Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal chromen-2-one functionality for efficient bulk-heterojunction devices

Article abstract of DOI:10.1016/j.dyepig.2017.07.047

Two simple semiconducting donor–acceptor–acceptor (D–A₁–A) modular, small molecule, non-fullerene electron acceptors, 2-(4-(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (P2) and 2-(4-(3,3-dicyano-1-(4-(diphenylamino)phenyl)-2-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (P3), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of P2 and P3 were directly compared with those of a structural analogue, 3-((4-((4-(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-one (P1), which was designed based on a D–A format. All of these new materials comprised an electron rich triphenylamine (TPA) donor core (D) and electron deficient chromen-2-one terminal core (A). In the simple D–A system, TPA and chromenone were the terminal functionalities, whereas in the D–A₁–A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A₁ units by keeping the D/A units constant. The inclusion of A₁ was primarily done to induce cross-conjugation within the molecular backbone and hence to generate low band gap targets. The physical and optoelectronic properties were characterized by ultraviolet–visible (UV–Vis), thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited broadened absorption spectra, for instance panchromatic absorbance in case of P3, excellent solubility and thermal stability, and energy levels matching those of the conventional and routinely used donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with P1, P2 and P3 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: P3-based device showed an impressive enhanced power conversion efficiency of 4.21%, an increase of around two-fold with respect to the efficiency of the best P3HT: P1-based device (2.28%). Our results clearly demonstrate that the D–A₁–A type small molecules are promising non-fullerene electron acceptors in the research field of organic solar cells.

Full text of DOI:10.1016/j.dyepig.2017.07.047

Accepted Manuscript
Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal
chromen-2-one functionality for efficient bulk-heterojunction devices
Pedada Srinivasa Rao, Akhil Gupta, Sidhanath V. Bhosale, Ante Bilic, Wanchun
Xiang, Richard A. Evans, Sheshanath V. Bhosale
PII:
S0143-7208(17)31088-4
10.1016/j.dyepig.2017.07.047
DYPI 6136
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 May 2017
Revised Date: 20 July 2017
Accepted Date: 20 July 2017
Please cite this article as: Srinivasa Rao P, Gupta A, Bhosale SV, Bilic A, Xiang W, Evans RA,
Bhosale SV, Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal chromen-2-
one functionality for efficient bulk-heterojunction devices, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2017.07.047.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal chromen-
2
-one functionality for efficient bulk-heterojunction devices
a,b
c
a
d
Pedada Srinivasa Rao, Akhil Gupta,* Sidhanath V. Bhosale,* Ante Bilic, Wanchun
e
f
g
Xiang, Richard A. Evans and Sheshanath V. Bhosale*
a Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500007, Telangana, India; Email: bhosale@iict.res.in
b Academy of Scientific and Innovative Research (AcSIR), CSIR-IICT, Hyderabad 500007,
Telangana, India
c Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216 Australia;
Tel: +61 3 5247 9542; E-mail: akhil.gupta@deakin.edu.au
d Data61 CSIRO, Molecular and Materials Modelling, Docklands, Victoria 8012 Australia
e State Key Laboratory of Silicate Materials for Architectures, Wuhan University of
Technology, 122 Luoshi Rd, Wuhan 430070 Hubei, P. R. China
f CSIRO Manufacturing, Bayview Avenue, Clayton South, Victoria 3169 Australia
g School of Science, RMIT University, GPO Box 2476, Melbourne Victoria 3001 Australia;
Tel: +61 3 9925 2680; E-mail: sheshanath.bhosale@rmit.edu.au

ACCEPTED MANUSCRIPT
Abstract
Two simple semiconducting donor–acceptor–acceptor (D–A –A) modular, small molecule,
1
non-fullerene electron acceptors, 2-(4-(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-
yl)ethynyl)phenyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (P2) and 2-(4-(3,3-dicyano-1-(4-
(diphenylamino)phenyl)-2-(4-((2-oxo-2H-chromen-3-
yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile
(P3),
were
designed, synthesized and characterized for application in solution-processable bulk-
heterojunction solar cells. The optoelectronic and photovoltaic properties of P2 and P3 were
directly
compared
with
those
of
a
structural
analogue,
3-((4-((4-
(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-one (P1), which was
designed based on a D–A format. All of these new materials comprised an electron rich
triphenylamine (TPA) donor core (D) and electron deficient chromen-2-one terminal core
(A). In the simple D–A system, TPA and chromenone were the terminal functionalities,
whereas in the D–A –A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane
1
(
TCNQ) derived functionalities were incorporated as A units by keeping the D/A units
1
constant. The inclusion of A was primarily done to induce cross-conjugation within the
1
molecular backbone and hence to generate low band gap targets. The physical and
optoelectronic properties were characterised by ultraviolet–visible (UV–Vis),
thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry.
These new materials exhibited broadened absorption spectra, for instance panchromatic
absorbance in case of P3, excellent solubility and thermal stability, and energy levels
matching those of the conventional and routinely used donor polymer poly(3-hexyl
thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with
P1, P2 and P3 as non-fullerene electron acceptors. Studies on the photovoltaic properties
revealed that the best P3HT: P3-based device showed an impressive enhanced power

