solvent was evaporated and the product
(
was purified by column chromatography with silica gel using hexane/ethyl acetate (vol. ratio
+
5
5
:1) as an eluent. Yield: 1.9 g (56 %) of yellow amorphous material. MS (APCI , 20 V):
1
65.3 ([M +H], 100 %). H NMR spectrum (300 MHz, CDCl , δ, ppm): 8.47 (d, 2H, J = 1.8
3
Hz, Ar), 8.30–8.22 (m, 2H, Ar), 7.81 (dd, 2H, J = 1.8 Hz, J = 8.4 Hz, Ar), 7.53–7.30 (m,
1
2
1
2H, Ar), 7.23–7.15 (m, 4H, Ar), 6.71 (dd, 2H, J = 11.1 Hz, J = 17.7 Hz, 2 × Ph-
1 2
CH=CH ), 5.74 (dd, 2H, J = 0.9 Hz, J = 17.7 Hz, Ph-CH=CH ), 5.59 (s, 4H, 2 × Ph-CH -
2
1
2
2
2
1
3
Ar), 5.24 (dd, 2H, J = 0.6 Hz, J = 10.8 Hz, Ph-CH=CH ). C NMR spectrum (CDCl , δ,
1
2
2
3
ppm): 141.12, 139.78, 139.94, 136.76, 136.32, 133.80, 126.69, 126,67, 125.99, 125.83,
23.66, 123.30, 120.54, 119.32, 119.06, 113.98, 109.15, 109.04, 46.51.
1
2
.2 Synthesis
The synthesis of the cross-linkable derivative 9,9′-bis(4-vinylphenylmethylen)[3,3′]bicar-
bazole (3) was carried out by the synthetic route shown in Scheme 1. Firstly the key starting
material 9H,9′H-[3,3′]bicarbazole (2) was obtained by chemical oxidation of commercially

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1
available 9H-carbazole (1) in the presence of FeCl using procedure described in literature.
3
The bicarbazole (4) was then treated with an excess of 9-(6-bromohexyl)carbazole to afford
the cross-linkable material (3), VyPyMCz. The synthesized derivatives were identified by
1
13
mass spectrometry, H NMR and C NMR spectroscopies and resultant data were found to
be in good agreement with the proposed structures. VyPyMCz inherited light yellow color
and are rationally soluble in common organic solvents such as dichloromethane (DCM),
toluene (TOL), chloroform (CHCl ), tetrahydrofuran (THF) and N,N-dimethylformamide
3
(DMF) but insoluble in alcohols. Transparent thin films of the derivative could be prepared
by spin coating from solutions.
Scheme 1. Schematic illustration of the synthesis of carbazole based cross-linkable hole
transporting material VyPyMCz.
2
.3 Photophysical and electrochemical characteristics
The photophysical property of the dye was investigated by measuring absorption and
emission spectra in THF at room temperature. The ultraviolet–visible (UV–Vis) and
photoluminescence (PL) spectra of VyPyMCz are displayed in Figure 1a and the pertinent
data are collected in Table 1. The molecule VyPyMCz show an energy band-gap of 3.38 eV,
which is calculated from its intersection point of the absorption and emission spectra.
The electrochemical property of the dye was examined by using cyclic voltammetry (CV).
The redox potential was calibrated by using the ferrocene/ferrocenium redox couple as an
internal standard in the measurement as shown in Figure 1b and selected data are compiled
in Table 1. The highest occupied molecular orbital (HOMO) energy level of this dye was
estimated by using the ferrocene/ferrocenium redox couple as a reference (-4.8 eV) and its
value was -5.25 eV. Its LUMO energy level was calculated from the energy difference

