A new cross-linkable 9,10-diphenylanthracene derivative as a wide bandgap host for solution-processed organic light-emitting diodes

Article abstract of DOI:10.1039/c8tc05013a

Efficient organic light-emitting diodes (OLEDs) can be obtained using multilayered architectures where the processes of charge injection, transport and recombination are separated and optimized in each layer. Processing these structures from solution requires strategies to avoid redissolution or damage of the previously deposited layers. Several reports have demonstrated the development of cross-linkable hole transport materials, while less literature describes the synthesis and applications of such wide bandgap host materials for multilayered OLEDs. In this work we introduce a cross-linkable derivative of 9-(4-(10-phenylanthracene-9-yl)phenyl)-9H-carbazole incorporating styrene moieties (SPhPC) which can be efficiently cross-linked via radical polymerization. We present the synthesis and characterization of SPhPC, as well as its application as the host in the light-emitting layer to prove the concept of solution processed blue OLEDs.

Full text of DOI:10.1039/c8tc05013a

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A new cross-linkable 9,10-diphenylanthracene
derivative as a wide bandgap host for
solution-processed organic light-emitting diodes†
Cite this: J. Mater. Chem. C, 2018,
6, 12948
d
Rawad K. Hallani,‡a Vahid Fallah Hamidabadi,‡bc Aron Joel Huckaba,
a
Gianmarco Galliani,
Azin Babaei,b Maria-Grazia La-Placa,b Ali Bahari,c
a
d
Iain McCulloch,
Mohammad Khaja Nazeeruddin,
Michele Sessolo *b and
b
Henk J. Bolink
Efficient organic light-emitting diodes (OLEDs) can be obtained using multilayered architectures where
the processes of charge injection, transport and recombination are separated and optimized in each
layer. Processing these structures from solution requires strategies to avoid redissolution or damage of
the previously deposited layers. Several reports have demonstrated the development of cross-linkable
hole transport materials, while less literature describes the synthesis and applications of such wide
bandgap host materials for multilayered OLEDs. In this work we introduce a cross-linkable derivative of
9-(4-(10-phenylanthracene-9-yl)phenyl)-9H-carbazole incorporating styrene moieties (SPhPC) which
can be efficiently cross-linked via radical polymerization. We present the synthesis and characterization
of SPhPC, as well as its application as the host in the light-emitting layer to prove the concept of
solution processed blue OLEDs.
Received 4th October 2018,
Accepted 11th November 2018
DOI: 10.1039/c8tc05013a
rsc.li/materials-c
polydispersity, and limited purity. This leads to poor perfor-
mance reproducibility, and makes industrial scale polymer
1. Introduction
Organic light-emitting diodes (OLEDs) have been showing OLED manufacturing more challenging. In contrast, organic
major industrial and salable progress in the field of organic small molecular weight compounds (small molecules) have a
electronics, especially in small and more recently even large area precise molecular weight, and can be purified to a high degree
flat panel displays.1–3 OLED displays have superior mechanical via recrystallization or sublimation. In commercial OLEDs,
flexibility, and can be applied in the fabrication of flexible or thin films of small molecules are sequentially deposited using
curved electronics. The main advantage over more established thermal vacuum methods leading to high efficiency multi-layer
liquid crystal displays (LCDs) is that OLEDs are electrolumines- devices.4 However, vacuum deposition can alter the morphology
cent devices and hence do not require backlight illumination, of the organic films, as molecules from one layer can diffuse into
leading to better contrast and higher power efficiency. Polymer another one during the process as a result of the high tempera-
semiconductors have been extensively studied as host materials tures applied during the use of this technique. In addition,
in fluorescent multilayered OLEDs. However, polymers face major vacuum deposition is a time-consuming and relatively expensive
drawbacks such as batch-to-batch molecular weight variation, method that at the moment makes OLEDs not compatible with
the lighting market. Solution processing can be a valid alter-
native to vacuum techniques since it meets the requirements of
a KAUST Solar Center (KSC), Division of Physical Sciences and Engineering,
fast, large area and potentially inexpensive manufacturing.5–9
King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900,
However, the use of solution deposition methods can be very
challenging in the fabrication of a multilayered device. The main
Saudi Arabia
b
´
Instituto de Ciencia Molecular, Universidad de Valencia, C/J. Beltran 2, 46980,
issue is the similar solubility of many organic semiconductors,
implying that a pre-deposited layer can partially be dissolved
while processing another layer on top. The use of orthogonal
solvents can alleviate this problem;10,11 nonetheless, this approach
is very challenging as most of the organic semiconductors present
very similar polarity, and the library of materials and molecules
that are suited for this method are very limited. An alternative
Paterna, Spain. E-mail: michele.sessolo@uv.es
c Department of Physics, Faculty of Basic Sciences, University of Mazandaran,
Babolsar 47416-95447, Iran
d Group for Molecular Engineering of Functional Materials, Ecole Polytechnique
Federale de Lausanne, Valais Wallis, CH-1951 Sion, Switzerland
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8tc05013a
‡ These authors contributed equally to this work.
