A new 2D-naphtho[1,2-b:5,6-b']dithiophene based donor small molecules for bulk-heterojunction organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2018.11.037

We present design and synthesis of three new symmetrical and linear A-D-A type π-conjugated donor small molecules (2D-NDT(TPD)₂, 2D-NDT(Ester)₂ and 2D-NDT(Amide)₂) containing two dimensional (2D) naphthodithiophene (NDT) unit as the central donor core, end-capped with electron deficient unit such as thieno[3,4c]pyrrole-4,6-dione (TPD), 2-ethylhexyl 2-cyanoacetate (Ester) and 2-cyano-N-(2-ethylhexyl)acetamide (Amide) group respectively. We characterized these small molecules and further investigated the optical, electrochemical, morphological and photovoltaic properties. When solution–processed bulk heterojunction organic solar cells are fabricated using these small molecules, the morphology of 2D-NDT(Ester)₂ or 2D-NDT(Amide)₂ and [6,6]-PhenylC₇₁-butyric acid methyl ester (PC₇₁BM) blend film was optimized using 1,8 Diiodooctane (DIO) additive. DIO additive promotes the formation of nanoscopically well-connected molecular domains in the active blend film. A device based on (1% DIO, 1:1) 2D-NDT(Ester)₂:PC₇₁BM exhibited highest the efficiency of 1.22% with a short-circuit density (J_sc) of 3.75 mA/cm², an open circuit voltage (V_oc) of 0.91 V and fill factor (FF) of 35.50. Similarly for (1% DIO, 1:3) 2D-NDT(Amide)₂:PC₇₁BM device efficiency of 0.55%, with J_sc of 2.36 mA/cm², V_oc of 0.64 V and FF of 36.95 was observed. Whereas for (1:2) 2D-NDT(TPD)₂:PC₇₁BM device, due to the improper blending and phase separation between donor and acceptor efficiency restricted to 0.33% with J_sc of 1.66 mA/cm², V_oc of 0.73 V and FF of 27.2.

Full text of DOI:10.1016/j.dyepig.2018.11.037

Accepted Manuscript
A new 2D-naphtho[1,2-b:5,6-b’]dithiophene based donor small molecules for bulk-
heterojunction organic solar cells
Sushil S. Bagde, Hanok Park, Van-Huong Tran, Soo-Hyoung Lee
PII:
S0143-7208(18)31819-9
https://doi.org/10.1016/j.dyepig.2018.11.037
DYPI 7181
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 16 August 2018
Revised Date: 19 November 2018
Accepted Date: 20 November 2018
Please cite this article as: Bagde SS, Park H, Tran V-H, Lee S-H, A new 2D-
naphtho[1,2-b:5,6-b’]dithiophene based donor small molecules for bulk-heterojunction organic solar
cells, Dyes and Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.11.037.
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ACCEPTED MANUSCRIPT
A new 2D-Naphtho[1,2-b:5,6-b’]dithiophene Based Donor Small Molecules for
Bulk-heterojunction Organic Solar Cells
Sushil S. Bagde, Hanok Park, Van-Huong Tran, and Soo-Hyoung Lee*
School of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-
gu, Jeonju-si, jeollabuk-do, 54896, Republic of Korea.
*Corresponding author; Telephone: +82-63-270-2435, Fax: +82-63-270-2306,
E-mail: shlee66@jbnu.ac.kr
Research highlights:
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) 3 new small molecules based on new 2D-NDT donor core are explored for the bulk-
heterojunction organic solar cell.
) Effect of intermolecular interaction in 2D-NDT(Ester) and 2D-NDT(Amide) was
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observed and affected the overall device performance.
) Power conversion efficiency up to 1.22 % achieved for 2D-NDT(Ester)2.