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between HOMO energy level and bandgap of the molecule. The resulted HOMO and LUMO
value of the molecules are -5.25 eV and -1.87 eV, respectively. The PL spectra of the HTM
polymer thin films before and after cross-linking are identical as shown in Figure S1. It
indicates that the cross-linking reactions of the vinyl moieties in the polymers produce
networked structures without any decomposition of the polymer backbones. Furthermore, the
robustness of the reported cross-linkable HTM also confirmed by solvent resistance
experiment as discussed in our earlier report [45]
2
.4 Theoretical calculations
The geometric and electronic properties of the synthesized polymer VyPyMCz at the
molecular level are studied by density functional theory (DFT) and time-dependent DFT
(TD-DFT) calculation by using B3LYP/6-31G (d, p) basis set [46] and obtained key
parameters are summarized in Table 1. Figure 2 and 3 display the optimized geometries and
spatial distributions of HOMO and LUMO energy levels of the synthesized polymer.
Compound VyPyMCz exhibited an energy band gap of 4.0 eV, a HOMO level of -5.25 eV,
and a LUMO level of -1.25 eV. It is notable that theoretical and experimental data are in
good agreement with each other.
2
.5 Thermal Properties
The thermal properties of the cross-linkable material, VyPyMCz, were investigated by using
differential scanning calorimetry (DSC) (Figure 1c) and thermogravimetric analysis (TGA)
(Figure 1d). The pertinent data are presented in Table 1. The compound VyPyMCz was
obtained as an amorphous material after synthesis and purification. During the first DSC
o
heating it became liquid at the glass transition temperature (T ) of 60 C, and then the liquid
g
o
sample polymerization started at about 230 C and formed cross-linked and insoluble as well
as not meltable polymeric network. This is well confirmed during the second DSC heating
scan. It is evident that the formed polymer is well thermally cross-linked network. It does not
have even glass transition state. The sample from the DSC pan was insoluble in any organic
solvents when we tried to dissolve it after the DSC experiment.
The result of TGA of the cross-linkable material VyPyMCz is shown in Figure 1d. As
o
mentioned earlier, the material is started thermally cross-linking from 230 C. The cross-
linking occurs also during the TGA test and finally thermal stability of the formed polymeric
network is established during the experiment. It could be seen that the cross-linked sample

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o
demonstrates very high thermal stability. Its mass loss of 5 % was determined at 423 C as
o
confirmed by the TGA test with a heating rate of 10 C/min.
The material, VyPyMCz, during and after cross-linking has always the same chemical
structure of chromophore, i.e. bicarbazole core, which is responsible for hole
injecting/transporting properties. It seems that different efficiencies of the OLEDs that
contain different layers of VyPyMCz cross-linked at various temperatures could be
o
connected with the morphology of the films. At temperature below 230 C the film is not
o
cross-linked. At 230-260 C, crosslinking reaction is rather fast; however, not all cross-
linkable vinyl groups are reacted and crosslinkers in the polymeric network are still rare. At
o
temperature higher than 260 C, level of the crosslinking is higher. Some shrinkage is
possible in the films, and maybe the changed morphology affects the performance of the
OLEDs.
2
.6 Surface Morphology
Surface topographies of the cross-linkable VyPyMCz thin films annealed at different curing
temperatures under nitrogen environment have been studied using atomic force microscopy
(AFM) in the tapping mode. As shown in Figure 4, coating of VyPyMCz over the
PEDOT:PSS layer decreases its roughness and hence increases its integrity and stability. We
also observed that roughness and stability of VyPyMCz thin films highly depend on the
annealing temperature. As we increased the temperature, roughness of the films decreased till
o
2
60 C. After that, its roughness started to increase. These observations show good agreement
o
with the DSC and TGA analysis. Meanwhile, the HTL film annealed in between 240-260 C
shows the lowest roughness along with no pinhole or crack, making it more suitable for
solution processed OLED fabrication.
2
2
.7 Electronics properties
.7.1 Ionization potential measurement
The ionization potential (Ip) of a thin layer of the VyPyMCz was measured by the electron
photoemission method. The photoemission spectrum is presented in Figure 1e. The bicarbazole based
VyPyMCz demonstrates Ip of 5.54 eV. It could be observed that Ip of the cross-linkable is notably
lower than that of derivatives having electronically isolated carbazole rings (Ip>5.9 eV) [47-48].
Aforementioned feature reveals that thin layers of the newly synthesized derivatives could be as hole
transporting layer materials for application in OLED devices.