12948 | J. Mater. Chem. C, 2018, 6, 12948--12954
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approach which has been investigated for multilayer solution The green crude solid was purified via chromatography using
processing of OLEDs is the introduction of cross-linkable hexane as an eluent to yield 8.1 g of light green crystalline
functional groups that helps insolubilizing the organic materials materials (94%). The structure of compound 2 was character-
after being deposited using thermal, chemical or light treat- ized by various spectroscopic methods (spectra not showed)
1
ments.12,13 Several reports have demonstrated the development including H NMR (400 MHz), UV-vis and mass spectrometry
of cross-linkable hole transport materials (HTMs), such as (EIMS m/z 257). All spectroscopic results agree with the reported
arylamine-based compounds,14–20 while less literature describes properties.31
the synthesis and applications of wide bandgap host materials
4-Carbazolyl-1-bromophenylboronic acid (2) was prepared
for multilayered OLEDs.21–23 In this work we present a cross- following a literature procedure.32
linkable derivative of 9-(4-(10-phenylanthracene-9-yl)phenyl)-9H-
9-(4-(Anthracen-9-yl)phenyl)-9H-carbazole (3): in a 100 mL
carbazole (PhPC). PhPC has been studied previously both as a round-bottom flask equipped with a stir bar and a condenser,
deep blue emitter (a deep-blue emission maximum at 436 nm)24 1 (1.24 g, 4.76 mmol, 1 eq.) and 2 (1.79 g, 6.21 mmol, 1.3 eq.)
and as a host for triplet–triplet annihilation (TTA) OLEDs, in were dissolved in 46 mL of dry toluene. K2CO3 was dissolved in
view of its low triplet level (1.74 eV).25 TTA is a mechanism 4 mL of water (5.14 g, 36.5 mmol, 10 eq.), and was added along
where the otherwise non-radiative triplet states in a fluorescent with 6 mL of ethanol. The mixture was purged for 30 minutes
material can be harvested to generate singlet excitons, substan- with nitrogen, and Pd(PPh3)4 (215 mg, 0.186 mmol, 0.05 eq.) was
tially contributing to the electroluminescence efficiency.26–29 added before the reaction was brought to reflux and left stirring
Incorporating styrene moieties into PhPC converts the molecule overnight. The reaction was cooled down to room temperature,
into an organic semiconductor (SPhPC) which can be efficiently and extracted using toluene. The organic phase was dried with
cross-linked via radical polymerization without any major MgSO4 and concentrated under vacuum. The crude solid was
alteration to the molecule energy levels.30 In this paper we purified via chromatography using hexane as an eluent to get rid
report the synthesis and use of SPhPC as a cross-linkable host of starting materials, followed by 4 : 1 hexane: dichloromethane
material for all solution processed fluorescent blue OLEDs.
