Asymmetric, efficient π-conjugated organic semiconducting chromophore for bulk-heterojunction organic photovoltaics

Article abstract of DOI:10.1016/j.dyepig.2017.09.048

A novel, asymmetric benzothiadiazole-core π-conjugated organic semiconducting chromophore (CF-BTz-ThR) containing an alkyl bithiophene donor and 3,5-bis (trifluoromethyl) benzene as acceptor unit was synthesized via Suzuki cross-coupling in a relatively cost-effective way. The synthesized chromophore was applied as an acceptor material in the solution-processed fabrication of bulk heterojunction (BHJ) organic photovoltaics (OPVs). The presence of terminal hexyl chain in CF-BTz-ThR induced its solubility in common organic solvents. The energy levels of CF-BTz-ThR were calculated as ?5.22 eV and ?3.19 eV for HOMO and LUMO, respectively. The fabricated devices attained power conversion efficiency (PCE) of ～3.52% for CF-BTz-ThR: PC₆₁BM (1:3, w/w) ratio with the short circuit current (J_SC) of ～10.38 mA/cm² and the open-circuit voltage (V_OC) of ～0.68 V. The reasonable J_SC and V_OC of the devices might be attributed to strong absorption and emission properties as well as the electrochemical properties due to the presence of strong electron-withdrawing benzothiadiazole as well as –CF₃ unit as electron-acceptor.

Full text of DOI:10.1016/j.dyepig.2017.09.048

Accepted Manuscript
Asymmetric, efficient π-conjugated organic semiconducting chromophore for bulk-
heterojunction organic photovoltaics
M. Nazim, Sadia Ameen, M. Shaheer Akhtar, Hyung Shik Shin
PII:
S0143-7208(17)31469-9
10.1016/j.dyepig.2017.09.048
DYPI 6274
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 July 2017
Revised Date: 19 September 2017
Accepted Date: 20 September 2017
Please cite this article as: Nazim M, Ameen S, Shaheer Akhtar M, Shin HS, Asymmetric, efficient π-
conjugated organic semiconducting chromophore for bulk-heterojunction organic photovoltaics, Dyes
and Pigments (2017), doi: 10.1016/j.dyepig.2017.09.048.
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ACCEPTED MANUSCRIPT
Asymmetric, efficient π-conjugated organic semiconducting chromophore for
bulk-heterojunction organic photovoltaics
M. Nazima, Sadia Ameena*, M. Shaheer Akhtarb, Hyung Shik Shina*
aEnergy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical
Engineering, Chonbuk National University, Jeonju, 54896, Republic of Korea
bNew & Renewable Energy Material Development Center (NewREC), Chonbuk National University,
Jeonbuk, Republic of Korea
__________________________________________________________________________________
Abstract
A novel, asymmetric benzothiadiazole-core π-conjugated organic semiconducting chromophore
(CF-BTz-ThR) containing an alkyl bithiophene donor and 3,5-bis (trifluoromethyl) benzene as
acceptor unit was synthesized via Suzuki cross-coupling in a relatively cost-effective way. The
synthesized chromophore was applied as an electron-donor material in the solution-processed
fabrication of bulk heterojunction (BHJ) organic photovoltaics (OPVs). The presence of terminal hexyl
chain in CF-BTz-ThR induced its solubility in common organic solvents. The energy levels of CF-
BTz-ThR were calculated as −5.22 eV and −3.19 eV for HOMO and LUMO, respectively. The
fabricated devices attained power conversion efficiency (PCE) of ~3.52 % for CF-BTz-ThR: PC61BM
(1:3, w/w) ratio with the short circuit current (JSC) of ~10.38 mA/cm2 and the open-circuit voltage
(VOC) of ~0.68 V. The reasonable JSC and VOC of the devices might be attributed to strong absorption
and emission properties as well as the electrochemical properties due to the presence of strong
electron-withdrawing benzothiadiazole as well as –CF3 unit as electron-acceptor.
Keywords: Benzothiadiazole, 3,5-bis (trifluoromethyl) benzene, electrochemical properties,
semiconducting chromophore, organic photovoltaics.