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2
.7.2 Carrier mobility measurement by time of flight method
The time-of-flight (ToF) measurement was used to elucidate the magnitudes of the hole mobility (µh)
in a layer of the compound VyPyMCz [49-50]. It is noteworthy that the room temperature hole drift
mobility shows linear dependencies on the square root of the electric field in the layers as shown in
Figure . The µ was calculated by the formula,
h
2
µ = d /(V.t )
(1)
h
T
where d is the layer thickness of the compounds, V applied bias, and t carrier transit time [51-52]. The
T
tT was calculated from the kink point in the double-logarithmic curve of transient photocurrent. Hole
mobility of the compound VyPyMCz determined from the Figure 1f and calculated data is listed in
-4
2
-1 -1
5
-1
Table 1, i.e. 3.6 x10 cm V s at an electric field of 3.3x 10 V cm . The synthesized compound
VyPyMCz exhibits sound hole mobility, it may be because of high hole mobility of the carbazole core
group [53-54].
2
.8 Electroluminescent properties
Inspired by noteworthy photophysical, thermal and crosslinakbe properties, high hole
mobility, high triplet energy and suitable HOMO-LUMO, VyPyMCz incorporated as the
hole transport layer into the solution processed multilayer blue, green and red multilayer
OLEDs. The molecular structure of compound VyPyMCz, schematic representation of the
fabricated OLED and electroluminescent spectra of the red, blue and green devices are
demonstrated in the Figures 5a, 5b and 5c, respectively. Figure 5d shows the step-to-step
patterning procedures of the fabricated OLED device, more detail information included in the
supplementary information (S1.2). The designed device structure, their corresponding energy
level alignment diagrams and proposed operational mechanism are displayed in Figure 6,
which schematically prove that VyPyMCz not only enable the significant injection of hole
but also successfully confine electron, which are one of the key factors in realizing high
efficiency. Figure 7 and Table 2 summarize the effect of absence and presence of cross-
linkable hole transporting material VyPyMCz on the electroluminescence characteristics of
all the studied blue, green and red OLED devices.
Remarkably, the incorporation of VyPyMCz HTL considerably improves the performance of
all the studied devices. For example, we fabricated monochromatic red phosphorescent
OLEDs with (R1) and without (R2) cross-linkable HTL, VyPyMCz, with following device
structure: indium tin oxide (ITO) (125 nm)/poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate) (PEDOT:PSS) (35 nm)/with (R1) and without (R2) VyPyMCz /7.5 wt% Ir(2-
phq) doped in CBP (20 nm)/ 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene (TPBi) (35
3