(DCM) to obtain a white solid powder (46%). 1H NMR (500 MHz,
CDCl3, d): 8.57 (s; 1H), 8.21 (d; J = 7.72 Hz; 2H), 8.10 (d; 8.46 Hz;
2H), 7.82 (m; 4H), 7.68 (m; 4H), 7.51 (m; 4H), 7.46 (m; 2H),
7.36 (m; 2H). 13C NMR (125 MHz, CDCl3, d): 140.9, 137.9,
137.05, 135.9, 132.8, 131.4, 130.3, 128.5, 127, 126.9, 126.6,
126, 125.7, 125.2, 123.5, 120.4, 120.1, 109.9. MALDI-TOF MS (m/z):
2. Experimental section
Synthesis of PhPC
The functionalization of PhPC was achieved following the syn- 419.12 [M]+.
thetic procedure depicted in Scheme 1. 9-Bromoanthracene
3,6-Dibromo-9-(4-(10-bromoanthracen-9-yl)phenyl)-9H-carb-
was reacted with n-phenylcarbazole boronic acid via Suzuki azole (4): in a flame dried 500 mL round bottom flask equipped
coupling to yield 46% of molecule 3, which was brominated with a stir bar, 3 (400 mg, 0.957 mmol, 1 eq.) was dissolved in
using N-bromosuccinimide (NBS) to produce 91% of molecule 4. 100 mL of dry dimethylformamide (DMF) and 100 mL of DCM.
Finally molecule 4 was reacted with 4-vinylphenylboronic The flask was covered with aluminum foil and cooled down to
acid through Suzuki coupling to generate 51% of the target 0 1C, then NBS (511 mg, 2.87 mmol, 3 eq.) was added and the
product, SPhPC.
reaction was left stirring for 30 minutes at 0 1C and then for
9-Bromoanthracene (1): in a 500 mL round bottom flask 3 hours at room temperature. The reaction mixture was quenched
equipped with a stir bar, anthracene (6.00 g, 33.7 mmol, 1 eq.) using water and extracted using DCM. The organic phase was
was dissolved in 300 mL of CHCl3. CuBr2 (15.1 g, 67.4 mmol, washed 5 times with water, dried using MgSO4, and concentrated
2 eq.) was added to the solution, and the reaction was stirred under vacuum to yield 572 mg of white-yellow crystalline material
overnight under reflux conditions. The reaction was filtered over (91%). 1H NMR (500 MHz, CDCl3, d): 8.66 (d; 8.85 Hz; 2H), 8.25
celite and the solution was concentrated under reduced pressure. (d; 1.84 Hz; 2H), 7.74 (m; 4H), 7.65 (m; 4H), 7.60 (d; 1.92 Hz; 1H),
Scheme 1 Reaction scheme for the synthesis of SPhPC.
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7.59 (d; 1.90 Hz; 1H), 7.50 (m; 4H). 13C NMR (125 MHz, was cooled to ꢀ78 1C. Then, n-butyl lithium (2.5 M in 2-MeTHF,
CDCl3, d): 139.8, 138.3, 136.4, 136.2, 132.9, 132.7, 131.0, 130.3, 0.49 mL, 1.07 mmol, 1 eq.) was added dropwise and the solu-
129.5, 128.1, 127.1, 126.9, 126.7, 126, 124.1, 123.3, 113.3, 111.6. tion was allowed to stir at ꢀ78 1C. After 4 h, 3-bromobenzo-
MALDI-TOF MS (m/z): 655.5 [M]+.
phenone (0.22 g, 0.86 mmol, 0.83 eq.) was added in one solid
3,6-Bis(4-vinylphenyl)-9-(4-(10-(4-vinylphenyl)anthracen-9-yl)- portion and let stir at ꢀ78 1C. After 30 min, the reaction was
phenyl)-9H-carbazole (SPhPC): in a 50 mL RBF equipped with a allowed to warm at rt under stirring and the reaction was
stir bar and a condenser K3PO4 was dissolved slowly in 4 mL of monitored via 1H NMR spectroscopy. After 30 min, the reaction
water (3.6 g, 16.98 mmol, 52 eq.). 4 (0.214 g, 0.325 mmol, 1 eq.) mixture was quenched using MeOH and concentrated on a
was dissolved in 10 mL of dry toluene and added to the solution rotary evaporator. Then, acetic acid (glacial, 20 mL) and HCl
along with 4-vinylphenylboronic acid (0.241 g, 1.63 mmol, (5 mL) were added to the crude product and the mixture
5 eq.), (o-tolyl)phosphine (45 mg, 0.18 mmol, 0.3 eq.), and warmed to reflux. After 24 h, the reaction mixture was cooled
2 drops of aliquat. The reaction mixture was purged for to rt and 10 mL water was added. The precipitate was filtered
25 minutes then Pd2(dba)3 (45 mg, 0.061 mmol, 0.15 eq.) was and dried to yield 0.262 g (88%) of a white solid. 1H NMR
added and the reaction was brought to 110 1C and left reacting (400 MHz, CDCl3) d 7.78 (d, J = 7.3 Hz, 2H), 7.42–7.26 (m, 5H),
overnight. The reaction was cooled down and extracted with 7.32–7.26 (m, 4H), 7.25–7.20 (m, 3H), 7.19 (m, 2H), 7.15–7.05
DCM. The organic phase was dried using MgSO4, concentrated (m, 2H). HRMS: calc. for C25H17Br: 396.0508. Found: 396.0508.