___________________________________________________________________________________________________
*Corresponding author. Tel: +82-63-270-2438, Fax: +82-63-270-2306
E-mail address: hsshin@jbnu.ac.kr (H.S. Shin), sadiaameen@jbnu.ac.kr
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Introduction
In the search of sustainable and environment friendly energy sources, the solar cells are
converting the solar energy to electricity and help to solve the energy and environmental issues [1,2].
The organic photovoltaics especially, bulk-heterojunction (BHJ) organic solar cells are of considerable
interest, both academically and industrially due to their versatile merits as low cost, large-scale
fabrication, flexibility and diversity of electron-donors [3-5]. Owing to their well-defined mono-
dispersed chemical structures as well as better solution-processability, organic chromophores have
attracted a lot of interest for the applications of organic light-emitting diodes (OLEDs), organic
photovoltaics (OPVs), organic field-effect transistors (OFETs) and organic photodetectors [6-10]. The
incorporation of the various electron-donating (D) and electron-accepting (A) units in π-conjugated
organic semiconducting chromophore plays a crucial role for the exciton-formation and its diffusion
towards D-A interface which strikes the efficient charge-transport [11]. The OPVs have been growing
attention owing to their simple and easy synthetic and purification techniques in solution-processed
bulk-heterojunction (BHJ) device fabrication [12]. Recently, W. Zhao et al. reported a record power
conversion efficiency of ~13% using a new polymer donor (PBDB-T-SF) and a new small molecule
acceptor (IT-4F) based fullerene-free [13]. Several fabrication factors such as the film thickness,
composition ratio of D/A ratio, device annealing temperature and time significantly influence the
overall
performance
of
the
photovoltaics
[14,15].
Poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is most often used as the buffer layer
because of its solubility, smooth film surface, and the matching work function to HOMO level of many
organic semiconductors [16], but the hygroscopic nature of PEDOT:PSS layer adversely affects the
intrinsic device stability in ambient conditions of photovoltaic devices and thus, degrades the low-
work-function metals [17]. To introduce a flexible long side chain such as alkyl, alkoxy etc. on the π-
conjugated backbone structure of organic chromophores help to improve the device film morphology
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and the charge transfer properties [18,19].
The organic materials with fluorine substituents shows strong polarizability and a high
electronegativity which might result to the high bond energy of the carbon–fluorine (C-F) bonds and
induces the self-assembly behaviour [20,21]. Recently, Wang and co-workers synthesized
trifluoromethyl-substituted non-coplanar biphenyl polymers for OFETs which enhanced the solubility
in common organic solvents [22,23]. Moreover, (trifluoromethyl) benzene group could be regarded as
an electron-acceptor due to their strong electron-withdrawing ability and a wide range of interesting
applications in medicinal and agricultural chemistry [24,25]. The CF3-based organic material contains
nanofibers morphological structures which form bicontinuous nanoscale film morphology and hence,
reduces the charge recombination [26]. Moreover, CF3-based material with PC61BM blend is of fibrous
morphology which might increase π−π interactions as well as intermolecular charge transfer from
donor to acceptor which could significantly influence the photovoltaic performance [27]. Of late, the
organic chromophores containing trifluoromethyl benzene as the end-group presented a high PCE in
BHJ fabricated small molecule solar cell (SMOSC) devices [28].
In addition, the benzothiadiazole (BT) has a strong electron-accepting capability with rigid
geometry which controls the energy levels but BT without alkyl chain shows poor solubility which
significantly affects the surface film morphology and hence, the performance of the devices [29,30].
The asymmetric benzodithiophene (BDT) have attracted much attention owing to their tuneable energy
levels, optical band gap as well as high hole mobilities which results to the high performance of the
devices [31-33]. Thus, the state-of-art structural modification is necessary for the solution-processed
BT based materials to achieve a better solubility and the device performance [34-36]. Zhou et al.
reported the organic conjugated materials with fluorine-substituted BT unit and demonstrated an
improvement in the performance than the unsubstituted analogues [37]. Recently, the fluorine
substituted BT based organic materials are reported by several research groups which shows the
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improved performance of solution-processed fabricated devices [38]. D-A-π-A type dyes are being
explored excessively and high solar efficiencies are obtained from the dyes containing auxiliary π-
bridge acceptors [39]. These substitutions induce stability and electron transport or ambipolar transport
in organic materials by lowering the energy levels (both the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels [40]. In this manuscript,
a novel and efficient benzothiadiazole-based D-A-A type organic chromophore, 4-(3,5-
bis(trifluoromethyl) phenyl)-7-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c] [1,2,5] thiadiazole (CF-BTz-
ThR) with alkyl bithiophene donor and 3,5-bis (trifluoromethyl) benzene as acceptor is synthesized in
a cost-effective way and utilized for the solution-processed fabrication of SMOSC devices. The
presence of alkyl chain in the organic chromophore has considerably improved its solubility in
common organic solvents and hence, controls the surface film morphology of the devices during the
fabrication.
