Dithieno[3,2-b:2′,3′-d]pyrrole-benzo[c][1,2,5]thiadiazole conjugate small molecule donors: Effect of fluorine content on their photovoltaic properties

Article abstract of DOI:10.1039/c7cp02729j

Two new small molecule donors, namely ICT4 and ICT6 with D₁-A-D₂-A-D₁ architecture having 2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole (EHDTP, D₁) and 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene (OBDT, D₂) as the terminal and central donor, and benzo[c][1,2,5]thiadiazole (BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as the acceptor (A) moieties, are synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and ICT6 have considerable solubility in various solvents and possess efficient light absorption ability [ε (×10⁵ mol^-1 cm^-1) is 0.99 and 1.06, respectively for ICT4 and ICT6] and appropriate frontier molecular orbital energy offsets with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). Bulk heterojunction solar cells (BHJSCs) are fabricated using ICT4/ICT6 and PC₇₁BM as donors and acceptors, respectively and BHJSCs with two-step annealed (thermal followed by solvent vapor annealing) active layers of ICT4 and ICT6 show overall power conversion efficiencies (PCEs) of 5.46% and 7.91%, respectively. The superior photovoltaic performance of the ICT6 based BHJSCs is due to the favourable morphology with a nanoscale interpenetrating network in the ICT6:PC₇₁BM active layer induced by the fluorine atoms on the BT acceptor, which significantly enhances the dissociation of excitons, charge transport and the charge collection efficiency, and suppresses bimolecular recombination in the BHJ. The observed higher PCE of 7.91% indicates that ICT6 is one of the best BT based donor material for small molecular BHJSCs.

Full text of DOI:10.1039/c7cp02729j

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ARTICLE
Study on the effect of fluorine substitution on the photovoltaic
properties of dithieno[3,2-b:2',3'-d]pyrrole-
benzo[c][1,2,5]thiadiazole conjugate small molecule donor
materials
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Manohar Reddy Busireddy,a Narendra Reddy Chereddy,a,* Balaiah Shanigaram,b Kotamarthi
www.rsc.org/
Bhanuprakash,b,* Subhayan Biswas,c Ganesh Datt Sharmac,* Vaidya Jayathirtha Rao a,d,
*
Two new small molecule donors namely ICT4 and ICT6 with D1-A-D2-A-D1 architecture having 2,4-
bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EHDTP, D1) and ethylhexyl)oxy)benzo[1,2-b:4,5-
b']dithiophene (OBDT, D2) as terminal and central donor and 4,8-bis((2- benzo[c][1,2,5]thiadiazole
(BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as acceptor (A) moieties are
synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and
ICT6 are have considerable solubility in various solvents and possess efficient light absorption ability
[ε
(x 105 mol-1 cm-1) is 0.99 and 1.06, respectively for ICT4 and ICT6) and appropriate frontier
molecular orbital energy offsets with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Bulk
heterojunction solar cells (BHJSCs) are fabricated using ICT4 ICT6 and PC71BM as donor and
/
acceptors, respectively and BHJSCs with the two step annealed (thermal followed by solvent vapor
annealing) active layers of ICT4 and ICT6 shows overall power conversion efficiency (PCE) of 5.46 %
and 7.91 %, respectively. Superior photovoltaic performance of ICT6 based BHJSCs is due to the
favourable morphology with nanoscale interpenetrating network in the ICT6:PC71BM active layer
induced by the fluorine atoms on BT acceptor, which significantly enhances the dissociation of
excitons, charge transport, charge collection efficiency and suppresses bimolecular recombination in
the BHJ. Observed higher PCE of 7.91 % portray ICT6 as one of the best BT based donor material for
small molecular BHJSCs.
donating organic semiconductors (polymers or small
molecules) and electron accepting fullerene derivatives. For
OBHJSCs with polymer donor material, PCEs have been
Introduction
In recent years, organic bulk heterojunction solar cells
(OBHJSCs) have been attracting immense interest from both
industry and academia. Even though power conversion
efficiencies (PCEs) reported for OBHJSCs are quite low than
conventional silicon solar cells, former are advantageous in
terms of their low production cost and simple fabricating
procedures.1-3 Generally, BHJ layer is a blend of electron
enhanced from below 1 % to over 10 % for single4-7 and 12 %
for multi-junction structure8,9 by optimizing the donor
materials molecular structure, BHJ morphology and device
structure. However, intrinsic polydispersity associated with
polymer donor materials due to batch-to-batch variation in
their synthesis and difficulties in their purification and
optimization of electronic properties intend to develop small
molecule donor materials. Superior characteristics like simple
synthesis, characterization and purification procedures
associated with small molecule donor materials make them
advantageous over their polymer counterparts.10,11 Broader
optical absorption for better exciton generation, suitable
energy level offsets with fullerene acceptor for efficient
aCrop Protection Chemicals Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad-500007, India. Email: chereddynarendra@gmail.com;
vaidya.opv@gmail.com
b Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad-500007, India
cDepartment of Physics, The LNM Institute of Information Technology, Jamdoli,
Jaipur, India. Email: gdsharma273@gmail.com
dissociation of excitons and extended π-electron delocalization
dAcSIR, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, India
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [NMR, MALDI-MS,
absorption and emission spectra and theoretical calculations] See
DOI: 10.1039/x0xx00000x
and planar backbone for high charge mobility are important
for donor materials to obtain high PCEs.10-14 Recently, PCE over
10 % has been reported for OBHJSCs with small molecule
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donor materials.12-14 However, PCE has to be improved further
for commercial applications. Several new findings have been
taking place in search of efficient donor/acceptor materials for
OBHJSCs.15-22 PCE of an OBHJSC can be enhanced by improving
the magnitudes of short circuit current density (JSC), open-
circuit voltage (VOC) and fill factor (FF) of the devices.
Generally, JSC of an OBHJSC can be increased by exploiting
donor materials with efficient light harvesting capacity and VOC
can be improved by lowering their HOMO energy to reduce
the exciton recombination.23,24
Experimental Section
Materials and instruments
Synthetic precursors reported in this study are obtained
either directly from commercial suppliers or synthesized as per
the literature procedures. All the experiments are performed
using dry organic solvents. NMR spectral analysis is carried out
on Bruker Avance (500 MHz) spectrometer and MALDI-MS
data is acquired using a Thermofinngan mass spectrometer.