ACCEPTED MANUSCRIPT
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A new 2D-Naphtho[1,2-b:5,6-b’]dithiophene Donor Based Small Molecules for
Bulk-heterojunction Organic Solar Cell
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Sushil S. Bagde, Hanok Park, Van-Huong Tran, and Soo-Hyoung Lee*
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School of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-
gu, Jeonju-si, jeollabuk-do, 54896, Republic of Korea.
*Corresponding author; Telephone: +82-63-270-2435, Fax: +82-63-270-2306,
E-mail: shlee66@jbnu.ac.kr
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Abstract: We present design and synthesis of three new symmetrical and linear A-D-A type π-
conjugated donor small molecules (2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide) )
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containing two dimensional naphthodithiophene (NDT) unit as the central donor core, end-
capped with electron deficient unit such as thieno[3,4c]pyrrole-4,6-dione (TPD), 2-ethylhexyl 2-
cyanoacetate (Ester) and 2-cyano-N-(2-ethylhexyl)acetamide (Amide) group respectively. We
characterized these small molecules and further investigated the optical, electrochemical,
morphological and photovoltaic properties. When solution–processed bulk heterojunction
organic solar cells are fabricated using these small molecules, the morphology of 2D-
NDT(Ester) or 2D-NDT(Amide) and [6,6]-PhenylC -butyric acid methyl ester (PC BM)
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2
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blend film was optimized using 1,8 Diiodooctane (DIO) additive. DIO additive promotes the
formation of nanoscopically well-connected molecular domains in the active blend film. A
device based on (1%DIO, 1:1) 2D-NDT(Ester) :PC BM exhibited highest the efficiency of
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2
1.22% with a short-circuit density (Jsc) of 3.75 mA/cm , an open circuit voltage (Voc) of 0.91 V
and fill factor (FF) of 35.50. Similarly for (1% DIO, 1:3,) 2D-NDT(Amide) :PC BM device
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efficiency of 0.55%, with Jsc of 2.36 mA/cm , Voc of 0.64 V and FF of 36.95 was observed.
Whereas for (1:2) 2D-NDT(TPD) :PC BM device, due to the improper blending and phase
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separation between donor and acceptor efficiency restricted to 0.33% with J of 1.66 mA/cm ,
sc
Voc of 0.73V and FF of 27.2.
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Keywords: 2D-NDT, Small Molecules, Organic solar cell, Intermolecular interactions.
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1. Introduction
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Organic solar cells (OSCs) have emerged as one of the finest ways of technology for production
of electricity through the abundant and eternal resource of sunlight to fulfill the energy demand
of the growing world [1-2]. OSCs have numerous advantages such as low production cost, high
flexibility and light weight with ease of solution-processability [3-4]. Bulk heterojunction (BHJ)
OSCs, where active layer typically composed of donor materials such as D-A conjugated
polymers or small molecules (SM) and fullerene acceptor (PCBM) have delivered high power
conversion efficiencies (PCE) as a result of meticulous development in device engineering
(interface modification by Interfacial modifiers or by using interlayers) and molecular energy
levels alignments through structure modification [5-7]. Undoubtedly SM possesses advantages
over polymers [8], also shows potential to deliver the high PCEs than polymeric solar cells,
which have opened new pathways for research and development for SM OSCs.
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Current enhancement in PCE of SM OSCs is mainly influenced by rational design and synthesis
of novel SM as photoactive materials. At glance, SM designs of linear analogs are generally
comprised of either D-π-A-π-D or A-π-D-π-A skeleton, [9-12] being as most effective molecular
motifs, where donor (D) and acceptor (A) are separated by π-conjugated spacer units. Among
these two strategies, A-π-D-π-A is appeared to be the effective one for obtaining the high-
performance photovoltaic material. In the recent years, a new class of two-dimensional (2D)
photovoltaic materials are developed, where along the vertical axis of molecular framework,
thienyl conjugated side chains are introduced to broaden the spectral coverage (for high Jsc), and
to enhance charge transport ability (high hole mobility) for improving the device
performance[13]. Similar 2D conjugation concept is applied and explored for BDT molecular
framework. For instance, an alkylthienyl-substituted benzo(1,2-b:4,5-b’)dithiophene (BDT-TH)
donor unit has emerged as an efficient 2D conjugated donor block for small molecules. Chen et
al. have demonstrated the excellent utility of 2D-BDT core for DR3TBDTT-HD and
DR3TBDTT donor small molecules in OSCs applications [14]. Similarly Sun et al. have
prepared new 2D nematic liquid crystalline BTR small molecule, where higher thickness of 400
nm active layer, shows PCE around 8% with fill factor up to 70%, which is quite appreciable
[15]. Furthermore, Liu. et al. showed an application of SMPV1, 2D small molecules in the single
junction and tandem solar cells, where PCE around 8 and 10% were recorded respectively [16].
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These results also assure the future mass scale production of 2D materials for commercial
applications.
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Among the rigidly fused polycyclic aromatic systems, naphthodithiophene (NDT) shows
desirable OSC properties, which have linear (lNDT) [17] and zigzag shape isomers (zNDT) [18].
zNDT unit shows unique properties, such as ideal HOMO energy level, balance charge transport
ability, and high space utilization than BDT [19]. Considering the vertical expansion of NDT
using alkyl thienyl moieties could expand the conjugation framework and increase the charge
transport ability due to improved π-π intermolecular interactions. Such example of an expansion
of zNDT based molecules are rarely studied and explored for SM [20]. Here in, we present a new
2D NDT donor core, coupled with strong acceptor thieno[3,4c]pyrrole-4,6-dione (TPD) to obtain
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new small molecule 2D-NDT(TPD) . TPD is one the successful acceptor in polymeric solar cells
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due to its excellent photovoltaic properties [21-23] and rarely used with 2D NDT based small
molecules for photovoltaic applications.
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The resultant small molecule, 2D-NDT(TPD) show limited solubility in common organic
2
solvents. To improve the solubility we replaced the TPD acceptor group with 2-ethylhexyl 2-
cyanoacetate (Ester) and 2-cyano-N-(2-ethylhexyl)acetamide (Amide) to obtain 2D-NDT(Ester)2
2
and 2D-NDT(Amide) respectively. The structural variation of ester and amide acceptor end
group can influence the photoactive layer by the intermolecular interactions between donor and
acceptor [24]. These intermolecular interactions (hydrogen (H)-bonding, π-π stacking and dipole-
dipole interactions) determine the molecular packing and orientation of molecules, also affect the
optical and electrical properties of the photoactive layer, thus relates to device performance.
Fused thiophene based D-A conjugated small molecules containing ester and amide acceptor
units have recently gained considerable attention due to their efficient light harvesting, effective
π-π stacking, superior charge transfer behavior and solar cell performance. For instance, Kim et
al. developed donors comprised of dithioenosilole (DTS) central core and π-conjugated spacers
end-capped with different acceptors, such as 2-ethylhexyl 2-cyanoacetate and 2-cyano-N-(2-
ethylhexyl)acetamide acceptor units, where the latter molecule is capable of forming strong H-
bonding [25]. A similar type of small molecules with dithieno thiophene (DTT) core was
synthesized by Xiao et al [26]. Furthermore, Stupp et al. synthesized DPP core based symmetric
and asymmetric donor molecules end-capped with ester and amide end groups respectively [27].
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So far small molecules based on 2D NDT donor coupled with ester and amide acceptor group are
rarely studied and explored for OSCs applications. In order to evaluate the effect of ester and
amide acceptor group on molecular interactions and photovoltaic performance in 2D NDT based
2
small molecules, we synthesized a new 2D NDT based small molecules as 2D-NDT(Ester) and
2D-NDT(Amide) with ester and amide end group respectively.
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In this contribution, we explored the use of new 2D NDT donor moiety to construct 2D small
molecules. Combining 2D NDT with TPD, ester and amide acceptor obtained new 2D small
molecules as 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide) respectively (Fig.1).
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These small molecules were used to fabricate BHJ OSC devices, (1:1) 2D-NDT(Ester) :PC BM
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1
1
1
1
1
blend device were prepared using a 1,8-diiodooctane (DIO) additive, which proved to be
effective and exhibited a PCE of 1.22%. Whereas for (1:3) 2D-NDT(Amide) :PC BM blend
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2
device with DIO additive PCE of 0.55% and for (1:2) 2D-NDT(TPD) :PC71BM device, PCE of
0.33% was observed. Design, synthesis and photovoltaic properties of these three new small
molecules are studied and presented.
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Fig. 1. Chemical structures of 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide) .
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2. Results and discussion
2.1. Materials preparation and thermal properties
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The synthesis of small molecules is shown in Scheme 1. 2-(2-Ethylhexyl)thiophene (1) was
obtain by alkylating thiophene with 3-(bromomethyl)heptane, using n-butyl lithium, which was
further converted to (5-(2-Ethylhexyl)thiophen-2-yl) trimethylstannane derivative (2) and reacted
with 3-bromothiophene to obtain 5-(2-Ethylhexyl)-2,3'-bithiophene (3) in toluene by Pd(PPh3)4
catalyzed reaction. Compound 3 was selectively brominated using N- bromosuccinimide (NBS)
to obtain bromine functionalized derivative 2'-Bromo-5-(2-ethylhexyl)-2,3'-bithiophene (4).
Compound 4 was reacted with compound 9 under Pd (PPh ) Cl catalyzed reaction in
2
3 2
2
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
chlorobenzene (CB) to obtain compound 10 which was further converted to stille derivative,
compound 11. The target compound 2D-NDT(TPD) was synthesized via stille cross-coupling
2
reaction of compound 11 and compound 13 using Pd
2 3 3
(dba) /P(o-tolyl) as a catalyst. Further
compound 11 was treated with 5-bromothiophene-2-carbaldehyde under Pd(PPh ) catalyzed
3
4
reaction in toluene to obtain compound 12, Then by Knoevenagel condensation, intermediate 12
was reacted with 2-ethylhexyl 2-cynoacetate acceptor in presence of catalytic triethyl amine in
dry chloroform to obtain
2D-NDT(Ester)2, whereas for acceptor 2-cyno-N-(2-
ethylhexyl)acetamide strong base such as piperidine in catalytic amount under similar condition
was required to produce 2D-NDT(Amide) . The molecular structures of small molecules were
2
1
13
confirmed by H and C NMR (Fig. S2-S9). The thermal properties of 2D-NDT(TPD) , 2D-
2
2 2
NDT(Ester) and 2D-NDT(Amide) were investigated by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) Fig. S1. The TGA curve reveals good thermal stability
o
with onset decomposition temperature, 5% weight loss (T
d
) occurring at 295 C for 2D-
NDT(TPD) , whereas 2D-NDT(Ester) and 2D-NDT(Amide) shows decomposition temperature
2
2
2
o
o
at 331 C and 350 C respectively under a nitrogen atmosphere. DSC was performed at the ramp
o
o
rate of 10 C/min, the melting endotherm for 2D-NDT(TPD)
2
occurs at 128.17 C, and upon
o
cooling it exhibit crystallization exotherm at 76.71 C, whereas for 2D-NDT(Ester) shows
2
o
o
exothermic peak at 196 C and sharp endothermic peak at 103 C respectively, which indicates
the tendency to crystallize for 2D-NDT(TPD) and 2D-NDT(Ester) . 2D-NDT(TPD) shows
2
2
2
lower exothermic and endothermic processes temperature, which could be assign to different
2
packing mode for this molecule. For 2D-NDT(Amide) no prominent peaks were observed,
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during heating and cooling cycle, which could be ascribed to the amorphous nature of 2D-
NDT(Amide) . The DSC results indicate, 2D-NDT(TPD) and 2D-NDT(Ester) molecules
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2
2
shows higher crystalline nature as a result of closer intermolecular π-π interaction. Varied
packing modes provide reasonable explanation for different DSC behavior for these small
molecules.
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Scheme 1. Synthesis routed to obtain 2D-NDT(TPD) , 2D-NDT(Ester) and NDT(Amide) .
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2.2. Optical and electrochemical properties
The absorption spectra of 2D-NDT(TPD) 2D-NDT(Ester) and 2D-NDT(Amide) in chloroform
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,
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and thin solid film are shown in Fig. 2a and b. and data is summarized in Table 1. In dilute
-5
-1
solution (10 mol L ) 2D-NDT(TPD) shows absorption profile from 350–500 nm, with a
2
-1
-1
maximum absorption peak (λmax) at 443 nm (ɛ = 108600 L M cm ) whereas ester and amide
compounds shows broad absorptions range between 350-580 nm due to intramolecular charge
transfer (ICT) from donor NDT to acceptor either ester or amide. A closer observation of spectra
showed vibronic shoulder around 360-380 nm. 2D-NDT(Ester) shows maximum absorption at
2
-1
-1
478 nm (ɛ = 236000 M cm ), while changing from ester to amide in 2D-NDT(Amide) shows
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-1
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maxima at 461 nm (105000 M cm ). This hypochromic shift (~17nm) in the absorption
maxima could be the result of weak electron accepting tendency of amide than ester group. In
thin film state, 2D-NDT(TPD) shows absorption peak at 480 nm with shoulder peak at 512 nm.
2
The red shift of 37 nm from solution to a thin film and prominent vibronic peak, indicate strong
intermolecular
π-π stacking of molecules in the thin film state. Effect of intermolecular
interaction due to oxygen and nitrogen can be seen in thin film state spectra for ester and amide
compound. As both molecules have a different degree of intermolecular interaction, the
molecular ordering and stacking order is significantly altered. The maximum absorption of 2D-
2
NDT(Ester) was 519 nm which red-shifted by 41 nm, along with the intense vibronic peak in
lower energy region, could be the result of planner framework and enhanced intermolecular π-π
stacking [28-29]. For 2D-NDT(Amide) broad absorption (Peak maxima at 479) without any
2
significant vibronic peak was observed with red shift of a 18 nm. Probably 2-ethylhexyl side
chain disturbs the molecular planarity and thus, π-π overlapping [30-31], which is strongly
influenced by hydrogen bonding in case of amide [25]. The optical band gaps were found to be
2.1, 1.9 and 2 for 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide) respectively. To
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estimate the molecular energy levels of 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide)
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2
2
cyclic voltammetry (CV) measurements were carried out using Ag/AgCl reference electrode in a
0.1 M solutions of Bu NPF in acetonitrile solution at room temperature under argon with scan
4
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-1
rate of 50 mV Fig. 2c. The CV instrument was calibrated using the ferrocene/ferrocenium
+
+
(Fc/Fc ) redox couple as an external standard. The onset potential of the Fc/Fc redox couple was
found to be 0.4 V relative to the Ag/AgCl reference electrode. HOMO/LUMO were calculated
ox
from the onset oxidation and reduction potential according to the equations HOMO = -e(E onset
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red
1
2
3
4
+ 4.4) and LUMO = -e(E
+ 4.4). From onset potential value, the HOMO energy level was
onset
found to be -5.36, -5.46 and -5.4 eV, whereas LUMO was found to be -3.58, -3.61 and -3.57 eV
for 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide) respectively. Optical band gap of
2
2
2
2.1, 1.9 and 2 eV was observed respectively.
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.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
2D-NDT(TPD) Sol
(b)
2D-NDT(TPD) film
2
2D-NDT(Ester) film
2
2D-NDT(Amide) film
2
2
2
D-NDT(Ester) Sol
2
1
.0
.8
.6
.4
.2
2
D-NDT(Amide) Sol
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0
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1
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2
0
0
0.0
300
400
500
600
300
400
500
600
700
Wavelength (nm)
(c)
2D-NDT(TPD) Oxi
2
2
D-NDT(TPD) Red
2
2
D-NDT(Ester) Oxi
2
2
D-NDT(Amide) Oxi
2
2D-NDT(Ester) Red
2
2
D-NDT(Amide) Red
2
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V)
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Fig.2. (a) UV-vis absorption spectra of 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide)
2 2
2
in chloroform solution. (b) UV-vis absorption spectra of 2D-NDT(TPD)
2
, 2D-NDT(Ester)
2
and
2D-NDT(Amide) in thin film state. (c) Cyclic voltammograms of UV-vis absorption spectra of
2
2D-NDT(TPD)
2
, 2D-NDT(Ester)
2
2
and 2D-NDT(Amide) in chloroform. (d) Energy levels of
small molecules with PC BM.
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Table 1. Photophysical and electrochemical data for small molecules.
a
ox
red
λ max
Sol
nm)
b
c
(Opt)
E
E
hole
mobility
electron
mobility
(µ )
e
onset
onset
5
-1
λ
max
λ
onset
E
g
Molecule
ɛ (× 10 M
/HOMO
(eV)
/LUMO
(eV)
-
1
Film(nm)
(nm)
(eV)
(
cm )
(µ
e
)
-9
-6
2
D-NDT(TPD)
2
443
1.08
480
570
2.1
0.86/-5.26
-0.82/-3.58
6.62×10
5.07×10
-
6
8
-6
2
D-NDT(Ester)2
478
461
2.36
1.05
519
479
624
609
1.9
2
1.06/-5.46
1.0/-5.4
-0.79/-3.61
-0.83/-3.57
2.91×10
5.20×10
8.08×10
7.04×10
-
-6
2D-NDT(Amide)2
3
a
b
c
4
5
6
7
Measured in chloroform solution. Spin-coated film from chloroform solution. Optical band
opt
d
gap, Eg = 1240/(λonset)film. Potential determined by cyclic voltammetry in 0.10 M Bu NPF -
CH
mobility ( e
4
6
ox
ox
f
3
CN. HOMO =-e(E onset+4.4). LUMO = -e(E onset+4.4). hole mobilities (
h
µ ) and electron
µ
) measured by SCLC technique.
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2.3. Photovoltaic properties
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2
3
2 2 2
To check the utility of 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide) in OSCs,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
photovoltaic devices were fabricated using small molecules as donors and PC BM as an
71
acceptor in chlorobenzene or chloroform solution using conventional device architecture of
ITO/PEDOT/small molecules:PC BM/LiF/Al. Different weight ratios (D:A) of systems were
7
1
-2
tested under a simulated AM 1.5 G illumination of 100 mW cm , to find the optimal device ratio
combination. The detailed Photovoltaic performance of small molecules is summarized in Table
2 and corresponding (J-V) curves are shown in Fig. 3a. For (1:1) 2D-NDT(TPD) :PC BM active
2
71
2
layer blend ratio PCE of 0.29 % with Jsc of 1.58 mA/cm , Voc of 0.71 and FF of 25.61 was
observed. Further increasing the blend ratio to (1:2) delivered the highest PCE of 0.33% with
2
improved J of 1.66 mA/cm , V of 0.73 and FF of 27.20. This PCE enhancement is attributed
sc
oc
to the improved device parameters (Voc, Jsc and FF). Increasing the PCBM content to (1:3) and
2
(1:4) affected the Jsc and FF, J dropped from 1.54 to 1.38 mA/cm along with reduced FF of
sc
27.01 and 26.02, thus PCE reduced to 0.3% & 0.26%; whereas Voc was nearly similar. For 2D-
2
NDT(Ester) , using (1:1) blend ratio gave highest PCE of 0.84% with J of 2.78 mA/cm , V of
2
sc
oc
0.98 and FF of 30.72. Further increasing the blend ratio to (1:2), PCE changes to 0.64 %, with
9

