Synthesis of new conjugated small-molecule-dyes based on 2-(2-methyl-4H-chromen-4-ylidene)malononitrile as the electron-withdrawing group and their application in photovoltaic devices

Article abstract of DOI:10.1016/j.dyepig.2018.12.060

A series of new acceptor-donor-acceptor (A-D-A) type conjugated small-molecule-dyes based on indacenodithieno[3,2-b]thiophene (IDTT) and 2-(2-methyl-4H-chromen-4-ylidene)malononitrile (CMCN), named IDTT-CMCN and IDTT-HT-CMCN, have been synthesized for application in organic solar cells. A-D-A type small-molecules with CMCN can effectively form a powerful push-pull structure that induces effective intramolecular charge transport (ICT) and enhanced optical absorption. In comparison with IDTT-CMCN, IDTT-HT-CMCN has 3-hexylthiophene (HT) as a π-bridge unit between the IDTT and CMCN moieties to increase the solubility and extend the π-conjugation length. The resulting dye (IDTT-HT-CMCN) with a hexylthiophene ring as a π-bridge showed a smaller band gap and better solubility. Thus, a power conversion efficiency (PCE) as high as 7.60% was achieved with a short circuit current of 12.7 mA/cm², open circuit voltage of 0.94 V, and fill factor of 63.8%. To the best of our knowledge, small-molecule-based donors with CMCN as an electron-withdrawing unit have never been reported before. In addition, the PCE of 7.60% reported in this work is highly efficient among the PCE reported for an organic solar cell using an IDTT-based small-molecule-donor material.

Full text of DOI:10.1016/j.dyepig.2018.12.060

Accepted Manuscript
Synthesis of new conjugated small-molecule-dyes based on 2-(2-methyl-4H-
chromen-4-ylidene)malononitrile as the electron-withdrawing group and their
application in photovoltaic devices
Dong Geun Kim, Ho Cheol Jin, Ratna Dewi Maduwu, Sabrina Aufar Salma, Doo
Kyung Moon, Joo Hyun Kim
PII:
S0143-7208(18)32459-8
https://doi.org/10.1016/j.dyepig.2018.12.060
DYPI 7265
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 7 November 2018
Revised Date: 26 December 2018
Accepted Date: 26 December 2018
Please cite this article as: Kim DG, Jin HC, Maduwu RD, Salma SA, Moon DK, Kim JH, Synthesis of
new conjugated small-molecule-dyes based on 2-(2-methyl-4H-chromen-4-ylidene)malononitrile as the
electron-withdrawing group and their application in photovoltaic devices, Dyes and Pigments (2019), doi:
https://doi.org/10.1016/j.dyepig.2018.12.060.
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ACCEPTED MANUSCRIPT
Synthesis of new conjugated small-molecule-dyes based on 2-(2-methyl-4H-chromen-4-
ylidene)malononitrile as the electron-withdrawing group and their application in
photovoltaic devices
1
1
1
1
Dong Geun Kim, Ho Cheol Jin, Ratna Dewi Maduwu, Sabrina Aufar Salma, Doo Kyung
2
1*
Moon, Joo Hyun Kim
1
Department of Polymer Engineering, Pukyong National University, Busan 48547, Korea
2
Department of Materials Chemistry and Engineering, Konkuk University, Seoul 05029, Korea
Corresponding Author
J. H. Kim: jkim@pknu.ac.kr
*
Key Words: Conjugated molecule; 2-(2-methyl-4H-chromen-4-ylidene)malononitrile; organic
solar cell
D. G. Kim and H. C. Jin equally contributed to this research.
1

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Abstract
A series of new acceptor-donor-acceptor (A-D-A) type conjugated small-molecule-dyes based
on
indacenodithieno[3,2-b]thiophene
(IDTT)
and
2-(2-methyl-4H-chromen-4-
ylidene)malononitrile (CMCN), named IDTT-CMCN and IDTT-HT-CMCN, have been
synthesized for application in organic solar cells. A-D-A type small-molecules with CMCN can
effectively form a powerful push-pull structure that induces effective intramolecular charge
transport (ICT) and enhanced optical absorption. In comparison with IDTT-CMCN, IDTT-HT-
CMCN has 3-hexylthiophene (HT) as a π-bridge unit between the IDTT and CMCN moieties to
increase the solubility and extend the π-conjugation length. The resulting dye (IDTT-HT-
CMCN) with a hexylthiophene ring as a π-bridge showed a smaller band gap and better
solubility. Thus, a power conversion efficiency (PCE) as high as 7.60% was achieved with a
2
short circuit current of 12.7 mA/cm , open circuit voltage of 0.94 V, and fill factor of 63.8%. To
the best of our knowledge, small-molecule-based donors with CMCN as an electron-
withdrawing unit have never been reported before. In addition, the PCE of 7.60% reported in this
work is highly efficient among the PCE reported for an organic solar cell using an IDTT-based
small-molecule-donor material.
2