Absorption measurements are performed on a Cary 5000 UV-
VIS-NIR spectrophotometer. Fluorescence experiments are
carried out on Cary Eclipse fluorescence spectrophotometer
and Horiba Jobin Yuvon Fluorolog Fluorimeter. Cyclic
voltammetry measurements, thermogravimetric analysis and
differential scanning calorimetry experiments are conducted
as previously described.17
From our recent findings, we observed that incorporation of
dithieno[3,2-b:2',3'-d]pyrrole (DTP) donor moiety can impart
intense visible and NIR absorption to donor materials.25,26
Moreover, recent reports revealed that attachment of fluorine
atoms to the molecular backbone of donor materials can lower
down their HOMO energy and thereby improves the VOC of
OBHJSCs and as well as fluorine atoms at appropriate location
of donor materials can improve JSC and FF of the devices.27-30
Extensive studies have been carried out to comprehend the
influence of fluorination on photovoltaic properties of
benzo[c][1,2,5]thiadiazole (BT) based polymer donor
materials.28,21-42 However, efforts to demonstrate the
importance of fluorination to obtain efficient BT based small
molecule donor materials are relatively low.43-49
BHJSC device studies
BHJSCs
ethylenedioxythiophene):polystyrene
(PEDOT:PSS)/ICT4 or
with
indium
tin
oxide
(ITO)/poly(3,4-
sulfonate
ICT6:PC71BM/poly[(9,9-bis(3'-(N,N-
dimethylamino)propyl)fluorene-2,7-diyl)-alt-(9,9–
dioctylfluorene-2,7-diyl) (PFN)/ aluminum (Al) architecture are
fabricated as follows: First of all the ITO coated glass
substrates are pre-cleaned in ultrasonicator containing de-
ionized water, acetone and iso-propanol, sequentially for 20
minutes each followed by UV/ozone treatment for 1 h to
remove organic residues. PEDOT:PSS is layered (thickness of
~40 nm) on the pre-cleaned ITO coated glass substrate using a
Considering all the factors mentioned above, we have
synthesized two new small molecule donors, ICT4 and ICT6
with subtle variations in their structure. Synthesized donor
materials have D1-A-D2-A-D1 linear architecture containing 2,4-
bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole
(EHDTP)
terminal donor (D1), BT with different extents of fluorine
spin coater and dried at 120 °C for 20 minutes. On the top of
substitution (BT for ICT4 and F2BT for ICT6) and 4,8-bis((2-
the ITO/PEDOT:PSS, active layer is spin coated from the blend
solution (chloroform solvent) consisting of different weight
ratios of donor (ICT4 or ICT6) and PC71BM, keeping its total
concentration as16 mg/mL. Active layer coated plates are
ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene
(OBDT)
as
acceptor (A) and central donor (D2) moieties, respectively. We
choose OBDT as central donor owing to its excellent
properties. With a flat and three ring fused structure, BDT can
trigger efficient intermolecular interactions among the donor
molecules in their thin films and there by enhances the charge
carrier dynamics in the BHJ.13,14,50-53 BT with different degrees
of fluorine substitution are selected as acceptor moieties to
explore the effects of fluorine atoms on BT acceptor moieties
of the synthesized EHDTP end-capped donor molecules on
their thermal, photophysical, electrochemical and photovoltaic
properties. Excellent thermal stability, efficient light harvesting
ability of ICT4 and ICT6 and their appropriate frontier
molecular orbital energy levels compared to [6,6]-phenyl-C71-
butyric acid methyl ester (PC71BM) portray them as suitable
donors for BHJSCs employing PC71BM acceptor. A detailed
structural analysis of donor materials in BHJ thin films has
been carried out using X-ray diffraction and transmission
electron microscope analyses to understand how morphology
of the BHJ changes with fluorine atom substitution. Also
influence of fluorine atom substitution on various photovoltaic
parameters of their BHJSCs is explained in detail. To our
knowledge, this is the first report demonstrating the influence
of fluorine substitution on optical, electronic and photovoltaic
properties of BT-DTP conjugate small molecule donors.
dried at 50
Two-step annealing (TSA) process has been carried out by
annealing the active layer coated ITO glass substrates at 120
°C for 10 minutes to remove the organic residue.
°C
for 10 minutes, followed cooling to RT and subsequently
keeping them for 10 minutes in a Petri dish present in a THF
contained chamber. A layer of PFN (thickness of ~5 nm) is spin
coated onto the active layer from its methanol solution. Finally
an aluminum (Al) electrode is deposited using thermal
evaporation technique under a vacuum of 1 x 10-6 Torr. The
shadow mark method is used to define the effective area of
the fabricated devices and is about 16 mm2. The devices are
characterized at ambient conditions under AM1.5 G stimulated
solar illumination (100 mW/cm2) using a xenon lamp based
solar simulator. The current-voltage (J-V) characteristics of the
fabricated devices are measured on a Keithley source meter
unit controlled by a PC. The incident photon to current
efficiency (IPCE) characteristics of the fabricated devices is
studied under short circuit conditions using
a Keithley
electrometer. Devices are illuminated with light source and
monochromator and resulting current is collected using the
electrometer unit.
Synthetic procedures
2 | J. Name., 2012, 00, 1-3
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Intermediates, N,N-diethylthiophene-3-carboxamide (1), 4,8- added and connected to a reflux condenser. Dry toulene (10
dihydrobenzo[1,2-b:4,5-b′]dithiophen-4,8-dione (
ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene (
ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-
diyl)bis(trimethylstannane) (
2), 4,8-bis((2- mL) is injected into the RB flask and the reaction mixture is
3
), (4,8-bis((2- bubbled with N2 for 20 minutes. Resulting solution is heated
and stirred overnight at reflux temperature. On the next day,
4
), 3,3′-dibromo-2,2′-dithiophene reaction mixture is cooled down to RT, added with chloroform
(
5
), N-(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (
6
) are (20 mL) and filtered through celite bed. Filtrate obtained is
partitioned between chloroform and water and organic
portion is collected. Aqueous portion is washed with
chloroform (3 x 10 mL) and all the organic portions are
combined, dried using anhydrous Na2SO4 and filtered. Filtrate
synthesized as per our previously reported procedures.25
2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (7):
Compound 6 (0.73 g, 2.5 mmol) and dry THF (30 mL) are added
obtained is concentrated using a rotary evaporator and the
residue formed is purified using a silica gel column (eluent:
to a double necked RB flask dried over flame and maintained
at -78 °C under N2 atmosphere. n-BuLi (2.5 M in hexanes, 2.75
mmol, 1.1 mL) is slowly added to the solution and stirred for 1
h at -78 °C followed by adding 2-ethylhexyl bromide (0.5 mL,
2.8 mmol). Resultant solution is stirred at RT overnight, added
with ice water (20 mL) and the layers formed are separated.
