Naphthalimide end-capped diphenylacetylene a versatile organic semiconductor for blue light emitting diodes and a donor or an acceptor for solar cells

Article abstract of DOI:10.1039/c8nj06496b

A novel compound 6,6′-(ethyne-1,2-diylbis(4,1-phenylene))bis(2-(2-butyloctyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) (NAI-PVP-NAI) based on an end capping group 1,8-naphthalimide and central building block diphenylacetylene was designed and synthesized b

Full text of DOI:10.1039/c8nj06496b

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Subbiah, T. T. Do, S. Chavhan, T. Ou, S. Manzhos, D. J. Jones, J. Jou and J. M. Bell, New J. Chem., 2019,
DOI: 10.1039/C8NJ06496B.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Napthalimide End-Capped Diphenylacetylene: A Versatile
Organic Semiconductor for Blue Light Emitting Diodes and
Donor or Acceptor for Solar Cells
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Thu-Trang Do,a Sudam Chavan,b Jegadesan Subbiah,c Tsu-Hao Ou,b Sergei Manzhos,d
David Jones, c John M. Bell,a JwoHuei Jou b and Prashant Sonara*
aSchool of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology (QUT), 2 George Street, Brisbane, QLD-4001, Australia.
bDepartment of Materials Science and Engineering, National Tsing-Hua University, 101 Sec-
2 Kuang -Fu Road, Hsinchu -30013 Taiwan
cSchool of Chemistry, University of Melbourne, Bio21 Institute, 30 Flemington Road,
Parkville, Victoria 3010, Australia
dDepartment of Mechanical Engineering, Faculty of Engineering, National University of
Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576.
*E-mail: sonar.prashant@qut.edu.au.
KEYWORDS: Napthalimide, Electron Acceptors, Electron Donor, Wide Band-gap
Molecules, Organic Solar Cells.
Thu-Trang Do, Sudam Chavan, and Jegadesan Subbiah all three authors contributed
equally for this work.
ABSTRACT:
A
novel
compound
6,6'-(ethyne-1,2-diylbis(4,1-phenylene))bis(2-(2-butyloctyl)-1H-
benzo[de]isoquinoline-1,3(2H)-dione) (NAI-PVP-NAI) based on end capping groups 1,8-
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napthalimide and central building block diphenylacetylene was designed and synthesized by
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Suzuki coupling. The newly synthesized NAI-PVP-NAI compound is characterized using
optical, thermal, and electrochemical techniques as well as ab initio modeling. This novel and
unique compound exhibits strong blue emission in solid state and has been successfully used
as an active light-emitting layer in organic light emitting diodes (OLEDs). Interestingly, the
newly developed compound shows both electron-donating and electron-accepting ability.
Therefore, it can act as a donor or as an acceptor organic semiconductor. Indeed, upon
evaluating this molecule in solution processable organic photovoltaic (OPV) devices, we
show that it acts as a donor when used with a PCBM acceptor and as an acceptor when used
with a P3HT donor. Thus, NAI-PVP-NAI is a versatile compound, which can play three
roles as a blue light emitting layer in OLED and a donor or an acceptor in OPV.
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INTRODUCTION
Low cost semiconducting materials are an important class of materials for todays and future
electronic industry for a wide range of electronic devices and for large area application.
These semiconducting materials can be classified as inorganic or organic. Compared to
inorganic (non-carbon based) semiconductors, organic (carbon based) ones can bring many
advantages for electronic devices such as low cost, preparation at low process temperatures or
from solution on flexible substrates (ink formulation and printing), large area coverage,
tunable optical and electronic properties by changing the molecular structure or
functionalization, and facile synthetic modification.1-3 Indeed, there are numerous organic
semiconductors, including small molecules and polymers, which have been designed and
applied in various organic electronic devices such as organic solar cells (OSCs),4 dye-
sensitized solar cells (DSSCs),5 perovskite solar cells (PSCs),6-7 organic field-effect
transistors (OFETs),8 organic light-emitting diodes (OLEDs)9 and so on. Basically, there are
plenty of studies based on which a single organic semiconducting material can be designed
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for a single specific application, but till date there are very few attempts or reports made
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available where single semiconducting material can be employed in various multifunctional
devices. Herein, we are reporting the use of a single compound for applications in two
different types of devices, OLEDs and OSCs.
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On the one hand, OLEDs have attracted considerable attention because of their potential
application in flat-panel displays for mobile phones and televisions.10-13 The key point of
material development for OLEDs is to achieve high efficiency, long lifetime, and saturated
colour purity. While a large number of green and red emitters satisfy the requirement for
OLEDs,14-16 efficient and stable blue emitters are still rare.17-19 Furthermore, outstanding blue
emitting materials are crucial not only as blue emitters but also as host materials to facilitate
efficient green and red emission.20 As a result, the improvement of blue light emitting
materials with high luminescent efficiency and pure bright color are some of the most critical
requirements.
At the same time, bulk heterojunction (BHJ) OSCs have gained particular interest as a
promising technology for harvesting solar energy and converting it into electrical energy due
to their apparent advantages of low-cost fabrication, light-weight, mechanical flexibility, roll-
to-roll production and the tunable chemical properties of the organic semiconducting
photovoltaic materials. Recently, power conversion efficiencies (PCEs) of OSCs have
significantly increased, to more than 12%21-24 following advances in syntheses of novel
photoactive materials, adoption of interface materials, and design of new device structures.