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conversion efficiency of 4.21%, an increase of around two-fold with respect to the efficiency
of the best P3HT: P1-based device (2.28%). Our results clearly demonstrate that the D–A –A
1
type small molecules are promising non-fullerene electron acceptors in the research field of
organic solar cells.
Keywords: solution-processable; triphenylamine; chromen-2-one; bulk-heterojunction;
donor–acceptor–acceptor; solar cells
1
. Introduction
There is a constant demand for the development of renewable technologies as humanity is
st
progressing through 21 century. The survival of the human race is often associated with the
supply of endless energy resources, and the natural resources such as wind, sunlight and
water can provide a definitive answer. Of such natural resources, sunlight or solar power
seems to be a strong candidate given the fact that our sun is still in its young age. Utilization
of solar energy or photovoltaic effect can in fact be dated back to Becquerel's 1839 work
when he observed similar effects while studying liquid electrolytes [1]. However,
developmental work to fabricate solar cell devices started with Tang’s 1989 work [2] and
provided a proof of concept that a variety of organic, inorganic or even tandem solar cells can
be a reality in the near future. Since then, a variety of solar cells have been designed,
fabricated, and have shown that the solar power indeed is the way to go as far as renewable
technologies are concerned. Among such solar cells, organic, or otherwise termed bulk-
heterojunction, solar cells [3] and dye sensitized solar cells [4] are the most studied, well
understood and widely accepted strategies. As far as bulk-heterojunction (BHJ) devices are
concerned, two types of developments – materials and device fabrication – have attracted
considerable attention over the past two decades. Therefore, it is apparent that this technology

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is worth studying and developing given the fact that such devices do promise multiple
advantages such as light weight, low cost and flexibility [5–7].
A typical BHJ architecture is an interlaced network of organic donor and acceptor
semiconducting materials which is formed during device fabrication mainly via solution
processing. Conventionally, semiconducting donor polymers, such as poly(3-hexylthiophene)
(
P3HT), and acceptors, such as fullerene derivatives, either [6,6]-phenyl-C -butyric acid
61
methyl ester (PC BM) or its C analogue (PC BM), have been employed to attain a strong
6
1
71
71
understanding of material design and device morphology [5–7]. As a result, conjugated
polymers and small molecules as alternative donors, and non-fullerene acceptors as
alternative accepting functionalities have been developed over time. It is judicious to point
out that the fullerene derivatives have been extensively used as acceptor materials due to their
intrinsic properties such as high electron affinity, isotropy of charge transport, and the ability
to form favourable nanoscale networks with a variety of donor materials [8–10]. Despite the
fact that fullerene acceptors are dominant in the research field of organic photovoltaics and
have been widely explored, they are visibly afflicted with a number of potential limitations
such as weak absorption in the visible spectrum, poor photo-stability in air, and restricted
electronic tuning via structural modification, to name a few [11,12]. Moreover, a large
electron affinity can result in low open-circuit voltage [13]. Such disadvantages with
fullerenes provide a strong support to the fact that alternate structural formats must be
designed and developed, for instance non-fullerene acceptors, which not only exhibit most of
the properties that make fullerene acceptors appealing but must satisfy the requirements such
as efficient absorption over visible spectrum, excellent solubility, high charge carrier mobility
and matching energy levels with those of a variety of donors.

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It is evident that past few years have seen a dramatic surge in the development of non-
fullerene acceptors. High power conversion efficiency (PCE) numbers, typically ranging 11–
1
2%, have been reported so far for organic solar cells based on non-fullerene acceptors
[
14,15]. Though this advancement is encouraging, incentives remain to develop materials
which will not only have better properties than fullerenes, but will also have crucial
characteristics such as solubility, chemical and thermal stability, and matching energy levels
with donors. Moreover, a newly designed non-fullerene acceptor should be compatible with
the commercially available polymeric donors such as P3HT. As per our understanding of this
research area, we theorize that a potential non-fullerene acceptor must be a conjugated
structure, either via the conjunction of a variety of building blocks (primarily donors and
acceptors) or should possess a rigid, extended fused-ring backbone. The advantages of using
donor/acceptor, or otherwise termed push/pull, functionalities lie in the fact that the
combination of such formats can reduce the optical bandgap, broaden absorption in the entire
visible region of the solar spectrum, and tune energy levels.
Having said this, we and others have been successful in demonstrating the use of a number of
push/pull formats to design a number of potential non-fullerene electron acceptors where
such targets have displayed promising device performance [16–34]. We realised that
functionalities
such
as
triphenylamine
(TPA),
diketopyrrolopyrrole
(DPP),
naphthalenediimide (NDI), and more recently, functionalities which contain cyano groups,
such as dicyanoindenedione, tetracyanoethylene (TCNE) and tetracyanoquinodimethane
(TCNQ), are promising building blocks (donors and acceptors), and have been used in recent
past. However, their amalgamation can be done in a variety of fashions. In this contribution,
we wish to report a new combination where we have used a new acceptor unit, 2-
chromenone, along with the terminal donor TPA. Overall, three targets were designed based

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on TPA- and chromenone-functionalized molecular systems of the types D–A and D–A –A.
1
In the simple D–A system, TPA and chromenone were the terminal functionalities, whereas
in the D–A –A system, TCNE and TCNQ derived functionalities were incorporated as A1
1
units by keeping the D/A units constant. It has been shown in the literature that cross-
conjugation is an efficient strategy to design low highest occupied molecular orbital–lowest
unoccupied molecular orbital (HOMO–LUMO) gap molecular systems [35–37]. Moreover,
the presence of cyano group in a molecular backbone can promote the formation of a
crystallite architecture and can increase electron affinity, thereby favouring efficient charge
transport and broadening absorption profile towards a longer wavelength region, near infrared
in particular. Such advantages are likely responsible for generating a focused interest in
designing of chromophores based on accepting units comprising cyano groups, and are an
inspiration to pursue research on the development of new and potential non-fullerene electron
acceptors.
Fig.
1
reveals the molecular structures of new materials, 3-((4-((4-
(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-one
(P1),
2-(4-
(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)buta-1,3-diene-
1
,1,4,4-tetracarbonitrile (P2) and 2-(4-(3,3-dicyano-1-(4-(diphenylamino)phenyl)-2-(4-((2-
oxo-2H-chromen-3-yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile
P3), where P1 was based on the D–A type, and P2 and P3 were based on the D–A –A
(
1
modular design. The optoelectronic and photovoltaic properties of P2 and P3 were directly
compared with P1. It was observed that the introduction of TCNE and TCNQ functionalities
in P2 and P3, respectively, resulted in strong intramolecular charge transfer transition when
compared with P1. The introduction of TCNQ in particular exhibited a broader absorption
profile extending up to 1000 nm, primarily ascribed to the stronger electron withdrawing