The aqueous layer collected is washed with chloroform (3 x 10
mL) and all the organic portions are combined, dried using
anhydrous Na2SO4 and filtered. Filtrate obtained is
concentrated using a rotary evaporator and the residue
formed is purified using a silica gel column (eluent: hexane) to
acquire the desired product as a colorless oil (yield 75%).
chloroform/petroleum ether, 1:6 v/v) to obtain
(yield 75%).
9 as a red solid
1H NMR (500 MHz, CDCl3,
δ ppm): 0.85-0.98 (m, 12H), 1.26-
1.44 (m, 16H), 1.62-1.70 (m, 1H), 1.94-2.04 (m, 1H), 2.82 (d, J =
7.0 Hz, 2H), 4.03-4.17 (m, 2H), 6.68 (s, 1H), 7.66 (d, J = 7.5 Hz,
1H), 7.80 (d, J = 8.0 Hz, 1H), 8.28 (s, 1H); 13C NMR (125 MHz,
CDCl3,
δ ppm): 10.74, 10.86, 14.09, 14.14, 22.97, 23.02, 24.09,
25.60, 28.61, 28.86, 30.63, 32.38, 35.75, 40.45, 41.50, 51.12,
109.33, 110.40, 112.76, 113.22, 116.09, 123.78, 128.50,
132.33, 134.40, 144.70, 145.01, 145.69, 151.61, 153.89.
1H NMR (500 MHz, CDCl3,
δ ppm): 0.82-0.95 (m, 12H), 1.22-
2-(7-bromo-5,6-difluorobenzo[c][1,2,5]thiadiazol-4-yl)-4,6-
bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (10)
1.43 (m, 16H), 1.56-1.69 (m, 1H), 1.86-1.97 (m, 1H), 2.80 (d, J =
7.0 Hz, 2H), 3.95-4.05 (m, 2H), 6.66 (s, 1H), 6.94 (d, J = 6.0 Hz,
1H), 7.03 (d, J = 6.0 Hz, 1H); 13C NMR (125 MHz, CDCl3,
δ
ppm): Similar procedure used to synthesize
10.67, 10.86, 14.00, 14.13, 22.97, 23.02, 24.01, 25.58, 28.64, using BrF2BT30 and
as the as synthetic precursors (yield 76%).
28.85, 30.65, 32.36, 35.56, 40.45, 41.53, 51.19, 109.34, 110.96, 1H NMR (500 MHz, CDCl3,
ppm): 0.85-0.97 (m, 12H), 1.25-
9 is adopted to obtain 10
8
δ
112.37, 114.98, 121.47, 142.69, 143.84, 144.43.
1.44 (m, 16H), 1.60-1.73 (m, 1H), 1.89-2.02 (m, 1H), 2.82 (d, J =
8.0 Hz, 2H), 4.00-4.13 (m, 2H), 6.67 (s, 1H), 8.25 (s, 1H); 13C
2,4-bis(2-ethylhexyl)-6-(trimethylstannyl)-4H-dithieno[3,2-
b:2',3'-d]pyrrole (8):
NMR (125 MHz, CDCl3, δ ppm): 10.65, 10.80, 14.03, 14.08,
22.89, 22.96, 24.04, 25.55, 28.53, 28.76, 30.53, 32.32, 35.67,
Under N2 atmosphere, n-BuLi (2.5 M in hexanes, 2.75 mmol, 40.33, 41.39, 50.84, 109.12, 112.60, 112.87, 113.18, 113.81,
1.1 mL) is slowly added to the dry THF (25 mL) solution of 116.95, 128.31, 128.50, 128.60, 130.07, 132.81, 144.36,
7
(1.0 g, 2.5 mmol) maintaining at -78 °C and stirred for 1 h. 145.59, 146.05, 148.44, 153.92, 153.99, 159.29, 161.78.
Trimethyltin chloride in hexane (1 M, 2.75 mL, 2.75 mmol) is
injected into the reaction vessel and the solution is allowed to 6,6'-(7,7'-(4,8-bis ((2-ethylhexyl)oxy) benzo [1,2-b:4,5-
reach RT and stirred overnight. Reaction mixture is added with b']dithiophene-2,6-diyl) bis(benzo[c] [1,2,5]thiadiazole-7,4-
ice water (20 mL) and washed with diethyl ether (3 x 20 mL). diyl)) bis (2,4-bis(2-ethylhexyl)-4H-dithieno [3,2-b:2',3'-
All the organic fractions are combined and dried using d]pyrrole) (ICT4)
anhydrous Na2SO4. Filtrate obtained upon filtration of this
To a thoroughly dried double necked RB flask (50 mL)
solution is concentrated using a rotary evaporator to achieve
connected to a reflux condenser,
4 (0.38 g, 0.5 mmol), 9 (0.93
the desired product as light brown oil (yield 82%).
1H NMR (500 MHz, CDCl3,
ppm): 0.39 (s, 9H), 0.85-0.93 (m,
g, 1.5 mmol) and tetrakis(triphenylphosphine)palladium (58
mg, 0.05 mmol) and dry toulene (10 mL) are added and the
solution is bubbled with N2 gas for 20 minutes. Resulting
solution is heated, stirred overnight at reflux temperature and
the reaction progress is monitored using TLC. After complete
δ
12H), 1.25-1.39 (m, 16H), 1.59-1.66 (m, 1H), 1.89-1.98 (m, 1H),
2.79 (d, J = 7.0 Hz, 2H), 3.96-4.05 (m, 2H), 6.64 (s, 1H), 6.95 (s,
1H); 13C NMR (125 MHz, CDCl3,
δ ppm): -8.16, 10.69, 10.88,
14.04, 14.13, 23.01, 23.99, 25.61, 28.55, 28.85, 28.88, 30.58,
32.37, 35.60, 40.40, 41.55, 51.12, 109.41, 112.43, 117.89,
120.50, 134.34, 142.49, 144.72, 146.58..
consumption of 4, reaction mixture is cooled down to RT and
chloroform (20 mL) is added. Solution obtained is filtered
through celite bed. Filtrate is washed with water and the
aqueous phase is extracted with chloroform (2 x 20 mL). All the
organic fractions are combined and dried using anhydrous
Na2SO4. The resultant solution is filtered, filtrate obtained is
2-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)-4,6-bis(2-
ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (9)
To a thoroughly dried double necked RB flask, BrBT42 (0.29 g, 1 concentrated using a rotary evaporator and the residue
mmol),
8
(0.57
g,
1
mmol)
and formed is purified using
a
silica gel column (eluent:
tetrakis(triphenylphosphine)palladium (58 mg, 0.05 mmol) are
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chloroform/petroleum ether, 1:6 v/v) to yield ICT4 as a blue-
black solid (yield 72%).