Traditionally, the photoactive layer of OSCs generally consists of a p-type electron donor and
n-type electron acceptor and optimization of both materials plays an important role in
enhancing performance of an OSC device.
1,8-Naphthalimide (NAI) derivatives have attracted great attention for their high
photoluminescence quantum yields (PLQY) and good optical, thermal and chemical
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stabilities.25-26 The 1,8-naphthalimide derivatives have unique photo-physical prDoOpIe: 1r0t.i1e0s39/aCn8dNJ06496B
have been used as yellow emitting layers27 in doped device or red emissive material.28
Herein, we are reporting a new compound based on diphenylacetylene or phenylene-
acetylene-phenylene (PVP) as a central building block and 1,8-naphthalimide as end capping
groups based acceptor-donor-acceptor (A-D-A) compound, namely, NAI-PVP-NAI, which
shows strong blue emission in solid state; we therefore attempted to use it as an active layer
in fluorescent OLEDs. The phenylene group in the PVP moiety is a weak donor and in this
molecular structure can act as an acceptor whereas the electron rich alkyne group is a good
electron donor29 and may also allow free internal rotation of two conjugated planes
suggesting better packing behavior.29 In addition, 1,8-naphthalimide has been studied in
recent years and its derivatives are used as n-type semiconducting materials for organic
electronics.30-34 To our best knowledge, however, there is no single report using NAI as a
candidate for designing p-type semiconductors. In our molecular design, we used NAI as an
end capping groups with phenylene-acetylene-phenylene as core moiety and found that this
compound demonstrates both electron-accepting and electron-donating abilities for OSCs
application. Our finding clearly suggests that a carefully designed single donor-acceptor
based compound can play three roles in various types of organic electronic devices, which is
very rare among materials reported in the literature.
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RESULTS AND DISCUSSION
Synthesis and Thermal Properties:
The synthetic route of the target compound is shown in Scheme 1. Firstly, 2-butyl-octylamine
(3) was prepared from respective alkyl alcohol via three steps following our previous report.31
Then, alkyl amine reacted with commercially available 4-bromo-1,8-naphthalic anhydride (4)
to form the alkyl substituted NAI derivative 6-bromo-2-(2-butyloctyl)-1H-
benzo[de]isoquinoline-1,3(2H)-dione (5). For the central PVP building block, its commercial
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dibromo PVP derivative was converted to its bisboronic ester analogous 1,2-bis(4-(4,4,5,5-
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tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethyne (6) using standard protocol. The final
compound NAI-PVP-NAI was synthesized by carrying out the Suzuki coupling reaction
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between
benzo[de]isoquinoline-1,3(2H)-dione and 1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)phenyl)ethyne over 48h under reflux condition, using
alkylated
monobromo
NAI
compound
6-bromo-2-(2-butyloctyl)-1H-
a
tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] as a catalyst in a mixture of toluene
and an aqueous 2 M potassium carbonate [K2CO3] solution. The final compound NAI-PVP-
NAI was obtained in 70% yield upon performing column chromatography and
recrystallization purification methods. The presence of branched alkyl chains such as butyl-
octyl on N-bay position of napthalimide as an end-capping group induces adequate solubility
for NAI-PVP-NAI compound, and indeed it is highly soluble in most common organic
solvents such as chloroform (CHCl3), chlorobenzene (CB), and o-dichlorobenzene (o-DCB).
The thermal properties of the compound NAI-PVP-NAI were evaluated by
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The 5%
weight loss temperature (decomposition temperature, Td) of the compound is 420 oC (Fig. 1)
under nitrogen, indicating an excellent thermal stability of NAI-PVP-NAI compound. In the
first DSC heating cycle, NAI-PVP-NAI exhibits a small melting peak at 102 oC and another
o
sharp melting point at 189 C (Fig. 2), suggesting that different polymorphs are present.
Meanwhile during the first cooling scan, the compound showed crystallization peaks at
o
151oC. A subsequent cycle showed a single melting transition at 188 C, indicating a
formation of single polymorph after the initial melt and recrystallization. In the second
o
o
cooling cycle, double exothermic peaks at 147 C and 155 C are observed, which might be
attributed to molecular conformation transformations.
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Scheme 1. Synthetic route of NAI-PVP-NAI. Reagent and conditions: (a)
o
Triphenylphosphine, Br2, DCM, 0 C  room temp., overnight, 95%; (b) Potassium
o
phthalimide, DMF, 90 C 24h, 90%; (c) NH2NH2·H2O, MeOH, refluxed, 8h, 100%; (d)
Anhydrous DMF, 110 oC, overnight, 60%; (e) bis(pinacolato)diboron, CH3COOK,
o
o
Pd(dppf)Cl2, dioxane, 84 C, 48 h, 72%; (f) 2M K2CO3, toluene, Pd(PPh3)4, 110 C, 48 h,
70%.
Optical and Electrochemical Properties:
The optical properties of NAI-PVP-NAI compound are studied using UV-vis absorption and
fluorescence spectroscopy in both solution and solid state. The UV-vis absorption spectra of
the compound in dilute chloroform solution and corresponding thin film deposited on a glass
substrate are shown in Fig. 3. NAI-PVP-NAI exhibits similar absorption spectra with a
maximum absorption peak at 367-368 nm in solution and thin film, suggesting weak
intermolecular aggregation of the compound in the solid state. Such weak intermolecular
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interaction is due to the distortion in the planarity of the backbone due to the attached end
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capping NAI group’s phenylene moiety to the central phenylene ring of PVP.
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Fig. 1: TGA thermogram of NAI-PVP-NAI
.