Experimental
Materials and Equipment
All the chemicals and reagents were purchased from the commercial sources and used without
further purification. The flash Column chromatography was performed on a column packed with silica
gel (300-400 mesh). The thin layer chromatography (TLC) plates of aluminum silica gel 60 F254
(Merck) were used to monitor the reaction progress.
Synthesis of organic chromophore
The synthetic route of benzothiadiazole-based organic chromophore, CF-BTz-ThR is shown in
Scheme 1. The monomeric precursors 2, 3, 5, 6, 7, and 8 were synthesized as reported elsewhere [41-
43]. The intermediate product, 3 (CF-BTz-Br) was obtained by the Suzuki cross-coupling reaction
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between 2,7-dibromo-9,9′-benzothiadiazole, 2 and 3,5-bis(trifluoromethyl)phenyl boronic acid using
Pd (0) catalyst (3 mol %) and potassium carbonate as a base in anhydrous toluene solvent. The
obtained CF-BTz-Br, 3 finally coupled with n-hexyl bithiophene boronic-acid pinacole ester, 8 by
Suzuki coupling using Pd(PPh3)4 catalyst (5 mol %) and aqueous potassium carbonate (2M) solution as
base in anhydrous toluene solvent under inert atmosphere to give the final organic chromophore, 9
abbreviated as CF-BTz-ThR in this manuscript. The synthesized chromophore was purified by flash
column chromatography and then recrystallized in the mixed solvent of dichloromethane/methanol
(2:1, v/v) as red solid (Yield: 72.2 %).
2-(7-bromo-9,9'-benzothiadiazole-2-yl)-5-(3,5-bis(trifluoromethyl)benzene (3).
3,5-bis(trifluoromethyl)phenyl boronic acid, 5 (0.411 gm, 2 mmol), and 2,7-dibromo-9,9′-
benzothiadiazole, 6 (0.948 g, 2 mmol) and Pd(PPh3)4 (15 mg, 5 mol%) were refluxed in degassed
toluene (∼10 mL) with an aqueous K2CO3 solution (2 M) (4 mL) for 24 h under inert atmosphere. The
reaction was cool and deionized (DI) water was added to quench the reaction and then diluted with
hexane (10 mL). The organic layer was extracted with dichloromethane (20 mL), washed with
deionized water, brine solution and dried over anhydrous MgSO4. The solvent was evaporated by
rotary evaporator and dried in vacuum. The crude product was purified by flash column
chromatography by using hexane:dichloromethane (2:1, v:v) followed by the recrystallization from
1
dichloromethane:methanol (5:1, v:v) to get a pale yellow solid (0.641 g, 68%). H NMR (600 MHz,
CDCl3, ppm) δ: 0.82–0.88 (t, 3H), 1.17–1.26 (m, 6H), 1.85 (t, 2H), 3.45 (t, 2H), 6.70 (d, 1H), 6.76 (d,
1H), 6.78 (d, 1H), 7.06-7.12 (t, 4H), 7.18-7.20 (d, 2H), 7.36 (d, 2H), 7.41-7.51 (d, 4H), 7.63 (s, 2H),
13
7.76 (d, 2H) . C NMR (100 MHz, CDCl3, ppm) δ: 150.62, 147.43, 147.13, 141.77, 139.72, 131.20,
128.36, 128.16, 127.41, 126.96, 126.67, 123.45, 121.96, 121.45, 120.88, 120.46, 120.35, 119.37, 31.65,
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30.69, 30.52, 30.27, 28.83, 28.46, 27.62, 27.34, 22.77, 22.69, 14.22 14.16. FTIR (KBr pellets, cm ):
3439, 3028, 2962, 2855, 1731, 1653, 1587, 1485, 1404, 1323, 1256, 1228, 1099, 1022, 976, 948, 836,
792, 691.