1H NMR (500 MHz, CDCl3,
δ ppm): 0.83-0.98 (m, 24H), 1.05-
1.14 (m, 6H), 1.21-1.45 (m, 36H), 1.55-1.92 (m, 24H), 2.74 (d, J
= 7.0 Hz, 4H), 3.56-3.71 (m, 4H), 4.20 (s, 4H), 6.43 (s, 2H), 7.30-
7.53 (m, 4H), 8.01 (s, 2H), 8.48 (s, 2H); 13C NMR (125 MHz,
CDCl3,
δ ppm): 10.99, 11.08, 11.96, 14.40, 14.72, 23.24, 23.32,
23.67, 24.33, 25.83, 28.87, 29.08, 29.12, 29.77, 29.94, 30.87,
31.01, 32.66, 35.96, 40.63, 41.11, 41.72, 50.76, 75.66, 109.51,
112.85, 112.97, 115.82, 121.81, 122.91, 124.06, 126.71,
127.58, 128.88, 133.25, 135.72, 138.63, 144.31, 144.40,
145.00, 145.63, 152.19, 152.67.
MALDI-MS (Positive mode, m/z): 1516.66 (M+), Calc. for
C86H112N6O2S8 is 1516.66.
Scheme 1. Synthetic scheme of ICT4 and ICT6
.
6,6'-(7,7'-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-
b']dithiophene-2,6-diyl)bis(5,6-
difluorobenzo[c][1,2,5]thiadiazole-7,4-diyl))bis(2,4-bis(2-
ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole) (ICT6)
Through characterization of ICT4 and ICT6 has been done using
NMR and MALDI-MS analyses (Figs. S2-S7, ESI†).
ICT6 is synthesized as per the procedure described for ICT4
To evaluate the thermal characteristics of ICT4 and ICT6
,
using
4
and 10 as synthetic precursors (yield 75%).
ppm): 0.76-0.88 (m, 12H), 0.90-
thermogravimetry (TGA) and differential scanning calorimetry
(DSC) analyses are performed and the obtained results are
1H NMR (500 MHz, CDCl3,
δ
0.98 (m, 12H), 1.10-1.16 (m, 6H), 1.21-1.41 (m, 36H), 1.57-1.96
(m, 24H), 2.44 (d, J = 7.0 Hz, 4H), 3.77-3.96 (m, 4H), 4.22 (s,
4H), 6.26 (s, 2H), 7.96 (s, 2H), 8.33 (s, 2H); 13C NMR (125 MHz,
presented in Fig. 1 (Fig. 1a and Fig. 1b are corresponding to
TGA and DSC analyses, respectively) and Table 1. Elevated
decomposition temperatures (Td) of ICT4 and ICT6 obtained
from TGA, confirms their superior thermal stability (Fig. 1a and
CDCl3, δ δ ppm): 10.77, 10.80,
ppm): 13C NMR (125 MHz, CDCl3,
o
Table 1). Similar Td values of ICT4 (342 C) and ICT6 (344 oC)
11.69, 14.15, 14.44, 23.04, 23.40, 24.13, 25.56, 28.64, 28.80,
28.83, 29.54, 29.68, 29.71, 30.66, 30.82, 32.37, 35.67, 40.42,
40.90, 41.44, 50.73, 75.35, 109.13, 109.41, 109.55, 112.61,
113.36,1132.50, 113.82, 116.74, 122.89, 122.96, 127.22,
127.32, 129.73, 129.80, 131.84, 132.38, 132.45, 133.85,
143.92, 144.62, 145.02, 145.92, 149.28, 153.34, 153.45,
158.42, 160.95.
indicates that fluorination of the BT acceptor unit has minimal
impact on their thermal degradation. Both ICT4 and ICT6
display glass transition temperatures (124 and 119 oC,
respectively for ICT4 and ICT6) during the second heating cycle
of their DSC analysis (Fig. 1b). Such a phase transition can
induce significant changes in their morphology upon thermal
or solvent vapor annealing processes during device
fabrication.54 Melting (Tm) and crystallization (Tc) temperatures
of ICT4 and ICT6 are also estimated from DSC analysis (Fig. 1b
and Table 1). Contrary to the thermal degradation, extent of
fluorine atoms has significant impact on their Tm and Tc values.
ICT4 shows clear melting (Tm) and crystallization (Tc)
MALDI-MS (Positive mode, m/z): 1588.66 (M+), Calc. for
C86H108F4N6O2S8 is 1588.62.
Results and discussion
Synthesis and thermal properties
o
thermograms at 260 and 249 C, respectively. However, ICT6
Structures of ICT4 and ICT6 is presented in Fig. S1, ESI† and
with four fluorine atoms does not show any melting or
crystallization behavior in the operated temperature range,
indicating its higher melting and crystallization temperatures.
scheme for their synthesis is provided in scheme 1
ICT6 are obtained from their corresponding mono-bromo
derivatives ( and 10, respectively for ICT4 and ICT6) by
. ICT4 and
9
reacting with distannyl intermediate
protocol. Mono-bromo intermediates
4
under Stille coupling
9
and 10 are obtained
100
from their corresponding dibromobenzothiadiazole precursors
(b)
(a)
4000
2000
0
(BrBT for ICT4 and BrF2BT for ICT6) by reacting with stannyl
90
80
70
60
ICT4
ICT6
DTP derivative
8
, which is obtained by reacting
is synthesized
with n-BuLi and 2-ethylhexyl bromide.
and are synthesized as per our
7 with
trimethyltin chloride and n-BuLi. Intermediate
7
by reacting
6
1
-2000
-4000
ICT4
ICT6
Intermediates
,
2
,
3
,
4,
5
6
previous procedures.25 Dibromobenzothiadiazole precursors
(BrBT and BrF2BT) are synthesized as per the procedures
available in the literature.49,39 Both ICT4 and ICT6 have fair
solubility in various organic solvents (>25 mg/mL). However,
50 100 150 200 250 300 350 400 450
Temp o
50
100
150
200
C
250
300
Temp o
C
Fig. 1. Thermograms of ICT4 and ICT6 obtained from TGA (a) and DSC
(b) analysis
solubility of ICT6 having four fluorine atoms is lesser than ICT4
.