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Fig. 2: Differential Scanning Calorimetric (DSC) analysis of NAI-PVP-NAI
.
It has been well established that the dihedral angle between two phenylene units is high and
this contributes to a relatively high band gap. In comparison with 2,2'-bis(2-ethylhexyl)-
1H,1'H-[6,6'-bibenzo[de]isoquinoline]-1,1',3,3'(2H,2'H)-tetraone in which two naphthalimide
groups are combined by a single bond, a slight red shift about 8-9 nm of the maxima peaks
are observed due to the increase in conjugation of diphenylacetylene bridge between two
naphthalimide units. The absorption band-edge of the cast film for NAI-PVP-NAI is 420 nm,
corresponding to an optical bandgap ( ) of 2.95 eV.
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Fig. 3: UV-Vis absorption spectra of NAI-PVP-NAI in chloroform solution and as thin
film and emission spectrum in chloroform solution and thin film.
The high band gap nature of this newly developed NAI-PVP-NAI compound is due to the
high dihedral angle between the end capping groups and the central conjugated building
block. NAI-PVP-NAI molecule is also investigated by fluorescence spectroscopy and it
exhibits maximum emission in chloroform solution at 450 nm and in thin film at 460 nm,
which is perfect blue to the human eye. In order to study the effect of solvent polarity on the
emission properties, we measured fluorescence using various solvents such as hexane,
toluene, tetrahydrofuran (THF), dichloromethane (DCM), and dimethylformamide (DMF),
respectively. The emission spectra of the compound clearly exhibits a bathochromic shift
upon increasing the polarity of the solvent (Fig. S8)
The electrochemical behavior of NAI-PVP-NAI was investigated by cyclic voltammetry
(CV) in dichloromethane solution. The CV curve was recorded with respect to the potential
of a standard calomel electrode (SCE), which was calibrated by the ferrocene-ferrocenium
(Fc/Fc+) redox couple, and its energy level was taken as -4.80 eV below the vacuum level. As
shown in Fig. 4, NAI-PVP-NAI exhibits irreversible reduction and oxidation waves. The
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HOMO and LUMO values for the compound are calculated from the onset oxidation and
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reduction potentials using the equations: HOMO = -EHOMO = e (Eonset (ox) + 4.8) eV and ELUMO
= e (Eonset(red) + 4.8) eV.
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Fig. 4: Cyclic voltammograms of NAI-PVP-NAI
.
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Fig. 5: Schematic energy level diagram of NAI-PVP-NAI based device.
The obtained HOMO and LUMO levels of NAI-PVP-NAI are -5.95 and -3.04 eV
respectively. Both the LUMO and HOMO of NAI-PVP-NAI are lower than those of the
classical polymer donor P3HT and higher than those of PC71BM (see Fig. 5), indicating this
compound could be used as an acceptor or as a donor in a low wavelength layer in tandem
cells. The optical (absorbance maxima, optical band gaps) and electrochemical (HOMO-
LUMO levels, electrochemical band gaps) properties of these newly synthesized materials are
shown in Table 1. To investigate the solid-state interactions and the thin film properties of the
material, we performed X-ray diffraction of pristine NAI-PVP-NAI as well as of thin films of
CBP doped with 3 wt % NAI-PVP-NAI. The recorded XRD spectra revealed the amorphous
nature of the material (Figure S9).
Ab Initio Calculations:
The density functional theory35-36 at the B3LYP37 level of theory using the basis set Lanl2dz
were performed to study the properties of the newly synthesized compound such frontier
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orbital energies and localization. A polarized continuum model (PCM) of theDOchI: l1o0.r10o3f9o/Cr8mNJ06496B
solvent was used. The calculations were performed in Gaussian 09.39 Because the alkyl
chains do not significantly affect the calculation result, we replaced long alkyl chains by a
simple ethyl group to reduce the computational load. From the Fig. 6, it is observed that
while both the HOMO and LUMO of the compound NAI-PVP-NAI are delocalized along
the entire backbone, HOMO is relatively more localized on the core ad LUMO on the end
capping groups.
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Table 1. Optical and electrochemical data for the molecules NAI-PVP-NAI
a
PLmax (nm)
Solution Film Solution Film
367 368 450 460

max (nm)
HOMO LUMO
Compound
(eV)
(eV)
(eV)
(eV)
2.95 2.91
-5.95
-3.04
NAI-PVP-NAI
a Electrochemical band gap = HOMO - LUMO

The computed HOMO and LUMO energy levels of NAI-PVP-NAI were -6.13 and -2.96
eV, respectively. The computed HOMO and LUMO energies and the gap (of 3.17 eV) are in
decent agreement with the measured values. The dihedral angle at the equilibrium geometry
between NAI moiety and central building block PVP is quite large, 50o, suggesting the
potential for the charge transport in more than one dimension.
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Fig. 6: Geometrical configuration (front view and side view) and calculated energy levels
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HOMO and LUMO orbital surfaces of NAI-PVP-NAI using the DFT method.
OLEDs Performance
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NAI-PVP-NAI revealed an excellent photoluminescence peak at 450 nm in solution and
460 nm in thin film upon exciting at 350 nm. The intense blue emission and strong
fluorescence inspired us to fabricate an OLED device to evaluate its potential for real-life
application in solid-state lighting. The multilayer OLED device was fabricated with poly(3,4-
ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as hole injection/transport
layer (HI/HTL), emissive layer (EML) comprising of different dopant concentration of NAI-
PVP-NAI blue emitter in 4,4'-bis(carbazol-9-yl)biphenyl (CBP) host, 2,2',2"-(1,3,5-
benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) as an electron transporting layer
(ETL), and lithium fluoride (LiF) as an electron injecting layer. The schematic representation
of device architecture and the energy level diagram are shown in Fig. 7.