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-2
nm)/lithium fluoride (LiF) (1 nm)/ AluminAl (100 nm). At 100 cd m , Device R-2 shows a
-1
-1
current efficiency of 13.5 cd A , which is increased to 20.6 cd A , an increment of 52%, as
-2
the VyPyMCz HTL is incorporated. At 1,000 cd m , the current efficiency is increased from
-1
8
.5 to 13.5 cd A , an increment of 59%, as shown in Figure 7 and Table 2.
Similarly, we also fabricated monochromatic blue and green phosphorescent OLED devices
with and without cross-linkable HTL, VyPyMCz, with following device structures:
Device B1: ITO (125 nm)/PEDOT:PSS (35 nm)/ 15 wt% FIrpic doped in CBP (20 nm)/ TPBi
(35 nm)/LiF (1 nm)/Al (100 nm).
Device B2: ITO (125 nm)/PEDOT:PSS (35 nm)/ VyPyMCz/ 15 wt% FIrpic doped in CBP (20
nm)/ TPBi (35 nm)/LiF (1 nm)/Al (100 nm).
Device G1: ITO (125 nm)/PEDOT:PSS (35 nm)/ 15 wt% Ir(ppy) doped in CBP (20 nm)/
3
TPBi (35 nm)/LiF (1 nm)/Al (100 nm).
Device G2: ITO (125 nm)/PEDOT:PSS (35 nm)/ VyPyMCz/ 15 wt% Ir(ppy) doped in CBP
3
(20 nm)/ TPBi (35 nm)/LiF (1 nm)/Al (100 nm).
-1
The current efficiency of the studied blue OLED device increases from 15.4 to 19.3 cd A , an
-2
-1
increment of 25.32%, at 100 cd m and from 12.8 to 15.7 cd A , an increment of 22.65%, at
-2
1
,000 cd m , as the VyPyMCz HTL is incorporated. Furthermore, the incorporation of
-1
VyPyMCz HTL also increases the CE of green OLED device from 48.1 to 55.3 cd A , an
-2
-1
increment of 14.90%, at 100 cd m and from 45.0 to 51.9 cd A , a increment of 15.3%, at
-2
1
,000 cd m .
We observed significant improvement in the efficiency of the all the studied OLED devices
consisting of low and high bandgap emitters, as the VyPyMCz HTL was introduced. The
reasons why the VyPyMCz HTL containing devices showed the improvement in efficiency
can be attributed to two important structural factors. First, the VyPyMCz HTL can effectively
facilitate the injection of hole because its incorporation breaks the original hole injection
barrier of 1.1 eV into two smaller ones, i.e. 0.35 eV and 0.75 eV, as shown in Figure 5(a).
Second, its incorporation also helps confine the electron to prevent the overflow of electrons
from EML into the hole injection layer especially at high current density or high brightness. It
is because VyPyMCz HTL possesses a relatively high LUMO value, -1.85 eV, which is 1.03

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eV higher than that of the CBP host, or 0.93 eV, 1.03 eV and 1.13 eV higher than that of the
red, blue and green emitters, respectively.
We observed an only 7.0 % efficiency-roll-off into the red OLED and 2.67 % into the blue
-2
-2
OLED, as brightness increases from 100 cd m to 1,000 cd m . These results confirm that
VyPyMCz HTL effectively facilitates the injection of the hole as well as effectively confine
the electron and prevent the overflow of the electrons entering into the hole injection layer
especially at high current density or high brightness. In the past, numerous attempts have been
made to elucidate the mechanisms behind these roll-off characteristics. These studies revealed
that triplet-triplet annihilation, triplet-polaron quenching or dissociation of excitons into free
charge carriers are the main reasons behind efficiency decrement at high current density. Both
these mechanisms are highly correlated with the balance dynamics of carrier injection. The
reason why the VyPyMCz HTL possesses an effective electron confining function is because
it has a relatively high LUMO value (-1.87 eV).
It is also interesting to note that all the VyPyMCz HTL contains OLED devices display higher
brightness when compared to reference device. It may be because of crosslinking properties of
the reported HTL, which significantly reduce the roughness of HIL layer and improved thin-
film integrity. AFM analysis of spin coated thin film also confirmed the aforementioned
hypothesis. These results show that designed and synthesized crosslinkable polymer is suitable
and advantageous to be used as HTL materials in OLEDs as it enables the subsequent solution-
processing of the emission layer without mixing in the previously deposited layer.
Conclusion:
In this study, a class of solution process feasible and thermally cross-linkable hole-
transporting material, VyPyMCz, has been designed, synthesized and successfully
incorporated in the multilayered OLEDs. This compound features a larger bandgap and a
higher triplet energy level. VyPyMCz can be thermally cross-linked to form a solvent-
resistant layer upon isothermal heating at 250 ̊C for 30 min. The crosslinked VyPyMCz layer
possesses not only hole-collecting ability (HOMO = -5.25 eV) but also electron-confining
capability (LUMO = -1.87 eV), which significantly improve the number of exciton into the
desired recombination zone, hence noticeable efficiency enhancement for red, green, and blue
OLEDs, especially at higher voltage. The OLED device consisting of low band gap red
-2
emitter displays 58 and 45% increments in the PE and CE at 1,000 cd cm , respectively, that
is 78 and 47% for high band gap blue emitter. The approach presented herein could become a
general method fabricate a highly efficient solution processed OLEDs by significantly

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injecting holes in the desired emissive layer and preventing the overflow of electrons into the
non-radiative HIL.
Acknowledgments:
This research was funded by a grant No. S-LLT-19-2 from the Research Council of Lithuania
and by grant 106-2119-M-007-011- from Ministry of Science and Technology (MOST) of
Taiwan.