2-(5'-hexyl-[2,2'-bithiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (8).
To a solution of 7 (1 g, 2.6 mmol), tetrahydrofuran (20 mL), n-BuLi (1.6 M, 3.17 mmol) was added
o
carefully by a syringe at -78 C under an inert atmosphere. The temperature of the reaction was
o
slowly increased up to -50 C within 20 min. Now, 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (0.58 mL, 5.27 mmol) solution was added dropwise by a syringe at -50 oC followed by
increasing the temperature up to the room temperature (RT) and stirred for 3 h. After completion of
the reaction, a solution of 2 N HCl (20 mL) was added. The reaction mixture was extracted with
diethyl ether (30 mL), washed with brine and DI water (500 mL), followed by dried over MgSO4, and
then evaporated under vacuum oven. The obtained white residue was then recrystallized in hexane
(15 mL) to obtain a white solid product (0.84 g, 93%). 1H NMR (400 MHz, CDCl3, d, ppm): 7.57 (d,
1H), 7.24 (d, 1H), 7.07 (d, 1H), 6.73 (d, 1H), 2.86 (t, 2H), 1.75 (m, 2H), 1.35 (m, 14H), 0.93 (t, 3H);
13C NMR (100 MHz, CDCl3, d, ppm): 146.3, 144.8, 137.7, 134.5, 124.7, 124.2, 84.4, 31.8, 31.3, 30.3,
29.8, 29.5, 29.2, 24.6, 22.5, 14.4.
4-(3,5-bis(trifluoromethyl)phenyl)-7-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (9).
A solution of 2-(5'-hexyl-[2,2'-bithiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 8
(0.381 g, 1 mmol), and intermediate bromide, 3 (0.642, 1 mmol) were added with aqueous solution
(2 M) of K2CO3 (5 mL) and Pd (PPh3)4 catalyst (10 mg, 0.01 mmol) in anhydrous toluene (∼10 mL)
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solvent. The reaction mixture was degassed several times, heated to reflux under inert atmosphere for
24 h. After completion, the reaction (as monitored by TLC) was cooled to room temperature and
diluted with hexane (10 mL). To ensure the completion of the coupling reaction, the boronic acid was
used in access. The organic layer was extracted with dichloromethane (20 mL) and washed with DI
water, brine and dried over anhydrous MgSO4. The crude product was purified by flash column
chromatography using hexane:dichloromethane (5:1, v:v) followed by recrystallization from
dichloromethane: methanol (2:1, v:v) to get the red solid (0.46 g, 72.2%). 1H NMR (600 MHz, CDCl3,
ppm) δ: 0.84–0.88 (t, 3H), 1.18–1.28 (m, 6H), 1.82 (t, 2H), 3.42 (t, 2H), 6.64 (d, 1H), 6.70 (d, 1H),
6.76 (d, 1H), 6.78 (d, 1H), 7.06-7.12 (t, 4H), 7.18-7.20 (d, 2H), 7.36 (d, 2H), 7.43-7.52 (d, 4H), 7.62 (s,
3H), 7.79 (d, 2H) . 13C NMR (100 MHz, CDCl3, ppm) δ: 150.62, 147.43, 147.13, 141.77, 139.72,
131.20, 128.36, 128.16, 127.41, 126.96, 126.67, 123.45, 121.96, 121.45, 120.88, 120.46, 120.35,
119.37, 119.26, 36.09, 30.69, 30.56, 30.42, 28.70, 28.46, 27.62, 27.34, 22.19, 21.69, 13.61, 13.07. FT-
IR (KBr pellets, cm−1): 3439, 3062, 3026, 2954, 2922, 2850, 1590, 1486, 1463, 1423, 1378, 1302,
1278, 1234, 1072, 972, 828, 786, 747, 693, 612, 522. MS (ESI): m/z calc. for [C47H34F6S2]+: 596.12;
found: 596.98 [M+].