under high vacuum for 15 minutes. The crude solid was purified
F1: spiro compound 5 (0.262 g, 0.66 mmol, 1 eq.) was dis-
via chromatography using 3 : 1 hexane : DCM as an eluent to obtain solved in dry toluene (3 mL). Then, KOtBu (0.22 g, 1.99 mmol,
118 mg (51%) of a white yellow powder. 1H NMR (500 MHz, CDCl3, 3 eq.) was added under N2, followed by Pd2dba3 (0.027 g,
d): 8.5 (s; 2H), 7.75 (m; 20H), 7.51 (m; 13H), 6.95 (dd; J = 17.64, 0.029 mmol, 0.12 eq.) and PtBu3 (1 M in Tol, 0.016 mL,
10.95 Hz; 1H), 6.83 (dd; J = 10.81, 6.77 Hz; 2H), 5.97 (d; J = 0.016 mmol, 0.055 eq.). Finally, m-toluidine (0.072 mL, 0.67 mmol,
17.69 Hz; 1H), 5.86 (d; J = 17.7 Hz; 2H), 5.42 (d; J = 11.2 Hz; 1H), 1.02 eq.) was added using a syringe and the solution was warmed
5.32 (d; J = 11.02 Hz; 2H). 13C NMR (125 MHz, CDCl3, d): 141.3, at 80 1C under stirring and monitored via 1H NMR spectroscopy.
140.8, 138.5, 138.4, 136.9, 136.6, 136.5, 136, 135.9, 133.3, 132.9, After 2 h, the reaction mixture was cooled to rt and passed
131.5, 130, 129.9, 127.4, 127.3, 127.1, 126.7, 126.3, 125.6, 125.4, through a SiO2 plug using dichloromethane. It was dried and
125.2, 124.2, 118.8, 114.2, 113.6, 113.5, 110.4: MALDI-TOF MS the solution was added into a flask. Atmospheric air was pump
(m/z): 725.89 [M]+; HRMS: calc. for C56H39N: 725.30825. Found: cycled three times, and then dissolved in toluene (3 mL). Then,
725.30695.
KOtBu (0.22 g, 1.99 mmol, 3 eq.) was added under an N2
atmosphere followed by Pd2dba3 (0.027 g, 0.029 mmol, 0.12 eq.)
and PtBu3 (1 M in Tol, 0.016 mL, 0.016 mmol, 0.055 eq.). Finally,
Synthesis of F1
The F1 emitter was synthesized as previously reported in two 1,6-dibromopyrene (0.12 g, 0.33 mmol, 0.5 eq.) was added in
one-pot steps.33 Lithium halogen exchange of 2-bromobiphenyl one portion, warmed to 80 1C under stirring and monitored via
and nucleophilic reactions with 3-bromobenzophenone at low 1H NMR spectroscopy. After 20 h, it was cooled to rt and passed
temperature followed by acid-mediated spirocyclization yielded through a SiO2 plug using dichloromethane. The solid was
compound 5 in good yield. Submitting this spirocyclic bromide dissolved in minimal dichloromethane and hexanes were
to sequential Pd catalyzed Buchwald–Hartwig C–N coupling added to precipitate an orange solid. The solid was washed
reactions afforded the F1 emitter in a modest yield after several using boiled hexanes (3 ꢁ 20 mL), then the solid was dissolved
purification steps (Scheme 2).
in minimal dichloromethane, B1 mL hexane was added, and
Spiro compound (5): a flame dried flask was charged with a the solution was concentrated on a rotary evaporator until
stirring bar and 2-bromobiphenyl (0.25 g, 1.07 mmol, 1 eq.). precipitates just started to form. This suspension was stored
After dissolving the white solid in THF (10 mL), the solution in a ꢀ15 1C freezer for 72 h, and dried to yield 0.062 g,
Scheme 2 Reaction scheme for the synthesis of F1.