4 | J. Name., 2012, 00, 1-3
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Table 2. Photophysical and electrochemical data of ICT4 and ICT6
.
λmax
ξ
λmax
Eo-o
Ered
(V)
EHOMO
(eV)d
ELUMO
(eV)
Material
(nm)a
(x 105 mol-1 cm-1
)
(nm)b
(eV)c
a
ICT4
ICT6
604
596
0.99
1.06
652
636
1.90
1.97
-1.43
-1.36
-5.55
-5.66
-3.65
-3.69
ain solution (chloroform, 10 µM), bin thin film (cast from chloroform
solution), ccalculated from the absorption and emission spectra
opt
intersection point, dEHOMO= ELUMO+ Eg
.
Fig. 2. Solution (chloroform, 10 µM) and thin film state absorption
ICT6 is measured using their onset reduction potential
spectra of ICT4 and ICT6
.
obtained from CV analysis (Table 2) and their HOMO energy is
opt
opt
calculated using EHOMO = ELUMO + Eg [Eg is the optical band
gap, measured from the intersection wavelength of donor
materials solution state absorption and emission spectra (Fig.
S9, ESI†)]. Introduction of fluorine atoms significantly lowers
down the HOMO energy of ICT6 (Table 2). Since the VOC of
BHJSC depends on the energy difference between the donor
HOMO and acceptor LUMO, observed deep-lying HOMO
energy of ICT6 can bestow high VOC for their corresponding
BHJSCs. The frontier molecular orbital energy levels of ICT4
and ICT6 are well aligned with HOMO (-6.1 eV) and LUMO (-4.3
eV) energies of PC71BM (Fig. S10, ESI†). This indicates the
applicability of ICT4 and ICT6 as donors for BHJSCs with
PC71BM acceptor. The higher magnitude energy gap between
Observed higher Tm and Tc values could be due to strong
intermolecular interactions associated with the fluorine atom
substituted donor material ICT6 55,56
High Td and Tm values of
.
the ICT4 and ICT6 reveals their suitability for application even
at elevated temperatures.
Photo-physical and electrochemical properties
Absorption spectra of ICT4 and ICT6 in solution and thin films
are presented in Fig. 2 and relevant data is given in Table 2. In
chloroform solution, ICT4 and ICT6 display absorption bands in
two distinct regions i.e 300-450 nm and 450-700 nm,
respectively. Compared to ICT4, longer wavelength absorption
band of ICT6 is faintly blue shifted and concomitant increase in
the LUMO of ICT4/ICT6 and PC71BM over the threshold value
(0.3 eV) reported for effective dissociation of excitons into free
charge carriers indicates efficient exciton dissociation at
molar extinction coefficient
(ε) is observed (Fig. 2).
ICT4
To understand the photo-induced charge transfer
characteristics between ICT4 ICT6 and PC71BM,
/
ICT6 and PC71BM junction.59
Enhancement in of ICT6 could be due to improvement in its
ε
molecular backbone planarity.57,58 Absorption bands of ICT4
and ICT6 are red shifted in their thin film state with a new
vibronic shoulder peak at longer wavelength regions (Fig. 2),
/
photoluminescence (PL) of pure donor materials and their
blend films with PC71BM are measured by exciting at 580 nm
(Fig. S8b, ESI†). As presented in Fig. S8b, ESI†, blend film of
indicating the strong intermolecular interactions due to
π-
stacking and efficient intermolecular packing between the
molecular backbones.46,47 Onset of thin film absorption spectra
ICT6/PC71BM demonstrate about 92
% PL quenching in
comparison with the PL spectrum of pristine ICT6, which is
is used to estimate the optical band gap of ICT4 and ICT6. To higher than that of ICT4:PC71BM blend film (83 %). These
results indicate that pronounced photo-induced charge
transfer can occur between ICT6 and PC71BM compared to
ICT4 and PC71BM, and indicates ICT6 as more suitable small
molecule donor for PC71BM acceptor for the application in
BHJSCs.
estimate HOMO (highest occupied molecular orbital) and
LUMO (lowest unoccupied molecular orbital) energies of ICT4
and ICT6, we have measured their electrochemical properties
using cyclic voltammetry (CV) analysis. Cyclic voltammograms
representing the reduction behavior of ICT4 and ICT6 are given
in Fig. S8, ESI† and relevant data is provided in Table 2.
Theoretical studies
Compared to ICT4, ICT6 shows decreased reduction potential
Theoretical studies are performed using Gaussian 09 package
(B3LYP functional along with 6-31G(d,p) basis set for DFT and
M06-2X functional and 6-311G(d,p) basis set for TD-DFT) and
detailed procedures described in our previous report.25 Various
conformations of ICT4 and ICT6 are optimized and the
conformer with lowest energy is used for theoretical
calculations (Fig. S11, ESI†). Optimized structures of ICT4 and
ICT6 is presented in Fig. S12, ESI†. Both ICT4 and ICT6 are
almost planar with zero dipole moment. The dihedral angle
between OBDT and outer BT-EHDTP or F2BT-EHDTP molecular
and it could be due to strong electron accepting tendency of
fluorine (Fig. S8a, ESI† and Table 2). LUMO energy of ICT4 and
Table 1. Thermal data of ICT4 and ICT6
.
Glass
transition
Decomposition
Melting
Crystallization
temperature
temperature
temperature
Material
temperature
o
o
o
o
(T )
C
(T )
C
(T ) C
d
(T )
C
m
c
g
ICT4
ICT6
342
344
124
119
260
249
-
-
planes are in the range of 0.06 - 4.54° and 0.09 - 2.51° for ICT4
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Material
L+1
ICT4
ICT6
(a)
(a)
(b)
L
H
Fig. 4. J-V characteristics (
a) under illumination and IPCE spectra (b) of
the devices under different conditions.
H-1
are provided Fig. 4a and Table 3, respectively. Without any
treatment, BHJSC with ICT4 donor shows only a PCE of 2.70 %
with JSC of 7.82 mA/cm2, VOC of 0.96 V and FF of 0.36. Under
same conditions, BHJSC with ICT6 shows PCE of 3.84 % with JSC
of 9.24 mA/cm2, VOC of 1.04 V and FF of 0.40. Higher VOC values
associated for both the devices could be due to the low lying
HOMO of donor materials. Since the VOC values of these
devices are significant, low PCE of these devices is mainly
related to their poor JSC and FF values and it could be due to
inappropriate nanoscale morphology of the BHJ. After the two
step annealing process (TSA), photovoltaic performance of the
BHJSCs with ICT4 and ICT6 donors is improved significantly i.e.