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V
oc of 0.98, FF of 31.69, and reduced Jsc of 2.05 mA/cm was observed. Increasing the blend
ratio to (1:3) and (1:4), PCE dropped to 0.51 and 0.39% respectively with concomitant decrease
in J and V ; whereas FF was nearly similar, which could be the result of high PCBM content.
sc
oc
Reduced PCE for these devices were attributed to the lower Jsc values obtain for these devices.
Additive treatment was given to (1:1) 2D-NDT(Ester) :PC BM to improve the efficiency. Using
2
71
2
0.5% DIO, PCE reaches to 0.97% with improved Jsc of 3.29 mA/cm and FF of 31.22, with
2
reduced V of 0.95V. Increasing to 1% DIO, J and FF were elevated to 3.75 mA/cm and
oc
sc
35.50, PCE reaches to a maximum of 1.22% with V of 0.91. Further increasing DIO to 1.5 and
oc
2%; Jsc and FF were dropped with similar Voc, PCE changes to 1.07 and 0.81% respectively. For
1
1
1
1
1
1
1
1
1
1
2D-NDT(Amide) system, (1:1) and (1:2) blend ratio system, shows average PCE of 0.25 and
2
0.30%. Further increasing the blend ratio to (1:3) highest PCE of 0.35 % with Jsc of 1.81
2
mA/cm , V 0.64 and FF of 30.79 was obtained. For (1:4) blend ratio, PCE dropped to 0.33 %
oc
2
as a result of reduced J of 1.66 mA/cm and V 0.62, whereas FF of 32.62 was observed. In
sc
oc
order to further improvise the device performance, DIO was used for optimized blend ratio of
(1:3). For 0.5 % DIO, (1:3) 2D-NDT(Amide) :PC BM system, PCE was reached to 0.41%,
2
71
with improved Jsc and FF. For 1% DIO, device parameters were improved and PCE elevated to
2
0.55% with higher J of 2.36 mA/cm and FF of 36.95, whereas V remain unchanged to 0.64V.
sc
oc
Further increasing DIO to 2%, all the device parameters were reduced and PCE was dropped to
0.33%.
20
21
22
23
24
25
26
27
External quantum efficiency (EQE) Fig. 3b was measured for best devices, the observed
wavelength range for the EQE spectra of the devices were consistent with UV data (from 300-
600 nm). For (1:2) 2D-NDT(TPD)
2
:PC71BM device shows a maximum value of 14% at 360 nm.
Whereas (1:1) 2D-NDT(Ester) :PC BM device, shows EQE almost at every wavelength with
2
71
highest 23% at 380 nm. 1% DIO (1:1) 2D-NDT(Ester)
2
:PC71BM device system, shows improved
EQE of over 27% at 380 nm. For (1:3) 2D-NDT(Amide) :PC BM device, 17% EQE at 380 nm
2
71
was observed which was improved to 21% at 380 nm, after addition of 1% DIO . Observed EQE
explains the improved Jsc for additive processed devices.
2
8
9
2
1
0