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1
. Introduction
Solution-processed organic solar cells (OSCs) based on bulk heterojunction (BHJ) structures[1]
composed of blended conjugated electron donors and acceptors have attracted considerable
attention as electricity-generating devices due to their unique properties such as their light
weight, mechanical flexibility, and low-cost fabrication over large areas.[2-3] Over the last few
decades, the development of conjugated polymers and small molecules as p-type organic
semiconductors has led to the progressive enhancements of device performances, with power
conversion efficiencies (PCEs) now reaching over 10%.[4-7] Compared to polymeric donors,
small molecule donors in OSCs possess intrinsic advantages, including monodispersity, a well-
defined and versatile structure, and no batch-to-batch variations.[8-9] In recent years, small
molecule-based donors having an acceptor-donor-acceptor (A-D-A) architecture with π-bridge
units have shown great potential for applications in OSCs. The optical and electrochemical
properties of A-D-A small molecule donors can be easily tuned by using different donating and
accepting units, which can also enable efficient intramolecular charge transport (ICT) within D-
A moieties.[10]
Many electron-donating units such as benzodithiophene (BDT)[11-14 (16)],
dithienosilole(DTS)[17-20], or oligothiophene [21-22] have been investigated for the design of
ideal molecular structures with the goal to produce high photovoltaic performance. So far, p-type
small molecules based on those electron donating units have shown high PCEs up to 11.3% in
single-junction OSCs.[23] Though various electron donating units have been applied to the
synthesis of small molecules, indacenodithieno[3,2-b]thiophene (IDTT) is a promising candidate
for the central building block in A-D-A small molecules owing to its coplanar structure and
extended backbone, which is beneficial to enhancing π-electron delocalization, light harvesting,
3

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and hole mobility.[24] In addition, four bulky side chains on the IDTT backbone prevent strong
self-aggregation, so that IDTT-based molecules have better solubility in common organic
solvents and form a favorable morphology. [25] These molecules are listed in Table S1, which
provides a summary of small-molecule-based OSCs with reported PCEs over 6%. Using IDTT
units with various strong electron-withdrawing units for A-D-A small molecule acceptors such as
ITIC has been successfully demonstrated in previous reports. [26] The devices based on those
acceptors with a polymeric donor showed excellent performance with greater than 11%
efficiency.[27-35] Unlike the application of IDTT units as acceptors, there have been few
reports for the application of small molecule donors in OSCs, [36,37] while devices with IDTT-
based donors and fullerene acceptors have been reported with a PCE of up to 7.05%. [38]
For the design of A-D-A small molecule donors containing IDTT as the electron donating unit,
it is necessary to develop end-capping units with proper energy levels to match the electron
acceptors and thus improve photovoltaic performance. From this perspective, the carbonyl
oxygen of chromone (CM) derivatives can be replaced with ylidene malononitrile, 2-(2-methyl-
4
H-chromen-4-ylidene)malononitrile (CMCN), to yield ideal end-capping unit because of their
strong electron accepting nature and planar structure. CMCN derivatives have been used for red-
emitting dyes and as dopants in organic light emitting diodes (OLEDs) [39,40]. However, they
are rarely applied as photoactive materials in OSCs. When CMCN derivatives are introduced to
the electron donating units in A-D-A small molecules, they can effectively form a powerful
push-pull structure that induces effective ICT and enhanced optical absorption.
Herein, we designed and synthesized two A-D-A small molecule donors, denoted as IDTT-
CMCN and IDTT-HT-CMCN, containing IDTT as the electron donating unit and CMCN as
the electron-withdrawing unit. In comparison with IDTT-CMCN, IDTT-HT-CMCN has 3-
4