Device fabrication
The synthesized CF-BTz-ThR chromophore was applied as photoactive materials for the
fabrication of ITO/PEDOT:PSS/PC61BM:CF-BTz-ThR/Au. First, CF-BTz-ThR chromophore was
dissolved in chlorobenzene and mixed in different amounts with PC61BM. Poly (3,4-ethylene-
dioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) solution as a buffer layer (thickness ~80 nm)
was deposited on cleanITO substrates by spin-coating with the scan rate of ~4000 rpm for 60 s and
annealed at 130 °C for 10 min in a vacuum oven. For depositing the photoactive layer, CF-BTz-
ThR:PC61BM solution with different weight ratios of 1:1, 1:2, and 1:3, w/w were deposited on
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PEDOT:PSS/ITO thin film by spin-coating with the scan rate of ~1000 rpm for ~ 40s. Finally, a top
Au layer of ~100 nm thickness was deposited on ITO/PEDOT:PSS/PC61BM:CF-BTz-ThR thin film by
a thermal evaporation. The active area of the solar cell devices was 1.2 cm2.
Characterizations
The nuclear magnetic resonance (NMR) spectra were obtained by using JEOL FT-NMR
spectrophotometer in CDCl3 as reference solvent (1H at 600 MHz and 13C at 100 MHz). The chemical
shift values were measured as δ values in parts per million (ppm) compared to the tetramethylsilane
(TMS) as internal standard. The internal structure and various bonds of CF-BTz-ThR were
characterized by Fourier-transform-infrared (FTIR) spectroscopy with FTIR-4100 (JASCO)
spectrometer. The ultra violet-visible (UV-vis) absorption spectra were measured by V-670 (JASCO)
spectrophotometer in dilute chlorobenzene solvent and the photoluminescence spectra (PL) was
obtained by the FP-6500 (JASCO) fluorometer in dilute chlorobenzene solvent. Thermo-gravimetric
analysis (TGA) was carried out with a TA instruments Q-50 thermogravimetry analyzer at a scan rate
o
of 10 C/min under inert atmosphere. The differential scanning calorimetry (DSC) was characterized
o
by TA instrument DSC-2910 at a heating and cooling rate of 10 C/min under nitrogen atmosphere.
The Cyclic Voltammetry (CV) measurements were done using WPG 100 Potentiostat/Galvanostat
(WonA Tech) at a scan rate of 100 mV/s with a three-electrode cell containing a glassy carbon
working electrode, a saturated calomel reference electrode (SCE) and a platinum wire as counter
electrode. Herein, CF-BTz-ThR was dissolved in chloroform solvent and thin film was deposited on
the glassy carbon working electrode by drop casting and dried at 60 oC for 4 h under nitrogen. The CV
measurement was performed in 0.1M solution of tetrabutylammonium hexafluorophosphate (TBAPF6)
in acetonitrile. The thin film morphology was performed by using Atomic force microscopy (AFM)
analysis system of AFM Nanoscope IV, Digital Instruments, Santa Barbara, USA. The photovoltaic
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properties of the cells were measured under simulated AM 1.5 radiation at 100 mW/cm using 1000
W metal halide lamp (Phillips) which was served as a simulated sun light source and its light intensity
(or radiant power) was adjusted with a Si photo detector fitted with a KG-5 filter (Schott) as a
reference calibrated at NREL (USA).
Results and Discussion
The thermal stability of CF-BTz-ThR is investigated by TGA under inert atmosphere, while the
phase transition and their interconversion are obtained by differential scanning calorimetry (DSC)
studies under inert atmosphere. Figure 1 (a and b) shows the TGA and DSC profiles of the synthesized
CF-BTz-ThR molecule, and the extracted data are summarized in Table 1. The synthesized
chromophore, CF-BTz-ThR shows a good thermal stability with ~2% weight loss to the decomposition
o
temperature at ~312 C. From DSC thermogram, the organic chromophore displays a glass transition
temperature (Tg) at ~61 oC which originates by the presence of hexyl chain and phenyl rings. The two
melting transitions (Tm) at ~178 oC and ~252 oC clearly evidence the liquid-crystalline (LC) properties
of CF-BTz-ThR which might due to its self-organization behavior due to LC-LC phase transitions [44].