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2
3
4
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7
8
9
0
4
0
(1:2) 2D-NDT(TPD) :PC BM
(
a)
(b)
2
71
0
(
1:1) 2D-NDT(Ester) :PC BM
35
2 71
1
%(1:1) 2D-NDT(Ester) :PC BM
2 71
30
(1:3) 2D-NDT(Amide) :PC BM
2 71
-1
1%DIO (1:3) 2D-NDT(Amide) :PC BM
2 71
25
-2
20
-3
15
10
5
(
1:2) 2D-NDT(TPD) :PC BM
2 71
(1:1) 2D-NDT(Ester) :PC BM
2
71
-4
1
%DIO(1:1) 2D-NDT(Ester) :PC BM
2 71
(1:3) 2D-NDT(Amide) :PC BM
2 71
-5
1%DIO (1:3) 2D-NDT(Amide)
2
:PC71BM
0
0.0
0.2
0.4 0.6 0.8
Voltage (V)
1.0
300
400
500
Wavelength (nm)
600
700
1
11
12
13
Fig. 3. (a) J-V curves of (1:2) 2D-NDT(TPD) :PC BM, (1:1) 2D-NDT(Ester) :PC BM and
2 71 2 71
(1:3) 2D-NDT(Amide)
PCEs.
2
:PC71BM. (b) EQE spectra of the corresponding devices with maximum
14
15
16
17
18
19
20
21
22
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25
26
1
1