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Fig. 7: a) Chemical structure of the materials and schematic representation of b) device
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architecture and c) energy level diagram of the studied blue OLED device.
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The electroluminescent characteristics of studied sky-blue OLED device are shown in Fig. 8
and Fig. 9 respectively. The obtained device data are shown in Table 2. Fig. 8a shows the J-V
characteristics of OLED devices for different NAI-PVP-NAI dopant guest concentration in
CBP host. The diode characteristics curve clearly exhibits that the studied devices are shunt-
free. The forward bias current of the device with 3wt% of NAI-PVP-NAI doped into the
CBP host is higher than that with the 7 wt % of NAI-PVP-NAI. As the doping concentration
of the guest increases in the host material, the self- aggression of molecules occurs. Because
of this sheet resistance of the device is increased and therefore, the forward bias current is
smaller compared to the other devices. The effect of NAI-PVP-NAI dopant concentration on
electroluminescence (EL) parameters of the blue OLED devices is summarized in Table 2.
The sky blue OLED devices were fabricated with four different NAI-PVP-NAI dopant
concentrations in CBP host. The studied devices revealed that the 1% NAI-PVP-NAI weight
doped device exhibits a sky-blue emission of CIE (0.17, 0.20) with power efficiency of 1.6
lm/W, current efficiency 2.0 cd/A, and an external quantum efficiency (EQE) of 1.5 % at 100
cd/m2. As the doping concentration increased to 3 weight %, the power efficiency, current
efficiency, and the EQE increased by 37, 40 and 6 % at 100 cd/m2 respectively, with a CIE
(0.17, 0.25). Further increase in doping concentration to 5 weight % did not lead to
significant improvement in EL parameters. In contrast, an increase in dopant concentration to
7 weight % leads to decrease in power efficiency, current efficiency, and the EQE by 13.6,
13.8 and 13.3 %, respectively, with a CIE (0.18, 0.28).
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Table 2. Effects of doping concentration on the operation voltage (OV), power efficiency
(p), current efficiency (c), external quantum efficiency (EQE), CIE coordinates, and
maximum luminance of the NAI-PVP-NAI doped with CBP host sky-blue OLED devices.
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Guest Conc.
(%)
EQE
(%)
CIE
(x,y)
L
p
c
max
Operation
Device
(cd/m2)
Voltage (V)
(lm/W) (cd/A)
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@100 and 1,000 cd/m2
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(0.17, 0.2)/
(0.17, 0.21)/
-
1
3
5
7
4.0/5.6/-
4.1/5.0/-
4.1/5.2/-
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1.6/0.7/- 2.0/1.2/- 1.5/1.0/-
2,083
3,620
3,945
4,318
I
(0.17, 0.25)/
(0.17, 0.24)/
-
2.2/1.3/- 2.8/2.0/- 1.6/1.2/-
2.2/1.4/- 2.9/2.3/- 1.5/1.3/-
1.9/1.3/- 2.5/2.0/- 1.3/1.1/-
II
(0.18, 0.27)/
(0.17, 0.26)/
-
III
IV
(0.18, 0.28)/
(0.17, 0.26)/
-
The overall decrease in EL parameters at high dopant concentration is attributed to
concentration quenching. More importantly, exciton generation is reduced due to self-
aggregation of the emitter materials at high dopant concentration.
The operating voltage for all OLED devices was found to be around 3.5 V. The low
operating voltage is attributed to appropriate energy levels and device structure that enables
to inject the charges into EML effectively. LUMO of CBP host is -2.9 eV, which is favorable
for electron injection because it is 0.2 eV lower than that of the electron transport layer,
TPBI, as shown in Fig. 7. Moreover, CBP possesses ambipolar characteristics with excellent
hole and electron mobility.40-41 Even though the difference between HOMO level of CBP
host and PEDOT:PSS hole injection/transport layer is large (~1.0 eV), the number of holes
injected into EML are fully utilized to generate excitons because of its high hole mobility (2 x
10-3 cm2/Vs). Due to this reason, effective charge carrier recombination is localized within
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the EML resulting in high power efficiency, current efficiency, and EQE of the studiedVsiekwyAr-ticle Online
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blue OLED device. The brightness of the devices steadily increased with increase in dopant
concentration. The maximum luminance is 4318 cd/m2 for 7 weight % doped device and it is
shown in Fig. 8b. At 1,000 cd/m2, the power efficiency roll-off significantly observed in 1
weight % device as compared to 3 and 5 weight % devices and similar trend is observed in
EQE of the devices, as shown in Fig. 8c and 8d.
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Luminance (cd/m2)
1000
Luminance (cd/m2)
Fig. 8: Effect of dopant concentration on a) current density, b) luminance, c) power
efficiency, and d) EQE of the studied sky-blue OLED devices.