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2
0
0
0
1.0
0.8
0.6
0.4
0.2
0.0
UV (a)
PL
Ferrocene (b)
VyPyMCz
(c)
cooling
nd heating
1
(1.02)
2
o
T = 230 C
pol
-
-
-
10
20
30
(0.77)
1st heating
2
50 300 350 400 450 500 550 600 650
0.0
0.3
0.6
0.9
1.2
0
50
100 150 200 250
Temperature ( C)
Wavelength (nm)
+
o
Potential (V vs Fc/Fc
)
(
d)
VyPyMCz
fitting
(e)
VyPyMCz
fitting
(f)
1
00
-
3
4
1
1
1
0
0
0
9
8
7
6
5
4
0
0
0
0
0
0
-
-5
-
3
α
µ
=(6.45±0.75)*10 cm/V
-
6
2
=(3.6±2.3)*10 cm /Vs
0
-6
10
5.2 5.4 5.6 5.8 6.0 6.2 6.4
0
200
400
600
800
1000
1
00 200 300 400 500 600 700 800
1
/2
1/2
1/2
o
Photon energy (eV)
Temperature ( C)
E (V /cm )
Figure 1. (a) UV-Vis and PL spectrum of the cross-linkable hole transporting material
VyPyMCz) recorded in chlorobenzene at room temperature. (b) Cyclic voltammograms of
the VyPyMCz dye. (c) and (d) show the TGA and DSC curves of the material VyPyMCz,
heating rate 10 C/min. (e) Electro-photoemission spectra of the VyPyMCz thin film. (f) The
hole mobility of the HTM VyPyMCz as a function of the square root of electric field.
(
̊

ACCEPTED MANUSCRIPT
Figure 2. Optimized structures of the synthesized cross-linkable dye VyPyMCz.
LUMO
HOMO
LUMO+1
HOMO-1
Figure 3. Electron Density Contours of frontier molecular orbitals (FMO).

ACCEPTED MANUSCRIPT
Figure 4. AFM images of cross-linkable hole transporting material (VyPyMCz) spin-coated
and cured on ITO, recorded in trapping mode. After coating the VyPyMCz HTL, the
o
substrate was baked at 120 C for 20 min to remove residual solvent, then it was annealed at
different curing temperature (T ) for 40 min: (a) PEDOT: PSS (b) PEDOT: PSS/ VyPyMCz
c
o
o
(
c) PEDOT: PSS/ VyPyMCz (T =160 C) (d) PEDOT: PSS/ VyPyMCz (T =200 C) (e)
c
c
o
o
PEDOT: PSS/ VyPyMCz (T =250 C) (f) PEDOT: PSS/ VyPyMCz (T =300 C). The root
c
c
mean square roughness are being 1.26 nm, 0.815 nm, 0.542 nm, 0.472 nm, 0.359 nm, and 1.1
nm, respectively.

ACCEPTED MANUSCRIPT
(
b)
(
a)
1.0
.8
.6
.4
.2
.0
Blue
Green
Red
0
0
(
c)
0
VyPyMCz
0
0
4
00 450 500 550 600 650 700 750
Wavelength (nm)
(
d)
Figure 5. (a) Chemical structure of synthesized cross-linkable hole transporting material
VyPyMCz), (b) schematic illustration of OLEDs using VyPyMCz as a hole transporting
(
layer, (c) electroluminescent spectra of fabricated red, blue and green OLEDs, and (d)
solution processed step-to-step fabrication of OLED using material, VyPyMCz.

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Figure 6. Schematic diagram of the energy levels of the fabricated solution-processed red, green, and
blue OLED devices consisting of CBP and VyPyMCz as the host and cross-linkable hole transporting
HTL, respectively.