5.46 % and 7.91 %, respectively. IPCE spectra of the BHJSCs
with active layers processed under different conditions are
shown in Fig. 4b. IPCE values for BHJSCs with TSA treated
active layer are significantly higher over their as cast
counterparts contributing to increase in their JSC values. J-V
characteristics (Fig. 4a) reveals that BHJSCs with the active
layer containing ICT6 donor processed under TSA conditions
Fig. 3. FMO distributions of ICT4 and ICT6
and ICT6, respectively indicating high planarity of ICT6. High
planarity generally improves intramolecular electron
delocalization and enhances molar extinction coefficient of the
electronic transition and it is well supported by the observed
high
ε values of ICT6 over ICT4 (Fig. 2). Calculated HOMO (H)
and LUMO(L) energies, allowed vertical electronic excitation
and optical band gaps using M06-2X functional are given in
Tables S1, ESI†. The first excitation neatly shows a strong
single-configuration nature (H to L) with a moderate secondary
(H-1 to L+1). Isosurface plots (isovalue 0.02) of the H-1, H, L
and L+1 orbitals of the ICT4 and ICT6 are illustrated in Fig. 3
.
For both ICT4 and ICT6, HOMO is extended over all the
fragments of donor molecules and LUMO is distributed over
the BT-OBDT-BT (ICT4), F2BT-OBDT-F2BDT (ICT6) moieties,
indicating intramolecular charge transfer form EHDTP moiety
to BT or F2BT moieties. However, H-1 and L+1 does not exhibit
significant contributions from OBDT moieties, but are rather
localized mainly towards EHDTP and BT or F2BT moieties.
Absorption spectra simulated by TD-DFT of ICT4 and ICT6
using M06-2X/6-311G(d, p) in chloroform solvent are given in
Fig. S13, ESI†. GaussSum 3.0 software is used to convolute
Gaussian function to produce spectra with a FWHM value of
3000 cm-1. Calculated spectra are in line with experimental
obtained. Though, absorption maxima values are slightly
underestimated by about 40 nm using M06-2X functional.
Excitation wavelengths corresponding to the strongest
oscillator strengths observed for these band values are given in
Table S1, ESI†.
have superior VOC, JSC and FF values (JSC = 12.23 mA/cm2, VOC
=
0.98 V and FF= 0.66) over the devices with ICT4 active layer (JSC
= 10.52 mA/cm2, VOC = 0.91 V and FF= 0.57). As ICT6 has
deeper HOMO level than ICT4, higher VOC for the devices with
ICT6 is expected. To understand the rationale for higher
photovolatic efficiency of ICT6 based devices, we have carried
out further investigations including IPCE response, charge
transport and morphology of BHJ thin films. The IPCE spectra
of the BHJSCs with ICT4:PC71BM and ICT6:PC71BM BHJ are
presented in Fig. 4b. Both the devices shows wide response
ranging from 350 to 750 nm. However, the device with ICT6
active layer shows higher IPCE maximum value (67 %) over the
Table 3. Photovoltaic parameters observed for BHJSCs with ICT4/ICT6
Photovoltaic properties
donor and PC71BM acceptor
BHJSCs with conventional structure ITO/PEDOT:PSS/ICT4 or
ICT6:PC71BM/PFN/Al are fabricated and characterized under
AM1.5 solar illumination (100 mW/cm2). PFN is employed as
cathode interface to improve photovoltaic performance of the
devices. First of all, we have optimized the donor to acceptor
weight ratio (1:1, 1:1.5 1:2 and 1:2.5) in chloroform solution
and best photovoltaic performance is achieved with 1:2 D/A
weight ratio. The current–voltage (J-V) characteristics of the
optimized devices and corresponding photovoltaic parameters
Active layer
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
2.70
2.62)a
ICT4:PC71BM (as cast)
ICT6:PC71BM (as cast)
7.82
9.24
0.96
1.04
0.36
0.40
(
3.84 (3.71)a
ICT4:PC71BM (TSA)
ICT6:PC71BM (TSA)
aAverage of 8 devices
10.52
12.23
0.91
0.98
0.57
0.66
5.46 (5.37)a
7.91 (7.83)a
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layers) leading to more balanced charge transport. The blend
film of ICT6 based active layers (both as cast and TSA treated)
has higher hole mobilities than ICT4 counterpart. This property
leads to better charge extraction and hence to higher JSC and
FF of the BHJSCs.25 Higher IPCE and JSC values of ICT6 based
device compared to ICT4 are attributed to rapid exciton
dissociation in blended active layer as confirmed by the PL
spectra (Fig. S8b, ESI†) and efficient charge transport as
evidenced by the hole mobility. These results suggest that
substitution of fluorine atoms onto the BT units of donor
molecule can enhance intermolecular charge transport in the
BHJ active layer.
Further, improved performance of BHJSCs with ICT6 donor and
its significant difference from ICT4 counterparts is supported
by photocurrent density (Jph, Jph = JL-JD, where JD and JL are the
current densities in dark and under illumination, respectively)
and effective voltage (Veff, Veff = Vo-Vapp, where Vo is the voltage
at zero Jph and Vapp is the voltage applied) characteristics. The
plots of Jph Vs Veff for the BHJSCs are presented in Fig. 6. In the
case of ICT6 based device, at low value of Veff, Jph has nearly
linear dependence on voltage and Jph starts to reach saturation
at Veff around 0.64 V and fully saturated at Veff = 2.4 V. This
indicates that photogenerated excitons are effectively
dissociated and the generated free charge carriers are
efficiently collected at electrodes with minute germinate or
bimolecular recombination. But, for the ICT4 based device, Jph
has greater field dependence over a larger bias range and Jph is
not saturated fully even at Veff = 2.4 V, indicating considerable
germinate and /or bimolecular recombination and poor
extraction/collection of charge carriers at electrodes. Efficient
charge dissociation and effective charge collection at
electrodes could improve the FF values of the BHJSCs with ICT6
over its ICT4 counterpart.60,61 The ratio of Jph/Jsat can represent
the overall exciton dissociation efficiency.62 The Jph/Jsat ratios
for the BHJSCs based on ICT4 and ICT6 measured under short
circuit conditions, are 0.86 and 0.93, respectively. Higher
Jph/Jsat ratio of ICT6 based BHJSCs indicates efficient exciton
dissociation for ICT6 based device compared to the device with
ICT4 active layer. At maximal power output conditions, Jph/Jsat
is 0.66 and 0.70 for ICT4 and ICT6 based devices, respectively
and it also indicates higher charge collection efficiency and
suppressed bimolecular recombination for the ICT6 based
BHJSC devices. Series resistance (Rs) and shunt resistance (Rsh)
of the ICT4 and ICT6 based BHJSCs are calculated from the
inverse slope near VOC and JSC of the J-V characteristics of the
respective devices under illumination, respectively. Compared
Fig. 5. J–V characteristics in the dark for hole-only devices with active
layers processed under different conditions
device with ICT4 active layer (54%) and it could be responsible
for the higher JSC value observed for ICT6 based devices. The
JSC values calculated from the integration of IPCE spectra are
10.41 mA/cm2 and 12.13 mA/cm2, respectively for
ICT4:PC71BM and ICT6:PC71BM based devices and are closely
resembles with the values obtained from J-V characteristics
(Table 3).