PL spectra of pristine NAI-PVP-NAI and CBP-host doped with different concentrations of
NAI-PVP-NAI guest obtained by employing 350 nm excitation wavelength are shown in
Figure 9 (a). All the PL spectra are normalized to peak guest NAI-PVP-NAI. It is noticed
that the maximum peak intensity of the neat film is obtained at 463 nm. The red shift is
observed in the host-guest doped film. The CBP host emission is noticed at 375 nm for doped
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thin films, which decreases with increasing NAI-PVP-NAI doping concentration. It
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demonstrates that the host-guest energy transfer depends on the amount of guest
concentration. At 1 weight % guest concentration, the amount of guest molecules present in
the host matrix is not enough to absorb the complete excitation originated from host by any
kind of energy transfer including Forster or Dexter energy transfer. The maximum emission
peak position for NAI-PVP-NAI emitter is roughly located at 477 nm. It is important to
mention that for 1 weight % doped film, the obtained PL spectrum is broader than that of
high concentration doped films. The photoluminescence quantum yield (PLQY) for a neat
film was found to be 92.72 % , whereas that of CBP doped with 3 wt % and 5 wt % NAI-
PVP-NAI was found around to be 91.42 and 88.52%, respectively.
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Fig. 9 (b) shows the EL spectra of studied sky-blue OLED devices, fabricated with four
different NAI-PVP-NAI dopant concentrations in the CBP host. The peaks of the EL spectra
indicate that the emission is from the sky-blue region for all the devices except the 1 weight
% doped device. The CBP host emission is observed at 390 nm along with a broad emission
peak at 470 nm. The intrinsic CBP emission spectrum is attributed to residual excitons in the
CBP host. The intrinsic emission can be minimized by increasing the dopant concentration in
the CBP host. As the dopant concentration increased to 3 weight %, single emission peak is
noticed at 470 nm. It is important to note that, as the concentration is increased from 3 to 7
weight %, the peak position of the EL spectrum shifts from 470 to 480 nm. This
bathochromic-shift is attributed to non-uniform dispersion of NAI-PVP-NAI dopant into
CBP host. NAI-PVP-NAI material is expected to be electron deficient; therefore, we
fabricated the electron only device with ITO/TPBi/NAI-PVP-NAI/TPBi/LiF/Al device
architecture (Figure S10). As shown in Fig. S10, the electron current density of the device
increases with increase in applied bias voltage confirming the excellent electron charge
carrier mobility of NAI-PVP-NAI material.
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1.0
0.8
0.6
0.4
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Neat
1%
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(a)
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Wavelength (nm)
1%
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1.0
0.8
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(b)
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500
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700
Wavelength (nm)
Fig. 9: a) Normalized PL spectra of CBP-host doped with NAI-PVP-NAI guest thin films
and b) effects of dopant concentration on EL spectra of the studied sky-blue OLED device
containing sky-blue emitter NAI-PVP-NAI. (Inset: working OLED device based on CBP
host doped with 3 wt % NAI-PVP-NAI)
Photovoltaic Properties:
To evaluate the potential of the novel compound NAI-PVP-NAI as an acceptor and a donor
in organic solar cells, bulk heterojunction (BHJ) solar cells with conventional structure
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consisting of ITO/PEDOT:PSS/P3HT:NAI-PVP-NAI/LiF/Al and ITO/PEDOT:PSS/NAI-
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PVP-NAI:PC71BM/LiF/Al were fabricated as shown in Fig. 11a. The ITO coated glass and
Al act as anode and cathode, respectively, while PEDOT:PSS and LiF are hole and electron
transporting layers, respectively. NAI-PVP-NAI is used as the electron acceptor when
combined with P3HT and is utilized as the donor when mixed with PC71BM. The active layer
blends (donor to acceptor ratio of 1:1 by weight, 20 mg/ mL) were spin-coated from
chloroform solution under ambient conditions without the use of additives, while the
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thickness of the active layers was kept at about 80 nm. The films were annealed at 80 C for
10 minutes before evaporation of the back contact. The OSC devices were characterized
using standard electrical current density (J) – voltage (V) characteristics. The current density–
voltage characteristics of the devices under solar illumination (AM 1.5G, 1000 W m-2) and
external quantum efficiency (EQE) spectra are shown in Fig. 11b and Fig 11c respectively.
The device characteristics, including short circuit current density (JSC), open circuit voltage
(VOC), fill factor (FF) and power conversion efficiency (PCE), are summarized in Table 3.
The P3HT:NAI-PVP-NAI based OSC devices achieved a VOC of 0.90 V, a JSC of 2.5 mA.cm-
2, a FF of 32% and a PCE of 0.7%. The higher VOC but low current and FF is likely due to the
relatively high lying LUMO of NAI-PVP-NAI approaching the LUMO of the P3HT donor,
limiting charge separation. Meanwhile NAI-PVP-NAI:PC71BM based device exhibited VOC,
JSC, FF and PCE of 0.64 V, 3.3 mA.cm-2, 47% and 1.0%, respectively. The current and FF are
somewhat higher in part due to a better driving force for charge separation in this donor-
acceptor pair. Although the LUMO/HOMO offset between acceptor and donor of
P3HT:NAI-PVP-NAI and NAI-PVP-NAI:PC71BM mixtures is comparable around 2.0 eV,
VOC of NAI-PVP-NAI:PC71BM is remarkably lower than that of P3HT:NAI-PVP-NAI.
Because many factors such as morphology and electrode contact could also affect the VOC
value of organic solar cells.42-43 The EQE spectra of blended films are presented in Fig. 11c.
OCS devices showed EQE spectra from 300-650 nm and 310-530 nm with a maximum value
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of 57.0% at ~390 nm and 33.8% at ~400 nm for the P3HT:NAI-PVP-NAI and NAI-PVP-
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NAI:PC71BM respectively. Both donor and acceptor contribute to the EQE spectrum. The
P3HT:NAI-PVP-NAI based device has a rather lower EQE onset than the NAI-PVP-
NAI:PC71BM, consistent with the broader absorption spectrum of P3HT than PC71BM.