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0.006 mmol (a 10% yield over two steps) of an amber colored
solid. 1H NMR (400 MHz, CDCl3) d 8.05 (d, J = 8.8 Hz, 3H), 7.87
(d, J = 9.2 Hz, 2H), 7.74 (d, J = 8.2 Hz, 2H), 7.62 (d, J = 7.5 Hz,
3H), 7.20 (m, 8H), 7.1–6.95 (m, 16H), 6.93 (s, 2H), 6.82 (t, J =
7.7 Hz, 4H), 6.74 (d, J = 7.6 Hz, 2H), 6.68 (d, J = 7.7 Hz, 2H), 2.23
(s, 6H). 13C NMR (400 MHz, CDCl3) d 151.23, 148.60, 147.07,
146.08, 141.33, 140.31, 139.17, 129.65, 129.12, 128.41, 128.34,
128.15, 127.79, 127.72, 127.57, 126.85, 126.71, 126.30, 123.55,
123.00, 122.89, 122.58, 121.46, 120.59, 120.26, 119.32, 65.64,
21.87. HRMS: calc. for C82H59: 1044.443. Found: 1044.445.
Material characterization and device fabrication
Solutions of all materials were prepared in a nitrogen filled
glove box, where the corresponding organic thin-films were
spin-coated and annealed. Prior to deposition, all solutions
were filtered using a 0.22 mm pore size filter. Optical absorption
spectra were collected in transmission mode using an optical
fiber system manufactured by Avantes. Photoluminescence
spectra and quantum yields in thin films were recorded in an
integrating sphere using a Hamamatsu absolute quantum yield
C9920 spectrometer. Poly(3,4-ethylenedioxythiophene):polystyrene
sulfonate (PEDOT:PSS, CLEVIOSt P VP CH 8000) was purchased
from Heraeus. The photolithography-patterned indium tin oxide
(ITO) glass substrates were purchased from Naranjo Substrates
(www.naranjosubstrates.com). The substrates were subsequently
cleaned using soap, deionized water, and isopropanol in an
ultrasonic bath for 5 minutes each, followed by 20 minutes of
UV-ozone treatment. Onto the clean ITO substrates, a 45 nm thick
film of PEDOT:PSS was spin-coated at 2500 rpm for 1 minute. The
PEDOT:PSS dispersion was filtered using a 0.45 mm pore size filter.
The layers were dried at 150 1C for 15 minutes. On top of that, the
hole transport layer and the emissive layers were subsequently
spin-coated upon cross-linking, as described below. The emissive
layer was then coated using an electron transport layer (ETL),
1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB). TmPyPB was
either solution processed by spin-coating or sublimed in a high
vacuum chamber. The devices were finished with a low work
function cathode (5 nm thick Ba capped with 100 nm thick Ag).
The thicknesses of all films were determined using an Ambios
XP1 profilometer. Devices were not encapsulated and were
characterized in a glove box. Current density and luminance
versus voltage were collected using a Keithley 2400 source meter
and a photodiode coupled to a Keithley 6485 pico-amperometer,
using a Minolta LS100 instrument to calibrate the photocurrent.
An Avantes luminance spectrometer was used to measure the
electroluminescence spectra.