Hole and electron mobility of the blend films are evaluated by
measuring the J-V characteristics of hole only
(ITO/PEDOT:PSS/ICT4 or ICT6:PC71BM/Au) and electron only
(ITO/Al/ICT4 or ICT6:PC71BM/Al) devices, in dark, employing
space charge limited current (SCLC) model (Fig. 5). Devices
with as cast ICT4:PC71BM and ICT6:PC71BM active layers shows
hole mobility (
respectively in donor phase, while corresponding electron
mobility (
e) in acceptor phase is 2.45 x 10-4 and 2.52 x 10-5
ꢀ
h) of 1.16 x 10-5 and 4.53 x 10-5 cm2/Vs,
ꢀ
cm2/Vs, respectively. The pronounced difference between ꢀh
and ꢀe indicates that holes and electrons transports are not
balanced in the as cast devices. Upon TSA treatment, the hole
mobility (
1.94
10-4 cm2/Vs, respectively for ICT4:PC71BM and
ꢀ
h) in the donor phase is increased (9.56 x 10-5 and
x
ICT6:PC71BM active layers). However, decrement in the
electron mobility is observed (2.40 x 10-4 cm2/Vs and 2.48 x 10-
4 cm2/Vs, respectively ICT4:PC71BM and ICT6:PC71BM active
to the device with ICT4 (Rs = 12.56
ICT6 (Rs =5.34
cm2) shows smaller Rs, indicating that
ꢁ
cm2), the device with
ꢁ
decrease in the hole collection barrier at the anode interfaces
induced by the more favorable nanoscale morphology and
vertical phase separation as observed in TEM images. The
observed decrease in Rs and increase in Rsh values for ICT6
based device also reflect the better PCE of ICT6, supports
better charge transport and efficient charge extraction by the
electrodes. As we have adopted same device structure and
processing treatment procedure, observed superiority in the
photovoltaic properties of the ICT6 based BHJSCs could be due
Fig. 6. Jph Vs Veff characteristics of BHJSCs with TSA treated
ICT4:PC71BM and ICT6:PC71BM active layers
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Fig. 7. Transmission electron microscopy (TEM) images of TSA treated
ICT4:PC71BM and ICT6:PC71BM thin films, bar is 100 nm
Fig. 8. X-ray diffraction of TSA treated ICT4:PC71BM and ICT6:PC71BM
blend thin films
ICT4:PC71BM and ICT6:PC71BM and it could be originated from
the small crystalline domains of ICT4 and ICT6 66
. Although both
to its structure driven favorable active layer morphology. For
efficient exciton dissociation and charge transport,
morphology of the BHJ should be bi-continuous,
interpenetrating with large interfacial area and regular
nanoscale D/A phase separation (domain size 10-20 nm).63,64
To get information about the morphology of the BHJ, we have
performed transmission electron microscopy (TEM) and X-ray
diffraction (XRD) analyses on TSA processed ICT4:PC71BM and
ICT6:PC71BM active layers. The TEM images of the active layers
processed with TSA treatment are presented in Fig. 7. The
bright and dark regions in the TEM images correspond to the
small molecule donor (ICT4 or ICT6) and PC71BM domains,
the small molecules possess same side chains, the intensity of
(100) diffraction peak of ICT6 is higher with shorter d-spacing
value compared to ICT4 67,68
This indicates that ICT6 has
.
improved lamellar stacking structure due to the self assembly
induced by the fluorine atoms of F2BT units during the film
formation.58 In both the blend films, a broad scattering at 2
θ =
18.62°
broad scattering band at 2
and a reflection peak at 2
θ
= 18.62
= 23.26
°
are observed. The
θ
the amorphous PC71BM agglomerate regions and reflection
°
could be originating from
peak at 2 = 23.26 corresponds to (010) reflection of the
θ
°
donor materials.67,68 The coherence lengths for these blend
films are estimated from the Scherrer’s equation and are
found to be 4.17 nm and 4.58 nm for ICT4:PC71BM and
ICT6:PC71BM, respectively. It can be seen from Fig. 8 that the
intensity of both (100) and (010) reflection peaks for ICT6 are
significantly higher than ICT4 indicating that fluorinated BT in
the backbone preferably adopted a face on orientation
towards the substrate with high degree of lamellar ordering
respectively. As shown in Fig. 7, ICT6:PC71BM blend film
exhibits more appropriate nanoscale morphology and phase
separation than ICT4:PC71BM counterpart, leading to more
effective charge separation at D/A interface. This result
indicates that the fluorination would induce better
compatibility between ICT6 and PC71BM due to the weak
interaction from fluorine atom in the BT unit.65 This favorable
nanoscale morphology can facilitate exciton dissociation and
charge transport and affords a higher JSC and FF values to the
corresponding BHJSCs. Favorable vertical phase separation and
nanoscale morphology observed for ICT6:PC71BM active layer
compared to ICT4:PC71BM may also be related to decrease in
Rs. However the nanoscale morphology of the as cast active
layer is effectively poor (as shown in Fig. S14, ESI†), leading to
the charge transport and low FF, resulting lower overall PCE
for OSC based on as cast film. XRD patterns of TSA treated
ICT4:PC71BM and ICT6:PC71BM films are presented in Fig. 8 and
XRD patterns of as cast blend films based on ICT4:PC71BM and
ICT6:PC71BM are also shown in Fig. S15, ESI†. The as cast active
and
π-π stacking. Therefore, we assume that fluorination of
small molecule donor (ICT6) can enhance the inter-chain
π-π
stacking attributed to relatively high co-planarity of the
molecular backbone induced by the intermolecular
interactions. These results indicate that in fluorinated small
molecule, the 'H' to 'F' exchange in the molecule backbone
promotes intermolecular interaction and packing and it is
presumably because of the lower steric effect of fluorine
atoms present in ICT6. The stronger intensity of the (100) peak
for ICT6 in the blend film than that for ICT4 indicates that the
crystallinity of former is higher than that for later. As a result,
charge transport is enhanced through the strong π-π stacking
interaction, compact stacking and high crystalline nature
layer film showed a broad diffraction peak at 2θ = 4.96° and
5.14 corresponding to (100) d-spacing. The broad diffraction
π-π
°
which are in line with the hole mobilities values obtained from
SCLC measurements. Hence these results collectively supports
that fluorine atom substitution on the donor materials
molecular backbone can induce favorable morphology to the
BHJ due to the increase inter-chain interaction of small
molecule leading to different molecular packing behavior in
the films thereby enhance exciton dissociation, charge
transport, charge collection efficiency and suppress
peak indicates that the as cast films exhibit poor crystallinity.