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The surface morphology of the active layer plays an important role in influencing device
performance and was characterized by atomic force microscopy (AFM) technique. The AFM
images of the blend films P3HT:NAI-PVP-NAI and NAI-PVP-NAI:PC71BM with the
weight ratio of 1:1 are shown in Fig. 11. The root mean square (RMS) roughness is 0.73 nm
and 1.83 nm for P3HT:NAI-PVP-NAI and NAI-PVP-NAI:PC71BM respectively.
Table 3. Solar cell performance of P3HT:NAI-PVP-NAI and NAI-PVP-NAI:PC71BM
JSC
(mA/cm2)
Active Layer Blend Ratio
VOC (V)
FF (%)
PCE (%)
P3HT:NAI-PVP-NAI
2.5 ± 0.10 0.90 ± 0.02
3.3 ± 0.20 0.64 ± 0.03
32 ± 1
47 ± 2
0.7 ± 0.10
1.0 ± 0.20
NAI-PVP-NAI:PC71BM
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Fig. 10: (a) Schematic diagram of conventional device geometry; (b) J-V curves of the BHJ
solar cells and (c) external quantum efficiency (EQE) curves of P3HT:NAI-PVP-NAI and
NAI-PVP-NAI:PC71BM based OSCs devices.
Fig. 11: AFM images AFM height images of (a) P3HT:NAI-PVP-NAI, (c) NAI-PVP-
NAI:PC71BM and AFM phase images of (b) P3HT:NAI-PVP-NAI, (d) ) NAI-PVP-
NAI:PC71BM
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CONCLUSIONS
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In summary, we have synthesized a new compound acceptor-donor-acceptor (A-D-A) NAI-
PVP-NAI based on diphenylacetylene as a central building block (donor) and 1,8-
naphthalimide (acceptor) as end capper. We have successfully demonstrated that the use of
NAI-PVP-NAI as a sky-blue emitter for high performance organic light emitting diode via
solution process technique. At 3 weight % dopant concentration of NAI-PVP-NAI with CBP
host, the fabricated OLED device demonstrate high power efficiency p = 2.2 lm/W, current
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efficiency,c= 2.8 cd/A, and EQE = 1.6 % at 100 cd/m2. The obtained OLED results are very
promising and NAI-PVP-NAI can be used as promising sky blue emitter to be used in
display and solid-state lighting. Since the HOMO and LUMO energy level of this material is
appropriate to be an ambipolar material so we explored to use NAI-PVP-NAI as both
electron donor and electron acceptor in BHJ OSC devices. The device based on NAI-PVP-
NAI as an acceptor showed the best PCE of 0.7% with VOC of 0.90 V, JSC of 2.5 mA.cm-2,
and FF of 32%. Meanwhile, when NAI-PVP-NAI is used as donor and combined with
conventional acceptor PC71BM, the device exhibited VOC, JSC, FF and PCE of 0.64 V, 3.3
mA.cm-2, 47% and 1.0%, respectively. All above results clearly indicates that NAI-PVP-NAI
is a versatlie organic semiconductor and can play a three role as an emiter in OLED and
donor or acceptor in OPV devices. Such a multifunctionality aspect in a single material is rare
feature and this makes this class of A-D-A materials useful for range of organic electronic
devices.
EXPERIMENTAL SECTION
Materials and Instruments
All staring materials were purchased commercially as analytical reagents and used
directly without any further purification. Synthesized compounds were characterized
1
by H-NMR and 13C-NMR spectrum, which were obtained with a Varian-400 or
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Bruker-600 spectrometer. High resolution mass spectra were recorded on an Orbitrap
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Elite mass spectrometer (Thermo Fisher Scientific) equipped with an atmospheric
pressure chemical ionisation (APCI) source, operating in the positive ion mode at a
resolution of 120,000 (at m/z 400). Thermal analysis was performed using a Pegasus
Q500TGA thermogravimetric analyzer under nitrogen atmosphere at a heating rate of
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10 C/min. Differential scanning calorimetry (DSC) was conducted under nitrogen
using a Chimaera instrument Q100 DSC. The sample was heated at 10 oC/min from 30
oC to 300 oC. Absorption spectra were recorded on Carry50 UV-Vis
Spectrophotometer. Cyclic voltammetry was carried out in tetrabutylammonium
hexafluorophosphate (0.1 M) as supporting electrolyte in dichloromethane and three-
electrode cell with a glassy carbon working electrode, a platinum-wire counter
electrode and an Ag/AgCl reference electrode was employed. X-ray diffraction
measurement was carried out with Bruker D8, which was operated at 30 kV and 10
o
mA with step of 0.02o from 5-50 . Photoluminescence quantum yield (PLQY) was
determined according to the procedure followed by Mello et al. 44. The absolute PLQY
was obtained via a calibrated integrating sphere system (Labsphere) attached with a
CCD spectrograph. ±2% measurement error is considered because of the stability of
the excitation source and the radiometric calibration source.