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nature of TCNQ relative to TCNE. Both P2 and P3 exerted low optical band gaps and low
lying HOMO energy levels when compared with P1. All of the investigated materials
displayed high chemical and thermal stability, and excellent solubility. Solution-processable
BHJ devices were fabricated with P1, P2 and P3 as non-fullerene electron acceptors. Studies
on the photovoltaic properties revealed that the best P3HT: P3-based device showed an
impressive enhanced power conversion efficiency of 4.21%, an increase of greater than two-
fold with respect to the efficiency of the best P3HT: P1-based device (2.28%). To our
knowledge, there are no reports on the connected use of TPA and 2-chromenone
functionalities where such functionalities have been used to design non-fullerene acceptor
formats comprising either TCNE or TCNQ units. Our results indicate that these 2-
chromenone-based small molecules are potential acceptors for efficient organic solar cells
using a commercially available donor polymer P3HT and can further be explored with a
variety of donors. The present work is a continuation of our efforts made in the design and
development of small molecular chromophores for organic photovoltaic applications [38–41].
>
Fig. 1<
2
. Experimental details
2
.1. Materials, methods and device details
All the reagents and chemicals used, unless otherwise specified, were purchased from Sigma-
Aldrich Co., Bengaluru, Karnataka, India, and were used without any purification. AR grade
1
solvents were purchased from Finar chemicals Limited, India and used as received. H and
1
3
C NMR spectra were recorded at 300, 400 or 500 MHz as indicated. The following
abbreviations were used to explain multiplicities: s = singlet, d = doublet, t = triplet, q =
1
3
quartet, m = multiplet, br = broad, dd = doublet of doublets, and dt = doublet of triplets. C

ACCEPTED MANUSCRIPT
1
13
NMR spectra were recorded at 75 or 101 MHz, as indicated. H and C chemical shifts were
calibrated using residual non-deuterated solvent as an internal reference and are reported in
parts per million (δ) relative to tetramethylsilane (δ = 0). All experiments were performed in
deuterated chloroform (CDCl ). ESI was used to ionize samples with a spray voltage. The
3
mass spectrometry measurements were performed using either Fourier transform-based high
resolution mass spectrometry or via atmospheric-pressure chemical ionization (APCI)
experiments which were carried out on FTMS, ionized by APCI from an atmospheric solids
analysis probe (ASAP). FTIR spectra were recorded using Thermo Nicolet Nexus 670
spectrometer in the form of non-hygroscopic KBr pellets. The UV–Vis spectra were recorded
using a Shimadzu UV-1800 spectrophotometer at room temperature. The TGA experiments
were carried out using Q-500 TGA instrument with nitrogen as a purging gas. The samples
were heated to 800 °C at 10 °C/minute under nitrogen atmosphere. AFM topographic maps
were performed directly on the active layers of P3HT: P1/P2/P3 blends using an Asylum
Research MFP-3D-SA instrument. The AFM was run in intermittent contact mode (tapping
mode) using MicroMasch NSC18 tips (typical resonant frequency ~100 kHz, typical probe
radius ~10 nm and typical aspect ratio 3:1). Transmission electron microscopy samples were
prepared by solvent evaporation on a holey carbon grid and micrographs were produced
using a JOEL 1010 100 kV TEM.
Though the general details of spectroscopic measurements and characterization were reported
previously [42], specific details of device fabrication are reported herein. Indium tin oxide
(
ITO)-coated glass (Kintek, 15 Ohms per square) was cleaned by standing in a stirred
solution of 5% (v/v) Deconex 12PA detergent at 90 °C for 20 minutes. The ITO-coated glass
was then successively sonicated for 10 min each in distilled water, acetone and isopropanol.
The substrates were then exposed to a UV–ozone clean at room temperature for 10 min.
UV/ozone cleaning of glass substrates was performed using a Novascan PDS-UVT,

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UV/ozone cleaner with the platform set to maximum height. The intensity of the lamp was
2
greater than 36 mW/cm at a distance of 10 cm. At ambient conditions the ozone output of
the UV cleaner is greater than 50 ppm. Aqueous solutions of PEDOT/PSS (HC Starck,
Baytron P AI 4083) were filtered (0.2 µm RC filter) and deposited onto glass substrates in air
by spin coating (Laurell WS-400B-6NPP lite single wafer spin processor) at 5000 rpm for 60
s to give a layer having a thickness of 40 ± 5 nm. The PEDOT/PSS layer was then annealed
on a hotplate in a glove box at 145 °C for 10 min. For OPV devices, the newly synthesized
organic n-type materials and P3HT were dissolved in individual vials by magnetic stirring.
Blend ratios and solution concentrations were varied to optimize device performance. The
solutions were then combined, filtered (0.2 µm RC filter) and deposited by spin coating onto
the ITO-coated glass substrates inside a glove box. The coated substrates were then
transferred (without exposure to air) to a vacuum evaporator inside an adjacent nitrogen-filled
glove box. Samples were placed on a shadow mask in a tray. The area defined by the shadow
2
mask gave device areas of exactly 0.2 cm . Deposition rates and film thicknesses were
monitored using a calibrated quartz thickness monitor inside the vacuum chamber. Layers of
calcium (Ca) (Aldrich) and aluminium (Al) (3 pellets of 99.999%, KJ Lesker) having
thicknesses of 20 nm and 100 nm, respectively, were evaporated from open tungsten boats
-6
onto the active layer by thermal evaporation at pressures less than 2 × 10 mbar. A
connection point for the ITO electrode was made by manually scratching off a small area of
the active layers. A small amount of silver paint (Silver Print II, GC Electronics, part no.: 22-
0
23) was then deposited onto all of the connection points, both ITO and Al. The completed
devices were then encapsulated with glass and a UV-cured epoxy (Summers Optical, Lens
Bond type J-91) by exposing to 365 nm UV light inside a glove box for 10 min. The
encapsulated devices were then removed from the glove box and tested in air within 1 h. The
OPV devices were tested using an Oriel solar simulator fitted with a 1000 W xenon lamp