Fig. 1 Chemical structures of the compounds used in the emissive layers
for solution-processed OLEDs.
temperatures and for different times, measuring the optical
absorption before and after rinsing with toluene. We observed
that after annealing at temperatures as high as 180 1C for a
prolonged time (1 hour), the absorbance of the layer diminishes
after the first rinsing step (Fig. S1, ESI†) and then stabilizes for
subsequent toluene rinsing cycles. This implies that the film is
only partially cross-linked under the annealing conditions
used. Hence, we attempted to quantitatively cross-link the films
by means of thiol–ene polymerization, adding pentaerythritol
tetrakis(3-mercaptopropionate) (PETMP, 1.5 wt%) and 2,20-
azobis(2-methylpropionitrile) (AIBN, 2 wt%), whose chemical
structures are shown in Fig. 2. PETMP allows the formation of a
hyper-branched polymer network, in the presence of the radical
photoinitiator AIBN.35
SPHPC films containing F1 (5 wt%), PETMP (1.5 wt%) and
AIBN (2 wt%) were exposed to UV light for 10 minutes and then
annealed at 150 1C for 30 minutes. After this treatment the layer
becomes completely resistant to toluene, as the optical absorp-
tion of the films is now unvaried (Fig. 3a). We used PETMP
(1.5 wt%) and AIBN (2 wt%) at these concentrations as they
were found to be the lowest still allowing for quantitative cross-
linking. We also measured the photoluminescence (PL) and its
quantum yield (PLQY) for the same layers, before and after
rinsing twice with toluene (Fig. 3b). Under excitation at 310 nm,
we observe a narrow PL signal with a maximum at 470 nm, and
an associated PLQY of 47% and 50% for the sample before and
after toluene dripping, respectively. The differences in the
intensity and quantum yield of the PL spectra are within the
sample-to-sample variation and can therefore be neglected.
3. Results and discussion
Thin-films of SPHPC doped with 5 wt% of the blue fluorescent These results confirm a quantitative cross-linking of the active
molecule F1 (Fig. 1) were deposited via spin-casting from materials in the presence of PETMP and AIBN, without com-
toluene solutions onto PEDOT:PSS-coated substrates. F1 was promising its optical properties.
initially chosen as it can be used in combination with the PhPC
The cross-linked SPhPC films were tested as the active
type host for efficient TTA OLEDs.25,34 At first we attempted to material in solution-processed multi-layered OLEDs (Fig. 3a).
cross-link the film by simple thermal treatment at different PEDOT:PSS was used as the hole injection layer (HIL) to smooth
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Fig. 2 (a) Optical absorption and (b) photoluminescence spectra (excitation wavelength 310 nm) of cross-linked SPhPC : F1 (5 wt%) thin films before and
after rinsing with toluene. In (b) the PLQY values are also reported.
Fig. 3 (a) Device structure with the corresponding (b) current density and (c) luminance versus applied voltage of OLEDs with a solution-processed or
sublimed ETL. (d) Efficacy as a function of luminance for the same devices. The inset in (c) is the electroluminescence spectrum of a blue OLED with a
solution-processed ETL. The corresponding color coordinates are reported in the CIE 1931 color space, see the inset of (d).
the indium tin oxide (ITO) electrode and enlarge its work func- deposited in a vacuum chamber using fully solution processed
tion. The hole transport (electron blocking) layer used was OLEDs with a spin-coated TmPyPB film from toluene.
KR386 (Fig. 2), as it can be solution processed and quanti-
The current density vs. voltage curves (Fig. 3b) for the
tatively cross-linked by a simple thermal treatment.36 The devices with sublimed or solution-processed ETLs show similar
emissive layer, consisting of SPhPC : F1 (5 wt%) : PETMP characteristics. We noticed a slightly lower current leakage for
(1.5 wt%) : AIBN (2 wt%), was spin-coated from toluene and the solution-processed device, which might originate from the
cross-linked as described above. The emissive layer was then device planarization during spin-coating of the ETL. Current
coated with TmPyPB and with a low work function cathode injection takes place at about 3 V reaching about 400 and
(5 nm thick Ba capped with 100 nm thick Ag). We compared the 600 A mꢀ2 at 10 V, for the fully-solution processed and for
performance of hybrid devices where TmPyPB is thermally the hybrid device, respectively. With our setup, the turn-on
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voltage for electroluminescence was detected at 2.7 V for both above (UV light exposure for 10 minutes and annealing at
types of devices (Fig. 3b). The light intensity was found to 150 1C for 30 minutes). The PLQY was measured under excita-
increase rapidly reaching a maximum of 623 cd mꢀ2 for the tion at 310 nm, and was found to be 85% for DPAVB and 61%
fully solution-processed OLED and 849 cd mꢀ2 for the one with for DPVBI (Fig. S2, ESI†). The corresponding OLEDs showed
the sublimed ETL. The electroluminescence spectrum of the low leakage and current injection at about 3 V (Fig. S3a, ESI†).