Device Fabrication
Organic light emitting diode
OLED devices were fabricated on pre-cleaned UV-Ozone treated, 3x3 cm2, 120 nm thick
pattern ITO substrates. PEDOT:PSS (AI4083) purchased from Uni-onward crop was used
without any purification to deposit a 40 nm hole injection/transport layer. It was spin-coated
o
on cleaned ITO substrate at 4000 rpm for 20s and dried at 120 C for 20 min. The host-guest
mixture solution consists of CBP and NAI-PVP-NAI was prepared in tetrahydrofuran
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solvent at 40 oC and subjected to sonication for 40 min. A 25 nm thick emission layer (EVMiewLAr)ticle Online
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was deposited via spin coating on ITO/PEDOT:PSS substrate at 2500 rpm for 20s in nitrogen
filled glove box. Electron transporting layer (ETL), 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-
1-H-benzimidazole) (TPBi), LiF and Al having thicknesses of 40, 1 and 100 nm,
respectively, were deposited via thermal evaporation technique under high vacuum (10-6 torr
). The EL characteristic of final devices measured in ambient atmosphere without
encapsulation. The current-voltage-luminance characterization was done by computer
interfaced Keithley 2400 electrometer and Minolta CS-100A luminance meter. PR-655
spectroradiaometer was used to record the emission spectra. The active OLED device area
was 0.09 cm2 calculated from the pattern ITO substrate and masked Al electrode.
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Organic Solar Cells
Bulk-heterojunction (BHJ) solar cells were fabricated by spin coating of 30-nm-thick layers
of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS; Baytron AI 4083
from HC Starck) on ITO (Indium Tin Oxide) patterned glass substrates which were washed
by acetone, and 2-propanol in an ultrasonication bath and UV/ozone-treated. The
PEDOT:PSS films were baked at 140 °C for 10 min in air. An active layer of the device
consisting of a blend of P3HT/NAI-PVP-NAI or NAI-PVP-NAI/PC71BM (20 mg/mL) with
a ratio of 1:1 was then spin coated from chloroform solvent with a thickness of ~ 80 nm.
Then, the active layer was annealed at 80 oC for 10 min. The films were transferred to a metal
evaporation chamber and top electrode consist of LiF (1 nm) and aluminum (100 nm) were
deposited through a shadow mask (device area was 0.1 cm2) at approximately 1 x10-6 torr.
Film thickness was determined by Veeco Dektak 150+Surface Profiler. The current density-
voltage measurements of the devices were carried out using a 1 kW Oriel solar simulator with
an AM 1.5G filter as the light source in conjunction with a Keithley 2400 source
measurement unit. Solar measurements were carried out under 1000 W/m2 AM 1.5G
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illumination conditions. For accurate measurement, the light intensity was calibrDaOteI:d10u.10s3i9n/gC8NaJ06496B
reference silicon solar cell (PV measurements Inc.) certified by the National Renewable
Energy Laboratory. Device fabrication and characterizations were performed in ambient
environment without any encapsulation.
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Morphology Characterisation
The AFM samples were prepared by spin-coating the blend solution on the PEDOT:PSS
layer. Tapping-mode AFM (NanoScope III, Dimension, Digital Instrument Inc.) was used
with commercially available tapping mode tips.
Synthesis
Synthesis of Compound 1
Triphenylphosphine (26.23 g, 0.1 mol) was dissolved in DCM (200 mL) under argon for 15
minutes. Then the reaction mixture was cooled to 0 oC and followed by the addition of
bromine (7.2 mL, 0.14 mol). After 2-butyloctan-1-ol (18.63 g, 0.1 mol) was added dropwise
via an additional funnel over 30 minutes, the reaction solution was stirred at room
temperature for overnight under argon. The organic solvent was removed under the reduced
pressure, and the residue was extracted with excess amount of hexane. The hexane was
evaporated and the resulting crude yellow oil was purified by silica gel column eluting with
1
hexane. The title compound was isolated as colourless oil (23.75 g, 95.3%). H NMR (400
MHz, CDCl3, ppm):  3.45-3.44 (d, J = 4.8 Hz, 2H), 1.61-1.56 (m, 1H), 1.28 (m, 16H), 0.92-
0.87 (m, 6H).
Synthesis of Compound 2
To a mixture of 1-bromo-2-butyloctane (7.48 g, 30.0 mmol) and potassium phthalimide (5.93
g, 32 mmol), 45 mL of DMF was added and stirred at 90 ºC for 10 hours. Then the reaction
mixture was cooled to room temperature and extracted with DCM and water. The combined
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organic layer was washed with 0.2 N KOH aqueous (100 mL), water (100 mL) aDnOdI:s10a.t1u03r9a/tCe8dNJ06496B
NH4Cl aqueous (100 mL). The organic layer was dried over anhydrous Na2SO4 and
concentrated by evaporation. The crude product was purified by silica gel chromatography
using a mixture of hexane and chloroform as eluent to obtain product as colourless oil (8.52
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g, 90%). H NMR (400 MHz, CDCl3, ppm):  7.85-7.83 (m, 2H), 7.71-7.69 (m, 2H), 3.57-
3.55 (d, J = 7.2 Hz, 2H), 1.87 (m, 1H), 1.26 (m, 16H), 0.88-0.83 (m, 6H).