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2
filtered to give an output of 100 mW/cm at simulated AM 1.5. The lamp was calibrated
using a standard, filtered silicon (Si) cell from Peccell Limited which was subsequently cross-
calibrated with a standard reference cell traceable to the National Renewable Energy
Laboratory. The devices were tested using a Keithley 2400 Sourcemeter controlled by
labview software. Film thicknesses were determined using a Dektak 6M Profilometer.
2
.2. Synthesis and characterization
Compound P1 was synthesized by reacting 4-((4-ethynylphenyl)ethynyl)-N,N-
diphenylaniline (compound 6) with 3-iodo-2H-chromen-2-one using Sonogashira coupling
reaction in dry tetrahydrofuran (THF) (Scheme 1). Compounds P2 and P3 were synthesized
by reacting P1 with TCNE and TCNQ, respectively (Scheme 2). The final compounds were
purified using silica gel chromatography, characterized spectrally, and their physical
properties were investigated. The materials were prepared in moderate to high yields and
were found to be highly soluble in a variety of conventional organic solvents such as
chlorobenzene, chloroform, dichloromethane and toluene. High solubility of organic
materials is an essential criterion for the fabrication of solution-processable organic
photovoltaic devices and newly designed materials P1, P2 and P3 fulfil this criterion. The
compounds 2, 3 and 4 were synthesised following the procedures reported in the literature
[
43–44].
>
>
Scheme 1<
Scheme 2<
2
.2.1 Synthesis of N,N-diphenyl-4-((4-((triisopropylsilyl)ethynyl)phenyl)ethynyl)aniline (5)
To the stirred solution of compound 4 (2.00 g, 7.07 mmol) in dry toluene (30 mL) was added
4
-iodotriphenylamine (2.62 g, 7.07 mmol), and the resulting reaction solution was stirred for
1
5 min under nitrogen atmosphere. N,N–diisopropylethylamine and catalytic amounts of

ACCEPTED MANUSCRIPT
bis(triphenylphosphine)palladium(II) chloride and copper iodide (ca. 5 mol% each) were
added to the reaction mixture. The reaction mixture was stirred overnight at room
temperature. After the completion of reaction (TLC monitoring), solvent was removed under
reduced pressure and the crude product was purified via silica gel column chromatography
(eluent: hexane) to afford compound 5 as a colourless liquid (3.30 g, yield: 91%). FTIR (neat;
-1
cm ): 3441, 2942, 2864, 2210, 2152, 1706, 1513, 1493, 1315, 1281, 1017, 835, 753, 696,
1
6
4
73, 507; H NMR (400 MHz, CDCl ) δ 7.42 (s, 4H), 7.37 (d, J = 8.8 Hz, 2H), 7.29–7.27 (m,
3
1
3
H), 7.12 (d, J = 7.4 Hz, 4H), 7.08 (t, J = 7.3 Hz, 2H), 7.00 (d, J = 8.8 Hz, 2H); C NMR
(
101 MHz, CDCl ) δ 148.0, 147.1, 132.5,131.9, 131.5, 129.3, 125.0, 123.5, 122.1, 115.6,
3
+
1
06.7, 92.5, 88.4, 29.6, 18.6, 11.3. ESI-mass (m/z) [M] : 525; HRMS calculated for
+
C H NSi [M+H] : 526.29245; found: 526.29362.
37
40
2
.2.2 Synthesis of 4-((4-ethynylphenyl)ethynyl)-N,N-diphenylaniline (6)
The compound 5 (1.00 g, 1.90 mmol) was dissolved in dry THF (10 mL) and tetra-n-
butylammonium fluoride (1.10 mL, 3.80 mmol, 0.1 M in THF) was added slowly at room
temperature. The reaction was monitored by TLC until all of the starting material
disappeared. After evaporation of solvent under reduced pressure, the residue was dissolved
in dichloromethane, washed with water followed by brine, and purified by column
chromatography (silica gel; hexane: DCM 9:1) to afford titled compound 6 (681 mg, 70%) as
-1
an off-white solid; mp: 161–162 °C; FTIR (neat; cm ): 3442, 2925, 2205, 1587, 1518, 1486,
1
1
316, 1277, 842, 758, 696, 534; H NMR (400 MHz, CDCl ) δ 7.44 (s,4H), 7.37 (d, J = 8.8
3
Hz, 2H), 7.29–7.27 (m, 4H), 7.12 (d, J = 8.6 Hz, 2H), 7.08 (t, J = 7.3 Hz, 2H), 7.01 (d, J =
1
3
8
.8 Hz, 2H), 3.15 (s, 1H); C NMR (101 MHz, CDCl ) δ 148.1, 147.0, 132.5, 132.0, 131.2,
3
1
29.3, 125.0, 124.1, 123.6, 122.0, 121.4, 115.5, 91.7, 88.1, 83.3, 78.7; ESI-mass (m/z)
+
+
[
M+H] : 370; HRMS calculated for C H N [M+H] : 370.15903; found: 370.15971.
28 20