ACCEPTED MANUSCRIPT
hexylthiophene (HT) as a π-bridge unit between the IDTT and CMCN moieties to increase the
solubility and extend the π-conjugation length. OSCs based on both of these donors and [6,6]-
phenyl-C71-butyric acid methyl ester (PC BM) as the acceptor were fabricated to investigate
7
1
the effect of alkylthiophenes on the photovoltaic properties. Consequently, IDTT-HT-CMCN
exhibits stronger absorption of the short wavelength region than that of IDTT-CMCN and
favorable film morphology when blended with PC BM. To the best of our knowledge, small-
71
molecule-based donors with CMCN as an electron-withdrawing unit have never been reported
before. In addition, the PCE of 7.60% achieved in this research is the highest PCE reported for
OSCs using the IDTT-based small-molecule donors.
Scheme 1. Chemical structures of IDTT-CMCN and IDTT-HT-CMCN.
2. Results and Discussion
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2.1. Synthesis and thermal properties
The detailed synthetic routes of the materials are illustrated in Scheme 1. Compound 4 was
synthesized by a condensation reaction between compounds 2 and 3 in the presence of piperidine
as a catalyst in acetonitrile (ACN), giving a yield of over 40%. Synthesis of IDTT-CMCN was
followed by similar procedures for the synthesis of compound 4. However, a mixed solvent of
chloroform (CF) and ACN was used to synthesize IDTT-CMCN because of its poor solubility in
ACN. IDTT-CMCN showed moderated solubility in CF (up to 5 mg/mL) but poor solubility in
toluene, chlorobenzene (CB), and o-dichlorobenzene (DCB). IDTT-HT-CMCN was prepared
by the Stille coupling reaction between compounds 4 and 6. IDTT-HT-CMCN showed better
solubility in CF (up to 10 mg/mL) due to the hexyl group on the thiophene ring also exhibited
poor solubility in toluene, CB, and DCB. Thermogravimetric analysis (TGA) of IDTT-CMCN
and IDTT-HT-CMCN was carried out at a heating rate of 10 °C/min under air atmosphere (See
Figure S1). IDTT-CMCN and IDTT-HT-CMCN showed good thermal stability with the onset
o
of decomposition temperatures of 407 and 393 C at 5 wt % loss, respectively. Differential
scanning calorimetry (DSC) thermograms of IDTT-CMCN and IDTT-HT-CMCN are
displayed in Figure S2. No discernible thermal transitions such as melting or glass transition
were detected during the heating processes up to 300 °C, due to their amorphous properties.
2
.2. Optical and electrochemical properties
As shown in Figure 1 (a), the UV-Vis spectra of both IDTT-CMCN and IDTT-HT-CMCN
solutions exhibit two absorption bands in the shorter (350-470 nm) and longer wavelength
regions (470-670 nm), respectively. The former corresponds to the π-π* transition of the
conjugated backbone and the latter corresponds to ICT, which is generally observed in
6

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conjugated materials with a donor and acceptor architecture. The maximum wavelength
corresponding to the π-π* transition of IDTT-HT-CMCN is 430 nm, which is longer than that
of IDTT-CMCN (400 nm). This is due to the extended π-conjugation length resulting from the
insertion of a thiophene ring between IDTT and CMCN. On the other hand, the maximum
position of the ICT band of IDTT-HT-CMCN (569 nm) appeared at a shorter wavelength than
that of IDTT-CMCN (601 nm). This is possibly due to the fact that the ICT in IDTT-CMCN is
stronger than that in IDTT-HT-CMCN.[32] The molar extinction coefficient (
ε
) of IDTT-
5
-1
-1
CMCN at 601 nm is 1.2 × 10 M cm , which is higher than that of IDTT-HT-CMCN (1.0 ×
5
-1
-1
1
0 M cm ) at 569 nm. The higher
ε of IDTT-CMCN supports the observation that ICT is
stronger in IDTT-CMCN. The UV-Vis spectra of IDTT-CMCN and IDTT-HT-CMCN films
(Figure 1 (b)) showed similar features to those of the solution phase spectra. The values of the
ꢇꢆꢈ
ꢃꢅꢆ
maximum wavelength of the π-π* transition, ꢀꢁꢂꢃꢁ, and ꢄ
of IDTT-HT-CMCN are 439 nm,
7
24 nm, and 1.71 eV, respectively, which are red-shifted compared with those of IDTT-CMCN
(400 nm, 700 nm, 1.77 eV). This is due to an increase in the number of thiophene rings in IDTT-
HT-CMCN. The positions of the ICT bands of IDTT-CMCN and IDTT-HT-CMCN exhibited
similar trends in their solution spectra. The position of the absorption peaks of the films are red-
shifted compared with those of the solutions owing to the intermolecular π-π stacking.
7