devices results in CIE coordinates x = 0.19 and y = 0.30, We noted that the current density at high bias (10 V) was rather
corresponding to the sky blue zone of the CIE 1931 color space low for these devices, about 22 A mꢀ2 and 100 A mꢀ2 for the
(Fig. 3d). We obtained a current efficiency (efficacy) of 2.18 cd Aꢀ1 OLEDs employing DPAVB and DPVBI, implying a very good
and 2.15 cd Aꢀ1, for the OLED with the sublimed ETL and for the charge selectivity at the hole and electron blocking interfaces,
fully solution-processed one, respectively (Fig. 3d). The essentially respectively (about 400 A mꢀ2 was obtained for devices with F1).
equal maximum efficacies validate SPHPC as a promising
As a consequence of the very efficient charge selectivity a
material for solution-processed multilayer organic devices. We promising maximum current efficiency of 3.73 cd Aꢀ1 was
do observe, however, a slightly lower efficiency at high lumi- obtained for a blue emitting solution processed LED. The
nance for the fully-solution processed devices, which can be corresponding luminance (104 cd mꢀ2) was modest as a con-
attributed to unavoidable impurities introduced during solution sequence of the somewhat low current density. A slightly blue
preparation and film casting.
shifted electroluminescence was obtained using DPVBI (color
In order to further validate SPhPC as a suitable host for coordinates 0.23, 0.32, Fig. 4b), even though the maximum
fluorescence blue emitters, we prepared fully solution-pro- efficacy was limited to 1.24 cd Aꢀ1
cessed devices with the same structure and materials depicted
.
in Fig. 3a, but using either 4,40-bis(2,2-diphenylethenyl)-1,10-
4. Conclusions
biphenyl (DPVBI)37 or 4-(di-p-tolylamino)-40-[(di-p-tolylamino)-
styryl]stilbene (DPAVB) (Fig. 1) as dopants. SPhPC films
containing the blue emitter (5 wt%), PETMP (1.5 wt%) and
AIBN (2 wt%) were spin-coated and cross-linked as described
We present a cross-linkable wide bandgap host organic semi-
conductor (SPhPC), suitable for the preparation of solution pro-
cessed multilayer OLEDs. The molecule is obtained by introducing
styrene moieties at the periphery of the wide bandgap material
9-(4-(10-phenylanthracene-9-yl)phenyl)-9H-carbazole. Upon func-
tionalization, SPhPC can be efficiently cross-linked via radicals
or polymerization, in the presence of suitable activators. The
optical properties of the film do not change upon this chemical
reaction which demonstrates its usefulness for use in multi-
layered opto-electronic devices. As a proof of concept, a multi-
layered blue light emitting OLED was prepared showing excellent
rectification and promising current efficiency.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We acknowledge the financial support from the European
Union H2020 project SOLEDLIGHT (grant 643791), the Spanish
Ministry of Economy and Competitiveness (MINECO) via the
´
Unidad de Excelencia Marıa de Maeztu (MDM-2015-0538 and
MAT2017-88821-R), and the Generalitat Valenciana (Prometeo/
2016/135). M. S. thanks the MINECO for his RyC contracts. R. K. H.
would like to acknowledge King Abdullah University of Science
and Technology (KAUST) for financial support.
Notes and references
1 B. Geffroy, P. le Roy and C. Prat, Polym. Int., 2006, 55,
572–582.
2 T. Tsujimura, OLED Display Fundamentals and Applications,
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Fig. 4 (a) Efficacy as a function of luminance for fully solution-processed
devices using different blue emitters. (b) Electroluminescence spectrum of
the same devices with (inset) corresponding color coordinates in the CIE
1931 color space.
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