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2
.2.3 Synthesis of 3-((4-((4-(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-
one (P1)
Compounds 6 (0.50 g, 1.3 mmol) and 3-iodo-2H-chromen-2-one (0.375 g, 1.3 mmol) were
added to the mixture of N,N-diisopropylethylamine (5 mL) and dry THF (10 mL) at room
temperature. The reaction mixture was subjected to several vacuum/nitrogen cycles and
catalytic amounts (ca. 5 mol %) of tetrakis(triphenylphosphine)palladium (0) and copper
iodide were added. The reaction mixture was stirred for 30 min at room temperature followed
by stirring at 65 °C for 24 h. The solvent was evaporated under reduced pressure and the
crude solid was filtered through Celite using dichloromethane. The organic layer was washed
with water followed by brine, dried over anhydrous MgSO and recovered to get a crude
4
mass which was subjected to column chromatography on silica using DCM: hexane (7: 3) to
-
afford titled target P1 as a yellow solid (0.49 g, 70%); mp: 209.1–209.9 °C. FTIR (KBr; cm
1
1
)
: 3038, 2206, 1737, 1588, 1513, 1488, 1286, 1045, 837, 755, 696; H NMR (500 MHz,
CDCl ) δ 7.95 (s, 1H), 7.55–7.52 (m, 3H), 7.51–7.47 (m, 3H), 7.38–7.35 (m, 3H), 7.32–7.26
3
1
3
(
m, 5H), 7.13 (d, J = 9.0 Hz, 4H), 7.07 (t, J = 7.3 Hz, 2H), 7.01 (d, J = 8.8 Hz, 2H); C NMR
(
101 MHz, CDCl ) δ 159.19, 148.17, 147.03, 144.74, 132.57, 132.20, 131.85, 131.34, 129.39,
3
1
27.72, 125.07, 123.65, 122.05, 18.88, 116.81, 115.49. 112.96, 95.59, 92.25, 88.26, 84.92;
+
+
ESI-mass (m/z) [M+H] 514; HRMS (APCI): calculated for C H NO [M+H] (m/z)
3
7
24
2
5
14.1802; found = 514.1795.
.2.4 Synthesis of
2
2-(4-(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-
yl)ethynyl)phenyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (P2)
To a solution of P1 (100 mg, 0.19 mmol) in dry DCM (10 mL) was added TCNE (74 mg,
0
.58 mmol) at room temperature. The reaction mixture was stirred at room temperature for 4
h. The progress of reaction was monitored by TLC. After completion of reaction the solvent
was evaporated off and the resulting dark brown mass was purified by column

ACCEPTED MANUSCRIPT
chromatography (silica gel; hexane: DCM 3: 7) to afford titled P2 as a brown solid (112 mg,
-
1
9
7
9
7
=
0%); mp: 196–198 °C; FTIR (KBr; cm ): 3435, 2854, 2223, 1730, 1586, 1488,1337,756,
1
00, 528; H NMR (500 MHz, CDCl ) δ 8.02 (s, 1H), 7.72 (q, J = 8.9 Hz, 4H), 7.65 (d, J =
3
.3 Hz, 2H), 7.58 (t, J = 8.5Hz, 1H), 7.53 ( d, J = 6.7 Hz, 1H), 7.40 (q, J = 7.78 Hz, 4H),
.30 (dd, J = 6.5 Hz, 7.6 Hz, 2H) 7.29 (d, J = 7.3Hz, 2H), 7.23 (d, J = 7.4 Hz, 4H), 6.90 (d, J
1
3
9.1 Hz, 2H); C NMR (125 MHz, CDCl ) δ 167.28, 163.14, 158.85, 153.91, 153.59,
3
1
1
46.21, 144.36, 133.00, 131.83, 130.11, 129.39, 128.04, 126.95, 125.03, 121.00, 118.62,
18.04, 116.94, 113.41, 112.69, 112.11, 111.72, 111.15, 93.74, 88.58, 87.45; ESI-mass (m/z)
+
+
[
M+H] 642, HRMS (APCI): calculated for C H N O [M+H] (m/z) 642.1925; found =
43 24 5 2
6
2
3
42.1912.
.2.5 Synthesis of 2-(4-(3,3-dicyano-1-(4-(diphenylamino)phenyl)-2-(4-((2-oxo-2H-chromen-
-yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (P3)
Compounds P1 (100 mg, 0.19 mmol) and TCNQ (119 mg, 0.58 mmol) were suspended in
,2- dichlorobenzene and the resulting reaction mixture was stirred at 100 °C for 12 h. The
1
progress of reaction was monitored by TLC. The solvent was evaporated under reduced
pressure and the crude residue was purified by column chromatography (silica gel; eluent:
-
DCM) to afford P3 as a bluish-black solid (97 mg, 70%); mp: 340–342 °C; FTIR (KBr; cm
1
1
)
: 3038, 2206, 1737, 1588, 1513, 1488, 1455, 1318, 1286, 1045, 837, 755, 696, 530; H
NMR (400 MHz, CDCl ) δ 8.00 (s, 1H), 7.67–7.49 (m, 7H), 7.39–7.28 (m, 7H), 7.24–7.12
3
1
3
(
m, 7H), 7.02–6.99 (d, J = 7.6 Hz, 1H), 6.95–6.93 (d, J = 8.9 Hz, 2H); C NMR (101 MHz,
CDCl ) δ 170.92, 159.09, 153.97, 153.73, 151.79, 150.06, 146.18, 145.42, 135.38, 134.53,
3
1
1
7
7
34.07, 133.48, 133.43, 133.07, 130.09, 129.76, 128.18, 127.91, 126.77, 126.28, 126.10,
25.21, 119.37, 118.78, 117.12, 114.18, 114.10, 112.81, 112.30, 112.27, 93.98, 88.21, 87.77,
+
+
5.18; ESI-mass (m/z) [M] : 717; HRMS (APCI): calculated for C H N O [M+H] (m/z)
4
9
28
5
2
18.2238; found = 718.2224. [Note: For experimental spectra, see Supporting Information].