The absorption spectral properties of CF-BTz-ThR are depicted in Figure 2(a) and are
summarized in Table 1. The organic chromophore exhibits two absorption peaks at ~360 and ~510 nm
in chlorobenzene solution, representing the π-π* transition and the intramolecular charge transfer
(ICT) transitions, respectively. In addition, CF-BTz-ThR thin film displays slight broad peaks as
compared to the solution phase. From UV plot, the optical band gap (Egopt) is ~2.03 eV by using the
absorption edge (~608 nm) of solid film. Figure 2(b) shows the photoluminescence (PL) spectra of CF-
BTz-ThR solution, thin films and CF-BTz-ThR:PC61BM thin films. A strong and sharp emission band
is observed at ~614 nm in CF-BTz-ThR solution which prominently broaden and red shifts at ~622 nm
in thin film. PL emission peak exhibits a strong fluorescence quenching behavior when CF-BTz-ThR
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chromophore is blended with PC61BM, facilitating the exciton-diffusion and makes a way to the
exciton-dissociation at D-A interface [45]. In support, the presence of 3,5-bis(trifluoromethyl)benzene
acceptor unit in CF-BTz-ThR might responsible for a strong fluorescence emission, resulting to a
strong electron-acceptor nature via hydrogen bond formation [46]. Moreover, the C–H....F type
hydrogen bonds play a crucial role in an ordered film morphology of the fluorine compounds which
improves their crystallinity and charge carrier mobility and hence, facilitates the charge transport
[47,48]. The emission peak is red-shifted with respect to the absorption peak by the Stokes shift in
organic materials which typically indicates the change in geometry from the ground state to the excited
state [49]. Hence, the energy losses upon electron excitation are larger in CF-BTz-ThR due to the
strong Stokes shift (~104 nm) and the presence of terminal hexyl side-chain.
The cyclic voltammetry (CV) spectrum of CF-BTz-ThR thin film is carried out using 0.1 M
tetra butyl ammonium hexaflouro phosphate [nBu4N]+[PF6]- in acetonitrile solution at a potential scan
rate of 100 mV/s. CV plot is presented in Figure 3 and summarized in Table 1. CF-BTz-ThR thin film
shows the oxidation potential peak at onset value Eox = +0.84 ± 0.02 V. By using the onset value and
the formula; LUMO = HOMO + Eg, HOMO, energy level is calculated as –5.22 eV while LUMO
energy level is computed as –3.19 eV. The obtained LUMO energy level of CF-BTz-ThR is deep
because of the presence of the thiophene and its derivatives whereas, the presence of benzothiadiazole
ring governs a deep HOMO energy level. Moreover, 3,5-bis(trifluoromethyl)benzene unit in CF-BTz-
ThR molecule acts as strong electron-withdrawing group which might cause a strong fluorescence
emission and deepens the HOMO energy level.
The photovoltaic parameters of the fabricated SMOSC devices are evaluated by the current
density (J)-voltage (V) measurements (Fig. 4) under the 1 sun light (100 mW/cm2, 1.5 AM). Among
the fabricated SMOSC devices, the highest efficiency of ~3.52% is achieved by CF-BTz-ThR:PC61BM
(1:3, w/w) ratio whereas, other fabricated SMOSC devices exhibit inferior PECs of ~1.16% for CF-
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BTz-ThR:PC61BM (1:1, w/w) ratio and ~2.75% for CF-BTz-ThR:PC61BM (1:2, w/w) ratio. The
fabricated devices of CF-BTz-ThR:PC61BM (1:3, w/w) ratio presents the high short-circuit current
density (Jsc) of ~10.38 mA/cm2 along with open circuit voltage (Voc) of ~0.68 V due to a high
absorption and the scattering behavior. The improved JSC and the performance of CF-BTz-
ThR:PC61BM (1:3, w/w) active layer might explain by the smooth film morphology and a fast
intermolecular charge transfer (ICT) between CF-BTz-ThR and PC61BM due to the introduction of
trifluoromethyl-acceptor unit with benzothiadiazole backbone [50]. The introduction of alkyl groups
on such donor groups might further reduce the interaction between an electrolyte and D-A interface,
which would further result to the suppression of the charge recombination and the improvement of
VOC. Moreover, the nonlinear benzothiadiazole based molecules and the terminal alkyl chain in
bithiophene unit might significantly facilitate the charge carriers mobility, resulting to the high
photocurrent density and thus, the performance of solar cell devices [51]. From Table 2, the fabricated
SMOSC with CF-BTz-ThR:PC61BM (1:3, w/w) thin film demonstrates the high FF of ~49%, which is
clearly related to its low Rs value of ~3.6 Ωcm2. However, the low FF in SMOSC with CF-BTz-
ThR:PC61BM (1:1, w/w) is resulted from the large series resistance (Rs) and low shunt resistance
(Rsh) of the solar cell devices, as summarized in Table 2. It could be assumed that the creation of
small barriers at the interfaces between the organic layers during the fabrication of solar cell might
result a large Rs in CF-BTz-ThR:PC61BM (1:1, w/w) thin film, premising the high recombination of
excitons at CF-BTz-ThR and PC61BM interface [52,53]. Hence, the blend film of CF-BTz-
ThR:PC61BM (1:3, w/w) achives a high JSC, high FF and power conversion efficiency due to good
interfacial contact between donor and acceptor layers.