ACCEPTED MANUSCRIPT
Figure 1. UV-visible spectra of IDTT-CMCN and IDTT-HT-CMCN (a) solutions in
chloroform and (b) films on glass substrates.
The HOMO energy level of IDTT-HT-CMCN, as determined from the oxidation onset potential
of the corresponding cyclic voltammogram (CV) (Figure 2), was -5.27 eV, which is higher than
that of IDTT-CMCN (-5.33 eV) owing to the incorporation of a thiophene ring into IDTT-HT-
CMCN. The LUMO energy levels of IDTT-CMCN and IDTT-HT-CMCN calculated from the
reduction onset potential of the corresponding CVs were -3.49 and -3.43 eV, respectively. The
optical and electrochemical properties of IDTT-CMCN and IDTT-HT-CMCN are summarized
in Table 1.
8

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Figure 2. Cyclic voltammograms of (a) IDTT-CMCN and (b) IDTT-HT-CMCN.
Table 1. Summary of optical and electrochemical properties of IDTT-CMCN and IDTT-HT-
CMCN.
a
ꢋꢇꢌꢍꢈꢎꢇꢏ
ꢀꢁꢂꢃꢁ
(nm)
ꢀ
ꢀ
(nm)
ꢉꢅꢊ
c
d
HOMO (eV)
LUMO (eV)
ꢇꢆꢈ
b
ꢐꢎꢌꢉ
(nm)
ꢉꢅꢊ
ꢄ
(eV)
ꢃꢅꢆ
7
1
7
00
.77
24
400, 601
400, 620
430, 569
439, 583
IDTT-CMCN
-5.33
-5.27
-3.49
-3.43
IDTT-HT-CMCN
1.71
a
b
c,d
Absorption edge of the film; Estimated from the ꢀꢁꢂꢃꢁ
;
Estimated from the CV.
Energy level diagrams of IDTT-CMCN, IDTT-HT-CMCN, and other materials in the OSC
are illustrated in Figure 3. From the energy levels, facile exciton dissociation from the donor to
PC BM as well as an energetically favored charge transfer process can be expected.
7
1
9

ACCEPTED MANUSCRIPT
Figure 3. Energy level diagrams of IDTT-CMCN, IDTT-HT-CMCN and the materials used in
this research.
2
.3. Theoretical calculations
Computational calculations based on density functional theory (DFT) were employed to
determine the optical and electrochemical properties. The DFT calculations were performed
using Gaussian 09 software with the hybrid B3LYP correlation functional and split valence 6-
3
1G(d) basis set. Due to the limits of calculation, the hexyl side chains on IDTT and thiophene
were replaced with methyl groups. The optimized geometries of IDTT-CMCN and IDTT-HT-
CMCN are displayed in Figure 4. The HOMO orbitals are delocalized in the conjugated
backbone. On the other hand, the LUMO orbitals are localized in the CMCN unit. The calculated
HOMO/LUMO levels of IDTT-CMCN and IDTT-HT-CMCN are -5.28/-3.07 and -5.03/-2.97,
respectively. Even though these values do not exactly match those observed in the UV-Vis
spectra and CVs, they are well correlated with the experimental results. It can be shown that the
shoulder observed in the absorption peak around 620 nm in the UV-Vis spectra of IDTT-CMCN
and IDTT-HT-CMCN is due to the formation of intermolecular aggregates. It is very interesting
that this shoulder peak in the spectrum of IDTT-CMCN is stronger than that of IDTT-HT-
10