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2
Table 2. Photovoltaic performances of small molecules based solar cells.
2
Voc
V)
PCE
(%)
Active layer
1:1) 2D-NDT(TPD) :PC71BM_CB
J
sc (mA/cm )
FF (%)
25.61
27.20
27.01
26.02
30.72
31.69
31.24
31.30
31.22
35.50
32.01
29.61
26.84
29.89
30.79
32.62
35.04
36.95
35.90
32.38
(
(
(
(
(
2
1.58
1.66
1.54
1.38
2.78
2.05
1.76
1.55
3.29
3.75
3.63
2.79
1.49
1.67
1.81
1.66
1.83
2.36
2.13
1.67
0.71
0.73
0.73
0.72
0.98
0.98
0.93
0.80
0.95
0.91
0.92
0.92
0.63
0.61
0.64
0.62
0.64
0.64
0.65
0.62
0.29
0.33
0.30
0.26
0.84
0.64
0.51
0.39
0.97
1.22
1.07
0.81
0.25
0.30
0.35
0.33
0.41
0.55
0.50
0.33
1:2) 2D-NDT(TPD) :PC BM_CB
2
71
1:3) 2D-NDT(TPD)
2
:PC71BM_CB
1:4) 2D-NDT(TPD) :PC BM_CB
2
71
(
(
(
(
1:1) 2D-NDT(Ester) :PC BM_CF
2
71
1:2) 2D-NDT(Ester)
2
:PC71BM_CF
1:3) 2D-NDT(Ester) :PC BM_CF
2
71
1:4) 2D-NDT(Ester) :PC BM_CF
2
71
0
1
1
2
.5% DIO, (1:1) 2D-NDT(Ester) :PC BM
2
71
.0% DIO, (1:1) 2D-NDT(Ester)
2
:PC71BM
.5% DIO, (1:1) 2D-NDT(Ester) :PC BM
2
71
.0% DIO, (1:1) 2D-NDT(Ester) :PC BM
2
71
(
(
(
(
1:1) 2D-NDT(Amide) :PC BM _CF
2
2
2
71
1:2) 2D-NDT(Amide)
1:3) 2D-NDT(Amide)
:PC71BM _CF
:PC71BM _CF
1:4) 2D-NDT(Amide) :PC BM _CF
2
71
0
.5% DIO,(1:3)2D-NDT(Amide) :PC BM
2 71
1
1
2
.0% DIO, (1:3) 2D-NDT(Amide) :PC BM
2
71
.5% DIO, (1:3) 2D-NDT(Amide)
2
:PC71BM
.0% DIO, (1:3) 2D-NDT(Amide) :PC BM
2
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To gain molecular ordering of the small molecules in a solid state, thin film-X-ray diffraction
(XRD) analysis on Si-wafer were performed and shown in Fig. 4. The pristine film display low
angle diffraction peak at 2
θ
= 4.58 arising from the (100) plane, corresponding to the
d-spacing
of 19.27 Å. The much stronger peak intensity of 2D-NDT(TPD)
2
indicate high crystallinity and
ordered molecular arrangement. The high crystallinity could be beneficial for carrier mobility,
which would lead to higher Jsc and FF as consequence of effective transport and collection of
charge carrier in OSC devices. 2D-NDT(Ester) pristine film shows low diffraction angled sharp
2
o
peak at 2
θ
= 5.18 arising from the (100) plane, corresponding to the
d
-spacing of 17.05 Å,
o
whereas 2D-NDT(Amide)
2
show the broad peak with low intensity at 2
θ
= 4.066 for the
d
-
1
1
1
1
1
1
spacing of 21.070 Å. The stronger intensity and sharp peak of ester suggest high crystallinity
with ordered molecular domain compared to amorphous amine molecule. Peaks at the lower
angle are generally attributed to the main planes of small molecules segregated by alkyl chains.
The
NDT(TPD)
packing in ester, than amine and 2D-NDT(TPD) molecules.
d spacing calculated by Bragg equation for 2D-NDT(Ester) (17.05 Å) is smaller than 2D-
2
2
2
(19.27 Å) 2D-NDT(Amine) (21.070 Å), which indicate effective interdigitated
2
16
17
18
19
20
21
22
23
24
25
26
27
28
2
D-NDT(TPD)2
D-NDT(Ester)2
D-NDT(Amide)2
2
2
5
10
15
20
25
30
2θ
Fig. 4. XRD pattern of 2D-NDT(TPD) , 2D-NDT(Ester) and 2D-NDT(Amide) on Si-wafer.
2
2
2
1
3

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2.4. Morphology
To gain inside into photovoltaic performance details, atomic force microscopy (AFM) of the
optimized active layer was measured as shown in Fig. 5. For (1:2) 2D-NDT(TPD)
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
2
:PC71BM
blend systems, small aggregates of PCBM domains can be seen with root mean square (RMS)
roughness values of 0.68. Higher crystallinity of 2D-NDT(TPD) resulted in improper miscibility
2
of donor-acceptor at the nanoscale, which might have affected the PCE for 2D-NDT(TPD)
2
device. For (1:1) 2D-NDT(Ester) :PC BM device, good interpenetrating morphology with rms
2
71
of 2.71 nm was observed. Small domains size of around 10-20 nm were seen, generally required
for effective charge generation and transfer to respective electrodes [32]. It also explains the
higher J and V obtained for this device. Whereas in 1% DIO (1:1) 2D-NDT(Ester) :PC BM
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
sc
oc
2
71
device optimal rms of 0.98 nm, with nanoscale morphology and phase separation, which is
comparable to exciton diffusion length is observed. The increased roughness of the surface is an
indication of self-organization of small molecule and PCBM domains in the blend, promoted by
addition of DIO, which in turn enhances the ordered structure orientations in active layer film
state, essential for promoting exciton dissociation and charge separation towards respective
electrodes. This nano-optimized, desired morphology improved Jsc and FF of this device and
resulted in highest efficiency. For (1:3) 2D-NDT(Amide) :PC BM device, higher rms of 3.47
2
71
nm and larger phase domains likely a result of the strong aggregation due to the intermolecular
hydrogen bonding of amide molecule was observed. Large aggregated domains of PCBM restrict
the charge generation and percolation towards the respective electrodes which restrict the J and
sc
2
FF, thus lower PCE for these devices. In 1% DIO (1:3) 2D-NDT(Amide) :PC71BM device, after
addition of DIO, significantly reduced domain size and lower rms of 0.38 nm was observed;
highly homogeneous system with optimized (D-A) interpenetrating network might have formed
for this system, which ensured the efficient percolation channels for charge transport. Thus
improved J was observed for this device.
sc
2
6
7
2
2
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1
1
1
1
1
1
1
1
1
2
2
0
1
Fig. 5. AFM images of (1:2) 2D-NDT(TPD) :PC BM, (1:1) 2D-NDT(Ester) :PC BM and
2
71
2
71
(1:3) 2D-NDT(Amide)
2
:PC71BM blend films.
2
2
2
2
2
3
4
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5

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2.5. Charge mobility
2
3
4
5
6
7
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0
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2
3
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5
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7
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9
0
1
2
3
4
To evaluate the influence of charge carrier mobility on photovoltaic performance, hole and
electron mobility of (1:2) 2D-NDT(TPD) :PC BM, 1% DIO (1:1) 2D-NDT(Ester) :PC BM and
2
71
2
71
2
1% DIO (1:3) 2D-NDT(Amide) :PC71BM device, were measured by space-charge limited
current (SCLC) method, using the device architecture ITO/PEDOT:PSS/active layer/MoO /Ag
3
for holes and ITO/ZnO/active layer/LiF/Al for electrons, respectively. J-V Characteristics of
hole-only and electron only devices are shown in Fig. S10 and results are summarized in table 1.
-9
2
-1 -1
(1:2) 2D-NDT(TPD) :PC BM device exhibits low hole mobility (µh) of 6.62 × 10 cm V S
2
71
-6
2
-1 -1
and electron mobility (
µ
e
) of 5.07 × 10 cm V S , with unbalanced electron to hole ratio (
µ
e
/µ
h
-6
2
-1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
= 765.86), which explain the lower efficiency of this device. Higher ( h
µ
) of 2.91 × 10 cm V
-1
-6
2
-1 -1
S and (µe) of 8.08 × 10 cm V S was observed for 1% DIO (1:1) 2D-NDT(Ester) :PC BM
2
71
device. The higher electron mobility could be attributed to the better planarity and improved
molecular packing of 2D-NDT(Ester) molecule in the photoactive layer [33-34] Well-balanced
2
e h
electron to hole ratio (µ /µ = 2.78), an important pre-requisite for achieving high efficiency,
which also explains improved J and FF for this device [35] These mobility values are in
sc
agreement with UV and XRD result. The UV absorption and XRD indicate 2D-NDT(Ester)2
molecule shows stronger π-π molecular stacking in the solid state. Closed molecular packing
favors better charge transport properties along with favorable film morphology, [26] explains the
2 2
higher performance for 2D-NDT(Ester) device. For 1% DIO (1:3) 2D-NDT(Amide) :PC71BM
-8
2
-1 -1
-6
2
-1 -1
device, lower ( h
µ ) of 5.20 × 10 cm V S and (µe) of 7.04 × 10 cm V S with unbalanced
ratio ( e µh = 135.38) was observed, than 2D-NDT(Ester) molecule based device. The lower
µ
/
2
charge carrier mobility for 2D-NDT(Amide) molecule can be attributed to disturbed π-π
2
stacking and random molecular ordering induced by strong intermolecular hydrogen bonding
between the amide molecules [25].
25
26
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29
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6