Synthesis of Compound 3
To a solution of 2-(2-butyl-octyl)isoindole-1,3-dione (6.31 g, 20.0 mmol) in MeOH (90 mL),
hydrazine monohydride (3.0 g, 60.0 mmol) was added and refluxed overnight. After cooling
to room temperature, the mixture was evaporated under vacuum. The residue was dissolved
in DCM and washed with 10 wt% KOH aq. (100 mL x2) and saturated NaCl aq. (100 mL
x2). The organic layer was dried with anhydrous Na2SO4 and concentrated by vacuum
evaporation to obtain colourless oil (3.89 g). This compound was used for subsequent
1
reaction without further purification. H NMR (400 MHz, CDCl3, ppm):  2.56-2.55 (d, J =
2.8 Hz, 2H), 1.22 (m, 19H), 0.87-0.82 (m, 6H).
Synthesis of Compound 5
To a solution of 4-bromo-1,8-naphthalic anhydride (4.16 g, 15.0 mmol) in DMF (60 mL), 2-
butyl-octylamine (3.34 g, 18.0 mmol) was added dropwise. Then the reaction mixture was
heated to 110 °C and stirred under nitrogen overnight. Then the reaction mixture was cooled
to room temperature and concentrated using a rotary evaporator. The product was
precipitated into 1N HCl, filtered off, and washed with 1N HCl. The crude product was
purified by silica gel column chromatography using ethyl acetate and hexane mixture (vol.
ratio 1:9) as an eluent. After recrystallization from the eluent mixture of the solvents, yellow
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crystals were obtained (4.0 g, 60%). H NMR (400 MHz, CDCl3, ppm):  8.66-8.64 (d, J =
7.2 Hz, 1H), 8.57-8.55 (d, J = 8.8 Hz, 1H), 8.41-8.40 (d, J = 8.0 Hz, 1H), 8.05-8.03 (d, J =
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8.0 Hz, 1H), 7.86-7.82 (t, J = 7.6 Hz , 1H), 4.10-4.08 (d, J = 7.2 Hz, 2H) 1.97 (m, 1H),V1ie.w2A2rticle Online
DOI: 10.1039/C8NJ06496B
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(m, 16H), 0.87-0.80 (m, 6H).
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Synthesis of Compound 6
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A mixture of 1,2-bis(4-bromophenyl)ethyne (2.76 g, 8.20 mmol), bis(pinacolato)diboron
(5.21 g, 20.5 mmol), Pd(dppf)Cl2 (603 mg, 0.825 mmol) and potassium acetate (2.95 g, 0.825
mmol) in 1,4-dioxane (40 mL) was stirred at room temperature for 30 minutes. The mixture
o
was then heated at 84 C for 48 hours under argon atmosphere. After cooling to room
temperature, the reaction mixture was diluted with DCM, washed with water and brine, and
dried over anhydrous Na2SO4. After removal of the solvent, the reaction mixture was purified
by flash chromatography on silica gel (chloroform: hexane = 1:1). This white-off powder was
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then washed by methanol to afford title compound (2.54 g, 72%). H NMR (600 MHz,
CDCl3, ppm): 7.72-7.70 (d, J = 8.2 Hz, 4H), 7.46-7.45 (d, J = 8.2 Hz, 4H), 1.27 (s, 24H).
Synthesis of NAI-PVP-NAI
In
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round bottom flask 1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
mg, 0.279 mmol), 6-bromo-2-(2-butyloctyl)-1H-
yl)phenyl)ethyne (120
benzo[de]isoquinoline-1,3(2H)-dione (310 mg, 0.698 mmol), and 2 M aqueous K2CO3
solution (9 mL) were dissolved in degassed toluene (18 mL). The solution was purged
with argon for 30 minutes, then tetrakis(triphenylphosphine)palladium (16 mg 0.014
o
mmol) was added. The reaction was stirred at 110 C for 2 days. Then the reaction
mixture was cooled to room temperature and extracted with chloroform and water. The
organic layer was dried over anhydrous Na2SO4 and concentrated by evaporation. The
crude product was purified through column chromatography (SiO2, hexane:
chloroform:EA= 10:2:1). Off-white powder was then washed by hot acetone to afford
title compound (180 mg, 70%). 1H NMR (600 MHz, CDCl3, ppm):  8.60-8.58 (t, J = 7.2
Hz, 4H), 8.22-8.20 (dd, J1 = 8.4 Hz, J2 = 0.6 Hz, 2H), 7.70-7.66 (m, 8H), 7.49-7.47 (d, J
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= 8.4 Hz, 4H), 4.09-4.07 (d, J = 7.2 Hz, 4H), 1.95 (m, 2H), 1.18 (m, 32H), 0.82-0.76 (mV,iew Article Online
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12H). 13C NMR (150 MHz, CDCl3, ppm):  164.61, 164.42, 145.84, 139.0, 132.19,
131.96, 131.34, 130.83, 130.03, 129.86, 128.76, 127.83, 127.07, 123.37, 123.07, 122.15,
90.20, 44.59, 36.63, 31.87, 31.74, 31.42, 29.76, 28.75, 26.52, 23.12, 22.67, 14.14, 14.11.
HRMS (EI, m/z) calcd.: 905.5252 (M+). Found: 905.5287.
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ASSOCIATED CONTENT
Supporting Information
1H-NMR and 13C-NMR spectra of all synthesized compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sonar.prashant@qut.edu.au.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
ACKNOWLEDGMENT
T.T.D is thankful to QUT for offering here QUTPRA scholarship to conduct her
research work. Special thanks to Jegadesan Subbiah for helping in manuscript preparation
and device fabrication discussion. We are thankful to CRC for Polymers, Central
Analytical Research Facility (CARF), and Institute of Future Environments,
Queensland University of Technology (QUT) for providing equipment support. P.S. is
thankful to QUT for the financial support from the Australian Research Council
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(ARC) for the Future Fellowship (FT130101337) and QUT core fundDinOIg: 10.(1Q039U/CT8N/J06496B
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322120-0301/07). S. M. is supported by the Ministry of Education of Singapore.
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