ACCEPTED MANUSCRIPT
CMCN. Based on the side view of IDTT-CMCN and IDTT-HT-CMCN in Figure 4, IDTT-
CMCN shows a more planar structure than IDTT-HT-CMCN. The theoretical calculations also
provide a good interpretation for the aggregation observed in the film state. This supports the
observation that the solubility of IDTT-CMCN is poor compared to that of IDTT-HT-CMCN.
Figure 4. Calculated frontier molecular orbitals of IDTT-CMCN and IDTT-HT-CMCN with
HOMO/LUMO energy levels.
2.4. Photovoltaic properties
To evaluate the performance of IDTT-CMCN and IDTT-HT-CMCN as the donor in OSCs,
we fabricated and tested inverted type polymer solar cells (PSCs) with a structure of ITO/ZnO
(
15 nm)/donor:PC BM/MoO (10 nm)/Ag (100 nm). Devices based on IDTT-CMCN with
71 3
different blend ratios of donor and PC71BM from 3:7 to 3:9 (w/w) were fabricated and tested to
optimize the device fabrication conditions; we fabricated OSCs with diphenyl ether (DPE) as a
processing additive as well. The current density-voltage (J-V) curves of the OSCs under AM
11

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1
.5G simulated illumination and the photovoltaic parameters are shown in Figure S3 and Table
S2. The PCEs were around 0.25 – 0.45%. Regardless of the blend ratio of the active layer, the
devices containing DPE as a processing additive exhibited decreased PCEs. This is due to the
morphology of the active layer. As shown in TEM images (Figure S4), no discernible
morphological changes were observed in the active layer with processing additive. One reason
for this is the poor solubility of IDTT-CMCN in CF (up to 5 mg/mL in CF). It should be noted
that the photovoltaic results achieved for devices based on IDTT-CMCN do not provide us with
important information. OSCs based on IDTT-HT-CMCN as the donor with different blend
ratios of donor and PC71BM from 3:3 to 3:12 (w/w) were fabricated to optimize the device
fabrication process. In addition, we fabricated devices with different concentrations of
processing additives in CF from 1.5 to 2.5 vol.% such as DPE and 1,8-diiodooctane(DIO). The
data are summarized in Table S3. The optimum blend ratio of donor to PC BM and the
7
1
optimum concentration of DPE in CF were determined as 3:8 and 2.0 vol.%, respectively.
Figure 5 (a) shows the current density as a function of the voltage of the OSCs showing the best
PCE under 1.0 sun illumination (inset: dark conditions) at the optimum blend ratio of donor and
PC71BM and optimum concentration of processing additives. The photovoltaic parameters are
summarized in Table 2.
12

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Figure 5. (a) Current density vs. voltage curves of OSCs under 1.0 sun illumination (inset:
under dark conditions) and (b) IPCE spectra of OSCs based on IDTT-HT-CMCN.
13

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Table 2. The best photovoltaic parameters of the OSCs based on IDTT-HT-CMCN at the
blend ratio of 3:8 (w/w) between IDTT-HT-CMCN and PC BM. The averages and error range
7
1
for the photovoltaic parameters of each device are given in parentheses.
Calculated
c
Rs
b
a
J
sc
Jsc
Voc
FF
(%)
PCE
(%)
Additive
2
(Ω
(
mA/cm )
(V)
2
2
cm )
(
mA/cm )
5
.01
0.89
47.3
2.11
none
DIO
5.24
4.52
3.97
3.75
(
4.91±0.08)
11.1
(11.0±0.03)
(0.88±0.01) (46.5±0.70) (2.02±0.06)
0.89 59.9 5.93
(0.89±0.01) (59.8±0.31) (5.91±0.02)
0.94 63.8 7.60
(0.94±0.01) (63.6±0.8) (7.27±0.21)
1
1
0.9
2.6
2
.0%
DPE
.0%
12.7
(12.2±0.33)
2
a
b
c
Thickness of the active layer was 50 nm; Calculated from the IPCE curves; Series resistance
estimated from the device showing the best PCE.
Even though the optical and electrochemical properties of IDTT-HT-CMCN are almost
identical to those of IDTT-CMCN, the presence of the HT group between IDTT and CMCN
improves the PCE from 0.36 to 2.11%. The enhancement of the PCE is presumably due to the
increase in the solubility of IDTT-HT-CMCN in CF (up to 10 mg/mL). The device containing
DIO as a processing additive showed a significant enhancement in PCE from 2.11% to 5.93%.
The improvement of the PCE mainly resulted from a 122% increase in the J . Similarly, the PCE
sc
of the device with DPE as a processing additive exhibited the best PCE of 7.60%. The R data
s
also agree well with the device data. The calculated J values based on incident photon-to-
sc
current efficiency (IPCE) curves (Figure 5 (b)) showed very good agreement with the Jsc data
measured from the devices under 1.0 sun illumination. Interestingly, the PCEs of the devices
treated with CF vapor for 30 and 60 seconds in the glove box were decreased (See Table S3).
Hole- and electron- only devices with a structure of ITO/PEDOT:PSS (40 nm)/IDTT-HT-
CMCN:PC BM (3:8 by weight, 2.0 vol.% DPE, 60 nm)/Au (50 nm) and ITO/ZnO (20 nm)/
7
1
14