ACCEPTED MANUSCRIPT
1
3. Conclusion
2 2
We developed, new 2D NDT donor based small molecules, 2D-NDT(TPD) , 2D-NDT(Ester)
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
and 2D-NDT(Amide) comprise of A-D-A main framework, with 2D-NDT core end capped
2
with TPD, ester and amide acceptor units. The molecules show good thermal stability, spectral
coverage, and sufficiently higher HOMO energy levels. For OSCs based on (1:2) 2D-
NDT(TPD) :PC BM device, an improper blending of donor and acceptor component, has
2
71
limited the J and FF, also low and unbalanced charge mobility has restricted PCE to 0.33 %. In
sc
case of ester molecules, 2D-NDT(Ester) controlled molecular interaction, that maintain high
2
crystallinity, effective π-π stacking along with the intermolecular compatibility with PCBM in
1
1
1
1
1
1
1
1
1
1
2
active blend were observed. For (1:1) 2D-NDT(Ester) :PC BM device, PCE of 0.84% was
2
71
observed. Further treating with 1% DIO of (1:1) 2D-NDT(Ester) :PC BM device, highest PCE
2
71
of 1.22% was achieved, as a consequence of phase separated morphology and balanced charge
mobility. Whereas, in 2D-NDT(Amide) , hydrogen bonding dominated the other intermolecular
2
interaction, also cause aggregations which give rise to unfavorable morphology, limits the charge
generation and charge transfer in active layer. For (1:3) 2D-NDT(Amide) :PC BM device, PCE
2
71
of 0.35% was observed, further with 1% DIO (1:3) 2D-NDT(Amide) :PC BM devices PCE
2
71
reached to 0.55%. This preliminary work demonstrates that this kind of two-dimensional small
molecules offers a good strategy to design new photovoltaic small molecule based donor
materials, further optimization of the morphology on an active layer would greatly improve the
performance of the devices.
21
22
23
24
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7

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1
2
4. Experimental
4.1 Materials and general methods
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
All the moisture sensitive reactions are carried in a dry vessel under nitrogen (N ) atmosphere,
unless stated otherwise, starting materials were obtained from Aldrich, across and used without
further purification. Anhydrous THF, toluene and chloroform were distilled over
Na/benzophenone prior to use. Chemical shifts were reported as
δ values (ppm) relative to the
internal standard tetramethylsilane (TMS). The UV-vis absorption spectra of films or solutions
were obtained using a Shimadzu UV-2550 spectrophotometer. Photoluminescence (PL) spectra
of films were obtained using an FP-6500 (JASCO). Thermogravimetric analysis (TGA) was
0
-1
1
1
1
1
1
1
1
1
1
1
2
carried out with a TA Instrument Q-50 at a scanning rate of 10 C min under a N atmosphere.
2
Differential scanning calorimetry (DSC) experiments were performed on a TA Instrument (DSC
0
-1
2910) at a heating rate of 100 C min under a N atmosphere. Cyclic voltammetry (CV)
2
measurements were made using a VersaSTAT3 (METEK) Electrochemical Analyzer under
-1
argon at a scan rate of 50 mV s at room temperature, where a Pt wire and Ag/AgCl were used
as counter and reference electrodes, respectively. The reference electrode was calibrated with the
+
ferrocene/ferrocenium (Fc/Fc ) redox couple (4.4 eV below the vacuum level) as an external
standard. The samples were prepared in chloroform solution with 0.10 M tetrabutylammonium
4 6
hexafluorophosphate (n-Bu NPF ) as the electrolyte. The surface morphology was measured
using a Digital Instruments Multimode atomic force microscope (AFM) controlled by a
Nanoscope IIIa scanning probe microscope 20 controller.
21
2-(2-Ethylhexyl)thiophene (1)
22
23
24
25
26
27
28
29
Under the protection of N , n-butyllithium (2.53 mL, 2.5 M), was added dropwise to the solution
2
o
of the dry THF solution (20 mL) containing thiophene (1 gm, 11.8 mmol) at 0 C. After stirring it
for 30 mins, 3-(bromomethyl)heptane (2.53 mL, 14.2 mmol) was added and warm to room
temperature for overnight.
The reaction was poured into water and extracted with
dichloromethane. The extracted organic phase was wash with brine, dried over MgSO and
4
concentrated to get crude product as brown liquid, which was further vacuum distillate to obtain
1
pure compound 1 as colorless oil (0.750gm, 32%). H NMR (400 MHz, CDCl ):
δ 7.13 (d, 1H),
3
1
3
6.93 (d, 1H), 6.77 (d, 1H), 2.78 (d, 2H), 1.3 (m, 9H), 0.89 (t, 6H). C (100 MHz, CDCl ):
δ
3
1
8

ACCEPTED MANUSCRIPT
1
2
144.37, 126.52, 124.93, 122.89, 41.46, 33.82, 32.33, 28.84, 28.81, 25.48, 25.15, 22.98, 14.11,
10.80.
3
(5-(2-Ethylhexyl)thiophen-2-yl)trimethylstannane (2)
4
5
6
7
8
9
0
1
2
In a dried, 2-neck round bottom (100 mL) under the protection ofN , n-butyllithium (1.22 mL,
2
2.5M) was added dropwise to the dry THF (20 mL) solution of compound 1 (0.5 gm, 2.54 mmol)
o
at -78 C for 0.5 hr. Then SnBu
3
Cl (3.31 ml, 3.3 mmol) was added to the reaction mixture at -78
o
o
C . The reaction mixture was further stirred at -78 C for 1 hr and then warm to room
temperature for overnight. Reaction was poured into water and extracted with dichloromethane.
Organic phase was wash with brine, dried over MgSO and concentrated to obtain brown crude
4
1
1
1
1
liquid (0.68 gm, 75 %). H NMR (400 MHz, CDCl ):
δ
6.97 (d, 1H), 6.83 (d, 1H), 2.76 (d, 2H),
): 150.31, 134.87, 126.52,
3
1
3
1.31-1.24 (m, 9H), 0.84 (t, 6H), 0.34 (s, 9H). C (100 MHz, CDCl
3
δ
124.92, 41.46, 32.89, 29.97, 28.88, 26.96, 23.82, 14.16, 10.80, -8.25.
13
5-(2-Ethylhexyl)-2,3'-bithiophene (3)
14
15
16
17
18
19
20
21
22
A mixture of compound 2 (0.5 gm, 1.39 mmol), 3-bromothiophene (0.2 gm, 1.25 mmol),
Pd(PPh (0.08 mg, 5 mol%) were added to a 50 mL flame-dried two-neck flask and subjected
3 4
)
to three vacuum/argon fill cycles. N degassed dry Toulene (10 mL) was added and the mixture
2
was stirred for 15 min flushing N . The reaction mixture was heated to reflux for 24 h. After
2
completion of the reaction, Toulene was removed under reduced pressure and the crude product
was purified by column using a hexane as an eluent afforded compound 3 as colorless liquid
1
(0.24 mg, 70%). H NMR (400 MHz, CDCl
3
):
δ
7.33 (m, 3H), 7.02 (d, 1H), 6.69 (s, 1H), 2.75(d,
144.24, 136.74, 134.73, 128.75, 126.51,
1
3
2H), 1.31 (m, 9H), 0.9 (t,6H). C (100 MHz, CDCl ):
δ
3
125.61, 122.88, 122.40, 41.44, 34.17, 32.34, 28.86, 25.48, 22.99, 14.13, 10.82.
23
2'-Bromo-5-(2-ethylhexyl)-2,3'-bithiophene (4)
24
25
26
27
28
29
30
Compound 3 (0.093 gm, 0.33 mmol) was dissolved in 10 mL of Chloroform:Acetic acid. NBS
(0.059 gm, 0.33 mmol) was added in portion and reaction was allow to stir in dark at room
temperature for overnight. The resultant brown solution was then diluted with 30 mL of water
and extracted with dichloromethane. Organic phase was washed with brine solution and dried
over anhydrous MgSO , evaporated to obtain brown crude. Purification by column
4
chromatography using silica gel with hexane as an eluent gave (0.065 gm, 55%) compound 4 as
1
yellow liquid. H NMR (400 MHz, CDCl ):
δ 7.22 (d, 1H), 7.17 (d, 2H), 6.66 (d, 1H), 2.69 (d,
3
1
9