ACCEPTED MANUSCRIPT
Figure 7. Effect of the presence and absence of the solution-processed cross-linkable hole
transporting and electron confining layer, VyPyMCz, on the (1) luminance-voltage-current
density, (2) power efficiency-luminance-current efficiency and (3) electroluminescent
characteristics of the (a-c) red device, (c-f) blue device and (g-i) green device consisting of
CBP as a host.

ACCEPTED MANUSCRIPT
Table 1 Thermal, photophysical, and electrochemical characteristics of the cross-likable hole
transporting material VyPyMCz.
a
b
c
d
e
e
f
f
g
f
Compound
VyPyMCz
λem
Tg
Td
µh
HOMO LUMO
[cm V s ] [eV] [eV]
-1.87
HOMO
[eV]
HOMO Band Gap
ET
°
°
2
-1 -1
[
nm] [ C]
[ C]
[eV]
[eV]
3.38
[eV]
2.87
-
4
402
60
423
3.6 x10
-5.25
-5.25
-1.25
a
b
c
d
Emission peak; Glass transition temperature; Decomposition temperature; Hole mobility
5
-1 e
at the electric field of 3.3 x 10 cm ; HOMO and LUMO were calculated from the redox
potentials measured by cyclic voltammetry (CV) method. The semi-oxidation potential
ox
(
E1/2 ) was calculated from (E + E )/2–0.48, where 0.48 is the correction value obtained in
p1
p2
+
the oxidation system as the Ferrocenium/Ferrocene (Fc /Fc) was added as the internal
ox
standard. The energy of HOMO was then obtained from the EHOMO = -(E1/2 + 4.8). The
energy of LUMO was estimated by subtracting the bandgap from the HOMO energy level,
f
[
(
ELUMO = -EHOMO + Eg].; Calculated by using B3LYP/6-31G (Density functional theory
g
DFT) and time-dependent DFT (TD-DFT)); Calculated from its intersection point of the
absorption and emission spectra.

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Table 2: Effects of the presence and absence of the cross-linkable hole transporting and
electron confining layer on the operation voltage (OV), power efficiency (PE), current
efficiency (CE), external quantum efficiency (EQE), maximum luminance, and CIE
coordinates of the fabricated solution-processed red, blue and green devices.
-
2
Device HTL
@ 100/1000/10,000 cd m
Maximum
Luminance
(cd m )
-
2
-
1
-1
OV[V]
PE[lm W ]
CE[cd A ]
EQE[%]
CIExy
coordinates
R1
R2
-
5.1/6.8/-
14.0/7.5/-
22.6/16.1/-
10.0/7.1/-
(0.57, 0.43)
(0.58, 0.42)
7,724
VyPyMCz 4.8/6.3/9.6
4.8/6.1/-
VyPyMCz 4.6/5.5/-
4.8/6.0/8.4
VyPyMCz 4.1/5.4/8.1
18.3/11.9/4.2
28.1/23.7/12.8
12.7/10.7/-
13,560
B1
B2
G1
G2
-
10.2/6.6/-
15.4/12.8/-
6.2/5.5/-
9.5/6.0/-
(0.17, 0.40)
(0.17, 0.39)
5,260
15.4/11.8/-
19.3/17.7/-
6,857
-
41.7/28.8/12.6
48.9/40.3/17.0
48.1/45.0/32.2
55.3/51.9/41.2
14.5/16.3/10.6 (0.33, 0.62)
13.4/14.6/9.7 (0.33, 0.62)
21,960
26,230

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ACCEPTED MANUSCRIPT
Highlights:
‹
Design, synthesis, and characterization of a thermally cross-linkable hole transporting
material VyPyMCz has been demonstrated.
‹
Preventing the dissolution of formerly deposited layers throughout the consecutive
deposition of multilayer stacks
‹
‹
Facilitating the injection of holes and effectively confine electrons to realize high efficiency
Solution process feasible highly efficient red, green and blue OLED devices have been
designed and fabricated.