ACCEPTED MANUSCRIPT
3
. Results and discussion
.1. Optical and electrochemical properties
3
The optical properties of P1, P2 and P3 were investigated by measuring their UV–Vis
absorption spectra in chloroform solution (Fig. 2) and in pristine as-cast films (Fig. 3). The
longest wavelength absorption maximum (λmax) exhibited by P3 in solution was red-shifted
when compared with the solution λmax of P1 and P2. Thin-film spectrum of P3 indicated
better light harvesting properties than either P2 or P1, and an obvious red-shift of ~30 nm
remained for P3. Two sets of absorption bands (<420 nm and >450 nm) in case of P2 and P3
can be assigned to π–π* transition and the intramolecular charge transfer (ICT) transition,
respectively, thus indicating that incorporation of additional acceptor units, TCNE and
TCNQ, can indeed be utilized for absorbing a larger amount of the solar spectrum. P3
exhibited a substantial bathochromic shift of the onset absorption wavelength, typically
termed as panchromatic absorbance, when compared to P2, which is mainly attributed to the
presence of the strong electron withdrawing TCNQ unit. Overall, P2 and P3 showed better
light harvesting properties when compared to P1 [film λmax of P1: 424 nm; P2: 431 nm, 523
nm; and P3: 345 nm, 661 nm]. According to the spectra shown in Fig. 3, the absorption
spectra of P2 and P3 complements well with the main absorption of P3HT (380–610 nm),
thus indicating that blend films can have more favourable optical absorption throughout the
entire visible spectrum and even tailing into the near infrared region. The optical bandgaps
estimated from the thin film spectra of P2 and P3 were 1.65 eV and 1.34 eV, respectively,
which were narrower than the optical band gap of P1 (= 2.43 eV), an observation that further
corroborates the strong accepting capacity of TCNQ given the fact that the other parts, for
instance D and A, of the molecular backbone were alike. Thus, the use of the D–A –A design
1

ACCEPTED MANUSCRIPT
can indeed be helpful for controlling absorption profile, fine tuning energy levels and
presumably enhancing BHJ performance.
>
>
Fig. 2<
Fig. 3<
Theoretical density functional theory (DFT) calculations using the Gaussian 09 suite of
programs and the B3LYP/6-311þG(d,p)//B3LYP/6-31G(d) level of theory [45] indicated that
the HOMO densities of all the materials were populated over TPA functionality. In case of
P1, the HOMO density was extended across the central phenyl unit whereas the LUMO
density primarily resided over the terminal chromen-2-one functionality. In case of P2 and
P3, the LUMO densities were spread along the A –A backbone, sparing the phenyl rings of
1
the TPA unit, thus representing an efficient segregation of orbital densities. Such
dissociations of theoretical densities can be considered ideal for ICT transition between donor
and acceptor fragments. In contrast, the HOMO and LUMO densities of P1 seem to hold the
entire molecular backbone, a result indicating a poor expression of ICT. It is important to
point out here that the ester group of chromen-2-one functionality does play a crucial role in
tuning the optoelectronic properties of the dyes reported herein. It is apparent from dye P1
where the LUMO density was completely delocalised on the ester group. Even is case of P2
and P3, it is evident that the ester group does share the LUMO distribution, albeit to a lesser
extent. For theoretical density distributions, see Fig. 4 and Section S1 (Supporting
Information (SI)). The time-dependant DFT (TD-DFT) calculations further indicated that the
dipole moment in the case of P3 (19.2943 Debye; field-independent basis) was larger than
the dipole moments of P2 (14.0198 Debye) and P1 (4.2926 Debye), thus indicating an
efficient intramolecular charge transfer transition in case of P3. Furthermore, the computed

ACCEPTED MANUSCRIPT
absorption spectra showed the major transition peaks at 465.27 nm, 623.99 nm and 722.25
nm, respectively, for P1, P2 and P3. The transition peak in case of P3 was observed to be
red-shifted more than the transition peak of P2 which in turn was more red-shifted than P1, a
result that gives support to our experimental finding of optical absorption. For the computed
absorption spectra, see Section S2, SI.
>
Fig. 4<
As such, the experimental estimation of the HOMO energies was carried out using photo
electron spectroscopy in air (PESA) and the LUMO energies were calculated by adding the
optical band gap (film spectra) to the HOMO values (for PESA curves see Section S3, SI).
The LUMO levels of P2 and P3 were deepest when compared with the LUMO level of P1.
This suggests the influence of additional accepting functionality in tuning the overall energy
levels. A similar drop of LUMOs was also observed while conducting DFT calculations
(Theoretical LUMOs of P1, P2 and P3 were -2.72 eV, -3.67 eV and -3.97 eV, respectively), a
result that provides a good support to our experimental findings. These theoretical and
experimental findings strongly indicate that the D–A –A format really plays a crucial role for
1
density segregation, efficient light-harvesting, and tuning optical energy levels when
compared with the D–A format. The energy level diagram (Fig. 5) indicated that the band
gaps of these materials are all in the range required of acceptor materials for BHJ devices.
The electrochemistry experiments revealed that both the compounds P2 and P3 undergo
reversible reduction processes, thus suggesting their suitability as n-type materials in BHJ
devices (see Section S4, SI). The HOMO and LUMO levels determined from cyclic-
voltammetry experiments were found to be consistent with the energy levels estimated using
PESA. We further believed that not only should organic semiconductors be soluble and
chemically stable, they should also be thermally stable in order to allow device processing