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3
1
2
3
3
2H), 1.32 (m, 1H), 1.27 (m, 8H), 0.84 (t, 3H). C (100 MHz, CDCl ): δ 145.44, 139, 129.67,
128.46, 128.21, 127.82, 126.28, 112.53, 108.09, 41.40, 34.15, 32.38, 28.86, 25.61, 22.98, 14.09,
10.82.
4
2,7-bis(5-(2-ethylhexyl)-2,3’-bithiophene-2-yl)naphtho[1,2-b:5,6 b’]dithiophene (10)
5
6
7
8
9
0
1
2
3
4
5
6
In a 100 mL two neck flask, Compound 9 (0.330 gm, 0.58 mmol), compound 4 (0.460 gm, 1.2
mmol, 2.2 eq) and dichlorobis-(triphenylphosphine)palladium(II) (0.020 gm, 0.029 mol) were
added and subjected to three vaccum/nitrogen fill cycles. Nitrogen degassed chlorobenzene (15
mL) were added and content were stirred for 15 min under nitrogen. The reaction mixture was
heated to reflux for 24 h. Further reaction was quenched with 30 mL of water and compound was
1
1
1
1
1
1
1
extracted with dichloromethane. Organic layer was washed with brine, dried over MgSO and
4
concentrated under reduced pressure. The resulting brown crude was purified by column
chromatography eluting with hexane to afford compound to afford compound 10 as yellow solid
1
(0.310 gm, 67%). H NMR (400 MHz, CDCl
3
):
δ
7.73 (dd, 2H), 7.75 (dd, 2H), 7.43 (d, 2H), 7.35
1
3
(d, 2H), 7.14 (d, 2H), 6.79 (s, 2H), 2.76 (t, 4H), 1.6 (m, 2H), 1.26 (m, 16H), 0.86 (t, 12H). C
(100 MHz, CDCl3): 145.95, 138.74, 136.90, 135.64, 134.69, 130.94, 129.20, 128.17, 125.40,
δ
123.95, 123.34, 120.95, 110.57, 40.79, 33.83, 31.92, 28.39, 25.03, 22.56, 13.71, 10.44.
1
7
8
2,7-bis(5-(2-ethylhexyl)-2,3’-bithiophene-2-yl)naphtho[1,2-b:5,6 b’]dithiophene distanane
(11)
1
o
19
20
21
22
23
24
25
26
27
28
29
30
To solution of compound 10 (0.319 gm, 0.40 mmol) in THF (10 mL) at -78 C, 2.5 M n-
2
butyllithium in hexane ( 0.386 mL, 2.4 equ) was added dropwise under N atmosphere. After 30
min, trimethyl tin chloride (0.965 mL, 2.4 equ) was added in one portion and stirred for 30 mins
o
at -78 C. The reaction mixture was warmed to room temperature and stirred for overnight. The
reaction was quenched with 20 mL of water and the compound was extracted with
Dichloromethane. The organic layer was washed with 10 mL brine and dried with anhydrous
MgSO
4
. Organic layer was concentrated under reduces pressure to obtain brown liquid, which
1
was used directly for next step 0.41 g (91%). H NMR (400 MHz, CDCl ):
δ 7.78 (d, 2H), 7.68
3
(d, 2H), 7.37 (s, 2H), 7.17 (d, 2H), 6.86 (s, 2H), 6.60 (s, 2H), 2.66 (d, 4H), 1.45 (m, 2H), 1.18
1
3
(m, 16H), 0.80 (t, 12H), 0.35 (s, 18H). C (100 MHz, CDCl3): δ 145.04, 138.37,137.11, 135.27,
134.76, 134.33, 126.63, 126.52, 125.66, 125.37, 125.22, 124.46, 122.51, 122.42, 121.20, 41.35,
34.12, 32.34, 28.81, 25.52, 22.98, 14.11, 10.85, -8.18.
2
0

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2D-NDT(TPD)
2
2
3
4
5
6
7
8
9
0
1
2
3
In 100 mL flame dried flask, compound 11 (0.165 gm, 0.147mmol), compound 13 (0.121 gm,
0.353 mmol), Pd (dba) (7 mg, 5 mol %) and P( -Tolyl)3 (5 mg, 10 mol %) were added and
2
3
o
subjected to three vacuum / N fill cycles. Previously N2 degassed chlorobenzene (10 mL) was
2
added and heated to reflux for 24 hr and was monitored by TLC. After completion of reaction, it
was quenched with water and organic layer was extracted with chloroform. Organic layer was
concentrated under reduced pressure. The resulting dark brown crude was purified by column
1
chromatography using (4:1) CF: hexane to obtain dark red solid compound. (80 mg, 42%). H
NMR (400 MHz, CDCl3):
δ 7.89 (s, 2H), 7.74 (dd, 2H), 7.69 (dd, 2H), 7.56 (d, 2H), 7.48 (d, 2H),
1
1
1
1
6.95 (s, 2H), 6.67(s, 2H), 3.49 (d, 4H), 2.70 (d, 4H), 1.78 (m, 2H), 1.3-1.22 (m, 34H), 0.84 (t,
1
3
3
24H). C (100 MHz, CDCl ): δ 163.04, 162.57, 146.17, 138.86, 137.47, 137.23, 134.77, 133.99,
133.20, 132.43, 131.18, 127.70, 125.77, 125.57, 125.53, 122.58, 121.43, 42.58, 41.36, 38.18,
34.14, 32.34, 30.54, 28.80, 28.53, 25.51, 23.86, 23.04, 23.01, 14.12, 14.04, 10.86, 10.43.
14
2D-NDT-CHO (12)
15
16
17
18
19
20
21
22
23
24
25
A solution of compound 11 (0.37 gm, 0.33 mmol) and 5-bromothiophene-2-carbaldehyde (0.176
gm, 0.92 mmol), PdPPh ) (0.05 equiv) in dry toluene was refluxed and stirred for overnight. The
3
4
reaction mixture was cool to R.T. and diluted with CH Cl . The organic layer was washed with
2
2
4
water and brine; dried over MgSO ; and concentrated in vacuum. The crude compound was
purified by flash column chromatography using the mixture of hexane/CH Cl (1:1.5) as the
2
2
1
eluent to afford the corresponding product as a dark red solid (0.16 gm, 48% yield). H (400
MHz, CDCl3): 8.9 (s, 2H), 7.78-7.74 (m, 2H), 7.72-7.67 (m, 3H), 7.43 (s, 2H), 7.31 (s, 2H),
7.21-7.18 (m, 3H), 6.92-6.91 (d, 2H), 6.66-6.65 (d, 2H), 2.28 (d, 4H), 1.4-1.1 (m, 18H), 0.89 (t,
δ
1
3
3
12H). C (100 MHz, CDCl ): δ 182.42, 146.09, 146.01, 142.17, 139.30, 137.25, 137.19, 134.53,
134.06, 133.32, 133.04, 129.11, 128.12, 127.46, 125.74, 125.55, 125.32, 124.55, 122.58, 121.43,
41.36, 34.09, 32.33, 28.79, 25.51, 22.99, 14.11, 10.86.
26
2D-NDT(Ester)2
27
28
29
30
A solution of compound (12) (0.08 g, 1 equiv.) and 2-ethylhexyl 2-cyanoacetate (0.155 gm, 10
equiv.) in dry CHCl was refluxed overnight with catalytic amounts of triethylamine. The
3
reaction mixture was cool to R.T. diluted with CH Cl . The organic layer was washed with water
2
2
4
and brine; dried over MgSO ; and concentrated in vacuum. The crude compound was purified by
2
1