Moreover, the external quantum efficiency (EQE) curves of CF-BTz-ThR:PC61BM (1:1, 1:2,
and 1:3 w/w) ratios based SMOSCs are investigated to measure the absorption and the incident photon
to current conversion properties. From the EQE curves of the fabricated SMOSCs (Fig. 5), CF-BTz-
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ThR:PC61BM (1:3, w/w) based SMOSC exhibits the highest EQE value as compared to CF-BTz-
ThR:PC61BM (1:1 and 1:2, w/w) ratios based SMOSCs. By the integration of EQE curve, CF-BTz-
ThR:PC61BM (1:3, w/w) based SMOSC exhibits the estimated JSC of ~11.12 mAcm-2, which shows
~6.6% mismatch with the JSC value obtained from J-V curves. The integrated JSC of other devices
match well with the obtained values from J-V curves. The EQE results are fully consistent with the J-V
parameters of the fabricated SMOSCs.
Fig. 6 shows the atomic force microscopy (AFM) to analyse the surface morphology and the
homogeneity of active layers of different blends ratios of CF-BTz-ThR:PC61BM (1:1, 1:2 and 1:3,
w/w) deposited as thin film by solution spin-coating techniques. From Fig. 6(f), the thin film
morphology of CF-BTz-ThR:PC61BM (1:3, w/w) ratio exhibits a nanoscale phase separation,
indicating a good blend properties of CF-BTz-ThR and PC61BM in chlorobenzene solvent [54]. As
shown in Fig. 6(f), the blend thin film of CF-BTz-ThR:PC61BM (1:3, w/w) exhibits smooth and
homogeneous film morphology with low root mean square roughness as compared to other blend films
of CF-BTz-ThR:PC61BM (1:1, and 1:2, w/w). Thus, the active layer CF-BTz-ThR:PC61BM (1:3, w/w)
supports a good miscibility of CF-BTz-ThR with PC61BM due to spontaneous phase-segregation,
which might facilitate the efficient charge transfer mechanism in organic solar cell devices. It is known
that the occurrence of low roughness in organic blend film provides a good platform at D-A interface
for an effective exciton-dissociation [55]. Herein, CF-BTz-ThR:PC61BM (1:3, w/w) ratio blend thin
film clearly displays the low roughness which might result to an effective transport and the
dissociation of excitons at D–A interface. Therefore, the improved thin film homogeneity and less
rough surface in CF-BTz-ThR:PC61BM (1:3, w/w) ratio blend thin film has considerably increased JSC
and the power conversion efficiency of SMOSCs, as summarized in Table 2.
Conclusions
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A novel, asymmetric, benzothiadiazole-based π-conjugated organic semiconducting
chromophore (CF-BTz-ThR) is synthesized and applied as photoactive material for the solution
processed bulk heterojunction fabrication of SMOSCs. The synthesized organic chromophore CF-
BTz-ThR is highly soluble in common organic solvents and possesses a reasonable HOMO and
LUMO energy levels of −5.22 eV and −3.19 eV, respectively. The fabricated SMOSCs with CF-BTz-
ThR:PC61BM (1:3, w/w) ratio exhibits the highest PCE of ~3.52% with JSC of ~10.38 mA/cm2 and
VOC of ~0.68 V. These improvements might attribute to the surface for the transport and dissociation
of excitons at the D–A interface in CF-BTz-ThR:PC61BM (1:3, w/w) ratio blend thin film.