ACCEPTED MANUSCRIPT
such as annealing. Keeping this criterion in mind, we conducted thermogravimetric analysis.
Thermogravimetric analyses revealed that all the materials are thermally stable chromophores
and can endure annealing conditions at a higher temperature, if required (See Section S5 (SI)
for TGA curves).
>
Fig. 5<
3
.2. Photovoltaic properties
Once it was established that new materials displayed promising optoelectronic properties and
are potential acceptors with energy levels matching those of the classical electron donor
P3HT, solution-processable BHJ devices were fabricated. BHJ architectures typically deliver
higher device power conversion efficiencies by maximising the surface area of the interface
between p- and n-type materials in the active layer. The BHJ device architecture used was
indium-tin
oxide
(ITO)/poly(3,4-ethylenedioxythiophene):
polystyrene
sulfonate
(PEDOT:PSS, 38 nm)/active layer (~60 nm)/Ca (20 nm)/Al (100 nm) where the active layer
was a 1: 1 blend of P3HT: P1/P2/P3, respectively, spin-cast from o-dichlorobenzene on top
of the PEDOT: PSS surface. It is apparent that we chose a simple device architecture to
observe initial performance and fabricating conditions of BHJ devices. Control devices based
on P3HT: PC BM were also fabricated. A promising and high PCE of 4.21% was achieved
6
1
when P3HT: P3 blend film (1: 1) was spin-coated from o-chlorobenzene solution. By
contrast, the maximum PCEs obtained from the devices based on P2 and P1 were 3.75% and
2
.28%, respectively, when fabricated under similar conditions.
We used o-dichlorobenzene as the processing solvent as the use of high boiling solvent is
preferred for optimal performance and to avoid formation of large-scale crystals on active
surfaces. The optimized donor: acceptor weight ratio was found to be 1: 1 where the BHJ
devices gave encouraging performance. Other donor: acceptor combinations, for example 1:

ACCEPTED MANUSCRIPT
2
(P2 and P3), afforded poor device outcome, thus indicating that 1: 1 blend is optimum for
the current study of P3HT and non-fullerene acceptors. We realized that this inferior
performance (1: 2 combination) may be attributed to low film quality where we physically
observed roughness on active surfaces. The PCE of P3-based devices (donor/acceptor 1: 1) is
higher than that of its counterparts, P2 and P1, primarily in terms of the enhanced short
circuit current density (J ), which might be attributed to the broader absorption profile of P3
sc
which in fact can facilitate the harvesting of more solar photons. The P2-based devices
afforded reasonable outcome too, almost two-fold efficiency when compared with P1-based
devices, thus indicating that the incorporation of an additional acceptor functionality, for
instance TCNE or TCNQ in the present study, in a given D–A system is advantageous not
only for tuning optoelectronic properties but for device outcome too. This validates the use of
D–A –A modular format as a favourable structural concept for the design and development
1
of efficient non-fullerene acceptors when compared with the D–A format. The maximum
PCE reached 3.08% for a device based on P3HT:PC BM when fabricated under similar
61
conditions. For current–voltage (J–V) curves see Fig. 6 and Table S1 (SI) represents
comparative photovoltaic data.
>
Fig. 6<
The incident photon to current conversion efficiency (IPCE) curves of the best BHJ devices
based on P2 and P3 under monochromatic light are shown in Fig. 7. For P2, the IPCE shows
a maximum of ~46% at 510 nm whereas for P3, the IPCE spectrum shows a much broader
response covering most of the visible region (400–800 nm) and to the near IR region with a
maximum value of ~60% at 700 nm. The IPCE spectra of the devices closely resemble the
blend film spectra of corresponding active layers (Fig. 7), thus demonstrating that both donor

ACCEPTED MANUSCRIPT
and acceptor domains contributed to the photocurrent generation. Better BHJ performance
and the higher peak IPCE in case of P3 can be held responsible by the synergetic effect of
better light-harvesting and the use of stronger TCNQ derived acceptor when compared with
P2 reported herein. The P1-based device afforded poor IPCE characteristics to be reported.
The broadness of these IPCE spectra, P3 in particular, indicates these non-fullerene acceptors
can definitely be matched with a variety of electron donors, such as conjugated polymers and
small molecules, in order to achieve charge generation over a broad range of wavelengths.
>
Fig. 7<
The physical microstructure of the blend surfaces was examined using atomic force
microscopy (AFM) in tapping mode. Fig. 8 displays the topographic and phase images of 1:1
blends of P2 and P3 with P3HT (1:1 w/w). P2 appears to have a granular morphology with
surface roughness of ~3.7 nm, whereas blend film of P3 appears to be smooth with better
phase separation and with a root-mean square (RMS) roughness of 1.2 nm. It is apparent that
both the materials blended well with their donor counterpart and that the superior
morphological properties of P3-based blend have had a greater effect on efficiency when
compared with the P2-based blend. Superiority of P3-based blend surface was also confirmed
by the transmission electron microscopy (TEM) analysis where we were able to observe a
finer texture, a finding that usually results in relatively higher values of J and FF, when
sc
compared with the P2-based blend (Fig. 9). It was also evident that the AFM analyses
supported our physical observation of the blend surfaces. To gain an insight into the effective
charge carrier mobilities, the space charge limited current (SCLC) method was applied to get
information about the charge transportation in the devices. The electron-only devices,
consisting of active layer sandwiched between a ZnO coated ITO electrode and LiF/Al