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2
3
4
5
6
7
flash column chromatography using chloroform as the eluent to afford the corresponding product
1
as a dark red solid (0.075 gm, 69% yield). H (400 MHz, CDCl ):
δ 8.1 (s, 2H), 7.76-7.61 (m,
3
6H), 7.45 (s, 2H), 7.36 (m, 2H), 7.22-7.21 (d, 2H), 6.93-6.92 (d, 2H), 6.67 (d, 2H), 4.06 (d, 4H),
1
3
3
2.7 (d, 4H), 1.6 (m, 4H), 1.25 (m, 32H), 0.89-0.79 (t, 24H). C (100 MHz, CDCl ): δ 163.02,
146.22, 146.17, 145.92, 138.91, 137.29, 134.25, 134.20, 134.12, 133.44, 133.20, 129.49, 127.56,
125.77, 125.58, 125.34, 124.74, 122.61, 121.47,115.83, 68.90, 41.40, 38.80, 34.14, 32.32, 28.92,
28.82, 25.52, 23.77, 23, 22.94, 14.13, 14.03, 11, 10.88.
8
2D-NDT(Amide)2
9
0
1
2
3
4
5
6
7
8
9
A solution of compound (12) (0.08 gm, 1 equiv.) and 2-cyano-N-(2-ethylhexyl)acetamide (0.271
1
1
1
1
1
1
1
1
1
1
mg, 20 equiv.) in dry CHCl was refluxed overnight with catalytic amounts of triethylamine. The
3
2 2
reaction mixture was cool to R.T. diluted with CH Cl . The organic layer was washed with water
and brine; dried over MgSO ; and concentrated in vacuo. The crude compound was purified by
4
flash column chromatography using chloroform as the eluent to afford the corresponding product
1
as a dark red solid (0.035 gm, 37% yield). H (400 MHz, CDCl ):
δ 9.8 (br, 2H), 8.2 (s, 2H),
3
7.79-7.69 (dd, 4H), 7.55-7.54 (m, 2H), 7.45 (d, 2H), 7.33 (d, 2H), 6.93 (d, 2H), 6.66 (d, 2H), 6.1
1
3
(d, 2H), 3.29 (d, 4H), 2.7 (d, 4H), 1.3-1.24 (m, 36H), 0.88-0.79 (t, 24H). C (100 MHz, CDCl
3
):
δ
160.96, 146.51, 145.29, 144.42, 138.60, 135.54, 133.64, 127.89, 125.94, 125.59, 125, 121.79,
117.71, 99.92, 43.76, 41.64, 39.67, 34.36, 32.59, 31.20, 29.11, 29.05, 25.74, 23.20, 14.38, 14.29,
11.09.
20
4.2 Solar cell fabrication and characterization
21
22
23
24
25
26
27
28
29
30
OSCs comprising a BHJ photoactive layer of small molecules: 6, 6- phenyl-C –butyric acid
7
1
methyl ester (PC BM) were prepared on a ITO-coated glass substrate with a structure of
7
1
ITO/PEDOT: PSS/ NDT(TPD) or NDT(TPDTT) :PC BM/ LiF/Al. Prior to use, the patterned
2
2
71
ITO-substrates were cleaned with deionized water, acetone, and isopropyl alcohol using
ultrasonication, followed by treatment with UV ozone. (PEDOT:PSS) (AI 4083, H. C. Starck)
was spin-coated (2600 rpm, 40 s) onto the cleaned ITO-glass at a thickness of 40 nm and dried at
O
140 C for 20 min in atmosphere and then transferred into a glove box filled with N . Blends of
2
NDT(TPD) or NDT(TPDTT) and PC BM (Nano-C, USA) with different weight ratios (from
2
2
71
-1
1:1 to 1:4 w/w) were solubilized overnight in chlorobenzene (20 mg mL ) filtered through a
0.45 mm poly(tetrafluoroethylene) (PTFE)filter and subsequently spin-coated (thickness, 60–70
2
2

ACCEPTED MANUSCRIPT
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2
3
4
5
6
7
8
9
nm) onto the PEDOT:PSS layer of the ITO. The resulting films were dried at room temperature
for 20 min under N and then under vacuum for 12 h. The devices were completed by deposition
2
of a 0.5 nm layer of LiF and a 120 nm Al layer. These layers were thermally evaporated at room
-6
2
temperature under high vacuum at (10 ) Torr. The active area of every device was 9 mm . The
current density–voltage (J–V) characteristics of the photovoltaic devices were measured in the
-
dark and under 1 sun illumination at AM 1.5G using a solar simulator (Newport) at 100 mW cm
2
2
adjusted with a standard PV reference (2 × 2 cm ), a mono-crystalline silicon solar cell
(calibrated at NREL, Colorado, USA) with a Keithley 2400 source-measure unit. The external
quantum efficiency (EQE) was determined using a Polaronix K3100 spectrometer.
10
Acknowledgements
11
12
13
14
15
This work was supported by the National Research Foundation of Korea (NRF) grant funded by
the Korea government (MSIT) (No. 2018R1A4A1025528). This research was also supported by
the Pioneer Research Centre Program through the NRF of Korea funded by the Ministry of
Science, ICT & Future Planning (NRF-2013M3C1A3065528).
16
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Supporting Information (SI)
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A new 2D-Naphtho[1,2-b:5,6-b’]dithiophene Donor Based Small Molecules for
Bulk-heterojunction Organic Solar Cell
5
6
7
Sushil S. Bagde, Hanok Park, Van-Huong Tran, and Soo-Hyoung Lee*
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School of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-
gu, Jeonju-si, jeollabuk-do, 54896, Republic of Korea.
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*Corresponding author; Telephone: +82-63-270-2435, Fax: +82-63-270-2306,
E-mail: shlee66@jbnu.ac.kr
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Material synthesis
2,7-Bis(trimethylstannyl)naphtho[1,2-b:5,6-b
-thieno[3,4- ]pyrrole-4,6(5 )-dione (13) [2], 2-Ethylhexyl 2-cyanoacetate [3], 2-Cyano-N-
′
]dithiophene (9) [1], 1-Bromo-5-(2-ethylhexyl)-
3
4
5
4
H
c
H
(2-ethylhexyl)acetamide [3] and Thiophene-2,5-dicarbaldehyde [4] were synthesized according
to the procedures reported in the literature with slight modification.
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