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IDTT-HT-CMCN:PC71BM (3:8 by weight, 2.0 vol.% DPE, 60 nm)/LiF (1 nm)/Ag (100 nm),
respectively, have been fabricated and tested to investigate the charge transporting properties. As
shown in Figure S5, the J-V curves showed a characteristic of space charge limited current
(SCLC) and can be expressed by the Mott-Gurney law;[41]
9
ꢄꢗ
ꢘ
ꢑ = ꢒ ꢔ ꢖ
ꢓ ꢕ
8
where J is the current density,
µ is the charge mobility, E is the electric field, ε ε is the
0 r
permittivity the active layer, and L is the thickness of the active layer. We calculated the charge
mobility using the permittivity of 3.0 for the active layer. The hole and electron mobility of the
-3
-3
2
-1 -1
device were 1.43 × 10 and 2.10 × 10 cm V s , respectively. Notably, the electron mobility of
the active layer is higher than that of hole mobility.
As shown in the UV-Vis spectra of the active layers (Figure S6), the absorbance at 400 – 490
nm and 560 – 750 nm of the active layer with additives are higher than that of the active layer
without additives. This is one of the reasons why the devices with additives exhibit higher PCEs
than the devices without additives. The morphologies of the active layers based on IDTT-HT-
CMCN were investigated by transmission electron microscopy (TEM) (Figure 6). The active
layer without additives showed significant phase separation and aggregation (Figure 6 (a)).
However, the active layers with DIO and DPE (Figure 6 (a), (b)) exhibited better nanoscale
phase separation and bicontinuous interpenetrating networks. It was observed that the active
layer with DPE showed the best nanoscale phase separated morphology. Thus, favorable
nanoscale phase separation in the active layer can result in a higher PCE of the related OSC
through efficient charge separation and transport.[42] Based on the grazing incidence wide angle
X-ray scattering (GIWAXS) of IDTT-HT-CMCN (Figure S7), we concluded that the IDTT-
HT-CMCN films do not show any packing behavior.
15

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Figure 6. TEM images of the active layers based on IDTT-HT-CMCN at various
magnification (a) without additive, (b) with 2.0% DIO, and (c) 2.0% DPE.
3
. Conclusion
In this work, two novel conjugated small-molecule-donor materials based on IDTT and
CMCN, labeled IDTT-CMCN and IDTT-HT-CMCN, were developed for organic solar cells.
16

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The electron-accepting CMCN group, when combined with IDTT, can effectively form a
powerful push-pull structure that induces effective ICT and enhanced optical absorption. In
comparison with IDTT-CMCN, IDTT-HT-CMCN has HT as a π-bridge unit between the
IDTT and CMCN moieties to increase the solubility and extend the π-conjugation length. The
PCEs of the devices based on IDTT-CMCN were poor, at around 0.25 – 0.45%. Even though
devices containing DPE as a processing additive were fabricated, the PCEs were not improved.
IDTT-HT-CMCN with the hexylthiophene ring as a π-bridge showed a smaller band gap and
better solubility, thus resulting in a PCE as high as 7.60% with a short circuit current of 12.7
2
mA/cm , open circuit voltage of 0.94 V, and fill factor of 63.8%.
Acknowledgement
This research was supported by the New & Renewable Energy Core Technology Program of the
Korea Institue of Energy Technology Evaluation and Planning (KETEP) under program number
(20153010140030, 2018201010636), and Basic Science Researches through the National
Research Foundation (NRF) of Korea under program number (201603930154).
Graphical Abstract
17

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References and notes
Experimental details of material synthesis, characterization, device fabrication, and analysis
are available in the Supporting Information.
Experimental details of material synthesis, characterization, device fabrication, and analysis
are available in the Supporting Information.
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ò A series of new conjugated small-molecule-donors were synthesized
ò Chromone malononitrile (CMCN) as the electron-withdrawing group
ò The PCE of OSCs accomplished was reached up to 7.6%