ACCEPTED MANUSCRIPT
counter-electrode as the hole-blocking contact, were fabricated as per the sketch depicted in
Section S6, SI. From the current density as a function of voltage data (Section S6 (SI)), the
electron mobility in the trap-free SCLC region can be estimated using the Mott-Gurney
2
3
equation, [(J = 9 (εꢀ)/8 × (V /d ); where ε is the dielectric constant, ꢀ is the charge-carrier
mobility, d is the sample thickness, and V is the applied voltage]. Using this expression, the
-4
2
electron mobilities of the order of 10 cm /Vs were observed for both P2 and P3. It is vital to
point out that the mobilities of the obtained order are well within the mobility range exhibited
-3
-4
2
-1 -1
by PC BM (typically ranging 10 –10 cm V s ). Evidently, the observance of good
6
1
electron mobility further supports the design principle of these new materials.
>
>
Fig. 8<
Fig. 9<
It is judicious to mention that though the PCE numbers steadily improve with the use of non-
fullerene acceptors combined with high performing donor polymers and small molecular
solids, we believe in continued emphasis on systematic material design and developing those
non-fullerene acceptors which can interweave well with the commercially available donors,
such as P3HT, hence demonstrating their foremost suitability to be an efficient acceptor.
Having said this, and taking into account the D–A –A-based acceptors reported in the
1
literature, it is justifiable to say that the device outcome reported herein provides strong
support and incentive for the current research strategy.
4
. Conclusions
We have been able to demonstrate the successful use of a D–A –A modular format in BHJ
1
solar cells. The new materials P2 and P3 were designed and synthesized based on the D–A1–

ACCEPTED MANUSCRIPT
A format and their optoelectronic and photovoltaic properties were directly compared with a
structural analogue P1, which was based on a simple D–A modular design. It was observed
that P2 and P3 exhibited superior properties, such as light-harvesting, enhanced photocurrent
density and overall device performance, when compared with P1. Overall, P3 performed
better than any of the three materials reported herein. It is notable to mention that not only are
P1, P2 and P3 the first examples in the literature which comprise promising building blocks,
such as TPA and terminal chromen-2-one, but the device parameters outlined herein are
among the highest numbers reported in this class of materials.
Acknowledgments
S. V. B. (RMIT) acknowledges financial support from the Australian Research Council
(ARC), Australia, under a Future Fellowship Scheme (FT110100152). A. G. is thankful to the
Alfred Deakin Fellowship Scheme at the IFM, Deakin University, Waurn Ponds Victoria
Australia. We also acknowledge Duong Duc La for TEM measurements. The CSIRO
Division of Manufacturing, Clayton Victoria, Australia is acknowledged for providing
support through a visiting fellow position for A. G. A. G. acknowledges a vast variety of
facilities at Deakin University, RMIT University and Bio 21 Institute, the University of
Melbourne, Melbourne Victoria Australia. S. V. B. (IICT) would like to thank the TAPSUN
programme for financial assistance under the project NWP0054. P. S. R. acknowledges to
CSIR, New Delhi for senior research fellowship (SRF).
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Figure and Scheme Captions
Fig. 1. Molecular structures of the newly designed and synthesized non-fullerene electron
acceptors, P1, P2 and P3.
Scheme 1. Synthetic protocol adopted to synthesize P1; Reagents and conditions: I)
trimethylsilylacetylene (TMSA), Pd(PPh ) Cl /CuI, Et N, RT, 20 h, 98%; II)
3
2
2
3
(triisopropylsilyl)acetylene (TIPS), Pd(PPh ) Cl /CuI, Et N, 4 days, reflux, 97%; III) K CO ,
3 2 2
3 2 3
THF: MeOH (1: 1), RT, 4 h, 82%; IV) Pd(PPh ) Cl /CuI, N,N-diisopropylethylamine
3
2
2
(
DIPEA), dry Toluene, RT, 12 h, 91%; V) tetra-n-butylammonium fluoride [C H ) NF], dry
4 9 4
THF, RT, 1 h, 70%; VI) Pd(PPh ) /CuI, DIPEA, dry THF, 65 ºC, 24 h.
3
4
Scheme 2. Synthesis of compounds P2 and P3.
Fig. 2. Absorption spectra of compounds P1, P2 and P3 in their chloroform solutions
equimolar solutions of 10 µM concentration).
(
Fig. 3. Absorption spectra of compounds P1, P2 and P3 for pristine as-cast films (films were
spin-casted at 2500 rpm for 1 min to give a film thickness of ~ 65 nm).
Fig. 4. Theoretical density distribution for the HOMOs (left) and LUMOs (right) of P1, P2
and P3.
Fig. 5. Energy level diagram showing alignments of different components of BHJ device
architecture.
Fig. 6. Characteristic current-density vs voltage (J-V) curves for the best BHJ devices based
-2
on P1/P2/P3 in blends with P3HT under simulated sunlight (100 mW cm AM1.5G). Device
structure is: ITO/PEDOT: PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm). The active
layer thicknesses were in the range of 60–65 nm with w: w = 1: 1.
Fig. 7. IPCE spectra of the best performing devices described in Fig. 6.
Fig. 8. AFM images of 1: 1 blend films with P3HT spin-cast from o-chlorobenzene at 2500
rpm atop annealed ITO/PEDOT: PSS substrates. Topographic (left) and phase images (right)
for P3HT: P2 (upper) and P3HT: P3 (lower) are depicted.
Fig 9. TEM images for 1: 1 blend of P3HT: P2 (left) and P3HT: P3 (right) showing superior
mixing of donor and acceptor domains in case of P3 when compared with P2-based blend.
Scale bar of 50 nm is represented.