Acknowledgements
Dr. Sadia Ameen acknowledges NRF Project # 1601002318. This work is also supported by
NRF Project # 2011-0029527. We acknowledge the Korea Basic Science Institute, Jeonju branch, for
utilizing the research supported facilities.
Author Contributions
M. N. performed all experiments and the characterizations; S. A. and H.S.S. interpreted the obtained
results. All authors reviewed the manuscript.
Additional Information
Competing Interests: The authors declare no competing financial interests.
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Table and Figure Captions
Table 1 Thermal, optical and electrochemical parameters of the CF-BTz-ThR chromophore.
Table 2 Photovoltaic parameters of fabricated SMOSCs.
Scheme 1. Synthetic scheme of benzothiadiazole-based organic chromophore.
Figure 1. (a) Thermogravimetric analysis (TGA) and (b) differential scanning calorimetry (DSC) plots of the
synthesized benzothiadiazole-based chromophore.
Figure 2 (a) UV-Vis spectra of CF-BTz-ThR in chlorobenzene solution (Black line) and film (Red line); (b)
Photoluminescence spectra of CF-BTz-ThR solution (Black line), thin film (Red line) and CF-BTz-
ThR:PC61BM thin film (1:3) ratio (Blue line) deposited by solution spin coating.
Figure 3 (a) Model structure representation of HOMO/LUMO energy levels of CF-BTz-ThR (b) Cyclic
voltammetry of CF-BTz-ThR with cathodic and anodic cycles in 0.1 M TBAPF6 anhydrous acetonitrile
electrolyte in acetonitrile solution at the scan rate of 100 mVs-1.
Figure 4. J-V curves of fabricated SMOSCs with the active layer of CF-BTz-ThR:PC61BM at various ratios of
1:1, 1:2 and 1:3 (w/w).
Figure 5. EQE curves of fabricated SMOSCs with the active layer of CF-BTz-ThR:PC61BM at various ratios
of 1:1, 1:2 and 1:3 (w/w).
Figure 6. Topographical and 3D AFM images of different active layer blend thin films of (a, d) CF-BTz-
ThR:PC61BM (1:1, w/w), (b, e) CF-BTz-ThR:PC61BM (1:2, w/w) and (c, f) CF-BTz-ThR:PC61BM (1:3,
w/w) ratio.
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Table 1
a
b
e
Td
(oC)
Tm
(oC)
λmax
λmax
Eox
(V)
HOMOc
(eV)
LUMOd
(eV)
Eg
Chromophore
CF-BTz-ThR
(nm)
(nm)
(eV)
312
178,252
360,510
614
+0.84
–5.22
–3.19
2.03
aAbsorption in chlorobenzene solution. bPhotoluminescence emmission in chlorobenzene solution. cEstimated from the onset of
oxidation wave of cyclic voltammogram. dCalculated from the formula, LUMO = HOMO + Eg. eCalculated from the formula,
Eg = 1240/λfilm(edge)
.
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Table 2
Photovoltaic parameters
CF-BTz-ThR:
PC61BM
JSC
(mA/cm2)
5.08
VOC
(V)
FF
(%)
36
Rs
(Ω cm2)
5.8
Rsh
(Ω cm2)
38.6
PCE
(%)
1.16
2.75
3.52
1:1, w/w
1:2, w/w
1:3, w/w
0.63
0.66
0.68
9.17
46
5.6
82.3
10.38
49
3.6
89.4
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Scheme 1
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Figure 1
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Figure 2
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Figure 3
25

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Figure 4
26

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Figure 5
27

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Figure 6
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Highlights
> An asymmetric benzothiadiazole-core π-conjugated organic (CF-BTz-ThR) semiconducting
chromophore > It is applied as an electron-donor material in the solution-processed organic solar
cells. > CF-BTz-ThR presents the good HOMO (−5.22 eV) and LUMO (−3.19 eV) values. >
Device with CF-BTz-ThR:PC61BM (1:3, w/w) ratio attains reasonable power conversion
efficiency (PCE) of ~3.52 %