However, after TSA treatment the intensity of diffraction peak
of the active layer films is increased considerably, indicating
that the crystallinity of the active layer increased. The TSA
treated active layer thin films shows a diffraction peak at 2θ =
4.96 and 5.14 , corresponding to the inter-chain separation
°
(d-spacing) of 1.56 nm and 1.45 nm, respectively for
°
8 | J. Name., 2012, 00, 1-3
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9
H. Zhou, Y. Zhang, C. K. Mai, S. D. Collins, G. C. Bazan, T. Q.
Nguyen and A. J. Heeger, Adv. Mater., 2015, 27, 1767.
bimolecular recombination of the BHJSCs, leading to the
higher PCE of the resultant devices.
10 Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y.
Yang and Y. Chen, Adv. Energy Mater., 2011, , 771.
1
11 J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li,
G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc.,
2013, 135, 8484.
12 L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street and Y. Yang,
Adv. Mater., 2013, 25, 6642.
Conclusion
Two novel, D1-A-D2-A-D1 structured, small molecule donor
materials (ICT4 and ICT6) with variations in the number of
fluorine atoms on the acceptor moieties (BT for ICT4 and F2BT
for ICT6) are synthesized and explored its effect on the
optical, electronic and photovoltaic properties of the donor
materials. BHJSCs are fabricated using ICT4/ICT6 donor and
PC71BM acceptor materials. BHJSCs with ICT6:PC71BM active
layer processed with TSA (PCE = 7.91 %, JSC = 12.23 mA/cm2,
13 Q. Zhang, B. Kan, F. Liu, G. Long, X. Wan, X. Chen, Y. Zuo, W.
Ni, H. Zhang, M. Li, Z. Hu, F. Huang, Y. Cao, Z. Liang, M.
Zhang, T. P. Russell and Y. Chen, Nat. Photonics, 2014,
14 Y. Liu, C.-C. Chen, Z. Hong, J. Gao, Y. Yang, H. Zhou, L. Dou, G.
Li and Y. Yang, Sci. Rep., 2013, , 3356.
9, 35.
3
15 Y. Patil, R. Misra, M. K. Singh and G. D. Sharma, Phys. Chem.
Chem. Phys., 2017, 19, 7262.
VOC = 0.98 V and FF= 0.66) shows superior photovoltaic 16 Y. Patil, R. Misra, F. C. Chen and G. D. Sharma, Phys. Chem.
Chem. Phys., 2016, 18, 22999.
17 P. Gautam, R. Misra, S. Biswas and G. D. Sharma, Phys. Chem.
Chem. Phys., 2016, 18, 13918.
18 P. Gautam, R. Misra and G. D. Sharma, Phys. Chem. Chem.
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performance over the device with ICT4:PC71BM active layer
(PCE = 5.46 %, JSC = 10.52 mA/cm2, VOC = 0.91 V and FF= 0.57).
Increasing the number of fluorine atoms significantly lowers
down the HOMO level of the donor material and improves the
VOC of BHJSCs. It also enhances the intermolecular interaction 19 T. Jadhav, R. Misra, S. Biswas and G. D. Sharma, Phys. Chem.
Chem. Phys., 2015, 17, 26580.
20 P. Gautam, R. Sharma, R. Misra, M. L. Keshtov, S. A. Kuklin
among the donor materials in their active layer thin films and
results in better hole mobility (9.56 x 10-5 and 1.94 x 10-4
cm2/Vs, respectively for ICT4:PC71BM and ICT6:PC71BM active
layers) and balanced charge transport. Further, fluorine atoms
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nanoscale morphology to ICT6:PC71BM active layer resulting
improved exciton dissociation, charge transport, charge
collection efficiency and reduces bimolecular recombination of
the BHJSCs. Reported PCE (7.91 %) of the BHJSCs with ICT6
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CSIR project NWP0054 is greatly acknowledged.
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Physical Chemistry Chemical Physics
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DOI: 10.1039/C7CP02729J
Graphical Abstract
Study on the effect of fluorine substitution on the photovoltaic properties of
dithieno[3,2-b:2',3'-d]pyrrole-benzo[c][1,2,5]thiadiazole
conjugate
small
molecule donor materials
Manohar Reddy Busireddy,a Narendra Reddy Chereddy,a,* Balaiah Shanigaram,b Kotamarthi
Bhanuprakash,b,* Subhayan Biswas,c Ganesh Datt Sharmac,* Vaidya Jayathirtha Rao a,d,
*
aCrop Protection Chemicals Division, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, India.
b Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007,
India.
cDepartment of Physics, The LNM Institute of Information Technology, Jamdoli, Jaipur, India.
dAcSIR, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, India.
Corresponding author Tel.: +91 40 27193933; Fax: +91 40 27193382
E-Mail: chereddynarendra@gmail.com (Narendra Reddy Chreddy); vaidya.opv@gmail.com (Vaidya Jayathirtha
Rao); gdsharma273@gmail.com (Ganesh Datt Sharma); bhanu2505@yahoo.co.in (Kotamarthi Bhanuprakash)
Two novel dithieno[3,2-b:2',3'-d]pyrrole tethered, benzo[c][1,2,5]thiadiazole based small
molecules (ICT4 and ICT6) are synthesized and successfully applied as donor materials for
BHJSCs with ITO/PEDOT:PSS/donor:PC71BM/PFN/Al structure. By adopting simple
fabrication procedures, PCE of 5.46 % and 7.91 %, respectively achieved for BHJSCs with ICT4
and ICT6 donors. Importance of fluorination on the donor materials backbone to improve the
photovoltaic parameters of the corresponding BHJSCs is described in detail.
S1